Cosmetic Corrosion of Welded Hot Dip Galvanized Steel Panels 912287

The cosmetic corrosion of welded and painted automotive hot-dip galvanized (HDG) panels in the GM scab test and outdoor exposure has been studied. A comparison was made between the performance of laser butt welds, mash seam welds and resistance spot welds. Other variables were the laser speed and the power input in the laser welding process. The underfilm corrosion in welded areas was compared with that in areas with standard scribes through the paint film. The weld areas were characterized by SEM and EDX before and after phosphating and also following corrosion testing by SEM and EDX. Corrosion products were identified by X-Ray diffraction (XRD). The best overall corrosion performance was exhibited by laser-welded panels, provided the weld area is protected from oxidation. Models for the corrosion propagation mechanisms are presented.

THE DEMAND FOR BETTER fuel consumption and production cost reduction in today's automobiles has led to an increased use of welded components, because welding allows part of the component to be made of a thinner gauge. Therefore, new processes, especially those that enable welding of dissimilar gauge panels, have been developed, such as laser welding (LW) and mash-seam welding (MSW).

At the same time, there has been an enormous improvement in the perforation and cosmetic corrosion performance of automotive steels, mainly as a result of the use of precoated steels and also because of recent improvements in phosphating processes and paint systems. Since the welding process can be expected to damage the zinc or zinc-alloy coating, there is concern in the automotive industry that welded areas in precoated steels may not perform acceptably in a corrosive environment.

Although corrosion studies of resistance-welded steels are abundant in literature, very little, if anything, has been published on the corrosion performance of laser-welded steels. Further, there is only sparse information on the cosmetic corrosion of weld areas in phosphated and painted precoated steels, welded by any technique. This is surprising, in view of the thousands of welds, mainly resistance spot welds, used in modern automobiles.

Electrochemical studies of laser-welded, cold-rolled steels in unpainted conditions were recently published (1, 2 and 3)*. In a recent paper, we reported on the cosmetic corrosion in the GM scab corrosion test of automotive painted hot-dip galvanized steel panels welded by mash seam and laser processes (4). It was shown that the laser-welded panels generally performed better. This difference was related to the presence or absence of phosphate crystals in the weld areas.

In the present study, we have extended our investigations so as to include a wider range of laser speeds and energy inputs and an additional welding process, standard resistance spot welding (SW). Because there is always a concern about the strength of welded steels, we also report here on the effects of the laser welding process on the microstructure of the base steel. Also, the first data on the corrosion performance of laser- and mash-seam-welded blanks in outdoor exposure is presented.