Resistance Spot Welding (RSW) Evaluation of Electro Galvanized (EG) 1.0 mm Dual Phase 780 (DP780) to Uncoated 1.0 mm Boron Steel for Automotive Body Structure Applications 2010-01-0445

There has been a substantial increase in the use of advanced high strength steel (AHSS) in automotive structures in the last few years. The usage of these materials is projected to grow significantly in the next 5-10 years with the introduction of new safety and fuel economy regulations. AHSS are gaining popularity due to their superior mechanical properties and use in parts for weight savings potential, as compared to mild steels. These new materials pose significant manufacturing challenges, particularly for welding and stamping. Proper understanding of the weldability of these materials is critical for successful application on future vehicle programs. Due to the high strength nature of AHSS materials, higher weld forces and longer weld times are often needed to weld these advanced strength steels.

In this paper, the weld current lobes, mechanical properties (shear tension and cross tension), metallographic cross-section and microhardness profile of Electro Galvanized (EG) 1.0 mm dual phase 780 (DP780) and uncoated 1.0 mm boron steel joint in a two-metal stackup are discussed. Weld lobes were developed with Medium Frequency Direct Current (MFDC) equipment, ISO-type B16 tips, weld force of 3.42 kN and hold time of 5 cycles. The weld times were varied at 12, 15 and 18 cycles, with each producing current ranges at or below 3.0 kA. Tensile shear and cross tension samples were made at weld time of 12 cycles, with samples showing average loads of 11.07 kN and 3.71 kN, respectively. Also, microhardness assessment using metallographic cross-sections were analyzed at three different weld cycles (12, 15, and 18 cycles). Similar heat affected zones (HAZ) and weld zones were observed for three different weld cycles.