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Gas Metal Arc Welding (GMAW) is widely employed for joining relatively thick sheet steels in automotive body-in-white structures and frames. The GMAW process is very flexible for various joint geometries and has relatively high welding speed. However, fatigue failures can occur at welded joints subjected to various types of loads. Thus, vehicle design engineers need to understand the fatigue characteristics of welded joints produced by GMAW. Currently, automotive structures employ various advanced high strength steels (AHSS) such as dual-phase (DP) and transformation-induced plasticity (TRIP) steels to produce lighter vehicle structures with improved safety performance and fuel economy, and reduced harmful emissions. Relatively thick gages of AHSS are commonly joined to conventional high strength steels and/or mild steels using GMAW in current body-in-white structures and frames.

In the North American automotive industry, various advanced high strength steels (AHSS) are used to lighten vehicle structures, improve safety performance and fuel economy, and reduce harmful emissions. Relatively thick gages of AHSS are commonly joined to conventional high strength steels and/or mild steels using Gas Metal Arc Welding (GMAW) in the current generation body-in-white structures. Additionally, fatigue failures are most likely to occur at joints subjected to a variety of different loadings. It is therefore critical that automotive engineers need to understand the fatigue characteristics of welded joints. The Sheet Steel Fatigue Committee of the Auto/Steel Partnership (A/S-P) completed a comprehensive fatigue study on GMAW joints of both AHSS and conventional sheet steels including: DP590 GA, SAE 1008, HSLA HR 420, DP 600 HR, Boron, DQSK, TRIP 780 GI, and DP780 GI steels.

In an attempt to develop gas-metal arc-welded (GMAW) sheet steel coupons with tightly controlled weld geometry for fatigue testing, it was discovered that the very slight changes in welding process parameters strongly influence the weld geometry. The extent of fusion zone, horizontal and vertical leg lengths, and depth of penetration were considered to define the weld geometry. In order to elucidate the sensitivity of weld geometry to process parameters, a study was conducted on welding of various types of specimens. The paper reports the results of variations in weld geometry with small changes in process parameters. In addition, through several examples, it is demonstrated that variability in the weld geometry is unavoidable even with very tight weld process control.

In this study fatigue tests of GMAW (Gas Metal Arc Welding) welded joints were conducted on both 1.6mm body sheet (DQSK-GA, DP590-GA, DP780-GI, and TRIP 780-GI) and 3.4mm frame materials (SAE1008 HR 240MPa, HSLA420 HR, DP600 HR, and uncoated Boron). Further, mixed thickness joints were tested which combined 3.4mm SAE1008 HR with each of the 1.6mm separately – with the exception of DQSK. A number of different joint configurations were tested including single and double lap-shear, start-stop lap shear, butt weld, and perch mount. Great care was taken in this study to ensure that the geometry of the welds was consistent, not only within a given material lay-up, but between all of the specimens of a given type – this effort was made in order to substantially reduce life scatter and provide a better understanding of the role base material plays in the fatigue life of GMAW joints.

Spot welding is the primary method of joining sheet metal for body and structural applications in the ground vehicle industry. A typical automobile may contain over 5000 spot welds. The fatigue failure of spot welded joints results in the degradation of both structural and Noise, Vibration, and Harshness (NVH) performance. Therefore, designers need reliable information about the total fatigue life of spot welded joints early on in the design phase. Currently, automotive structures are employing ever increasing amounts of Advanced High Strength Steels (AHSS) including dual-phase steels. As a result, automotive designers require fatigue strength information on AHSS spot welds. The Auto/Steel Partnership (A/SP) has conducted fatigue tests with lap shear and coach peel specimens made of AHSS, HSLA and low carbon steels. Overall, the test data showed good correlation with the applied load range for all of the materials, regardless of base material strength.

The potential of hot-stamped boron steel in aiding improved durability coupled with mass reduction is explored through two case studies involving a lightweight truck frame and a shock tower assembly. Through a computational approach, analyses are performed using service loading conditions. The stress fields calculated through the analyses are used to predict fatigue life cycles. It is shown that the advanced steel grades such as Boron can offer superior resistance to fatigue loading conditions while offering an opportunity to reduce the weight of the component.

Because of increasing fuel costs and environmental concerns, the automotive industry is under enormous pressure to reduce vehicle weight. One strategy, downgaging, substitutes a reduced gage (thickness) steel in place of a thicker one, and is usually accompanied by a material grade change to a higher strength steel. Thus, Advanced High Strength Steels (AHSS) are increasingly used for lightweight automotive body structures. The critical durability concern with steels is the spot welds used to join them, since fatigue cracks in body structures preferentially initiate at spot welds. Hence, the Auto/Steel Partnership (A/SP) Sheet Steel Fatigue Taskforce undertook an investigation both to study the fatigue performance of AHSS spot welds, and to generate data for OEM durability analysis. The study included seven AHSS grades and, for comparison, mild steels and a conventional High Strength Low Alloy grade, HSLA340.

The potential of advanced high strength steels such as TRIP versus conventional HSLA350 steel in achieving improved durability with weight savings is explored in the case of a shock tower assembly. Through a computational approach a linear analysis and a nonlinear analysis are performed. The nonlinear analysis takes into account the effect of geometry, material and contact nonlinearities. The results from both the linear and nonlinear analysis are used to predict fatigue lives. It is shown that the advanced steel grades, in particular, TRIP steels, can offer superior resistance to fatigue loading conditions while offering an opportunity to reduce the weight of the component.