Residual stresses and distortions due to time dependent and localised heating imposed during fibre laser welding a 2.0 mm thick titanium alloy Ti-6Al-4V sheet were studied. Sequentially coupled thermo-metallurgical-mechanical simulations were performed to predict welding induced residual stresses and distortion in the fibre laser weld sample, and validated using an experimental database including weld pool geometry and temperature fields. Residual stress measurements were taken using X-ray and neutron diffraction techniques and distortion measurements were recorded using a coordinate measuring machine (CMM). The influence of thermally driven non-isothermal diffusional and diffusionless solid state phase transformations on welding residual stresses and distortions were considered in the numerical model. An internal state variables approach was used to represent the transformed volume fraction of different microstructural phases as a function of cooling rate and peak temperature, and the volumetric change due to temperature variations and phase transformations were calculated. In addition, post weld heat treatment (PWHT) as a method for reducing residual stresses was examined

The effect of using argon and helium shielding gas on weld quality, defect formation and microstructure of laser welded aluminium alloy 2024-T3 was investigated. Full penetration autogenous welds were made at a constant laser power of 4.9 kW using a continuous-wave (CW) fibre laser at travel speeds of 3.0–5.0 m/min and focal positions of +4 to −4 mm. To investigate this effect, a comparison was made between Ar and He by examining the weld quality in terms of the face and root weld width, the weld width ratio; and the presence of welding defects including undercut, underfill, reinforcement, porosity and crack. Optical metallography, energy-dispersive X-ray spectroscopy and micro-hardness indentation testing on weld cross-sections were used to identify how the chemical and physical properties of the shielding gases and the characteristics of the fibre laser affect the overall weld geometry. Based on the results, it was believed that relatively small influence of ionisation on fibre laser induced plume enhanced the welding process stability and lowered the threshold power density for keyhole formation. Both Ar and He shielding gases could therefore, be used effectively to produce good quality welds. However, at the lowest speed and also at the maximum focal position, higher ionisation potential and thermal conductivity of helium resulted in an excessive weld width when He was used even though, the overall weld quality was better than that with Ar

The influence of aluminium alloy 4043 filler wire feed rate on the weld quality and mechanical properties of high power 5 kW fibre laser welded aluminium alloy 2024-T3 was investigated. Loss of volatile alloying elements such as magnesium and other elements including copper and silicon which all contributed to the hot crack sensitivity was measured using energy dispersive X-ray spectroscopy at different filler wire feed rates. High feed rates of above 4.0 m/min produced instabilities, whereas, low feed rates below 2.0 m/min did not sufficiently modify the chemical composition of the weld pool. The optimum feed rate was found to be in the range between 2 and 3 m/min, where the corresponding dilution ratio of around 9–12% in the weld pool with less than 0.6% silicon content reduced the percentage of Mg2Si and also decreased the solidification temperature and total shrinkage during freezing. The addition of filler metal reduced the risk of welding defects and improved ductility to over 3.5% and a fairly higher tensile strength of around 380 MPa than without. Microstructural examination showed that the addition of filler wire increased the number of finer dimples within the weld, resulting in a purely ductile fracture behaviour, as well as reduced micro hot cracks and porosities