Addition of small amounts of boron to cast Ti-6Al-4V alloy has shown to render a finer microstructure and improved mechanical properties. For such an improved alloy to be widely applicable for large aerospace structural components, successful welding of such castings is essential. In the present work, the microstructure and porosity of laser welds in a standard grade cast Ti-6Al-4V alloy as well as two modified alloy versions with different boron concentrations have been investigated. Prior-b grain reconstruction revealed the prior-b grain structure in the weld zones. In fusion zones of the welds, boron was found to refine the grain size significantly and rendered narrow elongated grains. TiB particles in the prior-b grain boundaries in the cast base material restricted grain growth in the heat-affected zone. The TiB particles that existed in the as cast alloys decreased in size in the fusion zones of welds. The hardness in the weld zones was higher than in the base material and boron did not have a significant effect on hardness of the weld zones. The fusion zones were smaller in the boron-modified alloys as compared with Ti-6Al-4V without boron. Computed tomography X-ray investigations of the laser welds showed that pores in the FZ of the boron modified alloys were confined to the lower part of the welds, suggesting that boron addition influences melt pool flow.

The effect of defects and microstructure on mechanical properties of Ti-6Al-4V welds produced by plasma arc welding (PAW) and tungsten inert gas welding (TIG) was studied. Both types of welds were fatigue tested at 250°C. Weld microstructure, fracture surfaces, crack initiation sites and internal defects such as porosity were studied using optical microscopy and SEM. Also microhardness measurements were performed. The fatigue performance of the welds tested at 250°C was found to be comparable to each other. Fatigue cracks in almost all samples were found to initiate at pores, especially the ones close to the sample surface. Large pores and pores close to the surface were found to have the largest effect on fatigue life. The weld microstructures consisted of coarse prior -grains.