Keyhole formation and thermal fluid flow-induced porosity during laser fusion welding in titanium alloys: Experimental and modelling

High energy-density beam welding, such as electron beam or laser welding, has found a number of industrial applications for clean, high-integrity welds. The deeply penetrating nature of the joints is enabled by the formation of metal vapour which creates a narrow fusion zone known as a “keyhole”. However the formation of the keyhole and the associated keyhole dynamics, when using a moving laser heat source, requires further research as they are not fully understood. Porosity, which is one of a number of process induced phenomena related to the thermal fluid dynamics, can form during beam welding processes. The presence of porosity within a welded structure, inherited from the fusion welding operation, degrades the mechanical properties of components during service such as fatigue life. In this study, a physics-based model for keyhole welding including heat transfer, fluid flow and interfacial interactions has been used to simulate keyhole and porosity formation during laser welding of Ti-6Al-4V titanium alloy. The modelling suggests that keyhole formation and the time taken to achieve keyhole penetration can be predicted, and it is important to consider the thermal fluid flow at the melting front as this dictates the evolution of the fusion zone. Processing induced porosity is significant when the fusion zone is only partially penetrating through the thickness of the material. The modelling results are compared with high speed camera imaging and measurements of porosity from welded samples using X-ray computed tomography, radiography and optical micrographs. These are used to provide a better understanding of the relationship between process parameters, component microstructure and weld integrity.