Manufacturing process of Nb521 niobium-tungsten alloy fastener

The material properties, machining properties, upsetting formability, room temperature and high temperature properties of Nb521 fasteners are described. Based on the traditional manufacturing process, the innovation and optimization have solved a series of problems in the manufacturing of Nb521 fasteners. The process research results show that the niobium-tungsten alloy has excellent process properties and high temperature properties, which can meet the requirements of aerospace fasteners for material properties.

1. Preamble

Nb521 niobium-tungsten alloy is an ultra-high temperature material, also known as refractory metal material. It has excellent high-temperature mechanical properties and excellent technological properties. The maximum service temperature is 1900 ℃, which can meet the requirements of the engine for the performance of ultra-high temperature materials, greatly reduce the flow of propellants used to cool the combustion chamber, and is conducive to improving the performance of the engine. With the increasing demand for the development of new aircraft, Nb521 niobium-tungsten alloy is more widely used in China’s aerospace field, not only in missile engines, but also in some high-temperature components of other high-speed aircraft. For example, the working temperature of the nozzle at the combustion chamber tail of a new missile engine is up to 1600-1800 ℃. It is proposed to use Nb521 material to make fasteners and connect the composite material at this position. Through the research on the manufacturing process of Nb521 fasteners, this paper aims to find the appropriate processing technology to meet the task requirements of new models.

2. Material properties

2.1 Chemical composition characteristics

Nb521 niobium-tungsten alloy has a density of 8.65-9.0g/cm³, and its matrix chemical composition is Nb. Pure metal Nb has a high solid solubility for W, Mo, Zr and other elements. These elements form a variety of solid solution strengthening phases with Nb, which can significantly improve the mechanical properties of the alloy at room temperature and high temperature; The melting point of W and Mo is high, and the atomic radius is close to that of Nb, which can improve the high-temperature strength and creep properties of Nb alloy; Zr mainly plays the role of forming strengthening phase and improving the strength of Nb matrix. See Table 1 for common chemical composition of Nb521 material.

Table.1 Common chemical composition (mass fraction) of Nb521 material (%)

Nb	W	Mo	Zr	C	Ta	Si	Fe
Allowance	4.5-5.5	1.7–2.3	0.7-1.2	0.05–0.12	≤0.10	≤0.01	≤0.02
Al	Ti	Cu	O	N	H	Other elements	Other elements
						Single	Sum
0.02	–	–	≤0.01	≤0.01	≤0.002	≤0.08	≤0.15

2.2 Mechanical properties and microstructure characteristics

According to the chemical composition of Nb521, it mainly contains Nb and W. With the increase of W content, the melting point of Nb alloy increases gradually. Nb and W form a solid solution with body-centered cubic structure (bcc), which cannot be strengthened by heat treatment and aging. Nb521 bar is delivered in recrystallization annealing, and its microstructure is fine equiaxed structure (see Figure 1).

Fig.1 Annealed microstructure
See Table 2 for the mechanical properties of Nb521 bar at room temperature. At room temperature, the material has low tensile strength, good plasticity and toughness, and good plasticity. It can be formed by forging, upsetting and other plastic processing methods.
Table.2 Mechanical properties of Nb521 bar at room temperature

Temperature/℃	Tensile property			Hardness/HBW
	Tensile strength Rm/MPa	Yield strength Rp0.2/MPa	Elongation after fracture A (%)
20	≥400	≥270	≥20	120一160

The C element in Nb 521 alloy and W, Mo and Zr form various types of carbides such as MC, M2C and M3C2, which are dispersed in the alloy matrix and play the role of pinning grain boundaries and dislocations. It is a relatively effective high-temperature strengthening method. However, with the increase of heating temperature, the strengthening phase begins to precipitate and grow up, which leads to the decrease of high temperature strength of the alloy. The high temperature mechanical properties of Nb521 bar are shown in Table 3. It can be seen that the material still has good tensile properties (Rm ≥ 70MPa) at 1600 ℃, which can meet the requirements of tensile strength ≥ 60MPa of the material at 1600 ℃ of the engine thrust chamber. It can be seen that Nb521 material fully meets the use requirements of the engine thrust chamber.
Table.3 High temperature mechanical properties of Nb521 bar

Temperature/℃	Tensile property
	Tensile strength Rm/MPa	Yield strength Rp0.2/MPa	Elongation after fracture A (%)
1500	≥100	≥90	≥25
1600	≥70	≥60	≥25
1800	≥45	≥35	≥30

3. Material forming characteristics

3.1 Upsetting formability

Nb521 material has good ductility and deformability in the solid solution state, and the upsetting ratio (the ratio of the original length and diameter of the upsetting part of the rod-shaped material) can reach more than 2.3. Upsetting can be used in the process of manufacturing fasteners. Upsetting can not only save materials and reduce costs, but also control the metal flow line at the joint of head and rod, and improve the fatigue life of products.
When machining hexagon head bolts with Nb521 material, three main problems should be solved in the process of cold heading:

• (1) The size of raw material blank. 
• (2) Surface lubrication of raw materials. 
• (3) Cold heading die design.

In the aspect of raw material diameter, it is mainly through calculating the material used for the bolt head and its deformation ratio to obtain an optimal value of the diameter of the bolt blank before thread rolling. As for the surface lubrication of raw materials, the binding force between Nb521 material and the working surface of the cold heading die is much stronger than that of heat-resistant steel and stainless steel, so before the cold heading forming, the cold heading blank must be lubricated to eliminate the sticking phenomenon and improve the service life of the upsetting die. Through verification and comparison, the scheme of coating molybdenum disulfide on the blank surface before cold heading was selected, which effectively solved the problem of cold heading blank sticking. In terms of die design, the blank deformation process of hexagon bolt products on the double-tap cold heading machine is formed by upsetting the cylindrical head (i.e. pre-punching and final upsetting) in two steps (see Figure 2). This method is mainly to design the pre-punching die reasonably, correct the shape after initial upsetting through adjustment and trial upsetting, and solidify the die, so as to realize the small batch production of such products.

Fig.2 Cold upsetting
1 – cutting knife 2 – stopper 3 – cutting die 4 – pre-punching die 5 – rod die 6 – final upsetting die 7 – ejector pin

3.2 Machining performance

The service environment of Nb521 high-temperature fastener is extremely bad, and the requirements for surface quality are very high. The surface processing trace and integrity have an important impact on the service quality. Due to the processing difficulties such as poor cooling performance, chatter of tool bar and difficulty in chip removal, the tool is easy to wear during turning and grinding, the product has obvious turning tool lines and poor surface quality, so it is necessary to use CNC lathe and centerless grinder to carry out precision machining on the fastener rod. The machining efficiency of fasteners can be effectively improved by selecting appropriate tools and formulating the most appropriate machining parameters.
(1) Select the appropriate tool and grinding wheel material. Because of the low hardness of the material, in the process of turning and grinding, you can choose a faster processing speed and a larger turning volume to ensure the surface roughness and dimensional consistency of the product. However, pay attention to cooling during processing to avoid grinding burn. The CNC lathe selects different tools such as DNMG110408MS, VNM G160404N, and adjusts the relevant speed and feed rate to ensure the dimension and appearance quality required by the CNC turning process. At present, the commonly used grinding wheel materials for fastener grinding are silicon carbide and white corundum. Through process verification, silicon carbide material grinding wheel is selected for grinding. The machining efficiency of centerless grinder can be greatly improved, and the appearance quality and size requirements of Nb521 fasteners can be well guaranteed.
(2) To determine the reasonable grinding allowance, the thread shall be formed by cold rolling, and the diameter of the screw blank shall be grinded. In the process of grinding the screw blank, whether the grinding allowance reserved by the CNC lathe is appropriate or not has an important impact on the qualified rate of the grinding products and the processing efficiency. If silicon carbide material grinding wheel is used, when grinding 0.03 mm on one side according to the conventional parameters, problems such as uneven product size, out-of-tolerance of local points and surface roughness can not be guaranteed, and the size cannot be guaranteed, and batch processing cannot be carried out. It has been verified for many times that under the premise of ensuring product quality, selecting the most reasonable machining allowance of 0.10 mm and adopting the processing method of rough grinding+fine grinding can effectively ensure product quality and improve grinding efficiency.

3.3 External thread rolling forming technology

Because thread is a typical feature of fasteners, its forming quality greatly affects the structural strength and performance level of fastener products, therefore, most high-performance fasteners in aerospace and other fields use rolling processing thread. Thread rolling refers to the processing technology that the metal blank is extruded radially on the precision thread rolling machine tool to make the metal plastic deformation to form the thread and strengthen the thread. Compared with turning thread, rolling thread has the characteristics of high material utilization rate, good surface quality, high thread strength and surface hardness, high fatigue life and high production efficiency. In the process of thread rolling forming, the plastic deformation resistance of the material is increased, the hardness and strength are improved, and the plasticity and toughness are decreased, that is, the work-hardening phenomenon occurs, which is very easy to cause cracks and folds on the top of the thread. For the thread processing technology of such difficult-to-machine materials, through the analysis of the thread forming process, the calculation and verification of the thread blank diameter of the rolled thread, the determination of the main process factors such as the rolling pressure, the roller speed, the roller feed speed and the selection of cutting fluid, solved the problems such as the tooth collapse of the rolling wheel and the excessive folding of the fastener thread in the cold rolling process, and ensured the surface quality of the external thread after the rolling of the thread. The bolt metal flow line is shown in Figure 3.

Fig.3 Bolt metal flow line

3.4 Forming difficulties and solutions of internal thread tapping

Nb521 alloy has good plasticity, but it is difficult to cut and has obvious tendency of chip scratch or adhesion. If the Nb521 material nut is processed with a common cutting tap, due to the high viscosity of the material, the chips are banded during processing, easy to scratch or stick on the cutting edge, resulting in increased cutting force, increased cutting temperature, and easy to tear on the surface of the workpiece. In the process of thread processing, there will be defects such as rotten teeth, side wrinkles and incomplete tooth shape, so in order to ensure the normal processing of the product, According to the characteristics and requirements of Nb521 material cutting, it is necessary to select appropriate taps and reasonable cutting parameters. Therefore, the special cutting tap and extrusion tap for Niobium-tungsten alloy material were developed. By adjusting the size of the bottom hole and other measures, the surface roughness value Ra of the processed thread flank can reach 1.6 μ m. The thread go-no go gauge has passed the inspection, solved the problem of sharp edges and burrs of thread teeth, reduced the difficulty of thread tapping, and the thread has no defects such as rotten teeth, side wrinkles and incomplete tooth shape, and the thread quality can meet the standard requirements.

4. Fastener performance index

The product structure of bolts, screws, studs, nuts and other types of fasteners made of Nb521 material shall refer to HB series standards. In order to verify the standard performance index and actual measurement of Nb521 fasteners, and determine the stability and rationality of Nb521 material performance, bolts, screws and nuts are selected according to the most commonly used head types, types and typical specifications of aerospace models to verify their performance, and their strength and hardness are tested. The measured performance of Nb521 product is shown in Table 4.
Table.4 Measured performance of Nb521 product

Product name	Specifications/mm	Tensile strength/MPa	Hardness/HBW
Hexagon headed bolt	M6X26	507, 508,  498	–
Screw	M6X30	510, 516, 508	–
Hexagon headed bolt	M8X44	509, 510,  508	–
Hexagon headed bolt	M8X40	505, 515, 510	–
Hexagon headed bolt	M8X36	508, 510,  508	–
Hexagon headed bolt	M8X52	498, 505,  514	–
Hexagon headed bolt	M10x40	496, 484,  496	–
Hexagon headed bolt	M10x75	505, 498,  504	–
Hexagon headed bolt	M10x59	499, 505,  510	–
Hexagon nut	M6	–	153, 156, 151
Hexagon nut	M8	–	145, 142, 142
Hexagon nut	M10	–	146, 150, 148

5. Surface treatment

At present, the main ways to improve the resistance of refractory metals to high temperature oxidation under high temperature and oxygen environment are alloying and surface coating. The alloying method can improve the oxidation resistance of the alloy, but the alloying elements must exceed a certain amount of critical value to protect the matrix, which will inevitably affect other properties of the alloy, especially the degradation of the high-temperature mechanical properties of the matrix. The coating on the alloy surface can not only protect the alloy matrix from high-temperature corrosion or slow down the corrosion rate, but also not change the composition of the alloy matrix, and can retain the high-temperature mechanical properties of the alloy to the maximum extent.
The surface treatment method for bolts, screws, studs and nut fasteners manufactured by Nb521 is surface coating, which is prepared by slurry melting method. It is a silicide composite coating with excellent performance, and the coating thickness is (60 ± 10) μ m。 The coating detection and high temperature performance meet the requirements of static oxidation resistance life at 1600 ℃ ≥ 1h in atmospheric environment; The static oxidation resistance life at 1800 ℃ is required to be ≥ 0.5 h; 1 800 ℃ cooling (water cooling) to room temperature, thermal vibration life ≥ 50 times.

6. Conclusion

In this paper, the characteristics of Nb521 material, upsetting formability, machinability and fastener structure are comprehensively analyzed, and the comprehensive performance index of Nb521 material fastener is formulated. A series of problems in the manufacturing of Nb521 fasteners have been solved through innovation and optimization of traditional processing technology, application of new molybdenum silicide coating and other measures. The fasteners produced meet the design and specification requirements, and have been successfully applied to the combustion chamber tail nozzle of a new type of equipment engine, connecting the composite materials at this position, and the working temperature is 1600-1800 ℃. The results show that Nb521 niobium-tungsten alloy has excellent processing properties and high temperature properties, which can meet the requirements of aerospace fasteners for material properties.

Authors: Hu Qingkuan, Xie Maoyang, Fu Jianjian, Xu Yongchun, He Kun

Source: China Alloy Fastener Manufacturer – Yaang Pipe Industry (www.epowermetals.com)