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Resistance spot welding (RSW) is an autogenous welding process used to lap weld sheet metals. The coalescence obtained by melting and solidifying a small region by passing very high currents using copper electrodes is known as a nugget. The strength of the spot welds is entirely dependent on these nuggets, and any reduction in the size of it may result in the reduction in its load-carrying capacity. Cavity/void is a defect formed in RSW nugget that reduces its load-carrying area. Although cavities are formed in all grades of steels, it has a devastating effect in advanced high-strength steels (AHSS). However, it is yet to understand the exact mechanism of its formation and influence on mechanical properties of AHSS spots. In this context, the present paper is aimed at carrying out an extensive microstructural and computed tomography analysis of the cavities formed in the AHSS spots to understand its mechanism of formation. The results indicate that the cavities formed in AHSS had two distinctive regions: (1) an equiaxed dendritic region with smooth edges and surfaces, which occupied a larger portion of the void, and (2) a rough and irregular dendritic region with dendritic gaps. A thorough microstructural analysis revealed that the dendrites, which grew inwards in the smooth region of the cavity is nucleated from the cavity, indicates the presence of porosity in nugget prior to the start of solidification, while the dendrites of the rough region grew and the dendritic gaps between them indicates the role of thermal contraction in void formation towards the end of solidification.

Among various defects formed in resistance spot welds of advanced high-strength steels (AHSSs), shrinkage void (cavity) is the least addressed topic. Although cavities are formed in numerous sizes and shapes, their influence on the strength and failure modes is yet to be investigated. The present study examines the effect of the cavity size and position on the peak tensile shear load and failure modes in 780 grade dual-phase steel used in automobile body structures. The size and position of the cavity in the nugget were varied carefully by altering the weld parameters and the sheet thickness. The cavity characteristics and its effect on the weld strength were evaluated from shear tensile tests and fracture surfaces. Results show that the size of the cavity increases with sheet thickness and nugget diameter. Further analyses revealed that the shear strength of the weld is influenced by the size and location of the cavity from the interface. A transition from plug-type failure to interfacial failure was observed with an increase in the cavity size. Consequently, the cavities reduce the load-carrying area of the nugget affecting the weld strength and failure mode. The effective nugget area, which is the actual load-bearing area of the nugget without any cavity inferred from the interfacial fracture surface, would be beneficial in determining the interfacial failures in resistance spot welds. Based on the results, a minimum nugget diameter of 5√t was proposed to obtain plug-type failures in AHSS spots with the cavity.

We demonstrate the feasibility of a technique that combines twin-roll strip casting, asymmetric rolling, and subsequent heat treatment, in obtaining Al alloy sheets with high-strength/high-formability. The precipitation- hardening Al alloy sheet thus obtained exhibited an excellent formability ( $\bar r$ = 1.20, Δr = 0.17) and mechanical properties (σTS = 265 MPa, e = 35%), which cannot be readily obtained via the conventional route based on direct-chill casting and heavy rolling operation. In this study, we examined the effects of the various process conditions used at different stages of the process that contribute to the development of specific textures. Simulation studies based on the generalized spherical harmonic series expansion method and the viscoplastic self-consistent (VPSC) model were conducted to arrive at a comprehensive understanding of the factors associated with the high formability realized in Al alloy sheet. It was found that specific textures evolved via twin-roll strip casting, asymmetric rolling, and heat treatment canceled out the anisotropic characteristics of the individual textures, resulting in the high sheet formability.

Among various defects formed in resistance spot welds (RSW), voids or cavities formed at the center of the nugget are scarcely studied. However, it is widely accepted that such cavities significantly influence the mechanical properties of the spots, especially in advanced high-strength steels (AHSS). Nonavailability of a proper characterization technique to quantify these cavities is one of the reasons for less research in this area. Although conventional metallographic analysis portrays cavities in the nugget, the associated destructive sampling results in erroneous interpretation. To date, there is no reliable technique to quantify the size, shape, and distribution of the voids in the nugget. In this context, the present study uses a computed tomography (CT) technique to visualize and quantify the cavities in spot-welded AHSS nuggets. The images obtained using a CT scan provide a new insight into the cavity and its characteristics, such as volume and surface topology. According to the CT results, the voids formed in AHSS spots increase with the increase in base metal yield strength. Furthermore, the CT results also reveal that the conventional critical diameter of 4 × thickness1/2 to obtain plug failure in RSW is not valid for AHSS spots because of the influence of the cavity in the faying interface.