Research On Process Multi-pass Rolling And Quality Control Of Sintered Tungsten Plate

As rare metal tungsten is an extremely important strategic resource, which in the form of pure metals and alloys are widely used in the field of science and modern technology, is increasingly being received most attention by all countries in the world. With the continuous development of high-speed rail, wind power, aerospace and other high-tech areas, tungsten plates have become increasingly demanding. Due to the high melting point, high hardness, tungsten slab is mainly made by the powder metallurgy process. Because of the internal pores, tungsten plate is prone to layered fracture, cracking and other defects in the rolling process. Therefore, to study how to optimize the rolling process and control fracture has important significance for the development of tungsten industry.The numerical simulation method is used to analyze the effects of different process parameters on the quality of the rolling process by DEFORM software. Comprehensive rolling processing status of sintered tungsten, model accuracy and other factors, it is better to use Zerilli-Armstrong constitutive relation model. Each parameter value of this model is calculated by the experimental stress-strain curve. Then a reasonable simulation geometry model is established and the typical process parameters are determined. In terms of shape and size, the stress and strain fields, and damage distribution, positions which can easily result in defects are pointed out, the corresponding laws are drawn, and the ways to optimize process are determined.According to the research object and analysis of the rolling process factors, the typical rolling process of sintered tungsten plate is proposed and numerically simulated. The results show that severe damage is mainly located at the edge of the blank side, and it is higher at the center in the thickness direction. The large equivalent stress area is mainly at the surface. With the increase of the cumulative reduction, the equivalent strain values and the large strain region significantly increased. The density is lower near the edge of the blank where cracks occur more easily.To analyze the underlying causes of sintered tungsten plate cracks, the tungsten billet after first forged was observed metallographically and the microstructure at different locations in the thickness direction was obtained. With analysis of three major faults-layered fracture, angle cracks and surface cracks-of tungsten plate rolling, and based on Normalized Cockcroft&Latham damage model, the numerical simulation method was used to analyze the fracture trend in terms of the maximum principal stress, equivalent strain, damage values, etc. The results show that reducing the maximum principal stress at the center of the slab can control the layered fracture, and reducing the strain difference between internal and surface can increase the uniformity of the overall deformation and reduce the trend of internal fine cracks.The effect of different process parameters on the quality of sintered tungsten plate rolling was investigated considering cogging temperature, pass temperature drop pass reduction rate, roll diameter, rolling speed, friction conditions and thickness difference. Numerical simulation results show that for five times rolling, the more ideal pass temperature drop is100℃,50℃,50℃,100℃, each pass rolling temperature is1550℃,1450℃,1400℃,1350℃,1250℃. With the pass reduction increases, the equivalent stress distribution is more uniform, equivalent strain increases, which is the maximum at the surface. For the different cogging reduction, it can select different roll diameter to reduce damage of cogging stage. With the increase of friction coefficient, equivalent strain, the maximum principal stress and the strain difference increases.According to the analytical results, the cross rolling process and concave roll process are proposed to improve the quality of rolling. The damage is mainly distributed around the blank in cross rolling process. Compare to unidirectional rolling, cross rolling has lower average damage, the tendency of cracking caused by cumulative damage is lower, and anisotropy of plate is very small. Using concave roll in the intermediate passes, the equivalent stress distribution is more uniform and shape change more fully. Numerical simulation results show that the two processes can significantly improve the quality of rolling. Then the ideal rolling process is suggested as an improvement program for sintered tungsten plate rolling to provide guidance for the actual production.