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WELDING RESEARCH

Fig. 1 — Secondary electron images showing the galvanized layers on the two DP600 steels and electron dispersive spectra indicating their approxi- mate chemical compositions.

Fig. 2 — Process map describing the effects of the current and weld time on zinc expulsion for both DP18 and DP20 (1200-lb weld force).

Fig. 3 — Process map describing the effects of the weld force on the zinc expulsion for the DP20 steel (1200-lb weld force).

Fig. 4 — Reduction in thickness or thinning vs. current and weld time for welds made in DP18 and DP20 steels (1200-lb weld force).

faces, causing the so-called “interfacial fracture.” The occurrence of weld interfa- cial fractures in DP600 steels was in con- trast with the traditional automotive steels, where weld fracture is known to occur consistently in the fusion zone pe- riphery, promoting the so-called “weld

button.” For spot welds to be reliable dur- ing vehicle lifetime, they are required to tear a button during quality control. The formation of a weld button during quality control indeed indicates that the same weld would have been able to transmit a high level of force, thus cause severe plas-

tic deformation in its adjacent compo- nents, and increased strain energy dissipa- tion in crash conditions.

In Part I of this investigation on the weldability of DP600 steels in production environments, fracture in spot welds from two commercial steels was induced mainly

WELDING JOURNAL 173-s

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