Abstract Zinc Penetration and Grain Boundary Embrittlement in QP980 Welding
Liquid Metal Embrittlement (LME) defects in high-strength steels coated with zinc are closely associated with the formation of Fe-Zn intermetallic compounds (IMCs). The relationship between LME-induced crack propagation and Fe-Zn intermetallic phases during the resistance spot welding (RSW) process of galvanized QP980 steel was specifically investigated. The results showed that LME cracks in the spot-welded joints of galvanized QP980 steel mainly occurred at the weld center (Type I cracks) and at the weld shoulders (Type II cracks). The longest Type I crack was approximately 1014.8 µm, and the longest Type II crack was 95.7 µm. Furthermore, the microstructural details and elemental distribution near the LME-induced cracks were analyzed. Fe-Zn intermetallic compounds were identified at grain boundaries, serving as sites for crack initiation. In addition, the brittle intermetallic compounds can weaken the grain boundaries of the steel substrate, resulting in the initiation and propagation of further LME-induced cracks.
Introduction: Zinc Penetration in QP980 Steel Welding
The advanced high-strength steel (AHSS) QP980 is a promising material for vehicle demands aimed at enhancing safety, environmental compatibility, and energy consumption, known for simultaneously providing high strength and low weight. In general, high-strength steel sheets with zinc-based coatings on both sides are used to resist corrosion. Resistance spot welding (RSW), with its advantages of high efficiency, reliability, and stability, is a crucial process in modern automotive manufacturing, widely used on assembly lines. However, LME-induced cracks can occur in galvanized QP980 steel during resistance spot welding, which may severely degrade the mechanical properties of these steels.
Resistance Spot Welding and LME Challenges
Resistance spot welding is a complex process with rapid heating and cooling cycles. Welding parameters, including pulses, current, and pulse patterns, can affect microstructural evolution, residual stress conditions, and the mechanical performance of the joint, particularly the formation of LME. Studies have shown that weld microstructure from single and pulse welds was fully martensitic with no significant microhardness changes. Finer austenite grain structures and martensite packets were formed in the coarse-grained heat-affected zones (HAZs) of dual-pulse welds compared to single-pulse welds. The higher cross-tensile strength and energy absorption capability of dual-pulse welds could be attributed to lower residual stress around the joint. Weld edges and a significant portion of high-angle grain boundaries in coarse-grained HAZs act as barriers to crack initiation and propagation. Multi-pulse welding current schemes can effectively reduce LME, but LME can only be slightly mitigated by adjusting welding parameters, and its initiation remains a contentious issue.
Zinc Penetration and Grain Boundary Interaction
The zinc coating, with a melting point around 420°C, melts easily while the steel substrate remains solid, leaving zinc in a liquid state. In this condition, molten zinc contacts the steel substrate and, under certain conditions, can penetrate into the solid metal, subsequently causing LME-induced cracks. Various studies have shown that intergranular crack propagation in high-strength steels is a primary feature of LME cracks caused by zinc presence. LME cracking is associated with solid-state diffusion of zinc atoms into grain boundaries, contributing to grain boundary sensitization. Zinc has a strong tendency to segregate at grain boundaries, negatively impacting grain boundary cohesion. Similarly, zinc wettability can significantly destabilize grain boundaries, indicating the role of grain boundary characteristics in LME crack formation and propagation. Moreover, liquid zinc can react with the steel substrate, forming Fe-Zn intermetallic compounds, including ξ (FeZn13), δ (FeZn10 or FeZn7), Γ1 (Fe5Zn21 or FeZn4), and Γ (Fe3Zn10 or FeZn3). LME crack formation in galvanized AHSS is driven by nucleation and growth of Fe-Zn intermetallic phases. In short, zinc penetration into grain boundaries can promote LME crack formation.
Experimental Analysis of Zinc Penetration in QP980 Steel
In this study, the behavior of zinc penetration and its interaction with grain boundaries during resistance spot welding of QP980 steel was analyzed. The chemical composition of QP980 steel, obtained using a spectrometer (Thermo Fisher, ARL4460), is shown in Table 1. The welding current used was an 8-step progressive pulse, starting at 5 kA and ending at 12 kA, with each pulse lasting 130 ms and a cooling time of 40 ms. The electrode force was set to 4.0 kN. The squeeze and hold times were 1000 ms and 250 ms, respectively. Thermal simulations were performed using SORPAS® software to illustrate the temperature and stress distribution in the welded joints. Welds were sectioned along the centerline to observe the location and length of LME cracks using an optical microscope. For detailed analysis of LME crack initiation, samples were prepared using focused ion beam (FIB) machining for high-resolution transmission electron microscopy (HRTEM).
Table 1. Chemical Composition of QP980 Steel Used in This Study
Element | C | Si | Mn | P | S | Fe |
wt.% | 21/0 | 8/1 | 0/2 | 015/0 | 001/0 | مابقی |
Characteristics of LME Cracks in QP980 Welds
To evaluate the characteristics of LME cracks, their location and length in the weld cross-section were determined. Two types of LME cracks were revealed from metallographic images. Type I cracks appeared at the weld center, while Type II cracks were found at the weld shoulder. The longest Type I crack was approximately 1014.8 µm, and the longest Type II crack was 95.7 µm. LME cracks longer than 500 µm can significantly reduce the mechanical properties of TRIP steel spot welds, including cross-tensile strength, energy absorption, and fatigue life. Hence, the considerable length of the Type I crack may significantly compromise the load-bearing capacity of the QP980 weld joint.
Microstructural Analysis of LME Crack Initiation
Further investigation of LME crack initiation revealed that LME cracks initiate at the surface and propagate nearly perpendicular to the tensile stress applied by the electrodes. Zinc-enriched grain boundaries were identifiable due to the higher contrast of zinc in BSE mode. FIB machining was used to prepare sections for electron microscopy. STEM images and elemental distribution maps showed that zinc was distributed along grain boundaries. Molten zinc penetrated the boundaries, and Fe-Zn interactions led to zinc accumulation at grain boundaries. Thus, identifying the nature of these boundaries is crucial for understanding LME crack formation and propagation mechanisms.
HRTEM Analysis of Zinc-Enriched Grain Boundaries
HRTEM was used to further analyze the zinc-enriched grain boundaries. The width of zinc-enriched grain boundaries was approximately 200 nm—much wider than typical metallic grain boundaries (~1 nm). Molten zinc diffusion can degrade and widen these boundaries. Additionally, areas were identified as the Γ-Fe3Zn10 phase, which is intrinsically hard and brittle, making grain boundaries filled with these brittle phases prone to failure under external stress.
Role of Residual Stress in LME Crack Formation
Residual stress plays a key role in LME crack formation. External tensile loading or internal tensile (residual) stress is required for LME crack formation, whereas compressive residual stress can resist crack initiation and propagation. Residual stress is unavoidably introduced due to thermal gradients and non-uniform plastic deformation during component manufacturing and service. Compared to residual stress, external stress has a greater impact on LME cracking. Thermal cycles and stress history of the welded joint were simulated using SORPAS software. Tensile stress was detected in weld locations, with only tensile stress observed in some positions during welding and holding, while both tensile and compressive stresses were observed in others.
At high tensile strain rates, insufficient time exists for Fe-Zn alloying reactions, leaving the steel substrate exposed to molten zinc, thus promoting LME. Clearly, tensile stress is a necessary condition for LME, and the brittle Fe-Zn intermetallic phases are vital mechanisms in LME crack formation.
Mechanism of Molten Zinc Penetration
QP980 steel contains coarse ferrite and an oxide layer formed after annealing. This pre-galvanizing microstructure can provide grain boundary paths for molten zinc penetration. During RSW, high current from copper electrodes generates significant heat due to material resistance, melting the lower-melting-point zinc coating. Under these conditions, molten zinc can react to form Fe-Zn alloy phases. Furthermore, molten zinc can penetrate grain boundaries in the steel substrate in certain locations, weakening boundary cohesion. The tensile strength and ductility of grain boundaries can be significantly reduced after zinc penetration.
author: Wufeng Dong a, Hua Pan b,c, Ming Lei b,c, Kai Ding a,*, Yulai Gao
