Effect of Process Variables on Residual Stress and Microstructure in Laser Additive Manufacturing of γ-TiAl Alloy

Abstract

Laser Metal Deposition (LMD) is used to fabricate intricate three-dimensional parts from metal powder by fusing material in a layer-by-layer manner of a digital Computer Aided Drawing (CAD) model. LMD process employed for processing of various materials such as metals, alloys, functionally graded materials, and repairing purpose. The LMD process involved numerous process conditions, mainly laser power, travel speed, and the powder flow rate. Effect on a layerwise variation of melt pool size, thermal cycle, the cooling rate is required to understand for producing a successful sound part. Experimentally determination of the effect of these process conditions on melt pool, thermal cycle, the cooling rate is extremely difficult. A remedy is to achieve a quantitative understanding of the process through computational modeling approaches. In this work, Laser Engineered Net Shaping (LENS), one of the LMD techniques is used to fabricate inherently brittle γ-TiAl alloy thin-wall structures at various processing conditions. These deposits are expected to develop residual stresses due to the rapid heating and cooling cycles involved in the LMD process. Towards this end, a 3-D nonlinear thermomechanical finite element analysis is performed to simulate the process under various process conditions. A commercially available ANSYS software utilized in conducting a sequentially coupled thermomechanical analysis. The melt pool, thermal gradients, and residual stresses are predicted from the developed FE models. Results indicate that laser absorption coefficient (αA) of γ-TiAl is obtained by a laser surface melting study, and an αA value is 0.13. The simulated thin-wall results show that thermal gradients increased with an increase in the number of deposited layers i.e., from the substrate to the last layer. Cooling rates decreased with increase in the number of deposited layers i.e., from the substrate to the last layer. Along the build direction, tensile stresses are generated at the edges and compressive stresses are generated at the centre region of the thin-wall which increase with increase in distance from the substrate. Along the laser travel direction maximum compressive stresses are observed at the centre of the wall and these stresses decreasein magnitude with increase in distance from the centre. Higher laser power input yields higher residual stresses due to high-thermal gradients, and hence, laser power has a significant impact on the development of residual stresses in the thin-walls. Residual stresses in the deposited thin-wall samples are measured using the X-ray diffraction technique. Reasonable agreement observed between the predicted and measured values of residual stresses. The microstructure, phases, and hardness of the LMD γ-TiAl alloy thin-walls are also analyzed. The microstructure analysis shows fine lamellar structure comprised of γ and α2 phases, which are matches with the existing studies. Microhardness in the bottom area is found higher than the middle and top areas of the thin-wall. The hardness values increased marginally (5%) with the increase in travel speed. Further, melt pool dimensions (length, width, and depth) increased with increase in laser power and decreased with increase in travel speed. During deposition of a layer (which consists of six tracks) the maximum temperature in the melt pool is observed in track 1. Maximum tensile residual stresses are observed in track 1 and these are lower than the yield strength of the material. The magnitude of these stresses decreased from track 2 to 6. Trends of residual stress are found to be independent of the scan strategy (Unidirectional and bidirectional) considered in this study. The state and magnitude of residual stress distribution in the thin-walls and plate are attributes to the transient thermal gradients encountered during deposition.