Improving the joint strength of the friction stir spot welding of carbon steel and copper using the design of experiments method

Purpose The purpose of this paper is to optimize the welding parameters: rotating speed and plunging depth of carbon steel and pure copper joints using friction stir spot welding (FSSW) with the aid of the design of experiments (DOE) method. Design/methodology/approach Carbon steel and pure copper sheets were welded using the FSSW technique with a cylindrical tool and without a probe. The welding parameters were: rotating speed: 1,120, 1,400 and 1,800RPM and plunging depth: 0.2 and 0.4mm. The welding process was carried out both with and without pre-heating. The welded specimens were analyzed using a shear tensile test. A microstructural investigation at the optimum conditions was carried out. The results were analyzed and optimized using the statistical software Minitab and following the DOE method. Findings Pre-heating the sample and increasing the rotating speed and plunging depth increased the tensile shear force of the joint. The plunging depth has the biggest effect on the joint efficiency compared with the rotating speed. The optimum shear force (4,560N) was found at 1,800RPM, 0.4mm plunge depth and with pre-heating. The welding parameters were modified so that the samples were welded at 1,800RPM and at plunging depths of 0.45-1mm in 0.05mm steps. The optimized shear force was 5,400N. The fractured samples exhibited two types of failure mode: interfacial and nugget pull-out. Originality/value For the first time, pure copper and carbon steel sheets were welded using FSSW and a tool without a probe with ideal joint efficiency (95 percent).