Browsing by Author "Lai, Wai Jun"

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    Knowledge-based bidirectional thermal variable modelling for directed energy deposition additive manufacturing
    (Informa UK Limited, 2024-09-05) Qin, Jian; Taraphdar, Pradeeptta; Sun, Yongle; Wainwright, James; Lai, Wai Jun; Feng, Shuo; Ding, Jialuo; Williams, Stewart
    Directed energy deposition additive manufacturing (DED-AM) has gained significant interest in producing large-scale metallic structural components. In this paper, a knowledge-based machine learning (ML) approach, combining both physics-based simulation and data-driven modelling, is proposed for a study on thermal variables of DED-AM. This approach enables both forward and backward predictions, which breaks down the barriers between the basic process parameters and key process attributes. Process knowledge plays a critical role to enable the prediction and enhance the accuracy in both prediction directions. The proposed ML approach successfully predicted the thermal variables of wire arc based DED-AM for forward modelling and the process parameters for backward modelling, typically within 7% errors. This approach can be further generalised as a powerful modelling tool for design, control, and evaluation of DED-AM processes regarding build geometry and properties, as well as an essential constituent element in a digital twin of a DED-AM system.
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    Mitigation of start/stop defects in circumferential laser and hybrid laser welding of tubular offshore structures.
    (2021-04) Lai, Wai Jun; Ganguly, Supriyo; Suder, Wojciech
    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.
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    Study of the effect of inter-pass temperature on weld overlap start-stop defects and mitigation by application of laser defocusing
    (Springer, 2021-03-08) Lai, Wai Jun; Ganguly, Supriyo; Suder, Wojciech
    Laser keyhole initiation and termination-related defects, such as cracking and keyhole cavities due to keyhole collapse, are a well-known issue in laser keyhole welding of thick section steels. In longitudinal welding, run-on and run-off plates are used to avoid this problem. However, such an approach is not applicable in circumferential welding where start/stop defects remain within the workpiece. These issues can hinder industry from applying laser keyhole welding for circumferential welding applications. In this paper, the effect of inter-pass temperature on laser keyhole initiation and termination at the weld overlap start-stop region was investigated. This study has identified that defects occurring within this region were due to laser termination rather than laser initiation because of keyhole instabilities regardless of the thermal cycle. The laser termination defects were mitigated by applying a laser defocusing termination regime to reduce the keyhole depth gradually and control the closure of the keyhole.
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    Study on effect of laser keyhole weld termination regimes and material composition on weld overlap start-stop defects
    (Elsevier, 2020-08-23) Lai, Wai Jun; Ganguly, Supriyo; Suder, Wojciech
    The effects of two different laser termination regimes; laser power ramp-down and laser defocusing, on the weld overlap start-stop region, have been studied on three different compositions of S355J2+N grade steel plates. The plates have varying amounts of deoxidisers, such as manganese and silicon, to allow for the effect of dissolved oxygen on melt pool dynamics and defect formation to be studied. Observations show that gas entrapment, manifesting as porosity, is the main issue in both the laser termination regimes studied, due to melt pool instabilities related to keyhole termination. However, it is more severe during laser defocusing due to the formation of porosity clusters below a specific power density and melt pool size. The percentage of dissolved oxygen and deoxidisers was found to be vital and could be correlated to the response of the melt pool during laser processing. Steel, with a higher percentage of deoxidisers, such as manganese and silicon, would combine with the dissolved oxygen in the weld pool and thereby affect the surface tension and melt pool dynamics. The effect of lower dissolved oxygen would lead to a reduction in penetration depth, but more importantly, porosity and porosity cluster formation, as observed for both laser termination regimes. Successful laser termination with defect-free weld overlaps at the start-stop region could be achieved using both termination regimes for the steel with a higher proportion of deoxidising elements