Optimization of friction stir welding parameters for toughness in super duplex stainless steel

In this paper, using the Taguchi method, the parameters affecting the toughness of super duplex stainless steels in friction stir welding were optimized. In order to achieve optimal conditions, maximum toughness, the quality characteristic was used as the higher the better. Analysis of Taguchi results showed that in order to achieve optimal conditions in super duplex stainless steel weldments must have a tool rotational speed of 500 rpm, a welding speed of 60 mm / min, an initial pressure of 70 MPa and a tool tilt angle with the workpiece is equal to 3 degrees. Under optimal conditions, the toughness obtained from the confirmation test was 61 J, which was very close to the predicted toughness (58 J). Analysis of variance was also performed on the results of signal to noise (S/N) ratio. According to the results of analysis of variance, the tool rotational speed parameter with an influence percentage of 64% was the most effective parameter on toughness in friction stir welding of super duplex stainless steels. On the other hand, the parameters of welding speed (with an influence percentage of 17 %), initial 2 pressure (with an influence percentage of 16%) and tool tilt angle to the workpiece (with an influence percentage of 3%) were in the next ranks. Also, SEM micrographs from fracture surface of the samples in the impact test proved that the sample that had the least toughness in the impact test had a cleavage morphology and as a result, brittle fracture. This was while the morphology of the fracture surface of the tested sample under optimal conditions (with the highest toughness in this study = 61 J) had a large amount of fine and deep dimples. The presence of these dimples in large quantities indicated ductile fracture and eventually reaching the highest toughness.