Spot Welding Different Sheet Metal Grades and Gauges A study looks at improving the welding of automobile body parts involving joints of mild and high-strength steels of varying thicknesses In order to design lighter automotive structures, to improve fuel economies and carbon dioxide (CO2) emissions, the use of new advanced high-strength steels (AHSS) is expanding, allowing weight savings through thinner sheet gauges. Spot welding remains the main joining process for body-inwhite construction. Most spot welds in automotive structures are dissimilar configurations (i.e., different sheet thicknesses and grades are welded together), but AHSS-to-AHSS configurations have become more common. Shear Load Testing The design of automotive body-in-white structures is usually based on simulated crash behavior. In this case, the fracture behavior of the welds has a major influence on the whole structure’s performance. Spot weld loading is usually divided into several individual loading scenarios, among which shear and opening modes are usually considered the most important (Ref. 1). Practically, shear and opening maximum loads are obtained through simple laboratory tests, i.e., cross-tension (Ref. 2) and tension-shear (Ref. 3). These tests are usually carried out during steel grade qualification for spot weldability. When upgrading the steel from soft drawing quality to high-strength and to AHSSs, the trend for tensile shear maximum loads is known and is consistent with the increase of base metal strength (Refs. 4, 5). However, in cross-tension tests there is not a clear trend between base material and cross-tension strength (CTS) (Ref. 6), but it is well known that AHSSs may show rather low CTS. These periodic poor strengths are usually attributed to the high alloying content of AHSS, leading to martensitic microstructure 38 JANUARY 2014 BY E. BIRO, L. CRETTEUR, AND T. DUPUY in the spot weld, which is sensitive to the notch effect when testing is performed in opening mode. However, most of these observations and conclusions are based on standard steel qualification results, i.e., tests on homogeneous welding configurations. When these data are then applied to dissimilar configurations, the most common approach in the automotive industry is to consider that the fracture behavior of a dissimilar configuration can be deduced from the corresponding two homogeneous configurations through a “minimum rule,” i.e., the load-bearing capacity of the heterogeneous configuration is supposed to be equal to the minimum strength of both homogeneous assemblies. This assumption is verified experimentally on material combinations using thin mild steels, as illustrated in Fig. 1. The mild steel sheet is the weakest point of the assembly and fails during the mechanical test, leading to the formation of a plug around the weld. Dissimilar Welds Studies on heterogeneous or dissimilar configurations are much fewer, although several studies have been published recently (Refs. 8–12). However most of these studies consider only tensile-shear testing of dissimilar welds. Only the work from Khan et al. (Ref. 9) considers cross-tension testing of a DP600-HSLA similar-thickness configuration, obtaining a loadbearing capacity almost equal to that of the maximum level among the similar configurations results, i.e., almost equal to that of DP600. This result does not agree with the minimum rule described above, which is attributed by the authors to the difference in microstructure and hardness of the spot weld nugget due to the dilution effect. This article focuses on AHSS E. BIRO (elliot.biro@arcelormittal.com), L. CRETTEUR, and T. DUPUY are with ArcelorMittal Global R&D, based in Hamilton, Ont., Canada; Montataire, France; and Maizières-lès-Metz, France, respectively. This article is based on a paper presented at the Sheet Metal Welding Conference XV, Livonia, Mich., Oct. 2–5, 2012.
Welding Journal | January 2014
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