The Dominant Factor Of Metal Powder Compaction Temperature Rise Under HVC

Products of powder metallurgy, with their unique chemical composition, mechanical behavior, physical properties, economical efficiency and environment protection, are used widely in various industries such as automobile, metal, aerospace and military manufacturing. The traditional processing technology can hardly do without the link of sintering, by means of which metal powder becomes a metal body through compacting and pressing of powder particles. However the process of sintering is consequentially accompanied by the volatilization of binder and thermal deformation of compact. Among these, the volatilization of binder affects the compactness of compacting and shaping, leaving blowholes in compact, and the volatilized gas also increases the burden on the environment. Therefore, the pursuit of once-molding process of metal powder has caught more and more attention. HVC of metal powder is the technology that achieves the purpose of compacting and pressing by means of impacting the metal powder particles with a high-speed punch. As the speed of compacting rising, the heat diffusion time produced in the compacting process is also shortened, and that means the contact surface temperature among the metal particles is very high, which makes once-molding, sintering and compacting of the metal powder possible. This paper is focused on studying the problem of temperature rise in HVC, investigating the rules of temperature rise under different compacting speeds and its dominant factors.We use the non-linear finite element software Marc as the analysis tool and respectively choose iron powder, copper powder, Ti6A14 V powder as analysis materials. We compare the Ballsjon compaction equation, Nishio Kitagawa equation, Huang Peiyun compaction equation with Johnson-Cook constitutive equation, which aims at study powder compaction and finally choose Johnson-Cook constitutive equation which suits HVC better. In view of the problem of perforation that occurs in large deformation simulation, we redefine the concept of contact among metal particles, putting forward the view that contact tolerance should be less than or equal to step displacement, which successfully resolve the problem. We build finite element models of 9 wholepacking and 9 close-packing, comparing and analyzing the stress state and deformation of these two models, we find the 9 close-packing model is more practical.Reviewing the friction models of contact interface among metal particles, comparing the advantages and disadvantages of revised Coulomb friction model and shear friction model, in view of the features of volatility and uncontinuity of the friction of particle interface in metal powder HVC, we consider the revised Coulomb friction model is more appropriate. According to the energy conservation law, this paper carries out research over the temperature rise on two aspects of particle friction work and plastic deformation, analyzing the temperature rise of three kinds of metal powder particle in different pressing speed ranges by means of numerical simulation. We discover when the speed is less than 5m/s, because the speed is too low and friction work is not obvious, then temperature rise is mainly resulted by plastic deformation. When the speed is between 5m/s and 60m/s, as the compacting speed rises, friction work increasing, then the temperature rise of friction occupies bigger and bigger proportion among the entire temperature rise. When the speed is more than 60m/s, plastic deformation becomes the dominant factor of temperature once again, which is the result of the occurrence of approximate adiabatic compression led by the high compaction speed.By means of reasoning and solution of the constitutive relation of thermo-piezoelectric coupling of metal materials, we find the dominant factor that affects the temperature rise in the constitutive in high-speed compaction is the yield strength of material. Meanwhile, by means of exploring the yield strength of material and the compaction speed needed by its corresponding once-molding with Marc, we discover that in order to realize the simultaneous completion of compacting and sintering, the temperature of contact surface of metal particles should be higher than the melting point of this metal. The results of research indicate that as the yield strength of material rises, the sintering and compacting speed first rises and then drastically falls. When the yield strength is higher than 400 MPa, the change of speed of sintering and compacting tends to be gentle. Comparing the curves of calculation results of different material simultaneously, we find that because the yield strength of iron powder and copper powder is rather low, their sintering and compacting speed is higher than 100m/s, which could be hardly achieved by using regular equipment, therefore once-molding is difficult to realize in traditional powder metallurgy technology. The metal powder with yield strength higher than 400 MPa can be implemented with electromagnetic pulse HVC technology, by means of adjusting the on-load voltage to realize the change of compaction speed, therefore also to realize once-molded sintering and compaction. These research offers reference to further development of metal powder HVC equipment.