Effects of aluminium in the Zn coating on electrode life in welding galvanized steel sheet

Summary The purpose of this study was to investigate effects of aluminium in the Zn coating on electrode life in welding galvanized steel sheet. Three hot‐dip galvanized and one electro‐galvanized steel sheet types were prepared for this study. Aluminium content in the coatings varied within the range 0.26–0.87 mass% for the three types of hot‐dip galvanized sheet. The approach used here included EDX, AEX analysis of the coating layers, electrode life tests and EPMA analysis of electrodes after 900 spots were welded. The results were as follows: The electrode lives of HDG materials were changed at approximately 0.3–0.4 mass% Al content in Zn coating. Materials with low‐Al coating content showed over three times longer electrode lives than materials with high‐Al coating content. Although the thickness of Al oxide layers was in proportion to the Al content in Zn coatings, the obvious correlation between electrode life and thickness of Al oxide layers was not observed. In the case of low‐Al coating content, it was observed that Fe‐Zn alloy grew from the steel‐coating interface to the Zn coating. It was considered that, in the initial stage of welding, the content of Fe in Zn coating increased immediately. In the case of high‐Al coating content, Fe‐Al alloy was observed at coating‐steel interfaces instead of Fe‐Zn alloy. It was known that Fe‐Al alloy suppresses the Fe‐Zn alloying reaction. Zn coating was not alloyed with Fe in initial stages of welding. From these results, it was concluded that aluminium in coatings affected electrode life by changing the melting point of coating layers between the electrode and the steel. The melting point of low‐Al content coating layers rose because of the diffusion of Fe into the Zn coating. This phenomenon decreased electrode wear and electrode life was long. In contrast, the melting point of high‐Al content coating layers remained low. This phenomenon caused electrode alloying easily and also increased electrode wear. As a result, electrode life became shorter.