Influence of α-contaminant Layer on the Microstructure and Properties of Ti-6Al-4V Titanium Alloy Casting

The microstructure,oxygen and hydrogen content,nano hardness trend of α-contaminant layer on the surface of Ti-6Al-4V titanium alloy by vacuum smelting（ZTC4 for abbreviation）were investigated,and the effects of α-contaminant layer on the tensile properties of ZTC4 titanium alloy were also analyzed.The results show that α-contaminant layer has a thick lamellar structure with the maximum thickness of 0.28 mm.Compared with the those of the heart zones of ZTC4 alloy,the oxygen content and average hardness of the α-contaminant layer respectively increases by 1.64 and 0.94 times while its hydrogen content decreases by 1 time.The α-contaminant layer has a significant effect on the tensile properties of ZTC4,and the yield strength,tensile strength,elongation and section shrinkage of ZTC4 respectively decreases by 14.2%,12.5%,20% and 35% in comparison with those of the alloy without α-contaminant layer.The hard and brittle α-contaminant layer is formed on the surface of the alloy by dissolving oxygen atoms,which is difficult for deformation under the tensile load,and then the macroscopic fracture sources are formed in the weakest zones of alloy and quickly expand to the matrix resulting in brittle cleavage fractures in the alloy finally.