Effects of temperature on the surface and subsurface of Al-Mg-Si welded joints

Abstract

Al-Mg-Si (6061) Al alloy represents a widely used light material which is highly dependent on the distribution of its alloying element. Yet, when it involved a welded structure, the performance was absolutely dissimilar due to the influence of alloying element from filler metal addition, especially when it is subjected to high temperature environment. Thus, the study focuses on the surface and subsurface oxide growth patterns, its morphologies, and phases that formed after oxidation exposure. The plates were joined by GMAW using Al-5%Mg (ER5356) filler metal, and then subjected to an oxidation process in air environment at 400 °C, 500 °C and 600 °C for 40 h; their mass gains were then measured. The oxidised parent and fused metal parts were subsequently analysed using the scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction technique (XRD). It was observed that with the increasing temperature, the fused metal parts exhibited different oxide morphologies on both surface and subsurface compared to the parent metal parts. Additionally, the less oxidation attack was found after exposure at 400 °C and poor oxidation behaviour showed at 500 and 600 °C. The d-Al2O3 phase was detected in both parent and fused metal samples after exposure at 400, 500, and 600 °C. Meanwhile, contradicted observation showed that the MgO phase was detected in fused metal part responsible for the poor oxidation behaviour. Thus, this would lead to a formation of non-protective oxide on the fused metal surfaces. This study suggested that the 6061 welded structure can withstand at 400 °C for long periods in air without serious deterioration but could not withstand exposure to 500 and 600 °C conditions in air.