Study On Powder Forging And Densification Forming Technology Of Ferrous Alloy

Powder metallurgy technology with low-cost, near net shape and other significant advantages has been widely used for production of engine transmission parts, general machinery parts and tools, etc. Density and distribution of powder metallurgy materials is a key factor to affect the mechanical properties and dimensional accuracy of powder metallurgy products. The development of high density and high performance powder metallurgy products has become the powder metallurgy technology development direction and research priorities. Powder forging technology combining the advantages of powder metallurgy and precision forging, improving the mechanical properties of porous powder metallurgy parts, simplifying the process of precision forging billet system, can produce the high performance complex shape parts with high efficiency and low cost. The research contents are as follows:the preparation of Fe-0.5C-2Cu alloy for powder forging; the hot deformation and densification behavior of Fe-0.5C-2Cu alloy. The constitutive equations of flow stress of the powder forged and sintered alloy at elevated temperature were established. The hot deformation and densification process of sintered alloy with pores was investigated, and the process parameters of powder forging were optimized by the means of FENS (finite element numerical simulation). Finally, the relationship between mechanical properties and density of porous materials and the effects of heat treatment process on mechanical properties of powder forged steel were studied. The following main conclusions are obtained:(1) The densification mechanism of metal powder compaction process was studied by the means of FENS. The low cost Fe-0.5C-2Cu alloy samples with density of6.43g/cm3were prepared through mechanical mixing, double-action pressing and sinter in the protection atmosphere. The mechanical properties and fracture behavior of sintered alloy were evaluated and analysed.(2) The hot compressive tests of powder forged and sintered Fe-0.5C-2Cu alloy at elevated temperature were carried out. The results indicated that the peak stresses on flow stress curves of high-density powder forged steel are found due to the effects of dynamic recovery and dynamic recrystallization. While the flow stress of sintered alloy continue to increase in the hot deformation process with the increase of deformation extent and density because of the effects of both material hardening and geometry hardening.(3) Utilizing hyperbolic sine function and introducing the strain with nonlinear fitting, the modified Arrhenius equation of flow stress of powder forged Fe-0.5C-2Cu alloy at high temperature was established. Considering the impact of deformation on the flow behavior, the constitutive equations of flow stress of sintered Fe-0.5C-2Cu alloy were also established using stepwise regression method. The average absolute relative error between predicted results and experimental data of powder forged and sintered alloy are3.07%and3.88%respectively, which indicates that the two constitutive equations have good predication ability.(4) The constitutive equations of flow stress of powder forged and sintered Fe-0.5C-2Cu alloy were implanted into finite elements analysis software to simulate the constant temperature and strain rate compression process. The simulative load-stroke curves agree well with the experimental results. The simulative densification and density distribution of sintered alloy during the hot compression process is basically consistent with the testing results. The max relative error of average density is3.97%, while the accuracy of prediction under the high strain rate condition (no more than0.53%) is higher than that under the low strain rate condition.(5) The powder forged alloy samples with density of7.85g/cm3were prepared under the optimum process conditions by the means of FENS. The values of ultimate tensile strength (UTS), yield strength (YS), elongation (EL), Brinell hardness (HB), reduction in area (RA) and impact toughness (IT) are932MPa,667MPa,9%, HB294.4,20%and22.7J/cm2respectively. The relationships between mechanical properties and density indicated that the UTS, YS and HB increases linearly, and the EL and AR increases in power function, as well as the IT increases exponentially with increasing density respectively. Fracture took place in sintered necks of the material at the low density and the mode of fracture is pure ductile. With the increase of density, the fracture morphology is a combination of ductile rupture as well as brittle fracture from fully dense pearlitic grains. When the density close to full dense, the fracture surface is composed of transgranular cleavage and the mode of fracture is complete brittle.(6) The mechanical properties of powder forged Fe-0.5C-2Cu alloy after different heat treatment were tested. The test results indicate that the IT, UTS, YS and HB of the specimens after oil cooling and tempering is higher than that after air cooling and tempering, while the plasticity is lower than the latter. The specimens cooled in oil and air after tempering at450℃have good mechanical properties.