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WELDING RESEARCH

Fig. 5 —

eld diameter (as measured after peeling) vs. current for welds

made in DP18 and DP20 steels (20-cycle weld time and 1200-lb weld force).

Fig. 6 — Fracture force (peak force recorded during tensile-shear testing) vs. current for spot welds made in DP18 and DP20 steels (20-cycle weld time and 1200-lb weld force).

Fig. 7 — Process maps showing effects of both current and weld time on the type of weld fracture encountered during chisel testing on DP18 (left) and DP20 (right) steels (1200-lb weld force).

Fig. 8 — Percent weld interfacial fracture vs. current and force for spot welds made on DP20 (20-cycle weld time).

by the chisel test (Ref. 9). When this study was initiated in 2003, it was primary to first establish a relationship between process parameters and occurrence of weld inter- facial fracture using a minimum of two DP600 steels. Since weld diameter is well- known to be a key player in weld fracture (Refs. 6, 9–14), weld diameters were mea- sured and correlated to the process para- meters, as were the weld microstructures. In this first study, the weld microstructures were thoroughly examined due to well-

established relationships with the me- chanical properties, and thus fracture (Refs. 5–8). In this article, the reader will discover that the weld microstructures are first discussed and linked to the steel com- positions, as suggested by the Nippon Steel carbon equivalent (CE) here pre- sented as Equation 1 (Refs. 12–14). This CE, developed from non-AHSS spot welds that were quasi-statically cross- tension tested (Refs. 12–14), connects chemical composition with one of the two

types of weld fracture. This CE, yet un- proven for AHSS steels, presented a means for measuring DP steel spot weld- ability with a load orientation that was not too different from the chisel test load ori- entation. Equation 1 for this CE infers that a relatively hard and brittle weld (i.e., one that is made of martensite) having enough sulfur and phosphorus (i.e., ele- ments that segregate to boundaries and in- crease solidification cracking susceptibil- ity, Refs. 15, 16) will fail interfacially

174 -s NOVEMBER 2005

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