Weld zone representation methods during the stretching of friction stir welded blanks with dissimilar sheet thickness: A study using numerical simulations

In this work, the validity of weld zone representation as single zone or double zone under varied weld conditions of friction stir welded (FSW) blanks with dissimilar thicknesses sheets is explored. During LDH test simulations, representative base material of 1.5 mm and 1.1 mm thickness is considered. It is observed that modeling FSW blanks with double zone assumption plays a vital role for accurate formability prediction. There are possibilities of obtaining multiple domains of weld conditions for the single zone and double zone assumption validity. The failure locations also affect the weld zone representation along with the critical error percentage difference between reference model (single zone) and double zone model FSW blanks. The number of single zone models has increased in transverse weld orientation and number of double zone models has increased in longitudinal weld orientation in the case of FSW blanks with dissimilar thickness as compared to similar thickness FSW blanks. A comparison is done between the limit strain values for single zone and double zone assumptions with reference model. The pattern that is observed in the load‐punch stroke behavior is also seen in limit strain comparison for weld zone representation.     

Modeling and fatigue assessment of weld start‐end locations based on the effective notch stress approach

The present paper contains a methodology for modeling and life assessment of fatigue loaded welded components providing distinct weld start and end locations. The proposed methodology follows the IIW recommendation regarding modeling and finite element meshing of weld toe and root by means of an effective notch radius and uses the corresponding Wöhler curve (FAT class) to assess the durability. Geometrical singularities and, therewith, numerical discontinuities, can be overcome especially when 3D weld root problems are treated. The fatigue life assessment is performed on the basis of normal stresses acting at the failure‐critical weld toe and root locations. Comprehensive experimental database containing stress and fatigue life results derived from motor truck's hypoid rear axles providing complex 3D welds subject to vertical, longitudinal, and torsional loading is used to verify the calculation accuracy of the proposed methodology. The agreement between experimentally determined and calculated fatigue results is satisfactory.     

Comparative experimental study on the modification of microstructural and mechanical properties of friction stir welded and gas metal arc welded EN AW‐6061 O aluminum alloys by post‐weld heat treatment

In this paper, the effects of post‐weld heat treatment on modification of microstructures and mechanical properties of friction stir welded and gas metal arc welded AA6061‐O plates were compared with each other. Gas metal arc welding and friction stir welding were used as the applicable welding processes for AA6061‐O alloys. The applied post‐weld heat treatment consisted of solution heat treatment, followed by water quenching and finally artificial aging. The samples were classified as post‐weld heat treated and as‐welded joints. The microstructural evolution, tensile properties, hardness features and fracture surfaces of both as‐welded and post‐weld heat treated samples were reported. The results clearly showed that friction stir welding process demonstrated better and more consistent mechanical properties by comparison with the gas metal arc welding process. The weld region of as‐welded samples exhibited a higher hardness value of 80 HV0.1 compared to the base material. In addition, the feasibility of post‐weld heat treatment in order to enhance the mechanical properties and to obtain more homogeneous microstructure of 6061‐O aluminum alloys was evaluated.     

Improvement of weld temperature distribution and mechanical properties of 7050 aluminum alloy butt joints by submerged friction stir welding

Submerged friction stir welding (FSW) in cold and hot water, as well as in air, was carried out for 7050 aluminum alloys. The weld thermal cycles and transverse distributions of the microhardness of the weld joints were measured, and their tensile properties were tested. The fracture surfaces of the tensile specimens were observed, and the microstructures at the fracture region were investigated. The results show that the peak temperature during welding in air was up to 380 °C, while the peak temperatures during welding in cold and hot water were about 220 and 300 °C, respectively. The temperature at the retreated side of the joint was higher than that at the advanced side for all weld joints. The distributions of microhardness exhibited a typical “W” shape. The width of the low hardness zone varied with the weld ambient conditions. The minimum hardness zone was located at the heat affected zone (HAZ) of the weld joints. Better tensile properties were achieved for joint welded in hot water, and the strength ratio of the weld joint to the base metal was up to 92%. The tensile fracture position was located at the low hardness zone of the weld joints. The fracture surfaces exhibited a mixture of dimples and quasi-cleavage planes for the joints welded in cold and hot water, and only dimples for the joint welded in air.     

Prediction of microstructural features and forming of friction stir welded sheets using cellular automata finite element (CAFE) approach

The main aim of the present work is to predict the microstructural features like grain size and dislocation density in the weld zone during friction stir welding (FSW) of similar (Al6061T6/Al6061T6) and dissimilar (Al6061T6/Al5086O) Aluminium grades using Cellular Automata Finite Element (CAFE) approach. The FSW process is not modelled with the stirring action, instead heat flux, strain-rate and strain are incorporated by analytical models. The grain size is controlled through cellular automata (CA) cells and dislocation density is related to this by two different (analytical and empirical) models. After FSW, four different methods are proposed for predicting the tensile behaviour of weld zone and the efficiency of these methods is evaluated through validations. The results indicate that the thermal, strain-rate, and strain models are accurate enough in their predictions when compared with existing results. The grain size predictions from CAFE model, which include the transition rule, are also consistent with the literature results, both for similar and dissimilar material combinations. The analytical model shows better dislocation density prediction than empirical model when compared with the experimental data. Of all the methods proposed for tensile behaviour prediction, the CAFE model that includes dislocation density evolution using the second model is efficient and accurate. The stress–strain data predicted from an averaged flow stress of many CA cells is also encouraging. Through these results, it has been demonstrated that the CAFE approach along with few validated analytical models can be used to predict the micro-features and forming aspects during FSW consistently.     

Effect of weld morphology on mechanical response and failure of friction stir welds in a naturally aged aluminium alloy

Friction stir butt welds in 6063-T4 aluminium alloy were obtained using square and two tapered tool pin profiles. Tensile tests at 0°, 45°, and 90° to the weld line, hardness contours in the weld cross-section, temperatures in the heat affected zones, cross-sectional macrographs, transmission electron micrographs, and X-ray diffraction studies were used to characterize the welds. In transverse weld specimen, tunnel defects appearing at higher weld speeds for tapered pin profiles, were found to result in mechanical instabilities, i.e. sharp drops in load–displacement curves, much before macroscopic necking occured. Further, in comparison to the base metal, a marked reduction in ductility was observed even in transverse specimen with defect free welds. Hardness contours in the weld cross-section suggest that loss in ductility is due to significant softening in heat affected zone on the retreating side. Transmission electron microscopy images demonstrate that while recovery and overaging are responsible for softening in the heat affected zone, grain size refinement from dynamic recrystallization is responsible for strengthening of the weld nugget zone. X-ray diffraction studies in the three weld zones: weld nugget zone, heat affected zone, and the base metal corroborate these findings. A weld zone model, for use in forming simulations on friction stir welded plates of naturally aged aluminium alloys, was proposed based on mechanical characterization tests. The model was validated using finite element analysis.     

Evolution of microstructures and mechanical properties in similar and dissimilar friction stir welding of AA5086 and AA6061

In this work, thermo-mechanical behavior and microstructural evolution in similar and dissimilar friction stir welding of AA6061-T6 and AA5086-O have been investigated. Firstly, the thermo-mechanical behaviors of materials during similar and dissimilar FSW operations have been predicted using three-dimensional finite element software, ABAQUS, then, the mechanical properties and the developed microstructures within the welded samples have been studied with the aid of experimental observations and model predictions. It is found that different strengthening mechanisms in AA5086 and AA6061 result in complex behaviors in hardness of the welded cross section where the hardness variation in similar AA5086-O joints mainly depends on recrystallization and generation of fine grains in weld nugget, however, the hardness variations in the weld zone of AA6061/AA6061 and AA6061/AA5086 joints are affected by subsequent aging phenomenon. Also, both experimental and predicted data illustrate that the peak temperature in FSW of AA6061/AA6061 is the highest compared to the other joints employing the same welding parameters.     

Microstructure simulation and ballistic behaviour of weld zones in friction stir welds in high strength aluminium 7xxx plate

The effect of friction stir welding (FSW) on the knockdown in properties and failure mechanisms of high strength aluminium plate (AA7010-T7651) under ballistic impact has been investigated by modelling and ballistic testing, simulated, and actual weld zone microstructures, in order to systematically study the link to weld microstructural variation. FSW of thick plate leads to an extremely wide heat affected zone (HAZ). The microstructures within the weld zones were successfully modelled and simulated. However, the maximum loss of hardness in the HAZ resulted in only a 20% reduction in the ballistic limit (V₅₀). The maximum V₅₀ was associated with the highest level of hardness reached prior to a change in failure mechanism, from ductile hole enlargement, using an armour piercing (AP) projectile, or plugging with a fragment simulation projectile (FSP) to excessive spalling and associated adiabatic shear banding at too high hardness levels. The behaviour of the nugget zone has also been examined. The trends in the ballistic data are discussed in the context of the weld zone microstructures and fracture mechanisms.     

Effect of welding wire and groove angle on mechanical properties of high strength steel welded joints

Mechanical properties of high strength steel welded joints strictly depend on the welding process, the filler material composition and the welding geometry. This study investigates the effects of using cored and solid welding wires and implementing various groove angles on the mechanical performance of weld joints which were fabricated employing the gas metal arc welding process. It was found that weld joints of low alloy, high strength steels using low alloy steel cored welding wires exhibited higher tensile strength than that of low alloy steel solid wire and chromium‐nickel steel bare welding wire when the method of gas metal arc welding is employed. The effect of groove angle on the strength and toughness of V‐groove and double V‐groove butt‐joints was investigated. V‐groove joints, with higher tensile strength than double V‐groove joints in the whole range of groove angles, were superior in toughness for small groove angles, but impact toughness values of both joints were comparable for large angles. The effect of heat input and cooling rate on the weld microstructure and weld strength was also investigated by performing thermal analysis employing the commercial software ANSYS. It was concluded that cooling rate and solidification growth rate determined the microstructure of the weld zone which had great consequences in regard to mechanical properties.     

A fatigue life prediction method for coke drum base,weld, and HAZ materials from tensile properties

The importance of material fatigue information in design has been well recognized. There are a few existing fatigue life prediction methods based on materials tensile properties. Some of these fatigue life prediction methods can be successfully applied for non-heat affected materials. However, industrial components, such as pressure vessel and pipelines are commonly constructed by welding parts together. The fatigue lives of welded section and its surrounding material could be greatly affected by the welding process. Therefore, it is beneficial to develop a fatigue life prediction model for the weld and surrounding heat affected zone (HAZ) materials based on their tensile testing data. In this paper, fatigue lives of base material and its weld and HAZ materials for constructing coke drums are studied. Mechanical properties are first obtained from the tensile tests. Then, fully-reversed strain-controlled fatigue tests were performed. It is found that the fatigue life of pure base material is roughly twice of the weld and four time of the HAZ at the same strain amplitude. Four-point correlation (FPC) method by Manson can reasonably predict the life of base material. However, it over-predicts the lives of weld and HAZ. By introducing two reduction factors R_plastic and R_elastic for the weld and HAZ material respectively into the FPC method, the over-prediction can be rectified. Therefore, the proposed modified FPC method could be applied in predicting fatigue lives of weld and HAZ materials.     

Microstructure and mechanical properties of friction stir welded copper

The main objective of this investigation was to apply friction stir welding technique (FSW) for joining of 2 mm thick copper sheet. The defect free weld was obtained at a tool rotational and travel speed of 1,000 rpm and 30 mm/min, respectively. Mechanical and microstructural analysis has been performed to evaluate the characteristics of friction stir welded copper. The microstructure of the weld nugget (WN) consists of fine equiaxed grains. Similarly, the elongated grains in the thermomechanically affected zone (TMAZ) and coarse grains in the heat-affected zone (HAZ) were observed. The hardness values in the WN were higher than the base material. Eventually HAZ shows lowest hardness values because of few coarse grains presence. Friction stir welded copper joints passes 85% weld efficiency as compared to the parent metal.     

Identification of the pin offset effect on the friction stir welding (FSW) via Taguchi – Grey relational analysis: A Case study for AA 7075 – AA 6013 alloys

The aim of this work is to present a case study relating to the dissimilar friction stir welding (FSW) ability of AA 7075‐T651 and AA 6013‐T6 by applying pin offset technique. An orthogonal array L18 was conducted to perform the overlapped weld seams using three different values of pin offset, welding speed and tool rotational speed along with two different pin profiles determine the impact of welding parameters on the tensile properties of friction stir welded joints. The nugget zone for each of overlapped weld seams exhibited a complex structure and also, the pin offset and profile also were found to have a great impact on the microstructural evolution of the nugget zone. The ultimate tensile strength, elongation at the rapture and bending strength of welded joints were measured in the ranges of 194–215 MPa, 1.79–3.34 % and 203–352 MPa. From the Taguchi based Grey relational analysis, the optimum welding condition was determined for the welded joint performed using a single fluted pin profile with the zero pin offset, tool rotational speed of 630 min^?1 and welding speed of 63 mm/min. Microstructural and macro‐structural observations revealed that welded joints exhibiting lower tensile strength are consistent of various types of defects (e. g. cracks, tunnels and cavities). The fracture location of welded joints was found to be on the heat affected zone and between the heat affected zone and AA 6013‐base metal. The tool and pin wear was not observed during the welding applications     

Metallurgical characterization of pulsed current gas tungsten arc,friction stir and laser beam welded AZ31B magnesium alloy joints

This paper reports the influences of welding processes such as friction stir welding (FSW), laser beam welding (LBW) and pulsed current gas tungsten arc welding (PCGTAW) on mechanical and metallurgical properties of AZ31B magnesium alloy. Optical microscopy, scanning electron microscopy, transmission electron microscopy and X-Ray diffraction technique were used to evaluate the metallurgical characteristics of welded joints. LBW joints exhibited superior tensile properties compared to FSW and PCGTAW joints due to the formation of finer grains in weld region, higher fusion zone hardness, the absence of heat affected zone, presence of uniformly distributed finer precipitates in weld region.     

Residual stress distribution and microstructure in the friction stir weld of 7075 aluminum alloy

In order to study the relationship between residual stress (RS) and the microstructure of friction stir weld (FSW), RS profiles through thickness in the un-welded aluminum alloy 7075 plate and in middle layer of its FSW joint were determined nondestructively by the short-wavelength X-ray diffraction (SWXRD) and neutron diffraction. Microstructure and mechanical properties of the FSW joint were also studied by optical microscopic analysis, and microhardness and tensile strength measurements. RS profiles measured by the two methods had the same distribution trend. The maximum tensile RS tested by SWXRD and neutron diffraction in transverse and longitudinal direction occurred in the weld nugget. Microhardness in the direction perpendicular to the weld line showed a “W” shape distribution. Position of the local maximal extremum of RS in thermo-mechanically affected zone corresponded to that of minimal microhardness. The grain-refined strengthening caused by the recrystallization in the weld nugget kept the joint from fracturing at this region notwithstanding the maximum tensile RS. And the tensile fracture occurred near the boundary of welding zone and thermo-mechanically affected zone where minimum of hardness and maximum of RS appear at the same position.     

A numerical analysis of sheet metal formability for automotive stamping applications

A theoretical failure model is presented for the numerical prediction of the forming limit strains of automotive sheets. The model uses the Swift’s diffuse necking and Hill’s localized necking concepts in describing tearing-type sheet metal failures and a computational scheme is proposed in which the failure conditions are expressed in incremental forms. The Bauschinger effect is included properly in the deformation modeling using an additive backstress form of the nonlinear-kinematic hardening rule. The necking conditions and plasticity model are transformed into a set of algebraic equations that may be applied both for proportional and non-proportional strain-controlled loadings. An iterative approach is employed in the incremental solution of algebraic equations. The formability analyses are conducted using the proposed theoretical model and the forming limit strains of two new generation auto sheets (Trip600 1.4 mm, DP980 1.15 mm) are estimated. The numerically generated FLC are compared with the experimental data and the FLC calculated with the Keeler–Brazier equation. For both steels, the model produced conservative plain–strain intercept values, FLC₀, when compared with the predictions of Keeler–Brazier equation. Also the negative minor strain part of the experimental FLD’s is estimated with sufficient accuracy. For the positive minor strain side, however, the predictions are lower than both the experimental fit and the standard curve.     

Prediction of hardness minimum locations during natural aging in an aluminum alloy 6061-T6 friction stir weld

This study describes a method that can predict the hardness minimum location as a function of natural aging time in a heat-treatable 6061-T6 Al alloy plate subjected to friction stir welding (FSW). First, temperature distributions were simulated in the FSW plate by finite element modeling. Second, to determine the natural aging kinetics, hardness changes were measured as a function of natural aging time from a number of Al specimens that had been isothermally heat treated at different peak temperatures. Finally, the simulated temperature profiles and the natural aging kinetics were correlated to predict the hardness profiles in the FSW plate. The predicted hardness minimum locations are consistent with the measured hardness profiles in that the hardness moves away from the weld centerline as the aging time increases. Moreover, the predicted hardness minimum is located at the similar position of failure in cross-weld tensile samples.     

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

Abstract

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.     

An investigation of the microstructures and properties of metal inert gas and friction stir welds in aluminum alloy 5083

Two different types of welds, Metal Inert Gas (MIG) and Friction Stir Welding (FSW), have been used to weld aluminum alloy 5083. The microstructure of the welds, including the nugget zone and heat affected zone, has been compared in these two methods using optical microscopy. The mechanical properties of the weld have been also investigated using the hardness and tensile tests. The results show that both the methods could successfully be used to weld such alloy. The strength of the joints is comparable to the strength of the base metal in both cases. However, FSWed samples have shown higher strength in comparison to the MIG samples. The results also show that the extension of the heat affected zone is higher in the MIG method in comparison to the FSW method. The weld metal microstructure of MIG welded specimen contains equiaxed dendrites as a result of solidification process during MIG welding while FSWed samples have wrought microstructures.     

Predicting laser weld reliability with stochastic reduced‐order models

Laser welds are prevalent in complex engineering systems and they frequently govern failure. The weld process often results in partial penetration of the base metals, leaving sharp crack‐like features with a high degree of variability in the geometry and material properties of the welded structure. Accurate finite element predictions of the structural reliability of components containing laser welds requires the analysis of a large number of finite element meshes with very fine spatial resolution, where each mesh has different geometry and/or material properties in the welded region to address variability. Traditional modeling approaches cannot be efficiently employed. To this end, a method is presented for constructing a surrogate model, based on stochastic reduced‐order models, and is proposed to represent the laser welds within the component. Here, the uncertainty in weld microstructure and geometry is captured by calibrating plasticity parameters to experimental observations of necking as, because of the ductility of the welds, necking – and thus peak load – plays the pivotal role in structural failure. The proposed method is exercised for a simplified verification problem and compared with the traditional Monte Carlo simulation with rather remarkable results. Copyright © 2015 John Wiley & Sons, Ltd.     

Application of response surface methodology to drive GTN model parameters and determine the FLD of tailor welded blank

Tailor welded blanks (TWBs) have found great usage in auto industries; however there are some drawbacks, low formability and weld line movement, which hinder their applications. Prediction of their formability through forming limit diagrams (FLDs) enables one to consider the effects of forming parameters on forming behavior of TWBs. Different numerical approaches have been applied to predict the experimental FLD of these blanks. In the present research Gurson-Tvergaard-Needleman (GTN) model was applied as failure criterion. The parameters of GTN model were firstly derived for both thin and thick monolithic blanks of studied TWB by application of response surface methodology. Then experimental method of obtaining FLD was simulated and FLD of transversely welded TWB was predicted using Abaqus software. The results indicate that the FLD of studied TWB is lower than those related to the parent sheets and there is an appropriate agreement between the predicted and experimental FLDs of the studied TWB.