Resistance Spot Welding Evaluation of Transformation Induced Plasticity 780 (TRIP780) Steel for Automotive Body Structural Applications 2009-01-0805

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 steels.

In this paper, the weld current lobes, mechanical properties (shear and cross tension), metallographic cross-section and microhardness profile of Transformation Induced Plasticity 780 (TRIP780) 1.6 mm electrogalvanized steel welded to itself in a two-metal stackup are discussed. Weld lobes were developed with Medium Frequency Direct Current (MFDC) equipment, ISO-type B20 tips, weld force of 5.8 kN and hold time of 5 cycles. The weld times were varied at 19, 21 and 24 cycles, with each producing current ranges at or above 2.0 kA. Shear and cross tension samples were made at 19 cycles, with samples showing average loads of 12.30 kN and 5.11 kN respectively. Microhardness assessment using metallographic cross-sections of 21-cycle welds were used to compare short and long hold times. Similar heat affected zones (HAZ) and weld zones were observed for both hold times.