For several decades, the automotive industry has made use of progressively more sophisticated high-strength sheet steels for body-in-white manufacturing. Using such steels has significantly helped reduce vehicle mass and improve crash performance. Unfortunately, these material developments have also been associated with a range of cracking behaviors in resistance spot welds. Spot weld cracking has historically been related to the weld's stress state, the microstructure's fracture toughness, and the presence of any preferential failure paths. In the face of that challenge, EWI had recently developed the H-Coupon test, a quantitative cracking susceptibility technique for sheet metal. The H-Coupon test essentially employs a small frame (spot welded together) that allows testing of candidate materials under varying levels of constraint, leading to quantifiable plastic strains. In the present work, different advanced high-strength steels were evaluated. Results showed that the test could successfully quantitatively compare and rank different advanced high-strength steels candidates in terms of cracking susceptibility. Such achievement shows the potential for the H-Coupon technique in aiding steel manufactures in the development of new high-strength sheet steels for body-in-white manufacturing.
All welding was performed using a standard pedestal-type system equipped with an MFDC power supply. The system was configured with the necessary electrodes for welding. In addition, and the system was instrumented to monitor welding currents, forces, and displacements. Three different GEN3 steels were used in this study: two galvanized (780 MPa and 1180 MPa) and one uncoated (1180 MPa). Initially, for each steel, baseline spot welding conditions were established for nominal 6√t button-size welds without any gap. The H-coupon is assembled completely from components sheared from the material(s) of interest. The test coupon uses multiples of the parent metal thickness to create a controlled separation between the test materials. These components include separation spacers, positioning straps, and the test material. Strain on the sheets prior to welding is then calculated assuming a nominally arc profiled setdown as the force is applied. Welds were then made using the H-coupons with increasing gaps (defined by the numbers of spacers). For each gauge of material, closure forces were established for each gap level of interest. Resulting welds were alternately peel tested and subject to nugget area measurement (using a Keyance microscope) or cross-sectional analysis. Cross-sectional samples were prepared by standard metallographic procedures.
Results and Discussion
Using the peel test results, the button-to-nugget diameter ratio was quantified. Results from all steels show a complete transition from a full button peel for flat coupons to an interfacial failure at nearly 6% augmented strain. Of note, numerous secondary cracks can also be seen on higher levels of strain. Corresponding metallographic sections shows that welding without augmented strain did not cause any LME cracking. However, higher strain levels (5.9%) resulted in a range of LME cracks. These included cracks located on the weld surface below the electrode surface; and cracks located on the weld surface at the indentation shoulder. Cracking susceptibility from the H-coupon results is presented as button-to-nugget ratio vs. augmented strain plots. Not surprisingly, the uncoated steel shows significantly higher button-to-nugget ratios than either of the galvanized grades. Metallographic results show that this difference is related to zinc related liquation cracking. Of interest, the two galvanized GEN3 steels performed nearly identically during H-coupon testing.
Cracking sensitivity assessments for a range of GEN3 advanced high strength steels have been examined using a new test methodology developed at EWI. The test is based on an H-coupon configuration and creates strain on the workpiece by welding into a defined gap. In these studies, the test was used to compare the performance of both bare and galvanized GEN3 steels. H-coupon testing easily discriminated between bare and galvanized variants of this grade, with the differences clearly related to zinc-LME cracking. Strength of the galvanized variant had little effect on the observed H-coupon cracking results. Achieved strain had more apparent impact on the potential for LME cracking than strength did. Overall, H-coupon testing has shown significant success in evaluating LME cracking for a different range of steels used in automotive industry. The test is conducted simply by using a resistance spot-welding system and the material of interest and produces a fast, reduced-cost, LME assessment procedure. Such can be used as a powerful weldability evaluation technique to support steel makers into the development of new advanced high strengths steels (AHSS).