Parameter optimization for laser welding of NiTi wires by the taguchi method

A preliminary study was conducted to find out the initial parameter window to obtain the full-penetrated NiTi weldment. A L27 Taguchi experiment was then carried out to statistically study the effects of the welding parameters and their possible interactions on the weld bead aspect ratio (or penetration over fuse-zone width ratio), and to determine the optimized parameter settings to produce the full-penetrated weldment with desirable aspect ratio. From the statistical results in the Taguchi experiment, the laser mode was found to be the most important factor that substantially affects the aspect ratio. Strong interaction between the power and focus position was found in the Taguchi experiment. The optimized weldment was mainly of columnar dendritic structure in the weld zone (WZ), while the heat affected zone (HAZ) exhibited equiaxed grain structure. The X-ray diffraction (XRD) and differential scanning calorimeter (DSC) results showed that the WZ remained the B2 austenite structure without any precipitates, but with a significant decrease of phase transformation temperatures. The results in the microhardness and tensile tests indicated that the mechanical properties of NiTi were decreased to a certain extent after fibre laser welding.