Development of low cost high specific gravity slender tungsten nail counterweight parts
High specific gravity components with high density and corrosion resistance are widely used in national defense and civil industry. Due to the complexity of the shape of the high specific gravity products of slender parts, the traditional process is usually made by injection molding, but for the high specific gravity tungsten nails with long parts and small outer diameter, the process control is difficult and the cost is high. Can use a cold forming, through the follow-up machining to solve the mass production problem, stabilize the quality of products, simplify the difficulty of production, improve the production efficiency, reduce the production cost.
1. Process and method
High specific gravity tungsten nails are characterized by large length-diameter ratio, steps and thin wall, so it is very difficult to direct forming at one time. In terms of forming technology, injection forming or powder pressing forming can be chosen. In addition to special equipment, injection molding requires high requirements for tungsten powder, high cost of mold production, complex mold processing, long cycle, low density of green billet, and large sintering deformation. The specific technological process is: material selection - mixing - ball milling - granulation - drying - pressing - degumming - sintering - machining.
1.1 Material selection
Due to the subsequent process of the product to mechanical processing, the material requirements have good toughness, machinability, the selected material composition is :W powder 2.5-3.5hm,Ni powder 0.047mm, sputum based Fe powder 3-5um.Material ratio :93% W,4.9% Ni, 2.1% Fe.
1.2 Mixing
The powder was mixed on the V-shaped mixer for 2h, and the binder SBP5% was added for pelletizing and drying after 24h ball grinding in the drum type ball mill to complete the preparation of powder.
1.3 Forming
According to the characteristics of the product, there can be two kinds of pressing scheme.
(1) overall suppression. Because the product is very long, so the use of radial pressing perpendicular to the axial direction, reduce the length of the negative die, easier to form, improve the green density. After sintering, machined to the size of the drawing gauge.
Although this process is easy to form, the subsequent machining allowance is large and the material loss is large, so the choice is abandoned.
(2) combined die pressing. Adopt the forming die structure of type III parts compaction, forming in 100T automatic powder forming press. Combination die stamping because of the die structure and use restrictions, can not form a complete circle, so the suppression of the remaining, do subsequent processing. The punch surface should not have sharp corners. The platform ligament of 0.2-0.3mm should be made to improve the service life. The blank density is designed to be 11.62g/ cm3, the forming pressure is 500MPa, and the shrinkage rate is 15%.
When loading tungsten powder, the negative die will rise to the position of loading tungsten powder driven by the template. The first floating die will also rise to the position of loading powder driven by the floating template. The second lower die will remain on the pressure plate. When pressing, the upper die is pressed down, the first upper floating die is pressed down by contact with tungsten powder for compression, and the first floating die is pressed down. The second upper die punch goes down at the same time, and the downward velocity of the first floating die punch is less than that of the second upper die punch. Implement floating up. When the floating plant rushed to the limit and the first cross with the speed of pressure down, powder pressure down, the negative mold is not moving. The first floating die rushes to the limit block, and the next floating stops. The upper die punch continues to press down to the required density. When demoulding, the negative template drives the negative die to pull down, the negative template forces the floating die punching plate to drive the first floating die to push down, the second and first die punching does not move, and the product is ejected.
1.3 Come unglued
The binder in the compaction was removed in a continuous mesh belt furnace at 800℃ for 4h under the protective atmosphere of ammonia decomposition.
1.4 The sintering
In high temperature molybdenum wire furnace, sintering at 1430-1450℃ for 1h, ammonia decomposition atmosphere protection, using buried sand sintering, reduce the sintering deformation of the product.
1.5. Mechanical processing
According to the size requirements of the drawing, the inner hole is machined by alloy drill and the outer circle is machined by turning.
2 Performance
According to the inspection standard of the finished product required by the customer: the finished product falls freely from a fixed position 2m high without deformation and crack, which fully meets the inspection requirements, and the tensile strength is 500-600m Pa.
3 Production and application
The high specific gravity slender tungsten nail parts were developed by traditional tungsten powder metallurgy compaction forming, which has the characteristics of fast mold adjustment and test, low mold cost, high production efficiency (10 pieces/min), high density of pressed green, small sintering deformation and so on.
4 Conclusion
Compared with injection molding, the traditional powder metallurgy pressing forming process can produce high specific gravity slender tungsten nail counterweight parts, which simplifies the production process, improves the production efficiency and reduces the production cost, and achieves good economic benefits.
