13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- 2. Experimental results 2.1. Dissimilar spot welded joints between ultra-high strength and low-alloyed steels The joints investigated in this work are dissimilar resistance spot welded joints between a hot-stamped ultra-high strength steel (22MnB5) with a nominal ultimate tensile strength (UTS) of 1500 MPa and a microalloyed high strength low-alloy steel (HC340LAD) with an UTS of 415 MPa. Spot welding and testing of spot welded KS2-specimens was carried out at the Laboratory for Materials and Joining Technology (LWF) at the University of Paderborn [7]. On the left Fig. (1) shows the etched cross section of the dissimilar joint. The upper sheet shows the microalloyed HC340LAD with a sheet thickness of 1.5 mm. The hot-stamped 22MnB5 steel sheet with a thickness of 1.0 mm can be seen as the lower sheet in Fig. (1) (left). The averaged weld nugget diameter measured from cross-sections of three different spot welds was 5.2 mm. Figure 1. Etched cross section (left) and hardness measurements along the dashed white line shown in the cross section of the investigated spot welded joint between hot-stamped 22MnB5 and HC340LAD (right) The base metal (BM) of the microalloyed steel shows a ferritic-perlitic microstructure with a hardness of 190HV0.1. Inside its HAZ the microstructure is ferritic-bainitic and a continuous increase in hardness up to 380HV0.1 adjacent to the weld nugget can be observed. The microstructures of the BM and the HAZ of the hot-stamped 22MnB5 are fully martensitic with averaged hardness values of 475HV0.1. Inside the subcritical HAZ a significant decrease in hardness can be seen. Due to the heat input during spot welding tempering of the martensitic microstructure occurs in this region (SHAZ) and leads to softening of the material. This phenomenon has also been observed during spot or laser welding of dual-phase steels as e.g. shown in [8]. The microstructure of the weld metal (WM) of the dissimilar weld is martensitic and has slightly lower hardness values than the BM and the HAZ of the hot-stamped manganese-boron steel. 2.2. Testing of dissimilar spot welded joints The mechanical behavior of the dissimilar spot welded joints was characterized under shear, axial and combined axial and shear loading conditions using different tests and specimens such as tensile-shear- (TS), LWF-KS2- and coach-peel (CP) specimens. The LWF-KS2 concept [9] is based on double-U formed specimens, which are spot welded in their center and tested under different loading angles and/or velocities using different clamping conditions. The CP- and the KS2-specimen as well as the fixtures of the KS2-specimen for tests with different loading angles
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