A study on the role of local residual stress on the local strength and defect growth in laser-welded Ti-6Al-4V

Residual stress and gas pores are major influencers to the mechanical responses of laser weldments, but their interdependency is often overlooked because of difficulties in local stress state characterizations. Here, the welding residual stresses in laser-welded Ti-6Al-4V were characterized at the micron scale by focused ion beam milling with digital image correlation (FIB-DIC), and the internal pore growth during uniaxial tension was tracked by in-situ X-ray computerized tomography (CT). This systematic experimental work revealed that the local residual stress in the fusion zone boundaries transformed from equibiaxial tension in the defect-free region to uniaxial tension immediately next to the pore. Such local residual stress state was shown to quantitatively correlate to the strength reduction next to the pore, which was obtained by CT monitoring of local plasticity initiations. This work demonstrated that the global failure behaviour is sensitive to the collective effects of the local residual stress and defects, which can provide valuable insights for residual stress control in laser butt welds.