Application of fibre lasers in fabrications and processing of thin gauge alloys for engineering applications

Micro-joining of thin metallic sheets has been growing due to the product weight reduction. Several methods are used to join aluminium and iron-based alloys, but most are limited on the workpiece dimensions, processing time and joint strength. Laser welding was selected as the joining tool for this study as a non-contact, productive and highly flexible process in spatial and temporal resolution of energy application for medical, automotive and aerospace applications. The digital control of the latest multi-pulse pulsed-wave (MPPW) fibre lasers allows different spatial and temporal resolutions to apply low pulse energy at a high repetition rate and narrow pulse width with high precision. However, it isn't easy to control each parameter's effect on the weld profile without understanding the underpinning science of the laser-material interaction. This study aims to predict the material response and establish a relationship between the total applied energy over a spot, the pulse energy, average peak power and pulse duration. The fundamental laser-material interaction parameters (FLMIP), which have proven to characterise the process in continuous-wave (CW) laser welding, have also been investigated in MPPW seam welding. The performance of MPPW and CW laser modes was compared under like-to-like conditions to correlate penetration and melting efficiency, productivity, joining flexibility and defects generation in the similar and dissimilar joining of 5251 H22 aluminium alloy and 304L austenitic stainless steel. In addition to this, an empirical model was applied in both laser modes to achieve a specific weld profile independent of the beam diameter. In MPPW mode, the weld pool profile could be correlated to the power density and interaction time considering the inter-pulse thermal losses. CW processing was revealed to have better flexibility to control the weld shape and joint strength, higher melting efficiency and productivity when compared to MPPW processing.