Study On Microstructure And Properties Of Near TC4 Alloy Based On Element Fine-tuning By Laser Solid Forming

TC4 titanium alloy has attracted much attention in the fields of aviation,aerospace,automobile,biomedical treatment and other fields.For complex parts,it is mainly obtained by forging/casting +machining and powder metallurgy,resulting in low alloy utilization rate,high production cost and long leading time.TC4 alloy fabricated by laser solid forming(LSF)technology meets the requirements of high efficiency,short leading time,flexibility and good performance in aerospace aircraftparts design.However,laser solid formed TC4 titanium alloy exhibits the characteristics of high strength and low plasticity,which was caused by the long needle-like ? laths in prior?grains due to the rapid solidification.In order to make the laser solid formed TC4 parts widely used in aerospace field,it is necessary to improve the characteristics of high strength and low plasticity of alloys by adjusting their microstructures.In this study,by adding pure Ti in TC4 with low Al and V content,Ti-5.6Al-3.8V,Ti-5.3Al-3.6V and Ti-5.0Al-3.4V alloys were designed.The evolution of the microstructure and its influence on the conventional mechanical properties of the material were studied.Finally,the optimized Ti-5.6Al-3.8V and Ti-5.3Al-3.6V alloys were heat treated,and the evolution of the microstructure after heat treatment and its effect on hardness and strength of the alloys were investigated,which will lay a theoretical foundation for the preparation of near TC4 alloy parts by laser solid forming with high strength and good plasticity.The main research achievements of this study are as follows:1.The laser solid formed Ti-5.6Al-3.8V,Ti-5.3Al-3.6V and Ti-5.0Al-3.4V samples are all dominated by the columnar grains growing vertically in the scanning direction with an average grain size of 600-700?m.With the decrease of the equal gradient of Al and V content,the average width of columnar grains changes little.When the deposited material changes from Ti-5.6Al-3.8V to Ti-5.3Al-3.6V,the average width of ? lath in prior ? grains is almost constant,but when Al content decreases from 5.3wt% to 5.0wt% and V content decreases from 3.6 wt% to 3.4wt%,the size of ? lathincreases sharply from 0.91?m to 1.26?m.It shows that the size of ? laths in prior ? grains obtained by laser solid forming does not change linearly with the increase of Al and V contents.2.Under the same process parameters,the microhardness,yield strength and tensile strength of laser solid formed near TC4 and TC4 alloys decrease gradually with the decrease of equal gradient of Al and V content,while the elongation increase significantly(from 10% to 14%).It shows clearly that the room temperature tensile properties of laser solid formed Ti-5.6Al-3.8V,Ti-5.3Al-3.6V and Ti-5.0Al-3.4V titanium alloys is nonlinear with the equal gradient of Al and V contents.The mechanical properties of the laser solid formed Ti-5.6Al-3.8V and Ti-5.3Al-3.6V alloys meet the forging standards under the process parameters,and those two titanium alloys are subsequently heat treated.3.The optimized laser solid formed Ti-5.6Al-3.8V and Ti-5.3Al-3.6V titanium alloys were subjected to subcritical annealing and aging treatment.The columnar grain size did not change significantly after heat treatment.The average width and average aspect ratio of the ? lath changed significantly,while the average width of ? lath changes from ~0.9?m to ~5?m,and the aspect ratio decreases significantly from ~9 to ~4.5.However,the volume fraction of ? phase of the two alloys change little before and after the heat treatment.4.After heat treatment,the microhardness of laser solid formed Ti-5.6Al-3.8V and Ti-5.3Al-3.6V titanium alloys is decreased,the strength of the alloy is slightly decreased(only about 25MPa),and the elongation is significantly increased(from 10% to 11% in the as-deposited state to 12% to 17%).The results show that the plastic properties of laser solid formed low Al and V content TC4 and near TC4 titanium alloy can be significantly improved after heat treatment,without sacrificing the strength of the alloy.