Resistance Spot Welding (RSW) Process Optimization for Uncoated 2.0mm Dual Phase 780 to 2.0 mm Uncoated 2.0 mm Dual Phase 780 (DP780) Steel for Automotive Applications

Abstract

There has been a substantial increase in the use of advanced high strength steel 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 new safety and fuel economy regulations. Advanced High Strength Steels (AHSS) are getting popular with superior mechanical properties and weight advantages compared to mild steel materials. These new materials have significant manufacturing challenges, particularly for welding and stamping. Proper understanding of the weldability of these materials is critical for successful application in future vehicle programs. Due to high strength nature of AHSS materials, higher weld forces and longer weld times are needed to weld AHSS materials. This work is in support of lightweight structures development and during the weld development phase various gages of coated and uncoated AHSS materials (DP780, DP980, TRIP780, Boron, Algoma 700B) were investigated. Both 2T and 3T stack-up conditions were investigated. Also, some combination of AHSS materials with High Strength Low Alloy (HSLA) and Bake Hardenable 210 (BH210) electro galvanized (EG) steels were also investigated. In this paper, weld lobe development for 2.0 mm DP780 bare to 2.0 mm DP780 bare 2T stack-up combination is discussed. Weld lobes were developed with Mid Frequency Direct Current (MFDC) equipment, ISO type B-20 tip, constant weld force of 6.36 kN (1430 lbf), hold time of 5 cycles and the weld times were varied 21, 24 and 27 cycles. Based on the tensile, cross-tension and nugget data, there were no correlations were observed between tensile load and button size and also between cross-tension and button size. Weld cross section data indicated heat affected zone (HAZ) at the weld nugget area and hardness drop of 17% was observed at the HAZ area. Irrespective of weld cycles, similar HAZ was observed close to the weld nugget. The weld lobes, mechanical properties (tensile shear and cross tension), cross-section examination, and microhardness of 2.0 mm DP780 bare to 2.0 mm DP780 bare weld 2T stack-up results are discussed.