Laser surface treatment of α-β titanium alloy to develop a β -rich phase with very high hardness

Abstract

• Laser surface treatment results microstructural gradient in laser affected zone (LAZ). • Plasma plume detachment at higher power result in surface roughness enhancement. • Beta enrichment in LAZ with higher hardness. • Rosette structure formation. The α - β titanium alloy VT31 is used in the aerospace industry due to its high strength to weight ratio and good mechanical properties at elevated temperatures. Surface microstructure plays an important role in the mechanical behavior of the material, especially fatigue. In this work, VT3-1 α - β titanium alloy surface microstructure was modified using laser surface heat treatment. Laser surface treatment (LST) was carried out at various laser powers (100−250 W) and scanning speeds (150−500 mm/min). Laser affected zones, surfaces and sub-surfaces were characterized using different techniques. Electron backscatter diffraction (EBSD) analysis of the laser affected zone exhibits a distinct microstructure across the depth of the specimen, and it changes with laser power and scanning speed. At a lower scanning speed and high laser power (200−250 W); almost complete β -containing microstructure was developed at the surface with very high hardness. At an intermediate power (150−200 W), a β -rich surface layer with dendritic features followed by a layer of martensitic microstructure was evident. Interestingly, at high power (250 W) a distinct shiny surface with a lower surface roughness was observed. Moreover, the microstructure under this condition corresponds to an α -phase (transformed microstructure) which is typical of melted and fast cooled α - β titanium alloy. Finite element analysis of the LST was also carried out to predict thermal history based on which the extent of the laser affected zone across the depth was predicted for various processing parameters and correlated with the hardness variation.