High strength and ductility are some of reasons that make ST14 steel one of the most widely used steels in automotive and aerospace industries. FSW is one of the new methods of solid-state welding that is proposed as a method with desirable mechanical properties.In this study, mechanical and microstructural properties of 1.5mm thick ST14 steel sheets were investigated in the friction stir welding process. The results showed that the welded specimen with rotational speed of 800 rpm and linear

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... 0 rpm and linear motion speed of 80 mm/min had the highest tensile strength of 305MPa. In addition, results of metallographic test showed that the sample with 1000 rpm rotational speed and 50 mm/min linear motion speed had the highest heat input to the piece, and therefore the tungsten carbide particles were separated from the instrument and entered the stirred zone. Also the results of micro hardness test showed that in the welded specimen with rotational speed of 1000 rpm and linear motion speed of 50 mm/min, hardness increased to 115H1 HV in the stirred zone, which is higher than hardness of other samples in same region. It can be claimed that tungsten carbide particles are present in this area.