Fusion Welding Processes Research Articles

Effects of Cold Gas Tungsten Arc Welding on Dissimilar DHP-CW024A Copper/304 SS Thin Lap Joint

Copper and stainless steel possess distinct properties that make them suitable for different applications (i.e. e-mobility, nuclear power plants, etc.). However, the high thermal conductivity of copper presents a significant challenge in welding. In fact, researchers have explored various fusion welding processes for joining copper to steel and concluded that fusion welding is generally difficult or even unsuitable for obtaining sound and defects free joints. The present study investigated the feasibility of dissimilar lap joint between copper and stainless-steel thin plates using Cold Gas Tungsten Arc Welding (CGTAW) without a filler material and with no significant geometrical distortion of welded plates. The weld was created by consecutive partially overlapped spots, whose welding time varied between 100 and 150 ms, in upgraded conventional TIG machine equipped with cold TIG welding function. Samples made with 150 ms welding time showed a near-uniform distribution of equiaxed copper grain microstructure, while those obtained with 100 ms exhibited significant differences in grain size with the presence of steel inclusions in globule and vortex shapes. The joints demonstrated exceptional flexibility, allowing it to be bent up to a 180º angle without any visible damage. The maximum tensile strength of sample obtained with a welding time of 150 ms was 220 MPa with a fracture located in the heat-affected zone. The sample welded with a welding time of 100 ms exhibited 171 MPa of tensile strength with the fracture along the melting spot area due to pronounce mixing of welded materials. All the samples showed ductile behavior in the fracture zone. Eventually, the application of CGTAW resulted in promising at obtaining joint with good mechanical properties.

Effect of laser beam incident angle on welding of Ti6Al4V with fiber lasers

Laser beam welding (LBW) is widely used for welding Ti6Al4V alloys in aerospace applications. LBW has localized high-energy fluence with low-energy input compared to other fusion welding processes, resulting in narrower heat-affected zones. On the other hand, most metals are highly reflective when the laser beam impinges perpendicular to the surface, making the process inefficient. Hence, this work proposes to employ shallow angle incidence to reduce the reflectivity during the welding of Ti6Al4V material. To explore the potential of this idea, the current study focuses on studying the effect of laser incident angle (15°-90°), power (300 W-1500 W), and feed rate (10 mm/s-25 mm/s) on autogenous weld bead geometry. For this purpose, bead-on plate (BOP) LBW is conducted on mill-annealed Ti6Al4V material of dimensions 25 mm × 25 mm × 3 mm by employing a fiber laser source with a maximum power of 3 kW and a wavelength of 1080 nm. It is observed from the results that at a normal incident angle and low laser power (< 600 W), the penetration depth is too low to generate a weld bead. Analyzing the cross-section of the weld bead, obtained from SEM, perpendicular to the weld direction reveals that the increase in laser incident angle up to an optimal angle resulted in increased bead dimensions (width and height), and beyond that, the dimensions decreased. However, the optimal incident angle changed when the laser power was changed. The major finding of this study is that at 600 W and a normal incident angle, the laser could not penetrate and generate a weld bead due to low absorptivity, while at an incident angle of 300, 450, and 600, weld beads are generated because of increased absorptivity. Similarly, the increase in weld dimensions with the increase in laser power is observed. At higher laser power, underfill and oxide formation are observed. The feed rate is less predominant than the incident angle and the power.

Effects of welding parameter on the microstructure and mechanical properties of friction stir-welded non-heat treatable alloy AA5083

A non-traditional friction stir welding (FSW) is used to joint the materials that are challenging to join together by utilizing the fusion welding processes. This study investigated the influence of the welding speed ratio (υ) to rotating speed (ꞷ) on the weld quality and found the relationship between mechanical qualities and welding peak temperature. With increase in (ꞷ/υ), there was a corresponding rise in peak temperature, leading to the expansion of both the nugget zone (NZ) and the softened region within the joint. As (ꞷ/υ) increased because of the increased heat, as a result in the joint hardness decreased. The joint ultimate tensile strength (UTS) increased with increasing (ꞷ/υ), then decreased, with the maximum value reaching 84.75% of the base material at a (ꞷ/υ) of 9.6. As a result of the low (ꞷ/υ), a flexural crack formed. In comparison to the base material, the thermos mechanically affected zone (TMAZ) and the nugget zone (NZ) showed greater impact strength. The impact strengths first increased and subsequently dropped with an increase in (ꞷ/υ). The tensile specimens’ fracture surfaces showed dimples, signifying a ductile mode of fracture.

Determination of the load acting on the probe by separating force and torque during FSW of AA 6060 T66

AbstractThe development of suitable welding processes is required to meet the ever-increasing demands on joining processes, particularly for lightweight construction and increasing environmental awareness. Friction stir welding (FSW) represents a promising alternative to conventional fusion welding processes, particularly for the joining of low-melting-point materials such as aluminium and magnesium alloys, which present a number of challenges, including the formation of pores and the occurrence of hot cracks. The central element of the process is the friction stir welding tool, which consists of a shoulder and a probe. The rotation and the simultaneous application of pressure during the joining process create a friction-based heat input through the tool. The excellent mechanical properties resulting from dynamic recrystallisation during the welding process are a major advantage of the process. As a result, strengths comparable to those of the base material can be achieved. However, FSW is subject to process-specific challenges, including high process forces, which result in the fabrication of complex and robust devices. Additionally, high dynamic loads on the friction stir welding tools must be considered. In many cases, the design of friction stir welding tools is based on empirical data. However, these empirical values are machine-, component- and material-specific, which often results in under- or overmatching of friction stir welding tools. Sudden probe failure, component scrap, and low process reliability are the direct consequences of undermatching. Overmatching results in enlarged tools with limited accessibility, high heat input, and high process forces, leading to component deformation. The aim of this study is to determine the load on the probe by separating the forces and torque of the shoulder and the probe in order to be able to make statements about the load acting on the probe and the resulting stress state. The knowledge of the stress state can be employed to design friction stir welding tools, both statically and dynamically, for a specific welding task. A strategy was devised to distribute the load exerted on the shoulder and probe. To this end, the length of the probe was gradually reduced between the welding tests. The investigations were carried out with a force-controlled robotized welding setup in which AA 6060 T66 sheets with a thickness of 5 mm were welded. A Kistler multicomponent dynamometer type 9139AA allows to measure the Cartesian forces to be recorded in the x-, y-, and z-directions with a sampling rate of 80 kHz. The weld seam properties were determined by visual and metallographic inspections as well as tensile and bending tests in accordance with DIN EN ISO 25239–5.

A Review on Metallurgical Issues in the Production and Welding Processes of Clad Steels.

Carbon and low-alloy steel plates clad with stainless steel or other metals are a good choice to meet the demand for cost-effective materials to be used in many corrosive environments. Numerous technical solutions are developed for the production of clad steel plates, as well as for their joining by fusion welding. For thick plates, a careful strategy is required in carrying out the multiple passes and in choosing the most suitable filler metals, having to take into account the composition of the base metal and the cladding layer. The specificity of the different processes and materials involved requires an adequate approach in the study of the metallurgical characteristics of clad steel, thus arousing the interest of researchers. Focusing mainly on ferritic steel plates clad with austenitic steel, this article aims to review the scientific literature of recent years which deals with both the production and the fusion welding processes. The metallurgical issues concerning the interfaces and the effects of microstructural characteristics on mechanical behaviour and corrosion resistance will be addressed; in particular, the effects on the fusion and thermally affected zones that form during the fusion welding and weld overlay processes will be analysed and discussed.

Tensile testing of S690QL1 HSS welded joint heterogeneous zones using small scale specimens and indentation methods

Abstract The welded joints are structures with significant heterogeneity, indicated by their fundamental segmentation into base metal (BM), heat affected zone (HAZ), and weld metal (WM). The heat affected zone, having width in millimeter scale for fusion welding processes, is further segmented into several characteristic regions, having differences in grain structure and size. The microstructural heterogeneity results in significant differences in mechanical properties of individual welded joint zones. Mechanical testing of such small material volumes is inconvenient, or even impossible, using the standard size specimens proposed in testing standards. Requirement to precisely position the specimens, even ones with subsize dimensions, and investigate mechanical properties of specific narrow HAZ regions presents certain challenge. This work investigates the X welded joint of S690QL1 grade high strength steel (HSS), welded with slightly overmatching filler metal. The material tensile properties are tested, using small scale specimens and indentation methods. Small scale specimens are ASTM E8 round tensile subsize and flat sheet mini tensile specimens (MTS). The indentation methods include hardness testing and profilometry-based indentation plastometry (PIP) method, to gain additional insights into material stress–strain behavior. Finally, paper evaluates the testing methods, comparatively processes the collected experimental data, and provides guidelines for heterogeneous structures testing.

Application of Pattern Search and Genetic Algorithms to Optimize HDPE Pipe Joint Profiles and Strength in the Butt Fusion Welding Process

The rapid spread of the use of high-density polyethylene (HDPE) pipes is due to the wide variety of methods for connecting them. This study keeps pace with the developments of butt fusion welding of HDPE pipes by exploring the relationship between the performance of the weld joints by studying ultimate tensile strength and exploring the joint welding profiles by studying the shape of the joint at the outer surface of the pipe (height and width of the joint cap) and the shape of the joint at the internal surface (height and width of the joint root). Welding pressure, heater temperature, stocking time, and cooling time were the parameters for the welding process. Regression was analyzed using ANOVA, and an ANN was used to analyze the experimental results and predict the outputs. Two optimization techniques (pattern search and genetic algorithm) were applied to obtain the ideal operating conditions and compare their performance. The results showed that pattern search and genetic algorithms can determine the optimal output results and corresponding welding parameters. In comparison between the two methods, pattern search has a limited relative advantage. The optimal values for the obtained outputs revolved around a tensile strength of 35 MPa (3.45 and 4.5 mm for the cap and root heights, and 8 and 6.98 mm for the cap and root widths, respectively). When comparing the effects of welding parameters on the results, welding pressure had the best effect on tensile strength, and plate surface temperature had the most significant effect on the welding profile geometries.

Comparison between Mechanical Properties and Joint Performance of AA 2024-T351 Aluminum Alloy Welded by Friction Stir Welding, Metal Inert Gas and Tungsten Inert Gas Processes.

The aim of this work is to study joining Al 2024-T3 alloy plates with different welding procedures. Aluminum alloy AA 2024-T351 is especially used in the aerospace industry. Aluminum plates are welded by the TIG and MIG fusion welding process, as well as by the solid-state welding process, friction stir welding (FSW), which has recently become very important in aluminum and alloy welding. For welding AA2024-T35 with MIG and TIG fusion processes, the filler material ER 4043-AlSi5 was chosen because of reduced cracking. Different methods were used to evaluate the quality of the produced joints, including macro- and microstructure evaluation, in addition to hardness and tensile tests. The ultimate tensile strength (UTS) of the FSW sample was found to be 80% higher than that of MIG and TIG samples. The average hardness value of the weld zone of metal for the MIG- and TIG-produced AA2024-T3511 butt joints showed a significant decrease compared to the hardness of the base metal AA2024-T351 by 50%, while for FSW joints, in the nugget zone, the hardness is about 10% lower relative to the base metal AA2024-T3511.

Process Parameters Influence the Mechanical Properties and Nugget Diameter of AISI 316 Stainless Steel During Resistance Spot Welding

Resistance spot welding (RSW) is an important fusion welding process used in many applications, including storage tanks, pipes, and medical tools. For this, should be improved the joints and developing the process parameters and through this research will be discussed the results and concept development. Herein, 1 mm-thick AISI 316 austenitic stainless steel was welded by RSW, and the effect of the welding process parameters, such as “welding current, pulses, squeeze time, and welding time”, on the mechanical properties, nugget diameter (ND), and microstructure of AISI 316 were investigated. As a result, there was a direct relationship between shear-tensile force and ND. The maximum shear-tensile force (12.5 kN) was obtained using 7500 A, 3 pulses, 1.8 s squeeze time, and 1 s welding time. The ND was maximized using 7500 A, 3 pulses, 1.6 s squeeze time, and 0.8 s welding time. The DOE analysis gives an indication about the relation between the parameters and ND on the hand and the parameters and shear strength on the other hand. The microstructure was investigated by optical microscopy, revealing the presence of martensitic, widmanstatten austenite, and ferrite structures.

Characterization of material flow behavior in friction stir welded AA2014 aluminum alloy joints

Abstract Steel rivets serve as a substitute material for connecting similar and dissimilar materials within the structural fabrication industries. However, the use of steel rivets can result in a significant increase in the structure’s weight and may trigger corrosion at the interface due to galvanic coupling. Combining dissimilar alloys through the fusion welding process poses numerous challenges for manufacturers and designers. A solid-state welding technique called friction stir welding (FSW) has been developed. FSW can effectively join materials without reaching their melting points, relying on severe plastic deformation and recrystallization to form a welded joint. The proper selection of the tool and process parameters is essential for achieving a sound weld. The findings of this study indicate that plastic deformation, material flow, and recrystallization play pivotal roles in the strength of the joint. This implies that FSW represents an ideal joining process for high-strength alloys and serves as a viable alternative to replace permanent joints like rivets.

Comparison of fatigue crack growth design curves on GMAW and EBW joints of high strength steels

There is a growing demand in the industrial sector for the use of high-strength structural steels (HSSSs), which can achieve a significant weight reduction in structures. These structural steels are usually produced by quenching and tempering (Q + T) or thermomechanical treatment (TM), and their applications in welded structures pose several challenges for the users. In industrial practice, gas metal arc welding (GMAW) is basically the most commonly used fusion welding process, which has a relatively high heat input. However, at HSSSs, there is a need for low heat input but, at the same time, productive welding processes. High-energy density welding processes, e.g., electron beam welding (EBW), offer a unique opportunity to weld these steels. The widespread use of HSSSs is also hampered by the fact that the benefits of high strength can be exploited primarily under static loading. At the same time, different welded structures made of HSSSs are often subjected to cyclic loading, and possible weld defects and material discontinuities are major risks in this case. During our experiments, GMAW and autogenous EBW processes were applied to make welded joints from S960 Q + T and TM structural steels. The fatigue resistance of the welded joints was characterized by fatigue crack growth (FCG) tests, considering the increased crack sensitivity of HSSSs. A statistical approach was followed both in the design of the experiments and in the evaluation of their results. Based on the test results fatigue crack propagation design curves were determined for the investigated GMAW and EBW welded joints. The design curves were compared with each other, with design curves of lower strength material (S690QL) and with the recommended fatigue crack growth laws of BS 7910.

Fuzzy logic-driven genetic algorithm strategies for ultrasonic welding of heterogeneous metal sheets

There are a lot of problems with the conventional fusion welding process, so ultrasonic welding has been used for about 20 years and has helped a lot of manufacturing industries, including aviation, medicine, and microelectronics. Ultrasonic welding takes less than one second, making it suitable for mass production. Poor weld quality and joint strength are common issues that industries encounter as a result of this process. Actually, the success and quality of the welding are determined by its control parameters. This research examines the impacts of weld time, vibrational amplitude, and weld pressure on the welding of 0.6 mm thick sheets of two different metals, specifically copper and aluminum (AA2024). Responses, including tensile shear stress, weld area, and T-peel stress, are acquired through experiments that follow a full factorial design including four replicas. The highest recorded tensile shear stress was 4.34 MPa, the maximum weld area measured was 63.6 mm2, and the peak T-peel stress reached 1.22 MPa. A second-order non-linear regression model was constructed using all of these data points, which related the responses to the predictors. Due to the importance of quality in the production sector, the process parameters were determined by the combination of genetic algorithm (GA) and fuzzy logic (FL) approaches. The impact of the weld zone temperature on various quality characteristics has been investigated through experiments. It has been noted from the confirmatory test that FL produces superior output outcomes compared to the genetic algorithm, with FL achieving a fuzzy multi-performance index of 0.94 compared to 0.61 for GA. By conducting microstructural analysis, weld quality levels, including “under-weld,” “good weld,” and “over-weld,” were established.

Comparative study on the performance of butt fusion-welding processes for nuclear safety class large-diameter thick-walled PE pipes

New technologies in polymer synthesis and pipe extrusion equipment have led to the commercialization of high-performance, large-diameter, thick-wall high density polyethylene (HDPE) pipes. They have been used in the field of seawater transport and cooling to replace metal pipes, due to their advantages of high corrosion resistance and extensibility. Connection of HDPE pipe is important as it determines the safety of the entire piping system. Butt fusion welding is commonly used for HDPE pipe connection but may cause the formation of weak points in the welded joints, interfering the reliability of the pipeline system in the application of nuclear power plants. At present, there is a lack of research on evaluating the performance of welded joint for large-diameter thick-wall HDPE pipes made by butt fusion-welding. The purpose of this study is to investigate the influence of three different butt fusion-welding processes, i.e., single low pressure (SLP), single high pressure (SHP) and dual low pressure (DLP), by evaluating the performance of their welded joints, including characterizing tensile strength, extensibility, crystallinity and hardness. In specific, a thick-wall HDPE pipe with OD of 812.8 mm and wall thickness of 74 mm which is certified for nuclear safety class was used for study. Representative specimen from the outer, middle and inner part across the wall of the main pipe body and welded joints were taken for testing. Different test methods and specimens were designed to assess the feasibility of evaluating the welding performance from different welding process. The results showed that the mechanical properties of different locations of the welded joints were different, and the tensile strength and fracture energy of the middle part of the joint were lower than that of the inner and outer parts, which could be caused by the difference in the crystallinity and thickness of the melting zone influenced by welding processes, as can be seen from the analysis of DSC test and morphology observation. Hardness testing was conducted on the section of the welded joints, and it revealed that the micromechanical properties of the welded joints in the region of the heat-affected zone were enhanced significantly, which may be due to the annealing effect caused by welding process. In summary, The DLP process resulted in the best extensibility of the welded joints among three processes, suggesting that the joining pressure from welding process plays an important role in affecting the extensibility of the welded joints.

Advancements in Friction Stir Welding Techniques for Enhanced Performance in Aluminum Alloy Structures

Abstract: Friction Stir Welding (FSW) is a solid- state welding method that can be utilized on both comparable and dissimilar materials. The technology is frequently utilized because it generates quality welds and avoids common issues like solidification and liquefaction cracks that are connected with fusion welding processes. The FSW of Aluminum 5086 andits alloys has been commercialized, and there is renewed interest in connecting dissimilar materials. However, before the process can be commercialized, research studies must be conducted to describe and set process windows. In particular, FSW has prompted researchers to try combining incompatible materials with different characteristics, such as Aluminum 5086, and sound welds with no or few intermetallic compounds have been formed.

Joining aluminum die castings and wrought aluminum by resistance spot welding

Resistance spot welding (RSW) is an economic, robust welding process, which is easy to automate and widely used in automotive industry. In this work, the resistance spot weldability of die-cast aluminum alloys EN AC-AlSi7MnMg, EN AC-AlSi9Mn and EN AC-AlSi10MnMg-T6/T7 to wrought aluminum alloy EN AW-AlSi1MnMg-T6 is investigated when applying the standard welding profile as recommended in VDA 238-401 considering main challenges in welding aluminum die castings, for example, porosity and inhomogenities. Weldability is best for EN AC-AlSi10MnMg-T6/T7, but the results show limited applicability of VDA 238-401 in general, especially for high total sheet thicknesses, because of invalid electrode force favouring spatter formation and weld spot irregularites. Still, all spot welds between die castings and 2 mm wrought aluminum sheets reach the recommended shear tension forces for spot diameter dw of 5·√t (2 mm: 5.95 kN; 3 mm: 8.96 kN; according to DIN EN ISO 18595), although being afflicted with solidification porosity and liquation cracking. Gas porosity is not observed and solidification porosity is only found to be significant for weld spot failure when being located close to the joining level, which is observed at welding depths of approximately 50% into the wrought aluminum sheet. Here, a failure path across the nugget is found. Weld porosity apart from the joining level is less significant and failure path is found along the fusion line between nugget and wrought aluminum. Instead, hardness difference between nugget and HAZ is the main reason for weld spot failure. Although the applied welding current profile is not suited best for RSW of mixed joints between aluminium die castings and wrought aluminium alloys, the results confirm its potential, for example, a significantly reduced gas porosity compared to applied fusion welding processes.

Exploring the boundaries of refill friction stir spot welding: influence of short welding times on joint performance

Refill friction stir spot welding is a solid-state spot-welding technique suited to lap joining of thin aluminium sheets, including difficult-to-weld 2xxx series alloys that are prone to hot cracking during fusion welding processes. Long welding time is an ongoing challenge that hinders industrial adoption of the process. To address this, the present study explores much shorter welding times than those previously reported in the literature and assesses the impact on joint quality. Joints of 1.8 mm thick AA2024-T3 sheet were produced with welding times from 3 s, down to 0.75 s and rotational speeds of 1000 rpm to 2500 rpm. Defect formations within the welds were studied with the aid of optical microscopy. The mechanical properties were evaluated using tensile lap shear testing and microhardness mapping, and failure modes were characterised using scanning electron microscopy. Various weld defects were found at all welding times and rotational speeds, and the defects enlarged with decreasing welding time and increasing RS. The highest lap shear strength of 9.21 kN was achieved with a welding time of 3 s and rotational speed of 2000 rpm; lap shear strengths of 7.02 kN and 6.37 kN were achieved for 1.5 s and 0.75 s welds, respectively.

Heat source models for numerical simulation of laser welding processes – a short review

In recent decades, numerical modeling and computer simulation have become an integral part of the design, analysis and optimization of fusion welding processes, including laser welding. In general, laser welding processes involve the interaction of multiple physical phenomena, such as thermal, fluid, metallurgical, chemical, mechanical, and diffusion effects, which makes the development of a simulation model difficult and complex. In addition to the geometric characteristics of the parts to be welded, their material properties must be specified in a wide temperature range, as well as the conditions for heat removal to the environment or shielding gas. One of the most complex tasks in the preparation of a simulation model of the laser welding processes consists in the selection of an appropriate heat source model to accurately determine the heat input to the weld. Very important is also the process of experimental verification and validation of the developed simulation models. In this paper, a short examination of significant mathematical heat source models for numerical simulation of laser welding is provided. Numerical analysis of laser welding of sheets made of S650MC steel is accomplished using conical 3D heat source model with the support of the ANSYS code. The effect of geometrical characteristics of the conical volumetric heat source model on the computed width, length, and depth of the weld pool is discussed, along with evaluation of maximum weld pool temperature.

Nanomechanical, mechanical and microstructural characterization of electron beam welded Al2219-T6 tempered aerospace grade alloy: A comprehensive study

As compared to traditional fusion welding processes, electron beam welding (EBW) is known to produce structurally robust microstructures and narrow heat-affected zone (HAZ) in metals. The process becomes more significant for the tempered alloys vulnerable to heat exposure. In the present investigation, Al 2219-T6 alloy was joined using the EBW process. The microstructural, mechanical, and nanomechanical characteristics of the resulting joint were investigated. EBW resulted in a narrow HAZ (22 μm) with a 430 mm fusion zone (FZ). A dendritic structure was observed in the FZ zone, while second-phase particles were absent indicating their dissolution during welding and interesting formation of Al2Cu mixture around the dendrites. The limited content of Cu in the base metal (BM) resulted in the formation of a solid solution in the FZ, along with the presence of fine equiaxed grains in the HAZ and equiaxed dendritic grains in the FZ zone. The X-ray diffraction analysis confirmed the absence of peaks corresponding to incoherent phases in the FZ. Compared to the BM, micro-hardness measurements revealed a 12.7 % increase in the hardness in the HAZ, while a significant decrease of approximately 19 % was observed in the FZ. The joint exhibited reduced tensile strength, ultimate strength by 42.2 %, and yield strength by 47.3 % when compared to the BM. The fracture analysis indicated a ductile failure mode with the presence of microvoids. Nano-indentation tests at various loads demonstrated a decrease in the nanohardness from the BM to the HAZ and FZ regions. Atomic force microscopy (AFM) analysis revealed significant pile-ups in the FZ, indicating the occurrence of plastic deformation during the welding process. The presented findings are valuable for the joint and structure design of Al −2219T6 alloy in particular and other Al alloys in general.

Development of Computational Approach for Analyzing In-Process Thermal-Mechanical Condition during Friction Stir Welding for Prediction of Material Bonding Defect

Unlike the conventional fusion welding process, friction stir welding (FSW) relies on solid-state bonding (SSB) to join metal surfaces. In this study, a straightforward computational methodology is proposed for predicting the material bonding defects during FSW using quantitative evaluation of the in-process thermal-mechanical condition. Several key modeling methods are integrated for predicting the material bonding defects. FSW of AA2024 is taken as an example to demonstrate the performance of the computational analysis. The dynamic sticking (DS) model is shown to be able to predict the geometry of the rotating flow zone near the welding tool. Butting interface tracking (BIT) analysis shows a significant orientation change occurring to the original butting interface, owing to the material flow in FSW, which has a major impact on the bonding pressure at the butting interface. The evolution of the interfacial temperature and the interfacial pressure at the butting interface was obtained to analyze their roles in the formation of material bonding. Four bonding-quality indexes for quantifying the thermal-mechanical condition are tested to show their performance in characterizing the bonding quality during FSW. When the BQI is below a critical value, a bonding defect will be generated. The paper indicates that the simulation-based prediction of a material bonding defect is highly feasible if the developed methodology is extended to quantitatively determine the critical value of the bonding quality index for successful SSB for various alloys.

Numerical Simulation of the Effect of Fusion Welding Processes on Residual Stress in Stainless Steel Pipe Weld Joints Used in Fast Reactors

The main structural material used in the building of Fast Breeder Reactors (FBR) is stainless steel of type 316L(N). Finite Element Model (FEM)-based simulations were carried out to understand the effect of various welding processes on the residual stresses induced in 316L(N) stainless steel primary sodium-carrying pipes with an inner diameter of 208 mm and a wall thickness of 5.6 mm. The welding processes considered include multi-pass Tungsten Inert Gas (TIG) welding, A-TIG, laser, hybrid laser-TIG, and Hybrid laser-Metal Inert Gas (MIG). First, single or hybrid heat sources are chosen based on the welding process, and then weld bead profiles obtained during simulation are made to match with that of the actual weld bead profiles obtained during experiments. Then, the optimised heat source is employed for the thermal analysis. The thermal analysis output is then successively linked and provided as input to the mechanical analysis, which uses an isotropic hardening model to predict hoop and axial residual stresses in pipe weld joints. The results show that the A-TIG weld joint exhibited lower hoop stress (165 MPa at inner diameter and -2 MPa at outer diameter) in comparison with that of the other weld joints. The lower residual stress is attributed due to the straight-sided weld bead, lower peak temperature, and slower cooling rate of the A-TIG welding. Therefore, A-TIG welding is recommended among the chosen fusion welding processes for welding of 316L(N) stainless steel primary sodium carrying pipes to minimise the residual stresses and mitigate the premature failure of the pipe weld joints.