The investigation of the effects of shot peening on the surface and mechanical properties of Ti6Al4V alloy produced by additive manufacturing method

In this study, the effects of different shot peening intensities applied after stress relief annealing on the microstructures and mechanical properties of Ti6Al4V parts produced by the Direct Metal Laser Sintering (DMLS) method, one of the Additive Manufacturing Methods, were examined. High porosity and surface roughness values occur in materials produced by additive manufacturing methods. In addition, residual stresses occur due to the high energy density resulting from production and short cooling times. Stress relief annealing was carried out to reduce residual stresses resulting from the manufacturing method, which may create limitations for high-stress applications exposed to variable loads. Stress relief annealing applied to Ti6Al4V samples was applied at 650°C under argon gas atmosphere and cooling was carried out in the furnace. After the heat treatment, shot peening process was applied, which causes cold plastic deformation on the outer surface layers. Four different Almen intensity parameters were applied in shot peening using compressed air. Two different parameters, 100% and 200% coverage rates, were used for the lowest and highest Almen intensity parameters (A12-14 and A24-26). The effect of different shot peening parameters on the surface and subsurface was revealed with the hardness results obtained using the Micro Vickers Hardness measurement device. The average surface hardness of the non-shot peened samples was measured as 415.9 HV, and the average hardness value for the A24-26 shot peened sample group was measured as 480.1 HV, and it was determined that the surface hardness increased by approximately 15%. Residual stress measurements were carried out by XRD on the surfaces of non-shot peened and shot peened samples with A12-14 and A24-26 Almen intensity. In addition, images were taken from the cross-section of the samples using optical microscope (OM) and scanning electron microscope (SEM). It is observed that there are fine-grained structures on the surface and subsurface due to extreme plastic deformation. Crystallite grain size measurements were carried out on shot-peened surfaces using XRD and it was revealed that the crystallite grain size decreased with increasing Almen intensity. After the three-point bending test, the average bending strength value of the non-shot peened samples was obtained as 2132 MPa. The highest bending strength value is 2178 MPa and belongs to the shot peened sample group with A20-22 Almen intensity. After tensile tests, scanning electron microscope was used to study fractured surface. It is observed that the fracture surface are characterized by many voids, unmelted powders and lack of fusion area.