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Porosity Reduction and Elimination in Laser Welding of AA6014 Aluminium Alloys for Automotive Components Manufacture and Industrial Applications
[Thesis]. Manchester, UK: The University of Manchester; 2017.
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Abstract
Automotive and aerospace industries consume a significant amount of Al alloys in structures and framing. There is, however, a significant challenge to join the alloy components by laser welding. A key problem is the presence of large amount of porosity in the welds. This research work aimed to understand factors affecting porosity formation in laser welding of AA6014 Al alloy and identification and verification of a suitable method for the porosity reduction and elimination. AC-170PX (AA6014) Al alloy was welded, for the first time, using a 5 kW disk laser in two different configurations: fillet edge and flange couch joints using a number of different filler wires. The experimental results showed that laser power (2-5 kW) and welding speed (20-50 mm/s) had a significant influence on porosity generation. Also, the introduction of a 0.2 mm gap between the sheets significantly reduced porosity for the fillet edge joint while it had a marginal effect for the flange couch joint. The effect of the chemical composition of the filler wire on the AA6014 weld quality was also evaluated for the first time by using different filler wires (AA3004, AA4043, AA4047 and AA5083) over a range of laser powers and welding speeds. The increase in Mg and Mn content in the filler wire’s composition was found to reduce porosity in comparison with high silicon content filler wires.Nanosecond pulsed Nd:YAG laser cleaning was investigated as a surface preparation method for laser welding for AA6061, and its effect on porosity at various welding parameters was examined. The effect of laser cleaning on porosity reduction during laser welding using a filler wire has not been reported before. The surface characteristics before and after laser cleaning were analysed. The results showed that laser cleaning played an essential role in significantly reducing porosity in both the fillet edge and flange couch joints at different levels of power and laser welding speed. The developed surface preparation technology as a method for porosity reduction in laser welding has been successfully implemented in one of the largest UK/international car manufacturers.To study the laser cleaning process, a novel Smoothed particle hydrodynamics (SPH) meshless model has been implemented using a new 3-D multi-phase transient model. For the first time, a study was conducted to validate the temperature field distribution predicted in SPH method under nanosecond pulsed laser heating. The need for special surface treatment of the kernel truncation was also investigated. The proposed model accurately predicted the laser ablation depth and the crater shape and was validated using a significant number of experimental and numerical data reported in the literature. Moreover, a primitive laser welding model has been created to predict the material flow inside the welding pool.The research work has resulted in four publications in peer-reviewed journals. The research highlighted that future work should include the development of a more advanced SPH model for the prediction of porosity in laser welding and to fully describe the relationship between laser cleaning and porosity reduction in laser welding.