Influence of selected alloying elements and starting microstructure on Zn-assisted liquid metal embrittlement susceptibility of advanced high strength steels

Abstract

The rate of implementation of zinc (Zn) coated third generation advanced high strength steels (AHSS) is being reduced due to the susceptibility of these steels to Zn-assisted liquid metal embrittlement (LME), which leads to surface cracking during resistance spot welding. LME in this application is manifested by an instantaneous “loss” in the ductility (or toughness) of Zn-coated steels at elevated temperatures above the melting temperature of Zn (419.5 °C), triggered by the simultaneous action of liquid Zn and tensile stress. The premature failure of Zn coated steels is usually activated by “intergranular” liquid Zn penetration into a steel substrate. Classical LME literature recognizes that strong metals are, in general, more sensitive to LME, and this trend has also been noted in the case of Zn-assisted LME of steels, i.e. AHSS are considerably more LME-sensitive compared to conventional (lower strength) mild steel alloys. However, it is not clear whether strength is the “primary” factor related to LME sensitivity, since strength is dependent on both the microstructure and chemical composition. AHSS have complex multiphase microstructures and rich alloy compositions, and it is possible that both of these variables affect LME susceptibility through “independent” mechanisms. Therefore, there was a need to decouple the effects of strength, microstructure, and alloy content, and experiments were developed to examine these effects and better understand the mechanism and factors controlling Zn-assisted LME in AHSS.

In the first phase of this research, the LME characteristics were compared for two Zn coated AHSS having comparable strength (1 GPa tensile strength) but different alloy compositions and microstructures: a dual-phase “high ductility” (DH1000) steel and a conventional dual-phase (DP1000) steel. The DH1000 is a “third-generation” AHSS, having a transformation-induced plasticity (TRIP)-assisted bainitic ferrite (TBF) type microstructure, and alloyed with a high Si content (approximately 1 wt. %) to enhance the retained austenite fraction in the steel. On the other hand, the DP1000 is a “first-generation” AHSS having a ferrite martensite dual-phase microstructure, and produced with a lean (approximately 0.05 wt. %) Si concentration. Both the DH1000 and DP1000 steels exhibit similar tensile strengths at room temperature and particularly at elevated temperatures (when LME cracks are formed in the AHSS substrate during deformation). Hot tension tests were performed in a Gleeble® 3500 thermomechanical simulator using simulated spot weld thermal cycles to determine the critical ranges of temperatures and strain rates capable of triggering LME in these AHSS. Despite the fact that the two investigated AHSS, i.e. DH1000 and DP1000 had similar strengths, distinct differences were observed in their LME-associated ductility trough characteristics and LME cracking severity. Therefore, this comparison helps isolate the contributions of factors other than strength on Zn-LME susceptibility, such as chemical composition or microstructure.

In the second phase of research, high temperature tension tests were performed to investigate the specific influence of AHSS starting microstructure on Zn-LME susceptibility for a fixed chemical composition. A cold-rolled C–Mn–Si steel alloy was heat treated via continuous-annealing simulations in a controlled atmosphere to generate four different AHSS starting microstructures: martensitic, quenched and partitioned (Q&P), TBF, and dual phase (DP). The annealed panels were electroplated with Zn prior to Gleeble® hot tension tests. The hot tension test data revealed comparable Zn-LME susceptibility for different martensite and bainitic-ferrite based microstructures generated through full austenitization, such as martensitic, Q&P, and TBF. Electron backscatter diffraction (EBSD) analysis indicated that LME cracks propagated through these microstructures via intergranular fracture along prior austenite grain boundaries, with no apparent involvement of microstructural components like retained austenite or martensite and/or carbide-free bainite lath or packet boundaries in the LME cracking behavior. In contrast, the DP microstructure variant of the same steel, generated through intercritical annealing (partial austenitization), exhibited somewhat suppressed LME, revealing that the starting microstructure can influence Zn-LME susceptibility. The lower LME susceptibility of the DP steel compared to the martensitic, Q&P, or TBF steels is explained by the presence of ultrafine ferrite grains and discrete martensite islands, and a smaller area fraction of prior austenite grain boundaries in the DP microstructure. LME cracking involved crack propagation both along prior austenite boundaries as well as transgranularly n through ferrite.

In the third phase of research, the specific influence of chemical composition on the Zn assisted LME susceptibility of AHSS alloys was assessed. Q&P heat treated panels of AHSS alloys of varying C, Mn, Si, and Al contents were subjected to hot tension testing in a Gleeble® 3500 simulator. The LME behavior of each AHSS variant was characterized from 600 to 900 °C, a temperature range established through SYSWELD® simulation to be extremely relevant in the context of spot weld LME behavior. The Base (0.25C-2.7Mn-1.5Si-0.05Al, all in wt. %), Low-C (0.15C-2.7Mn-1.5Si-0.05Al), and Low-Mn (0.25C-2.0Mn-1.5Si-0.05Al) steels, each alloyed with 1.5 wt. % Si, exhibited the highest LME susceptibility among all the investigated AHSS. The Low-Si AHSS (0.25C-2.7Mn-0.5Si-0.05Al) was the least LME sensitive of all investigated AHSS, while the Low-Si High-Al AHSS (0.25C 2.7Mn-0.5Si-1.3Al) exhibited low to moderate LME susceptibility. Thus, C or Mn variation relative to the base alloy composition did not remarkably influence LME susceptibility. On the other hand, the LME sensitivity correlated strongly with the Si content of the AHSS. In depth characterization of cracking behavior in relation to the microstructure and elemental distribution in the coating and coating-substrate interface layers was performed to study the mechanism by which Si influences LME behavior of AHSS. Scanning (SEM) and transmission electron microscopic (TEM) examination of the microstructure of the Fe/Zn alloyed coating layer in AHSS samples with high and low Si contents, revealed that Si in the AHSS substrate suppresses Fe-Zn alloying reactions and retards the nucleation and growth of intermetallic phases at the coating-substrate interface. Elemental characterization using scanning-TEM energy dispersive spectroscopy (STEM-EDS) and time-of-flight secondary-ion-mass spectroscopy (TOF-SIMS) indicated the occurrence of Si enrichment in the substrate and coating-substrate interface layers of galvanized samples during thermal cycling at elevated temperatures. The mechanism of Si-enrichment is associated with low solubility of Si in liquid zinc, and was understood through thermodynamic assessments of the Fe-Zn-Si ternary system and DICTRA® numerical simulations of the interaction between liquid and Si-alloyed AHSS substrate. Thereafter, the large difference in the LME sensitivity of High-Si and Low-Si AHSS was explained in the context of this Si enrichment behavior. The presence of Si in large concentrations in the AHSS substrate reduces the driving force for Fe dissolution into the liquid, diminishes the Zn solubility (or increases in the activity of Zn) in the AHSS matrix, and destabilizes intermetallic phases at the coating/substrate interface. These effects lead to a greater overall availability of liquid Zn for embrittlement and an increased likelihood of direct contact between liquid Zn and the steel substrate for LME to be activated. Finally, Si-enrichment of the substrate by liquid-steel interaction increases the (local) strength of the substrate layers in contact with liquid Zn, which leads to a much greater driving force for “stress-assisted” liquid penetration along the grain boundaries of the steel substrate.