The CTS for the main DP980 configurations are shown as a function of the weld diameter in Figs. 4 and 5. Other DP980 results (i.e, 1+1.25-mm configuration) have been left out of the figures for the sake of clarity, but are considered in the discussion. As in the case of the TRIP800 steel welds, these results show the CTS is mainly dependent on weld diameter. Again, the dissimilar configurations performances appear obviously above the minimum rule assumption. Three-sheet configurations based on 1-mm DP980 LCE results are shown in Figs. 6 and 7, and again show that the CTS obtained for these configurations are higher than expected from the minimum rule. Figure 6 shows that compared to the standard similar 1-mm, two-sheet configuration, both the patch and the 1+1+0-mm configurations bring an improvement in CTS. Figure 7 shows that the 1+++1+1-mm configuration shows a performance very close to the 1+2-mm two-sheet configuration, whereas the 1+1+1 configuration shows a more limited improvement, highlighting the importance of restraint in the case of three-sheet assemblies. The observation that the CTS is greater than predicted by the minimum rule has been called a “positive deviation” from the expected strengths. This positive deviation turns out to be almost systematic when AHSSs are welded together. In the following discussion, a detailed analysis of this phenomenon is described, and a tentative explanation for it is proposed. Analysis of Results Several hypotheses can be proposed to explain the “positive deviation”: 1. Dilution Effect: when spot welding an AHSS sheet to a lowcarbon sheet, the dilution (mixing of both chemistries) reduces 40 JANUARY 2014 the carbon content of the molten nugget, which may improve its mechanical behavior. Although this explanation is probably true, it cannot explain the results of the present study, since they were obtained with similar grades joined together (Ref. 9). 2. Thermal Effect: when comparing 1+2-mm to 1+1-mm configuration, the overall assembly thickness is increased, leading logically to increased thermal mass slowing the cooling rate. Reducing the cooling rate can, in turn, reduce the brittleness of the microstructures formed during welding (bainite may be tougher than autotempered martensite, which may be tougher than quenched martensite). 3. Notch Effect: in the case of dissimilar thickness spot welds, the solidification plane, where the columnar grains in the weld nugget meet, is assumed to be roughly at the midthickness of the whole assembly, which is away from the notch end at the faying surface. As the solidification plane may be weaker than the bulk of the molten nugget, a dissimilar thickness joint may be stronger in opening mode than a similar thickness joint. 4. Mechanical Effect: in the case of dissimilar thickness spot welds, the stress concentration at the notch will be different than in a similar thickness joint due to uneven loading. This will in turn influence the cross-tension strength. As this effect is not obvious it will be shown further in the discussion. The analysis of three-sheet configurations results helps in understanding the relative importance of these effects — Fig. 8. The 1+1+0-mm configuration is especially interesting as the loading conditions during cross-tension are strictly identical to the reference 1-mm similar configuration, but its CTS performance is greater than the strength of the similar gauge joint — Fig. 6. This positive deviation can only be attributed to the thermal (since overall thickness during welding was 3 mm instead of 2) and notch effects. However, the positive deviation for this 1+1+0-mm configuration is limited compared to the 1+2, Table 1 — Steel Sheet Samples Grade Coating Sample # Thickness YS (MPa) UTS (MPa) C (%) Mn (%) Si (%) Cr (%) Al (%) (mm) DP980 Hot Dip LCE Galvanized AR1084 1 650 995 0.08 2.49 0.27 0.28 0.14 AS160 1.25 833 1076 0.08 2.44 0.26 0.29 0.15 AN2157 2 704 1037 0.07 2.54 0.25 0.31 0.14 TRIP Electrogalvanized AL761 1 520 828 0.19 1.67 1.63 0.02 0.03 800 Bare AL351 2 546 832 0.19 1.71 1.68 0.03 0.04 Table 2 — Welded 2-Sheet Configurations Grade Thickness DP980 LCE TRIP800 (mm) 1 1.25 2 1 2 1 AR1084/ AR1084/ AR1084/ AR1084 AS160 AN2157 DP980 1.25 AS160/ AS160/ LCE AS160 AN2157 2 AN2157/ AN2157 1 AL761/ AL761/ TRIP800 AL761 AT351 2 AT351/ AT351
Welding Journal | January 2014
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