The Study, Based On Fractal Theory Hvc Powder Heat Transfer And Friction

High Velocity Compaction is a new manufacturing technique for producing high density powder metallurgy material efficiently at low cost. On the basis of summarizing the principle, characteristic and research situation of HVC and application of fractal theory, this paper studies on characteristic of granular powder space, heat conduction and friction in HVC by the fractal theory.Firstly, fractal dimension is used to describe the characteristic of powder granular media space. The calculating formula of the fractal dimension, accumulated density, particle density and pore media density is derived which avoids computing filling rate. Then with the method of equivalent resistance, a fractal model of thermal conduction between powder particles and pore media is established. And influence of heat radiation on effective heat conductivity is taken into consideration. The numerical simulation indicates that effective thermal conductivity gradually decreases with fractal dimension increasing. Furthermore, the effect of heat radiation makes the effective heat conductivity proportional to the cube of temperature.Secondly, the Weierstrass - Mandelbrot function is introduced to describe the statistic self'- affinity fractal feature of powder particles' surface. According to Hertz contact theory, the expression is generated to represent the relationship of compaction load, real contact area and fractal parameters when powder particles' elastic and plastic deformation occur. It provides theoretical foundation for determining the size of pressing load.Finally, a fractal model of adhesive friction coefficient is proposed by comparing the normal elastic load with the normal plastic load acting on particle contact points. Results show that for certain fractal dimension, the total adhesive friction coefficient reduces sharply as the normalized contact area increases. Meanwhile, a fractal model of powder particles' internal friction is built. And the change tendence of powder particles' internal friction with the change of powder particles' surface temperature and fractal dimension is analyzed. It has certain significance for studying compacts density and energy transformation in HVC.