A Comprehensive Guide: What is Aluminum Alloy?

What is aluminum alloy?

Aluminum alloy is an alloy based on aluminum and added with a certain amount of other alloying elements. It is one of the light metal materials. In addition to the general characteristics of aluminum, aluminum alloys also have some specific characteristics of alloys due to the different types and quantities of alloying elements. The density of aluminum alloy is 2.63 – 2.85 g/cm and has high strength（σ_b is 110 – 650 MPa), the specific strength is close to that of high alloy steel, and the specific stiffness is higher than that of steel. It has good casting performance and plastic processing performance, good electrical and thermal conductivity, good corrosion resistance and weldability. It can be used as structural materials. It is widely used in aerospace, aviation, transportation, construction, electromechanical, lightweight and daily necessities.

Aluminum alloy is one of the most widely used non-ferrous structural materials in industry, which has been widely used in aviation, aerospace, automobile, machinery manufacturing, shipbuilding and chemical industry. With the rapid development of science and technology and industrial economy, the demand for aluminum alloy welded structural parts is increasing day by day, which makes the research on the weldability of aluminum alloy in-depth. The wide application of aluminum alloy promotes the development of aluminum alloy welding technology. At the same time, the development of welding technology expands the application field of aluminum alloy. Therefore, the welding technology of aluminum alloy is becoming one of the research hotspots.

Name, composition and application of commonly used structural etchants for aluminum and aluminum alloys

Serial number	Name	Form	Condition	Purpose
1	Dilute sodium hydroxide aqueous solution	Sodium hydroxide 15-25g, water 75-85g	Room temperature Time: 1 – 4min	The macrostructure of aluminium and its alloys is shown
2	0.5% hydrofluoric acid aqueous solution	Hydrofluoric acid 0.5ml, distilled water 100ml	Room temperature Time: 10 – 40min	It is suitable for most aluminum and aluminum alloys. Compounds containing Fe and Ni are brown yellow and Si is dark red
3	Sodium hydroxide aqueous solution	Sodium hydroxide 10-15g, distilled water 100ml	Temperature: 50 – 70 ℃, Time: about 5 seconds	Exposure of grain boundaries of aluminum and aluminum alloys
4	25% nitric acid aqueous solution	distilled water 75ml, nitric acid 25ml	Temperature: 70 ℃, Time: 40s	It is suitable for most aluminum and aluminum alloys, especially for aluminum alloys containing copper

Classification of aluminium and aluminium alloys

Wrought aluminum alloy

Aluminum alloy with high strength, high specific strength and suitable for plastic forming.

Wrought aluminum alloy can be divided into:

1. Industrial pure aluminum
2. Non strengthening aluminum alloy by heat treatment
3. Aluminum alloy strengthened by heat treatment

Cast aluminum alloy

It is suitable for filling the mold in molten state to obtain aluminum alloy with certain shape and size.

Cast aluminum alloy is divided into:

1. Al Si alloy
2. Aluminum copper alloy
3. Aluminum magnesium alloy
4. Al Zn Alloy

Grades of aluminium and aluminium alloys

According to the content of aluminum and other elements in aluminum alloy:

• (1) Pure aluminum: pure aluminum is divided into high-purity aluminum, industrial high-purity aluminum and industrial pure aluminum according to its purity. The welding is mainly industrial pure aluminum. The purity of industrial pure aluminum is 99.7% to 98.8%, and there are six brands, such as l1.l2.l3.l4.l5.l6.
• (2) Aluminum alloy: aluminum alloy is obtained by adding alloying elements to pure aluminum. According to the processing characteristics of aluminum alloys, they can be divided into deformation aluminum alloys and casting aluminum alloys. Wrought aluminum alloy has good plasticity and is suitable for pressure machining.

Wrought aluminum alloys can be divided into antirust aluminum (LF), hard aluminum (ly), super hard aluminum (LC) and forged aluminum (LD) according to their performance characteristics and applications. Cast aluminum alloys can be divided into aluminum silicon system (Al Si), aluminum copper system (Al Cu), aluminum magnesium system (Al Mg) and aluminum zinc system (Al Zn).
The main aluminum alloy grades are: 1024, 2011, 6060, 6063, 6061, 6082, 7075

Group	Brand series
Pure aluminum (aluminum content not less than 99.00%)	1XXX
Aluminum alloy with copper as the main alloy element	2XXX
Aluminum alloy with manganese as the main alloy element	3XXX
Aluminum alloy with silicon as the main alloy element	4XXX
Aluminum alloy with magnesium as the main alloy element	5XXX
Aluminum alloy with magnesium and silicon as main alloying elements and Mg2Si phase as strengthening phase	6XXX
Aluminum alloy with zinc as the main alloy element	7XXX
Aluminum alloy with other alloy elements as main alloy elements	8XXX
Spare alloy set	9XXX

• The first digit of the brand indicates the alloy system (main alloy element).
• The third and fourth digits of the brand name represent the identification of alloys with different chemical compositions.
• The second letter of the brand indicates the modification of the original pure aluminum or aluminum alloy, and the last two digits indicate that the last two digits of the brand identify different aluminum alloys in the same group or indicate the purity of aluminum.
• 1×××: the last two digits of the series brand are expressed as percentage points of the lowest aluminum content. For example, 1050,1070 indicates that the aluminum purity is more than 99.50% and 99.70% respectively. The second letter of the brand indicates the modification of the original pure aluminum.
• 2××× – 8×××: the last two digits of the series brand have no special significance and are only used to distinguish: different aluminum alloys in the same group. The second letter of the brand indicates the modification of the original pure aluminum.

*Status of aluminium and aluminium alloys:

• Code F×× Is: free machining status;
• O×× As: annealing state;
• H××As: work hardening state;
• W×× As: solid fusion heat treatment state;
• T×× Is: heat treatment state (different from F, O and H States).

*H×× Subdivision status of: the first digit after H indicates the basic handler to obtain this status, as shown below.

• H1: simple work hardening state;
• H2: work hardening and incomplete annealing state;
• H3: work hardening and stabilization state;
• H4: work hardening and painting state h.

The second digit after H indicates the work hardening degree of the product. For example, 0 – 9 represents that the work hardening degree is getting harder and harder.
*Rolling of aluminum alloy plate and strip: according to the rolling temperature, it can be divided into hot rolling, warm rolling (medium temperature rolling) and cold rolling. According to the arrangement of rolling mills, it can be divided into single stand rolling, semi continuous rolling and continuous rolling.

Characteristics of aluminum and aluminum alloy (aluminum profile)

Compared with other metal materials, aluminum and aluminum alloys have the following characteristics:
1. Low density
The density of aluminum and aluminum alloy is close to 2.7g/and about 1/3 of that of iron or copper.
2. High strength
Aluminum and aluminum alloys have high strength. After a certain degree of cold working, the matrix strength can be strengthened, and some brands of aluminum alloys can also be strengthened by heat treatment.
3. Good conductivity
The electrical and thermal conductivity of aluminum is second only to silver, copper and gold.
4. Good corrosion resistance
The surface of aluminum is easy to naturally produce a layer of dense and firm Al2O3 protective film, which can well protect the matrix from corrosion. Through artificial anodizing and coloring, cast aluminum alloy with good casting performance or deformed aluminum alloy with good processing plasticity can be obtained.
5. Easy processing
After adding certain alloying elements, cast aluminum alloy with good casting properties or deformed aluminum alloy with good processing plasticity can be obtained.

Application of aluminum alloy

Aluminum alloy has the characteristics of low density, good mechanical properties, good processability, non-toxic, easy recovery, excellent conductivity, heat transfer and corrosion resistance. It is widely used in marine industry, chemical industry, aerospace, metal packaging, transportation and other fields.
1. Aerospace
Aluminum alloy is the main material for making aircraft. Compared with mild steel for automobile, aluminum alloy has expensive price and low density, with a relative density of 2.8. Compared with mild steel with a relative density of 7.8, it is about one-third lighter. Although the strength difference is not much, for aircraft, light material is the most important, with strong corrosion resistance and convenient processing. Therefore, aluminum alloy is the most ideal material for aircraft.
Duralumin can manufacture rivets, aircraft propellers and high-strength parts on aircraft according to its different alloy element content; Superhard aluminum is a kind of hard aluminum containing zinc. Its hardness and strength are higher than that of hard aluminum. Different kinds of superhard aluminum are used to manufacture various structural parts and high load parts. It is one of the important materials in the aviation industry.
2. Marine industry
Aluminum and aluminum alloys are more and more widely used in the shipbuilding industry, ranging from motor boats to 10000 ton oil tankers, from offshore hovercraft to submarine, from civil to military, from fishing ships to marine mining ships. Aluminum alloys with excellent comprehensive performance are used to produce ship shells, support structures, supporting facilities, pipelines, etc.
The application of aluminum alloy in marine industry can reduce the overall weight of the ship, improve the running speed of the ship, and resist the corrosion of seawater to the ship. Aluminum alloys used in marine industry are mainly aluminum copper alloy, aluminum magnesium alloy and aluminum silicon alloy. Aluminum copper alloy is widely used in ships in China and Russia, but its poor seawater corrosion resistance hinders its development in the shipbuilding industry. Aluminum magnesium alloy is mainly used for hull shell, water pump conduit, pump shell and base support, with grades of 5083, 5086, 5456, 5466, etc. Aluminum silicon alloy has moderate structural strength, good fluidity and strong filling capacity. It is easy to produce parts with high density and complex structure, such as high-pressure valve, cylinder block, pump, reducer housing, turbine blade, etc.
3. Application in chemical industry
Aluminum has good thermal conductivity. Aluminum and aluminum alloys are widely used in the production of heat exchange equipment in chemical equipment, storage tanks resistant to concentrated nitric acid corrosion, adsorption filters, fractionators, pipelines and many linings. Cast aluminum alloy has good fluidity, strong mold filling ability, small shrinkage, not easy to form cracks, good corrosion resistance (aluminum oxide and silicon dioxide protective film can be formed on its surface), light weight and good mechanical properties. It is widely used to manufacture corrosion-resistant parts with complex structure, such as cylinders, pipe fittings, valves, pumps, pistons, etc. Aluminum has many special uses in chemical production. Aluminum does not produce sparks, and aluminum alloy can produce containers containing easily volatile substances; Aluminum is non-toxic, will not cause food deterioration, will not affect the appearance of goods and will not corrode goods. Therefore, aluminum alloy is widely used to make relevant equipment in food chemical industry.
4. Application in metal packaging industry
Aluminum alloy can be used for metal packaging, which has the following excellent characteristics: good mechanical properties, light weight, high compressive strength, durability, easy to store and transport goods; Good barrier performance, can block the damage of sunlight, oxygen and humid environment to articles, and can prolong the shelf life of articles; Good texture and aesthetic feeling. Aluminum alloy is used as packaging, with unique metallic luster, good touch feeling and beauty, so as to improve the quality of goods; Non toxic, easy to recycle, recyclable, saving resources and reducing environmental pollution. Aluminum alloy is widely used in beer, beverage and other food cans, mostly in stamping and drawing structure. Aluminum foil utensils are beautiful, light and good heat transfer. They are used in the packaging of fast food. They have the effects of fresh preservation, taste preservation and non-toxic. They are used by more and more food industries. The aluminum alloy metal hose can be extruded and deformed, and the contents can be used after extrusion. It is simple and convenient. It is commonly used in the packaging of paste cosmetics.
5. Application in other industries
Aluminum alloy has high specific strength, light weight, good fluidity, strong filling capacity, good corrosion resistance and low melting point. It is widely used in tractor, locomotive parts, electronic products, medical devices, building decoration and other industries. Aluminum alloy has excellent ductility and is widely used in daily necessities industry and food industry. In the field of power transmission, aluminum alloy conductor has attracted more and more attention because of its low cost, light weight, good corrosion resistance, heat transfer, easy conductivity and wear resistance. In the field of power transmission, aluminum alloy is used the most, and up to 90% of high-voltage conductor materials are aluminum products. Al Si alloy can be used as a good deoxidizer to reduce the sensitivity of steel to subcutaneous bubbles, deoxidize steel and improve the quality of steel. The market consumption of Al Si alloy is large, and the national annual demand is up to one million tons.
I. JISA.A1000 series – pure aluminum series

• 1. 1060 as a conductive material, IACS guarantees 61%, and 6061 wire is used when strength is required.
• 2. 1085,1080,1070,1050,1N30,1085,1080,1070,1050 – good formability and surface treatment, and its corrosion resistance is the best among aluminum alloys. Because it is pure aluminum, its strength is low. The higher the purity, the lower the strength. Daily necessities, aluminum plate, lighting appliance, reflector, decoration, chemical industry container, heat sink, solution wiring, conductive material.
• 3. 1100,1200al general purpose aluminum with purity of more than 99.0% is slightly white after anodizing, which is the same as the above. General utensils, heat sink, bottle cap, printing board, building materials and heat exchanger assembly 1n00 – strength is slightly higher than 1100, good formability, and its chemical characteristics are the same as 1100.

II. Daily necessities 2000 series – alxcu series

• 1. 2011 fast cutting alloy, good machinability and high strength. But the corrosion resistance is poor. When corrosion resistance is required, 6062 series alloy volume shaft, optical components and screw head shall be used.
• 2. 2014,2017,2024 contains a large amount of Cu, which has poor corrosion resistance, but high strength. It can be used as structural materials. Forgings can also be used for aircraft, gears, oil, pressure components and axles.
• 3. After solid solution heat treatment, 2117 is used as hinge nail material, which is an alloy to delay the aging rate at room temperature.
• 4. 2018,2218 alloy for forging. It has good forging property and high high temperature strength. Therefore, it is used for forgings requiring heat resistance. It has poor corrosion resistance, such as cylinder head, piston and VTR cylinder.
• 5. 2618 alloy for forging. High temperature strength is superior, but corrosion resistance is poor. Piston, die for rubber forming, general heat-resistant components.
• 6. 2219 has high strength, good low and high temperature characteristics, excellent solubility, but poor corrosion resistance. Cryogenic vessels, aerospace machines. 7. 2025 alloy for forging. Good forging property and high strength, but poor corrosion resistance. Propeller, magnetic barrel. 2n01 – alloy for forging. It has heat resistance and high strength, but poor corrosion resistance. Aircraft engine, oil pressure components.

III 3000 series – alxmn series

• 1. The strength of 3003,3203 is about 10% higher than 1100, and its formability, solubility and corrosion resistance are good. General utensils, heat sink, cosmetic board, photocopier drum, marine materials.
• 2. 3004,3104 has higher strength than 3003, superior formability and good corrosion resistance. The strength of aluminum tank, lamp cap, roof plate, color aluminum plate 3 and 3005,3005 is about 20% higher than that of 3003, and the corrosion resistance is also better. Building materials, color aluminum plate.
• 4. The strength of 3105,3105 is slightly higher than that of 3003, and other characteristics are similar to that of 3003. Building materials, color aluminum plate, bottle cap.

IV 4000 series – alxsi series

• 1. 4032 has good heat resistance, abrasion resistance and low coefficient of thermal expansion. Piston, cylinder head.
• 2. 4043 has less solidification shrinkage and is treated with sulfuric acid anodizing, showing a natural gray color. Dissolving wiring, building panel.

V 5000 Series – alxmg series

• 1. The strength of 5005 is the same as that of 3003, with good processability, solubility and corrosion resistance. The modification processing after anodizing is good, which is commensurate with the color of 6063 shape material. Interior and exterior decoration for construction, interior decoration for vehicles and interior decoration for ships.
• 2. 5052 is the most representative alloy with medium strength. It has good corrosion resistance, solubility and formability, especially high fatigue strength and good seawater resistance. General sheet metal, ships, vehicles, buildings, bottle caps, honeycomb plates.
• 3. 5652 restricts the impure elements of 5052 and inhibits the separation of hydrogen peroxide. Other characteristics are about 20% higher than 5052 in strength with 5052 and hydrogen peroxide containers 4 and 5154. Other characteristics are the same as 5052 and pressure vessel
• 5. 5254 limits the impure elements of 5154 and inhibits the decomposition of hydrogen peroxide. Other properties are the same as 5154.
• 6. The strength of the 5454 is about 20% higher than that of the 5052, and its characteristics are roughly the same as those of the 5154, but its corrosion resistance in vicious environment is better than that of the 5154. Car wheels.
• 7. 5056 has excellent corrosion resistance, cutting and working surface modification, good anodizing and dyeing. Camera body, communication machine components, zipper.
• 8. 5082 has similar strength to 5083 and good formability and corrosion resistance. Tank cover.
• 9. The strength of 5182 is about 5% higher than that of 5082, and other characteristics are the same as that of 5082. Tank cover.
• 10. 5083 alloy for fusion construction. It is the highest strength corrosion-resistant alloy in practical non heat treated alloys and is suitable for solution bonding structures. Ships, vehicles, low temperature vessels and pressure vessels with good seawater resistance and low temperature characteristics.
• 11. 5086 has higher strength than 5154. It is a non heat treatment fusion structure alloy with good seawater resistance. Ship, pressure vessel and magnetic disc 5n01 – the strength is the same as that of 3003, and the anodizing treatment after brilliant treatment can have high brilliance. Good formability and corrosion resistance. Kitchen supplies, cameras, decorations, aluminum 5n02 hinge nail alloy, with good seawater resistance.

Ⅵ 6000 series – alxmgxsi series

• 1. 6061 heat treated corrosion resistant alloy. T6 treatment can have a very high endurance value, but the strength of the fusion interface is low. Therefore, the extrusion alloy used in 6n01 for screws, hinged nails, ships, vehicles and land structures has the middle strength of 6061 and 6063. It has good extrusion, stamping and quenching properties. It can be used as a large thin meat material with complex shapes, with good corrosion resistance and fusion properties. Vehicles, land structures, ships.
• 2. 6063 is a representative extrusion alloy with lower strength than 6061 and good extrudability. It can be used as a shape material with complex section shape. It has good corrosion resistance and surface treatment. It is used in buildings, highway guardrails, high hurdles, vehicles, furniture, household appliances and decorations
• 3. 6101 high strength conductive material. 55% IACS guaranteed wires.
• 4. 6151 has excellent forging processability, corrosion resistance and surface treatment. It is suitable for complex forging products. Mechanical and automotive components 5 and 6262 have better corrosion resistance and surface treatment than 2011, and their strength is the same as 6061. Camera body, oxidizer assembly, brake assembly and gas appliance assembly.

VII 7000 series – alxznxmg series

• 1. 7072 electrode has low potential. It is mainly used for anti-corrosion covering leather materials. It is also suitable for heat sink of heat exchanger. Aluminum alloy sheet leather, heat sink.
• 2. 7075 aluminum alloy is one of the alloys with the highest strength, but its corrosion resistance is poor. The coating material with 7072 can improve its corrosion resistance, but the cost is increased. Aircraft and ski poles 7050, 7050 are alloys that improve the quenchability of 7075. They have good stress corrosion cracking resistance. They are suitable for 7N01 fusion structure alloys for thick plates, forged aircraft and high-speed rotating bodies. They have high strength. Moreover, the strength of the fusion part can be returned to the strength close to the base metal when placed at room temperature. Corrosion resistance is also very good. Vehicles, other land structures and aircraft.
• 3. 7003 extrusion alloy for fusion structure has slightly lower strength than 7N01, but good extrudability. It can be used as a large shape material with thin meat. Other properties are roughly the same as 7N01. Outer ring of vehicle and locomotive wheel.

Manufacturing process of aluminum alloy

Aluminum alloy is obtained by refining raw aluminum and recovered aluminum obtained by electrolysis, and then adding elements to it.
The process of electrolytic raw aluminum is to extract alumina from bauxite by Bayer method, and then electrolytic aluminum by ERU hall method.
Electrolysis is an application of electrochemistry. The so-called electrolysis is a process in which the electrolytic cell is driven by electric energy and the product is separated by the oxidation-reduction reaction on the cathode or cathode of the electrolytic cell. These products are useful to us.
The principle of electrolytic aluminum smelting is the same. But electrolytic aluminum uses a special electrolyte, not an aqueous solution. Aluminum cannot be obtained by electrolysis in aqueous solution. The method of electrolytic melting alumina is adopted. Alumina (alumina) is an ionic compound. It is usually solid. Ions cannot move easily in solids. Only when they are heated above the melting point and melted into liquid, can ions migrate between cathode and anode under the drive of electric field to form ionic conductors. However, the melting point of alumina is very high. Before heating it to melting, most of the structural materials used to make electrolytic cells, such as steel, are melted first. Therefore, it should be dissolved in a suitable solvent to form a special solution. The solvent used is cryolite and sodium hexafluoroaluminate. In this way, the working temperature of electrolytic aluminum can be reduced below 1000 ℃ (generally 950 ℃ – 970 ℃).

Heat treatment and quenching process of aluminum alloy

The heat treatment of aluminum alloy is to select a certain heat treatment specification, control the heating speed to rise to a certain corresponding temperature, hold it for a certain time and cool it at a certain speed to change the structure of the alloy. Its main purpose is to improve the mechanical properties of the alloy, enhance the corrosion resistance, improve the processing function and obtain the dimensional stability.
It is well known that for steels with high carbon content, high hardness and low plasticity can be obtained immediately after quenching. However, it is not the case for aluminum alloy. After quenching, the strength and hardness do not increase immediately, but the plasticity does not decrease, but increases. However, when the quenched alloy is placed for a period of time (e.g. 4 – 6 days and nights), the strength and hardness will be significantly improved, while the plasticity will be significantly reduced. The phenomenon that the strength and hardness of aluminum alloy increase significantly with time after quenching is called aging. Aging can occur at room temperature, called natural aging, or in a temperature range higher than room temperature (such as 100 – 200 ℃), called artificial aging.
Age hardening of aluminum alloy is a very complex process. It depends not only on the composition and aging process of the alloy, but also on the defects caused by shrinkage in the production process, especially the number and distribution of vacancies and dislocations. At present, it is generally believed that age hardening is the result of the segregation of solute atoms to form hardening zone.
The size and quantity of hardening zone depend on quenching temperature and quenching cooling rate. The higher the quenching temperature, the greater the vacancy concentration, the more the number of hardened zone and the smaller the size of hardened zone. The larger the quenching cooling rate is, the more vacancies are fixed in the solid solution, which is conducive to increasing the number of hardening zone and reducing the size of hardening zone.
A basic characteristic of precipitation hardening alloy system is the equilibrium solid solubility varying with temperature, that is, the solid solubility increases with the increase of temperature. Most aluminum alloys that can be strengthened by heat treatment meet this condition.

Basic knowledge of aluminum and aluminum alloy heat treatment

1. Function of heat treatment of aluminum and aluminum alloy
Aluminum and aluminum alloy materials are heated to a certain temperature and held for a certain time to obtain the expected product microstructure and properties.
2. Main methods and basic principles of heat treatment of aluminum and aluminum alloys
(1) Basic principle of heat treatment of aluminum and aluminum alloys
Annealing: the product is heated to a certain temperature and held for a certain time, and then cooled to room temperature at a certain cooling rate. Through atomic diffusion and migration, the structure is more uniform, stable and internal stress is eliminated, which can greatly improve the plasticity of the material, but the strength will be reduced.

• ① Ingot homogenization annealing: keep warm at high temperature for a long time, and then cool at a certain speed (high, medium, low and slow), so as to homogenize the chemical composition, structure and properties of the ingot, which can improve the plasticity of the material by about 20%, reduce the extrusion force by about 20%, increase the extrusion speed by about 15%, and improve the surface treatment quality of the material at the same time.
• ② Intermediate annealing: also known as local annealing or inter process annealing, it is to improve the plasticity of materials, eliminate the internal processing stress of materials, and keep them warm for a short time at a lower temperature, so as to facilitate continuous processing or obtain a combination of certain properties.
• ③ Complete annealing: also known as finished product annealing, it is held at a higher temperature for a certain time to obtain the softened structure in the state of complete recrystallization, with good plasticity and low strength.

Solution quenching: heat the heat treatable and strengthened aluminum alloy material to a higher temperature and keep it for a certain time, so that the second phase or other soluble components in the material can be fully dissolved into the aluminum matrix to form a supersaturated solid solution, and then maintain the supersaturated solid solution to room temperature by rapid cooling. It is an unstable state because it is in a high-energy state, Solute atoms may precipitate at any time. However, at this time, the plasticity of the material is high, and the cold working or straightening process can be carried out.

• ① On line quenching: for some alloy materials with low quenching sensitivity, solid solution can be carried out at high temperature during extrusion, and then quenched with air cooling (T5) or water mist cooling (T6) to obtain certain microstructure and properties.
• ② Off line quenching: some alloy materials with high quenching sensitivity must be reheated to a higher temperature in a special heat treatment furnace and maintained for a certain time, and then quenched into water or oil with a transfer time of no more than 15 seconds to obtain a certain structure and performance. According to different equipment, they can be divided into salt bath quenching, air quenching, vertical quenching and horizontal quenching.

(2) Aging: when the material after solution quenching is maintained at room temperature or higher temperature for a period of time, the unstable supersaturated solid solution will decompose, and the second phase particles will precipitate (or precipitate) from the supersaturated solid solution and distribute in α( Al) around the aluminum grain, resulting in strengthening, which is called precipitation (precipitation) strengthening.
Natural aging: some alloys (such as 2024, etc.) can produce precipitation strengthening at room temperature, which is called natural aging.
Artificial aging: some alloys (such as 7075, etc.) precipitate at room temperature without obvious strengthening, but the precipitation strengthening effect is obvious at higher temperature, which is called artificial aging.
Artificial aging can be divided into under aging and over aging.

• ① Under aging: in order to obtain certain properties, lower aging temperature and shorter aging time are controlled.
• ② Over aging: aging at higher temperature or holding for a long time in order to obtain some special properties and better comprehensive properties.
• ③ Multistage aging: in order to obtain some special properties and good comprehensive properties, the aging process is divided into several stages. It can be divided into two stages and three stages.

(3) Regression treatment: in order to improve plasticity, facilitate cold bending or correct geometric tolerance, the quenched and aged products can be restored to the new quenched state after heating at high temperature for a short time, which is called regression treatment.

Quenching cooling rate of aluminum alloy

The cooling rate of aluminum alloy quenching furnace must ensure that the supersaturated solid solution is fixed and does not decompose. Prevent the precipitation of strengthening phase and reduce the mechanical properties after quenching and aging.
Therefore, the faster the cooling rate during quenching, the better. However, the greater the cooling rate, the greater the residual stress and residual deformation of quenched products. Therefore, the cooling rate should be determined according to different alloys and products of different shapes and sizes.
The quenching of general alloys is highly sensitive to the cooling rate, and the cooling rate should be large. For example, the quenching cooling rate of 2A11 and 2A12 alloys should be above 50 ℃ / s, while 7A04 alloy is very sensitive to the cooling rate, and its quenching cooling rate should be above 170 ℃ / s.
For products with different shapes and sizes, different cooling rates should be adopted, which is usually realized by adjusting the temperature of quenching medium. For simple shape, small and medium-sized bars, room temperature water quenching can be used (water temperature is generally l0 – 35 ℃). For profiles with complex shape and large wall thickness difference, water quenching at 40 – 50 ℃ can be used.
For products that are particularly prone to deformation, the water temperature can even be raised to 75 – 85 ℃ for quenching. The test shows that the mechanical properties and corrosion resistance of quenched products decrease with the increase of water temperature.
The common quenching medium of aluminum alloy quenching furnace is water. Because the viscosity of water is small, the heat capacity is large, the heat of evaporation is fast, the cooling capacity is strong, and the use is very convenient and economical.
However, its disadvantage is that the cooling capacity decreases after heating. The cooling of quenched and heated products in water can be divided into three stages: the first stage is film boiling stage. When the hot product just comes into contact with cold water, an uneven superheated steam film is immediately formed on its surface. It is very firm and has poor thermal conductivity, which reduces the cooling speed of the product.
The second stage is bubble boiling stage. When the vapor film is destroyed, the liquid near the metal surface will produce violent boiling and strong heat exchange. The third stage is the heat convection stage. The circulation of cooling water, or the product swings left and right, or moves up and down, increases the convective heat exchange between the product surface and water, so as to improve the cooling speed.
According to the above analysis, in order to quickly break through the first stage, step by step cooling and ensure uniform cooling of quenched products, compressed air pipes need to be installed in the quenching water tank for mixing, and the products should swing properly after entering the water tank.
In addition, in order to ensure that the water temperature will not rise too much, the quenching tank shall have sufficient capacity (generally more than 20 times the total volume of quenched products). Moreover, the cooling water shall have a circulating device.
In addition to adjusting the water temperature to control the quenching cooling rate of aluminum alloy quenching furnace, different solvents can also be added to the cooling water to adjust the cooling capacity of the water.
Usually, the polyethylene alcohol aqueous solution is used as the cooling medium. At the same time, the concentration of polyethylene alcohol aqueous solution can be adjusted to control the cooling rate of product quenching. Generally, products that are easy to deform are often quenched with this aqueous solution of polyethylene alcohol.

What is the difference between air cooling and water cooling quenching of aluminum alloy profiles

The quenching of aluminum alloy is from high temperature to low temperature. For example, the steel blade should be quenched in water before it is hard and sharp. The quenching of aluminum also includes water cooling quenching and air cooling quenching.
6063 aluminum profile for construction is quenched by air cooling. The quenched aluminum profile is aged in the aging furnace for a certain time. The internal crystallization of aluminum profile is rearranged, and the mechanical strength is significantly improved. Among all metal alloys, only aluminum alloy has aging state.
The forming temperature of aluminum alloy extruded profile is 460 – 500 degrees. Generally, the temperature after quenching is 200 degrees. The state of artificial aging after high temperature extrusion and solution heat treatment (quenching) is air cooling (T5); The state of cooling through the high-temperature extrusion process and then artificial aging is water cooling (T6).
As the saying goes: T5 is air-cooled after extrusion. T6 is water-cooled after extrusion, and its hardness is improved after water quenching. It’s that simple. However, if the aluminum material needs to be pulled and bent, try not to T6.
Some people think that the difference between T5 and T6 is only the difference of cooling speed, not the difference of air cooling and water cooling. If the cooling speed of air cooling is large enough, it can also achieve T6 effect. On the contrary, if the cooling speed of water cooling is not large enough, it can only be T5 effect!
In fact, T6 status can be quenched online (water cooling or strong air cooling) or offline (quenched in quenching furnace), but it should be determined according to customer requirements and product varieties and specifications.
In actual production, no matter which production process is adopted, the key is to meet the mechanical strength requirements of the aluminum profile product.
Hardness has little to do with air cooling speed and water cooling speed.
The better the cooling effect is, the better the hardness will be after aging. Why are there T5 and T6? Because the size of T5 air-cooled products will not deform, T5 will be selected for almost all profiles such as doors and windows. T6 is a profile with relatively thin water-cooled thickness, which will deform in case of water, especially those with openings are easy to deform in case of water.
Of course, you can also use floating water in the form of water, that is, the spray way of water. Of course, the effect of the product after water is much better than that of air cooling. 6063-T5 is between 10-13, and 6063-T6 can reach more than 13.

Why is aluminum alloy difficult to electroplate?

As a widely used metal material, aluminum needs metal surface treatment in order to have better performance. In most cases, aluminum materials will be anodized instead of electroplating. Electroplating on aluminum and aluminum alloy is much more difficult and complex than electroplating on steel, copper and other metal materials. The main reasons are as follows.

• 1. Aluminum and aluminum alloys have high affinity for oxygen and are easy to form oxide film. Once this oxide film is removed, a new oxide film will be produced in a very short time, which will seriously affect the adhesion of the coating.
• 2. The electrode potential of aluminum is very negative. When immersed in electroplating solution, it is easy to replace with metal ions with positive potential, which affects the adhesion of coating.
• 3. The expansion coefficient of aluminum and aluminum alloy is larger than that of other metals, so it is not suitable for electroplating in the range of large temperature change. The difference of expansion coefficient between aluminum and aluminum alloy coating and other metal coatings will cause large stress, so that the bonding force between coating and aluminum and aluminum alloy is not strong.
• 4. Aluminum is an amphoteric metal, soluble in acid and alkali, and unstable in both acidic and alkaline electroplating solutions.
• 5. Aluminum alloy die castings have sand holes and pores, which will leave plating solution and hydrogen, easy to bubble, and reduce the adhesion between the coating and the base metal.

Therefore, aluminum cannot be plated by electroplating. This is the same reason that electrolytic salt water can not get metal sodium, but sodium hydroxide. Therefore, aluminum alloy needs to be coated by anodic treatment.

Anodizing of aluminum alloys

Principle of anodic treatment

Anodizing is a process in which aluminum or aluminum alloy is placed in the electrolyte with an anode and an oxide film is formed on its surface by electrolysis. The cathode in the device must be materials with high chemical stability in electrolytic solution, such as lead, stainless steel, aluminum, etc. The principle of aluminum anodizing is essentially the principle of water electrolysis. When the current passes through, hydrogen is released on the cathode; On the anode, the precipitated oxygen is not only molecular oxygen, but also atomic oxygen and ionic oxygen, which are usually expressed as molecular oxygen in the reaction. Aluminum as an anode is oxidized by the oxygen precipitated on it to form an anhydrous oxide film. Not all the generated oxygen acts with aluminum, and some will precipitate in gaseous form.

Purpose of anodizing

The original intention of anodic treatment is antioxidant treatment, but due to the rapid change of science and technology, the anodic treatment process is becoming more and more colorful. After dye dyeing treatment, dyes can be selected according to different uses and product grades, and then color matching and dyeing treatment can be carried out. Therefore, it is mostly used in high-grade aluminum products, such as gold pens, cosmetics, hearing aids, optical instruments, jewelry and 3C products. Therefore, in addition to strengthening its functionality, it can show its color and brightness on the surface of various products, so as to increase the charm of commodity sales.

Advantages of anodic treatment

Generally, aluminum alloy is easy to oxidize in the air. Although the natural oxide layer has a certain passivation effect, the oxide layer will still be damaged and peeled off as a result of long-term exposure;

Therefore, the purpose of anodic oxidation treatment is to quantitatively and forcibly form an oxide layer on the product surface by electrochemical method, so as to prevent further natural oxidation of aluminum and increase the mechanical properties (functionality) of the surface; Another purpose is to produce various colors and enhance beauty (decoration) through different back reactions.

Classification of anodic oxidation

Classification by process
A. Anodizing:

• 1. Natural Color Anodizing
• 2. Black anodizing
• 3. Oxidation of other colors (blue oxidation, red oxidation, gold oxidation, etc.)

B. Hard anodizing

• 1. Hard oxidation of natural color (the color of oxide film presents different colors according to the material)
• 2. Black hard oxidation (hard oxidation dyeing black)

Classification by substrate
1. Anodizing of extruded aluminum 2. Anodizing of die cast aluminum
Characteristics of anodic oxidation
Color of natural oxide film
Aluminum products do not dye after conventional anodizing, showing the color of the product oxide film itself, which is not always colorless. Different aluminum materials show slightly different colors.

• 1100-pure aluminum: transparent and colorless
• 3003-aluminum manganese: light yellow
• 3004-aluminum manganese: transparent and colorless
• 4043-aluminum silicon: gray
• 5052-aluminum magnesium: transparent and colorless
• 6061-aluminum magnesium zinc: milky yellow with gray
• 6063-aluminum magnesium zinc: light yellow
• 7072-aluminum zinc: transparent, colorless and striped
• ADC12-cast aluminum: brown gray, hairy, macular (silicon)
• ZL105-cast aluminum: brown yellow

Color of natural hard oxide film
Aluminum products do not dye after hard anodizing, showing the color of the oxide film itself. Different aluminum materials show great color differences. From yellow to brown to gray to black, the thicker the film, the darker its color will be.

Fig. 1 appearance color of hard oxide film of different materials
Oxidation characteristics of die cast aluminum

• 1. Compared with extruded aluminum, the impurity content of die-casting aluminum is much higher (for example, ADC12, silicon content is 9.5-12.5%, copper content is 1.5-3.5%).
• 2. There is release agent on the surface.
• 3. These impurities cannot be removed by conventional chemical pretreatment.

Therefore, if the oxide film layer is formed on the surface of such material, there will be uneven, irregular, spotted, variegated and other phenomena. Moreover, because silicon is not conductive, the overall conductivity of the product becomes poor. Under the same oxidation conditions, the appearance of the film will be much worse than that of extruded aluminum (as shown in the figure below). This is also the characteristic of die casting aluminum oxidation.

Fig. 2 appearance of die cast aluminum after oxidation

Anodizing process

The general oxidation process of extruded aluminum is as follows:

Figure 3 extrusion aluminum oxidation process
The general oxidation process of die cast aluminum is as follows:

Figure 4 die casting aluminum oxidation process
See here, the new people should have a general grasp of the anodizing process and application, and will gradually refine the details in the follow-up, so that we can have a better understanding of it.

The heat treatment of aluminum alloy

The metallographic structure of cast aluminum alloy is larger than that of wrought aluminum alloy, so it is different during heat treatment. The former has a long holding time, generally over 2h, while the latter has a short holding time, as long as tens of minutes. Because the metallurgical structure of metal castings, low-pressure castings, and cast aluminum alloys is coarser than that of wrought aluminum alloys, they are different during heat treatment. The former has a long holding time, generally over 2h, while the latter has a short holding time, as long as tens of minutes. Because metal castings, low pressure castings, and differential pressure castings are crystallized and solidified under a relatively large cooling rate and pressure, their crystalline structure is much thinner than that of gypsum and sand casting, so their heat preservation during heat treatment is also short a lot of. Another difference between cast aluminum alloy and deformed aluminum alloy is that the wall thickness is uneven, and there are complex structural shapes such as special-shaped surfaces or internal channels. In order to ensure that there is no deformation or cracking during heat treatment, special fixtures are sometimes designed for protection and quenching medium. The temperature is also higher than that of deformed aluminum alloy, so artificial aging is generally used to shorten the heat treatment cycle and improve the performance of the casting.

Purpose of heat treatment

The purpose of heat treatment of aluminum alloy castings is to improve the mechanical properties and corrosion resistance, stabilize the size, and improve the machining performance of cutting and welding. Because the mechanical properties of many as-cast aluminum alloys cannot meet the requirements of use, except for the ZL102 of the Al-Si series, the ZL302 of the Al-Mg series and the ZL401 alloy of the Al-Zn series, the rest of the cast aluminum alloys must be further improved by heat treatment. The mechanical properties and other service properties of the casting are as follows:

• 1) Eliminate the uneven cooling rate of the casting caused by the uneven cooling rate during the crystallization and solidification due to the structure of the casting (such as uneven thickness of the wall and thick transition) 2) Improve the mechanical strength and hardness of the alloy, improve the metallographic structure, and ensure that the alloy has a certain degree of plasticity, cutting performance, and welding performance;
• 3) Stabilize the structure and size of the casting, prevent and eliminate high-temperature phase transformations. The volume changes;
• 4) Eliminate intergranular and component segregation, and homogenize the structure.

Heat treatment method

Annealing treatment

The effect of annealing treatment is to eliminate the casting stress of the casting and the internal stress caused by machining, stabilize the shape and size of the processed part, and spheroidize part of the Si crystals of the Al-Si alloy to improve the plasticity of the alloy. The process is: heating aluminum alloy castings to 280-300°C, holding for 2-3 hours, and cooling to room temperature with the furnace, so that the solid solution slowly decomposes, and the precipitated second particles gather, thereby eliminating the internal stress of the casting and achieving a stable size , The purpose of improving plasticity, reducing deformation and warpage.

Quenching

Quenching is to heat the aluminum alloy castings to a higher temperature (usually close to the melting point of the eutectic, mostly above 500 ℃), and hold for more than 2 hours to fully dissolve the soluble phase in the alloy. Then, it is quenched into water at 60-100℃ to make the castings cool rapidly, so that the strengthening components can be dissolved to the maximum extent in the alloy and stored at room temperature. This process is called quenching, also called solution treatment or cold treatment.

Aging treatment

Aging treatment, also known as low temperature tempering, is a process that heats the quenched aluminum alloy castings to a certain temperature, heats them out for a certain period of time and air-cools them to room temperature to decompose the supersaturated solid solution and stabilize the alloy matrix structure.
During the aging treatment process of the alloy, with the increase of temperature and the extension of time, after the recombination of atoms in the supersaturated solid solution lattice, the solute atom enrichment zone (called G-PI zone) and G-PⅠ zone disappear. The atoms of the second phase segregate according to a certain rule and form the G-PII region, and then a metastable second phase (transition phase) is formed. A large number of G-PII regions are combined with a small amount of metastable phase and the metastable phase is transformed into a stable phase. , The second phase particles gather in several stages.
The aging treatment is divided into two categories: natural aging and artificial aging. Natural aging refers to aging strengthening at room temperature. Artificial aging is divided into three types: incomplete artificial aging, complete artificial aging, and overaging.

• 1) Incomplete artificial aging: heat the casting to 150-170°C for 3-5h to obtain better tensile strength, good plasticity and toughness, but a heat treatment process with lower corrosion resistance;
• 2) Complete artificial aging: heat the casting to 175-185°C and keep it for 5-24 hours to obtain sufficient tensile strength (that is, the highest hardness) but a heat treatment process with lower elongation;
• 3) Over-aging: heat the casting to 190-230°C and keep it for 4-9 hours to reduce the strength and increase the plasticity to obtain better resistance to stress and corrosion. It is also called stabilized tempering.

Loop processing

Cool the aluminum alloy casting to a certain temperature below zero (such as -50°C, -70°C, -195°C) and keep it for a certain period of time, and then heat the casting to below 350°C to make the alloy’s moderate solid solution lattice repeatedly shrink and expand. And make the crystal grains of each phase move a small amount, so that the atomic segregation zone and the particles of the intermetallic compound in the solid solution crystal lattice are in a more stable state, so as to achieve the purpose of increasing the size and volume of the product parts. This heat treatment process of repeated heating and cooling is called cyclic treatment. This kind of treatment is suitable for parts that require very precise and stable dimensions in use (such as some parts on testing instruments). Generally castings are not treated in this way.

Code and meaning of heat treatment state of cast aluminum alloy

T1 – artificial aging

Alloys cast in metal molds or green sand molds have obtained a certain degree of supersaturated solid solution due to the faster cooling rate, even if there is a partial quenching effect. After artificial aging and demelting strengthening, the hardness and mechanical strength can be improved, and the machinability can be improved. Effective for improving the strength of alloys such as ZL104 and ZL105.

T2 – Annealing

The main function is to eliminate the internal stress of the casting, stabilize the size of the casting, and spheroidize the Si crystals of the Al-Si series alloy to improve its plasticity. The effect on Al-Si series alloys is obvious, the annealing temperature is 280~300 ℃, and the holding time is 2~4h.

T4 – Solid solution treatment plus natural aging

Through heating and holding, the soluble phase is dissolved, and then quenched, so that a large amount of the strengthening phase is solid-dissolved in the α solid solution to obtain a supersaturated solid solution to improve the hardness, strength and corrosion resistance of the alloy. For Al-Mg series alloys, it is final treatment, and for other alloys that require artificial aging, it is preliminary treatment.

T5 – Solid solution treatment plus incomplete artificial aging

It is used to obtain higher strength and plasticity, but the Cornish property will decrease, especially the intergranular corrosion will increase. The aging temperature is low, the holding time is short, the aging temperature is about 150-170 ℃, and the holding time is 3 to 5 hours.

T6 – Solid solution treatment plus complete artificial aging

Used to obtain the highest strength, but the plasticity and corrosion resistance are reduced. Perform at higher temperature and longer time. Suitable for parts that require high load, the aging temperature is about 175~185 ℃, and the holding time is more than 5h.

T7 – Solution treatment plus stabilized tempering

Used to stabilize the size and structure of castings, improve corrosion resistance, and maintain high mechanical properties. It is usually carried out at close to the working temperature of the part. Suitable for parts that work at high temperatures below 300 ℃, tempering temperature 190-230 ℃, holding time 4-9h.

T8 – Solution treatment and softening and tempering

The solid solution is fully decomposed, and the precipitated strengthening phase is aggregated and spheroidized to stabilize the size of the casting and increase the plasticity of the alloy, but the tensile strength decreases. Suitable for castings that require high plasticity, the tempering temperature is about 230-330 ℃, and the holding time is 3-6h.

T9 – Loop processing

Used to further stabilize the size and shape of the casting. The repeated heating and cooling temperature and the number of cycles should be determined according to the working conditions of the parts and the properties of the alloy. Suitable for parts requiring precise and stable appearance.

Heat treatment process

Heat treatment process parameters of cast aluminum alloy

Alloy code	Heat treatment	Quenching process			Aging or annealing process
		Heating temperature (℃)	Holding time (h)	Cooling medium (water)	Heating temperature (℃)	Holding time	Cooling method
ZL101	T1	—	—		230±5	7—9	Air cooling
	T4	535±5	2—6	60—100		—	—
	T5	535±5	2—6	60—100	155±5	2—7	Air cooling
	T6	535±5	2—6	60—100	255±5	7—9	Air cooling
	T7	535±5	2—6	60—100	250±5	2—4	Air cooling
ZL102	T2	—	—	—	290±10	2—4	Air cooling
ZL103	T1	—	—	—	180±5	3—5	Air cooling
	T2	—	—	—	290±10	2—4	Air cooling
	T5	515±5	3—6	60—100	175±5	3—5	Air cooling
	T7	515±5	3—6	60—100	230±5	3—5	Air cooling
	T8	515±5	3—6	60—100	230±5	3	Air cooling
ZL104	T1	—	—	—	175±5	5—15	Air cooling
	T6	535±5	2—6	60—100	175±5	10—15	Air cooling
ZL105	T1	—	—	—	180±5	5—10	Air cooling
	T5	525±5	3—5	100	160±5	3—5	Air cooling
	T6	525±5	3—5	60—100	180±5	5—10	Air cooling
	T7	525±5	3—5	60—100	240±5	3—5	Air cooling
ZL107	T6	515±5	10	60—100	155±5	10	Air cooling
ZL108	T1	—	—	—	200±10	10—14	Air cooling
	T6	515±5	3—8	60—80	205±5	6—10	Air cooling
ZL109	T6	500±5	5	80	185±5	16	Air cooling
ZL110	T1	—	—	—	210±10	10—16	Air cooling

Technical points of heat treatment operation

• 1) Before heat treatment, check whether the heat treatment equipment, auxiliary equipment, instruments, etc. are qualified and normal, and whether the temperature difference across the furnace is within the specified range (±5°C);
• 2) Before installing the furnace, sand blowing or washing should be carried out, and there should be no oil, dirt, and soil, and alloy grades should not be mixed;
• 3) Castings that are prone to warping in shape and shape should be placed on a dedicated chassis or bracket, and suspended cantilever parts are not allowed;
• 4) Single casting or attached casting test bars for checking the performance of castings should be treated together with the parts in the same furnace to truly reflect the performance of the castings;
• 5) During the heat preservation period, check and correct the temperature around the furnace at any time to prevent local high temperature or burning;
• 6) When it cannot be recovered in a short time after the power is cut off, the castings in the heat preservation should be quickly out of the furnace for quenching, and after the return to normal, the furnace is installed, warmed and heat treated;
• 7) Castings that have been quenched in the nitrate bath should be rinsed with hot water immediately after quenching to eliminate residual salt and prevent corrosion;
• 8) If the casting is found to be deformed after quenching, it should be corrected immediately;
• 9) The parts to be aging treatment should be aging treatment within 0.5h after quenching;
• 10) If the performance is found to be unqualified after heat treatment, the heat treatment can be repeated, but the number of times shall not exceed 2;
• 11) The heat treatment should be carried out according to the heat treatment process established by the casting structure shape, size, alloy characteristics, etc.

Causes of heat treatment defects and their elimination and prevention methods

The annealing state δ₅ is low, and the strength and elongation after quenching or aging treatment are unqualified. The annealing temperature is too low or the holding time is insufficient, or the cooling is too fast; the quenching temperature is too low or the holding time is not enough, or the cooling rate is too slow (the temperature of the quenching medium is too high); the incomplete artificial aging and the complete artificial aging temperature are too high, or the holding If the time is too long, the chemical composition of the alloy will deviate. Re-anneal, increase the temperature or extend the holding time; increase the quenching temperature or extend the holding time, and reduce the temperature of the quenching medium; if quenching again, adjust the subsequent aging temperature and time; if there is a deviation in the composition, it must be based on the specific deviation element , Deviation, change or adjust repeated heat treatment parameters.

Deformation, warpage

The size and shape change of the casting reflected in the heat treatment or subsequent machining. The heating rate or quenching cooling rate is too fast (too intense); the quenching temperature is too high; the design structure of the casting is unreasonable (for example, the wall thickness of the two connecting walls is too different, and the ribs in the frame structure are too thin or too small; during quenching Improper launching direction of the workpiece and improper charging method. Reduce the heating rate, increase the temperature of the quenching medium, or change to a quenching medium with a slower cooling rate to prevent residual stress in the alloy; apply paint or use asbestos to thick or thin walls Fibers and other thermal insulation materials cover the thin-walled parts; choose a reasonable launching direction according to the structure and shape of the casting or use special anti-deformation fixtures; the parts with small deformation can be corrected immediately after quenching.

Cracks

The obvious cracks on the surface of the quenched casting that can be seen with the naked eye or the micro cracks that can not be seen by the naked eye through fluorescent inspection. The cracks are often tortuous and not straight and appear dark gray. The heating speed is too fast, and the cooling is too fast during quenching (the quenching temperature is too high or the temperature of the quenching medium is too low, or the speed of the quenching medium is too fast); the design of the casting structure is unreasonable (the wall thickness difference between the two connecting walls is too large, and the middle of the frame Reinforcing ribs are too thin or too small); the method of loading the furnace or the direction of launching is wrong; the temperature of the furnace is uneven, which makes the temperature of the casting uneven. Slow down the heating rate or adopt an austempering process; increase the temperature of the quenching medium or replace it with a quenching medium with a slow cooling rate; apply paint on the wall thickness or thin-walled parts or cover the thin-walled parts with heat insulation materials such as asbestos; use special anti-corrosion materials Cracked quenching fixture, and choose the correct direction of launching.

Overburning

There are nodules on the surface of the casting, and the elongation of the alloy is greatly reduced. The content of low-melting impurity elements in the alloy such as Cd, Si, Sb, etc. is too high; the heating is uneven or too fast; the local temperature in the furnace exceeds the overburning temperature of the alloy; the instrument for measuring and controlling the temperature fails, making the furnace actual The temperature exceeds the value indicated by the meter. Strictly control the content of low-melting alloy elements not to exceed the standard; slowly increase the temperature at a rate of no more than 3°C/min; check and control the temperature of each zone in the furnace to not exceed ±5°C; regularly check or calibrate the measurement and control instrument to ensure that the instrument temperature, temperature, display, The temperature control is accurate.

Surface corrosion

The surface of the casting appears streaked or lumpy, which is different from the surface of the aluminum alloy casting. The chloride content in the nitrate solution exceeds the standard (>0.5%) and causes corrosion to the surface of the casting (especially the loose and shrinkage parts); after being taken out from the nitrate tank, it is not sufficiently cleaned, and the nitrate adheres to the surface of the casting ( Especially narrow gaps, blind holes, channels) cause corrosion; nitrate salt solution is mixed with acid or alkali or castings are corroded around concentrated acid or alkali. Try to shorten the time for castings to move from the furnace to the quenching tank; check whether the chloride content in the nitrate salt exceeds the standard, if it exceeds the standard, the content (or concentration) should be reduced, and the castings heated from the nitrate salt tank should be immediately heated with warm water or Rinse with cold water; check the content of acid and alkali in the nitrate salt. If there is acid or alkali, neutralize or stop using it; do not place aluminum alloy castings around where there is concentrated acid or alkali.

Uneven quenching

The elongation and hardness of the thick parts of the castings are low (especially the inner center), and the thin-wall parts have high hardness (especially the surface layer). The heating and cooling of the castings are uneven, the thick parts cool slowly, and the heat permeability is poor. Re-heat treatment, reduce the heating rate, extend the heat preservation time, and balance the temperature of the thick and thin parts; apply heat-insulating paint or cover heat-insulating materials such as asbestos to the thick-walled parts to try to cool all parts of the casting at the same time; make the thick parts Start the water first; change to an organic quenching agent to reduce the cooling rate.

Heat treatment equipment and metal materials

Main technical requirements for heat treatment equipment

• 1) Because the temperature difference between the quenching and aging temperature of aluminum alloy is not large (because the quenching temperature is close to the melting point of the low melting eutectic component in the alloy), the temperature difference in the furnace should be controlled at ±5°C;
• 2) The temperature measurement and temperature control instruments are required to be sensitive and accurate to ensure that the temperature is within the above-mentioned error range;
• 3) The temperature of each zone in the furnace should be uniform, and the difference should be within the range of 1-2°C;
• 4) The quenching tank has a heating device and a circulation device to ensure the heating and temperature of the water is uniform;
• 5) The contaminated cooling water should be regularly checked and replaced.

Quenching medium

Quenching medium is an important factor to ensure the realization of various heat treatment purposes or functions. The higher the cooling rate of the quenching medium, the more intense (faster) the cooling of the casting, the higher the degree of supersaturation of the α solid solution in the metal structure, and the better the mechanical properties of the casting, because a large amount of intermetallic compounds and other strengthening phases are solid solution Into the Al α solid solution. Quenching medium according to the cooling rate of the casting is: dry ice and acetone mixture (-68℃), ice water, water at room temperature, water at 80-90℃, water at 100℃, water after atomization , Various oils (rapeseed oil, etc.), various oils heated to 200-220℃, air, etc.

Temperature measurement and temperature control instruments and materials

The accuracy of temperature measurement and temperature control instruments should not be lower than 0.5. The heat treatment furnace should be equipped with automatic recording, automatic alarm, automatic power-off, and power-up devices and meters that can automatically measure and control temperature to ensure the temperature in the furnace. Accurate display and control and uniform temperature.
The thermocouple uses nickel-chromium-nickel-silicon, nickel-chromium-nickel-aluminum couple wires with a diameter of 2.0-0.5 mm. In order to improve the sensitivity of the thermometer and reduce the temperature fluctuation range, it is best to use the above-mentioned material of Ф0.5-1.0 mm. And before use and during use (once every 3 months) test and calibration once.

What are the differences between aluminum alloy heat treatment characteristics and steel heat treatment?

As we all know, for steels with higher carbon content, high hardness is obtained immediately after quenching, while plasticity is very low. However, this is not the case for aluminum alloys. After quenching, the strength and hardness of aluminum alloys do not immediately increase. As for the plasticity, instead of decreasing, it increases. But this kind of quenched alloy, placed for a period of time (such as 4-6 days and nights), the strength and hardness will be significantly improved, while the plasticity will be significantly reduced. The phenomenon that the strength and hardness of the aluminum alloy after quenching increases significantly with time is called aging. Aging can occur at room temperature, which is called natural aging, or it can occur in a certain temperature range (such as 100 to 200°C) higher than room temperature, and is called artificial aging.
The age hardening of aluminum alloy is a very complicated process, which not only depends on the composition and aging process of the alloy, but also depends on the defects caused by the shrinkage of the alloy during the production process, especially the number and distribution of vacancies and dislocations. At present, it is generally believed that age hardening is the result of the segregation of solute atoms to form a hardened zone.
The size and number of hardened zones depend on the quenching temperature and quenching cooling rate. The higher the quenching temperature, the greater the vacancy concentration, the greater the number of hardened zones, and the size of the hardened zone decreases. The greater the quenching cooling rate, the more vacancies fixed in the solid solution, which is beneficial to increase the number of hardened zones and reduce the size of the hardened zones.
A basic feature of precipitation hardening alloys is the equilibrium solid solubility that changes with temperature, that is, the solid solubility increases with the increase of temperature. Most aluminum alloys that can be heat treated and strengthened meet this condition.

Welding and Defect Control Measures of Marine Aluminum Alloy

Aluminum alloy material is widely used in shipbuilding because of its low density, high strength, strong corrosion resistance, good plasticity, and other characteristics. However, aluminum alloy is easy to oxidize. It has a large coefficient of linear expansion, which results in aluminum alloy weldability being worse than carbon steel, and defects such as cracks, pores, and deformation are prone to occur when the hull is welded. Therefore, it is of great practical significance to study the types of marine aluminum alloys, welding defects, causes, and control measures. This paper starts with an analysis of aluminum alloy properties. It analyzes the common welding methods, defects, and preventive measures of aluminum alloy, to provide a reference for the construction and maintenance of ships.
Ships are divided into two categories of civil and military ships according to their use; civil ships such as transport ships, fishing boats, and harbor ships with large transport volumes, long voyage distances, and other characteristics; military ships because of the need to carry out the task of maritime safety and security, maritime law enforcement, maritime crime detection, and so on, and therefore require that in the process of design and manufacture of the ship to be considered in the tonnage, speed, and smoothness. Traditional ship welding, due to the use of a large number of steels, greatly limits the ship’s carrying capacity and sailing speed; at the same time, some ships need a higher superstructure to ensure the erection of a variety of radar and antenna, the result is the center of gravity of the ship upward, affecting the stability of the ship’s navigation, tonnage, size and smoothness is difficult to take into account at the same time the contradictory needs. Aluminum alloy specific gravity is about 1/3 of iron and steel, so aluminum alloy ship is lightweight, have low fuel consumption, and have fast sailing speed, which solves the contradiction of ship tonnage, size, and sailing speed to a certain extent. Therefore, some warships and speedboats at home and abroad use aluminum alloy hulls, for example, 022 missile boats with aluminum alloy hulls in our country, whose sailing speed can reach 50 knots.

1. Requirements and types of marine aluminum alloy

Due to the long-term in the marine environment, subject to seawater mechanical impact, chemical corrosion, electrochemical corrosion, and biological corrosion of seawater planktonic microorganisms, the marine aluminum alloy must have strong corrosion resistance. Generally speaking, aluminum alloy in seawater will occur in the protocell reaction, the surface of the formation of dense passivation film, preventing the aluminum alloy from further corrosion; this self-repairing ability to make the aluminum alloy has a strong corrosion resistance to avoid the internal components of the ship due to corrosion failure to affect the service life of the hull. On the other hand, relevant tests show that even a small amount of chloride ions will make the aluminum alloy produce obvious pitting corrosion and intergranular corrosion, so when discussing the selection of aluminum alloy for shipbuilding, it is necessary to take into account the difference in corrosion resistance of different series of aluminum alloys.
Comprehensive consideration of marine aluminum alloy corrosion resistance, weldability and strength, and other factors, the ship welding manufacturing process commonly uses 3000 series, 5000 series, 6000 series, and 7000 series aluminum alloy.

1.1 Al-Mg Aluminum Alloy

Al-Mg aluminum alloy is an aluminum alloy with Mn as the main alloying element, and the content of the Mn element is about 1%-1.5%; Mn can improve the strength of aluminum alloy through solid solution reinforcement in Al. Meanwhile, it can improve the corrosion resistance of the aluminum alloy. The commonly used grades are 3003 and 3004 aluminum alloys, for example, the seamless tubes used in aircraft for oil guiding (3003 alloys) and cans (3004 alloys). Al-Mn aluminum alloys cannot be strengthened by heat treatment. Still, they can be eliminated by cold working, annealing, and other processes to eliminate elemental segregation to get close to the plasticity of super aluminum alloys. At the same time, the 3000 series aluminum alloy is easy to obtain good organizational properties; even cast and rolled billet plate has a fine grain size (Figure 1), cold working treatment and can eliminate surface streaks, improve the tendency of internal segregation, and improve the corrosion resistance of the surface of the aluminum alloy, so it is a more widely used rust-resistant aluminum alloy series, for example, the top and side plates of the offshore ship vessel (3003 or 3004 alloy) Figure.1

Figure.1 Metallographic organization of 3003 alloy cast and rolled billets

1.2 Al-Mg Aluminum Alloys

Al-Mg aluminum alloy has Mg as the main alloying element, and its content is about 3% to 5%. Al-Mg aluminum alloy cannot be heat-treated to strengthen the performance but only cold-worked to improve the strength of the cold-worked Al-Mg aluminum alloy has high tensile strength, high elongation, good fatigue strength and good heat-resistant performance and welding performance, and it can be used in the aircraft tank conduit, bullet-proof vests and so on. And in the marine environment, Al-Mg system aluminum alloy, in addition to the resistance to general corrosion performance, also can eliminate spalling corrosion and intergranular corrosion, so in the ship structure is often used in the bottom of the outer plate, keel, rib plate, and rib manufacturing, commonly used grades are 5083, 5086, 5052 and 5456 and so on.

1.3 Al-Mg-Si Aluminum Alloy

Al-Mg-Si aluminum alloy is an aluminum alloy with Mg and Si as the main alloying elements, and the alloying elements will form Mg₂Si as the main reinforcement item, the most widely used marine aluminum alloy today. The most prominent disadvantage of Al-Mg-Si aluminum alloy is that it is prone to intergranular corrosion, so it is commonly used to weld the ship’s upper structure, for example, some aluminum alloy vessel parts on the ship.

1.4 Al-Zn-Mg Aluminum Alloy

Al-Zn-Mg aluminum alloy is an aluminum alloy with Zn as the main alloying element, and sometimes a small amount of Mg and Cu is also added. The super-hard aluminum alloy is close to the hardness of steel due to the elements of Zn, Pb, Mg, and Cu. Because of its excellent aging and weldability at normal temperatures, Al-Zn-Mg aluminum alloy is also used in the deck structure of ships in recent years. However, compared with other series of aluminum alloys, the corrosion resistance of Al-Zn-Mg aluminum alloy is not strong, and the welding should consider the use position.

2. Aluminum alloy welding methods

Aluminum alloy welding methods, ship welding, according to the thickness of the weldment, aluminum alloy grade, welding conditions, and joint quality requirements, and other factors to consider the welding method; commonly used aluminum alloy welding methods are fused electrode gas shielded welding, tungsten argon arc welding, gas welding, welding electrode arc welding, plasma welding and laser welding and so on.
2.1 Melting pole inert gas shielded welding (MIG)
Melting pole inert gas shielded welding is a wire as an arced pole, and inert gas as the arc medium and protective gas, the arc melting wire and the base material to achieve welding arc welding method, referred to as MIG welding. The use of MIG for aluminum alloy welding, you can use a variety of shapes of flux-cored wire and current density, so the wire deposition speed is fast, welding deformation is small, conducive to improving the weld metal and base material heat-affected zone organization and performance, to meet the requirements of the ship’s strength.
2.2 Tungsten argon arc
Tungsten arc welding is a tungsten or tungsten alloy rod as an arc pole, and Ar gas is a protective gas welding method referred to as TIG welding. Due to TIG welding tungsten electrode does not melt, tungsten tig welding arc length can be kept unchanged; arc stability is better, and the welding current is not affected by the wire melting transition factors, especially suitable for welding, including aluminum alloys, including a variety of non-ferrous metal alloy welding. Joint strength is better than the melting pole inert gas shielded welding, tungsten argon arc welding aluminum alloy with less spatter, fine grain, and small heat-affected zone (Figure 2).

Figure.2 TIG welding principle
TIG welding aluminum alloy, taking into account the welding depth requirements and the “cathode crushing” effect, generally using alternating current welding. Welding of some important structures on the ship of medium-thick aluminum alloy plate, for example, with 5083 aluminum alloy welding LNG ship vessel tanks, weld formability is good, and the surface is bright and beautiful.

2.3 Gas welding

Aluminum alloy gas welding is prone to oxidation, porosity, thermal cracks, and other problems. Still, because of the advantages of gas welding, low cost, simple structure, and flexible welding, it is also commonly used in welding aluminum alloy thin plate (0.5 – 10 mm), which does not require high quality. Welding to prevent burn-through can choose to weld carbon steel with a welding nozzle smaller than one and use neutral flame for welding.
Gas welding must use flux to dissolve and eliminate the oxide film covering the surface of the molten pool and generate a layer of slag on the surface of the molten pool to protect the molten metal from oxidation. Gas welding aluminum alloy commonly used flux grade CJ401, welding with water into a paste coated in the wire and the surface of the weldment.

2.4 Friction Stir Welding

The stir friction welding principle is the use of a special stirring head pressed into the welded parts after the use of friction between the two heats to make the welded material thermoplasticization, stirring head along the weld interface to move forward, thermoplasticization of the base metal cooling connection to form a solid-phase joint, the principle of which is shown in Figure 3.

Figure.3 Principle of Friction Stir Welding
In addition to noise, the friction stir welding process does not produce pollution factors such as arc light, smoke, and radiation, which is a relatively environmentally friendly welding technology. In addition, stir friction welding is highly efficient and has a wide range of applications; it can weld almost all grades of aluminum alloys and is commonly used for welding aluminum alloy decks, maneuver room floors and portholes, and other profile components.

2.5 Steel – aluminum alloy transition joints explosion welding

The use of aluminum alloy superstructure of high-performance ships, aluminum alloy superstructure, and steel ship body connection deserves attention. Due to the large difference between the density of steel and aluminum, with the traditional fusion welding method of welding aluminum alloy and steel, Al will float on top of the steel; after cooling, the weld composition is not uniform. In addition, the difference in linear expansion coefficient between Al and steel is large and easy to crack after welding; Al is easy to oxidize, hindering the fusion with steel; these factors lead to the difficulty of welding aluminum alloy superstructure and steel ship body.

Figure.4 Explosion welding principle
The steel-aluminum alloy joint made by explosion welding method has the features of high strength of composite layer, wear resistance, corrosion resistance, and good water tightness, which can significantly improve the steel-aluminum connection performance and shipbuilding efficiency, and it is one of the main methods of welding aluminum alloy and steel at present. Its welding principle is shown in Figure 4; the explosion of explosives produces shock waves, so the two workpiece surface collision removes the surface oxide film and the formation of a metallurgical connection. Explosive welding equipment is simple, has low production costs, high bonding strength of the welded section, and is more suitable for aluminum alloy and steel connection.

3. Aluminum alloy welding defects and preventive measures

Common welding defects such as biting edge, unfused, unwelded through, weld aneurysm, etc., can be improved by adjusting the process parameters, improving the proficiency of the welders as well as the appropriate welding specifications. Due to its physical and chemical properties, the aluminum alloy welding process and the above defects are especially prone to burn-through, porosity, deformation and cracks, and other defects.

3.1 Burn-through

Unlike steel, aluminum melting color does not have obvious changes; welders rely mainly on the melt pool shape to determine the melting of aluminum alloy; aluminum heat heating, strength, and plasticity decline very quickly, at 400 ℃ when the strength of less than 10% of the strength of the room temperature, so the aluminum alloy welding is very easy to burn through the melting pool temperature is too high.
To prevent aluminum alloy welding burn-through can be prevented from the following aspects:

• (1) During the welding process, pay close attention to the molten pool changes. In particular, no wire, and the molten pool size appears to become a larger or concave trend, signaling the imminent emergence of the phenomenon of burn-through to control the welding torch’s moving speed.
• (2) Select the appropriate welding process parameters.
• (3) Take the method of pad plate. In the welded aluminum alloy workpiece placed below the pad, even if the molten pool through the aluminum alloy workpiece, the liquid metal in the aluminum alloy blocking will not be lost to avoid burn-through defects.

3.2 Porosity

Aluminum has a strong affinity for oxygen and reacts easily with moisture in the air to form hydrogen. Hydrogen can be dissolved in much liquid aluminum and is almost insoluble in solid aluminum; cooling due to rapid crystallization, many hydrogens cannot be excluded from the molten pool promptly, causing the formation of hydrogen pores.
To prevent hydrogen porosity, the main idea is to reduce the source of hydrogen. Before welding, the weldment carefully rusts, oil, and preheating moisture removal, standardizes the welding process, and welding process as far as possible continuously; these measures can effectively prevent the generation of aluminum alloy welding from producing hydrogen gas holes.

3.3 Welding deformation and thermal cracking

Aluminum alloy coefficient of linear expansion is about twice as much as iron, and shrinkage is about three times as much as iron, which is the intrinsic factor of aluminum alloy welding deformation; Aluminum alloy structural parts in the roll rolling, bending, shearing and other forming process will also produce stress, the combined effect of these two factors, make aluminum alloy welding more prone to deformation and stress. Al-Mg and Al-Mn aluminum alloy cannot be strengthened by heat treatment, rarely produce thermal cracks; Al-Mg-Si, aluminum alloy cannot be strengthened by heat treatment, and rarely produce thermal crack; Al-Mg-Si, aluminum alloy can be strengthened by heat treatment. Aluminum alloys of the Al-Mg-Si and Al-Zn-Mg systems often have thermal cracking problems, especially when the rigidity or thickness is large and the cracking tendency is obvious.
To reduce the deformation and cracking tendency, the following aspects can be started.
(1) Welding method.
Argon arc welding heat concentration, and the role of protective gas strong cold, heat affected zone is small, welding deformation is small; gas welding flame is wider, the heat is relatively dispersed, resulting in a wide range of heat-affected zone, the welded workpiece is unevenly heated prone to deformation and cracking. Under the conditions allowed, the priority choice of argon arc welding, followed by manual arc welding, and finally, choose gas welding, can reduce welding deformation and cracks.
(2) Welding process parameters.
Welding current, voltage, and welding speed is the determinant of heat input to the molten pool, and the amount of heat input is positively correlated with the tendency to produce welding deformation or cracks. Therefore, to ensure that the aluminum alloy weldments do not appear unfused and unwelded, selecting a smaller current, voltage, or appropriately large welding speed can effectively reduce the deformation and cracking tendency.
(3) Weld size, position, and number of layers.
Welding deformation is related to the weld size: the larger the weld size, the larger the longitudinal and transverse shrinkage, the more likely to produce welding deformation. Therefore, according to the thickness of the welded part, choose the appropriate bevel.
Welding position is the main factor affecting welding deformation. When the welding position does not coincide with the center of the structural member, it easily leads to distortion and deformation. The welding position should be arranged symmetrically or in a suitable welding sequence.
The number of weld layers will also affect the welding stress and deformation. When multi-layer welding, the shrinkage of the first layer of the weld is the largest, the shrinkage of the second layer of the weld is about 20% of the first layer, and the shrinkage of the third layer, the fourth layer, etc., decreases in turn. Because the first layer of weld hinders the shrinkage of the latter layer of weld, when welding a workpiece with large stiffness, choose multi-layer welding to reduce welding deformation.

3.4 Clamping tungsten

If the welding current is too high in tungsten arc welding, part of the molten tungsten metal is deposited in the weld, resulting in tungsten entrapment. Therefore, in the tungsten argon arc welding aluminum alloy process, we should focus on controlling the welding current and tungsten electrode – the distance between the molten pool, to prevent the tungsten electrode from melting into the molten pool. Due to the tungsten electrode shrinkage in the nozzle, plasma arc welding is not in contact with the workpiece, can reduce the tungsten pinch defects, and is a more suitable aluminum alloy welding method.

3.5 Other welding defects and preventive measures

The aluminum alloy welding process will also appear common other welding defects, for example, not welded through, not fused, biting edge and weld tumor, etc., mostly welding current, voltage selection is unreasonable, or welding operation of the transport bar is not standardized resulting in the welding process should pay attention to the control of the welding process parameters. The selection of reasonable welding specifications can effectively avoid these welding defects.

4. Conclusion

In recent years, due to the outstanding contribution of aluminum alloy in reducing weight and speeding up, improving stability, and reducing the energy consumption of ship hulls, as well as its excellent corrosion resistance, aluminum alloy welding has been increasingly widely used in ship hull structures. For aluminum alloy, the welding process is prone to welding defects, and welding manufacturing and maintenance work should comprehensively consider welding materials, location and performance requirements, and other factors to ensure that the welding quality meets the use requirements. The development of ship welding technology tends to be mechanized, intelligent, and high-precision; scientific workers can research and develop new aluminum alloy, innovative welding methods, and post-welding treatment process, in-depth study of aluminum alloy welding, aluminum alloy welding to help China’s shipbuilding industry flourish.
Author: Fu Hao