Effect of Welding Speed on the Properties of Laser Beam Welded 3 mm Thick Ti-6Al-4V Butt Joints

The effect of welding speed on weld geometry, microhardness, tensile properties, and fracture behaviour of laser beam welded 3 mm thick Ti-6Al-4V butt joints was investigated. Autogenous joints were produced at 1.5–4.0 m/min using a fibre laser at a power of 3.3 kW, while the remaining process parameters were kept constant. Full penetration was obtained under all conditions. Increasing welding speed markedly narrowed the fusion zone and heat-affected zone; between 1.5 and 4.0 m/min, the fusion-zone width decreased by approximately 42% at the weld face and 66% at the root. Microhardness profiles shifted from bilateral HAZ softening at lower speeds to a distinct fusion-zone hardness peak of approximately 450 HV0.1 at 4.0 m/min, consistent with faster cooling and an increased tendency for α′ martensite formation. Ultimate tensile strength showed a statistically significant decreasing trend, from 881 to 769 MPa (joint efficiency from 73% to 64%), and the total strain at fracture decreased from 1.02% to 0.77%. Fracture occurred along the fusion line or adjacent HAZ; fractography revealed predominantly quasi-cleavage features in these regions, whereas the base material exhibited ductile dimpling. Within the investigated range, higher welding speed reduced weld width but adversely affected tensile performance through evolving local hardness gradients and geometric stress concentration.