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This paper underlined the corrosion behavior of aluminum alloy/galvanized steel joints fabricated by resistance spot welding (RSW) and self-piercing riveting (SPR) exposed to different salt-spray cycles. It was found that the crevice generated by different joining methods has an important impact on corrosion behavior. Compared with the RSW joint, less corrosion happened on the coupled regions in SPR joints because of the higher local pH value resulted from the smaller crevice. The crevice, however, has less impact on the types of corrosion products. Galvanic corrosion happened in the coupled region for both RSW and SPR joints. The Pourbaix diagram explained the stable corrosion products of Al and Zn Fe alloy, which are Al2O3, ZnO, and Fe3O4. Additional corrosion products observed from the experiment included α-Al(OH)3, Ca4Al2(CO3)(OH)12(H2O)5, Zn5(OH)8Cl2(H2O), Zn5(CO3)2(OH)6, and Fe2(OH)2(CO3), which can be understood by considering the metastable corrosion products, Pourbaix diagram, and possible chemical reactions.

This paper studied the effects of two welding modes, i.e., keyhole penetration and full penetration, on laser welding of two zinc-coated steel stack-ups of the same total sheet thickness but different sheet thickness combinations. The effects of welding modes on keyhole and spatter behavior were studied. It was found that keyhole penetration welding led to little spatter and mass loss for a thick-gage stack-up of the same top and bottom sheet thickness (two 1.5 mm zinc-coated steel sheets, viz., Stack-up S). This was confirmed by numerically calculated low Zout values which indicate a low potential of spatter due to zinc outgassing insufficiency. For a stack-up of thin top and thick bottom sheet combination (1.1 mm/1.9 mm, viz., Stack-up D), full penetration mode is more preferred generating less spatter than the keyhole penetration mode. This was attributed to an enlarged keyhole size at the faying interface in the full penetration mode and the relatively thinner top sheet (1.1 mm thick) compared to the bottom sheet (1.9 mm thick). It was confirmed by the low average and maximum values of Zout. In summary, to reduce the spatter in laser welding of zinc-coated steel, the keyhole penetration mode welding is preferred for the stack-up with the top and bottom sheets of similar thickness, and the full penetration mode is more suitable for the stack-up having a much thinner top sheet than the bottom sheet.

Resistance spot welding of aluminum alloys (Al RSW) is known to be very challenging, in part due to the undesirable and hard-to-predict asymmetric growth of the weld nugget. In this paper, two 1.2 mm thick AA6022-T4 aluminum alloy sheets were joined by RSW using multi-ring domed (MRD) electrodes to experimentally investigate the effect of sheet surface condition, electrode surface morphology, electrode material, weld schedule, and electrode tip condition on asymmetric nugget growth, as well as the resulting weld microstructure and coach peel strength. It was found that asymmetric nugget growth was most strongly influenced by sheet surface conditions and electrode materials at the positive polarity electrode side. The asymmetric nuggets exhibited a smaller nugget angle in relation to the faying surface and a wider partially melted zone (PMZ) in the sheet adjacent to the negative electrode which reduced weld mechanical strength and toughness. In addition, the asymmetric nugget growth also caused asymmetric electrode/workpiece interaction.

This paper describes an experimental study in which a traditional resistance spot weld process with Multi-Ring Domed (MRD) welding electrodes was used to join AA5754-O to low carbon steel sheets in a coach peel configuration. A linear relationship was identified for the resistance spot weld C-gun stationary arm deflection with force applied by the movable gun arm. Due to the relative rotation of the electrodes during deflection, the point of electrode/coupon surface contact moved away from the original centerline to a point inboard towards the C-gun arm. This led to asymmetric features in aluminum/steel resistance spot weld morphology, intermetallic compound (IMC) characteristics, weld defect distribution, and aluminum hardness. The aluminum weld nugget solidified closer to the weld gun with respect to the centerline of the MRD electrode imprint and exhibited a softer thermo-mechanically affected zone (TMAZ) and expulsion which produced a thinner IMC layer and fewer oxide film defects. This in turn resulted in strong button pull-out strength and the welds were able to absorb 36% more energy under an applied directional load in comparison to aluminum weld nuggets that solidified further away from the weld gun with respect to the electrode imprint centerline. The mechanism responsible for these weld asymmetries was further revealed through analysis of the variation in contact condition between electrodes and workpieces due to weld gun deflection.

To achieve high quality resistance spot welds (RSW) through intelligent manufacturing of automotive body structures, adaptive control for RSW processes becomes imperative and has gained global attention. Tracking ideal process signal profiles predetermined offline is a commonly-used strategy to realize the adaptive control of RSW and is found effective in compensating for the adverse impacts caused by shunting and electrode wear. However, the validity of this control strategy under other types of disturbance is still unproven. In this article, typical abnormal welding conditions, i.e. initial sheet gaps and edge proximity, were designed to test the performance of output tracking control strategies for steel RSW. Two types of process signals including dynamic resistance and electrode displacement were tracked to approach the referenced signal profiles through the adjustment of weld parameters. Comparative studies were conducted based on metallographic analysis and mechanical tests to validate the two tracking control strategies. Results demonstrated that gap and edge proximity conditions decreased the nugget diameter and the tensile-shear strength of the welds. When satisfactory tracking of the reference resistance signal was realized under these two types of abnormal conditions, weld properties are further degraded, owing to a reduction in welding current. On the contrary, tracking the electrode displacement signal mitigated the negative influences from the disturbance through increasing welding current properly. When the disturbance was too severe, the displacement-based strategy raised the risk of expulsion. The findings suggest that tracking the dynamic resistance signal is not optimal for adaptive control of steel RSW, while tracking the electrode displacement signal is more effective. However, to achieve a more satisfactory adaptive control result, the output tracking control should be used in combination with expulsion suppression techniques.

The laser ablation treatment has been validated to be an effective surface treatment method in improving adhesive bonding performance for metal or composite materials. This study was further conducted to investigate the effect of laser spot overlap ratio on surface characteristics and resultant adhesive bonding strength for AA6022-T4 Al alloy. The laser spot overlap ratio was changed by varying the laser processing speed and line spacing between laser scanning lines. Surface characteristics analyses show that the laser ablation treatment at higher overlap ratio produces rougher surface morphology and thicker aluminum oxide layer on substrate surface, which are considered to be more favorable for adhesive bonding strength. Lap-shear strength results show that the laser ablation treatment at overlap ratio of -49 % (corresponding with separated laser spots) improves the joint strength by 6.8 % and 17.1 % before and after water soak exposure compared to as-received condition. Meanwhile, the desired cohesive fracture mode is also achieved which indicates that the joint strength reaches to the saturated value depending on mechanical performance of adhesive. This explains that the continuous increase of laser spot overlap ratio from -49 % to 63 % exhibits negligible improvement on joint strength. Nevertheless, although inducing a higher degree of surface physicochemical modifications on Al substrates, the laser ablation treatment with an overlap ratio beyond 63 % decreases the joint strength. Detailed studies of SEM micrographs verified that the decreased joint strength is attributed to insufficient wetting of adhesive on the rough surfaces of laser-treated substrates resulting in entrapment of air between substrate and adhesive layer, which forms micropores defects in the adhesive layer and consequently weakens the adhesive-bonding joints. In addition, the improved corrosion resistance of adhesive-bonded Al joints by laser ablation treatment is attributed to the barrier effect of laser-produced rough surface morphology to water diffusion along bonding interface.

Ultrasonic metal welding (UMW) has been widely applied as a high throughput solid-state joining technology for multilayers of sheet metal. During a typical UMW process, multilayer work materials are mechanically compressed by a knurl-patterned horn (also known as a sonotrode) onto an anvil tool, and a simultaneous in-plane sliding is applied to the horn at an ultrasonic frequency (20 kHz or higher) to help form the weld at the material interfaces. There is a great challenge in modeling and simulating the dynamic behavior of the work material and the whole weld formation process is subject to ultrasonic mechanical loadings imposed by the knurl-patterned horn tool. In this work, finite element (FE) models are developed to simulate the multilayer UMW process using knurl-patterned tools by directly applying the ultrasonic vibration as a model input. For a short weld duration of 0.1∼0.5 s, a high-fidelity FE modeling approach is developed using ABAQUS/Explicit to simulate the dynamic material response under the 20 kHz horn vibration. For an extended long welding duration of approximately 1.0 s, a computationally efficient hybrid approach is developed using both ABAQUS/Explicit and DEFORM-3D in order to leverage the strengths of each software package. The developed models are validated using experimental data of dynamic welding force, temperature, and weld geometry from in-situ process measurements of UMW. The 3D FE models developed in this study are the most comprehensive solution to date to simulate the complex material response subject to UMW process conditions and provide engineering guidance for the design of UMW applications.

Laser brazing has recently become a preferred process for the joining of thin-gauge sheet metal for outer closures in automotive body structures. One of the key concerns is the very small and often hard-to-predict surface distortions resulting from the transient and high-powered laser heat input. In this study, we established an efficient shell-element-based sequential thermal-structural finite element simulation procedure to predict the laser-brazing-induced surface distortion. The procedure was experimentally validated via both temperature and distortion measurements in laser brazing of two 0.65 mm thick L-shape steel plates. Following the established procedure, a series of simulation cases were carried out to parametrically study the effects of laser beam offset, clamping conditions and trailing cooling on the predicted distortions. It was found that strong fixturing and trailing cooling can effectively reduce brazing-induced distortion.

In this paper, a traditional resistance spot welding (RSW) process in combination with a GM-patented Multi-Ring Domed (MRD) electrode was used to join two types of aluminum alloys, AA5754-O and AA6022-T4, to interstitial-free low carbon steel (LCS). Parallel studies were carried out for AA5754-LCS and AA6022-LCS resistance spot welds to investigate the effects of aluminum contact resistance on the weld profile, interfacial microstructure, defect distribution, and coach peel performance. The results indicated AA5754-O develops a higher contact resistance when ex-posed to atmospheric conditions. This resulted in a degradation of the RSW process due to increased internal expulsion of the molten aluminum nugget and concurrent reduction in aluminum nugget size that contributed to a loss in joint mechanical performance. By contrast, AA6022-T4 exhibited a lower contact resistance, which minimized internal expulsion and promoted the retention of larger aluminum nuggets. The larger AA6022 weld nuggets exhibited improved mechanical performance in comparison to the smaller 5754 nuggets.

Resistance spot welding (RSW) is the predominate joining method used for steel auto body assembly. Significant effort has been made to control and inspect the RSW quality in real time to guarantee the security and the reliability of body structure joints. Among the various methods, monitoring of the surface indentation is proven to be a promising method to actualize online weld quality assessment. In this article, a new approach to online measurement of indentation depth for the RSW process using a servo gun is proposed. The approach entails the development of a modified measurement system to eliminate the transmission error of the toothed belt drive and compensate the deflection of the fixed electrode arm, which enables precision measurement of the electrode tip displacement (ETD). Furthermore, consideration is given to explaining the downtrend of the displacement curve during the hold time within the RSW process, and the indentation depth at ambient temperature is finally estimated through curve fitting and experimental correction. The experimental results show that the error of indentation estimation could be limited to $\pm 30~\mu \text{m}$ ( $\pm 2\sigma$ ) at 95% confidence level, which far surpasses the performance of the previous method. Based upon sensitivity analysis, the precision is proven to be robust to different workpieces ranging from low-carbon steel to high-strength steel.

Resistance spot welds were produced between dissimilar 1.2-mm-thick wrought aluminum alloy AA6022-T4 and 2.0-mm-thick interstitial free (IF) steel with acceptable joint strength using symmetric and asymmetric electrode combinations that incorporate a multiring domed (MRD) electrode and multiple solidification weld schedules. The focused ion beam and transmission electron microscopy results indicated the intermetallic layer that was created consisted of two distinct layers, i.e., a needle-like FeAl3 adjacent to the AA6022-T4 sheet and a tongue-like Fe2Al5 adjacent to the IF steel sheet. A new mini-shear test was developed to directly measure the shear strength of the intermetallic layer formed at the interface of these welds produced using symmetric and asymmetric welding electrodes. The results showed the intermetallic layer near the aluminum weld nugget periphery, which is the region critical for joint strength, had an intermetallic layer less than 2 μm thick and an average shear strength of 64 MPa, irrespective of the welding electrodes used. In addition, the tensile shear strengths of the aluminum-steel welds were greater than comparable aluminum-aluminum resistance spot welds.

To prevent zinc vapor–induced spatter, pre-scanning was proposed as a means to evaporate the zinc coating prior to the welding process. This paper studied the effectiveness of various pre-scanning parameters such as beam defocusing distance, beam power, scanning velocity, and geometric scanning path. The gap conditions during pre-scanning were also studied in regard to outgassing of zinc vapor during pre-scanning. To complement the experimental testing, a thermomechanical numerical model was developed and validated by comparing the size of the predicted versus the physical fusion zone. The effectiveness of the pre-scanning process was evaluated by examining the temperature distribution at both the top and faying surfaces of the stack-up under the condition that the temperature at the faying interface is greater than the zinc boiling point through the entire pre-scanning process to prevent condensation of the zinc vapor back onto the faying interface. The study found that no gap at the faying interface should be allowed with the use of pre-scanning to ensure an effective heat conduction across the faying interface. When the pre-scanning path is spiral or circular, the beam translation should initiate at the center and work outward towards the periphery. The line energy is suggested to decrease as the radius increases to avoid excessive accumulation of heat in the path center.

The automotive industry is developing designs and manufacturing processes for new generations of electric motors intended for use in hybrid and electric vehicles. This paper focuses on solid-state welding to join copper end rings to copper spokes in the fabrication of copper rotors. Friction stir welding was explored to examine weldability of these copper components. Defect free welds were produced on 8-9.5 mm thick copper plates with copper spokes through slots at a travel speed of 225 mm/min and a tool speed rotation of 2250 rpm using lanthanated tungsten as the welding tool. The H13 and nimonic tool materials underperformed due to higher welding temperatures. The conditions were optimized by monitoring the tool and copper plate temperatures. Several tool geometries were investigated and the scrolled shoulder with tri-flat threaded pin performed the best. The strength and weld characteristics were sufficient for this application. Finally, this work puts forth major welding process for induction copper rotors.

Resistance spot welding (RSW) of Al-Si-coated PHS was undertaken, and the effect of Al-Si coating on nugget formation and mechanical properties was investigated. Press-hardened steel (PHS) has long been applied to automotive body structure construction to support mass and corresponding greenhouse gas emission reductions. PHS materials are often combined with an Al-Si coating applied as an oxidation barrier though unfortunately, the Al-Si coating poses a challenge to the resistance spot welding (RSW) of PHS containing stack-ups. As a newly developed coating for the hot stamping process, the property of Al-Si coating is different from base metal and traditional coatings, and the influence mechanism of Al-Si coating on the welding process is not clear. It would remain at nugget edge and might cause severe stress concentration. To investigate this problem, RSW of 1.5-mm Al-Si-coated PHS was undertaken, and the results indicate that a large portion of the Al-Si coating is extruded and forms a sharp notch close to the nugget edge during the welding process. During the post-weld cooling stage, a thin layer of residual coating is formed between the nugget and notch root. The mechanical performance of the welded joints is limited by the thin residual Al-Si layer which acts as a preexisting crack and supports the interfacial fracture. The presence of the Al-Si coating at the faying interface also significantly delays nugget formation, though it contributes to a larger nugget size by inhibiting expulsion events at the faying interface.

Laser triangulation and image processing are two major methods for surface quality inspection. The former has the advantage of three-dimensional information, while the latter can provide more detailed characteristics. However, there are few researches on effective integration of the two methods. In this paper, a lab scale laser braze quality inspection system utilizing information from laser triangulation scanning and optical image acquisition was able to inspect an automotive laser braze at a scanning speed of 1.5 m/min with 100% accuracy. The process started with defect zoning to sort out braze areas with and without defect based on the joint surface profile reconstructed from laser triangulation. Feature extraction was then carried out on the images of defect-containing braze areas, in which the location, size, geometry, and texture features of the defects were extracted from the gray-scale images, and since these features were extracted from only defect-containing areas, a lot of computational cost was saved, which reduced time consumed greatly. For defect classification, in the training stage the extracted features were processed by the principal component analysis and the first three principal components were calculated for 5 typical defects: surface cavity, pore, hump, burn-through, and lack-of-braze. In the test stage, the first three principal components of the images of interest were calculated and compared with the established principal component values of the 5 defect types for defect classification.

Al-Si coated press hardened steel (PHS) were introduced into vehicle body structures for a balance of lightweighting and crashworthiness. However, the weldability of PHS is relatively poor, due to its high carbon equivalent and high strength and is characterized by severe heat affected zone, HAZ softening, and interfacial fracture. The interfacial fracture mode is defined in part by the presence of retained Al-Si coating as a sharp notch at the faying interface following the welding process, which limits the mechanical performance of the joint. To improve the weldability of PHS, a stepped current pulse schedule was proposed for the resistance spot welding (RSW) of 1.5 mm Al-Si coated PHS as a means to effectively promote nugget growth, inhibit expulsion and eliminate the residual Al-Si layer. In this manner the mechanical performance of joints were significantly improved and the desirable button pullout fracture mode was realized.

For the automotive industry, it is important to predict the fracture mode of resistance spot-welded joints. Traditional 4t0.5 (where t is the sheet thickness) criterion for the transition from interfacial fracture (IF) to button pullout fracture (BPF) mode is not applicable for press hardened steel, press-hardened steel (PHS), welds. This study aims to investigate the effect of Al–Si coating and critical heat affected zone (CHAZ) location on the lap-shear fracture mode transition mechanism of Al–Si-coated PHS welds. It was found that in-situ tempering pulse after the welding stage could change the relative location of the CHAZ and weld nugget, which in turn, had a significant effect upon the fracture mode transition. Thus, a new analytical model was built to predict the critical nugget size for Al–Si-coated PHS, wherein the Al–Si coating and the CHAZ location are considered as critical factors for predicting the fracture mode transition of PHS spot welds and are incorporated into this model. A reasonable correlation of the model to experimental data was achieved.

Friction surfacing is a new variation of friction stir processing for surface property modification of metallic substrates. There is an increasing body of literature about friction surfacing by deposition of metal from a consumable tool to a solid substrate. Friction surfacing has many potential applications in joining, coating for corrosion resistance, and repair of degraded components. This article presents a review of the basic principles and latest research organized by processing techniques and variations, thermomechanical transfer and deposition of material, and finally metallurgical, mechanical, and chemical properties of the resulting deposition. Different friction surfacing processes are reviewed of novel tool–substrate configurations for material deposition for noncoating purposes like keyhole filling and joining dissimilar materials. Possible future topics of study for this area are discussed, which include deeper understanding of material transfer through metallurgy, FEM, and scale up of the technique for practical application.

Resistance spot welds with three aluminium to steel sheet thickness ratios were welded using electrodes featuring a plurality of rings and multiple solidification weld schedules. The averag...

This paper discusses the unique application of numerical modeling of remote laser spiral spot welding of zinc-coated steels to reduce spatter occurrence. A thermo-fluid process simulation model was developed to simulate the laser-material interaction, material melting, keyhole formation and molten pool dynamics occurring in remote laser spiral spot welding of zinc-coated steels. Based upon numerically calculated thermal profiles during welding, the instantaneous zinc vapor pressure between faying surfaces was estimated. It was found that high vapor pressure points correlated well with spattering events observed in high-speed videos taken of the molten pool during welding. Using the estimated zinc vapor pressure as an indicator of spatter occurrence, the welding schedule was redesigned to limit high zinc vapor pressure and rapid pressure increase for spatter prevention. The revised schedule was verified by physical testing and resulted in greatly reduced spatter.

Surface treatment of the Al surface by pulsed Yb: fiber laser has proven to be effective in improving the adhesive bonded performance of sheet Al substrates. This study was conducted to expand the initial investigation to include sheet, extrusion, and cast Al alloys and measure the effect of laser ablation treatment on the bond strength both in ambient and after water soak. It is found that laser ablation treatment improved the joint strength for all three Al alloys through modification of the surface characteristics and exceeded the target bond strength. Moreover, the results after water soak support the conclusion that the laser ablation treatment exhibited a significant improvement in corrosion resistance of the adhesive-bonded Al joints compared to as-received condition and resulted in the required cohesive fracture mode even after water soak. The reason for this is attributed to the retarding effect of rough laser-ablated surfaces on water diffusion along the bonding interface of laser-treated joints. Besides, the improvement of laser ablation treatment on joint strength before and after exposure increases with the laser fluence. The optimized laser fluence for all three Al alloys is 26.3 J/cm2, which is attributed to a higher degree of laser-induced surface modifications.

With growing demand for better fuel economy for automobiles, multimaterial solutions are increasingly being utilized in the automotive industry for reducing weight in the vehicle body structure. This poses challenges in terms of joining dissimilar metals, especially those with vastly different properties such as aluminum to steel joining. General Motors has developed a new resistance spot-welding technique for dissimilar materials using a multi-ring domed (MRD) electrode and multiple solidification weld schedules to address this challenge. Originally developed for aluminum to aluminum resistance spot welding, this technology is being deployed as the mainstream aluminum joining solution to leverage existing infrastructure and workforce competency in resistance spot welding. With the recent expansion of MRD technology to aluminum to steel resistance spot welding, there is an ever-greater need to experimentally verify the quality of each aluminum to steel resistance spot-weld application with limited time and resources. Nondestructive evaluation (NDE) would enable the transfer of resistance spot-welding technology to dissimilar aluminum to steel joints. This article describes the current state of the art of aluminum to steel resistance spot welding and the challenges in developing a robust NDE process for this technology.

Several types of materials are commonly used in automotive body structures, driving the need for joining of dissimilar material joints of aluminum alloys to steels by various techniques including resistance spot welding. Al-steel spot welds can exhibit variable structures and properties, even when produced with the same welding schedule, welding equipment, and with the same combination of materials. Laboratory test conditions can correct for this variability; however, this is not always a realistic representation of production conditions. This study evaluated the influence of production conditions such as gaps between sheet metal, angles between the sheet metal and welding electrode, changes in current, and cap length on the microstructural and mechanical properties of Al-steel spot welds. A designed experiment was used to weld tensile (lap) shear, coach peel, and metallographic specimens under various combinations of production conditions. Comparative t-test and quadratic regression statistical analyses were performed to quantify the production conditions that significantly affect the robustness of the Al-steel spot welding process. Compared to analogous Al-Al specimens, the Al-steel tensile shear specimens exhibited greater strengths and were robust to being produced under various production conditions. However, the Al-steel coach peel specimens were sensitive to some of the production conditions, especially those that combined off normal sheet angles and gaps between the metal sheets. These differences in behavior are discussed, including the underlying fundamental metallurgical and physical mechanisms.

A 2-step schedule, consisting of a pre-cleaning step followed by a welding step, and an electrode design incorporating a multistep face geometry were developed to address issues of inclusions in the aluminum fusion zone and over thickening of intermetallic compound (IMC) layer in weld bonding of aluminum to steel. The pre-cleaning phase used a multi-pulse current to expel the adhesive from the contacting faces. The subsequent welding phase created a sound joint by growing aluminum nugget diameter while limiting IMC thickening and breaking down remaining inclusions. The multistep electrode contributed to effectiveness of the cleaning by boosting expulsion in the pre-cleaning stage. It also reduced reaction between the electrode and the outer surface of the aluminum via breaking down the oxide film of the aluminum and hence reducing contact resistance. The uncured RSWB joints, generated from the 2-step schedule together with the multistep electrode, had approximately 95% improvement in lap shear peak load compared to joints fabricated using the conventional 1-step welding schedule with a dome electrode. The cured RSWB joints achieved approximately 20% and 80% improvement in the peak load and energy absorption, respectively, in coach peel tests.

Resistance spot welds were produced using GM’s patented welding process. The stack-ups consisted of 0.8 mm thick X626 aluminum and 0.9 mm thick low carbon steel (LCS) with three different coating types: hot dipped galvanized (HDG), Zn-Ni coating, and Zn-Ni coating with a passive layer containing trivalent clear chromate (Zn-Ni-Cr). The addition of Ni or Cr to the Zn coating provides a unique microstructure compared to that for welding to HDG coated steel by creating an increased porosity, forming agglomerated particulates near the weld nugget periphery, eliminating the oxide film defect, and retarding the formation of the Fe2Al5 layer. The RSW-HDG samples exhibited the largest nugget size and least thinning, which increased tensile strength for tensile-shear and coach-peel specimens. The shorter fatigue life of RSW-ZnNi and RSW-ZnNiCr compared to RSW-HDG was attributed to defects observed along the weld nugget perimeter and a smaller weld nugget diameter which was demonstrated using the structural stress concept.

Resistance spot welding of aluminum alloys is increasingly used in the automotive industry for vehicle lightweighting and better fuel efficiency. In this contribution, General Motor’s aluminum spot welding process including Multi-Ring Domed (MRD) electrodes and Conditioning, Shaping, and Sizing (CSS) weld schedule were used for welding of 0.8-mm thick X626-T4 aluminum alloy sheet. The resultant spot welds had an equiaxed grain structure adjacent to the weld nugget periphery. Within the weld nugget, fine columnar grains were found at the nugget perimeter, while welding defects were concentrated in the centre of the nugget. Using modified shear specimens, it was determined that the weld nugget is the weakest location while the heat affected zone is enhanced by precipitation aging due to a combination of welding and paint bake thermal cycles. We then derived a new equation for calculating minimum weld nugget diameters to ensure that interfacial fracture would not occur during tensile and fatigue testing. Load controlled fatigue test results showed that using structural stress analysis, fatigue life curves for both lap-shear and coach-peel configurations fell onto a single master curve indicating that the weld nugget size is the controlling parameter for fatigue life. While fracture mode varied with load amplitude from full button pullout at high amplitude to eyebrow-like fracture with kinked crack paths at low amplitude, no interfacial fracture occurred during fatigue testing due to the large size of the weld nugget even though the weld nugget is the weakest location.

With increasing demands to improve vehicle fuel economy, multi-material body structures are increasingly utilized in the automotive industry for structural lightweighting purposes. These multi-material structures pose challenges in dissimilar material joining, particularly aluminum to steel. General Motors (GM) developed a new resistance spot welding technique using a multi-ring domed electrode and multiple solidification weld schedules to address these challenges. In aluminum-steel resistance spot welds (RSWs), an iron-aluminum intermetallic compound (IMC) layer is formed at the interface and its strength affects tensile shear specimen fracture modes, i.e. interfacial versus pull out fracture. Based upon the experimental heat affected zone (HAZ) and IMC shear strengths using a new mini-shear test specimen, it was observed that it was not suitable to use the critical weld nugget diameter of 4√t recommended by the American Welding Society (AWS) to determine the fracture modes of these unique aluminum-steel spot welds. In the present study, a new formula considering the shear strength of intermetallic layer in aluminum to steel RSWs is derived to calculate a critical aluminum-steel weld nugget diameter based upon experimental results. The calculated critical weld nugget diameters were then compared with experimental results to predict fracture modes for aluminum-steel stack-ups with different sheet thicknesses.

Unique spindle microstructures with an apex angle of ∼20° bring the ability of directional water collection to various biosystems (i.e., spider silk and cactus stem). This has great potential to solve the insufficient interfacial wetting for mechanical interlocking formation between polymers and substrates. In this study, the bioinspired spindle microstructures were easily fabricated through the deposition of molten materials by a nanosecond laser with an overlap ratio of 21% between laser spots and achieved superior interfacial wetting for commercial epoxy adhesive on aluminum substrates. Detailed analyses show that there are four mechanisms responsible for the superior interfacial wettability of bioinspired spindle microstructures: the Laplace pressure difference, newly formed aluminum oxide, the capillary effect, and no extra pressure from a trapped atmosphere. Consequently, the bioinspired spindle surface microstructures achieve a maximum improvement of ∼16 and ∼39% in interfacial bonding strength before and after water soak exposure compared to the as-received condition. Moreover, the stable interfacial wettability of bioinspired spindle microstructures ensures that the improved joint strength varied little with an increase in surface roughness from ∼1.7 to ∼12.8 μm. However, the interfacial wettability of common dimple microstructures deteriorated with an increase in surface roughness, which is indicated by the decreasing rule in the quadratic polynomial function of the interfacial bonding strength as the surface roughness increases from ∼2.1 to ∼18.2 μm.

Laser spot welding method was employed to fabricate Al Si coated press-hardened steel (PHS) joints in overlapped configuration. Al and Si segregate at the notch root of a laser spot weld of Al Si coated PHS resulting in pre-mature cracking and weakening of the joint strength. The thickness-oriented δ-ferrite strips induced by the Al and Si segregation was mainly responsible for the mechanical deterioration though the dispersive δ-ferrite in the fusion zone also contributes. A supplemental annular stirring with beam oscillation is proposed to break up the segregation. With the annular stirring, the lap-shear peak load and energy absorption increased 57% and 80% respectively, which is attributed to the removal of δ-ferrite strips and the reduction in the fraction of dispersive δ-ferrite. Thermo-fluid numerical simulations suggest that the annular stirring promoted the melt flow and extended the duration of the melt pool, which accelerated and prolonged the diffusion of Al and Si atoms into the fusion zone, thus preventing the segregation of Al and Si and formation of δ-ferrite strips. Meanwhile, the prolonged melt flow pulled additional dissolved Al onto the molten pool surface forming the Al oxide film, resulting in further reduction of Al content and fraction of dispersive δ-ferrite.

Al/steel joints are increasingly used in the automotive industry to meet the requirement of energy saving and emission reduction. Among various joining technologies, self-pierce riveting (SPR) and resistance spot welding (RSW) are two promising technologies to fabricate dissimilar joints with stable and high mechanical performance. However, corrosion will occur in these joints inevitably due to different electrochemical properties, which can degrade the surface quality and the mechanical performance, including strength, ductility, etc. In this paper, 1.2 mm AA6022 and 2.0mm HDG HSLA340 are joined by SPR and RSW. After the fabrication of these Al/steel joints, cyclic corrosion tests are performed, which lasts 26 cycles and 48 cycles. By comparing the microstructure of the joints with and without corrosion, different corrosion mechanisms in SPR and RSW are revealed, including the corrosion initiation and propagation.

Resistance spot welding (RSW) of aluminum–aluminum (Al–Al) is known to be very challenging, with the asymmetric growth of the weld nugget often observed. In this article, a semicoupled electrical–thermal–mechanical finite element analysis (FEA) procedure was established to simulate the RSW of two layers of AA6022-T4 sheets using a specially designed Multi-Ring Domed (MRD) electrodes. Critical to the modeling procedure was the thermoelectric (including the Peltier, Thomson, and Seebeck effects) analyses to simulate the asymmetric nugget growth in the welding stage. Key input parameters such as the Seebeck coefficients and high-temperature flow stress curves were measured. Simulation results, experimentally validated, indicated that the newly developed procedure could successfully predict the asymmetric weld nugget growth. Simulation results also showed the Seebeck effect in the holding stage. The simulations represent the first quantitative investigation of the impact of the thermoelectric effects on resistance spot welding.

In a comparative study of resistance spot welded (RSW) 1.2 mm AA6022 joined to 2.0 mm high strength low alloy (HSLA) steel and 1.2 mm AA6022 joined to 2.0 mm low carbon steel (LCS) it was found that quasi-static tensile tests and mini shear tests exhibited similar intermetallic compound (IMC) strength but different load capacity, i.e., nominal shear strength in tensile shear tests. This phenomenon was related to the Fe2Al5 structure and properties at the Al-steel interface which were unique for each stack-up. The thicker Fe2Al5 layer in 1.2 mm AA6022-2.0 mm LCS was comprised of fewer but larger grains which contributed to a larger difference between the Young's modulus of FeAl3 and Fe2Al5 than in 1.2 mm AA6022-2.0 mm HSLA. The Fe2Al5 in the AA6022-LCS stack-up was more brittle and rigid with greater nanohardness values and smaller variation in elastic modulus that may have contributed to lower overall joint strength with large variation. In addition, AA6022-LCS welds produced with non-optimized and optimized weld schedules produced different IMC shear strengths which was attributed to the different mismatch in Young modulus values that accentuated the differences in shear stresses at the FeAl3/Fe2Al5 interface.

General Motors (GM) has developed a proprietary resistance spot welding process using a Multi-Ring, Domed (MRD) electrode geometry that is capable of producing welds between aluminum alloys and steel materials with acceptable joint strength. This work presents fatigue properties of resistance spot welds (RSWs) produced between dissimilar 1.2-mm thick wrought aluminum AA6022-T4 and 2.0-mm thick IF steel RSWs in tensile-shear and coach-peel configurations. The results were compared to those of spot welds made of 1.2-mm thick and 2.00-mm thick AA6022-T4. The tensile-shear and coach-peel spot welds exhibited limited scatter in fatigue life for both stack-ups. The overall fatigue life of the tensile-shear AA6022-T4 to IF stack-ups was much greater than that of the AA6022-T4 to AA6022-T4 stack-ups. However, in coach-peel, there was no significant difference in fatigue life between the two stack-ups. In both tensile-shear and coach-peel fatigue tests the fracture mode for both stack-ups was primarily crack growth through the 1.2-mm thick AA6022-T4 sheet. The superior performance of the tensile-shear AA6022-T4 to IF steel RSWs was most likely a result of larger weld nugget size along with microstructural features that improved performance. These included more favourable notch root openings as well as columnar grain growth and alloying of the aluminum nugget with iron that could retard fatigue crack propagation.

As the stress-strain data of the fusion-brazed filler material AA4047 could not be easily or directly measured, an isotropic strain-hardening model of the fusion zone material AA4047 was estimated by an inverse method to quantify the tensile behavior of AA6111-T4 coach-peel joints. The Hill’s 1948 quadratic anisotropic yield stress function for the base sheet metal AA6111-T4 was first determined by conducting uniaxial tensile tests on the sheet metal in the rolling, diagonal, and transverse directions. Based on the yield stress function and isotropic strain hardening behavior determined for the base sheet metal, the linear-exponential strain hardening model was used to identify the fusion zone material AA4047 by an iterative correction and image-based finite element analysis of the welded joint. Modeling of joints with and without flawed brazing interface showed that the load-carrying capacity of coach-peel joints was insensitive to the existence of the brazing zone metal AA4047. When the fusion zone model was also applied to the brazing zone, the predicted and measured force-displacement curves of the coach-peel joint matched well. The failure stress of the coach-peel joint was identified as the von Mises fracture stress of 100 MPa at the brazing interface.

A multi-scale finite element modeling approach was developed to study the deformation and fracture behavior of Al-steel resistance spot welds. First, a micro-scale model was applied to simulate the mechanical responses of the intermetallic compound (IMC) layer having various morphologies and thicknesses under tensile and shear loading conditions. Second, the predicted tensile and shear strength of the IMC layer, that varied along the joint interface per the IMC layer morphology and thickness variation, was then introduced into 3D macro-scale models to predict the overall mechanical performance of weld coupons under coach peel, lap shear and cross-tension testing conditions. The numerical predictions agreed reasonably well with the experimental data.

The effects of surface topography modified by laser ablation on adhesively bonded joint strength and toughness are quantified. Model joints consisting of AA7075-T6 substrates, a high strength aluminum alloy, and a commercial structural adhesive, were investigated with both tensile-shear and double cantilever beam (DCB) specimens. Surface topography was manipulated through varying single crater morphology and crater-to-crater spacing on the substrates with a pulsed Yb-fiber laser. Relationships between single crater morphology and energy related process parameters were qualitatively established. Three regimes of crater spacing, i.e. overlapping, tangential and separated, were identified and connected to key process parameters that control roughness amplitude and spacing. For this purpose, a new energy parameter that links laser pulse energy to resulting crater geometry and spacing was developed. It was found that both joint strength and toughness increase with S a , the 3D arithmetic mean roughness height deviation, at low and mild (

In present study, the interfacial microstructural evolution with intermetallic compound (IMC) growth and its influence on mechanical properties was investigated in resistance spot welding of aluminum to steel. Fe2Al5 was identified as the dominant phase in the reaction layer formed at the joint interface. It was observed that as Fe2Al5 grew, it evolved from randomly oriented fine grains to coarse grains with preferred orientation along [001] direction, perpendicular to the Al/steel interface, while the volume fraction of iron phase between the Fe2Al5 grains decreased. Via numerical simulation, it was found that the majority of Fe2Al5 growth occurred when interfacial temperatures was over 900℃. Fe2Al5 growth tended to reduce the joint strength and lead to interfacial fracture. Cracks within Fe2Al5 mainly consisted of primary cracks propagating along the interface and secondary cracks approaching the steel side, while the primary cracks became the dominant ones as the IMC thickness increased. The estimated in-situ fracture toughness of Al/steel interface decreased from 1.07 MPam1/2 to 0.35 MPam1/2 as the Fe2Al5 layer thickness increased from 3 μm to 6.5 μm, where a dramatic drop to 0.56 MPam1/2 happened when the thickness reached 4 μm. The IMC growth-induced degradation of mechanical performance was attributed to the increasing texture and grain size of the Fe2Al5 phase and diminishing iron phase between the Fe2Al5 grains.

Under tensile shear loading, fracture modes of dissimilar lap welds produced by friction stir scribe technology were studied. Three fracture modes were observed. For zone A fracture, the initial cr...