Liquid Metal Embrittlement in Resistance Spot Dissimilar Welds on Advanced High Strength Steels: Microstructure and Fracture Characteristic
Keywords:Liquid Metal Embrittlement
Advanced High Strength Steels
Resistance Spot Welding of Dissimilar Metals
Advanced High Strength Steels & Zn-Galvanized Steel
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Publisher:The Ohio State University
Series/Report no.:The Ohio State University. Department of Materials Science and Engineering Undergraduate Research Theses; 2018
Advanced High Strength Steels (AHSS) are being utilized in automotive production in an effort for body-in- white weight reduction while being able to attain an increase in structural strength. During the Resistance Spot Welding (RSW) of these AHSS in dissimilar metal combinations such as with Zinc (Zn) coated low alloy steels, it is believed that liquid Zn penetrates along the grain boundaries in the Heat Affected Zone (HAZ), resulting in the embrittlement of the AHSS and subsequent cracking, a phenomenon known as Liquid Metal Embrittlement (LME). Although widely studied, the mechanisms behind LME are still not well understood. The objectives of this research are to (1) reproduce LME induced cracking in a laboratory environment, (2) develop procedures for the metallographic examination of the cracks, and (3) characterize the cracking morphology (e.g. crack location, crack path, fracture surface) to identify the weld microstructures susceptible to LME. The resistance spot welding process will be varied in terms of welding parameters and the application of additional stresses during welding to introduce LME induced cracking. Metallographic evaluation includes light optical microscopy and scanning electron microscopy. The testing involved three different AHSS including two Dual Phase (DP) steels (DP590, DP980) and a Press-Hardened Boron Steel (PHBS) Usibor-1500 in dissimilar metal combination with a Zn-galvanized low alloy steel. Results revealed that varying weld parameters alone were unsuccessful at reproducing LME cracking in the laboratory setting. With the introduction of external tensile strains using an offset electrode setup, cracking was achieved in the HAZ of all three base metals. Although not enough samples were produced so far to conclusively determine which microstructures are most susceptible to LME cracking, the results agreed with previous research on LME, such that cracking occurred in the HAZ of the welds with an intergranular crack path.
2nd Place AWS Fabtech Poster Competition
Academic Major: Welding Engineering
The Ohio State College of Engineering Research Scholarship Program
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