Determination of α to β phase transformation kinetics in laser-powder bed fused Ti–6Al–2Sn–4Zr–2Mo-0.08Si and Ti–6Al–4V alloys

Laser powder bed fused (L-PBF) Ti–6Al–2Sn–4Zr–2Mo-0.08Si (L-PBF-Ti-6242) and two grades of L-PBF-Ti-6Al–4V (L-PBF-Ti-64) alloys are studied using differential scanning calorimetry (DSC), to determine the kinetics of α to β phase transformation to design and develop post-process heat treatments. Both non-isothermal and isothermal kinetics models are developed using the DSC results and employing the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. The mechanism of non-isothermal phase transformation is discussed based on the local Avrami parameters. The transformation process is divided into three stages showing a high rate of transformation in the initial stage (dominated by nucleation) followed by a steady-state rate (dominated by growth) and again a rapid increase in the transformation rate in the final stage. The effect of the β stabilizing elements on α to β phase transformation kinetics is discussed for all three alloys in both non-isothermal and isothermal transformations. L-PBF-Ti-6242 needs higher activation energy to initiate the transformation and the process occurs at a comparatively higher temperature range since this is a near-α Ti alloy. The oxygen homogeneity in the L-PBF-Ti-64 shows a clear effect on the transformation kinetics. The initial microstructure also affects the kinetics of α to β phase transformation, as the as-built L-PBF microstructure results in the highest transformation kinetics. Any heat treatment that alters the as-built microstructure results in slower transformation kinetics. The results of the isothermal kinetics models are employed to design a two-step heat treatment for L-PBF-Ti-6242 to enhance the ductility with minimal strength loss. The properties are then correlated to the microstructure. This study provides great insight into the importance of considering the effect of the material system, alloying elements, and initial microstructure on the kinetics of phase transformation in L-PBF titanium alloys and the role of phase transformation kinetics in designing heat treatments to achieve the desired microstructure and properties