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A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y 2 0 1 9 2 9 exhibited roping, while others had fre- quent or less severe issues. Research eventually traced the source of the problem to preferably oriented grains in the sheet, which originated in the hot rolling process. Some grains were “softer” than others due to their crys- tallographic orientation; when pres- ent as bands, they gave rise to surface roughening (Fig. 9a). When grains were more randomly oriented, roping was not observed (Fig. 9b). Unlike pre-aging, a roping-free appearance could not be immediately associated with any performance im- provement. It simply made the metal look better, although some would say that it finally looked like it was sup- posed to. Further, the “defect” could only be seen by trained observers un- der specialized lighting conditions. Because the introduction of any modi- fied material meant having to conduct lengthy production prove-out runs to determine any unintended secondary Fig. 7 — Effect of pre-aging temperature on yield strength of alloy 6111. Excerpt fromUS Patent 5,718,780. Fig. 8 — In this image, roping may be observed after stretching of 6xxx-T4 automotive sheet. (a) (b) Fig. 9 — Surface grain structures of 6xxx ABS showing (a) extensive roping or (b) no roping at all. Scanning electron microscope, EBSD images. Courtesy of Novelis. required are currently pre-aged. These products may be called by other trade names, e.g., Strongalex or Advanz s615. During this time, the steel industry successfully introduced bake harden- able steels, which like aluminum, could be formed at relatively low strength and harden during the paint bake cy- cles. As a result, steel outer skin thick- nesses had dropped from 0.9 to about 0.7 mm, encouraging the rapid adop- tion of pre-aging for 6xxx ABS on all continents. As the industry settled on 6xxx al- loys and began production of more complex parts such as fenders and trunk lids, the higher surface strain on these parts would sometimes reveal a discrete surface roughening. The effect was subtle, best observed on dark col- ors under the merciless lighting of the paint inspection booths in the assem- bly plant. But once seen, it was difficult for a trained eye to miss it. The effect was very much like the part had been painted with a brush, hence the initial name “paintbrush lines,” even though its origin was the sheet itself, not the painting process. Other de- scriptive names for the phe- nomenon include “ridging” and “roping” because the roughened areas are linear in appearance (Fig. 8). Because there is no in- dication of a problem be- fore forming or in a simple tensile test, finding and fixing the root cause of roping was initially a chal- lenge. Some suppliers pro- duced sheet that never