Mechanical Properties and Failure Behavior of Resistance Spot Welded Third-Generation Advanced High Strength Steels

Abstract

Acceptable crash performance and fuel efficiency are vital requirements for any modern automobile. To meet these requirements, the automotive industry is designing lighter vehicles by further adopting third-generation advanced high strength steels (3G-AHSS) within their vehicle assemblies. 3G-AHSS possess multiphase microstructures that provide a favorable combination of strength-ductility relative to existing commercial AHSS. A safe and reliable migration to 3G-AHSS within automotive body-in-white (BIW) structure demands, among other requirements, the ability to predict the onset of failure from components fabricated using common joining techniques such as resistance spot welding (RSW). A fast and reliable approach for RSW failure prediction within the automotive industry is utilizing force-based RSW failure criteria that are calibrated using critical loads/moments at the onset of RSW failure from various mechanical tests. Aside from conventional tensile shear (TS) and cross tension (CT) mechanical tests, characterizing the 3G-AHSS RSW failure strength components at various complex loading conditions can improve the calibration accuracy of experimental RSW failure loci. Some of such complex loading conditions include various ratios of shear-tension loading, characterized by KS-II tests, and tension-bending loading mode, characterized by coach peel (CP) tests. Accurate quantification of RSW mechanical performance indices, such as load-bearing capacity and energy absorption capability from single spot weld characterization technique is accompanied by unique challenges due to rotation of the joint and plastic work due to coupon deformation at regions away from the joint during mechanical testing. The influence of such unintended phenomena on extracted mechanical performance indices is commonly acknowledged but not accounted for. In this research program, the RSW process parameters were optimized for two grades of 3G-AHSS, referred to as 3G-980 and 3G-1180, via the development of a weldability lobe, and performing traditional TS and CT mechanical tests for various RSW nugget diameters while following the welding schedule recommended by AWS D8.9 standard. Thereafter, the mechanical performance of optimized and sub-optimal 3G-AHSS spot welds were characterized under various combinations of shear/tension loading ratios as well as different combinations of tension-bending loading modes. The rotation and slippage of combined loading specimens within the testing fixtures posed a challenge leading to overestimation of spot weld performance indices, such as failure load components and absorbed energy during failure. These challenges were overcome via viii quantification of rotation during mechanical tests and proposing novel post-processing methodologies that approximate local nugget displacement fields by coupling tests with stereoscopic digital image correlation (DIC) techniques. Upon attainment of critical load components and moments at various shear-tension and tension-bending loading modes, the accuracy of various force-based RSW failure criteria was evaluated independently. It was shown experimentally that while the commonly used force-based RSW failure criteria, proposed by Seeger, is fairly accurate in shear-tension loading mode, it loses accuracy by a relatively large margin in determining critical bending moments the spot welds withstand at the onset of failure. Alternative mathematical functional forms of RSW failure loci were proposed that can be readily implemented in finite element analysis for the potential improvement of 3G-AHSS RSW failure predictions. Calculations related to quantifying the energy absorption capability of the joints showed that brittle propagation of cracks into the columnar structure of fusion zone (FZ), leading to partial interfacial- partial pullout failure, significantly limits the post-failure energy absorption capability of the investigated joints in both shear-tension and tensile-bending loading conditions. The understanding of single spot weld characterization techniques were expanded to weld group (component) tests that evaluate the mechanical performance and failure characteristics as groups of spot welds separate under tensile-bending loading conditions. It was shown that the energy absorption capability of groups of spot welds is a function of the extent to which the base materials involved in the tests dissipate energy by plastically deforming throughout the tests, as well as the failure mode of the spot welds. The components made of the more ductile 3G-980 material exhibited superior energy absorption capability due to a higher degree of parent metal deformation and ductile pullout failure mode compared with the less parent metal plastic work and partial pullout failure of components from 3G-1180 material. This research program is comprised of various sections including the 3G-AHSS RSW process optimization, detailed microstructural characterizations of optimized joints, mechanical performance and failure characterization of the joints under combined shear-tensile loading using KS-II tests, tensile-bending loading using various geometries of CP test, weld-group tests, and novel post-processing techniques used for improving the accuracy of force-based RSW failure criteria, which were the key takeaways of this research.