Resistance Spot Weldability of Three Metal Stack Dual Phase 600 Hot-dipped Galvanized Steel 2007-01-1363

Fuel economy and federal safety regulations are driving automotive companies to use Dual Phase and other Advanced High Strength Steels (AHSS) in vehicle body structures. Joining and assembly plays a crucial role in the selection of these steels. Specifications are available for the resistance spot welding (RSW) of lower strength sheet steels, covering many aspects of the welding process from the stabilization procedure to endurance testing. Currently, specifications in the automotive industry for RSW with AHSS are limited. It is well known that welding of a thickness ratio greater than 1:2 poses a challenge. To utilize thinner gauge AHSS panels on body-in-white, welding schedules to join the thin to thick sheet steel stack-up are needed. Most of the existing published work was conducted on uncoated sheets and welded to the same thickness. This project was initiated to understand the RSW of a three-metal stack-up using AHSS - Dual Phase 600 (DP600) steel with a hot dip galvanized coating. As the use of welded joints involving thin to thick AHSS sheets become more common for vehicle body structures, the possibility of an extreme case of sheet thickness differential involving 1.9 and 0.8 mm gauges, both galvanized, is more likely. This study, through a Design of Experiments (DOE), quantified the effects of factors that included welding equipment (AC vs. MFDC), electrode shape (dome, 45º truncated and ISO 30º truncated), hold time, pulsing and weld current on joint strength (lap-shear and coach peel) for a three-metal thickness stack-up (0.8/1.9/1.9 mm). During the study a modified electrode stabilization procedure was developed and the hold-time sensitivity behavior at optimum button size was investigated. The current range, lap-shear tensile load, coach peel load and electrode life values were compared for each electrode type. The study was successful in identifying that the dome shaped electrode stabilizes faster, produces more consistent welds, and consumes less current, primarily due to its higher current density capability. The amount of weld current needed by the truncated electrodes was influenced by the electrode face angle. The dome shaped electrode produced a higher current range than the truncated cone electrodes. The dome electrode always produced the largest current range with the AC power source; no effect of the MFDC power source on the current range was observed. The number of pulses, or cool time between pulses, did not influence the current range. No hold time sensitivity was seen during the testing at 5 and 90 cycles hold time. A minimum of 2000 lbf (∼907 kgf) lap-shear tensile and 200 lbf coach peel loads were obtained with the dome electrode. Although the truncated cone electrodes produced an average load similar to the dome electrode, the results were more scattered. The endurance testing showed that the 45º truncated cone electrode provided the highest number of welds (∼1500), with the dome electrodes producing ∼1000 welds. Increasing weld current gradually and monitoring the voltage/resistance across the electrode was most effective in stabilizing the resistance spot welding electrodes. Overall, the dome electrode performed better than the truncated cone electrodes in welding a 0.8-1.9-1.9 mm DP600 HDG weld stack-up. The dome electrode is suggested for resistance spot welding of stack-ups with high thickness ratios.