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Fig. 13 — Percent voids in weld fusion zones vs. weld time for spot welds made in DP18 and DP20 steels (12-kA current and 1200-lb weld force).

Fig. 14 — Percent voids in weld fusion zones vs. current and weld force for spot welds made in DP20 steel (20-cycle weld time).

ative shortfall of strengthening elements such as manganese, chromium, and vana- dium. In both DP18 and DP20, the boron contents were also less than 10-wt-ppm; a concentration that therefore neither could increase steel hardenability (Ref. 17) nor cracking susceptibility (Ref. 14).

Table 1 provides the carbon equiva- lents for DP18 and DP20. Although the CE was determined with Equation 1 that normally applies to non-AHSS steels (Refs. 12–14), the CE could still be uti- lized as a relative indicator of hardenabil- ity and weldability. Since the CE for both steels was close to the limit of 0.24, vari- ability in the types of weld fracture was ex- pected to be greater than in non-AHSS steels, where the CE is typically smaller. Also, note that the carbon equivalent of DP18 not only exceeded that of DP20, but also the value of 0.24 given by Equation 1 (Refs. 12–14). Consequently, with only the CE to describe weld fracture, the spot welds in DP18 could have been expected to fracture interfacially more frequently than in DP20. The question as to whether this statement is correct or not is answered later in this paper.

As for the as-received microstructures (not shown), they were comparable in DP18 and DP20. Both steels had average grain sizes of about 10 µm and contained approximately 15% martensite. However, hardness of the DP18 steel was greater than that of DP20, averaging 205 kg/mm2 vs. 185 kg/mm2 for the DP20 steel. Coinci- dentally or not, this difference in hardness

agrees with the CE values.

SEM observations revealed that the galvanized layer was 9 µm thick in DP20 and nearly twice as thick in DP18 (Fig. 1), while EDS and XRF measurements showed that coating compositions were similar with at least 99 wt-% zinc and less than 1 wt-% aluminum. Therefore, the possibility that coating compositions af- fected weld fracture in the two DP600 steels could be eliminated.

Spot Weld Expulsion

In the resistance spot welding of galva- nized steels, the expulsion of zinc (i.e., the element that melts and vaporizes first) is important to examine, as it limits the se- lection of the process parameters. Zinc can be expulsed both at the electrode- sheet interfaces, and at the interface be- tween the two sheets where the weld forms (Refs. 18–20). Due to water cooling within the electrodes, the temperature is nor- mally greater at the interface between the sheets and expulsion occurs there first (Ref. 18). Because this expulsion is more difficult to detect when it first occurs, only the expulsion from the electrode-sheet in- terface is considered here.

Figure 2 describes the effects of weld current and weld time on zinc expulsion. To determine with precision the threshold of zinc expulsion for the two steels, each data point was generated from at least three welds. Judging from Fig. 2, the pos- sibility that the thicker zinc coating of

DP18 (Fig. 1) could have promoted more expulsion was ruled out since a single boundary between the expulsion-free and the expulsion regions was found for the two steels. Although both DP18 and DP20 were indistinguishable here, Fig. 2 was later found to be extremely useful for se- lecting appropriate process parameters, and to further compare DP18 and DP20.

In Fig. 2, the solid line demonstrates that the current at the beginning of expul- sion is inversely related to the weld time. Consequently, to create large welds, cur- rent and weld time must be selected such that they are positioned close to this boundary line, and, to prevent expulsion, they must be also on the left of this bound- ary line. Moreover, to enhance weld process repeatability, the best-controlled variable (i.e., weld time) must be extended as much as possible, whereas the current, the second variable of Fig. 2, must be min- imized to alleviate its relative contribution in the weld formation, and to prevent ex- tensive cap wear (Ref. 6). Unlike the weld time, the current is self-regulated and varies as contact resistance, material prop- erties, alignment, and cap wear gradually change from one weld to another (Refs. 6, 15, 18–20). For this study, where a high level of reproducibility between welds was needed to compare DP18 and DP20, ex- tended weld times (> 20 cycles) and cor- respondingly small currents (<9 kA) were particularly appropriate.

Figure 3 is a complementary figure de- scribing weld force effect on zinc expul-


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