Analysis of the research status of the deformation of metal parts in laser additive manufacturing
Laser additive manufacturing is a near-net-shape technology that manufactures solid parts based on material discrete-gradual accumulation. This technology usually uses metal powder as the raw material, sets the laser scanning path through the pre-layered processing of the three-dimensional model, and uses a high-energy laser beam to melt the metal powder layer by layer according to the set scanning path, so that it is rapidly solidified and accumulated to form high-performance components .
Laser additive manufacturing is mainly divided into laser direct energy deposition (LDED) and laser selective area melting (SLM). LDED technology has the following characteristics: high-efficiency moldless forming, unlimited forming size, suitable for forming large-size workpieces; it can realize the mixed processing of multiple materials and realize the manufacture of gradient materials; it can quickly repair damaged parts; and the workpiece is formed The complexity, precision and surface quality are low. In contrast, SLM technology has high forming accuracy, and the formed parts have good mechanical properties, and the tensile properties are generally up to the level of forgings; but its forming efficiency is low, and the size of the formed parts is limited by the powder work box , So it is not suitable for manufacturing large integral parts. At present, laser additive manufacturing technology has been widely used and developed in the fields of automobiles, biomedicine, and aerospace.
In the additive manufacturing process, metal materials undergo rapid heating, solidification, and cooling processes, during which large thermal stresses and organizational stresses caused by solid-state phase transitions are formed. These stresses remain inside the workpiece after forming and become residual stresses. If the residual stress exceeds the yield strength of the material itself, the formed part will be deformed, resulting in a decrease in dimensional accuracy and performance. Therefore, the deformation of additive manufacturing metal parts has always been one of the research hotspots in the field of additive manufacturing at home and abroad.
Deformation mechanism and detection methods of metal parts
Table of Contents
- Deformation mechanism and detection methods of metal parts
- Influencing factors of deformation of metal parts
- The effect of material properties and phase change on deformation
- Deformation prediction
- Concluding remarks
In the additive manufacturing process, the metal material undergoes a cyclical rapid heating and cooling process, which will generate a large temperature gradient and a large residual stress, which will eventually lead to deformation and even cracking of the workpiece.
Under the action of the laser, the metal material melts to form a molten pool. The molten pool shrinks during the solidification process and further shrinks during the subsequent cooling process; due to the uneven temperature distribution and the complex shape of the formed part, different areas of the formed part shrink Uneven.
At the same time, additive manufacturing is a layer-by-layer processing. In the latter laser scanning process, the pre-solidified material (deposited layer) adjacent to the molten pool undergoes a complex thermal cycle again and continues to expand and contract, resulting in an increase in residual stress. When the residual stress exceeds the yield strength of the material, the formed part deforms, and even cracks in severe cases, thereby reducing the dimensional accuracy and integrity of the formed part. In the SLM additive manufacturing process, if the amount of deformation of the deposited layer warping upwards is too large and it collides with the powder spreading or powder feeding device, the additive manufacturing process will be stopped and even the powder spreading or powder feeding device will be damaged.
The measurement of the deformation of the additive manufacturing parts mainly includes the measurement of the contour of the formed part after the forming and the in-situ measurement during the forming process. The contour measurement of the formed part mainly includes three methods: three coordinate measurement (CMM), 3D laser scanning measurement and computer tomography (CT) measurement.
3D laser scanning measurement technology and CT technology construct a geometric model of the formed part by scanning the formed part, and compare it with the designed manufacturing model to analyze the deformation of the formed part; CMM technology uses a three-dimensional probe to select points on the formed part When measuring, its system software will automatically calculate the deformation of the selected point. In-situ measurement technology mainly includes laser displacement sensor (LDS) and digital image correlation (DIC) technology. The deformation of parts in the laser additive manufacturing process is a dynamic accumulation process. In-situ measurement can monitor the deformation of the workpiece after each layer is processed in real time, and can better study the influence of interlayer effects on deformation. Therefore, real-time monitoring of deformation through in-situ measurement to feedback the influence of process parameters and other factors on deformation is the main direction of future research.
Influencing factors of deformation of metal parts
The influence of process parameters on deformation
The process parameters will directly affect the temperature gradient, cooling rate and size of the molten pool in the laser additive manufacturing process, and then affect the mechanical properties and dimensional accuracy of the formed parts. The process parameters that affect the deformation of metal parts mainly include laser power, scanning speed, powder layer thickness, scanning distance and scanning strategy.
The influence of laser power, scanning speed, powder layer thickness, scanning distance and scanning interval time between layers
In the laser additive manufacturing process, the volume energy density is usually used to comprehensively consider the laser power, scanning speed, powder layer thickness and scanning The effect of spacing on the forming quality of parts is defined as:
Formula: EV is the volumetric energy density; P is the laser power; v is the scanning speed; h is the scanning distance; t is the thickness of the powder layer.
The laser power represents the size of the laser energy and directly controls the temperature of the molten pool; the scanning speed refers to the distance scanned by the laser per unit time, and the temperature of the molten pool is affected by controlling the time of the interaction between the laser and the powder. The size and peak temperature of the molten pool increase with the increase of laser power, and decrease with the increase of scanning speed.
Studies have shown that: the thermal strain of metal increases with the increase of laser power, and decreases with the increase of scanning speed; increasing the laser power will produce a larger temperature gradient, and a larger temperature gradient will produce a larger Thermal stress, when the thermal stress exceeds the yield strength of the material, it will cause deformation or cracking of the alloy; when the laser scanning speed increases, the interaction time between the laser and the powder is shortened, the temperature of the molten pool decreases, the size decreases, and the deformation of the alloy decreases. small. In the laser additive manufacturing process, when the thickness of the powder layer increases, the heat absorbed by the powder particles on the lower surface of the powder layer decreases, which causes the temperature gradient on the upper and lower surfaces of the molten pool to increase, and the deformation of the molded part also increases. Others, when studying the influence of process parameters on the warping deformation of the cantilever beam, found that the amount of deformation increases with the increase of the thickness of the powder layer; the residual stress of the parts decreases with the decrease of the thickness of the powder layer, and the amount of deformation also increases with the thickness of the powder layer. It reduces. The scanning distance directly affects the overlap rate of adjacent molten pools. The smaller the scanning distance, the greater the overlap rate, the heat input and temperature gradient increase, and the corresponding thermal stress and deformation also increase. In addition, researchers have found that when the scanning distance is reduced, the degree of warpage of the formed part increases significantly.
In the laser additive manufacturing process, the scanning interval between layers will also have a certain effect on deformation. The deformation of laser additive manufacturing parts is mainly caused by residual stress, and the residual stress is constantly accumulated and released during the deposition process; the amount of accumulation or release depends on the stress relaxation behavior of the deposited layer and the substrate. The scanning interval and temperature between the layers affect the stress relaxation behavior of the deposited layer and the substrate, thereby affecting the deformation of the formed part. Some researchers have used in-situ measurement technology to study the effect of interlayer scanning interval time on deformation, and found that the amount of deformation of Ti6Al4V alloy produced by laser additive manufacturing increases with the extension of the interlayer scanning interval; others have passed in-situ neutron Diffraction studies the stress relaxation mechanism of the additively manufactured Ti6Al4V alloy samples, and speculates that the stress relaxation may be achieved by the slip and climb of dislocations.
The influence of process parameters such as laser power, scanning speed, powder layer thickness, scanning distance and scanning interval time between layers on the deformation of the formed part is more complicated. Generally, the amount of deformation increases with the increase of the laser power, the thickness of the powder layer and the scanning interval time between layers, and decreases with the increase of the scanning speed and scanning interval.
The impact of laser scanning strategy
In the SLM process, the scanning strategy will have an important impact on the forming quality and dimensional accuracy of metal parts. Common scanning strategies include unidirectional scanning, zigzag scanning, spiral scanning and island scanning, as shown in the figure below.
One-way scanning and Z-scanning are relatively simple traditional scanning strategies. The heat transfer in the helical scanning process is more uniform than that of the unidirectional and Z-shaped scanning, so the temperature gradient generated is smaller, the temperature field is more uniform, and the residual stress and warpage deformation of the final parts are also smaller. The island scanning strategy refers to a strategy that divides the area to be scanned into multiple small square areas (also called islands), and then scans these islands according to a preset scanning sequence. The island scanning strategy makes the heat distribution in the processing process more uniform and reduces the heat concentration; and the adjacent islands of the upper and lower layers have mutually perpendicular scanning directions, which reduces the anisotropy between different deposition layers and the entire metal parts. Helps reduce the deformation of additively manufactured components. Studies have shown that scanning at a certain angle (usually 67° or 90°) between layers can also reduce the anisotropy between the deposited layers, improve the adhesion between layers, and reduce delamination and warping. Defects such as bending deformation.
Based on the above-mentioned different scanning strategies, researchers have carried out a lot of research on the deformation of additive manufacturing parts. For example, it is found that for flat-shaped parts, the spiral scanning strategy from the outside to the inside can reduce the deformation; The Z-shaped scanning strategy with an inter-rotating 67° can effectively reduce the warpage deformation; when the laser melts the powdered layer, a certain temperature gradient is generated in the depth direction of the substrate, and the thermal expansion of some areas on the substrate is more significant, so The substrate bends upwards. When the molten pool is solidified and cooled, its contraction will “pull” the substrate to bend in the opposite direction, so the deformation curve is fluctuating. Studies have also found that compared with the Z-shaped scanning strategy, the island scanning strategy can effectively reduce the deformation of the formed part, and the island scanning sequence has a more significant influence on the deformation and residual stress than the island size. Some researchers also used SLM to prepare Ti6Al4V alloy thin plates, and designed an interval island scanning strategy based on mathematical algorithms. By controlling the scanning sequence to ensure that the next scanned island is not adjacent to the two newly scanned islands, thereby weakening the additive manufacturing process The heat concentration effect in, reduces the deformation; at the same time, it is found that the amount of deformation parallel to the scanning direction is larger than that perpendicular to the scanning direction, and the residual stress in the formed part increases with the increase of the scanning vector length. This is because the smaller scan vector length produces a smaller weld pool length, and the residual stress and deformation are also smaller. Generally, in the laser additive manufacturing process, the deformation of the parts formed by the island scanning strategy is minimal.
The influence of substrate preheating
The substrate is a construction platform for laser additive manufacturing of metal parts, and it also undergoes a complex thermal cycle during the additive manufacturing process; the deformation of the substrate directly affects the dimensional accuracy of the formed part.
The study found that in the laser additive manufacturing process, the distortion of the substrate along the deposition direction (Z direction) is much greater than along the other two directions (X and Y directions), and the substrate deformation caused by laser processing is permanent and cannot be heat treated. eliminate. The preheating of the substrate can reduce the temperature gradient and cooling rate during the forming process, thereby reducing the residual stress and deformation of the formed part. In addition, it has been found that the degree of warping deformation of the cantilever beam decreases with the increase of the substrate preheating temperature; and the use of substrate preheating can effectively reduce the residual stress, and the residual stress increases with the preheating temperature in a certain temperature range. If the height is reduced, the deformation of the formed part is also reduced; when using SLM technology to form cemented carbide tools, a CO2 laser is added to the equipment to preheat the substrate, which can significantly increase the bending strength of the formed part. The degree of warpage deformation is relatively reduced.
In addition, the in-situ annealing treatment can also reduce the residual stress and deformation of the formed part.
At present, substrate preheating has become an effective method to reduce the residual stress and deformation of the formed part, and in-situ annealing of the deposited layer during the forming process provides a new way to control and solve the residual stress and deformation problems.
The effect of material properties and phase change on deformation
Different metal materials have different thermodynamic properties, and the forming properties in the laser additive manufacturing process are also different.
Research has found that metal materials with lower heat capacity and higher thermal diffusivity are easier to reach peak temperature in additive manufacturing, and produce larger molten pool size and thermal strain; some researchers believe that the deformation of Ti6Al4V alloy forming parts varies with layer The obvious increase of the inter-scan interval time is caused by the solid phase transformation of Ti6Al4V alloy during the forming process. At the same time, it is also found that the deformation of Inconel 625 alloy formed parts under the same inter-layer scan interval time and heat input conditions It is twice that of Ti6Al4V alloy formed parts.
In the laser additive manufacturing process, the high-energy laser beam heats and melts the metal powder. When the laser beam is removed, the molten metal rapidly solidifies at a higher cooling rate. During the cooling process after solidification, some metal materials will undergo solid phase transformation. For example, in the process of additive manufacturing of Ti6Al4V alloy, when the temperature drops to near the β phase transformation point (980 ℃), β phase to α phase or The α’phase transition, the lattice type changes from body-centered cubic (BCC) to close-packed hexagonal (HCP), and the resulting lattice strain will affect the overall deformation.
Studies have found that austenite to martensite solid phase transformation occurs during the forming process of H13 tool steel manufactured by LDED. The phase transformation process is accompanied by an increase in volume, thus forming a residual compressive stress. At present, there are few researches on the influence mechanism of solid-state phase transition on deformation, which should be one of the research hotspots in the future.
The problem of residual stress and deformation is a thorny problem faced by laser additive manufacturing technology. At present, numerical simulation technology is often used to study the influence of process parameters on residual stress and deformation, and deformation compensation is used to control the deformation of parts.
The specific steps are: predict the deformation of the part during the additive manufacturing process through numerical simulation, design a part manufacturing model with deformation compensation based on the simulation result and perform laser additive manufacturing forming, thereby offsetting the deformation of the formed part and increasing the size Accuracy.
Some researchers used a verified three-dimensional transient heat transfer and fluid flow model to simulate the residual stress and deformation of the formed part during the additive manufacturing process, and found that the residual stress decreased with the increase of heat input, while thermal deformation Increase with the increase of heat input. Others have established a multi-scale finite element model to simulate and analyze the influence of four different scanning strategies on the deformation of metal forming parts. It is found that the island scanning strategy corresponds to the smallest deformation of the formed part, and the simulation results are consistent with the experimental results. In addition, studies have shown that the deformation compensation method can significantly reduce the deformation of the formed part.
Compared with the traditional trial and error method, numerical simulation can save manufacturing cost and time cost to a large extent, shorten the time from design to forming of parts, and improve the forming accuracy of formed parts. However, numerical simulation needs to establish different thermodynamic models according to the thermodynamic properties of different materials, and need to consider the influence of phase transformation of multiphase alloys. For example, when establishing the thermodynamic model of Ti6Al4V alloy, it is necessary to consider the effect of solid phase transformation during solidification. The effect of deformation. At present, the numerical simulation of additive manufacturing mainly adopts thermal-mechanical coupling small-scale model. With the development of computer technology and the continuous improvement of simulation models, numerical simulation will be more accurate in predicting the deformation of additively manufactured parts.
The main factors that affect the deformation of metal parts in laser additive manufacturing include molten pool temperature, temperature gradient, cooling rate, etc. The focus of future work is to achieve macro-scale “shape control” and micro-scale “controllability” by optimizing process parameters. .
There are still great challenges in the deformation control of large and complex parts in laser additive manufacturing: the deformation will continue to accumulate as the size of the parts increases, and there are complex mutual constraints between different parts of the parts with complex shapes and structures. Its deformation law is very complicated, so it is difficult to predict and control.
At present, the deformation of laser additive manufacturing parts is often predicted by establishing a thermal-mechanical coupling small-scale model, and then the deformation of the formed part is controlled by deformation compensation.
With the development of computer technology and the in-depth research of laser additive manufacturing process by researchers, numerical simulation technology will play an increasingly important role in the field of deformation control of large thin-walled parts. Multi-scale and multi-physics coupling numerical simulation will It is a research hotspot in the future.
In the future, the deformation control of laser additive manufacturing forming parts mainly includes the following research directions: development of in-situ detection technology to realize real-time control of component deformation during forming; establishment of material gene database, design and development of new types of high yield strength and low thermal expansion The coefficient of the metal material improves the deformation resistance of the parts during the forming process; in the part design process, the influence of the shape and structure factors on the dimensional accuracy of the formed parts during the additive manufacturing process is considered.
Source: Network Arrangement – China Metal Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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