| Titanium and its alloys have high specific strength and high corrosion resistance and good biocompatibility,which are widely used in aerospace,ocean,medical and other fields.However,titanium materials have poor processing performance and high cost of raw materials,which limits their applications.Titanium alloy with low cost and high performance can be produced by thermomechanical powder consolidation with high efficiency and low cost.In this thesis,the microstructure and mechanical properties of Ti-6Al-4V(TC4 titanium alloy)parts which prepared by Panzhihua Mengmeng Technology Co.,Ltd.using the new thermomechanical powder consolidation and forming process developed by the advanced powder metallurgy materials and technology team of Northeastern University were evaluated,focusing on the effect of component inhomogeneity on the microstructure and mechanical properties of parts before and after heat treatment.This study has guiding significance for the adjustment of the technological parameters of the thermomechanical powder consolidation process.It is found that there are elliptical alloy elements aggregation areas in the samples prepared by extrusion-forged.The size of those areas is about 200 ?m.According to EDS analysis,those areas come from Al-V master alloy powder particles in the powder compact,which are mainly caused by the uneven powder mixing process.The lamellae in the element enrichment areas are narrow with high hardness.While the lamellae in other areas are wide and discontinuous with low hardness.The difference of microstructure and microhardness in different aeras results in the yield strength of top and bottom part of horn-mouth parts and top-cover parts prepared by extrusion-forged is 934,861 and 919 MPa,respectively.The ultimate tensile strength is 1044,992 and 1010 MPa,respectively.The elongation to fracture is 7.9%,6.7%and 8.3%,respectively.It can be seen that the strength of top and bottom part of horn-mouth parts and top-cover parts basically reaches the intrinsic strength of ingot metallurgy TC4 titanium alloy,but the tensile ductility is significantly lower than that of ingot metallurgy TC4 titanium alloy(elongation to fracture is generally more than 10%).After 700? vacuum dehydrogenation annealing,alloy elements diffused only at low concentration and small scope in the top and bottom part of horn-mouth parts and top-cover parts sample.While the high concentration of alloy elements aggregation areas in the material are not eliminated.The strength of the samples cut from top and bottom part of horn-mouth parts and top-cover parts prepared by extrusion-forged decreases slightly(the yield strength:885?893 MPa,the ultimate tensile strength:963?991 MPa).The tensile ductility doesn't increase but slightly decreases(elongation to fracture:6.7%?7.4%).The search results show that the uneven distribution of elements in the top and bottom part of horn-mouth parts and top-cover parts has not been eliminated after annealing.Therefore,the cooperative deformation effect of soft and hard phases still exists when the annealed materials deform.The effect of interface strengthening is produced during the annealing process,which improves the hardness of different areas.Therefore,the elongation to fracture of the material is decreases slightly due to the improved hardness of the soft phase.At the same time,the strength decreases slightly because of the widening lamellae after annealing.After 955?/4 h homogenization treatment and furnace cooling,the lamellar continuity of the areas with less V content in the top and bottom part of horn-mouth parts was improved.This helped to make more uniform deformation.Therefore,the elongation to fracture of the top and bottom pars of the horn mouth parts after homogenization treatment was increased by about 20%compared with that before heat treatment,reached 10.7%and 8.1%respectively.But the lamellae grew wider and the corresponding strength decreased by 10%.The yield strength of those samples is 813 and 804 MPa respectively,and the ultimate tensile strength is respectively 904 and 897 MPa.While the lamellar continuity of the areas with less V content in the top-cover parts was not significantly improved.At the same time the lamellae became wider,which made its strength and elongation to fracture decreased compared with that before heat treatment.The yield strength,ultimate tensile strength and elongation to fracture of the top cover parts is 845 MPa,934 MPa and 7.2%respectively.Under the same heat treatment conditions,the difference of lamellar continuity between different samples is mainly related to the deformation of the parts that sample taken from.More deformation means better diffusion effect of alloy elements and the lamellar continuity of the discontinuous lamellae areas in the original samples is improved to a greater extent.For the extruded rod with the same uneven element distribution,homogenization treatment followed by air cooling and furnace cooling respectively was carried out.The strength and ductility of the as-extruded sample is close to that from the samples cut from extrusion-forged samples.The yield strength,ultimate tensile strength and elongation to fracture are 891 MPa,1006 MPa and 8.2%,respectively.After homogenization treatment of 955?/2 h and air cooling,the yield strength and ultimate tensile strength are 863 MPa and 1004 MPa,and the elongation to fracture increased to 12.1%.After homogenization treatment of 955?/4 h and furnace cooling,the yield strength and ultimate tensile strength are 852 MPa,961 MPa,and the elongation to fracture is 17.2%.Compared with the as-extruded sample,the strength of the sample solution treated and furnace cooled is only reduced by 4%,but its elongation to fracture is increased by 110%.This shows that homogenization treatment of 955?/4 h and furnace cooling leads to excellent comprehensive mechanical properties of the extruded sample of Ti-6Al-4V alloy.The main reasons can be attributed to the elimination of uneven distribution of alloy elements and the resulting uneven distribution of microstructure of the material by homogenization treatment and the ? transformed structure produced during furnace cooling.The ? transformed structure has a back stress strengthening effect to the a phase,and the widening of ?lamellae after heat treatment also has a toughening effect associated with coarse grains. |
PDF Full Text Request