Influence of scanning strategies on L-PBF processing of titanium aluminides

Abstract

Titanium Aluminides (TiAls) are highly appealing alloys in aerospace and automobile industries due to their combination of high specific strength and low density. However, their poor ductility is challenging during subtractive processing. Additive Manufacturing (AM) technology such as Laser Powder Bed Fusion (L-PBF) offers new horizons for processing different materials with complex shapes and high quality. Nevertheless, the TiAl parts fabricated by L-PBF process were typically characterized by low density and internal defects due to rapid cooling after laser melting. Recently, the sequential thermal scanning (STS) has been introduced as a cost-effective L-PBF strategy to produce high quality TiAl parts with low defects. Despite the success of the presented strategy, the literature still has a shortage of knowledge about the application of other STS strategies. Therefore, this work proposes novel STS strategies, besides the presented in the literature, and investigates the influence of using different geometrical scanning strategies. The results showed that the double post-heating STS strategies could produce parts with higher density, better surface quality, and less internal defects. The microstructure investigation demonstrated that the cracks formed inside the TiAl samples are solidification and thermal induced cracks. These cracks could be reduced significantly when applying the pre-heating STS strategy at higher laser melting energy or the post-heating STS strategy at lower melting energy. The XRD spectra and EBSD analysis demonstrated that the samples fabricated by STS strategies are dominated by the α2-Ti3Al phase due to the Al vaporization that reached up to 6.71 %. In addition, the equiaxed grain growth and alloy's isotropy increased with the application of the STS strategies. On the other side, the investigation of the parts fabricated by STS strategy using different geometrical scanning showed that the chessboard (Ch) strategy could produce dense TiAl sample with 99.3 %, while the sequential bidirectional tracking (X-Y) could enhance the surface quality. In addition, the 45° layer rotation ensured homogenous texture on the side surfaces. A three-steps heat treatment (HT) was performed to enhance the microstructure and relieve the residual stresses. A duplex microstructure with enhanced ductility could be achieved. In addition, compressive residual stresses were induced, which had a beneficial effect on the crack reduction. Finally, the microhardness has been measured and a high value of 562 HV was recorded for the STS samples. Besides, the microhardness homogeneity significantly increased when applying the post-heating STS strategies.