Laser powder bed fusion of titanium aluminides using sequential thermal scanning strategy

Abstract

Titanium Aluminides (TiAls) are good candidates for many applications in the automobile, aerospace, and energy industry because of their high strength, toughness, corrosion, and oxidation resistance at high temperatures. Nevertheless, the low ductility and fracture toughness makes processing of TiAls extremely difficult. Laser powder bed fusion (L-PBF) can additively manufacture high-quality components of various materials. The micro layer-based nature of this process allows for design freedom and near-net shape products. The rapid cooling rates during the L-PBF process may cause defects during the manufacturing of hard materials such as TiAls. These rapid cooling rates lead to high induced residual stresses and crack formation. Several heating strategies and post-heat treatments methods are presented in the open literature for reducing the cracks formed during the L-PBF of TiAl. This study presents a new, novel scanning strategy for mitigating the cracks and reducing residual stresses in the L-PBF of Ti-48Al-2Cr-2Nb. This scanning strategy, referred to as sequential thermal scanning (STS) strategy, applies three thermal cycles in each layer: pre-heating selective area of the powder, melting a layer of powder, and post-heating the solidified layer. The laser process parameters used in each step of the STS strategy were determined and explained in this study. The thermal cycles applied in this scanning strategy were verified using an infra-red thermal camera. Sets of process parameters were studied in the melting step of the STS strategy using a factorial design of experiments. These sets of process parameters cover a large range of volumetric energy density (17–2917 J/mm3). The powder morphology, composition, and behavior were studied. The TiAl fabricated parts were investigated for bulk density, surface and internal cracks, balling defects, internal pores, and surface quality. The spatter formation was studied using a high speed camera. Process maps for the relationship between the laser process parameters and part defects were formulated. Residual stresses were measured to understand the influence of the STS strategy and its effects on the surface features and cracks. The results showed that the STS strategy could be used to produce parts of approximately 99 % density without internal pores. In addition, the STS strategy minimized the amount of internal micro-cracks in TiAl parts. This study contributes to the mitigation of internal defects in the L-PBF of TiAl using a sequential thermal scanning (STS) strategy.