PSI - Issue 5

Jidong Kang et al. / Procedia Structural Integrity 5 (2017) 1425–1432 Jidong Kang/ Structural Integrity Procedia 00 (2017) 000 – 000

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Porosity and hot cracks in the center portion of the weld nugget are evident in Fig. 2(a), which are common features observed in other aluminum to aluminum RSWs [10,13]. Eutectic stringers rich in alloy content were also observed on the periphery of the weld nugget as seen in Fig. 2(b). The partially melted zone (PMZ) just outside of the weld nugget boundary was characterized by large recrystallized grains as seen in Fig. 2(c). A microscopic crack-like feature, i.e., the notch slit that forms at the weld notch root adjacent to the edge of the weld nugget (indicated by the dark arrow) is shown in Fig. 2(d). In addition, the periphery of the weld nugget exhibited columnar grain structure as seen in Fig. 2(d). The center portion of the weld nugget exhibited a mixed equiaxed grain structure and traces of columnar grains as seen in Fig. 2(e). The MRD electrode design and CSS weld schedules are designed to work together to produce just such a nugget structure, i.e., one that has fine columnar grains with no defects along the nugget periphery transforming to an equiaxed structure with porosity and micro-cracks contained within the central region of the nugget. This creation of a sound, fine-grained nugget periphery results in good mechanical performance.

Fig. 3. A representative cross-section of an untested AA6022-T4 to IF steel resistance spot welds (a) overall view with numerical regions for further study, (b) magnified view of interface in region 1, (c) magnified view of the interface showing the increase in thickness of the intermetallic compounds in region 2, (d) magnified view of the intermetallic compounds layer in the center portion of the weld nugget and, (e) magnified view of the columnar grains in weld nugget on the AA6022-T4 sheet. Unlike aluminum-to-aluminum or steel-to-steel spot welds, the AA6022-T4 to IF steel welds lacked the distinctive large, central nugget joining the two sheets as seen in Fig 3(a). As such, the bonding between the AA6022-T4 and IF steel appears more as a welded-brazing joint due to wetting of the molten aluminum and spreading on the IF steel sheet surface [2,8]. The aluminum nugget in the AA6022-T4 sheet exhibited columnar grains as seen in Fig. 3(e), which indicates that rapid cooling occurred for the aluminum sheet after welding. Compared to the corresponding Al to-Al weld shown in Fig. 2a, the Al-to-steel weld shows very little porosity or cracking in the weld nugget, Fig. 3a. Comparison of Fig. 2 and Fig. 3 also reveal that the notch root opening is much more pronounced for the AA6022-T4 to IF steel welds. This can benefit both static and dynamic mechanical properties since the stress concentration for the more open notch root would be less. A magnified view of the interface between the faying sheets shows the formation of a thin continuous layer of IMCs along the faying surface. The IMCs are much thinner (~2 μm) clo ser to the periphery of the weld nugget as seen in Fig. 3(b) and grow in thickness moving closer to the center of the weld nugget (~ 9 μm) as seen in Fig. 3(c) and (d) respectively. During welding, the center region of the weld nugget experiences higher temperatures, which resulted in thicker IMCs as compared to the periphery of the weld nugget [1,8]. The IMC layer consisted of two layers, FeAl 3 formed adjacent to the AA6022-T4 sheet and Fe 2 Al 5 formed adjacent to the IF steel sheet surface. A high magnification TEM image of the IMC layer formed between the top aluminum and bottom steel sheet is shown in Fig. 4(a). The FeAl 3 layer that formed adjacent to the AA6022-T4 sheet has a “needle” like ap pearance as observed in Fig. 4(b) while the Fe 2 Al 5 layer that formed adjacent to the IF steel sheet has a “tongue” like appearance. The exact chemical composition of the IMC layers is provided in Table 2.

Table 2. Nominal chemical composition of the two intermetallic compounds (wt%) Elements Fe Al Si Mn Cu

O

Total 100% 100%

FeAl 3 Fe 2 Al 5

40.61 36.47

51.91 56.32

0.64 0.64

0.05

3.16 3.32

3.63 3.24

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