Porosity morphology in laser-based thermal joining of steel/thermoplastic and steel/composite joints characterized by X-ray computed tomography

The morphology of porosity formed during laser-based thermal joining of thermoplastics and metals has not been understood well and causes considerable uncertainty in the design and manufacturing of hybrid structures. In this work, high-resolution X-ray computed tomography (μ-CT) was used to analyse the porosities inside polycarbonate (PC) and glass fibre reinforced polycarbonate (GFRPC) welded with DP590 steel using a laser, and the quantitative characteristics of the morphology and distribution of the porosities were extracted. Significant differences were found between the two types of laser joints. For the DP590/PC joints, below a critical line energy (LE), the load-bearing capacity of the joint increased with increasing LE, above which it maintained nearly constant. The maximum tensile strength equivalent to approx. 75% of the tensile strength of the bulk PC was achieved. The load-bearing capacity for the DP590/GFRPC joints was much lower than that of the DP590/PC joints. The DP590/PC joints had a large number of round porosities caused by polymer pyrolysis. In contrast, the DP590/30%GFRPC joints had more irregular porosities caused by shrinkage. A transient thermal conduction model was developed to predict the temperature field during laser joining. The predicted melting region was then correlated with the porosity distribution characterized by μ-CT. The spatial distribution of the porosities was found to match the predicted region of resin melting in the modelling closely.