Spot-welded Steel Research Articles

Failure mode transition in AHSS resistance spot welds. Part I. Controlling factors

Failure mode of resistance spot welds is a qualitative indicator of weld performance. Two major types of spot weld failure are pull-out and interfacial fracture. Interfacial failure, which typically results in reduced energy absorption capability, is considered unsatisfactory and industry standards are often designed to avoid this occurrence. Advanced High Strength Steel (AHSS) spot welds exhibit high tendency to fail in interfacial failure mode. Sizing of spot welds based on the conventional recommendation of 4t0.5 (t is sheet thickness) does not guarantee the pullout failure mode in many cases of AHSS spot welds. Therefore, a new weld quality criterion should be found for AHSS resistance spot welds to guarantee pull-out failure. The aim of this paper is to investigate and analyze the transition between interfacial and pull-out failure modes in AHSS resistance spot welds during the tensile–shear test by the use of analytical approach. In this work, in the light of failure mechanism, a simple analytical model is presented for estimating the critical fusion zone size to prevent interfacial fracture. According to this model, the hardness ratio of fusion zone to pull-out failure location and the volume fraction of voids in fusion zone are the key metallurgical factors governing type of failure mode of AHSS spot welds during the tensile–shear test. Low hardness ratio and high susceptibility to form shrinkage voids in the case of AHSS spot welds appear to be the two primary causes for their high tendency to fail in interfacial mode.

Real-time ultrasonic control of spot-welded steel joints

Real-time ultrasonic control of spot-welded steel joints

About the choice of mode parameters at resistance spot welding of zinced steels with covering preservation

About the choice of mode parameters at resistance spot welding of zinced steels with covering preservation

Expulsion Reduction in Resistance Spot Welding by Controlling of welding Current Waveform

There is a welding problem such as expulsion in resistance spot welding of high strength steel. This is due to overheating induced by increase of total heat input. Recently, many studies are carried out to solve the problems using electrode force control and current control. In this study, we tried to achieve expulsion reduction through controlling welding current. Steel sheet coated by Al-Si that has strength of 1500MPa was selected as a base material. The control results were compared with conventional welding method. Also, it is characterized how the variation of the heating and cooling by pulse current waveform control have an effect on expulsion.

Fracture mechanics of spot-welded steel sheets of equal thickness in Coach-peel specimens

In the current paper, a numerical investigation based on the finite element method was constructed to analyse fracture mechanics of two L-shaped sheets welded with resistance welding. The fracture behaviour of the welded structure is considered with the assumption of a crack at the interface of the welded sheets. The study was treated as a linear elastic fracture mechanics problem in mixed mode, under plane stress conditions. Computations were performed using the commercial FE software package ABAQUS. The modelling results highlight the dependence of the stress intensity factors on key design parameters of spot welds such as nugget diameter, bent curvatures, sheet thickness, material properties and the applied load.

Resistance spot welding of high-strength steels through new corrosion-resisting coatings

Resistance spot welding of high-strength steels through new corrosion-resisting coatings

Resistance spot welding and weldbonding of advanced high strength steels

AbstractThe resistance spot welding procedure is one of the most important joining techniques in lightweight car body shell mass production. Especially for newly developed high strength multiphase steels, also called advanced high strength steels (AHSS), and ultra high strength steels (UHSS), this joining technique has more advantages than other thermal and mechanical joining procedures for thin steel sheets. Additionally, the technique of adhesive bonding and its combination with the technique of resistance spot welding called weldbonding becomes more and more important. One of the targets of the contribution is to show the influence of joined advanced high strength steels on the process reliability for both the resistance spot welding process and the weldbonding process. Based on welding current ranges and on results of electrode wear tests, statements concerning the resistance spot weldability of some special AHSS will be given. The mechanical behaviour of spot welded and weldbonded joints for different AHSS will be studied. Furthermore, some statements regarding the fracture behaviour, the hardness and the fatigue behaviour of both spot welded and weldbonded joints for different AHSS will be given. Finally, some results on the mechanical properties of spot welded and weldbounded joints under corrosive attacks with be discused.Widerstandspunktschweißen ist die am häufigsten eingesetzte Fügetechnik in der Massenfertigung von Rohkarosserien. Insbesondere beim Einsatz von neuentwickelten, hochfesten Mehrphasenstählen, auch Advanced High Strength Steels (AHSS) genannt, und Ultra hochfesten Stählen (UHSS) hat das Widerstandspunktschweißen eindeutige Vorzüge gegenüber anderen, thermischen oder mechanischen, Fügetechniken für Feinbleche. Darüberhinaus gewinnt das Kleben und die Kombination des Klebens mit dem Widerstandspunktschweißen, Punktschweißkleben genannt, zunehmend an Bedeutung. Ein Ziel dieses Beitrags ist es, den Einfluss der gefügten hochfesten Stähle auf die Prozesssicherheit sowohl des Widerstandspunktschweißprozesses als auch des Punktschweißklebprozesses zu zeigen. Anhand der Schweißbereichsdiagramme und der Ergebnisse von Standmengenversuchen werden Aussagen zur Widerstandspunktschweißeignung einiger besonderer AHSS getroffen. Die mechanischen Eigenschaften von punktgeschweißten und punktschweißgeklebten Verbindungen aus verschiedenen AHSS und UHSS werden untersucht. Darüberhinaus werden noch einige Aussagen zum Bruchverhalten, zur Härte und zur Ermüdungsfestigkeit sowohl der punktgeschweißten als auch der punktschweißgeklebten Verbindungen für verschiedene AHSS getroffen. Abschließend werden einige Ergebnisse zum mechanischen Verhalten punktgeschweißter und punktschweißgeklebter Verbindungen in korrosiver Umgebung diskutiert.

Influence of initial gap on weld expulsion in resistance spot welding of dual phase steel

The weld expulsion is prone to occur and severely affects the nugget quality when the initial gap between dual phase (DP) steel sheets exist in resistance spot welding (RSW). To investigate the effect of initial gap on weld expulsion, a finite element model was developed to analyse the weld nugget formation process with different initial gaps for DP steels. An estimation method of expulsion occurrence based on the ratio of the nugget radius Rn and the contact radius Rc between sheets was proposed to get the critical initial gap without expulsion. The simulation and experimental results showed that the weld expulsion would not happen until the gap spacing reaches the critical value. The critical initial gap of DP steel is much smaller than that of low carbon steel. For both DP steel and low carbon steel, the critical initial gap would increase with the thickening of the steel sheet.

Friction stir spot welding of DP780 carbon steel

Friction stir spot welds were made in uncoated and galvannealed DP780 sheets using polycrystalline boron nitride stir tools. The tools were plunged at either a single continuous rate or in two segments consisting of a relatively high rate followed by a slower rate of shorter depth. Welding times ranged from 1 to 10 s. Increasing tool rotation speed from 800 to 1600 rev min–1 increased strength values. The 2-segment welding procedures also produced higher strength joints. Average lap shear strengths exceeding 10·3 kN were consistently obtained in 4 s on both the uncoated and the galvannealed DP780. The likelihood of diffusion and mechanical interlocking contributing to bond formation was supported by metallographic examinations. A cost analysis based on spot welding in automobile assembly showed that for friction stir spot welding to be economically competitive with resistance spot welding the cost of stir tools must approach that of resistance spot welding electrode tips.

Improving the working efficiency of electrodes in resistance spot welding of steels

Zinc-plated carbon steels and also steels with other protective coatings are used widely not only in the manufacture of general goods but also in the manufacture of components for important applica...

Similar and dissimilar RSW of low carbon and austenitic stainless steels: effect of weld microstructure and hardness profile on failure mode

In this paper, the failure behaviour of similar and dissimilar resistance spot welded joints of low carbon and austenitic stainless steel sheets was studied under tensile shear test with attention focused on the failure mode. Results showed that the microstructure of the fusion zone and the hardness distribution across the weld have a profound effect on the failure behaviour. Similar spot welds of stainless steel sheets exhibit the highest tendency to fail in interfacial failure mode, compared to low carbon steel similar spot welds and dissimilar low carbon and stainless steel spot welds. This behaviour is explained by the consideration of pullout failure location and hardness profile characteristics of each joint. It was shown that the failure mode transition is controlled by the hardness ratio of the fusion zone and the pullout failure location. In the case of dissimilar resistance spot welding, the hardness of the fusion zone which is governed by the dilution between two base metals, and the fusion zone size of the low carbon steel side are the dominant factors determining the failure mode of the joint.

Effect of dynamic contact resistances on advanced high strength steels under medium frequency DC spot welding conditions

The effect of dynamic contact resistance (DCR) during MFDC spot welding of dual phase and martensitic steels was evaluated. A comparative analysis of DP590 to DP590 with DP780 to DP780 steel welds, and DP780 to DP780 with M1200 to M1200 steels welds was carried out. The DCR of DP780 steel is higher than DP590 steel during the initial stages of weld time, but is reversed later. The bulk resistance component, which is higher in DP780 steel, is dominant and generates more energy early in the process and controls melting. Although the total energy input is almost same, the higher β-peak and its early occurrence ensures better heat utilisation resulting in larger nugget size. Contrarily, in martensitic steel the interface resistance component remains high throughout the entire welding process and compensates for the lower bulk resistance effect. Even with relatively lower energy input the nuggets produced in M1200 steel are comparable to DP780 steel.

Ultrasonic Imaging Technique of Spot Weld Structure for a High Strength Galvanized Steel Sheet

Abstract In this paper, ultrasonic C-scan imagery technique is applied to measure the nugget diameter and image the defects in a spot weld of a high strength galvanized steel sheet. In order to enhance the test precision, the ultrasonic C-scan image is divided into six characteristic color regions and the related A-scan signals are analyzed according to the structural characteristics of the high strength galvanized steel spot weld. The results indicate that the nugget diameter can be measured accurately, and the shape of inner defects of nugget is evaluated based on the C-scan image and the time domain analysis of A-scan signal. The method proposed in the research is not only a comprehensive and visualized method for spot welding evaluation, but also a kind of important auxiliary method for spot weld research since the inner structure of the spot weld can be investigated without devastating it.

Dissimilar friction stir spot welding of low carbon steel and Al–Mg alloy by formation of IMCs

Dissimilar lap joints of low carbon steel and Al–Mg alloy were obtained by friction stir spot welding. Mechanically mixed layer between top and bottom plates was not formed at the weld nugget due to the limited tool penetration and the pin height of welding tool lower than the thickness of Al plate laid in top side. These welding conditions made it possible to weld steel plate using welding tools made out of a general tool steel. With increasing tool penetration depth (TPD), tensile shear force of joint increased and maximum value of 3·0 kN was obtained at the TPD of 0·5 mm, but excessive tool penetration beyond 0·5 mm was caused in a deformation of Al plate of top side. In the result of interface observation, interaction layer was formed between Fe and Al alloy, which was constituted by various intermetallic compounds (IMCs). Consequently, the size of strongly bonded area containing Fe3Al and Fe4Al13 IMCs increased with TPD, which resulted in the increase in joint strength under the limited TPD.

Real Time Monitoring Weld Quality of Resistance Spot Welding for Stainless Steel

Resistance spot welding (RSW) is one of the most widely used processes in sheet metal fabrication. Although used in mass production for several decades, RSW has a major problem of inconsistent quality from weld to weld, which results from both the complexity of basic process as well as from various process conditions, noise and errors. There have been a number of investigations on monitoring and controlling the resistance spot welding of low carbon steel by using dynamic resistance, but those for stainless steel are limited because the dynamic resistance has descent property. Dynamic resistance curve in spot welding for low carbon steel has a typical shape: the resistance drops sharply at the beginning, and then rises; before the current is terminated, the resistance starts to drop, which results in a peak. Unlike low carbon steel, dynamic resistance for stainless steel decreases rapidly at the beginning and then decreases at a reducing rate.The objective of this research is to explore the effects of various process conditions in spot welded stainless steel on quality by using dynamic resistance. The process conditions studied in this research are chosen to be the most often observed in production, such as variations of welding parameters, edge weld, small weld spacing, poor fitup and axial misalignment. A series of experiments will be conducted to research how process conditions affect the dynamic resistance. The results show that dynamic resistance responds well to the variations of process conditions and can serve as an important indicator of weld quality.

인장전단시험을 이용한 TRIP1180강의 계면파단특성 평가

The weldability of resistance spot welding of TRIP1180 steels for automobile components investigated enhance in order to achieve understanding of weld fracture during tensile-shear strength (TSS) test. The main failure modes for spot welds of TRIP1180 steels were nugget pullout and interfacial fracture. The peak load to cause a weld interfacial failure was found to be related to fracture toughness of the weld and the weld diameter. Although interfacial fracture occurred in the spot welded samples, the load-carrying capacity of the weld was high and not significantly affected by the fracture mode. Substantial part of the weld exhibits the characteristic dimple (or elongated dimple) fractures on interfacial fractured surface also, dimple fracture areas were drawmatically increased with heat input which is propotional to the applied weld current. In spite of the high hardness values associated with the martensite microstructures due to high cooling rate. The high load-carrying ability of the weld is directly associated with the area of ductile fracture occurred in weld. Therefore, the judgment of the quality of resistance spot welds in TRIP1180 steels, the load-carrying capacity of the weld should be considered as an important factor than fracture mode.

DYNAMIC MATERIAL PROPERTIES OF THE HEAT-AFFECTED ZONE (HAZ) IN RESISTANCE SPOT WELDING

This paper is concerned with a methodology to identify the dynamic material properties of the heat-affected zone (HAZ) near the base metal in a resistance spot weld process at various strain rates. In order to obtain the dynamic material properties of the HAZ in the spot-welded steel sheet, specimens are prepared to have similar material properties, hardness and microstructure to the actual HAZ. Such thermally simulated specimens are fabricated with the material thermal cycle simulator (MTCS) and compared with the real one for the hardness and microstructure. Dynamic tensile tests are then conducted with a high speed material testing machine. Stress–strain curves of the thermally simulated HAZ are obtained at various strain rates ranged from 0.001/sec to 100/sec. Obtained material properties are applied to the finite element analysis of the spot-welded tensile-shear specimen in order to verify validity of the proposed testing methodology and obtained results. Analysis results demonstrate that the material properties obtained are appropriate for the FE analysis of spot-welded specimens.

Experimental investigation of fatigue performance of spot welded dual phase sheet steels

Spot weld fatigue performance of dual phase steels is of great interest to worldwide automotive manufacturers due to their expanding use in automotive industry. Given that the majority of the fatigue life is spent propagating the crack through the microstructure, changes in microstructure of welding steels could result in significant changes in spot weld fatigue life compared to that of low carbon steel spot weld. In the present study, tensile shear spot welds of DP600GI, DP780GI with different thickness were used for fatigue test under various amplitude tensile loads. For comparison tensile shear fatigue test was also conducted for spot welds of low carbon steel. Microhardness was measured for spot weld of dual phase steels to study the hardness change across the weld nugget. Failure mechanism and crack propagation path of dual phase steel spot weld was studied. For spot welds of low thickness, the fatigue strength of dual phase steel spot weld is higher than that of low carbon steel specimen in high cycle fatigue test. When thickness increases, the fatigue strength of dual phase steel spot weld and low carbon steel spot weld are almost the same, and fatigue strength of dual phase spot welds also increases with the thickness. And it is also shown in the present study that static tensile interfacial fracture of spot weld does not necessarily lead to interfacial fracture of high cycle fatigue tests. Crack propagation path of tensile shear spot weld of dual phase sheet steels under fatigue test was examined. Crack initiates at the faying surface around the weld nugget, and then passes through the thickness of sheet, which leads to the failure of specimens.

Surface modified long-life electrode for resistance spot welding of Zn-coated steel

In order to increase the life of conventional copper electrodes in resistance spot welding of Zn-coated sheet steel, a multi-layer Ni/(TiC P/Ni)/Ni composite coating was deposited onto the copper electrode top surface by electro-spark deposition. Scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction and micro-hardness tests were employed to characterize the microstructure and property of the coating. The results showed that the copper electrodes with a dense Ni/(TiC P/Ni)/Ni coating slightly increased the resistance of the weld system and hence the welding current could be reduced to produce a weld with the same button size as that made by uncoated electrodes at a high welding current. The coating was gradually cracked during welding under the action of welding force, forming Ni/(TiC P/Ni)/Ni composite islands which were strongly adherent to and further, punched into copper substrate. The coating could significantly reduce the alloying between copper electrode substrate and molten Zn. As a result, coated electrode showed a much longer life than an uncoated electrode even though the welding current for coated electrode was lower than that for uncoated one.

On-line monitoring method for electrode invalidation during spot welding of zinc-coated steel

In the process of spot welding of zinc-coated steel, because of the existence of zinc coat, the electrode invalidation is accelerated greatly, which causes unacceptable welding quality. A collecting system is developed with a C8051F020 microcomputer as the controlling core, which is used to collect such parameters as welding current, supply voltage, dynamic resistance, etc. Through the comprehensive processing, the whole variety trends of those parameters are obtained. Finally, an on-line monitoring method is given through the relationship between electrode wear and welding parameters, which is valuable for the industrial production.