Zn-coated Steel Research Articles

Evaluation of liquid metal embrittlement-free weld by micro-plasma arc welding of Zn-coated interstitial free steel

The liquid metal embrittlement (LME) severity is a significant concern in the fusion welding of Zn-coated steel. The LME severity in galvannealed (GA) interstitial free (IF) steel processed through microplasma arc welding (MPAW) is investigated. The weld geometry, microstructure and mechanical properties of the 0.8 mm thin butt-welded structure are evaluated to check the welding quality. Optical microscopy and field emission scanning electron microscopy (FESEM) of the weld surface are considered to observe any LME cracks. The high-temperature tensile test is executed on GA-IF steel to simulate external stress-free MPAW conditions. The ultimate tensile strength of the welded joint is more than that of the base material, and it is LME-free, which signifies that the current MPAW system can be successfully used for joining GA-IF steel. The microstructure of the weld zone is having large grain structure and much more hardness than the base material. Even the high-temperature tension test does not show any LME cracks. Approximately, one-third of yield strength is reduced as the temperature rises from 600 °C to 700 °C. Overall, the parametric domain for MPAW of GA IF steel is successfully evaluated, which can produce any LME-free weld.

Effect of Zn-coating type on intergranular Cu-penetration in steels during weld-brazing

Weld-brazing has emerged as a promising alternative to fusion welding for joining Zn-coated advanced high strength steels (AHSSs) due to its lower heat input, reduced Zn burn-off, and elimination of defects like porosity and blowholes. Additionally, the weld-brazing process minimizes HAZ softening, a phenomenon that can affect joint strength. While weld-brazing can potentially eliminate the liquid metal embrittlement (LME) caused by the penetration of molten Zn into grain boundaries (GBs), this study found that galvanized (GI) DP600 steels were more susceptible to intergranular penetration of Cu into the substrate during weld-brazing compared to galvannealed (GA) DP600. Although no Zn-related LME was observed during arc-brazing with Si-Bronze filler, the potential for Cu-related LME cracks to form in GI coated steels may affect the mechanical integrity of the joint.

Towards a dependable TEM characterization of hot-dip galvanized steels with low and high Si content

Designing Advanced High-Strength Steels assumes fine-tuning of alloying elements to enhance performance properties. This affects the kinetics of Fe-Zn interaction during further galvannealing. A key role here belongs to the inhibition layer at the steel/coating interface. Applying mutually-reinforcing TEM techniques, we traced the evolution of Zn-Fe phases in low- and high-Si steels during galvannealing. It has been shown that conventional room temperature Ga FIB for TEM sample preparation is applicable only to annealed low-Si steel, a coating of which comprises equilibrium Zn-Fe compounds. The phases were validated via electron microscopy and diffraction methods. In contrast, even relatively long annealing of the high-Si steel was not sufficient to form stable Zn-Fe phases pointing to a hampering of the interphase reactions. As-galvanized and annealed high-Si specimens contain almost pure Zn and are affected by Ga via a eutectic reaction during FIB preparation. Thus, low-temperature FIB has been applied for reliable Zn-coated steel preparation. We refined with HRTEM the inhibition layer structures specific for low- and high-Si steel grades. While the typical Fe2Al5-xZnx interface layer has been found in the low-Si samples, for high-Si ones the formation of a quite stable Si-Mn mixed oxide membrane, which drastically hinders Fe-Al reaction, has been disclosed.

Effect of gap clearance on the mechanical properties of weld-brazed lap joints

Gas metal arc brazing (GMAB) is a non-fusion joining process used in the automotive industry for joining thin-gauge Zn-coated steel sheets to other steels or dissimilar metals. Unlike torch brazing, GMAB does not rely on capillary action to form a joint. Consequently, gap clearance has not been considered important for lap joining applications. In this study, the effect of gap clearance on the mechanical properties of arc-brazed lap joints was investigated experimentally and verified using finite element modeling (FEM). The results showed that the gap clearance of lap joints played a critical role in controlling the joint strength.

Resistance Spot Welding of Zn-Coated Third Generation Automotive Steels Using Mid-Frequency Direct Current Technology

Resistance Spot Welding of Zn-Coated Third Generation Automotive Steels Using Mid-Frequency Direct Current Technology

Influence of selected alloying variations on liquid metal embrittlement susceptibility of quenched and partitioned steels

In this work, the influence of selected alloying variations on Zn-assisted liquid metal embrittlement (LME) susceptibility of Zn-coated advanced high strength steels (AHSS) is investigated. Cold-rolled AHSS alloys of different carbon (C), manganese (Mn), silicon (Si), and aluminum (Al) concentrations were continuous-annealed to generate a third generation AHSS microstructure (composed of martensite and retained austenite) via quenching and partitioning. High temperature tension tests using simulated spot-weld thermomechanical cycles revealed no significant influence of C and Mn variations on the Zn-LME susceptibility of AHSS. On the other hand, Zn-LME susceptibility was strongly correlated with the Si content of AHSS. A direct comparison of the (reacted) coating microstructures of the Si-alloyed and Low-Si AHSS variants revealed that Si in the AHSS substrate suppresses Fe–Zn alloying reactions and retards the nucleation and growth of Fe-Zn intermetallic phases at the coating-substrate interface in these spot weld simulations. The suppressed intermetallic formation at elevated Si concentrations is consistent with phase equilibria considerations in the Fe-Zn-Si ternary system. In the context of Zn-assisted LME, therefore, Si is hypothesized to aggravate LME behavior by increasing the liquid Zn availability for embrittlement and promoting direct contact between liquid Zn and the AHSS steel substrate.

Role of the internal oxidation layer in the liquid metal embrittlement during the resistance spot welding of the Zn-coated advanced high strength steel

Advanced high strength steel (AHSS) with the Zn coating can produce high susceptibility of liquid metal embrittlement (LME) particularly when the resistance spot welding (RSW) is involved. The internal oxidation layer formed in the subsurface of AHSS is believed to be one of the most important factors to control the Zn-assisted LME susceptibility. The role of the internal oxidation layer in the formation of the LME for the resistance spot welded joint of the Zn-coated AHSS was comprehensively investigated in the present study. The results showed that increasing the dew point of the annealing atmosphere from −40 to −10 °C can successfully produce the internal oxidation layer in the subsurface of the steel. The Zn-coated AHSS without the internal oxidation layer revealed higher LME susceptibility during the RSW compared with the one with internal oxidation layer. In particular, the elements Si and O exhibit a synchronous segregation tendency, forming an internal oxidation layer in the subsurface of the steel. The internal oxidation layer together with the internal oxides distributed at the grain boundaries can decrease the penetration motivation of the liquid Zn, reducing the LME susceptibility.

Effects of semi-solid structure on interface formation of dissimilar aluminum to galvanized steel welds produced by load-controlled Refill Friction Stir Spot Welding

Refill Friction Stir Spot Welding (Refill FSSW) allows welding dissimilar materials providing excellent bonding between both parts. On this work, a multi-scale analysis of load-controlled Refill FSSW was performed to analyze dissimilar AA6016-T4 and Zn-coated DX56D steel joints. These were produced using optimized process parameters and analyzed in both as-welded and bake-hardened conditions. During the process, fusion and subsequent dispersion of Zn favored the formation of a semi-solid structure characterized by an intense microsegregation. Therefore, incipient melting of Zn-rich phase followed by eutectic reaction was observed. The presence of liquid phases along the grain boundaries led to a complex relationship between mechanical properties, microstructure and processing variables. Joints with enhanced mechanical properties were produced by limiting the growth of intermetallic compounds (IMC) at the interface, which coupled with stir zone (SZ) strengthening due to Zn dispersion, led to less pronounceable secondary bending effects. The bake hardening process was also found to have a substantial influence on diffusion-dependent mechanisms and, consequently, on the final performance of the welded joint. The results highlighted a great potential of load-controlled Refill FSSW for producing high-strength dissimilar joints in short cycles, which is desirable for applications in the automotive industry.

Superhydrophobic polyaniline/TiO2 composite coating with enhanced anticorrosion function

The combination of superhydrophobicity and electrical conductivity to enable new functionalities of conductive polymers is of great potential in anticorrosion applications of metal and alloy. In this work, a rough Zn coating was fabricated on carbon steel substrate through an electrodeposition process. The Zn-coated carbon steel substrate was further coated by a modifier that contains polyaniline/TiO2 composite (PTC) and stearic acid (STA), to develop a superhydrophobic and corrosion resistant STA/PTC (SPTC)-Zn coating. The surface morphology study found that there were numerous stamen/pistil-like protrusions on the SPTC-Zn coatings. The results of wettability test show that the SPTC-Zn coating has a water contact angle (WCA) of 159.8° and a water sliding angle (WSA) of 6.2°, which exhibits gratifying superhydrophobicity. Moreover, the superhydrophobic SPTC-Zn coating also displays superior scratching/peeling resistance, abrasion resistance, acid-alkali resistance, corrosion resistance and self-cleaning properties, indicating that the modifier-containing PTC could significantly improve the mechanical and chemical stabilities, antifouling and anticorrosion properties of Zn coatings. It can be inferred that the excellent performance of SPTC-Zn coating was mainly derived from the synergistic effect of the following three aspects: the air cushion stored in its hierarchical micro/nanostructures transforms the traditional solid-liquid contact into solid-gas-liquid separation, the formation of series hydrophobic complexes on the coating surface, and the composite passivation film generated by the oxidation zinc substrate of conductive polyaniline. Based on this new protection mechanism, the obtained superhydrophobic SPTC-Zn coating displays promising potential in practical industries.

Zn penetration and its coupled interaction with the grain boundary during the resistance spot welding of the QP980 steel

The liquid metal embrittlement (LME) defect in the Zn-coated high-strength steels is intimately associated with the formation of Fe-Zn intermetallic compounds (IMCs). The relationship between LME crack propagation and Fe-Zn IMCs during the resistance spot welding of the galvanized QP980 steel was systematically investigated. The results showed that LME cracks were mainly generated in the center of the weld (type I crack) and the shoulders of the weld (type II crack) for the galvanized QP980 resistance spot welded joint. The longest type Ⅰ crack was around 1014.8 and 95.7 μm for type Ⅱ crack. Moreover, the details of the microstructure and element distribution near the LME crack were analyzed. The brittle Fe-Zn IMCs were traced at the grain boundaries to initiate the cracks. In addition, these brittle IMCs could further weaken the grain boundaries of the steel substrate and resultantly promote the formation and further propagation of LME cracks.

Influence of Process Parameters on the Formability of Zn‐Coated Hot Stamping Steel

The cover image illustrates thermal behavior and micro-morphology of Zn-coated hot stamping steel after thermal tensile tests. The stress-strain relationship at elevated temperature, crack morphology, micro-cracks and micro-voids are analyzed in detail. The damage and fracture mechanism and other details can be found in the article number 2100474 by Ma and co-workers.

On the origin of micro-cracking in zinc-coated press hardened steels

Zn-coated press hardened steels (PHS) are in high demand for automotive mass reduction and enhanced passenger safety applications while the Zn coating supplies robust cathodic corrosion protection. However, the mechanism of micro-crack formation during direct hot-press forming (DHPF) has not been adequately described. Thus, the objective of this work was to determine the mechanism for micro-crack formation in Zn-coated DHPF PHS that addressed the relationship between micro-cracking and the coating microstructure created during substrate austenitization. Zn-coated 22MnB5 steel sheets were annealed at 900 °C for annealing times ranging from 30 to 780 s and DHPF at 75 °C s−1 to obtain a fully martensitic substrate microstructure. The inward diffusion between the Zn coating and the substrate during annealing resulted in a dual phase coating microstructure initially comprising Γ-Fe3Zn10 + α-Fe(Zn), transitioning to a single phase α-Fe(Zn) coating after annealing for 240–420 s. Coincident coating α-Fe(Zn) and substrate Zn-enriched austenite (γ-Fe(Zn)) grain boundaries became Zn-enriched, forming a thin layer of α-Fe(Zn) along the γ-Fe(Zn) grain boundaries. It is proposed that coincident coating α-Fe(Zn) and substrate prior austenite grain boundaries (PAGBs) were weakened by this grain boundary α-Fe(Zn) layer. Upon the application of tensile stress, intergranular fracture occurred along the coincident coating α-Fe(Zn) and Zn-enriched PAGBs in the Zn-enriched martensite (M(Zn)) layer. It was further determined that crack propagation ceased and the crack tip was blunted when Zn-enrichment along the PAGBs in the M(Zn) layer was exhausted.

Macro-galvanic corrosion of tower grounding device consisting of graphite and Zn-coated steel in a simulated soil environment

The galvanic corrosion of a tower grounding device consisting of a flexible graphite grounding electrode and Zn-coated steel grounding wire was investigated in a simulated soil environment using corrosion tests and numerical simulation. The significant potential differences between graphite and Zn-coated steel suggest that macro-galvanic corrosion could occur readily in soil, while the dissolution of Zn-coated steel parts was accelerated under the galvanic interaction. The most preferential fracture site resulted from galvanic corrosion of such grounding device was the metallic grounding wire. The conductivity of the soil environment also had a significant influence on the galvanic effect of the couple. The results suggest that the coupling structure of graphite and Zn-coated steel should be avoided in the grounding device design.

Zinc coating on steel by atmosphere plasma spray and their anti-corrosion behavior

In this paper, we report a simple process for Zn coatings based on atmosphere plasma spray (APS) techniques. The Zn coating, prepared at the optimal spraying current of 120A, shows excellent structure characteristics with low porosity and high adhesion strength of 10.42 MPa. After 3000 h of NaCl solution immersion test, no obvious corrosion damage occurred on the surface. The corrosion evolution of Zn-coated steel was mainly transformed from spontaneous passivation of the oxide film to pitting damage by corrosion medium. However, due to the isolation effect and lower self-corrosion potential, the Zn coating could protect steel from corrosion damages, which demonstrates the excellent anti-corrosion property.

A systematic study on the effect of coating type and surface preparation on the wettability of Si-Bronze brazing filler material on GI and GA-coated DP600

Zn-coated advanced high strength steels are popular in the automotive industry due to their excellent combination of mechanical strength and ductility as well as superior corrosion resistance provided by the Zn-coating – with hot-dip galvanized and galvannealed coatings being the most popular. Gas metal arc brazing technology is a non-fusion joining method, proposed as an alternative to the gas metal arc welding process due to several advantages: The arc-brazing process delivers significantly lower heat input to the substrate, which minimizes the Zn-coating burn-off leading to higher corrosion protection for the joined parts, and reduces the HAZ softening phenomenon which affects the mechanical integrity of the substrate, while at the same time reduces welding defects such as porosity and blowholes. The strength of an arc-brazed joint depends on the spreading of the molten filler material over the joint to create a bond between the parts as the molten braze solidifies. Existing literature on the subject suggests that heat input is the main driving factor controlling the wettability of the molten braze material with the type of shielding gas used also having an effect. However, the influence of different types of Zn-coatings and their respective surface condition (i.e., clean, or unclean) on the wettability of Cu-based molten braze materials during arc-brazing has not been investigated. The present work investigates the effect of surface morphology of galvanized and galvannealed DP600 steel in the as-received and plasma cleaned conditions on the wettability of molten Si-Bronze (CuSi3Mn1) brazing filler material in the bead-on-plate configuration. The findings of this work clearly demonstrate that the type of coating and its surface condition has a significant effect on the spreading of the molten filler material and on the growth of the intermetallic compound layer at the joint interface and therefore, should be taken into consideration as a factor that can impact the efficacy of an arc-brazed joint. • Evaluating surface morphology of different Zn-coatings to calculate their surface free energy. • Establishing a clear link between the surface roughness of Zn-coated steels on the wettability of molten Si-Bronze filler material. • The study shows that the wettability of the molten filler material is also affected by the presence of organic contaminants on the coating surface. • This work shows the relevance of surface morphology in controlling the wetting length of weld brazed joints. • The thickness of the intermetallic compound layer at the braze interface can be controlled by using oxygen plasma cleaning.

Humic acid: A new corrosion inhibitor of zinc in chlorides

Aiming to simulate the corrosion of Zn-coated steels in soils containing high quantities of organic matter, we studied the corrosion of Zn in 0.1 M NaCl containing 2 g•L−1 humic acid (HA). Up to around 4 h exposure, HA slightly promotes corrosion, while after 20 h, it acts as an inhibitor reaching efficiencies around 90%, as inferred from EIS and mass loss data. Voltammetry and EIS evidenced the slow formation of a secondary passive layer that explains the HA inhibition. FTIR and XPS spectroscopy showed that the additional layer contains Zn humate, which blocks the lateral growth of pits, as observed by SVET (scanning vibrating electrode technique).

Liquid metal embrittlement susceptibility of two Zn-Coated advanced high strength steels of similar strengths

Advanced high strength steels (AHSS) exhibit an increased susceptibility to Zn-assisted liquid metal embrittlement (LME). The strength of an AHSS is believed to be one important factor controlling LME sensitivity; this study investigated the LME behavior of two AHSS having different chemical compositions, while maintaining similar tensile strength at elevated temperatures. Hot tension tests were performed 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 had similar strengths, distinct differences were observed in their LME-associated ductility trough characteristics and LME cracking severity. This comparison helps isolate the contributions of chemical composition on the Zn-LME susceptibility. Specifically, the AHSS grade with higher-LME sensitivity had a Si content of 1 wt%, while the AHSS grade with lower-LME sensitivity had a Si content of 0.06 wt%. Three-dimensional elemental characterization using time-of-flight secondary ion-mass-spectroscopy indicated Si-enrichment in the coating-substrate interface layers of galvanized samples during thermal cycling at elevated temperatures. The difference in the LME sensitivity of the two AHSS is discussed in the context of this Si-enrichment behavior, which potentially suppresses Fe–Zn alloying reactions and increases the likelihood of direct contact between liquid Zn and the steel substrate.

Effects of laser beam defocusing on high-strain-rate tensile behavior of press-hardened Zn-coated 22MnB5 steel welds

Zn-based coatings have emerged as viable alternatives to the conventional Al-Si-coating for 22MnB5 press-hardening steel. However, excessive weld concavity associated with laser beam welding of Zn-coated 22MnB5 imposes an adverse effect on dynamic tensile behavior of the welds after press-hardening. In the present study, laser beam defocusing is used to mitigate excessive weld concavity during laser welding of Zn-coated 22MnB5 steel. The high-speed digital image correlation (DIC) technique during high strain rate tensile testing confirmed that the modified laser weld concavity by beam defocusing can successfully shift the failure location from the fusion zone to the base material. Laser beam defocusing reverses the negative strain rate sensitivity of the welds which drastically improves the dynamic loading performance of Zn-coated 22MnB5 laser joints in automotive crash situations.

Optimization of Tensile-Shear Strength in the Dissimilar Joint of Zn-Coated Steel and Low Carbon Steel

The present study features analytical and experimental results of optimizing resistance spot welding performed using a pneumatic force system (PFS). The optimization was performed to join SECC-AF (JIS G 3313) galvanized steel material with SPCC-SD low carbon steel. The SECC-AF is an SPCC-SD (JIS G 3141) sheet plate coated with zinc (Zn) with a thickness of about 2.5 microns. The zinc coating on the metal surface causes its weldability to decrease. This study aims to obtain the highest tensile-shear strength test results from the combination of the specified resistance spot welding parameters. The research method used the Taguchi method using four variables and a combination of experimental levels. The experimental levels are 2-levels for the first parameter and 3-levels for other parameters. The Taguchi optimization experimental results achieved the highest tensile-shear strength at 5049.64 N. It properly worked at 22 squeeze time cycles, 25 kA of welding current, and 0.6-second welding time and 12 holding-time cycles. The S/N ratio analysis found that the welding current had the most significant effect, followed by welding time, squeeze time, and holding time. The delta S/N ratio values were 1.05, 0.67, 0.57 and 0.29, respectively.

On the Joint Formation and Interfacial Microstructure of Cold Metal Transfer Cycle Step Braze Welding of Aluminum to Steel Butt Joint

Cold metal transfer cycle step braze welding process was modified to successfully produce an aluminum alloy/Zn-coated steel linear joint with a closed square butt joint configuration. The braze-welded joint shows wide-spreading and low wetting angle of molten filler alloy on both sides of the steel. The butt joint is formed by the overlapping of individual spot braze welds during cold metal transfer cycle step braze welding. The microstructure at the front, backside, and face side interface of the Al/steel joint were characterized in terms of the crystal structure, morphology, and thickness of intermetallic compounds by scanning electron microscopy, analytical transmission electron microscopy, and energy dispersive X-ray spectroscopy. The interfacial structure of Al/τ5(H)-Al8Fe2Si(film-like)/θ-Al3Fe(rod-like)/θ-Al3Fe(500 nm)/η-Al5Fe2(2.5 µm)/Fe is formed at the front interface associated with high-temperature interaction between liquid Al alloy and Zn-coated steel. On the contrary, Al/τ5(C)-Al8(Fe,Mn)2Si/τ5(H)-Al8Fe2Si/θ-Al3Fe(1.2 µm)/η-Al5Fe2(500 nm)/Fe is present at the backside interface associated with relatively low-temperature interaction between liquid Al alloy and Zn-coated steel. The intermetallic compounds and morphology at the face interface associated with medium temperature interaction are similar to those at the backside interface. However, the thickness of the η-Al5Fe2 layer is only 220 nm, which is caused as a result of the interaction of molten Al filler alloy with the bare steel sheared surface without Zn coating. The mechanism for the formation of varied intermetallic compounds at the different interfaces is discussed.