Unveiling the hot cracking mechanism in Ti2AlNb-based alloys during fusion welding

Solidification cracks in fusion welds of Ti2AlNb-based alloys severely deteriorate joint mechanical properties. This study reveals through systematic experiments that the cracking mechanism stems from the coupling of metallurgical and mechanical factors. Results show that low-melting-point liquid films and thermal stress are the main causes of solidification cracking. During non-equilibrium solidification, Al solute segregation forms low-melting-point liquid films, weakening grain boundaries. Specifically, high-angle grain boundaries (HAGBs) with high grain boundary energy increase the critical coalescence undercooling, keeping liquid films stable at lower temperatures. Intense thermal stress from rapid heating/cooling and structural stress from coarse columnar grains during welding easily induce liquid film rupture. Thermodynamic simulations confirm that steep temperature gradients during solidification drive the formation of columnar grains with high aspect ratios in welds, which hinder residual liquid from backfilling ruptured liquid films, increasing hot cracking susceptibility. This study provides fundamental insights into microstructural determinants of weld cracking and lays a theoretical foundation for optimizing welding parameters to enhance joint integrity of Ti2AlNb-based components.