PSI - Issue 17

Liting Shi et al. / Procedia Structural Integrity 17 (2019) 355–362 Liting Shi/ Structural Integrity Procedia 00 (2019) 000 – 000

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ensure pull out fracture. The weld nugget diameter and peak load were normalized by dividing the critical weld nugget diameter and calculated fracture load, respectively. Comparison of these results exhibit good agreement using the new prediction method (Fig. 7).

Fig. 7 Normalized diameter vs normalized peak load.

For the 0.8-mm X626-0.8-mm X626 and 0.8-mm X626-0.9-mm LCS stack-ups, the aluminum sheets were the same thickness of 0.8 mm. The critical weld nugget diameter is the same if only the sheet thickness is considered. However, the actual critical weld nugget diameters were 6.0 mm and 7.1 mm, respectively. The critical weld nugget diameters in 1.2-mm AA6022-1.2-mm AA6022 and 1.2-mm AA6022-1.2-mm LCS were also different even though the aluminum sheet thickness was the same. The difference of the critical weld nugget diameter for the two stack-ups with the same aluminum sheet thickness results from differences between the shear strength in the aluminum weld nugget and the IMC layer. The effect of the IMC layer in aluminum-steel RSWs on the fracture modes cannot be ignored. Mortazavi et al. (2011) and Qiu et al. (2009) pointed out that a thinner and more uniform IMC layer led to a stronger spot weld. The welds described in this study exhibited IMC layers of 2µm thick or less at the weld edges. The higher shear strength of this thin IMC lead to a smaller critical weld nugget diameter and promoted pull out fracture. While a new formula is now developed to predict the fracture modes of RSWs, it can be unrealistic to produce welds having a weld nugget as large as the calculated critical weld nugget diameter considering many other complex factors. It is interesting to note that even though the experimental weld nugget is smaller than the critical weld nugget diameter, the fracture load is still greater than the minimum average fracture load required in the standard recommended by the Aluminum Association (1982). In this standard, the recommended minimum average fracture load is 1310 N and 2400 N for 0.8-mm and 1.3-mm aluminum sheet, respectively. The fracture loads in the present study were all larger than the two values, even though interfacial fracture occurred. The spot welds were thus still considered to have an acceptable strength. 5. Conclusion Aluminum- steel RSWs were successfully welded with an acceptable strength using GM’s MRD and multi solidification weld schedules. Interfacial and pull out fracture modes were observed in the static tensile tests. The