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Organosilane-based thin coating on top of galvannealed (GA) coating, known as secondary coating, provides extra protection from corrosion and thus is used for manufacturing automotive fuel tanks. In the present investigation, resistance seam weldability of 0.8-mm-thick secondary-coated GA interstitial free steel has been evaluated and compared with conventional GA sheets of identical thickness, chemistry, and mechanical properties. It has been found that the 1-3-µm-thick layer of organosilane-based secondary coating increases the electrical contact resistance by up to 50%. Consequently, the heat input increases by 34% when welding secondary coated steel as compared to conventional GA steel. This higher heat-input ensures suitable nugget formation over a wide range of parameters viz. a wider weldability lobe. This paper also addresses the test results on the service life of the welding electrodes. It has been found that the electrodes can withstand up to 30 m of continuous welding of secondary-coated steel without affecting the weld quality in terms of the joint strength. However, for conventional GA sheets, the same electrodes lay up to 20 m of welding length prior to dressing. Higher electrode face widening takes place during welding of plain GA sheets which lowers the current density and thereby affects formation of high-quality weld nugget.

Liquid Metal Embrittlement (LME) is a complex phenomenon which may occur during spot welding of zinc-coated Advanced High Strength Steels (AHSS) developed for the Automotive market. Zn or Zn-rich alloy in liquid state penetrates along the steel grain boundaries and may cause, depending on conditions, grains’ decohesion and cracks’ initiation and propagation under given mechanical stresses. LME sensitivity depends on the steel microstructure and mechanical properties, its bulk and surface composition, the coating composition and the spot welding process itself. In this study, a comparative assessment of conventional GalvanIzed (GI) and GalvAnnealed (GA) coatings in terms of LME behaviour has been done on the same sensitive AHSS material with the same welding process configuration and parameters. The intrinsic effect of the type of coating (GI vs GA) on the LME performance during spot welding could therefore be properly evaluated. It has been observed that no significant difference exists between both coatings.

In advanced high strength automotive steels, small amounts of diffusible hydrogen can lead to a deterioration of mechanical performances, especially a loss of ductility, in the simultaneous presence of internal stresses and of a sensitive microstructure. In the hot-dip galvanizing process, hydrogen is mainly absorbed during high temperature operations in hydrogen-containing atmospheres before hot-dipping when the solubility of hydrogen in steel is the highest. After hot-dipping, the metallic zinc-based coating can impede hydrogen diffusion out of the metal. As a result, an excess of diffusible hydrogen remains in the steel substrate and can subsequently lead to a possible embrittlement. In this contribution, the effects of the coating nature on the hydrogen diffusion of a 980 MPa dual phase steel (DP980) are investigated. The attention is focused on three Zn-based coating alloys: galvanized (Zn‑0.23%Al), galvannealed (Fe-Zn based on Zn‑0.12%Al) and Zn1.2Al1.2Mg. The hydrogen permeability of the coatings is first assessed through degassing experiments at room temperature and highlights the tightness of all coatings at room temperature. Secondly, cyclic SAE J2334 corrosion testing is performed and enables to highlight a stronger corrosion of GI and GA samples with respect to Zn1.2Al1.2Mg-coated samples, which is however the sole coating that seems to promote a hydrogen uptake during corrosion experiment.

In this study, the influence of the microstructural evolution of galvannealed (GA) coating and the thermomechanical characteristics of liquid metal embrittlement (LME) cracking during resistance sp...

The dissimilar metal joining of aluminium alloy and steel sheets is an essential aspect of weight reduction in the automobile industry to increase the strength-to-weight ratio. The large gap in thermo-physical properties, development of intermetallic compounds (IMC) layer at the interface, and severe distortions make it challenging to produce an acceptable joint. In this study, the low heat input cold wire gas tungsten arc welding (GTAW) process is used to successfully join the 1 mm thick Al6061-T6 alloy sheet to galvannealed sheet using AC welding current. The weld-braze joint is developed in lap joint configuration, and various mechanical and physical aspects of the weld bead like weld thermal cycle, micro-hardness, IMC layer thickness, and severe surface and edge distortions are shown. This work's novelty is to report the accurate surface and edge distortions of the joined sheets by a novel technique using a 3-D scanner and CAD model of the workpiece. An overall successful joint is produced using a cold wire GTAW process and the 3-D scanner technique to measure surface and edge distortion found to be precise and accurate.

Electrode degradation during the resistance spot welding (RSW) of coated steels has been a major challenge in automobile manufacturing as it affects the weld quality in terms of nugget size, defects, and mechanical properties. In this work, we investigate the effect of electrode degradation on such a defect, Zn-assisted liquid metal embrittlement (LME) cracking, in RSW by producing 400 consecutive welds of galvannealed TRIP steel. Infrared thermography, carbon imprinting, dynamic resistance, optical imaging, SEM, and EDS were used to characterize various phenomena during consecutive welding. LME crack severity was found to be unaffected in the first 80 consecutive welds. However, the severity declined sharply between 80–200 welds, and further, it remained low in 200–400 welds. The electrode experienced geometrical degradation (increase in the radius of curvature and face diameter) and metallurgical degradation (Cu-Zn-Fe alloy layer formation) during consecutive welding. The geometrical degradation, specifically the radius of curvature, was confirmed as the main factor affecting LME cracking. On the other hand, metallurgical degradation did not affect the LME cracking in the first 200 welds as its influence was overcome by the geometrical degradation. However, a small impact of metallurgical degradation was detected in 200–400 welds in the form of local rise and fall in the temperature and LME crack length.

We conduct high-resolution microscale tensile testing of galvannealed steel/Al–Mg–Si alloy dissimilar joints and find that the interfacial reaction layer is sufficiently strong to induce microscale aluminum fractures. The interfacial reaction layer with the 5-µm-thickness generated by friction stir spot welding contains Fe–Al intermetallic compounds (IMCs) with grain diameters of 10 nm to 2 µm, Zn–Mg compounds present around the Fe–Al IMC, and microscale defects within the layer. The microtensile test demonstrates the slip fracture of aluminum regardless of defects present within the reaction layer. The maximum stress withstood by the specimen is 309 MPa.

A warm stamping process of 1 and 1.2 GPa ultra-high strength steel sheets at comparatively low temperatures using rapid resistance heating was investigated to produce high strength parts. The effects of the heating temperature and rate on mechanical and surface properties of warm-stamped parts were examined. For a rapid heat rate of 100 °C/s around a heating temperature of 300 °C, the hardness of the formed parts increased from that of the as-received sheets, and the hardness for the 1.2 GPa sheet approached that of conventional hot-stamped parts from 22MnB5 steel sheets. The increase in hardness appeared only for rapid heating, and not for slow heating using a furnace. The increase in hardness is due to the transformation of retained austenite into martensite at comparatively low temperatures using rapid resistance heating and cooling. For the comparative low heating temperature, the non-coated sheet hardly oxidised, and the galvannealed sheet having high corrosion resistance did not exhibit the exfoliation of the coating layer, whereas the springback did not improve.

Press-hardening steels (PHSs) are used in modern passenger vehicles to increase part strength while reducing vehicle weight to meet both environmental and safety regulations. To prevent oxidization and decarburization during heat treatment, some PHSs are coated with Zn (galvanized or galvannealed). Heating during the press-hardening process drives interdiffusion of Zn from the coating and the Fe from the steel substrate, forming a diffusion layer composed of -Fe phase (a Zn-Fe solid solution). The electrical resistance of the diffusion layer is a function of its thickness and Zn-Fe composition. Both the diffusion layer thickness and the Zn-Fe composition are dependent on the initial coating thickness and heat-treatment time/temperature conditions. Changes to the heat-treatment process shift the resistance spot welding process window by altering the resistance behavior of the material. If the shift in the process window is not accounted for during assembly welding, the welds produced may either be too small or exhibit expulsion, both of which will reduce the strength of the weld. This study showed increasing heat-treatment time shifted the process window toward lower current. A final combined processing window of 1.5 kA, which is suitable for industrial application, was obtained when taking into account the variation in heat-treatment time. The tensile shear performance was not affected by the heat treatment, as increased softening in the heat-affected zone at longer heat-treatment time canceled the strength gain from increasing nugget size.

Flaking of galvannealed steel due to interfacial bond failure during press-forming results in a deterioration in product quality and reduced productivity. To solve this problem, an understanding of bonding at the interface must be gained in terms of the microstructure. In this research, the relationship between coating adhesion strength and the evolution of interface microstructure was investigated using an interstitial free steel sheet. Hat channel drawing with a bead machine and stud pin tests were used to measure the flaking resistance and adhesive bond strength, respectively. A strong relationship was found between flaking resistance and interfacial bond strength. High bond strength samples exhibited two distinctive microstructures, Zn-penetrated grain boundary, and rough, island-like knoll, with the knoll always surrounded by the grain boundaries. The rough knoll, with etch pits on the top, formed underneath a crater, and a deep pit was observed at the grain boundary. After stud pin testing, coating residues remained mostly on the Zn penetrated and rough knoll substrates, indicating strong coating adhesion reinforced by a mechanical interlocking effect that originated from the two distinctive microstructures.

The microstructure and tensile shear strength of the joint by laser brazing between dissimilar metals of aluminum alloy and galvannealed (GA) steel were investigated. A blowhole was formed by zinc vaporization during laser brazing; the tensile shear strength was significantly affected not by intermetallic compounds (IMCs) at the filler metal/GA steel interface but by the blowhole. The vaporization of zinc was suppressed, and the blowhole was controlled by inserting a Ti interlayer. The joint strength was improved through the suppression of blowhole, the thermal deformation of the Ti interlayer, and the increase in brazed filler metal thickness by optimizing the brazing parameter. The hardness test revealed that fracture occurs preferentially in the brazed filler metal; however, as the brazed filler thickness increases, fracture begins to occur near the heat-affected zone of A5052 because the volume of filler metal increases. The maximum strength of the joint was 185 N/mm, which is about 73% of the joint efficiency to 254 N/mm of the A5052 base material, and the base material partly fractured.

To protect the base metal from corrosion, different types of coating systems and coating process are an area of continuous development. Galvannealed (GA) coating is one of the most suitable coated sheet to satisfy the requirement in automobile industry. It was observed that the zinc starts to oxidise or corrode during the transport or storage time. To delay this oxidation or corrosion of zinc, a secondary coating is being given after GA sheet production, that also improve certain properties of the GA coating like, welding, high temperature performance etc. Many times, it is seen that the adhesion of secondary coating like electroplating deteriorates due to use of older and oxidized GA sheets. Generally, before electroplating, the GA sheet was cleaned by convention degreasing (Ammonium chloride–NH4Cl, pH 4.0) process. In the present work, it was investigated that zinc oxide layer (i.e. corroded zinc) was formed on top of older GA surface cannot be removed or dissolved by conventional alkaline solution. This has resulted in poor adhesion between GA and secondary electroplated coating. Those secondary coating peeled off during high temperature processes and 90° bend test. In this work, modifications were done during degreasing process which improve the adhesion of secondary electroplated coating. High concentration degreasing solution (pH 12.5) was used and the dipping time of the sample into the degreasing solution was increased from 20–30 s to 40–90 s. This enhances the dissolution of the zinc oxide layer from top of the GA coating and improve the adhesion between GA and electroplating coating. Ni anode, conventional Watt’s bath and current density 400 A/m2 for 8 min were maintained for electroplating. Ni coating thickness was maintained 6–8 µ. Coating thickness, cross-section, compositional analysis was done by using scanning electron microscopy (SEM), adhesion was checked by high temp 90° bend test.

Understanding phase evolution and growth kinetics of Zn-coating during press-hardening is essential in predicting welding behaviour of press-hardened 22MnB5 steel. The results showed that d...

The environmental regulations and safety requirements have enhanced the requirement of advanced and ultra-high strength steels. The hot forming process is known to deliver ultra-high strength steels. The boron added steels are heated beyond the austenitization temperature and subsequently deformed and quenched to form deformed components having fully martensitic structure. However, the bare steel suffers from oxidation and decarburization if no coating is applied. The conventional zinc based coatings like, galvanised, galvannealed are not suitable as zinc or iron-zinc phases melt at high temperature of austenitization and give rise to microcrakcs. The present work deals with a new zinc based coating system formed by a combination of electrodeposition and galvanising incorporating nickel in the coating. The coating microstructure is comprised of different nickel-zinc phases having difference in melting points as well. The evolution of coating microstructure at different soaking temperature and time is studied. The coating microstructure and compositions are observed by optical and confocal laser scanning microscope along with Secondary Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS). The different coating phases undergo melting and there is homogeneous nucleation of iron-rich solids in the coating. The composition and phase fractions of iron and zinc-rich phases are verified from ThermoCalc calculations. The coating transforms to complete iron-rich solid after 300 s of soaking at the austenitizing temperature. However, the initial zinc content largely determines the final fraction of zinc-rich liquid phases. The hot deformation study shows microcrack free regions at different areas of the hat sections.

In order to reduce the weight of parts in the automobile and electronic industries, various research on dissimilar welding techniques of aluminum and steel is being carried out. Since dissimilar materials have different physical and electrochemical characteristics, joining through conventional fusion welding is challenging, and there is a high probability of a decrease in strength of the welded joints. To solve this problem, a mechanical fastening method is mainly applied to join dissimilar parts with different material properties, but this process has disadvantages in terms of productivity improvement and cost reduction because additional consumables, such as rivets, are required. In this research, we investigated the optimization of the weld bonding conditions of joints using epoxy-based adhesive bonding and DeltaSpot welding for Al/Fe dissimilar materials. For each experimental condition, the corrosion resistance and tensile shear strength of the welded joints were evaluated according to salt spray test times of 0 h, 640 h, 1280 h, and 1920 h. As a whole, as the salt spray test time increased, the tensile shear strength of the welded joints decreased. It was confirmed that weld bonding, after manual polishing of the aluminum side, resulted in the highest average tensile shear strength of 5.88 kN at 1920 h, which was an increase compared with other conditions.

This research describes the development of a combined process of forming and thermal joining without adhesives of high strength steels and organosheets. In order to achieve cycle times below 20 s the temperature of the forming tool remains constant at 110 °C throughout the process. Thus, the removal of the component can take place immediately after the forming process without cooling of the forming tool. The thermal energy required to assure the thermal joining process at 280 °C at the beginning of the intermediate forming step is applied by means of heating the steel in a furnace before the transfer to the forming tool. By choosing certain coating depended heat treatment parameters during the heating step, galvannealed rough surfaces of the steel blanks can be generated in order to increase the joint strength. The paper includes a surface analysis and lap-shear testing results to quantify the respective joint strength to be achieved with the proposed surface treatment. In addition tensile test results of the heated DP800 steel to characterize the thermal influence on the mechanical properties of the heated high-strength steels will be shown. Furthermore the paper presents a tool concept with a graded blank holder to perform stretching of the metal on the one hand and draping of the organosheet on the other hand as well as preliminary forming results.

The galvannealed coating has been extensively used in automotive industries because of its superior performance for weldability, paintability and corrosion resistance. However, surface defect of the galvannealed coating is a major concern for the steel manufacturers and the automotive industries. The galvannealing coating process comprises several process steps and each process step may incorporate different types of defects on the coated surface. In the present study, an uncommon surface defect which looks like an indentation mark on the coating surface has been investigated. Scanning Electron Microscopy (SEM) coupled Energy Dispersive X-ray Spectroscopy (EDS) has been used to analyse the morphology and the chemical compositions. Confocal Laser Scanning Microscopy (CLSM) has been used to measure the depth and the morphology of the defect. It was found that the defect was developed on the steel sheet at the end section of the annealing furnace because of mechanical abrasion with rolls. In-depth analysis suggested that zinc vapour from zinc bath were forming zinc oxide particles in the snout region. Zinc oxide particles were able to travel to the end section of the annealing furnace due to improper snout operation. These particles were getting entrapped in-between the roll surface and the steel strip and forming the surface defect on the galvannealed steel surface.

The zinc coating layer fracture properties of a galvannealed steel and an electrolytically galvanized steel are analyzed by conducting the in-situ bending test with newly designed samples. It is found that the fracture develops much earlier in the coating layers of the galvannealed steel than that of the electrolytically galvanized steel. Using transmission electron microscope and energy dispersive X-ray spectroscopy, the intermetallic phases of the coating layers are characterized and it is found that the early crack initiation in a galvannealed steel is mainly triggered in the gamma phase. Combining with nanoindentation tests and corresponding simulation, the deformability of intermetallic phases are analyzed to explain the failure behavior of coating layers in the two steels.

Microstructural evolution of a 5 wt pct Al-Zn coating on press hardening steel was studied in comparison to hot-dip galvanized (GI) and galvannealed (GA) coatings. The results show that the presence of 5 wt pct Al effectively suppresses oxidation during austenitization; meanwhile, the presence of Fe resulting from galvannealing accelerates oxidation. Alloying with Al or Fe in the coating prior to austenitization reduces the susceptibility to liquid metal embrittlement (LME). The presence of Al in the as-coated Zn coating enhances the corrosion resistance in HCl solution and reduces the cathodic kinetics in NaCl solution. However, for sacrificial protection, the austenitized GI steel outperforms the other austenitized-coated steels. Nevertheless, the 5 wt pct Al-Zn coating exhibits better overall performance including high-temperature oxidation resistance, less LME susceptibility, and cathodic protection.

Electrode misalignment, induced from the flexibility of welding machine gun of spot welder or the potential poor fit-up, leads to the high tendency of expulsion and asymmetric nugget shape. Yet very few studies have focused on the influence of misalignment on the electrode degradation and microstructure evolution of the electrode, especially when welding galvannealed steel. This paper investigates the degradation of such misaligned electrodes when spot welding galvannealed steel. To reveal its mechanism, the electrodes welded with galvannealed steel were examined after 10 and 200 welds with the slight misalignment. The electrodes were found to experience more severe degradation compared to the results from previous studies with the aligned electrodes. The results from energy-dispersive spectrum (EDS) analysis further confirmed that δ-Fe–Zn phase, a barrier from galvannealed coating for isolating Cu and Zn formation, was not uniformly distributed on the electrode tip. As a result, the initial electrode pitting took place after 50 welds. Furthermore, electron backscatter diffraction (EBSD) quantitatively analyzed the recrystallized grains of the worn electrodes, which underwent rotation under the asymmetric pressure distribution under misalignment. The calculated Taylor factor via EBSD mapping also indicated the declined portion of grains accounted for the low deformation resistance of the worn electrode. Finally, electrode displacements were simultaneously collected in the experiments, of which the peak values accurately predicted the heat generation for each spot weld and accordingly predicted the electrode life.

The laser brazing process is successfully applied in automotive industry for joining of roofs and hatchbacks of vehicles. The bad wetting of the brazing alloy during the process can lead to the formation of random external porosities which are not allowed on visible parts. This paper describes the wettability and reactivity mechanisms at short contact time of Cu and Al matrix brazing alloys with different reactive elements (Si, Sn) on different steel products such as hot-dip galvanized steels, galvannealed steel and bare steel. Wetting experiments were carried out by the dispensed drop technique. The effects of alloying elements and brazing alloy matrix on interfacial reactivity are discussed. It was found that Cu matrix containing 3 wt.% Si is the most favorable for short time liquid/solid adhesion relatively to the other studied brazing alloy compositions. The brazing ability of different steel products is well correlated with the wettability and interfacial reactivity results.

Tensile strengths of up to 1.5 GPa and fracture elongation of 8% in press-hardening steel (PHS) opened up a great opportunity to improve the crash performance of vehicles. However, due to the increased susceptibility to welding defects and undesired transformation as a result of coating mixing into the weld pool, laser welding of PHS is still challenging. Hence, the present study focuses on the effect of Galvanneal (GA)-coating weight on the microstructure and mechanical performance of fiber laser welded PHS. It was observed that GA-coating weight considerably affects the laser welding process window as well as weld geometry. Weld penetration decreased and concavity increased with increasing coating weight, which is attributed to intensified Zn-plasma and laser interaction at higher coating weights. Moreover, a model has been developed to interpret the correlation between the GA-coating weight of the PHS and the penetration depth based on the heat input per unit thickness of the sheet. The size of fusion zone and heat affected zone decreased slightly with increasing coating weight due to lower energy absorption by the material. Furthermore, GA-coating weight did not affect the tensile strength of all welded joints as the failure happened in the base metal.

In order to evaluate the galvanisability of a CMnSiCr DP780 steel, which is originally designed for manufacturing continuously annealed bare steel sheet, hot-dip galvanising experiments were conducted on a hot-dip process simulator. The process and galvanising behaviours were investigated. The results indicate that the alloying elements such as Si, Mn and Cr segregate and selectively oxidise at the surface during annealing. Besides external oxidation, internal oxidation of Cr and Mn is observed. Aluminothermic reducing reaction is effective to reduce the negative effect of surface oxides on the galvanisability of this steel and improve the coating quality. According to the measurement results and discussions, it is believed that this CMnSiCr DP780 steel could possibly be used to produce the corresponding hot-dip galvanised or galvannealed product when further increasing the Al content in Zn bath slightly, especially for galvannealed product.

The coatings of boron steels play an important role in affecting the quality of hot stamping parts, so it is important to evaluate the hot stamping performance of coatings before designing processes. Taking the U-type hot stamping part of boron steel as research objects, the surface quality, microstructure and temperature variation of samples with GA (galvannealed), GI (galvanized) and Al–Si coatings were observed and analyzed to evaluate the anti-oxidation, forming and quenching performances of different coatings. The results show that all the GA, GI and Al–Si coatings could provide good oxidation protection and also act as the lubricants for avoiding the friction damage of sample substrates and die-surface. But the different compositions of GA, GI and Al–Si coatings will contribute the different colors. Under the same deformation degree, the Al–Si coating can provide the best substrate protection and the GI coating will induce cracks in the substrate because of the liquid metal-induced embrittlement phenomenon. There is no significant difference between the quenching performances of GA, GI and Al–Si coatings, and the thermal conductivity of the GI coating is slightly better than Al–Si and GA coatings.

Structural-adhesive-assisted DeltaSpot welding was used to improve the weldability and mechanical properties of dissimilar joints between 6061 aluminum alloy and galvannealed HSLA steel. Evaluation of the spot-weld-bonded surfaces from lap shear tests after long-term exposure to chloride and a humid atmosphere (5% NaCl, 35 °C) indicated that the long-term mechanical reliability of the dissimilar weld in a corrosive environment depends strongly on the adhesive–Al6061 alloy bond strength. Corrosive electrolyte infiltrated the epoxy-based adhesive/Al alloy interface, disrupting the chemical interactions and decreasing the adhesion via anodic undercutting of the Al alloy. Due to localized electrochemical galvanic reactions, the surrounding nugget matrix suffered accelerated anodic dissolution, resulting in an Al6061-T6 alloy plate with degraded adhesive strength and mechanical properties. KrF excimer laser irradiation of the Al alloy before adhesive bonding removed the weakly bonded native oxidic overlayers and altered the substrate topography. This afforded a low electrolyte permeability and prevented adhesive delamination, thereby enhancing the long-term stability of the chemical interactions between the adhesive and Al alloy substrate. The results demonstrate the application of excimer laser irradiation as a simple and environmentally friendly processing technology for robust adhesion and reliable bonding between 6061 aluminum alloy and galvannealed steel.

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.

In this study, the galvanizing and galvannealing ability of dual phase steel sheet (DP 590), with and without Fe pre-coatings, is investigated with varying dew points using a hot dip process simulator (HDPS). Good quality defect free adherent galvanized coating on the substrate surface is obtained by appropriate selection of the inter-critical annealing atmosphere consists of N2-5%H2 gas mixture with +10 °C dew point without substrate pre-coating. However, the production of defect-free coating on DP 590 substrates is highly challenging when the substrates are without any pre-coating. It is observed that the selective oxidation of manganese on the substrate surface during intercritical annealing is primarily responsible for the appearance of bare spots on the galvanized surface in case of the coated specimen produced without pre-coating. A significant improvement in the reactive wetting behaviour between the liquid zinc alloy and pure Fe results in defect free adherent coatings (both galvanized and galvannealed) with electrodeposited Fe pre-coating on the substrate surface irrespective of dew points. Both the galvannealed specimens prepared with and without iron pre-coatings, exhibit partial passive-active metal characteristics in potentiodynamic polarization corrosion test. Iron pre-coating on the substrate surface is observed to improve the quality of galvannealed coatings by reducing defects and enhancing the formation of more compact and dense delta (δ) phase compared to galvannealed coatings produced without pre-coating under identical conditions. This leads to the improvement in corrosion resistance of galvannealed coating produced with pre-coating by lowering the corrosion potential and corrosion current density compared to that of galvannealed specimens produced without iron pre-coating.

In recent years, ultra-high-strength steels are used as structural components more and more. For automobile industry and for different carmakers, different coating layers are chosen for corrosion resistance, such as galvanized (GI) and galvannealed (GA) layers. In this study, a hot-dip galvannealed complex phase 1000 MPa grade (CP1000) steel was chosen as the model material. Different GA parameters were adopted to get different Fe–Zn coating microstructures on CP1000, and bending and tensile loading were loaded on CP1000. Microstructure of the coating layers was investigated using SEM, and the cracking mechanisms of the coating layers were also observed using SEM. It was found that, with increasing GA time, ζ phase decreased on the surface of the coating layers, and more and more δ phase was formed from ζ phase on the surface. Furthermore, with increasing GA temperature, the formation velocity of δ phase got faster and faster. Bending tests showed that cracks could be found at the interface between the coating layers and CP1000 steel, especially when GA temperature was 550 °C. Cracks in δ phase, which were perpendicular to the interface, could also be found. Tensile tests showed that when the tensile strain was low enough, interface delamination could not be found. At the interface, when the GA temperature was 510 °C, cracks perpendicular to the interface could also be found.

Joining of aluminium alloys 7075-T6 and galvannealed dual phase 980 steel was achieved by friction bit joining (FBJ) and weld-bonding (FBJ + adhesive) processes. Accelerated laboratory-scale corrosion tests were performed on both FBJ only and weld-bonded specimens to study joint strength under a corrosive environment. Static lap shear tests showed that both FBJ only and weld-bonded cases generally retained more than 80% of the joint strength of non-corroded specimens at the end of corrosion testing. The presence of Zn/Fe coating on the steel substrate resulted in improved corrosion resistance for FBJ specimens, compared to joints produced with bare steel. An optical microscopy was used for cross-sectional analysis of corroded specimens. Some corrosion on the joining bit was observed near the bit head. However, the joining bit was still intact on the steel substrate, indicating that the primary bond was sound.

Al-Si coated and galvannealed (GA) hot stamping steels were resistance spot welded, and the welding current, properties and mechanical performance of the joints produced were compared. The coating significantly affected the welding current required for attaining the conventionally acceptable weld fusion zone size of 4 t , and the welding current range. The coating also influenced the energy absorption and failure mode transition but had no effect on the peak failure load provided the stack consisted of sheets having similar coating. For dissimilar combination of Al-Si and GA coated sheet stack, the joint mechanical performance was significantly deteriorated.

In the last several years, automobile industries have increasingly focused on galvannealed (GA) steel sheet due to their superior properties such as weldability, paintability and corrosion protection. To improve the properties further, different coatings on GA have been reported. In this context, an electroplating process (flash coating) of bright and adherent Ni plating was developed on GA steel sheet for covering the GA defects and enhancing the performances such as weldability, frictional behavior, corrosion resistance and phosphatability. For better illustration, a comparative study with bare GA steel sheet has also been carried out. The maximum electroplating current density of 700 A/m2 yielded higher cathode current efficiency of 95-98%. The performances showed that Ni-coated (coating time 5-7 s) GA steel sheet has better spot weldability, lower dynamic coefficient of friction (0.07 in lubrication) and three times more corrosion resistance compared to bare GA steel sheet. Plate-like crystal of phosphate coating with size of 10-25 µm was obtained on the Ni-coated GA. The main phase in the phosphate compound was identified as hopeite (63.4 wt.%) along with other phases such as spencerite (28.3 wt.%) and phosphophyllite (8.3 wt.%).

Phosphate ion containing acidic polyphenol coating solutions were developed as to mitigate the deleterious effect of corrosion on galvannealed iron surface. The influence of manganese sulphate, manganese phosphate and polyethylene wax in polyphenol coatings were investigated to reduce the powdering of complex-shaped galvannealed structures and corrosion effect. Moreover, the reactivity of coating at the surface was evaluated by glow discharge optical emission spectroscopy, scanning electron microscopy and energy dispersive spectroscopy. Low surface integration within 3.5% NaCl environment and frictional behaviour was achieved with 0.5% manganese phosphate in polyphenol coated coupon by electrochemical and mechanical testing respectively.

The galvannealed coating have gained importance as a potential solution for auto body panels owing to their superior performance like weldability, paintability and after painting corrosion resistance. Efficient production of galvannealed strip through hot dip galvanising and subsequent annealing root is a challenge as occurrence of surface defects results in material downgrading and rejection. The present work deals with one type of surface defect present at the bottom surface of the galvannealed strip with high defect severity. The defect appearance was like a shining spot by naked eye. It was characterised using Optical Microscope, Scanning Electron Microscope coupled with Energy Dispersive Spectroscopy to analyse shape, size, morphology and composition. The defect was further analysed by Confocal Laser Scanning Microscope (CLSM). The features of CLSM like high resolution, high depth of focus, 3D surface topography were useful in determining the relative depth of the defect at different regions. The results indicated that the defect occurrence was due to mechanical abrasion of coating with deposited dust of Fe–Zn intermetallic phases at the galvannealing tower top roll. The production line parameters were modified to maintain the temperature and cleanliness of the top roll and a reduction in defect severity was achieved.

During a galvannealed sheet metal forming, the failures of coating layers (powdering, flaking and cracking) frequently affect the strain state of sheets and deteriorate the frictional characteristic between sheets and tools. Two FE-models in this study were suggested to investigate the effects of the mechanical behavior of coating layers on the formability and friction of the coated steel sheets in FE analysis; the first is one-layer model to express the coated sheet as one stress-strain curve and the second is a multiple-layer model which is composed of substrates and coating layers, separately. First, the frictional properties and the formability of the coated sheets were experimentally investigated using a cup deep-drawing trial. After, the drawing process was simulated by FE analysis of the two models. In the multiplelayer model, the mechanical behavior of the coating is defined as a stress-strain curve which was determined using the nanoindentation test of the coating, its FE analysis and artificial neural network method. The result showed that the multiple-layer model provides more accuracy predictions of drawing loads than the one-layer model in the FE analysis, compared to the actual cup drawing test.

The galvannealing process results in thermal residual stress in zinc–iron alloy coating on steel substrate due to differential thermal behaviour of the coating and the substrate. The magnitude of biaxial tensile residual stress generated just after cooling has been estimated theoretically and the value is found to be high enough to cause through-thickness cracks in the coating. Energy model has been adopted to determine the distribution of cracks and crack spacing in the galvannealed (GA) coating. The same was investigated from the microstructural analysis using scanning electron microscope. It is shown that the estimated crack spacing is always lower than the actual values. Subsequently a stress field develops around each crack tip which induces a non-uniform triaxial stress state in the coating.

The plastic deformation behavior of single crystals of the δ1p and δ1k phase compounds in the Fe–Zn system, which are the major constituent phases in the coating layer of galvannealed (GA) steels, has been investigated by micropillar compression tests at room temperature as a function of crystal orientation and specimen size. (0001)[11 2 ‾ 0] basal slip and (10 1 ‾ 0)[ 1 ‾ 2 1 ‾ 0] prism slip are observed to operate in the δ1p phase compound while (0001)[11 2 ‾ 0] basal slip is observed in the δ1k phase compound. For all cases, a significant strain burst occurs immediately after yielding, forming giant steps on the slip planes along the slip direction and leading in many cases to instantaneous ‘slip plane (shear) failure’. The extent of such strain burst as well as instantaneous ‘slip plane failure’ is reduced by introducing dislocations (pre-straining) prior to micropillar testing, accompanied by the reduction in the stress at which such strain burst occurs as well as that in the extent of strain burst. The critical resolved shear stress (CRSS) for basal and prism slip in the δ1p and δ1k phase compounds all show an inverse power-law scaling against the specimen size with an exponent in the range of 0.02–0.06. The bulk CRSS values are estimated respectively to be approximately 320, 430, and 765 MPa for the basal slip in the δ1p and δ1k compounds and the prism slip in the δ1p compound by taking into account the specimen size effects of CRSS.

Galvannealed (GA) steel sheets have been widely used in the automotive industry because of their good weldability, paintability, and corrosion resistance. However, coating failure and exfoliation originating from the brittle and hard Fe Zn phases during press forming is a serious problem. This study investigates the mechanism of flaking deterioration in GA coatings with high Fe content. For this, specimens with various Fe contents were fabricated with different galvannealing temperatures. As the temperature increased, the η and ζ phases became extinct and both δ and Γ phases grew simultaneously. Accordingly, surface became homogeneously flattened with crater degeneration. Simultaneously, the interface was flattened by lateral Γ phase growth. The flat region where the Γ existed was widened by broadening the Γ phase area, by which the etch-pit shape rough knoll found under craters was gradually eliminated. As a result, the Γ phase growth resulted in interface flattening, which deteriorated the flaking resistance.

The 22MnB5-galvannealed steel is extensively used in the hot stamping process to produce car anti-collision structure parts. Furthermore, the resistance spot welding (RSW) is an important process in the automobile industry, especially in body construction, and the 22MnB5-galvannealed steels are a big challenge for the joining methods because their microstructure and mechanical properties are different from those of the conventional steels. In view of this, the present paper aims to optimize the parameters of the RSW process of the 22MnB5-galvannealed steel. Initially, the goal was to remove the galvannealed coating and in the next stage, the following responses were maximized: the nugget width, the nugget cross-sectional area, the penetration, the strength, the joint efficiency, and the energy absorption, whereas the indentation, heat affected zone and separation were used as constraints. The process parameters selected were the effective welding time, the effective welding current, the quenching time, and the upslope time. Response surface methodology (RSM) was applied jointly with the global criterion method based on principal components. The results of the multiobjective optimization are close to the individual targets for each response, highlighting the importance of considering the correlation structure presented in the responses.

Zn assisted liquid metal embrittlement (LME) has characteristics that differ from the general characteristics of other LME systems: the formation of Fe-Zn intermetallic compounds and the high solubility of Zn in Fe. Consequently, there is a need to investigate the influence of the Fe-Zn interaction and resulting phase composition on LME behavior. However, little research has been conducted to reveal such relationships. This paper explores the influence of high temperature phases in galvannealed coating, specifically liquid Zn and α-Fe(Zn) on the LME sensitivity of medium Mn steel using a high temperature tensile test with eight combinations of temperatures (700 °C to 900 °C) and hold times (1 s, 50 s, and 100 s). The test results are categorized as LME, microcrack and LME free cases in terms of LME behavior. The study reveals that the LME sensitivity is governed by the fraction of effective liquid Zn, which is a consequence of isothermal solidification owing to the thermodynamic stability of liquid Zn, diffusivity of Zn in Fe and the partitioning coefficient of liquid Zn. Moreover, LME is influenced by the breaking of α-Fe(Zn) layer, which directs the liquid Zn into the substrate steel. Finally, a water-tank analogy is proposed to demonstrate the effect of the fraction of liquid Zn and the breaking of α-Fe(Zn) layer on LME.

As more emphasis is placed on thinner gauges and stronger steel grades for more fuel efficient, and crash-worthy vehicles, corrosion becomes a more important consideration. This demand has led to the development of advanced high strength steel (AHSS) grades such as dual-phase (DP) and transformation-induced plasticity (TRIP) aided steels. To protect these steels from corrosion, a hot-dip galvanizing (HDG) or galvannealing (GA) process is employed. However, alloying with Mn and Si can cause complex surface oxides during annealing, which may result in defects in the zinc coating1-8. In order to increase AHSS galvanizability, selective oxidation of alloying elements in DP and TRIP-aided steel needs to be understood. Depending on annealing atmosphere, the mechanism (internal or external) of oxidation can change, as well as the thermodynamic stability of the oxide. Wagner’s theory of oxidation enables prediction of the mechanism of oxidation for binary oxide systems9. The defects can be present after pickling and fluxing, causing wettability issues during the HDG process. Coating defects may also form or be further exacerbated by the galvannealing operation. For this work, oxidation mechanism predictions were performed using Wagner’s theory of oxidation. In the present study a 0.07C-1.9Mn-0.2Si-0.3Cr galvannealed dual-phase steel which exhibited streaking defects in the coating was analyzed. Light optical metallography (LOM) and scanning electron microscopy (SEM) were performed to characterize defects within the galvannealed coating. Static spectra and 3-D profiling using time of flight secondary ion mass spectrometry (TOF-SIMS) was employed. It was found that Mn rich Si containing oxides are present on the steel-coating interface. It does not appear that these oxides affect wettability, however they may have an influence on intermetallic growth during galvannealing. This, combined with an influence from the micro-grooves of the sink roll in the Zn pot, may lead to the streaking defects observed.

The Fe–Zn and Fe–Al-based alloying phases are formed on galvanized/galvannealed steels which are extensively used as automobile bodies. This paper investigated the effects of Si on the formation of intermetallic phases in the alloying reaction between an iron substrate and liquid Zn containing 0.2 wt% Al and partitioning of Si between the phases. The addition of Si to the iron substrate was found to scarcely influence the formation and morphology of the Fe2Al5 intermetallic phase layer but to reduce the amount of the δ1-FeZn7–10 intermetallic phase layer at the substrate/liquid interface and to disperse the phase as grains in the Zn–0.2 wt% Al coating matrix. STEM–EDS analysis demonstrates that Si partitions less into the Fe2Al5 and the δ1 phases than into the iron substrate. Enrichment of Si around an Si-depleted Fe–Zn intermetallic phase product was also observed in the beginning of the alloying reaction. The results obtained allow us to explain the effect of Si on the intermetallic phase formation in terms of a difficulty in nucleating the δ1 phase at the substrate/liquid interface.