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Joining technology is a key factor to utilize dissimilar materials in vehicle structures. Adaptable insert weld (AIW) technology is developed to join sheet steel (HSLA350) to cast magnesium alloy (AM60) and is constructed by combining riveting technology and electrical resistance spot welding technology. In this project, the AIW joint technology is applied to construct front shock tower structures composed with HSLA350, AM60, and Al6082 and a method is developed to predict the fatigue life of the AIW joints. Lap-shear and cross-tension specimens were constructed and tested to develop the fatigue parameters (load-life curves) of AIW joint. Two FEA modeling techniques for AIW joints were used to model the specimen geometry. These modeling approaches are area contact method (ACM) and TIE contact method.

Friction stir linear welding (FSLW) is widely used in joining lightweight materials including aluminum alloys and magnesium alloys. However, fatigue life prediction method for FSLW is not well developed yet for vehicle structure applications. This paper is tried to use two different methods for the prediction of fatigue life of FSLW in vehicle structures. FSLW is represented with 2-D shell elements for the structural stress approach and is represented with TIE contact for the maximum principal stress approach in finite element (FE) models. S-N curves were developed from coupon specimen test results for both the approaches. These S-N curves were used to predict fatigue life of FSLW of a front shock tower structure that was constructed by joining AM60 to AZ31 and AM60 to AM30. The fatigue life prediction results were then correlated with test results of the front shock tower structures.

Due to magnesium alloy's poor weldability, other joining techniques such as laser assisted self-piercing rivet (LSPR) are used for joining magnesium alloys. This research investigates the fatigue performance of LSPR for magnesium alloys including AZ31 and AM60. Tensile-shear and coach peel specimens for AZ31 and AM60 were fabricated and tested for understanding joint fatigue performance. A structural stress - life (S-N) method was used to develop the fatigue parameters from load-life test results. In order to validate this approach, test results from multijoint specimens were compared with the predicted fatigue results of these specimens using the structural stress method. The fatigue results predicted using the structural stress method correlate well with the test results.

The fatigue performance of gas metal arc welding (GMAW) joints of advanced high strength steels (AHSS) are compared and analyzed. The steel studied included a number of different grades of AHSS and baseline mild steels: DP600, DP780, DP980, M130, M220, solution annealed boron steel, fully hardened boron steels, HSLA690 and DR210 (a mild steel). Fatigue testing was conducted under a number of nominal stress ranges to obtain the S/N curves of the weld joints. A two-phase analytical model is developed to predict the fatigue performance of AHSS welds. It was found that there are appreciable differences in the fatigue S/N curves among different AHSS joints made using the same welding practices, suggesting that the local microstructure in the weld toe and root region plays non-negligible role in the fatigue performance of AHSS welds.

Typical automotive joints are lap, coach, butt and miter joints. In tubular joining applications, a coach joint is common when one tube is joined to another tube without the use of brackets. Various fusion joining processes are popular in joining coach joints. Common fusion joining processes are Gas Metal Arc Welding (GMAW), Laser and Laser Hybrid, and Gas Tungsten arc welding (GTAW). In this study, fusion welded 2.0 mm uncoated DP780 steel coach joints were investigated. Laser, Gas metal arc welding (GMAW), and laser hybrid (Laser + GMAW) welding processes were selected. Metallurgical properties of the DP780 fusion welds were evaluated using optical microscopy. Static and fatigue tests were conducted on these joints for all three joining processes. It was found that joint fit-up, type of welding process, and process parameters, especially travel speed, have significant impact on static and fatigue performance of the coach joints in this study.

With the increasing demand for safety, energy saving and emission reduction, Advanced High Strength Steels (AHSS) have become very attractive materials for automobile makers. Welding of AHSS remains one of the technical challenges in the successful application of AHSS in automobile structures, especially when durability of the welded structures is required. In this study, 2.0 mm uncoated DP780 was investigated. GMAW welding parameters for lap joints of this steel were developed in order to obtain different weld geometries defined by weld toe angle, weld leg sizes, and weld penetration. Metallurgical properties of the joints were evaluated using optical microscopy and scanning electron microscopy (SEM). Static and fatigue tests were conducted on the welded joints. Effect of weld geometry and HAZ softening on fatigue performance including fatigue life, crack initiation site and propagation path of the joints will be analyzed.

With the increasing demand for safety, energy saving and emission reduction, Advanced High Strength Steels (AHSS) have become very attractive materials for automobile makers. The usage of AHSS materials is projected to grow significantly in the next 5-10 years with new safety and fuel economy regulations. These new materials have significant manufacturing challenges, particularly for welding and stamping. Welding of AHSS remains one of the technical challenges in the successful application of AHSS in automobile structures, especially when durability of the welded structures is required. In this study, 2.0 mm uncoated DP600 and 2.0 mm uncoated boron (heat treated) steel lap joint configuration was investigated. Metallurgical properties of the DP600 to boron steel dissimilar steel lap joints were evaluated using optical microscopy. Static and fatigue tests were conducted on these joints.

Gas Metal Arc Welding (GMAW) is commonly used in the automotive industry for joining heavier gauge mild and High Strength Low Alloy (HSLA) uncoated steels, where it is recognized for its versatility and speed. The only constraints typically encountered relate to fatigue performance of the joint as a result of poor design or manufacturing fit-up. Advanced High Strength Steels (AHSS), now being considered for more and more applications, however, do not offer the same ease of welding and process control is significantly more critical. They differ from mild steels in chemical composition and thermal processing, resulting in a different microstructure; designed with a richer metallurgy to have higher strength at equivalent thickness. As a result, the sensitivity to heat input is greater and the process window in which acceptable welds can be achieved is narrower.

Gas Metal Arc Welding (GMAW) is used in the automotive industry, for joining heavier gauge mild and HSLA steels, where it is recognized for its versatility and speed. The only constraints typically encountered relate to fatigue performance of the joint as a result of poor design or manufacturing fit-up. Advanced High Strength Steels, however, do not offer the same ease of welding, and process control is significantly more critical. The process window represents the range of acceptable process parameters, primarily heat inputs, to achieve an acceptable weld; which is a measure of the robustness of the process. AHSS differ from mild steels in chemical composition and thermal processing, resulting in a different microstructure; designed with a richer chemistry to have higher strength at equivalent thickness compared to mild steels. As a result, the sensitivity to heat input is greater and AHSS has a narrower process window in which acceptable welds can be achieved.

In this study, the Gleeble test was used to investigate the effect of cooling time, which is an indication of welding heat input, on fracture toughness at the simulated HAZ of different test materials, including one mild steel and three DP600 steels from three different suppliers. One of the important findings is that the three DP600 steels have similar tensile properties and similar base metal microstructures. After different simulated welding thermal cycles, however, the microstructure, the microhardness and thus the fracture toughness of the simulated HAZ of the steels showed significant variations among the steels tested, which indicates that DP600 steels from different suppliers can have different responses to the welding heat input.

In this study, effects of resistance spot welding parameters on weld performance, metallurgical features and formation of imperfections in DP600 spot welds were investigated. Optical microscopy, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to characterize the weld. No heat-affected zone (HAZ) softening was found for DP600. Weld nugget microhardness profile results indicated that DP600 welds had no hold time sensitivity. Shrinkage, solidification cracks and liquid metal embrittlement were also found in some of the welds. Factors affecting formation of the weld imperfections were discussed. Similarly to those found in HSLA steels, imperfections in resistance spot welds of DP600 can be minimized or even eliminated using a proper welding schedule.