Abstract
This study presents the results of welding high-strength steel type TRIP 700 with 1.56 mm in thickness welded by friction stir welding (FSW) technology. Welding was performed by application of tungsten carbide (WC) welding tool at constant spindle revolution 400 rev min^–1 on zinc coated and uncoated steel sheets. Welding results have revealed the occurrence of inhomogeneities, which were observed to more or less extent on all welds, regardless the welding parameters. However, the certain improvement was observed on uncoated sheets. This study analyses the reason for defects formation and degradation of welding tools owing to welding parameters. The results from weld defects analysis were an incentive for the design of a modified welding tool geometry, which was subsequently used in the new experiments. The results of new experiments were positive and are presented within this work. Mechanism of WC tools damage was considered together with their life.
The microstructure of welded joints is documented in this study, together with hardness profiles in dependence on selected welding parameters.

Key words
FSW, high-strength steel, lifetime of WC tool, microstructure

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