PSI - Issue 51

Liting Shi et al. / Procedia Structural Integrity 51 (2023) 102–108 L. Shi et al. / Structural Integrity Procedia 00 (2022) 000–000

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1. Introduction Multi-material solution for structural lightweighting is attractive for automotive industry to improve fuel economy thus reduce Greenhouse gases emission. Joining of dissimilar materials, in particular aluminum alloy sheet to steel sheet joining poses challenges. While many new joining techniques are being explored, e.g., self-pierce riveting (Xing et al. (2015), mechanical clinching (Abe et al. (2014), Gao et al. (2022)), flow-drilling screw-bonding joining (Li et al. (2022)), etc., resistance spot welding (RSW) remains as a dominating joining technology in automotive industry (Emre, et al. (2020), Duran, (2022), Ghanbari et al. (2022)), thus it is naturally an attractive solution to join dissimilar metals without additional capital cost. Overcoming the challenges in welding dissimilar metals due to disparity material properties, resistance spot welding has been successfully used to join dissimilar metals, such as aluminum dissimilar alloys (Kang et al. (2016)), aluminum alloy to steel, by using the Multi-Ring Domed (MRD) electrode (Sigler et al. (2010, 2013)), and the multiple solidification weld schedules (Sigler et al. (2017, 2018)). The effect of intermetallic compound (Pouranvari (2017), Chen et al. (2018), Shi et al. (2022)), weld nugget diameter (Qiu et al. (2009), Shi et al. (2020)), specimen configuration (Rao et al. (2018)), sheet thickness ratio (Manladan et al. 2017) and notch root angle (Shi et al. (2020)) etc. on the mechanical performance of spot welds has been reported on two-sheet aluminum to steel RSWs. Multiple sheets assembled spot welds are worth great attention, for example, Lei et al. (2011) pointed that three sheets stack-up RSWs are often used in vehicle manufacturing, such as front longitudinal rails (Li et al. (2015)) and base beam. However, (Nielsen et al. (2011)) demonstrated that joining three sheets was more complicated due to the extra interface introduced and asymmetrical heat distribution during welding process. Kang et al. (2010), Pouranvari et al (2011) and Lei et al. (2011) performed researches on fatigue behavior, critical sheet thickness and transient thermal characteristics of three sheet stack-ups with similar materials of equal thickness, respectively. Nielsen et al. (2011) and Yu et al. (2017) studied the weldability of dissimilar steels, i.e. low carbon steel and two high strength steels. Considering that aluminum alloy is increasingly used in the automotive industry, Li et al. investigated weld nugget formation (Li et al. (2015)) and failure mode (Li et al. (2016)) in three sheet aluminum alloy RSW. With the development of aluminum-steel hybrid white body in automotive industries, stack-ups of three-sheet aluminum to steel sheets is inevitable, however, the formation of brittle intermetallic compound at the interface of aluminum alloy and steel sheet presents more challenges to resistance spot welding process and is yet to explore. The purpose of this study was to investigate the tensile and fatigue behavior of three-sheet aluminum to steel dissimilar RSWs. A 1.2 mm thick AA6022 to 0.65 mm thick high strength low alloy (HSLA) and 1.4 mm thick CR780T was resistance spot welded successfully using MRD and multiple solidification weld schedules. The baseline 1.2mm AA6022 to itself RSWs and two-sheet stack-up of 1.2 mm AA6022 to 2.0 mm HSLA RSWs were used as a comparison. Microstructure of the intermetallic compound at the interface of aluminum alloy and steel sheets was characterized and tensile and fatigue properties were compared. Finally, fatigue life of all stack-ups was assessed using the structural stress method derived by Rupp et al. (1995). 2. Materials and experimental procedures 2.1 Materials and welding schedules The materials used in the present study were commercially available 1.2 mm thick AA6022, 0.65 mm and 2.0 mm thick high-strength low-alloy (HSLA) steel sheets, as well as 1.4 mm CR780T.

Table 1 Welding schedules used for specimen fabrication

Squeeze time (ms)

Weld time (ms)

Welding current, RMS i (kA)

Holding time (ms)

Parameters

Weld force (N)

Preheat stage 10ms @16% VS 40ms at 7.5kA, 10ms cool 40ms @8.5kA, 10ms cool

AA6022 AA6022 AA6022 HSLA AA6022 HSLA CR780T

3336

1500

151

23.3

250

3781

1500

1705

10.7

250

5783

1500

1435

11.4

250