Microstructure and mechanical properties of Ti-6Al-4V welds produced with different processes

Licentiate thesis, 2016

Titanium alloys are widely used for components in the fan and compressor sections of aeroengines mainly because of their superior strength-to-weight ratio. Large static compressor components can be manufactured by welding together smaller subcomponents, which has potential to provide benefits such as higher buy-to-fly ratio and improved performance of the components. This is the background for why welding and the mechanical properties of welds have been investigated in this project. Fusion welding involves localized melting of materials which produces changes in microstructure, geometry of the surface, residual stresses and defects in the material, which all can affect the mechanical properties of weld zones. In this study, fusion welds have been produced with tungsten inert gas welding (TIG), plasma arc welding (PAW), electron beam welding (EBW) and laser beam welding (LBW) of Ti-6Al-4V sheet material. In addition, investigation of LBW and TIG welds in cast Ti-6Al-4V material with different boron contents have also been performed within this work. The mechanical properties of the different weld types have been evaluated with respect to microhardness, yield strength, ultimate tensile strength, ductility and fatigue at room temperature and at elevated temperatures. Metallographic investigation was carried out to characterize the microstructures of the different weld types and weld zones. The fractographic investigation was conducted in order to relate the effect of defects and microstructure on fatigue performance. High energy beam welding processes render in finer weld material microstructure in comparison to the coarser microstructure produced by arc welding processes, and finer weld microstructure was found to be beneficial for tensile ductility and low cycle fatigue performance. Porosity was found in all the welds. Fatigue life in arc welds was found to be more sensitive to porosity then the high energy beam welds. Large pores and pores located close to the specimen surface were found to be most detrimental for fatigue strength. The boron addition was found to render in significantly finer prior β grain size, and smaller α colonies and α plates in the weld zones, as compared with that of standard Ti-64 welds.