Svetskommissionen är en teknisk branschorganisation för fogande industri. Vi jobbar med standardisering, forskning och utbildning. Svetskommissionens viktigaste uppgift är att verka för våra medlemmars intressen.
Joakim Hedegård, Linus Gemvik
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 Joining Technology Centre project, these problems were focused on to seek ways to improve the weldability.
This is the final report of a continuing project concerning improved MAG-welding of Zncoated steels. The project also included 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.
To challenge the process, the welding trials were performed on lap joints with 2 mm material thickness and zero-gap between the plates. The latter was considered to be the most difficult task, since vaporised zinc is trapped which causes heavy arc disturbances. Two different Zn-coating thicknesses were used, 7 µm and 20 µm (double-sided, Z100 and Z275). The welding was performed with the same methodology as in the earlier project, with the exception of an improved fixture. The plates were clamped in the fixture (welding position PA) and the first side was welded. The specimen was then turned after cooling, whereafter the second side of the lap joint was welded. The last weld in each joint was of main interest in the weld quality examinations (visual inspection and X-ray). Here, the weldments were compared to the demands in Volvo Corporate Standard 5605,515. The results showed slightly narrowed process tolerances, compared to the earlier project. This was an expected effect due to the better controlled zero-gap in the joint during welding.
In this project, a further evaluation of welding with solid filler materials was performed. Here, possibilities with low heat input and high travel speeds were mainly explored. The results showed that if the chemical composition of a solid wire was more adapted to the task of welding coated steels, both the process stability and the surface condition of the weldment was improved (i.e. no surface breaking pores on the 7 µm coating). The internal porosity level remained high, however, and the process tolerances were still rather limited for the zero-gap lap joint. The best results in the solid wire tests were achieved with thin wires and a minimised heat input. No, or very little, spatter remained on the 7 µm Zncoated plates after welding. With the solid wires, the 20 µm Zn-coating could not be welded with acceptable results.
A number of cored wires have also been investigated. Some of these were specially developed for welding of coated steels. With cored wire welding, high quality weldments were achieved with no, or little, internal porosity. Furthermore, the process ranges were wide, compared to the results with solid wire welding.
Acceptably low, and very low, levels of internal porosity have so far only been achieved with cored wires. Metal cored wires gave good results on the 7 µm Zn-coatings. Here, the internal porosity was low and the welding was performed with very low spatter level and no remaining slag on the plates. With specially designated flux cored wires, both the 7 µm and the 20 µm Zn-coatings could be welded with excellent results. After process optimisation, a low spatter level was achieved even for the 20 µm Zn-coating. Apart from some small spatter, a remaining problem for the flux-cored wires is some surface slags. The amount is, however, limited and the type and position varies depending on the filler material.
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