| Ti6A14V alloy has a wide range of applications in aerospace,shipbuilding,and biomedical sectors due to its high strength-to-weight ratio and excellent corrosion resistance and good biocompatibility.However,it is rather difficult to develop complex Ti6A14V parts by traditional processing methods due to its low thermal conductivity and work hardening.Laser powder bed fusion(L-PBF)is an ideal method for the preparation of complex components due to its high design freedom and manufacturing flexibility.In addition,the components prepared by L-PBF have high dimensional accuracy and good surface quality.However,the intrinsic high cooling rates and high thermal gradient of the L-PBF process often leads to coarse primaryβ-columnar grains and needle-like α’ martensite in Ti6Al4V alloys.This results in high strength but poor ductility for LPBF fabricated Ti6A14V alloys,which could not meet the requirements for practical applications.Regulation of the chemical composition of L-PBF fabricated Ti6A14V alloys has been proven to refine the microstructure and lower the phase transformation temperature.This leads to the formation of β-phases at room temperature,which improves mechanical properties.Mo element is an effective β stabilizers in Ti alloys,which can largely reduce the martensitic transformation temperature.Meanwhile,no intermetallic compounds will be formed since Mo and β-Ti have a very similar crystal structure,and Mo and Ti are fully mutually soluble at high temperatures.Focusing on the abovementioned issues,Mo addition was used to modify the microstructure and performances of the L-PBF fabricated Ti6A14V alloys.The effects of Mo addition on the microstructure,phase composition,and mechanical properties of the LPBF fabricated Ti6A14V alloys were systematically investigated.The strengthening mechanism of the L-PBF fabricated Ti6Al4V alloy by Mo addition was investigated.In addition,the influence of heat treatment on the microstructure and performances of the Ti6Al4V-xMo alloy was investigat:(1)Optimization of process parameters for laser powder bed melting of Ti6A14V alloy.By varying the laser power(120 W,140 W,160 W)and scanning speed(600 mm/s～1400 mm/s),27 different sets of process parameters were used to study the effects of process parameters on the microstructure and mechanical properties.The results show that the highly dense parts(relative density>99%)were obtained at the energy densities varying between 50 J/mm3～60 J/mm3.Under the optimum process parameters(i.e.,P=160 W,v=1100 mm/s),the ultimate tensile strength of the as-fabricated alloy exceeded 1200 MPa and the elongation exceeded 8%.(2)Effect of Mo addition on the microstructure and performances of the LPBF fabricated Ti6Al4V alloys.Powder mixtures with different Mo contents(0.5 wt.%,1 wt.%,2 wt.%,3 wt.%,5 wt.%,7 wt.%,10 wt.%,15 wt.%)were prepared by mechanically mixing the prealloyed Ti6Al4V powders and Mo powders.The Ti6A14V-xMo alloys were prepared by L-PBF processing.Depending on the amount of Mo addition,the microstructure evolution could be divided three stages:(ⅰ)the microstructure is dominated by needle-like α’ martensite in the alloys with Mo content less than 3 wt.%.Due to the refinement of primary β columnar crystals and acicular α’ martensite by Mo addition,the strength of the alloy increases with the increase of Mo content while the elongation remains essentially unaffected.(ⅱ)When the Mo content is between 3 wt.%and 7 wt.%,the alloy consists of two phases α’ and β.The alloy undergoes stress-induced β→α’ martensitic phase transformation(SIMT)during loading,with enhanced work-hardening capacity and increased elongation.(ⅲ)When the Mo content exceeds 10 wt.%,the alloy is mainly composed of β-phase,resulting in an increase in elongation but a decrease in tensile strength.The above results indicate that the modulation of the microstructure of the LPBF fabricated Ti6A14V alloy can be achieved by controlling the amount of Mo addition.(3)The effects of heat treatment temperatures(650℃,850℃ and 1050℃)on the microstructure and performances of the LPBF fabricated Ti6A14V-xMo alloy were investigated.At the lower heat treatment temperature(i.e.650℃),the α’ martensite was only partially decomposed.With the increase of Mo content,the grain size in the alloy decreases,the strength increases and the elongation decreases.With increasing heat treatment temperature to 850℃,α’ martensite decomposes completely into α+β phase,leading to a decrease in the strength and an increase in the elongation of the alloy.With the increase of Mo content,the strength remains the same and the elongation decreases.After the heat treatment temperature exceeded the Tβline(i.e.,1050℃),the primary β columnar grains transitions to equiaxed grains,and massiveα-martensite was produced during the cooling process.This results in a sharp decrease in mechanical properties. |
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