C. C. P. Mazzaferro, T. S. Rosendo, F. D. Ramos, J. A. E. Mazzaferro, M. A. D. Tier, J. F. dos Santos, GKSS Research Centre, Geesthacht, Germany; A. Reguly, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil
TRansformation Induced Plasticity (TRIP) steels combine high-strength levels with excellent ductility, making them very attractive for the automotive industry. Friction Stir Spot Welding is a solid-state process, used for joining two or more sheets in overlap configuration. The heat generated by a rotational tool (formed by pin and shoulder) plasticizes the base materials, stirring them to form the joint.
A TRIP steel containing 0,21C, 1,60Mn and 1,42Si (%wt) was friction stir spot welded at three different rotational speeds – 1600, 2000 and 2400 rpm. Besides microhardness, lap-shear and cross-tensile tests were performed to evaluate the mechanical response of the joints.
After friction stir spot welds, the joint region could be divided in three parts: stir zone (SZ), termomecanically affected zone (TMAZ) and heat affected zone (HAZ). The size of each zone increased with increasing rotational speed, but the transition between them was more gradual with greater rotational speeds. For all rotational speeds, the microstructure in SZ was martensite, however some “mixed zones” constituted by martensite and material from the sheet coating were also observed. These mixed zones were located mainly near pin and shoulder; they were very small for the rotational speed of 1600 rpm, but larger and more dispersed for 2400 rpm, indicating that a better intermixing was obtained when using higher rotational speeds.
The best performance in lap-shear and cross-tensile tests was observed for the rotational speed of 2000 rpm, although similar values in cross-tensile tests for 2000 and 2400 rpm have been found. This indicates that the dispersed coating material did not affect negatively the joint performance.
Summary: A high-strength, low-alloy TRIP steel was friction stir spot welded at three different rotational speeds. A better material mixing was obtained when using higher rotational speeds. The best mechanical performance was observed for the intermediate rotational speed.
