PSI - Issue 37

Liting Shi et al. / Procedia Structural Integrity 37 (2022) 351–358 L Shi et al/ Structural Integrity Procedia 00 (2022) 000 – 000

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Nomenclature M x bending moments in x direction in a tensile shear specimen M z bending moments in z direction in a coach peel specimen F y maximum applied force in y direction, i.e. loading direction in a tensile shear specimen F z maximum applied force in z direction, i.e. loading direction in coach peel specimens R load ratio, i.e. ratio of minimum to maximum load in a fatigue test d weld nugget diameter t thickness of aluminium sheet

1. Introduction Multi-material solutions are a common approach in the automotive industry to lighten vehicles. This enables automakers to achieve stringent fuel economy requirements or offsets use of heavier sensors and technology associated with smart vehicles. One multi-material solution that has received considerable attention is joining aluminum and steel sheets. When joining dissimilar materials such as aluminum to steel by resistance spot welding (RSW), a thin layer of brittle intermetallic compound forms at the aluminum-steel interface and dominates mechanical behavior of the joint, see Miyamoto, et al., (2009), Qiu, et al. (2009), Wan, et al. (2017), and Chen, et al. (2018). Resistance spot welding of aluminium to steel was successfully developed by Sigler, et al. (2017, 2018) using a multi-ring domed (MRD) electrode and multiple-solidification welding schedules to obtain high-quality weld nugget with acceptable joint strength. Kang, et al. (2020) developed a mini-shear test specimen that can directly measure the shear strength of the aluminium-steel intermetallic compound layer. It was revealed by Shi, et al. (2020a) that when the sheet thickness or sheet alloys are dissimilar, the maximum plastic strain reflects the change of the stiffness of the weld joint due to the sheet thickness asymmetry or mixed metal combinations. Further, Shi, et al. (2020a) determined that maximum plastic strain correlates better with fatigue life compared to the structural stress approach. Shi, et al. (2020b) demonstrated in RSW of AA6022 to a high strength low alloy steel (HSLA) that increasing notch root angles produced a slight increase in static tensile peak load, improved fracture modes, and were beneficial for increasing fatigue life because the principal strain decreased with increasing notch root angle. It was discovered that changing weld polarity in the aluminum-steel RSWs produced different notch root angles. In this contribution, we extend our study to RSW of 1.1 mm thick AA5754 to 2.0 mm thick HSLA steel sheets, with a focus on the effect of specimen configuration and notch root angle on fatigue behaviour of the welds. Multi ring domed (MRD) electrodes and multiple solidification weld schedules were used to produce RSWs with AA5754 sheet connected to either positive or negative electric polarity. Tensile and load-controlled fatigue tests are carried out and the results are compared. Structural stress analysis followed Rupp’s method for fatigue life correlation in Rupp , et al. (1995). In addition, finite element analysis using ABAQUS 2020 was performed to obtain the maximum principal strain at the notch root, and the results are used to highlight the relationship between maximum principal strain and fatigue life of the RSWs. 2. Experimental procedures 2.1. Materials and welding schedules The materials used in the present study were 1.1 mm thick AA5754 and 2.0 mm thick high-strength low-alloy (HSLA) steel sheets. The resistance spot welds were produced using multi-ring domed (MRD) electrodes and multiple solidification weld schedules with aluminum sheet connected to either positive or negative polarity to produce RSWs (hereafter denoted as positive and negative polarity Al-steel RSWs, respectively) as outlined in Shi, et al. (2020b). As a baseline, RSWs comprised of 1.1 mm thick AA5754 joined to itself were also produced and tested. Table 1 provides the welding schedules. More details of the welding processes can be found in Shi et al.