Linus Gemvik

Abstract

Arc welding of hot dip zinc-coated steel sheets faces a number of challenges, such as minimising the generation of spatter and porosity. These problems are caused by zinc being vaporised during the welding process. In an earlier SIMR-JTC (Swedish Institute for Metals Research - Joining Technology Centre) project, these problems were focused on to seek ways to improve the weldability.

This report is a master thesis covering a continuing project on improved weldability for Zn-coated steels. The report also includes a literature study of MIG-brazing, as an alternative to MAG-welding. In addition, an attempt was made to identify key parameters for indicating suitable filler materials for welding of Zn-coated steels.

Welding trials were performed on lap joints with zero gap between the plates as this is considered to be the most difficult task to manage (vaporised Zn is trapped which causes heavy arc disturbances). The welding in this project was performed in an improved fixture to maintain the zero-gap during welding. It resulted in slightly narrowed tolerances for the process, compared to the results from the earlier project. Two Zn-coating thicknesses were used, 7 µm and 20 µm (double-sided, Z100 and Z275). The quality of the weldments was compared to the demands in Volvo Corporate Standard 5605,515.

In this work, a further evaluation of welding with solid filler materials has been performed.The results show that if the chemical composition of a solid wire is more adapted to the task of welding coated steels, it can improve the process stability and the surface conditions of a weldment. The internal porosity level remains high, however, and the process tolerances are still rather limited. The best results in these tests were achieved withthin wires and a low heat input.

In the project, a number of cored wires were also investigated. Some of these were specially developed for welding of coated steels. With cored wire welding, it was possible to fulfil the demands in the Volvo Standard. Furthermore, the process ranges were much wider compared to solid wire welding.

With the above mentioned methodology, an acceptably low level of internal porosity has only been achieved with cored wires. In this project, the metal cored wires gave good results on 7 µm Zn-coatings. Here, the internal porosity was low and the welding was performed with very low spatter levels. With specially designated flux cored wires, both 7µm and 20 µm Zn-coatings could be welded with good results. A remaining problem for the flux-cored wires is some (small) spatter and surface slags. The amount varies from case to case.