| As a promising new near-net forming technology,Semi-solid powder forming combines advantages of powder metallurgy and semi-solid forming,such as the fine grains of materials,low pressure and short process,etc.At present,there are many researches focusing on the process,microstructures,and mechanical properties of semi-solid powder forming,but few studies are on forming theories.Because the semi-solid forming is a complicated process during which samples deform,solidify and densify simultaneously,the forming mechanism is still unclear.Semi-solid compression of porous materials is an effective method to study semi-solid powder forming due to the simple stress state,controllable porosity and the feasible numerical simulation.Therefore,in the present study,semi-solid compression of porous materials was used to investigate the basic principles of semi-solid powder forming and then mathematical model was established to simulate the processing of semi-solid powder rolling,which plays an important role in the development of semi-solid powder forming theory and lays a technical foundation for application and promotion.Liquid fraction,microstructures,grain sizes and coarsening rate of powders under different semi-solid conditions were studied.The results show that the DSC method is the optimal suitable method to determine the liquid fraction of semi-solid powders in this work.The grain coarsening rate of semi-solid powders is much lower than that of dense semi-solid materials,which is one of the reasons for obtaining fine grain size during semi-solid powder forming.The porous and dense compression specimens with different densities were heated to the semi-solid temperature and then compressed.The microstructures,fracture morphology and stress-strain curves of specimens before and after compression were compared and the constitutive equation for semi-solid compression of porous and dense materials was studied.The results show that the porous specimens prepared by spark plasma sintering have refined grains at the center and more spherical powders at the edge after compression,which is different from grain coarsening and even deformation of dense materials.The stress for semi-solid compression of the porous material is still a power function with strain rate and exponentially related to temperature.Then,a constitutive equation is established,with respect to the influence of liquid fraction and relative density on semi-solid compression of the porous materials.Taking compression specimens with initial relative densities of 68% and 83% as examples,the effects of processing parameters(such as heating temperature,holding time,strain and strain rate)on semi-solid compression of porous materials were investigated.Deformation behavior,process,deformation mechanism,densification mechanism and pores generation mechanism were analyzed.The results show that the heating temperature has a complex effect on the grain size of porous materials after semi-solid compression.The deformation mechanism includes the breakup of solid skeleton or powders,and flowing of liquid with powders or fragments.Consequently,the rearrangement of powders or fragments and filling of liquid result in the increment of relative density and the appearance of new holes.The crushing methods of semi-solid powders were studied,and the deformation resistances of semi-solid porous materials and the crushing resistances of semi-solid powders at different temperatures were calculated.The crushing coefficient was introduced and the relationship between crushing coefficient and strain was determined.Then,mathematical models of strain vs.relative density during semi-solid compression of porous materials,strain vs.relative density and relative density vs.rolling force during semi-solid powder rolling were deduced and experimentally vevificated.The results show that there are three kinds of crushing methods of semi-solid powders: a small amount of powders break into fine particles from the stress concentration points(liquid phase),powders slide along grain boundaries and are torn through grain boundaries.The break resistance of powders is a power function with the solid fraction and the breaking factor is 0<?<1.The numerical analysis model based on the ellipsoid yield criterion is not suitable for the semi-solid compression of porous materials,but relative density and rolling force of strips after semi-solid powder rolling can be calculated.Because the contact temperature between the powder and the roll cannot be accurately obtained and the assumption of uniformly distributed rolling force is not in accordance with the actual situation.The difference between calculated and experimental results is large.Based on Fourier heat conduction equation and related parameters of semi-solid materials,the temperature field distribution of dense semi-solid rolling was simulated by using Marc finite element software platform,and subsequently compared with experimental results to verify the validity of the model.Then,the process of semi-solid powder rolling was simulated based on the yield criterion of porous materials,considering influences of relative density on each parameter.The temperature,rolling force and relative density were compared and verified,and the influences of different processing parameters on the semi-solid powder rolling process were analyzed.The temperatue and relative density distribution are in good agreement with the experimental data,which verifies the accuracy of the model. |
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