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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.

Galvannealed (hot dip galvanized and annealed) dual phase (DP600) steel and interstitial free (IF) steel are two of the widely used materials in automotive industries owing to their high specific strength, toughness, formability and corrosion resistance. This inevitably requires their joining to form larger components. Resistance spot welding has emerged as the predominant welding technique for joining of sheet metals in automotive industries. There are complexities regarding microstructural variation of the fusion zone and across heat affected zones during spot welding of fundamentally dissimilar steels. This is further aggravated by the prospect of zinc redistribution during the welding process. Current study presents the evolution of microstructure in different zones of welding in IF and DP600 steel. Micro-hardness profiles and electron back scattered diffraction studies were performed. Furthermore, redistribution of elements in different welding zones is studied using electron probe micro analysis. The effect of welding current and time on the nugget size and the strength of the weld during tensile–shear is determined. It was observed that there was an increase in nugget diameter and maximum load bearing capacity of the joint with an increase in heat input, until the occurrence of expulsion. Furthermore, critical nugget diameter for pull-out failure was determined and existing empirical models were verified for the applicability in this case. None of the existing empirical models could predict the critical nugget diameter correctly and thus a new empirical relationship between sheet thickness and critical nugget diameter for zinc coated dissimilar steels has been proposed. Failure behaviour of the spot-welded sheets were studied under tensile–shear configuration under quasi-static and fatigue mode of loading and it showed substantial difference in location of crack initiation and fractographic features. Tensile–shear specimen at optimum welding parameter failed from the base metal of the IF steel side with dimples on the fracture surface, whereas fatigue specimen failed from the heat affected zone of the IF steel side with transgranular striations on the fracture surface.

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.

The microstructures, tensile shear properties, and tensile shear fatigue properties of dissimilar A6061/Galvannealed steel joints fabricated by friction stir spot welding (FSSW) were investigated. Fe4Al13 phases form as the intermetallic compound (IMC) layer at the joint interface between the A6061 matrix and the galvannealed layer consisting of FeZn7, Fe, and Zn. At the edge of the joint, the stirred layer in which the A6061 matrix and the galvannealed layer are stirred also forms. Moreover, the solidified part of the residual melt discharged from the joint area forms at the outer peripheries of the joint. In this study, FSSW was conducted for two total welding durations: 9 and 10 s. Although the thickness of the remaining A6061 sheet in the welded area decreased with an increase in the welding time, the effects of the total welding time on tensile shear and tensile shear fatigue properties were negligible. A fatigue fracture occurred in the A6061 matrix and at the joint interface at the high cycle fatigue region and the low cycle fatigue region, respectively. In the case of the interfacial fracture, the crack was generated in the solidified part of the residual melt or at the interface between the solidified part and the stirred layer.

The present work studied the influence of the geometric design of the electrode on Zn-assisted liquid metal embrittlement (LME) cracking during resistance spot welding (RSW). LME cracking of the galvannealed transformation-induced plasticity (TRIP) steel welds, induced by two types of electrodes, a radius type with different radius of curvature (R), and a dome type with variable tip diameter (d), was studied both experimentally and by simulation. The current density decreased and the contact area at the electrode/sheet (E/S) interface increased with the increasing R, resulting in low temperatures and thermal stress, which subsequently led to decreased LME tendency. On the contrary, the current density decreased but the initial contact area at the E/S interface remained unchanged with increasing d, causing only a minor reduction in the temperature and hence less influence on LME cracking. These results suggested that R is the most critical design parameter of the electrode that controls LME cracking. Moreover, the radius type electrode displayed lower LME sensitivity as compared with the dome type electrode. This is attributed to the fact that the radius type electrode provides the benefits of increase in both R and d.

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.

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.

Galvanized steel sheets are increasingly used in the automotive industry due to excellent corrosion resistance, sufficient weldability, and formability. During sheet forming of galvanized metals, material failures such as exfoliation and cracking can be found in the zinc coated layers of the sheet under severe friction. Thus, the evaluation of the influences of surface topography, pressure, and lubrication on friction could provide valuable guidance in forming of galvanized metals. A hot-dip galvannealed sheet and a hot-dip galvanized sheet are chosen in this tribology study. The surface topographies of the materials are measured using profile meter and the fractal parameters are calculated using the fractal theory. The contact model between the sheet and the die is established using the real contact area. The friction model concerning the surface topography of galvanized sheet and the pressure is thus formulated. The friction coefficients of the theoretical values are in reasonable agreement with the experimental.

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.

In the process of welding zinc-coated steel, zinc vapor causes serious porosity defects. The porosity defect is an important indicator of the quality of welds and degrades the durability and productivity of the weld. Therefore, this study proposes a deep neural network (DNN)-based non-destructive testing method that can detect and predict porosity defects in real-time, based on welding voltage signal, without requiring additional device in gas metal arc welding (GMAW) process. To this end, a galvannealed hot-rolled high-strength steel sheet applied to automotive parts was used to measure process signals in real-time. Then, feature variables were extracted through preprocessing, and correlation between the feature variables and weld porosity was analyzed. The proposed DNN based framework outperformed the artificial neural network (ANN) model by 15% or more. Finally, an experiment was conducted by using the developed porosity detection and prediction system to evaluate its field application.

In this work, the wettability of the galvannealed surface pretreated with the hybrid silane APTES/GPTMS film was measured by sessile drop method. The coatings were applied on the substrate from dip-coating method and dried in an oven. The study followed an experimental factorial design, 3³, with contact angle as response. The corrosion resistance was evaluated by electrochemical techniques and the surface morphology was characterized by scanning electron microscopy. The wettability results showed the best condition for galvannealed steel pretreated with the hybrid silane film for 2:1 APTES:GPTMS ratio, 2% silane concentration and 150 min of hydrolysis time, because the result may be related to a higher rate of hydrolysis of the silanes that is influenced by both the functional group present and by the type and number of hydrolyzed groups, resulting consequently in a higher availability of silanol groups in solution, promoting a greater crosslinking of the film on the metallic substrate. Keywords: hybrid silane, corrosion, galvannealed steel, pretreatment, wettability.

It was attempted to produce galvannealed dual-phase (DP) steel through a commercial continuous galvanizing line (CGL). The main challenge was to form the DP microstructure even with the slow cooling rate after annealing in the CGL. A two-pronged approach was adopted for this, viz. design of the steel chemistry with calculations and optimization of the CGL parameters with simulation using Gleeble-3800 thermomechanical simulator. The steel thus produced in CGL revealed ferrite and 18 ± 2% martensite microstructure, with tensile strength exceeding 600 MPa and 24% total elongation along with good formability. Weldability study using resistance spot welding indicated the possibility of achieving sound joint with a 4-kA width in weld time–current lobe despite the high carbon equivalence.

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.

Depth-sensing indentation (DSI) is used in this work to determine the change of local mechanical properties by annealing of hot-dip galvanized IF steel sheets. The influence of annealing conditions (temperature and time) on: (i) coating composition; (ii) local mechanical properties and (iii) roughness of the coating surface was quantified. Annealing of steel samples (Ti-IF steel and Ti-Nb-P-IF steel alloyed with phosphorus) was performed with different holding times (10, 60, and 300 s) by both temperatures 450 and 550 °C. The zinc in coating transformed during annealing to the intermetallic phases ZnxFey. Annealed Zn-Fe coatings, wherein the iron concentration falls towards the surface, consist of different intermetallic phases.

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.

Bare spots defect of galvanealed (GA) in High Strength Dual Phase (DP) steel strip with 1.5 % Mn contain was studied in detail. The surface morphologies of spot defects before and after partial and complete removal of the Zn layer, as well as the interface between the outermost coating layer and the sheet substrate were analyzed by optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray Photoelectron spectroscopy (XPS), glow discharge optical emission spectrometry (GDOES) and laser scanning confocal microscope(LSCM). It was found that the bare spots defect was composed of a large number of pits with different sizes and depths. There were a lot of Fe - Zn alloy particles distributed in the pits, and many MnO formed on the surface, no effective Fe2Al5 inhibition layer formed. The results showed that the main reasons for the bare spots defect of the GA in High Strength DP steel strip are as follows: a silicon oxide film forms on the substrate during annealing prior to hot dipping because of Mn gathered on the surface of steel strip, thus strip surface wettability with liquid zinc is deteriorated and preventing the formation of the Fe2Al5 inhibition layer during hot dipping. In this paper, the dew point control process is introduced creatively, by increasing the dew point and the hydrogen content in the furnace area, and the bare spot defects on the surface of the high strength duplex steel galvanized sheet are solved.

The crystal structures of the Γ- and Γ1-phase compounds in the Fe-Zn system have been refined by single-crystal synchrotron X-ray diffraction. The Γ-phase compound consists of four Fe-centred (Zn,Fe)12 icosahedra connected with one another by face-sharing with the vertex sites of the shared triangle face being occupied by both Fe and Zn atoms. The Fe occupancy at the vertex sites increases with increasing Fe composition of the compound. The stoichiometry of the Γ phase is described as Fe4Zn9 (Zn-30.77 at.%Fe), realized when all the vertex sites of the shared triangle face are occupied exclusively by Fe atoms. The Γ1-phase compound consists of Zn12 and (Zn,Fe)12 icosahedra whose central site is exclusively occupied by Fe atoms, which is different from the previously reported structural model where some centres of these icosahedra are occupied by both Fe and Zn atoms. The crystal structures of all Fe-Zn intermetallic compounds can thus be best understood by considering the packing of the common structural unit; Fe-centred Zn12 and/or Fe-centred (Zn,Fe)12 icosahedron. Although the crystal structures of the α-Fe, Γ and Γ1 phases with cubic symmetries are related with each other in terms of the arrangement of structural vacancies, there is no orientation relationship either between the Γ and Γ1 phases or between the α-Fe and Γ phases in the coating layer of galvannealed steel sheet.

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.