Weld Processing and Mechanical Responses of 1-GPa TRIP Steel Resistance Spot Welds

Abstract

It is essential to assess the weldability of newly developed steels, which are candidates for manufacturing vehicle autobodies. This work aims to study the weld processing and weld mechanical performance of a 1-GPa grade, transformation-induced plasticity steel. It also tried to disclose the relationships between the thermophysical properties of the alloy and its response to the resistance spot welding process. The rich chemistry and complex thermomechanical processing of the alloy result in changes to its thermophysical properties. It was found that the steel had higher electrical resistivity and lower heat conductivity in contrast to the conventional low carbon steels, resulting in more weld penetration depth. The special thermophysical properties of the alloy also allowed for nugget formation at lower welding currents. Due to the enhancement of the metallurgical bond induced by the weld penetration, the weld samples exhibited more load bearing and energy absorbing capacities in the tensile-shear tests in contrast to the conventional and high-strength steels (HSS). The relationships between the sheet intrinsic properties, metallurgical bond characteristics, and mechanical responses of the welds are disclosed here. In studying the mechanism of the nugget formation and growth, it was seen at high heat input conditions, the rates of the nugget growth and penetration were almost unchanged, an indication of the balance between the heat generation and heat transfer by the water-cooled electrodes. The results indicate a higher nugget penetration depth was obtained in the resistance spot welds of 1-GPa grade, transformation-induced plasticity steel in contrast to those of conventional low-carbon steels. This was a key factor in limiting the nugget size because the higher nugget penetration depth reduced the distance between the water-cooled electrodes and molten nugget, and thus enhanced the efficiency of the heat transfer by the water-cooled electrodes. The welds showed high load and energy bearing capacities; however, the size of the weld button in the tensile experiments was less than the corresponding nugget size. It was seen that both cross-tension and tensile-shear strength of the welds increased with the nugget size, but their ratio (ductility ratio) decreased at high welding currents. This can be an indication of the existence of the weld discontinuities, inhomogeneities, and stress concentration factors in the welds obtained at high-heat input conditions, which need to be assessed with more detail.

Investigation on microstructure and tensile shear properties of 9Cr oxide dispersion strengthened steel resistance spot welded joint

“…However, as the nugget size increases, the electrode has a gradually increasing cooling effect on the nugget owing to the decreasing distance between the nugget and the electrode. As a result, the increase in penetration ratio has gradually become slower [24]. Figure 3(b) and (e) represent the effect of welding time on nugget size (under the constant welding current of 6.8 kA and electrode force of 3 kN).…”

Evaluation of resistance spot weldability and weld performance of zinc-coated martensitic steels

“…Owing to easily activated stress concentration factors in tensile loading, the peak load achieved in CTT is low compared to the TST (Figure 6 g, h). Additionally, the sensitivity of the CTS to weld defects and discontinuities [1,7] may be the reason for the clear scatter in the CTT data. To address the large scatter in CTT data of MS-2 steel, more replicates were tested.…”

“…The ductility ratio, CTS/TSS, of MS-3 was the highest, and MS-2 was the lowest (Figure 6 i). The CTT and TST results are used in combination to calculate the ductility ratio, which is defined as the ratio between strength in the normal direction (CTS) to strength in the shear direction (TSS) [1,[17][18][19]. The ductility ratio is an indication of the weld's response to the tensile stresses, with a higher ratio being desirable [1].…”

“…Increased vehicle safety and emission standards have led to the development of advanced automotive steel grades that can balance the overall weight of the vehicle with meeting the performance requirements. The automotive industry can produce both lighter and safer cars by employing AHSS in manufacturing [1].…”

The investigation on the ideal spot weld numerical model in resistance welding

“…the analysis of the weld in terms of shear strength, deliberately ignoring the thermal cycle. Numerous researchers use numerical modelling for the analysis of resistance welding process, detailed analysis of processes taking place during welding and optimization of the welding process [6][7][8][9][10][11]. Many authors attempt to achieve the optimal, i.e.…”