Effect of applying second-pass welding and welding parameters on the defects and mechanical properties in friction-stir lap-welded AA5456 sheets

Abstract

In this research, effect of applying second-pass welding and rotational and travel speeds for second-pass welding on defects and mechanical properties of lap joint were studied. Lap joint of 5456-H321 and 5456-O, with thicknesses of 5 and 2.5 mm, respectively, were made. Specimens were welded using two passes: in the first pass, all specimens were considered with rotational speed of 650 rpm and travel speed of 50 mm/min, while in the second pass, rotational speeds of 250, 450, and 650 rpm and travel speeds of 25, 50, and 75 mm/min were applied. Macrostructure and microstructure of weld cross-section were studied by optical microscope and scanning electron microscopy, and the effects of the second pass, rotational speed, and travel speed of welding on material flow way in the nugget zone and defect formation in the second pass were studied. The results showed that an increase in the upward material flow, upon applying the second pass, resulted in an increase in the hooking size. The hooking defect size in second pass was a function of rotational speed and travel speed of the tools as well. A tensile shear test was taken from the specimens. The results of the test showed that the failure load with increasing rotational speed decreased due to a decrease of effective sheet thickness at the upper region of the cold worked sheet at the retreating side. Lap joint produced in the second pass parameter at 250 rpm for 75 mm/min showed the best tensile properties with tensile shear test.