Search

In recent years, the offshore wind industry's growth and increased demand for larger and stronger offshore wind turbines have led to significant research into the joining and manufacture of thick section tubular structural steels. Advances in joining processes, such as high productivity laser and hybrid laser welding, will reduce lead times and manufacturing costs of offshore wind turbine support structures compared to conventional arc welding processes. However, defect formation associated with laser keyhole initiation and termination at the weld intersection of circumferential welds and methods of mitigation is not fully understood nor documented. Therefore, to enable high productivity laser and hybrid laser welding for circumferential welding applications, further research into the mitigation of defects at the weld intersection of circumferential welds due to laser and hybrid laser weld start and stops are necessary. In this thesis, investigations into factors affecting the formation of laser keyhole initiation and termination related defects at the weld overlap start/stop region and methods of mitigation were carried out, by controlled experimentation, for both laser and hybrid laser-arc welding processes. Defect formation at the weld overlap start/stop region were monitored using weld cameras and characterised using optical microscopy. The underlying cause of defects at the weld overlap start/stop region in laser keyhole, and hybrid laser-arc welding is associated with keyhole collapse and instability during laser termination. These defects can be successfully mitigated using various weld termination regimes, such as laser power ramp-down and laser defocusing in autogenous laser keyhole welding, and a combination of laser power ramp-down and arc current sloping in hybrid laser-arc welding. Results show that the amount of deoxidising elements in the steel influences the weld penetration depth and defect formation in the steady-state welding condition and during the application of a laser termination regime in autogenous laser keyhole welding.