Microstructure And Adiabatic Shear Susceptibility Of Ti6Al4V Titanium Alloy Fabricated By Additive Manufacturing

Ti6Al4V titanium alloy bar fabricated by selective electron beam melting（SEBM）was selected as the research object in this paper,and the microstructure of the material was regulated by heat treatment process.The internal defect characteristics of Ti6Al4V sample before and after heat treatment were detected by X-ray computed tomography（XRCT）technology.Combined with the scanning strategy used in the SEBM manufacturing process,the formation and evolution mechanism of pores in different positions of the bars before and after heat treatment were analyzed.Optical microscopy（OM）,X-ray diffraction（XRD）and split Hopkinson pressure bar（SHPB）were used to investigate the effects of microstructure,phase composition and orientation of printed and heat-treated samples on their adiabatic shear anisotropy.The effects of microstructure and printed pores on the localized adiabatic shearing behavior of the printed,heat-treated and rolled Ti6Al4V alloy were investigated with the split Hopkinson compression bar,metallurgical microscopy and X-ray computed tomography technology.The results showed that for the spherical pores,the porosity of the peripheral position in the mid-layer part of the printed bar sample（0.00585%）was lower than that of the central position（0.203%）,because the contour point scanning strategy with more concentrated electron beam was adopted on the peripheral part of the powder layer,which produced deeper molten pools,promoted the remelting of the previous layer and gave more opportunities for bubbles to escape,so the corresponding porosity was lower.However,the cooling rate of the material was higher and the subsequent powder layer was reduced at the top-layer part of the bar,so the remelting effect of the powder layer was poorer than that of the mid-layer part.In addition,because the width of the scanning track generated by the contour point scanning strategy was uneven,the fluidity of the molten pools was poor and the molten pools were more prone to uneven shrinkage,so the bubbles were more difficult to escape,resulting in the porosity of the peripheral position in the top-layer part of the sample（0.0142%）was higher than that of the central position（0.00895%）.During the air cooling process of heat treatment,the near-surface part of the bar was subjected to additional tensile stress,which enlarged the pores.Correspondingly,the interior of the bar was subjected to additional compressive stress,which compressed the pores here.Finally,the reduction of pore volume in the center part was greater than the increase of pore volume in the near-surface part,so the porosity of the whole sample was reduced after heat treatment.The adiabatic shear behavior of printed Ti6Al4V was anisotropic,which was caused by the orientation of the microstructure.For the printed longitudinal sample,the c-axis of the hcp-αphase unit cell was parallel to the loading direction.At this time,the plastic deformation of Ti6Al4V mainly depended on the pyramidal slip system,and the critical resolved shear stress（CRSS）required to start this slip system was high,so the flow stress of the material increased,the deformation work increased,and the corresponding thermal softening effect was strong,resulting in high adiabatic shear susceptibility.The original columnar grains in the printed sample were transformed into equiaxed grains after heat treatment,and the microstructure orientation decreased.Therefore,the adiabatic shear susceptibility of the heat-treated sample loaded in different directions was basically the same.The lamellar spacing in the widmanstatten structure decreased and the aspect ratio ofαphases became larger after heat treatment,so the resistance to dislocation movement during plastic deformation was stronger,resulting in the increase of flow stress and the enhancement of thermal softening effect.As a result,the heat-treated samples were more prone to adiabatic shear at the strain rate of 2700s^-1,and the adiabatic shear susceptibility was higher than that of the printed samples.The adiabatic shear susceptibility was determined by the microstructure,orientation,density and porosity.The rolled sample had fine equiaxed grains,no printed pores and the highest density,so its adiabatic shear susceptibility was the lowest at the strain rate of 1700s^-1.The existence of printed pores was equivalent to omitting the void nucleation stage of microcrack formation during dynamic deformation,and the pores located in the 45^oshear direction would grow and converge directly along the shear direction to form microcracks,while the pores located in the adiabatic shear direction would also accelerate the propagation of microcracks and more easily cause adiabatic shear fracture.The number of pores in the sample increased after heat treatment,and the probability of pores in the shear direction was greater,so the heat-treated sample was more likely to initiate microcracks in the adiabatic shear band,and the crack propagation rate would be faster and more likely to cause adiabatic shear fracture than the printed sample.