Low chromium ferritic stainless steel has great potential in automobile structures for improved strength. In this study, quenching and partitioning (Q-P) treatment was applied to a low-carbon-chromium ferritic stainless steel and compared with traditional heat treatment (quenching-tempering [Q-T] and annealing) in terms of microstructure, mechanical properties, corrosion resistance, and deformation of plate. The results show that the quenching and partitioning (Q-P) treatment has a series of advantages over conventional heat treatments (quenching-tempering and annealing). In terms of mechanical properties, it achieves a good match between strength and plasticity by combining the advantages of “soft state” with high elongation resulting from conventional annealing and high strength "hard state” through the traditional quenching-tempering process. The material possesses better crash safety; for the quenching-partitioning (Q-P) process, quenching-tempering process, and annealing process, the production of strength plasticity is about 16 GPa%, 15 GPa%, and 14 GPa%, respectively. The material has low yield strength, high work hardening index (compared with Q-T), a smooth tensile curve, and no yield plateau (compared with annealing), so it has better forming performance and processing surface, and the corrosion resistance has also improved. The pitting potential of the samples produced by the quenching treatment of Q-P and Q-T increased by about 0.2 V, which is about 20% higher than the one by the traditional annealing process.

Both conventional friction stir welding (C-FSW) and stationary shoulder friction stir welding (S-FSW) were employed to join the Al-7075 butt-lap structure, then the microstructural evolution and mechanical characterization of all FSW joints were systematically studied. The C-FSW joint exhibited a rough surface with flashes and arc corrugations, while the surface of the S-FSW joint became smooth. Moreover, for the S-FSW joint, the shoulder-affected zone got eliminated and the material flow mode during FSW was changed owning to the application of stationary shoulder. Furthermore, in comparison to C-FSW, the lower welding heat input of S-FSW decreased the average grain size in the nugget zone and inhibited the coarsening of strengthening precipitates in the heat-affected zone, elevating the overall hardness for the S-FSW joint. In addition, the tensile strength of the S-FSW joint became higher compared to the C-FSW joint, and all the FSW joints failed inside the nugget zone attributing to the existence of hook defect. The sharp-angled hook defect deteriorated the plasticity of the C-FSW joint further, which was only 70% that of the S-FSW joint.