This study aims to investigate the lapped joint characteristics of friction stir spot welding (FSSW) of the steel to the Al alloy using the embedded-rod (ER) tool. First, to challenge the lapped joint and welding of over 2 mm thick carbon steel, FSSW was conducted on a low carbon steel plate pair with different thicknesses using the ER tool under a downward force of 8 kN, a rotating speed of 1200 rpm and a dwell time of 100 s. Result showed that the temperature at 2 mm below the center of the stir surface rapidly increased to about 900°C using the ER tool, which was higher than that of 510°C using the plain tool. The depth of TMAZ using the ER tool was about 6 mm, which was 2 times that using the plain tool. When the thickness of the upper plate was less than 3 mm, the failure load using the ER tool was about 35 kN, which was about 1.7 times higher than that using the plain tool. As the coefficient of sliding friction and adhesion role between the ER tool and contact surface of workpiece are high, the interface temperature and TMAZ increased, which was favorable to the diffusion reaction of materials. Thus, with ER, the upper plate with the thickness as much as 4 mm can be welded, while its failure load using the ER tool was still greater than a 2 mm upper plate welded using the plain tool.
On the other hand, as of the materials which are difficult to be welded, such as steel/Al alloy, the ER tool was used to melt the Al alloy below the rotating center so as to cause pyrolysis of oxidation film and bond low carbon steel and Al alloy. This process was called friction stir spot fusion welding (FSSFW). Then, a low carbon steel sheet with the thickness of 2 mm was lapped on a 6061-T6 Al alloy sheet with the thickness of 5 mm or 10 mm using the ER tool under a downward force of 12 kN and a rotating speed of 1200 rpm at different dwell times. Result showed that when the 6061-T6 Al alloy was 5mm thick and the interface temperature of the low carbon steel and Al alloy rapidly rose to the melting point (652oC) of the 6061-T6 Al alloy in about 8 s and then achieved about 740oC in about 15 s. After welding, the failure load achieved 15 kN at the dwell time of 55 s. In contrast, when the 6061-T6Al alloy was 10 mm thick, and the interface temperature rapidly rose to 652oC in about 20 s and then achieved about 740 oC in about 45 s. After welding, the failure load achieved 22 kN at the dwell time of 55 s. The Al alloy sheet was melted at the central area of the faying surface, so that two IMC layers were formed at this surface and identified as Fe2Al5 and Fe4Al13, as detected by XRD and SEM images. Result showed that the IMC thickness increased along with the dwell time, but its increment rate became slower for IMC thickness larger than 25 μm. The failure load increased along with the IMC thickness, but the failure load decreased quickly for IMC thickness larger than 17 μm, because the thicker the IMC, the larger the crack was.
To understand the growth mechanism of the IMC layer, the numerical simulation was used to analyze the speed of plastic flow and the temperature distribution within the workpiece. The interface temperature achieved the melting point of the Al alloy at 10s and 20s for the lower Al alloy sheets with thickness of 5mm or 10mm, respectively, and then the temperature increased continuously, because the theory could not predict the steel sheet was broken through. The interface temperature of 5mm Al alloy sheet was about 100oC higher than that of 10mm at dwell time of 35s, so that the 5mm Al alloy sheet had a thicker IMC layer due to temperature has a most profound effect on the diffusion rate.