Welding of Ti-6Al-4V: Influence of welding process and alloy composition on microstructure and properties

Doctoral thesis, 2018

Titanium alloys are widely used in aviation industry mainly because of their superior combination of high strength and low weight. Titanium alloys can be found on parts for landing gear, internal components of wings, and engine components. Large titanium alloy components used in aircraft engines have been traditionally manufactured out of large single piece casting that are machined into final shape. A significant amount of the initial casting is machined away in order to achieve the finished geometry of the part. This is neither environmentally friendly nor economically viable. An alternative method to manufacture large aeroengine components is to weld together smaller subcomponents. The benefit of this approach is that less scrap material is produced and the performance of the components can be improved. However, fusion welding involves localized melting which produces changes in geometry and microstructure, weld defects and residual stresses in the welded material. These factors are detrimental to the mechanical performance of welded structures. This is the background for why welding and the mechanical properties of welds have been investigated in this project. The aim of the work was to study what kind of microstructures and defects are formed in welding of Ti-6Al-4V with different welding processes and how these aspects affect the mechanical properties of the welds. Furthermore, the influence of chemical composition of the alloy on the formation of microstructures and defects was studied.

The welding processes compared were electron beam welding, laser beam welding, plasma arc welding and TIG welding. High energy beam welding processes produced a finer weld microstructure in comparison to the coarser microstructure produced by arc welding processes. The finer weld microstructure was found to be beneficial for tensile ductility and low cycle fatigue performance. Porosity was observed in welds produced by all the processes and this caused variation in the fatigue performance of the welds. Large pores and pores located close to the specimen surface the most detrimental to the fatigue strength.  Fatigue life in the welds produced by arc weld processes was more sensitive to porosity than in the high energy beam welds. The finer microstructure has a higher resistance to micro crack initiation and micro crack growth which contributed to the better fatigue performance of welds produced by laser beam welding and electron beam welding.

The chemical composition of the base material had a significant influence on the microstructure of the welds and the formation of defects. A small boron addition induced significant grain refinement in boron alloyed welds. Narrow columnar prior-β grains were formed in the fusion zones in the welds in boron alloyed materials. The α colonies and α plates were also refined, as compared to the standard Ti-6Al-4V welds. TiB particles located in the prior-β grain boundaries restricted the grain growth in the heat affected zone. A significant batch to batch variation in amount of porosity was observed in laser welding of Ti-6Al-4V. The material batches that were most susceptible to formation of porosity had increased amount of carbon and oxygen. The formation of porosity could be minimized in all material batches by optimizing the welding parameters.