Showing papers on "Spot welding published in 2013"

Critical review of automotive steels spot welding: process, structure and properties

Abstract: Spot welding, particularly resistance spot welding (RSW), is a critical joining process in automotive industry. The development of advanced high strength steels for applications in automotive industry is accompanied with a challenge to better understand the physical and mechanical metallurgy of these materials during RSW. The present paper critically reviews the fundamental understanding of structure–properties relationship in automotive steels resistance spot welds. The focus is on the metallurgical characteristics, hardness–microstructure correlation, interfacial to pullout failure mode transition and mechanical performance of steel resistance spot welds under quasi-static, fatigue and impact loading conditions. A brief review of friction stir spot welding, as an alternative to RSW, is also included.

Failure mode of spot welds: interfacial versus pullout

Abstract: Spot welds produced via resistance welding have been widely used in the joining of sheet metal for autobodies since the 1950s. Every modern car contains over 2000 spot welds. Failure of the spot weld is therefore an important concern in relation to autobody durability and safety design. Spot welds can fail in two completely distinct modes, namely, nugget pullout failure and interfacial failure. In the present paper, it is first shown that the nugget pullout failure is caused by plastic collapse and the interfacial failure is governed by crack or fracture mechanics. These two failure mechanisms compete with each other and failure of a spot weld occurs when the fracture criterion for one of the mechanisms is satisfied first. Test data from available literature are used to validate the theoretical predictions. Recommendations are made for minimum weld nugget size for a given sheet metal thickness so that nugget pullout failure, the acceptable mode of failure in industry, is ensured.

Induction Spot Welding of Metal/CFRPC Hybrid Joints

Abstract: Today's areas of application for lightweight materials range from consumer goods and sports to high technology applications in transportation like aerospace or automotive. Thermoplastic composites manufactured by compression molding, thermoforming, tape laying, or injection molding will play a major role due to their weldability, their suitability for automated production (robot), and their recyclability. To reach a further step of weight reduction, the use of carbon fiber reinforced polymer composites (CFRPC) is unavoidable. A full substitution of metal is unlikely and new developments will consist of a combination of metal and CFRPC. This paper shows new developments in joining hybrids of metal (steel DC01 and aluminum AlMg3) and CFRPC (organic sheets CF-PA66 and CF-PEEK). Induction heating is chosen as appropriate joining technology for the bonding of metal/CFRPC as it is characterized by a rapid heating. An explanation of the process, the equipment, the influence of surface treatments, the characterization of the bonding mechanisms, as well as a first step to automation are presented. Basic experiments on the influence of pretreatments and process parameters show great influence of corundum blasting, acidic pickling and temperature control on the shear tensile strength. Joints shear tensile strength of 14.5 MPa for AlMg3/CF-PA66 and of 20 MPa for DC01/CF-PEEK, respectively is measured. The documentation of the process parameters shows a high reproducibility and reliability of the developed equipment and demonstrator parts are successfully manufactured.

Improving aluminum resistance spot welding in automotive structures

Abstract: The majority of automotive body shop welding consists of resistance spot welding (RSW) steel sheet due to its inherently low cost and high speed. With the introduction of aluminum, it is desirable ...

Dissimilar Metal Joining Technologies for Steel Sheet and Aluminum Alloy Sheet in Auto Body

Abstract: Multi-material structure of auto body partially employing aluminum alloy sheets may be adopted in order to satisfy requests at the same time improving crash safety and lightweight. This paper described the dissimilar metal joining technologies required to achieve multimaterial structure. In resistance spot welding, high current & short time welding condition is conducted to avoid formation of brittle Fe-Al intermetallic compound layer at joint interface and joint strength is investigated. In mechanical joining, current application situation of SPR, Tog-L-Loc ® , TOX ® and FDS ® methods are outlined and application examples of Blind rivet in automobile body are introduced. In addition to that, joint strengths in shear direction of SPR joints for cold rolled steel sheets and 6000 series aluminum alloy sheets are described. In adhesion bonding, it was shown that joint strength of TSS over 20 MPa is obtained by using of recent adhesive for automobile in sheet combinations of cold rolled mild steel sheets and 6000 series aluminum alloy sheets. In FSSW, effects of holding time, rotating speed and coating layer on joint strength were clarified in sheet combinations of Super Dyma ® steel sheets and 6000 series aluminum alloy sheets. Also, in laser brazing, it was shown that joining of cold rolled or GA mild steel sheets and 6000 series aluminum alloy sheets are possible by using 4000 series aluminum alloy filler (A4043) with fluoride type NOCOLOK ® flux. Finally, direction of development needed in the future in the field of dissimilar metal joining technologies of steel and aluminum alloy was described based on past development history.

Microstructures and Mechanical Properties of Friction Stir Spot Welded Aluminum Alloy AA2014

Abstract: Friction stir spot welding (FSSW) is a relatively recent development, which can provide a superior alternative to resistance spot welding and riveting for fabrication of aluminum sheet metal structures. In the current work, FSSW experiments were conducted in 3-mm thick sheets of aluminum alloy 2014 in T4 and T6 conditions, with and without Alclad layers. The effects of tool geometry and welding process parameters on joint formation were investigated. A good correlation between process parameters, bond width, hook height, joint strength, and fracture mode was observed. The presence of Alclad layers and the base metal temper condition were found to have no major effect on joint formation and joint strength. Friction stir spot welds produced under optimum conditions were found to be superior to riveted joints in lap-shear and cross-tension tests. The prospects of FSSW in aluminum sheet metal fabrication are discussed.

Microstructure and Mechanical Performance of Friction Stir Spot-Welded Aluminum-5754 Sheets

Abstract: Friction stir spot welding (FSSW) is a recent trend of joining light-weight sheet metals while fabricating automotive and aerospace body components. For the successful application of this solid-state welding process, it is imperative to have a thorough understanding of the weld microstructure, mechanical performance, and failure mechanism. In the present study, FSSW of aluminum-5754 sheet metal was tried using tools with circular and tapered pin considering different tool rotational speeds, plunge depths, and dwell times. The effects of tool design and process parameters on temperature distribution near the sheet-tool interface, weld microstructure, weld strength, and failure modes were studied. It was found that the peak temperature was higher while welding with a tool having circular pin compared to tapered pin, leading to a bigger dynamic recrystallized stir zone (SZ) with a hook tip bending towards the upper sheet and away from the keyhole. Hence, higher lap shear separation load was observed in the welds made from circular pin compared to those made from tapered pin. Due to influence of size and hardness of SZ on crack propagation, three different failure modes of weld nugget were observed through optical cross-sectional micrograph and SEM fractographs.

Feasibility study of resistance spot welding of dissimilar Al/Mg combinations with Ni based interlayers

Abstract: Microstructure and mechanical properties of the dissimilar aluminium–magnesium resistance spot welds made with gold coated and bare nickel interlayers are investigated. Welds were made with different welding currents in a range from 16 to 24 kA with a fixed welding time of five cycles. No joints were achieved with a bare nickel interlayer; after welding, specimens were separated without applying any force. Addition of gold coating on nickel surface greatly contributed to the metallurgical bonding at the interfaces and welds easily met requirements of AWS D17·2 standard. Average lap shear strength reached 90% of similar AZ-31B spot weld strength. Fusion nugget size, interfacial microstructure and fracture surface morphology of the welds were analysed.

Number size distribution of fine and ultrafine fume particles from various welding processes.

Abstract: Studies in the field of environmental epidemiology indicate that for the adverse effect of inhaled particles not only particle mass is crucial but also particle size is. Ultrafine particles with diameters below 100 nm are of special interest since these particles have high surface area to mass ratio and have properties which differ from those of larger particles. In this paper, particle size distributions of various welding and joining techniques were measured close to the welding process using a fast mobility particle sizer (FMPS). It turned out that welding processes with high mass emission rates (manual metal arc welding, metal active gas welding, metal inert gas welding, metal inert gas soldering, and laser welding) show mainly agglomerated particles with diameters above 100 nm and only few particles in the size range below 50 nm (10 to 15%). Welding processes with low mass emission rates (tungsten inert gas welding and resistance spot welding) emit predominantly ultrafine particles with diameters well below 100 nm. This finding can be explained by considerably faster agglomeration processes in welding processes with high mass emission rates. Although mass emission is low for tungsten inert gas welding and resistance spot welding, due to the low particle size of the fume, these processes cannot be labeled as toxicologically irrelevant and should be further investigated.

Online nugget diameter control system for resistance spot welding

Abstract: The aim of this paper is to provide an integrated real-time control system for resistance spot welding (RSW), which is capable of producing welds with predetermined nugget diameters. Nugget diameter is a commonly used criterion for estimating the weld quality; thus, the system can be used for online quality control of RSW. The proposed system consists of two parts: a constant current controller and an online nugget diameter estimator. The constant current controller is used to guarantee the consistency of the nugget formation and growth during the welding process, while the online nugget diameter estimator can be used as a sensor for online estimation of the nugget diameter. The proposed online nugget diameter estimator is a mathematical function of the heat energy absorbed by the weld, which is taken into account only after the time when the first melting point of a weld appears. This time can be obtained by means of the dynamic resistance curve with a high resolution. A mathematical model of the online nugget diameter estimator is proposed and realized based on the experiment results and mathematical analysis. The welding process would be terminated when the difference between the actual nugget diameter and the predetermined desired value is within a given tolerance. According to a series of experiments, the welds used in the experiments had nugget diameters, which were well within the limits and the errors were within the given tolerance.

Metallographic and fracture characteristics of resistance spot welded TWIP steels

Abstract: In this study, the microstructural characterisation, mechanical testing and fractography investigation were performed on twinning induced plasticity (TWIP) steels, fabricated with resistance spot welding. Failure mode during the cross-tensile test was found to follow the sequences of strain localisation of both sheets, crack initiation at notch tip, crack following along the fusion boundary and, finally, ductile shear fracture along the sheet thickness direction. On the other hand, failure in the tensile shear test was always directed along the sheet/sheet (s/s) interface; the interfacial failure and shear deformation were observed at the weld centreline. Solidification occurred as a primary austenitic solidification mode, and no martensitic transformations were detected through electron backscatter diffraction analysis. The fusion zone was mainly composed of austenite with directional solidification towards the centreline; the columnar dendritic and equiaxed structures were identified. Interdendr...

Ultrasonic spot welded AZ31 magnesium alloy: Microstructure, texture, and lap shear strength

Abstract: The structural application of ultra-lightweight magnesium alloys inevitably involves welding and joining. A solid-state welding – ultrasonic spot welding (USW) – was conducted on AZ31B-H24 Mg alloy sheet by varying a key parameter of welding energy in this study, aiming to identify the changes in the microstructure, crystallographic texture, and lap shear tensile strength. The grain size was observed to increase with increasing welding energy. Crystallographic texture determined via X-ray diffraction showed a significant change that corresponded well to the change in the deformation and recrystallization mechanisms of the Mg alloy. The lap shear strength first increased with increasing energy input, reached the maximum value at a welding energy of 2000 J, then decreased. The welds generally fractured along the joint interface when the welding energy was lower than 2000 J, while fracture occurred at the periphery of the joint (button pull-out) for the samples made with an energy input of higher than 2000 J.

The Effectiveness of Surface Coatings on Preventing Interfacial Reaction During Ultrasonic Welding of Aluminum to Magnesium

Abstract: High power ultrasonic spot welding (USW) is a solid-state joining process that is advantageous for welding difficult dissimilar material couples, like magnesium to aluminum. USW is also a useful technique for testing methods of controlling interfacial reaction in welding as the interface is not greatly displaced by the process. However, the high strain rate deformation in USW has been found to accelerate intermetallic compound (IMC) formation and a thick Al12Mg17 and Al3Mg2 reaction layer forms after relatively short welding times. In this work, we have investigated the potential of two approaches for reducing the IMC reaction rate in dissimilar Al-Mg ultrasonic welds, both involving coatings on the Mg sheet surface to (i) separate the join line from the weld interface, using a 100-μm-thick Al cold spray coating, and (ii) provide a diffusion barrier layer, using a thin manganese physical vapor deposition (PVD) coating. Both methods were found to reduce the level of reaction and increase the failure energy of the welds, but their effectiveness was limited due to issues with coating attachment and survivability during the welding cycle. The effect of the coatings on the joint’s interface microstructure, and the fracture behavior have been investigated in detail. Kinetic modeling has been used to show that the benefit of the cold spray coating can be attributed to the reaction rate reverting to that expected under static conditions. This reduces the IMC growth rate by over 50 pct because at the weld line, the high strain rate dynamic deformation in USW normally enhances diffusion through the IMC layer. In comparison, the thin PVD barrier coating was found to rapidly break up early in USW and become dispersed throughout the deformation layer reducing its effectiveness.

Influence of structural integrity on fatigue behavior of friction stir spot welded AZ31 Mg alloy

Abstract: In this paper the fatigue behavior of the friction stir spot welded (FSSW) coupons of magnesium AZ31 alloy manufactured under different welding process conditions is investigated. Two sets of lap-shear coupons were made based on variation in plunge depth, tool rotation speed and tool geometry. Metallographic analysis of the untested FSSW coupons revealed differences in weld geometry and microstructure. Interfacial hooking of the faying surface was found to vary significantly between the two sets of process conditions. Microhardness test data revealed a large gradient in hardness profile. Results from the load controlled cyclic tests showed that lower tool rotation and shallower shoulder plunge depth led to better fatigue performance. Optical fractography of the fatigued coupons revealed that fatigue cracks initiated at the weld interface in both sets of coupons. However, the fracture mode showed variability between the two process conditions. The fractography analysis suggests that the effective top sheet thickness, interfacial hook and microstructure, which are largely determined by the process conditions, significantly affect the fatigue behavior of the friction stir spot welds in magnesium alloys.

Surface breaking cracks in resistance spot welds of dual phase steels with electrogalvanised and hot dip zinc coating

Abstract: In normal production of resistance spot welded galvanised structures, it is difficult to completely avoid surface breaking cracks. Known key factors to cause cracking are zinc coating, electrode we ...

Model development for quality features of resistance spot welding using multi-objective Taguchi method and response surface methodology

Abstract: In this research, the effect of parameters in Resistance Spot Welding (RSW) on the weld zone development was first investigated using Taguchi Method. Further, the RSW parameters were to be optimized based on multiple quality features, focusing on weld nugget and Heat Affected Zone using multi-objective Taguchi Method (MTM). The optimum welding parameter for MTM was obtained using Multi Signal to Noise Ratio and the significant level was further analyzed using Analysis of Variance. Lastly, Response Surface Methodology was employed to develop the mathematical model for predicting the weld zone development. The experimental study was conducted under varied welding current, weld time and hold time. To validate the predicted model, experimental confirmation test was conducted for plate thickness of 1 and 1.5 mm. Based on the results, the developed model can be effectively used to predict the size of weld zone which can improve the welding quality and performance in RSW.