This study characterizes the interfaces of Al-steel joints generated by resistance spot welding (RSW) processes. Two types of intermetallic compound (IMC) layers at the interface are characterized and discussed. The first type, located in central area of the interface, is composed of tongue-like Fe2Al5 adjacent to the steel and serrated-like FeAl3 adjacent to the Al; the second type, a mixture of FeAl3 and Al, lies in the periphery of the joint interface. Formation mechanisms of the two types of IMC layer observed are proposed and discussed in the paper. The thickness distributions of the IMC layer generated by different welding parameters are predicted based on the dynamic interfacial temperature histories from Al-steel RSW process simulation. The predicted IMC thickness distributions are validated against physical measurements and show good agreement. A bimodal IMC thickness distribution is found when an adequately long welding time is applied. This is found to be related to lower interfacial temperature at the center of Al-steel interface than what in the surrounding area after about 200 ms of welding due to strong cooling effect by the electrode.

•The grain evolution in refill stir spot welding was dominated by grain subdivision and geometric dynamic recrystallization.•The shear texture formation beneath the sleeve and pin was governed by the sleeve inner wall.•The shear texture at the sleeve periphery was associated with the material flow imposed by the thread on the rotating sleeve.•The strength of refill friction stir spot welds mainly depends on the size of the hook defect.The evolution of microstructure, texture and mechanical properties during refill friction stir spot welding (RFSSW) of 6061-T6 alloy was investigated in the present paper. The Electron backscattering diffraction results revealed that a significant volume of A/-A, and B/-B component of shear textures was formed in the weld. The sleeve inner wall governed the formation of shear texture beneath the sleeve and pin. Whereas, the shear texture at the sleeve periphery was associated with the material flow when material was discharged from the bottom of the thread on the rotating sleeve. The microstructure evolution in the RFSSW of 6061-T6 alloy was dominated by continue recrystallization. However, the grain boundary bulging and presence of cube texture indicated that the discontinue recrystallization also participate in the grain evolution. The tensile shear strength had a negative correlation with the hook height.

Material flow during friction spot welding (FSpW) of the aluminum alloy Al 6061-T6 was examined by a combination of numerical modeling and experimentation. The finite element model was developed according to coupled Eulerian-Lagrangian formulations, where the Johnson-Cook equation was used to describe the dependence of stress on temperature, strain, and strain rate. The modeling results revealed that the hook defect was formed due to the insufficient material deformation and low material diffusion rate. A model of the material flow in FSpW was developed based on the numerical and experimental results. The onion structure near the sleeve periphery was formed during the plunge stage when materials beside and below the sleeve were combined.

•Micro-/nanostructures are produced by picosecond laser.•Surface textures and local quenching are simultaneously induced by picosecond laser.•Tribological properties of stainless steels can be tuned by picosecond laser treatment.We reported on the manipulation of tribological properties of stainless steel by picosecond laser texturing and quenching. The micro-grooved textures were formed on the steel surface, accompanied with the appearance of nanostructures with grain sized of 80–400 nm, oxides thin film (Fe2O3 and Cr2O3) and martensite during picosecond laser surface texturing process. The tribological tests indicated that the average friction coefficients and wear rates of textured surfaces initially increased then decreased with the increase of spacing of micro-grooves. The well-controlled friction and wear properties are attributed to the combined effects of induced micro-/nanostructures, martensitic transformation and oxides thin film in the local quenched zone of stainless steel samples.

The microstructure of a high strength dual phase steel resistance spot welded with tempering-pulse technology is characterized in this paper. In the fusion zone, there is a needle-like microstructure identified as acicular or side plate ferrite that has a cube-on-cube orientation relationship with respect to the surrounding martensite. In contrast to the microstructures produced by the lower cooling rate arc or laser welding techniques, the nucleation of this fine intragranular ferrite takes place independent of inclusions. Further, a leaf-like microstructure within the martensitic matrix is found to contain primitive orthorhombic Cr3C2 and face-centered cubic CrC chromium carbides, rather than Cr23C6 or Cr7C3 as is commonly observed in steel alloys. The formation histories of both the ferrite phase and the chromium carbides are analyzed.

This study investigated failure modes, mechanical strength and fracture mechanism of Al/steel resistance spot welds in lap shear test. Three major fracture mechanisms and failure modes were identified: semi-brittle or brittle fracture in the intermetallic compound (IMC) layer which led to interfacial failure, ductile fracture in the aluminum fusion zone (FZ) which led to thickness failure, and ductile fracture in the aluminum heat affected zone (HAZ) which led to the engineering preferred button pullout failure. The failure mode and mechanical strength of the weld depended highly on the dimension and properties of the IMC layer, aluminum FZ, and aluminum HAZ. The IMC thickness was the dominant factor of the failure mode and mechanical properties. The button pullout failure or thickness failure was related to thin IMC thickness of no more than 3 μm. Meanwhile, increasing the Al nugget size or steel bulge height helped to increase lap shear peak load and promote button pullout failure. Thick IMC layer with thickness more than 3 μm tended to cause interfacial failure in the IMC layer, even with large Al nugget size and severe steel bulging.