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WELDING RESEARCH

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Fig. 9 — Microhardness profiles for welds in DP18 and DP20 produced using widely different welding conditions (microhardness was measured using a 200-g load and 30-mm away from the weld centerline).

Fig. 10 — Optical micrographs of microstructures in a spot weld made in DP20 steel (12-kA current and 20-cycle weld time), and EDS results revealing the presence of nontransition metals, including sulfur.

during testing if CE is greater than 0.24 (Refs. 12–14).

CE

0.24limit

= C+

Si 30

+

Mn 20

  • +

    2P + 4S

(1)

and verified using a combination of tech- niques: X ray fluores- cence (XRF), induc- tively coupled plasma (ICP) spectroscopy, and wavelength dispersive spectroscopy (WDS).

Although not detailed here, but sug- gested elsewhere (Ref. 14), zinc contami- nation from the coatings is also another potential contributor to weld fracture. Consequently, the effects of process para- meters, sheet thickness, steel composi- tion, and coating were all important to consider for fully explaining the weldabil- ity of DP600 steels, since they all influence weld dimensions, weld microstructures, and weld mechanical properties, including fracture characteristics (Refs. 4–19).

Experimental Procedure

Materials

Following suggestions by manufactur- ing engineers, two hot-dipped galvanized DP600 steels of commercial availability were selected with two distinct gauges. The thinner steel, designated as DP18, was 1.8 mm thick, whereas the second, DP20, was supplied in a 2.0-mm sheet thickness. For testing, the steel sheets were sheared into 38 ¥ 127-mm coupons. Guided by Equation 1, the steel chemical compositions were precisely measured

Welding

All resistance spot welds were fabricated using a mid-frequency DC welding machine, where current was de- livered through C15000 Cu-Zr alloy caps with a 5.0-mm face diameter. To minimize weld-to- weld variations intro- duced by cap wear, the caps were regularly in- spected using reference welds, which were gen- erated each time new caps were inserted. These reference welds were all produced using typical process parame- ters (e.g., 9 kA, 20 cycles and 1200 lb, or 5.3 kN). When the repeat of a reference weld with used caps resulted in a weld with a new di- ameter and type of weld fracture, new caps were systematically mounted. This pre- caution guaranteed that the welding con- Fig. 11 — X-ray compositional maps near the center of an unetched spot weld made in DP20 steel (12.5 kA current and 20-cycle weld time) with line scans for three alloying elements, all revealing some mi- crosegrgation. ditions were not drifting over time, and that the two steels could be compared under identical conditions at any time if needed. Each welded sample was made of two

WELDING JOURNAL 175 -s

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