Investigation of Liquid Metal Embrittlement during Resistance Spot Welding of Martensitic Steel with Zn Jet Vapor-Deposited Coating

Abstract

Advanced high-strength steels protected by zinc coatings have contributed to a reduction in CO2 emissions in the automotive industry. However, the liquid metal embrittlement (LME) of the Fe/Zn couple induced by simultaneously acting stresses and high temperatures during resistance spot welding could be the cause of unexpected failure. We investigated the possible risk of LME in spot-welded martensitic steel with Zn jet vapor-deposited coating and its influence on weld strength. The weld nugget cross-sections were analyzed (optical microscopy, SEM-EDS), and their tensile shear strengths were compared with their uncoated counterparts. LME cracks were observed in all samples meeting the process window (6, 6.5, 7 kA) located at the edge of the sheet/electrode indentation area. The frequency and length of cracks increased with current, and the occurrence of Zn within cracks indicated the LME mechanism. The shear tests showed the Zn-coated sample underwent a decrease in tensile shear strength that was most evident at a welding current of 7 kA (13.2%). However, LME was excluded as a cause of lower strength. The decrease was attributed to the smaller nugget diameter and the thin slit of Zn coating remaining in the weld notch.

Evaluation of resistance spot weldability and weld performance of zinc-coated martensitic steels

“…The liquation crack seems to have assisted the crack propagation during the CTT. Other researchers have reported that steels with austenitic microstructures, such as austenitic stainless steel and TWIP steels, have a liquation crack susceptibility [25,29]. Further, adding alloying elements like Mn and C in the AHSS tends to stabilise the austenite phase and widen the partially melted zone (PMZ), promoting the susceptibility of liquation crack formation.…”