Cross Tension Research Articles

고Si DP980강 스폿 용접 특성에 미치는 Phosphorus (P) 및 in-situ 후열처리 펄스 조건의 영향

Recently, application of UHSS(Ultra High Strength Steels) whose tensile strength is over 1000MPa to car body structure are growing due to great needs for light weighting and improved crash worthiness. However, their poor weldability is one of obstacles to expand selecting to car body. In this study, effect of Phosphorus contents on resistance spot weldability of high elongation DP980 steel whose Si content is over 1% was investigated. The cross tension strength (CTS) was decreased showing partial interface fracture as Phosphorus content increase because of solidification segregation of Phosphorus. In order to improve resistance spot weldability by modification of welding condition, in-situ post-weld heating pulse was introduced after main pulse. The optimum cooling time between main and post pulse and post-pulse current condtion were determined through FEM welding simulation and DOE tests. The CTS was increased about 1.5 time showing plug fracture. The decrease of Phosphorus segregation was found to be a major reason for weld ductility and CTS improvement.

Elemental segregation during resistance spot welding of boron containing advanced high strength steels

The partitioning behaviour of carbon, phosphorous and boron during the solidification of a resistance spot weld pool was studied using experimental simulations and a phase field model. Steels with varying carbon, phosphorous and boron contents were designed and subjected to a range of resistant spot welding thermal cycles. Mechanical properties were evaluated by hardness and cross tension tests and correlated with the weld microstructure. Phase field modelling results and experimental predictions show that the phosphorus concentration in the last area in the weld pool to solidify can reach about 0.38 wt% for a steel with a bulk concentration of 0.08 wt%. Elemental analysis indicates that in the absence of boron, the grain boundaries of columnar grains in the weld pool are decorated with phosphorous. As a result, a complete interface failure occurs during cross tension testing. With the addition of boron, apart from an increase in weld strength and plug diameter, the failure mode switches to a complete plug mode, resulting from the phosphorous depletion at the grain/inter-phase boundaries.

Resistance Spot Weldability of Hot-Formed Ultra High Strength Steel Based on Virtual and Physical Experiment

In this study, virtual and physical experimental investigations were performed on the resistance spot weldability of hot-formed ultra high strength steels (UHSS). The sheets were joined by the resistance spot welding as lap or cross joint. The mechanical properties were assessed in terms of cross tension strength (CTS) and tensile shear strength (TSS). The micro structural profile and weld nugget diameter were also investigated. Both virtual and physical methods yielding results are in good agreement. The results are discussed in the light of earlier work published in the literature and contribute to a better understanding on the evaluation of resistance spot weldability.

Effect of pin length on Friction Stir Spot Welding (FSSW) of dissimilar Aluminum-steel joints

Friction stir welding (FSW) has produced a great impact in several industries due to the advantages that this process presents. In particular, the automotive industry has developed a variant of the original process, called Friction Stir Spot Welding (FSSW), which has a strong interest related to the welding of aluminum alloys and dissimilar materials in thin sheets. Aluminum-steel welding is an actual challenge, being FSSW an alternative to produce these joints. However, the information available related to the influence of process parameters on the characteristics of aluminum-steel joints is scarce. The aim of this work was to study the effect of the pin length of the welding tool and its penetration depth, during friction stir spot welding (FSSW) of overlaps joints of AA6063 with galvanized low carbon steel. FSSW was done by changing the pins length between 0.65 and 1.5mm, and also by modifying the tool penetration depth in the welded joints. On the welded spots macro and microstructural characterization was performed, Vickers microhardness profiles were determined and Peel and Cross Tension Tests were also done. The maximum loads increased when the tool penetration depth goes up and the pin length decreased. The fracture mode was, at first, interfacial meanwhile it changed to a circumferential mode when the tool penetration depth increased and the pins length was reduced.

Effect of formation and state of interface on joint strength in friction stir spot welding for advanced high strength steel sheets

The tensile shear strength and cross tension strength of friction stir spot welded joints were evaluated in the cases of lap joints of 270 N/mm2 grade and 980 N/mm2 grade cold rolled steel sheets with respect to the stir zone area, hardness distribution, and interface condition between the sheets. The results suggested that both the tensile shear strength and cross tension strength were based on the stir zone area and its hardness in both grades of steel. The "hook" shape of the interface also affected the joint strength. However, the joining that occurred across the interfaces had a significant influence on the value of the joint strength in the case of the 270 N/mm2 grade steel.

Influence of the moisture content on flat-clinch connection of wood materials and aluminium

The paper describes the joining of composites made of different types of wood-based materials and aluminium. The wood materials were conditioned in a climate chamber or in a chamber furnace to set specific moisture contents. Afterwards, the conditioned wood materials were joined with aluminium (Al 99.5) by means of flat-clinching. After a relaxation period of 48 h under standard climatic conditions the joint strengths at different moisture contents were quantified by cross tension tests. Based on the well-founded experimental studies the understanding of the flat-clinching technology for this kind of material combination was improved.

短時間・高電流後通電による発熱形態および十字引張強さの向上

短時間・高電流後通電による発熱形態および十字引張強さの向上

Cold Forge Spot-bonding of High Tensile Strength Steel and Aluminum Alloy Sheets

Recently automobile components need to have various characteristics that are hard to be fulfilled using a single material for satisfying the several demands like weight saving, cost cutting and crash safety with trade-off relationship. Thus bonding or joining methods of dissimilar materials are necessary. However it is difficult to weld steel and aluminum together. Therefore the authors developed an alternative solid phase bonding method “spot-bonding”. In this method, the plastic deformation by cold forging would enable metallic bonding of two dissimilar metals. In this study, this method was applied to bonding of 590MPa-class high tensile strength steel and A2017P aluminum alloy sheets. The joint strength was measured by cross tension testing. When the amount of punch indentation is increased, the joint strength became higher. With the same depth of punch indentation as the total thickness, the joint was broken in the steel base metal blank but not at the interface. Finite element analysis was performed in order to consider the deformation behavior during the bonding process. The parameters on the bonding interface such as surface expansion ratio, surface pressure and relative slippage of two blanks became larger with increase in amount of punch indentation. Furthermore, the bonded region was predicted by calculating the total friction work and the evaluation factor of real contact area that depended on surface expansion and surface pressure.

ナゲット端部の開き角度に着目した抵抗スポット溶接重ね継手の十字引張強度向上

This paper deals with the strengthening of spot welded lap joints by increasing the opening angle on the nugget edge. Based on the equation proposed by past researchers to predict the strength, the factor necessary for the strengthening was extracted. And, based on this factor, the strengthening of spot welded lap joints was examined. First of all, in order to increase the opening angle on the nugget edge, the vicinity of the nugget edge was bended with a bending machine. Next, cross tension strength tests were conducted into the spot welded lap joints with various opening angles on the nugget edge. Finally, the relationship between cross tension strength and the opening angles on the nugget edge was clarified. As a result, by increasing the opening angles on the nugget edge, we found that the cross tension strength could be improved by 20%.

Joining of Various Kinds of Metal Plates Using Ultrasonic Vibrations

Joining of dissimilar metal plates by using ultrasonic vibrations was investigated. Plastic flow appeared when joining two aluminium plates by applying a zero amplitude ultrasonic wave (stress amplitude is at a maximum). Over time, the plastic flow became larger and resulted in a stirring phenomenon of the materials. By utilizing this stirring phenomenon, an aluminium plate was joined to a steel plate. The resultant plate was analysed with cross tension test and cross section observation. These results revealed the relation between ultrasonic conditions and cross tensile load. When joining an aluminum plate to various kinds of metal plate, the cross tensile load of the joint reached a maximum value of 200N. The cross tension tests reveal that the fracture of the materials resulted in plug failure mode. Therefore, the achieved maximum cross tensile strength is determined by the tensile strength of the aluminum plate. The strength of the bonding was stronger than the tensile strength of the aluminum plate. These results led us to the conclusion that joining of dissimilar metal plates has been attained using ultrasonic vibrations.

A synthesis of data from steel spot welded joints of reduced thickness by means of local SED

In some previous works two fatigue scatterbands were given in terms of the local strain energy density (SED) for welded joints made of structural steel and aluminium alloy. The material-dependent radius R0 of the control volume was identified with reference to conventional arc welding processes. It resulted equal to 0.28mm for welded joints made of structural steel tested under prevailing mode I, with fatigue failures both at the weld root and the weld toe. The scatterband did not include data from thin welded joints, the main plate thickness being greater than 6mm. The SED approach is used here to summarise more than 400 fatigue data from steel spot welds in lap–shear joints, coach–peel specimens and cross–tension specimens of small thickness. Due to the typical crack initiation and propagation phases, an appropriate choice to summarise the data in a narrow scatterband seems to be the use of a toroidal control volume. The volume circumferentially encapsulates the slit tip along the weld spot nugget. The SED value at 2 million cycles is found to match that of the previous scatterband for welded joints of higher thicknesses.

OS1506 溶接欠陥付十字引張型スポット溶接継手の疲労特性に関する破壊力学的検討

OS1506 溶接欠陥付十字引張型スポット溶接継手の疲労特性に関する破壊力学的検討

Design of an in situ mechanical test for spot-welded joints

Facing the need of a better understanding of spot-weld strength and failure mechanisms of advanced high strength steels, we have developed a novel test for the in situ failure analysis of spot-welded joints. Providing loading conditions similar to the standard cross tension test, this new test is based upon the use of wedges to peel plates apart in a welded sample that has been cut in half, revealing the weld in the scanning electron microscope. We first perform finite element modeling and show why the wedge test approach is suitable for our purpose. We then perform experimental tests and observe failure in the scanning electron microscope chamber. We are able to both follow crack propagation and control the failure mode. As a corollary, this approach enables the use of local strain mapping, using digital image correlation, to measure local strains in the vicinity of the crack path. The wedge test should be valuable for assessing failure processes in spot-welded joints.

Susceptibility to interfacial failure mode in similar and dissimilar resistance spot welds of DP600 dual phase steel and low carbon steel during cross-tension and tensile-shear loading conditions

Abstract The paper investigates the failure mode transition from interfacial to pullout in similar and dissimilar combinations of DP600 dual phase steel and low carbon steel (LCS) under tensile-shear (TS) and cross-tension (CT) loading conditions. In both CT and TS loading conditions, a transition in the failure mode from interfacial to pullout was observed with increasing fusion zone size beyond a critical value (DC). The tendency to fail in interfacial mode during the CT loading was increased in the order of LCS/LCS, DP600/LCS and DP600/DP600. It was shown that interfacial to pullout failure mode transition during the CT test is governed by the fracture toughness of the fusion zone and strength of the pullout failure location (i.e. heat affected zone). It was shown that increasing carbon equivalent of the fusion zone promoted interfacial failure mode in CT loading condition. The high carbon equivalent of DP600 steel led to formation of hard and brittle martensite, which in turn promotes crack propagation through fusion zone. In DP600/LCS combination, decreased carbon equivalent and fusion zone hardness through dilution with the LCS promotes pullout failure at smaller weld sizes. DC during the TS loading is increased in the order of DP600/LCS, LCS/LCS and DP600/DP600. No correlation between fusion zone carbon equivalent and the tendency to fail in IF mode during TS loading was found. Failure mode transition during the TS loading is controlled by hardness of fusion zone and stiffness of the joint. The lowest DC for DP600/LCS is a function of its high fusion zone hardness (in comparison to LCS/LCS combination) and its low stiffness (in comparison to DP600/DP600 combination).

A finite element model for the prediction of Advanced High Strength Steel spot welds fracture

A finite element model has been developed for the simulation of resistance spot welds behavior in Cross Tension. The model is able to capture the competition between semi-brittle fracture at the faying surface and ductile failure at the weld boundary. For this purpose, ductile damage and fracture are considered in the different weld zones in addition to a cohesive zone at the faying surface and an evaluation of the Rice integral (J) at the notch tip. The model is run for different weld configurations made of TRIP780 steel. It reproduces the proper level of load at the onset of cracking and predicts the governing damage mechanism leading to final fracture as a function of the geometrical features of the welds.

Differences between Laser and Arc Welding of HSS Steels

Abstract Conventional welding processes often fail to provide adequate joints in high strength steels with multiphase microstructures. One of the promising techniques is laser beam welding: working without filler metal and with sufficient capacity for automotive and transportation industry (where the amount of AHSS steels increases each year, as well as the length of laser welds). The paper compares microstructures and properties of HSS (high strength steel) joints made by MAG (Metal Active Gas) and laser welding. The effects of main welding parameters (heat input, welding speed and others) are studied on multiphase TRIP 900 steel tubes and martensitic sheets DOCOL 1200, advanced materials for seat frames and other automotive components. Whereas the strength of conventional welds is significantly impaired, laser welding leaves strength of the base material nearly unaffected. As the nature of fracture changes during loading and depending on the welding method, failure mechanisms upon cross tension tests have been studied as well.

Experimental and modeling investigation of the failure resistance of Advanced High Strength Steels spot welds

Damage of Advanced High Strength Steel resistance spot welds is investigated in Cross Tension by means of coupled microtomography, metallography and fractography. Three main failure mechanisms and failure zones are identified: (i) strain localization in the base metal/sub-critical Heat Affected Zone (HAZ), (ii) ductile shear around the weld and (iii) semi-brittle fracture in the weld nugget. A finite element model is developed in order to illustrate how the mechanisms compete and lead to a given failure type. The local constitutive behavior is obtained from tensile tests on simulated HAZ microstructures. The model enables capturing the main trends in the transition between failure types as a function of weld geometry as well as a reliable estimation of the load bearing capacity.

Resistance Spot Welding between Aluminum Alloy and Low-Carbon Steel

Aluminum alloy A5052 and low-carbon steel (SPCC) was welded using resistance spot welding with a cover plate. The interfacial microstructure was observed using electron microscopy. A thick two-layered reaction layer contains Fe2Al5 and FeAl3was observed at the welding interface, respectively. Mechanical property analysis suggested that the reaction layer has no effect on the tensile shear strength of the A5052/SPCC joint and affects the cross tension strength of the joint.

Compatible finite element modelling of laser welded region for crash analysis of autobody assemblies

This paper proposes a compatible modelling method for finite element analysis of the laser welding for autobody assemblies in the crash analysis. The fracture modes of a laser weld were investigated for application to finite element modelling. A single laser weld in a stitch shape was modelled by a cluster of beam elements or hexahedron elements. The model was then applied to a cross tension test and a lap shear test under the quasi‐static loading condition both experimentally and numerically. Finite element analyses were carried out with respect to the element type, the number of elements and the panel mesh size. The numerical result was compared with experimental results under the same condition. Analysis results demonstrate that the load acting on the laser welded element is accurately calculated by the change of the element type and the location of welded constrains. The method proposed provides a guideline for accurate finite element modelling of the laser welded region for the crash analysis of vehicles.

OS2312 レーザー溶接継手の十字引張疲労試験における疲労特性および内部疲労き裂進展挙動の三次元的観察(OS23-3 疲労強度,座屈,OS-23 自動車車体の構造強度・構造形成・疲労信頼性と軽量化)

OS2312 レーザー溶接継手の十字引張疲労試験における疲労特性および内部疲労き裂進展挙動の三次元的観察(OS23-3 疲労強度,座屈,OS-23 自動車車体の構造強度・構造形成・疲労信頼性と軽量化)