Strength Of Resistance Spot Welded Joints Research Articles

EFFECTS OF HOLDING TIME ON MICROSTRUCTURE AND SHEAR STRENGTH OF RESISTANCE SPOT-WELDED JOINTS BETWEEN MILD STEEL AND ALUMINUM WITH ZINC POWDER FILLER

This study aimed to determine effects of holding time on microstructure and shear strength of resistance spot-welded joints between mild steel and aluminum with zinc powder filler. The materials used are 0.9 mm thick mild steel, 1.2 mm thick 3000 series aluminum, and 200 mesh zinc powder. Welding parameters were selected as welding time of 0.5 seconds (s), electric currents of 8000 A and 9000 A, with holding time variations of 1 s, 3 s, and 5 s. Macro and scanning electron microscope (SEM) images follow ASTM E3-01, while ASME IX was used as tensile test standards. The macro images showed that holding time affected quality of the welded joints. Applying a longer holding time was able to reduce defects on the welded joints. SEM energy-dispersive X-ray spectroscopy (SEM-EDX) analysis exhibited that the holding time of 1 s produced a lack of density in the spot-welded joint, resulting in the existence of porosity. In contrast, increasing the holding time to 5 s gives a denser spot-welded joint with no defects found. The tensile test results showed that applying electric current of 8000 A, welding time of 0.5 s, and holding time of 1 s produced the joint with the weakest shear strength of 130 N. The highest joint shear strength of 790 N is obtained with the condition: electric current of 9000 A, welding time of 0.5 s, and holding time of 5 s.

Investigation on shearing strength of resistance spot-welded joints of dissimilar steel plates with varying welding current and time

The present research investigated the effects of welding current and welding time on the shearing strength of resistance spot-welded joints of low carbon steel and stainless steel plates. The macro characteristics, microstructure and micro-hardness of welded joints were also examined. It was found that the welded joints produced with a welding current of 10 kA and welding time of 80 ms showed the highest shearing strength. The diameter of welded joints and nuggets increased with increasing welding current and welding time. For evaluation of the microstructure, different regions including the nugget, coarse heat-affected zone, fine heat-affected zone and base materials were observed within the welded joints. Also, the nugget and coarse heat-affected zone showed higher hardness than the fine heat-affected zone and base materials due to the presence of martensite. The findings obtained in this research are applicable in the development of resistance spot welding processes for joining dissimilar steel plates.

Correlation of Local Constitutive Properties to Global Mechanical Performance of Advanced High-Strength Steel Spot Welds

Recent results have shown that the strength of resistance spot-welded joints made from advanced high-strength steels (AHSS) do not increase linearly with their base metal strength. While the martensite tempering and subsequent softening in a narrow heat affected zone (HAZ) of these joints has been reported as a primary cause for this degradation, the quantitative effects of HAZ softening on above nonlinearity in different steels have not been explored. In this research, the role of material heterogeneity on load-displacement characteristics of dual-phase and martensitic AHSS with initial martensite volume fractions of 16, 58, and 100 pct during tension-shear (TS) and cross-tension (CT) testing was modeled with finite element method and compared with experimental measurements of global deformation and fracture behavior. Results from low-strength steels showed that the location of HAZ failure transitions from hardened to softened regions, as the nugget diameter increases from 4 to 6 and 8 mm, even with the presence of softening in the HAZ. At the same time, the results from higher strength steels showed more sensitivity to the softened region in larger nugget diameters. This result elucidates that the nonlinearity in strength of high-strength AHSS spot welds in comparison to lower strength of mild steel, is due to their intrinsic brittleness, as well as the overall geometry of the weld nugget. Our results also suggest that, while HAZ softening plays a detrimental role in DP steels, it helps to improve the ultimate load and global extension in high strength grades. Finally, several uncertainties related to finite element simulations and experiments have been highlighted. The results from this study can help in optimizing the resistance spot-welding process parameters and the design of the part made from AHSS.

Effect of Initial Gap on Tensile Strength of Resistance Spot Welded Joints

It has been proved that the initial gap has obvious influence on nugget formation, but little works focused on the effect of initial gap on the tensile strength of resistance spot welded (RSW) joints. In this paper, a 3D FE model was built for solving this question. The results show that, even though there are some fluctuations of weld diameter and tensile strength of RSW joints with initial gap, the tensile strength and weld diameter of welded joints with initial gap are still larger than that of welded joints without gap, which confirm that the influence of initial gap on tensile shear strength is little significant. The computation results agree well with experiment.

Impact strength measurement of spot welds

The impact strength of spot welds is a very important quality index in the automotive industry. However, impact testing is not routinely performed for weld quality testing because of its complexity, cost and relatively low reliability and repeatability. In this paper, the impact strength of steel spot welds was measured using a newly designed impact tester. Besides the impact energy of the welds, detailed impact process characteristics, such as impact force and displacement profiles, which may be important to understanding the impact performance of a weld for vehicle design and analysis, were also recorded. A brief description of the new impact tester and experimental results on welded joints using this device are presented. A sensitivity study of testing results to specimen size has been conducted, prior to experiments on the influence of weld size, shape and orientation on the impact strength of resistance spot-welded joints. Results of impact testing were also compared with those of static tensile shear tests. The correlation between weld attributes and impact performance also provides important information for the quality definition of spot welds. Initial tests using this impact tester show promise that it can be used to measure impact strength of a large variety of welds (spot welded, laser welded, brazed, etc.), and other joints (such as mechanical fasteners and adhesion).