Weldability of a cast gamma titanium aluminide alloy using gas tungsten arc welding.

摘要

This study investigates the weldability of a cast gamma titanium aluminide alloy of nominal composition Ti-48Al-2Cr-2Nb using microstructural characterization, mechanical testing, and mathematical modeling techniques. Autogenous gas tungsten arc welding was used to produce the welds. A procedure for obtaining sound, crack-free welds was established. This procedure consisted of pre-stress relief treatment of 615°C for 2 hours, welding at arc currents equal to or higher than 75 A, and postweld heat treatment (PWHT) at 1000°C, for at least 10 hours, or 1200°C for a duration no longer than 10 hours.; The welded microstructure exhibited dendritic structures along with small amounts of gamma grams and metastable structures. The solid state transformations that occurred were heavily dependent on the cooling rate. It was found that the selected welding parameters should ensure that the heat affected zone cooling rate will be less than 500 K s−1. Such cooling rates prevent the development of deleterious α₂ structures, which promote cracking.; Mechanical properties of the weld were decreased relative to the base metal. However, postweld heat treatment was effective in improving the mechanical properties of the fusion zone. All PWHTs resulted in the transformation of the as-welded structure into equiaxed recrystallized gamma grains.; Finally, a realistic mathematical model was used to predict the transient development of the weld pool shape, temperature and velocity fields for any given arc current and arc length. The model was based upon the numerical solution of Maxwell's equations, Navier-Stokes equations and the transient energy balance equation. The boundary conditions for the current and velocity fields at the arc-striking zone were deduced from experimental measurements of the arc radius. The model predicted that electromagnetic or Lorentz forces dominate fluid flow in the weld pool of TiAl, as shown by the predominant counterclockwise fluid flow. The model was validated by the reasonably good agreement obtained between theoretical predictions and measurements of the dimensions of the weld pool.