Abstract
This work aims at investigating the spot weldability of a new advanced Quenching and Partitioning (Q&P) steel and a Transformation Induced Plasticity (TRIP) steel for automotive applications by evaluating the effects of the main welding parameters on the mechanical performance of their dissimilar spot welds. The welding current, the electrode tip voltage and the electrical resistance of sheet stack were monitored in order to detect any metal expulsion and to evaluate its severity, as well as to clarify its effect on spot strength. The joint strength was assessed by means of shear and cross tension tests. The corresponding fracture modes were determined through optical microscopy. The welding current is the main process parameter that affects the weld strength, followed by the clamping force and welding time. Metal expulsion can occur through a single large expulsion or multiple expulsions, whose effects on the shear and cross tension strength have been assessed. Longer welding times can limit the negative effect of an expulsion if it occurs in the first part of the joining process. The spot welds exhibit different fracture modes according to their strengths. Overall, a proper weldability window for the selected process parameters has been determined to obtain sound joints.
Highlights
Advanced high strength steels (AHSSs) are used extensively in the automotive industry for the fabrication of more resistant and lighter components with the main aim of reducing fuel consumption and gas emissions, and of improving passenger safety
The aim of this study was to investigate the weldability of dissimilar Quenching and Partitioning (Q&P)/Transformation Induced Plasticity (TRIP) spot welds by evaluating the effects of the welding current, clamping force and welding time on their microstructure and mechanical strength
As already known from the literature, the Q&P/TRIP nuggets widen as the welding current is increased
Summary
Advanced high strength steels (AHSSs) are used extensively in the automotive industry for the fabrication of more resistant and lighter components with the main aim of reducing fuel consumption and gas emissions, and of improving passenger safety. Plasticity (TRIP), martensitic, complex phase and hot stamping boron steels are the most commonly used AHSS grades for such applications. New AHSSs are currently under research and development to achieve better combinations of ductility (e.g., crashworthiness and sheet formability) and mechanical strength (e.g., impact resistance). In this context, Quenching and Partitioning (Q&P) steels appear to be one of the most innovative and promising solutions. They are characterized by a microstructure that consists of retained austenite and martensite: the former phase provides ductility and toughness, the latter mechanical strength. Only car body prototypes are made of Q&P steels
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