Role of Volumetric Energy Density and Post-Heat Treatments to Achieve Stable Microstructure on Additive Manufactured Ti6Al4V Alloy

The present study aims to analyse of the role of volumetric energy density (VED) in the achievement of denser titanium alloy (Ti6Al4V) through the use of Laser based powder bed fusion (L-PBF) process. Rectangular blocks were fabricated with five different VEDs, namely, 35.07 J/mm3, 40.46 J/mm3, 44.19 J/mm3, 55.11 J/mm3 and 60.45 J/mm3. A highly intensified laser beam causes a faster cooling rate resulting in excessive formation of unstable needle-like martensite (α') phases inside the columnar beta (β) grains. Reduction in scan speed from 990 to 630 mm/sec with constant laser power as 250 W triggered the formation of keyhole pores. Also, a similar kind of observation was noted when the scan speed was reduced from 1270 to 850 mm/sec with constant laser power of 370 W. The sample fabricated with a VED of 44.19 J/mm3 resulted in a minimum fraction of porosity, i.e. 0.04%.The sample fabricated with a VED 44.19 J/mm3 was subjected to solution heat treatment (HT) at 950 °C/1 h and 1050 °C/1 h, i.e. below and above the β transition temperature for the achievement of stable equilibrium microstructure.Solution HT at 950 °C resulted in the growth of thin and thick needles of Widmanstätten α phases followed by water quenching (WQ) and air cooling (AC), respectively. The columnar β grains were disintegrated at 1050 °C leading to the equiaxed grain boundary α along with newly formed α' phases after WQ and lamellar α phases after AC. The findings of the study showed the role of energy densities and post L-PBF HT in the achievement of stable microstructure on Ti6Al4V alloy.