Phase transformations and stress evolution during laser beam welding and post heat treatment of TiAl alloys
g TiAl material is an attractive alternative candidate for aerospace and automotive applications because of its low density, high specific yield strength and high creep and oxidation resistance. It has been used successfully in the manufacture of low pressure turbine blades in the GEnx engine, which represents a major advance in propulsion efficiency, with a 20% reduction in fuel consumption, a 50% reduction in noise, and an 80% reduction in NOx emissions compared with prior engines of the same class. In future, g TiAl will most likely be applied as a structural material in the combustion turbine of aircraft; for this purpose, the challenge of determining a method for the proper joining of this material must be met. Laser beam welding is considered to be a promising joining method. However, because of the low ductility and fracture toughness of g TiAl alloys at ambient temperature, cracks are frequently observed in welding seams. The high cooling rate further reduces the weldability of the alloy, as a large amount of brittle phase and high residual stresses are formed. Thus, there are four problems that must be solved: (1) How can a crack free TiAl butt joint be produced? (2) How can the microstructural and mechanical properties of the weld be improved? (3) How do the phases transform during welding? (4) How to perform a fast and reliable welding? The objective of this thesis is to methodically address these challenges. First, because of the intrinsic brittleness of the alloy, several heat treatments using a furnace and a defocused laser are applied to heat the alloys above the brittle to ductile transition temperature and reduce the cooling rate. The butt joint welds are investigated via radiography and found to be free of cracks. Second, the task is the modification of the microstructural and mechanical properties of the welds. The grain refinement induced by borides, which is widely exploited during casting, is suppressed by the high cooling rate. Additionally, a large amount of a2 phase and residual stress are detrimental to the weld. Thus, post weld heat treatments are applied to the aswelded specimens to refine the grain size, modify the microstructure and relieve the residual