Investigation of Liquid Metal Embrittlement in Advanced High Strength Steels
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Date
2020-04-17
Authors
He, Liu
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Publisher
University of Waterloo
Abstract
Third-generation advanced high strength steels (AHSS) are typically given a zinc coating that provides excellent resistance to corrosion. During the resistance spot welding (RSW) process, the melted zinc coating enables liquid metal embrittlement (LME) where the liquid zinc, acting as an embrittling agent, induces cracking in the weld indent, compromising weld strength. This work investigates the various factors that influence LME in AHSS and provides a viable solution to suppress LME.
Hot tensile testing was first used to evalute the LME susceptiblility of the studied steels. It was discovered that the austenite content of the steels’ microstructure, Si content in the steels’ chemistry and the type of Zn coating all influence the behavior of the ductility trough of the examined steels. As the austenite content of the steel increased, the ductility loss caused by LME increased as well. Approximately 18 vol.% to 31 vol.% austenite is the minimum amount required to trigger the rise in ductility loss of all the studied steels. In addition, steels containing a low Si content are more likely to form a layer of Fe-Zn intermetallic that acts as a barrier to suppresses LME at temperatures below 670°C. It was also discovered that the GA coated steels are far less susceptible to LME than their GI coated counterparts due to it being thinner and containing 25 wt.% Fe in its coating.
A mathematical model capable of estimating the crack index within the weld lobe of each material was also developed through resistance spot welding. The model showed that the weld lobe of materials where not equally affected by LME. Furthermore, it identified regions within the weld lobe where welds of sufficient size could be made while minimizing LME cracks.
Using the hot tensile testing data and the results from RSW, LME susceptibility of the studied steels are ranked. QP1180GA is the most LME susceptible steel while DP980GA is the least. The ductility loss obtained via hot tensile testing shows good correlation with the intensity of LME cracks found in resistance spot welds.
Finally, LME was suppressed in AHSS by placing aluminum interlayers added between the electrode and steel contact surface. Compared to welds exhibiting LME, TRIP 1100 with aluminum interlayers showed complete strength recovery while TRIP 1200 with aluminum interlayers resulted in a recovery of strength by 90%. Aluminum interlayers suppress LME of TRIP steel by formation of iron aluminides that hinder liquid zinc from coming in contact with the steel substrate, thus preventing LME.
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Keywords
advanced high strength steel, liquid metal embrittlement, resistance spot welding, hot tensile testing