In situ study of phase transformations during laser-beam welding of a TiAl alloy for grain refinement and mechanical property optimization

• At high heating rate, the material disintegrates directly into the liquid phase.
• High heating rate generates superheated γ, serving as nucleation sites for β grains.
• Lamellae colonies are refined and mechanical properties are improved by fast heating.
• Low heating rate leads to phase transformation and no visible superheated γ.

In situ time-resolved X-ray diffraction by synchrotron radiation was used to monitor the phase transformations and grain-refining processes during laser-beam welding of a γ-TiAl-based alloy. The heating rate plays an important role of grain refinement. A high heating rate suppresses solid–solid phase transformations. The superheated γ grains serve as heterogeneous nuclei for β grains on subsequent solidification and refine the lamellar colonies. At low heating rate, diffusion-based transformations are observed on heating and coarse lamellae are formed after welding. The refined lamellar colonies improve the mechanical properties.