Abstract
Defects associated with friction stir welding of two steel grades including DH36 and EH46 were investigated. Different welding parameters including tool rotational and tool traverse (linear) speeds were applied to understand their effect on weld seam defects including microcracks and voids formation. SEM images and infinite focus microscopy were employed to identify the defects types. Two new defects associated with the friction stir welding process are introduced in this work. The first defect identified in this work is a microcrack found between the plunge and the steady state region and attributed to the traverse moving of the tool with unsuitable speed from the plunge-dwell to the steady state stage. The tool traverse speed has recommended to travel 20 mm more with accelerated velocity range of 0.1 from the maximum traverse speed until reaching the steady state. The maximum recommended traverse speed in the steady state was also suggested to be less than 400 mm/min in order to avoid the lack in material flow. The second type of defect observed in this work was microcracks inside the stirred zone caused by elemental precipitations of TiN. The precipitates of TiN were attributed to the high tool rotational speed which caused the peak temperature to exceed 1200 °C at the top of the stirred zone and based on previous work. The limit of tool rotational speed was recommended to be maintained in the range of 200-500 RPM based on the mechanical experiments on the FSW samples.
Highlights
Despite many advantages associated with the friction stir welding (FSW) process, the technique does not always produce defect free joints [1]
Possible defects of high tool speeds of DH36 (W2D) in the longitudinal direction between plunge/ steady state and in the steady state are shown in Fig. 4a and b, respectively
Insufficient heat coming from low tool rotational speed and the sudden increase in traverse speed have been caused a lack in material flow of W1D which in turn has resulted in the mentioned crack initiation
Summary
Despite many advantages associated with the friction stir welding (FSW) process, the technique does not always produce defect free joints [1]. A significant amount of data is reported in the literature on investigations into the types and sources of defects in FSW including numerical and experimental techniques. Toumpis et al [9] studied the formation of flaws during the FSW process of DH36 steel experimentally by examining the microstructure and carrying out fatigue test. Lower embedded flaws (Type 1) as a result of the lack in material stirring due to lower heat input were reported. The authors found in another experiment including hot compression test with wide ranges of temperatures and strain rates that the flow stress increases with the decrease in temperature and the increase in strain rate This was the case when tool traverse speed increases, leading to a lack in material flow and defects formation [5]. Gibson et al [14] showed that flaws in FSW are the result of unsuitable welding parameters and the source of surface breaking flaws comes from surface oxides penetration into the stirred zone
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