Effects of plasma electrolytic polishing on the surface and mechanical properties of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si specimen fabricated by laser powder bed fusion

Abstract

The high roughness of titanium alloy specimen fabricated by laser powder bed fusion (LPBF) may limit the application of LPBF technology. Plasma electrolytic polishing (PEP) can effectively reduce the surface roughness of metal samples. In this study, an orthogonal test was designed to investigate the effects of polishing time, voltage and electrolyte temperature on the reduction of roughness and wall thickness of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si specimens fabricated by LPBF. The surface roughness of the specimen decreased from Ra = 49.1 μm to Ra = 9.1 μm and the thickness decreased from 1.72 mm to 1.38 mm after the PEP. Due to the recoil pressure induced by rupture of vapor-gaseous envelope (VGE) and the annealing effect caused by electron impact, the width of α/α' grains on the surface of TC11 specimen decreased from 0.51 μm to 0.46 μm; the mean value of geometrically necessary dislocations (GND) increased by 9.8 %; the proportion of high angle grain boundaries (HAGBs) increased from 75.2 % to 80.8 %; the proportion of twin grain boundary length increased from 2.9 % to 11.9 %, and the activated twin variant type increased from 2 to 4. The number of pores in the samples reduced from 85 to 2 after PEP, which resulted in the improvement of tensile and fatigue properties. The ultimate strength and elongation of the tensile specimens increased by 6.2 % and 53.5 % respectively. The infinite fatigue life maximum stress of the high cycle fatigue specimen increased by 62.5 %. Another reason for the improvement of mechanical properties may be the self-healing of microcracks and the electroplasticity in the specimens.