Occurrence Of Expulsion Research Articles

Effect of Heat Treatment Conditions on Resistance Spot Weldability of 1.8 GPa-grade Al-Si Coated Hot Stamping Boron Steel

This study entailed resistance spot welding conducted using 1.8 GPa-grade hot stamping boron steel heat-treated under varied conditions. The relationship between weldability and melting behavior of the intermetallic layer during resistance spot welding was examined. As regards the four heat treatment conditions beyond the austenitic temperature, it was found that the intermetallic layer thickened with increased time and temperature. Furthermore, the contact resistance also increased with increased time and temperature of the heat treatment. This was mainly because of the expansion of the area of Al-Fe-based intermetallic phases (e.g., FeAl2 and Fe2Al5) within the intermetallic layer. Contact resistance induces the thickening of the intermetallic layer and results, even with a low current, in the occurrence of expulsion due to the high heat generation in the faying surface. Subsequently, the weldable current range became narrow, and satisfying the property requirements was challenging. The welding using the heat treatment condition of 900 °C and 5 min showed sufficient current path area. This can be attributed to the phenomenon that the intermetallic layer was forced out to the rim of the corona bond area at an early stage of the welding. However, the welding using other conditions showed that the intermetallic layer remained on the border of the nugget and corona bond areas it was observed and verified via electron probe micro analysis. Consequently, the nugget was insufficiently formed and the fracture mode was a partial interfacial fracture. On the contrary, it was found that the adoption of the pre-pulse could enhance the weldability of all conditions by obtaining a larger contact area.

Weld-Quality Prediction Algorithm Based on Multiple Models Using Process Signals in Resistance Spot Welding

An efficient nondestructive testing method of resistance spot weld quality is essential in evaluating the weld quality of all welded joints in the automotive components of a car body production line. This study proposes a quality prediction algorithm for resistance spot welding that can predict the geometrical and physical properties of a spot-welded joint and evaluate weld quality based on quality acceptance criteria. To this end, four statistical models that predict the main geometrical and physical properties of a spot-welded joint, including tensile shear strength, indentation depth, expulsion occurrence, and failure mode, were estimated based on material information, dynamic resistance, and electrode displacement signals. The significance of the estimated models was then verified through an analysis of variance. The prediction accuracies of the models were 94.3%, 93.4%, 97.5%, and 85.0% for the tensile shear strength, indentation depth, expulsion occurrence, and failure modes, respectively. A weld quality evaluation methodology that can predict the properties of a spot-welded joint and evaluate the overall quality requirements based on authorized welding standards was proposed using the four statistical models.

A Novel Expulsion Control Strategy With Abnormal Condition Adaptability for Resistance Spot Welding

Abstract Welding expulsion is a common problem in the resistance spot welding (RSW) process, which severely impacts weld quality and surrounding facilities. Existing expulsion control strategies are ineffective for complex and changeable welding conditions. This article studied the growth relationship between weld nugget and corona bond under two abnormal conditions: edge proximity (EP) and initial sheet gaps (IG). It is testified that expulsion would occur when the nugget size exceeds the corona bond size under EP and IG conditions. Reducing the welding current before the expulsion time can increase the size difference between the corona bond and the weld nugget, thereby delay and even eliminate the occurrence of expulsion. In this way, a novel online expulsion control strategy, named short-time current regulation (STCR), is proposed through the expulsion moment analysis of historical weld data. The effect of the new control strategy is verified with workpieces ranging from low carbon steel to ultra-high strength steel. Experimental results showed that STCR can effectively reduce the amount of expelled metal, decrease the indentation depth, and increase the nugget diameter. The method not only works well under one specific abnormal condition but also adapts to the transition between different welding conditions. This novel expulsion control strategy can help achieve the expulsion-free RSW process in mass production without frequent manual offline optimization of welding parameters.

Microstructure, Tensile and Fatigue Behaviour of Resistance Spot Welded Zinc Coated Dual Phase and Interstitial Free Steel

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.

Expulsion rate of intrauterine device: mediate vs. immediate puerperium period

Objective:To evaluate the rate of expulsion of intrauterine device (IUD) inserted during the immediate and mediate puerperium. To evaluate whether the type of delivery is a predictor of expulsion of the IUD when inserted in the puerperium period.Material and Methods:This was a prospective observational study. Patients whose IUD (TCU-380 copper) was placed during the puerperal period were divided in two groups according to the time of placement: immediate and mediate puerperium. The decision regarding the time of IUD insertion was made in a non-randomized manner. Analysis was performed using chi-square, Mann-Whitney U, and Spearman’s correlation tests and logistic regression analysis.Results:The total rate of IUD expulsions was 28.8% (49/170). There was no significant association between the occurrence of expulsion and the timing of IUD insertion (immediate vs mediate puerperium; 26.6% vs 34.78%, p=0.296). Among patients experiencing expulsion, 79.6% (39/49) underwent insertion after vaginal delivery and 20.4% (10/49) during cesarean section (CS). The type of delivery was a significant predictor for IUD expulsion (p<0.0001). Vaginal delivery was fourfold more likely to be associated with IUD expulsion inserted in the puerperal period than CS (odds ratio: 4.23, 95% confidence interval: 1.94-9.25). There was no significant correlation between the period between IUD insertion and the diagnosis of expulsion in regard to number of pregnancies (r=-0.160, p=0.271) or gestational age at delivery (r=-0.058, p=0.939).Conclusion:Vaginal delivery was the most prevalent type of delivery in patients who underwent IUD insertion during the immediate and mediate puerperium. The risk of IUD expulsion after vaginal delivery was greater than CS.

Cross-Wire Projection Welding of Aluminium Alloys in Relation to Pneumatic and Electromechanical Electrode Force Systems

The cross wire projection welding of wires (Al 5182, =4 mm) performed using the conventional (i.e. pneumatic) electrode force system was subjected to thorough numerical analysis. Calculations were performed until one of adopted boundary conditions, i.e. maximum welding time, maximum penetration of wires, the occurrence of expulsion or the exceeding of the temperature limit in the contact between the electrode and the welded material was obtained. It was observed that the ring weld was formed within the entire range of welding parameters. The process of welding was subjected to optimisation through the application of a new electromechanical electrode force system and the use of a special hybrid algorithm of electrode force and/or displacement control. Comparative numerical calculations were performed (using SORPAS software) for both electrode force systems. Technological welding tests were performed using inverter welding machines (1 kHz) provided with various electrode force systems. The research also involved the performance of metallographic and strength (peeling) tests as well as measurements of welding process characteristic parameters (welding current and voltage).The welding process optimisation involving the use of the electromechanical force system and the application of the hybrid algorithm of force control resulted in i) more favourable space distribution of welding power, ii) energy concentration in the central zone of the weld, iii) favourable (desired) melting of the material within the entire weld transcrystallisation zone and iv) obtainment of a full weld nugget.

Criterion for predicting expulsion in resistance spot welding of steel sheets

Resistance spot welding (RSW) is frequently accompanied by expulsion, an undesirable event taking place in manufacturing processes, including car body production. Many variables are involved in RSW, all of which can affect expulsion. This investigation aimed at developing an analytical model for predicting expulsion in RSW. Accordingly, based on force and moment balances, a criterion was established. It was proposed that expulsion took place when the molten metal ejected as a result of the pressure inside nugget. The model presented here considered that the pressure inside the nugget was mainly due to melting transformation and liquid thermal expansion. Hence, the model took into account the effects of nugget diameter, electrode force, base material yield stress, sheet thickness, and joint fit-up to predict the occurrence of expulsion. To assess the model, a number of experiments were done, where the effects of welding current, electrode force, sheet thickness, and galvanization coating of the steel were studied. Regarding the effects of electrode force and welding current, the prediction made by the analytical model was consistent with the results obtained from the experiments. However, it was observed that expulsion in the galvanized steel sheets occurred at larger nugget diameters. The macroscopic examination of the welded samples showed that, with galvanization the ability for a better electrode fit-up was increased, and according to the model this could result in thereby increasing the critical nugget size.

Prediction of the Weld Qualities Using Surface Appearance Image in Resistance Spot Welding

The quality of the resistance spot weld is predicted qualitatively using information from the weld’s external apparent image. The predicting tool used for weld qualities was a convolution neural network (CNN) algorithm with excellent performance in pattern recognition. A heat trace image of the weld surface was used as information on the external apparent image of welds. The materials used in the experiment were advanced high strength steel (AHSS) with 980 MPa strength, and uncoated cold-rolled (CR) steel sheets and galvannealed (GA) steel sheets were used. The quantitatively predicted weld quality information contained tensile shear strength, nugget diameter, fracture mode of welds, and expulsion occurrence. The predicted performance of the verification step of the model determined through the learning process was as follows; the predicted error rate for tensile shear strength and nugget diameter were 2.2% and 2.6%, respectively. And the predicted accuracy on fracture mode and expulsion occurrence was 100%.

Cross-wire projection welding of aluminium alloys\u2014pneumatic and electromechanical electrode force systems

The article discusses the optimisation of the conventional resistance welding process performed using the pneumatic (electrode) force system (PFS) by applying the electromechanical force system (EFS) and a new algorithm enabling the control of the force and/or displacement of welding machine electrodes, particularly during the flow of current. The research work described in the article aimed at the in-depth numerical analysis of the projection cross-wire welding of bars made of aluminium alloy Al 5182, having a diameter of 4 mm, in relation to two electrode force systems, i.e. the PFS and the EFS. The research-related tests involved experimental verification. In addition, the article presents advantages of the new control system. Calculations were performed until one of the adopted boundary conditions, i.e. the maximum welding time, the maximum penetration of wires (bars), the occurrence of expulsion or the exceeding of the temperature limit in the contact between the electrode and the welded material was obtained. For the PFS, it was observed that the ring weld was formed within the entire range of welding parameters. Numerical calculations were performed using SORPAS software. Technological welding tests were performed using inverter welding machines (1 kHz) provided with various electrode force systems. The research also involved the performance of metallographic and strength (peeling) tests as well as measurements of welding process characteristic parameters (welding current and voltage). The welding process optimisation involving the use of the EFS and the application of the hybrid algorithm of force control resulted in (i) more favourable space distribution of welding power, (ii) energy concentration in the central zone of the weld, (iii) favourable (desired) melting of the material within the entire weld transcrystallisation zone, (iv) obtainment of a full weld nugget and (v) over 20% longer weld nugget diameter.

Cross-Wire Projection Welding of Aluminium Alloys in Relation to Pneumatic and Electromechanical Electrode Force Systems

The cross wire projection welding of wires (Al 5182, =4 mm) performed using the conventional (i.e. pneumatic) electrode force system was subjected to thorough numerical analysis. Calculations were performed until one of adopted boundary conditions, i.e. maximum welding time, maximum penetration of wires, the occurrence of expulsion or the exceeding of the temperature limit in the contact between the electrode and the welded material was obtained. It was observed that the ring weld was formed within the entire range of welding parameters. The process of welding was subjected to optimisation through the application of a new electromechanical electrode force system and the use of a special hybrid algorithm of electrode force and/or displacement control. Comparative numerical calculations were performed (using SORPAS software) for both electrode force systems. Technological welding tests were performed using inverter welding machines (1 kHz) provided with various electrode force systems. The research also involved the performance of metallographic and strength (peeling) tests as well as measurements of welding process characteristic parameters (welding current and voltage).The welding process optimisation involving the use of the electromechanical force system and the application of the hybrid algorithm of force control resulted in i) more favourable space distribution of welding power, ii) energy concentration in the central zone of the weld, iii) favourable (desired) melting of the material within the entire weld transcrystallisation zone and iv) obtainment of a full weld nugget.

New methods of resistance spot welding using reference waveforms of welding power

The use of high strength steels in car bodies has increased gradually in recent years, contributing to lighter weight vehicles. In general, the resistance spot weldability of high strength steels is poor when compared to that of low-carbon steels due to a number of factors including narrow welding range (weld lobe), irregular occurrence of expulsion, high resistivity of base metals, and zinc coating. To improve the weldability of high strength steels, technologies such as adaptive resistance spot welding and optimization of welding conditions have been developed and applied within the automotive industry. This study focuses on developing two methods of resistance spot welding with the aim of improving the weldability of high strength steels. The first method is reference welding, which uses the reference power waveforms obtained from preliminary welding. The second is modified reference welding, which uses modified power waveform from the reference welding method. The features and effects of these welding methods were evaluated through a tensile shear test, analysis of welding signals, and an investigation into the frequency of expulsion. Both welding methods showed high tensile shear strength results and reductions in expulsion over a wide variety of different steel types including GI coated steels.

The weldability of dissimilar three-steel sheet using constant power control method for resistance spot welding

Resistance spot welding is one of the most commonly used processes for joining-metal the automotive, and electronics industry. When joining a car body, the method of welding dissimilar three sheets, including high strength steel is frequently used. Studies on the dissimilar welding of high strength steel and low carbon steel are being conducted in an attempt to improve the weldability. However, the dissimilar welding of TWIP steel with other steel grades is extremely challenging due to the high resistivity of the base metal. Therefore, further studies are required to improve the weldability of three sheets including the low carbon steel. In general, the resistance spot welding process is conducted in the field by the constant current control method. In this study, the constant power control method was applied in order to reduce the occurrence of expulsion and to improve the weldability of three dissimilar steel sheets consisting of SGACEN, DP steel, and TWIP steel. The constant current control was compared with constant power control, through the evaluation of weldability by using the suitable welding range lobe curve, which takes into account the tensile shear strength, and nugget diameter. Additionally, the current, voltage, resistance, and power signal were also analyzed. The constant power control prevented the occurrence of expulsion as a result of high heat not being generated at the early stage, compared to the constant current control method. Regarding the welding time being long, an increase in heat was applied so that there was a considerable improvement of weldability, compared to the constant current control.

Development of single-side resistance spot welding technology applying in-process welding current and electrode force controls

An indirect resistance spot welding process with single-side electrode access was developed for automotive applications. The variable controls of electrode force and welding current during welding were studied in order to achieve the promotion of weld nugget formation and the suppression of expulsion without sacrificing the productivity and design flexibility of automobiles. The welding experiments were performed on lapped test coupons of 0.7-mm-thick cold-rolled sheet with tensile strength of 270 N/mm2 and 1.6-mm-thick cold-rolled sheet with tensile strength of 980 N/mm2 using a resistance spot welding system consisting a servo-motor-controlled welding gun and an inverter DC power supply. Welding experiments verified that the occurrence of expulsion and formation of molten nugget were significantly influenced by the heat generation and melting process at an initial period during welding and manageable by applying the variable patterns of electrode force and welding current. When welding was performed under the large shunting condition simply with the constant force and current pattern of 400 N in electrode force, the appropriate current range was less than 1 kA. On the other hand, it extended to 2.6 kA when performed with the variable force and current pattern of 800 N in force and 4 kA in current at the first stage and 400 N in force at the second stage, confirming the fact that the variable pattern successfully suppressed the expulsion and promoted the nugget formation. Numerical simulations were conducted to compare the difference in welding phenomena between the constant force and current pattern and the variable force and current pattern and clarified that the effect of variable force and current pattern on the promotion of nugget formation and the suppression of expulsion.

Development of a new program control setting of welding current and electrode force for single-side resistance spot welding

For the indirect-type single-side resistance spot welding, the new program control setting of welding current and electrode force were developed in order to increase the welding current range, suppress the expulsion, and promote the weld nugget growth. Welding experiments verified that the occurrence of expulsion and formation of molten nugget were significantly influenced by the contacting conditions at the interface between the electrode tip and upper sheet and at the faying surface between the sheets and the resulted heat generation at the early stage of welding. Focusing on better controlling the contacting conditions at the early stage, the two-stage program control setting of welding current and electrode force was developed and successfully improved the weldability, increasing the welding current range, compared with constant current and force setting. Numerical simulations were conducted to study the welding phenomena and clarified the effect of two-stage current and force setting on the suppression of expulsion and promotion of nugget growth.

Expulsion characterization in resistance spot welding by means of a hardness mapping technique

Expulsion is an undesired event during resistance spot welding because the weld quality deteriorates. It is the ejection of molten metal from the weld nugget which usually occurs due to applying a high current for a short welding time. Expulsion has a significant impact on the final yield strength of the weld, thus the detection and characterization of expulsion events is significant for the quality assurance of resistance spot welds. In this study, hardness mapping, using a scanning hardness machine, was used as a quality assurance technique for resistance spot welding. Hardness tests were conducted on a resistance spot welded sample to prepare a hardness map. The test results showed good correlation between the hardness map and metallographic cross sections. The technique also provided further fundamental understanding of the resistance spot welding process, especially regarding the occurrence of expulsion in the nugget.

The applicability of welding force for spot weld quality assurance

This paper presents research about the possible usage of welding force for the estimation of the weld strength. Other signals such as welding current or electrode displacement are commonly used for this purpose. Our previous research has, however, shown that welding force is a very good indicator of expulsion occurrence. In this paper, we presented the relationship between time needed for expulsion occurrence and weld strength. The results are better than in the case of welding current. In order not to limit this approach only to welds with expulsion, another variable is calculated from the welding force signal. The welding force variation is the sum of square differences of welding force and mean welding force. Although slightly less accurate than time needed for expulsion variable, it is not limited to expulsion welds.

Effect of Electrode Force on Expulsion in Resistance Spot Welding with Initial Gap

Compared with the conventional low carbon steels, the expulsion is more prone to occur in resistance spot welding (RSW) of dual phase (DP) steels. Especially when the initial gap exists between the steel sheets, the weld expulsion would reduce the weld quality of the joint. It is important to decrease or inhibit the occurrence of the weld expulsion in auto-body assembly line in order to guarantee the joint quality. In this study, the experiments of RSW with different initial gaps have been done to study the effect of the electrode force on the expulsion using different thicknesses of dual phase steels. The results show that the increment of the electrode fore would enable to decrease the occurrence of expulsion with the increase of the sheet thickness and gap spacing in resistance spot welding of DP steel sheets.

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.

고장력 강판 적용에 따른 자동차용 4겹 다층 점용접물의 용접성 및 적정 용접조건

Spot-welding is a widely used manufacturing method for thin-sheet components, especially in mass-production industries such as the car industry. Automobiles are often constructed by multi-lap spot welding to secure the passenger from the accident, where optimisation of the welding conditions is a major economic consideration. This research is conducted to investigate weldability characteristics with various welding conditions on the 4-lap spot welded joint of structural steel sheets in automobile. The relationship between the tensile-shear strength and the indentation depth has been investigated to propose the optimum welding conditions. The welding current and the welding time have a greater effect on the welding characteristics than the electrode force. It was found that the electrode force has a relatively close relationship with the expulsion occurrence. The design curves for optimum welding are proposed for the 4-lap spot welded joint.

Modeling small-scale resistance spot welding machine dynamics for process control

Electrode displacement is generally regarded as a variable that can provide real-time information useful for monitoring and controlling resistance spot welding (RSW) process quality. However, in small-scale RSW production, it is difficult to measure the displacement because its magnitude is very small. By contrast, force signals are relatively large and thus are less susceptible to measurement noise. In this article, an empirical model is proposed to simulate the dynamics of an SSRSW head with the objective of calculating electrode displacement from the variation of electrode clamping force measured during welding. The parameters in the model were determined by fitting experimental force and displacement signals with polynomials and then performing an optimization search for parameters of first-order dynamic models. To verify the models’ accuracy, they were subsequently applied to simulate the electrode displacement curves of welds with expulsion and without expulsion. The calculated displacement curves agreed well with experimental measurements, and the occurrence of expulsion was clearly indicated by the model predictions. A more comprehensive model is under construction with an objective to eliminate displacement sensor in the monitoring and control of SSRSW process.