Influence of LaB6 inoculant on the thermodynamics within the molten pool and subsequent microstructure development and cracking behavior of laser powder bed fused TiAl-based alloys

Abstract

A LaB6 particles decorated Ti48Al2Cr2Nb composite almost free of any cracks has been successfully fabricated by LPBF in this work. To better understand the role of LaB6 inoculant in LPBF-fabricated γ-TiAl alloys, the effects of LaB6 inoculant on the thermal behavior, microstructure evolution and crack sensitivity were investigated by simulation and experimental methods. Based on a single particle model, it was predicted that the addition of LaB6 particle could induce a significant change in temperature gradient G. Due to the LaB6→La2O3 in-situ transformation, the temperature field was severely perturbed, thus causing random G orientations of the matrix surrounding the particle. Furthermore, based on the multi-particle model, predictions indicated that the 0.5 wt% LaB6 inoculant led to a significant decrease of the Gmax from 1.14× 107 °C/m to 6.9 × 106 °C/m but a little influence on solidification rate R at solidified front, therefore contributing to a lower G/R and resultant larger undercooling. In this condition, fine equiaxed grains with random orientations were expected to dominate in the solidified microstructure, which got well verified by the experiments where the maximum texture index dropped from 15.06 to 2.23 and the average grain size decreased from 16.13 ± 8.52 μm to 1.52 ± 1.10 μm due to the addition of LaB6 inoculant. What's more, the LaB6 inoculant also induced an apparent reduce in crack density from 7.02 ± 1.06 mm/mm2 to 0.52 ± 0.12 mm/mm2 and a rise in relative density from 97.65 ± 0.58% to 99.88 ± 0.10%, which could be mainly attributed to grain refinement, grain purification and texture attenuation.

Keywords

• Laser powder bed fusion
• TiAl-based composites
• Thermal behaviour
• Cracking behavior
• LaB6 inoculant
• Numerical simulation