Dissimilar Metal Welds Research Articles

Thermal ageing effect on solute segregation and precipitation in the heat-affected-zone of dissimilar metal welds for nuclear power plants

Solute segregation in the carbon-depleted (CDZ) heat-affected zone of dissimilar metal welds (DMW) designed for nuclear power plant EPRTM reactors has been investigated by atom probe tomography (APT) and related to the mechanical properties (toughness test). The analysis of grain boundaries (GBs) by APT allowed the quantification of the P segregation and other solute elements, in all the analyzed samples, before and after long-term ageing heat treatments. The post-weld stress-relief heat treatment applied to the samples before the ageing heat treatments leads to important GBs segregation levels. Following thermal ageing treatments allow a relative increase of GB segregation. P segregation increases with the various thermal ageing conditions. The experimental data were used to model P segregation in high-angle GBs. The simulations show that P segregation is kinetically limited by bulk diffusion, explaining the observed P segregation increase with temperature. P segregation in GBs in the CDZ of the DMW should remain low compared to the expected equilibrium segregation level during the entire reactor service. In addition to solute segregation in extended defects, APT measurements revealed the presence of Cu-rich clusters in GBs, dislocations, and grains in all the aged samples. These clusters were not observed in the sample before ageing.

Effect of assisted heating on microstructure, material flow and intermetallic formation during friction stir welding of copper alloy and AA 6063

The friction stir welding (FSW) is a newly developed solid-state welding technique, which addresses a number of practical issues such as reducing melt related defects by reducing welding temperature. This provides an opportunity for dissimilar metal welding and provides high quality joints. In the present study friction stir welding was performed on AA6063 and copper alloys by conventional FSW method and with the aid of external flame heating during the FSW process. The concept behind the experimentation is to perform the welding at low RPM to improve the material mixing and joint properties. The microstructure, phase constituents and mechanical properties of FSW joints were studied through optical metallography, SEM, X-ray diffraction, micro hardness, and tensile tests. Al2Cu and Al4Cu9 intermetallic were formed during the conventional and assisted flame heating FSW process which were distributed in the nugget zone. Due to the flame heating, copper and aluminum alloys get softer and leads to distribution of intermetallic as lamellar structure. The values of tensile strength for conventional friction stir welding, and assisted flame heating FSW are 69 MPa, and 102 MPa respectively.

Residual stress assessment for dissimilar metal welds in nuclear power plant

The European Network on Neutron Techniques Standardization for Structural Integrity (NeT) aims to develop experimental and numerical techniques and standards for the reliable characterization of residual stresses in structural welds. NeT has developed over 20 years and involves over 30 organizations including industrial, academic, and research facilities around the world. The network tackled each specific problem by creating a dedicated Task Group (TG), which undertakes both measurement and modelling studies. NeT TG8 follows on from the successful TG1 and TG4 benchmarks, which studied the dissimilar metal welds (DMW) between 18MnD5 and Alloy 52. The material properties characterization, residual stress measurements and modelling have been conducted for NeT TG8. This paper only presents the NeT TG8 benchmark residual stress measurement in the as welded condition and preliminary simulation results.

Electron beam welding of P91 steel and incoloy 800HT and their microstructural studies for advanced ultra super critical (AUSC) power plants

This study characterizes an electron beam welded dissimilar joint between P91 ferritic/martensitic steel and Incoloy 800HT Nickel-based alloy. Such dissimilar metal welds (DMW) have applications in nuclear and thermal powerplants where steam-carrying pipes lead to header sections operating at around 600–650 °C. The study employed an advanced welding process to obtain a clean, high-quality dissimilar metal. The microstructure characterization of the DMW involving Nickel-based alloys is prone to solidification cracking in the weld fusion zone (WFZ), which affects the overall weld properties and leads to failure. In the present study, the WFZ microstructure revealed a mixed solidification mode with a homogenous presence of equiaxed dendritic and cellular-type structures. Advanced analyses of WFZ microstructure were done to account for microsegregation. Nb, Ti, Cr, and Mo microsegregation was found in the WFZ. The Ti-rich grain boundaries in the WFZ provided the pinning action during plastic deformation. The evolution of Cr, Mo-based carbide phases affected the microhardness distribution. Microsegregation under rapid solidification of the WFZ resulted in the formation of secondary phases in the interdendritic region. The effect of the evolution of precipitates deteriorated the mechanical strength. High and uneven dislocation distribution was observed in the WFZ near the IN 800HT weld interface. Under tensile loading, the DMW failed from the IN 800HT weld interface due to coarsening of precipitated in the IN 800HT HAZ region near the weld interface and grain boundary thickening due to the deposition of Ti over them. The tensile strength of WFZ was 592 MPa. The tensile strength of WFZ was 592 MPa. The post-weld heat treatment (PWHT) homogenized the microhardness across the weld. Impact toughness after PWHT slightly increased from 63 J to 69 J. The dissimilar joint obtained using the EBW process had sufficient mechanical strength to be considered for Generation IV power plants under the American Society for Testing and Materials (ASTM) standards. The study concludes that the dissimilar metal welded joint was highly prone to failure despite sufficient strength, as IN 800HT weld interface was susceptible to grain boundary liquation.

Microstructure evolution of weld root in dissimilar weld metal in S690QL high-strength steel under multiple welding thermal cycles

Microstructure evolution of weld root in dissimilar weld metal in S690QL high-strength steel under multiple welding thermal cycles

High contrast corrosion mapping of dissimilar metal weld joints using alternating current scanning electrochemical microscopy: A case study with Zr-4–Ti–304L SS weld

Intermittent contact alternating current scanning electrochemical microscopy (IC-AC–SECM) has been used in the present study to image the local electrochemical activity of Zr-4–Ti–304L SS dissimilar weld joint without a redox mediator at an open circuit potential. The perturbation frequency and bias potential applied to the Pt ultramicroelectrode (UME) are found to affect the contrast of IC-AC-SECM imaging. The optimum parameters for high contrast imaging with high sensitivity were determined by performing potentiodynamic polarization and approach curves of Pt UME. The potentiodynamic polarization behaviour of Pt UME carried out in tap water revealed that the Pt UME biased with potential at −0.5 V was ideal for obtaining better sensitivity due to the maximum reduction of dissolved oxygen in the electrolyte. The effect of applied perturbation frequencies on Pt UME examined by the approach curves using the AC-SECM technique showed a transition from negative to positive feedback over 304L SS at about 10 kHz whereas for Zr-4 and Ti the transition was observed at 50 kHz but with only a minor difference in their feedback. Better contrast in mapping local current and impedance magnitude across the weld joint has been achieved by applying an optimum dc bias (–0.5 V) to the Pt tip and perturbation frequency (10 kHz). The IC-AC-SECM results have been compared with the conventional potentiodynamic polarization and electrochemical impedance spectroscopy results of Zr-4, Ti, 304L SS and the weldment in tap water, where the weld sample undergoes preferential corrosion of 304L SS region due to the galvanic coupling. The present work provides new insight into the optimal IC-AC-SECM parameters for best lateral imaging to map electrochemical activity with good contrast across a multilayer weld joint in tap water.

Computational modeling of laser welding for aluminum–copper joints using a circular strategy

Dissimilar metal welding poses a significant challenge for researchers in the welding and materials science communities due to the different properties of the metals involved. In this study, we present a novel approach for determining the optimal processing parameters for continuous laser welding (CLW) of dissimilar metals, specifically aluminum and copper, using a circular strategy. The approach utilizes computational fluid dynamics (CFD) simulations to model the welding process and predict joint quality. We validated a CFD-based numerical model for predicting the melt-pool shape across a range of laser power and circular welding speed values. Additionally, we developed artificial neural network (ANN) models that can predict melt pool interface width, penetration depth, and copper concentration for any combination of processing conditions. Using surrogate modeling, we identified the optimal values of laser power and welding speed that result in the highest quality joint. Our results showed that the optimal processing conditions were a laser power of 2950W and a circular speed of 55 mm/s, which resulted in a high-quality Al–Cu joint with maximum shear strength of 2200 N. Microstructural analysis and tensile-shear strength testing confirmed that the optimal conditions resulted in a low degree of intermixing and the absence of large intermetallic compounds (IMCs) in the weld seam. Overall, study offers a valuable contribution to the welding and materials science communities by presenting a novel approach to address a common challenge in dissimilar metal welding. Our approach can be applied to other types of dissimilar metal welding and may inform the development of new welding techniques in the future.

Mechanical Properties and Corrosion Characteristics of Dissimilar Friction Stir Welded AA1000/AA6061-T6 Aluminum Alloys

This paper is aimed to investigate microstructure, corrosion and mechanical properties of dissimilar metal weld joints between AA1000 and AA6061-T6 under different tool rotational speeds of 910, 1500, and 2280 rpm at a constant transverse speed of 30 mm/min. Several experimental works have been conducted including microstructure observation using optical microscopy and Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray Spectroscopy (EDS), Vickers microhardness measurements, tensile tests, and corrosion measurements using Potentiodynamic Polarization method. Results show that increasing tool rotational speed from 910 rpm to 1500 rpm increases the degree of homogeneity in nugget zone and improves strength of the weld joint but further increase in the tool rotational speed up to 2280 rpm degrades the mechanical properties due to coarsening of the microstructure. Furthermore, corrosion rate of the nugget zone (NZ) is in between the two base metals.

Study of fusion boundary microstructure and local mismatch of SA508/alloy 52 dissimilar metal weld with buttering

A SA508/Alloy 52 dissimilar metal weld (DMW) mock-up with double-sided Alloy 52 butterings, which is fully representative of Ringhals pressurizer surge nozzle DMW repair solution, was studied. The microstructure, crystal structure, elemental diffusion, carbide formation and macro-, micro- and nano-hardness of the SA508/nickel-base Alloy 52 buttering fusion boundary (FB) were investigated. Three types of FBs were analyzed, i.e., narrow FB (∼80–85% of whole FB), tempered martensitic transition region (∼15%) and wide partially mixed zone (∼1–2%). The different FB types were induced by the local heat flow and respective elementary diffusion, which significantly influence the local hardness mismatch across the DMW interface and the local brittle fracture behavior.

Probabilistic Graphical Model for Sizing Assessment in Ultrasonic Testing in Dissimilar Metal Weld

This work investigates an approach for evaluating the uncertainty associated with flaw sizing in Ultrasonic Testing in nuclear inspections using Bayesian Belief Networks (BBN). BBN graphically represents the probabilistic relationship between the sizing accuracy and some of the variables that cause its deviation. The data for BBN training was collected from sizing information of Round-Robin trials reported in the literature and supplemented with sizing results obtained using the Ultrasonic Testing module of CIVA Non-Destructive Evaluation software. The test specimen used in the Round-Robin exercise and simulations was a dissimilar metal welded pipe. Given the available data, the variables considered causing sizing inaccuracy are the flaw’s actual depth, tilt angle, and position in relation to inner and outer surfaces.

Corrosion and wear resistance of the Al/steel dissimilar weld metals by using multi-principal filler materials

Abstract Multi-principal filler materials via a high-entropy design have been reported to successfully finish the dissimilar metal joining of Al alloy to steel and to reduce the amount of Fe-Al IMCs in weld metals. However, few studies have concentrated on the corrosion and wear resistance of the Al/steel dissimilar weld metals by using multi-principal filler materials. In this study, the joining of Al 6061 alloy to 304 stainless steel served as the research object. Three types of filler materials including CoZnCuMn0.8Si0.2, FeCoCrNiMn, and AlSi12 powders were used. The effects of multi-principal filler materials on the corrosion behavior and wear resistance of the weld metals were evaluated. It was found that the weld metals by using multi-principal filler materials presented the contents of chemical elements in the range of 5–35 at%. The thermodynamic environment with a low ΔG mix was formed in the weld metals. Compared to the AlSi12 sample, the FeCoCrNiMn sample had excellent corrosion resistance in NaCl solution, whereas the CoZnCuMn0.8Si0.2 sample had excellent corrosion resistance in HCl solution. Moreover, the weld metals by using multi-principal filler materials had a better wear resistance compared to that of the AlSi12 sample. The wear loss of the CoZnCuMn0.8Si0.2 and FeCoCrNiMn samples was 4.5% and 11.4% of that of the Al 6061 alloy, respectively. Abrasive wear was the main wear mode for the weld metals by using multi-principal filler materials.

Microstructure and Mechanical Properties of TA2/Q235 Laser Weld Joint with Copper Interlayer.

For the dissimilar metal welding needs of TA2 titanium and Q235 steel, preliminary trials were conducted using laser welding methods, and the results showed that the addition of a copper interlayer and the bias of the laser beam toward the Q235 side allowed for an effective connection. The welding temperature field was simulated using the finite element method, and the optimum offset distance of 0.3 mm was obtained. Under the optimized parameters, the joint had good metallurgical bonding. Further SEM analysis showed that the microstructure of the bonding area between the weld bead and Q235 was a typical fusion weld pattern, while that of the bonding area between the weld bead and TA2 was in brazing mode. The microhardness of the cross-section showed complex fluctuations; the microhardness of the weld bead center was higher than that of the base metal due to the formation of a mixture microstructure of copper and dendritic Fe phases. The copper layer not involved in the weld pool mixing had almost the lowest microhardness. The highest microhardness was found at the bonding site of TA2 and the weld bead, mainly due to the formation of an intermetallic layer with a thickness of about 100 μm. Further detailed analysis revealed that the compounds included Ti2Cu, TiCu and TiCu2, showing a typical peritectic morphology. The tensile strength of the joint was approximately 317.6 MPa, reaching 82.71% of that of the Q235 and 75.44% of the TA2 base metal, respectively. The fracture occurred in the unmixed copper layer.

Structural integrity assessment of Inconel 617/P92 steel dissimilar welds for different groove geometry

The work is focused on examining the effect of the weld groove geometry on microstructure, mechanical behaviour, residual stresses and distortion of Alloy 617/P92 steel dissimilar metal weld (DMW) joints. Manual multi-pass tungsten inert gas welding with ERNiCrCoMo-1 filler was employed to fabricate the DMW for two different groove designs: Narrow V groove (NVG) and Double V groove (NVG). The microstructural examination suggested a heterogeneous microstructure evolution at the interface of the P92 steel and ERNiCrCoMo-1 weld, including the macrosegregation and element diffusion near the interface. The interface structure included the beach parallel to the fusion boundary at the P92 steel side, the peninsula connected to the fusion boundary and the island within the weld metal and partially melted zone along Alloy 617 fusion boundary. An uneven distribution of beach, peninsula and island structures along the fusion boundary of P92 steel was confirmed from optical and SEM images of interfaces. The major diffusion of the Fe from P92 steel to ERNiCrCoMo-1 weld and Cr, Co, Mo, and Ni from ERNiCrCoMo-1 weld to P92 steel were witnessed from SEM/EDS and EMPA map. The Mo-rich M6C and Cr-rich M23C6 phases were detected in inter-dendritic areas of the weld metal using the weld’s SEM/EDS, XRD and EPMA study, which formed due to the rejection of Mo from the core to inter-dendritic locations during solidification. The other phases detected in the ERNiCrCoMo-1 weld were Ni3(Al, Ti), Ti(C, N), Cr7C3 and Mo2C. A variation in the microstructure of weld metal from top to root and also along the transverse direction in terms of composition and dendritic structure and also due to the composition gradient between dendrite core and inter-dendritic areas, a significant variation in hardness of weld metal was observed from both top to root and also in the transverse direction. The peak hardness was measured in CGHAZ of P92 while the minimum was in ICHAZ of P92 steel. Tensile test studies of both NVG and DVG welds joint demonstrated that failure occurred at P92 steel in both, room-temperature and high-temperature tensile tests and ensured the welded joint’s applicability for advanced ultra-supercritical applications. However, the strength of the welded joint for both types of joints was measured as lower than the strength of the base metals. In Charpy impact testing of NVG and DVG welded joints, specimens failed in two parts with a small amount of plastic deformation and impact energy of 99 ± 4 J for the NVG welds joint and 91 ± 3 J for the DVG welded joint. The welded joint met the criteria for boiler applications in terms of impact energy (minimum 42 J as per European Standard EN ISO15614-1:2017 and 80 J as per fast breeder reactor application). In terms of microstructural and mechanical properties, both welded joints are acceptable. However, the DVG welded joint showed minimum distortion and residual stresses compared to the NVG welded joint.

A new friction processing method for welding of dissimilar metals

A friction processing method is proposed for joining of dissimilar metals which does not involve direct mechanical processing, significant deformation or thickness reduction of parent metals. A rotating tungsten carbide tool is plunged into a sacrificial top sheet to generate heat and pressure required for dissimilar metal welding. Constant plunge rate is applied till melting initiates at the interface. Interdiffusion of dissimilar metals at the interface, results in interface melting at (near eutectic) temperatures well below the melting of either of the base metals. Once the interface starts melting, plunging is stopped and tool is maintained at constant position for a prescribed dwell time. Rapid cooling, from self quenching, occurs in the weld zone once the tool is retracted post the dwell time. In this work, the proposed method is used for lap welding of copper and aluminium sheets. While low dwell times are seen to result in insufficient bonding, excessive dwell times are seen to result in higher amounts of brittle intermetallic phases that degrade the joint strength. High bond strength is achieved for 0.05 mm/s plunge rate and 10s dwell time. Cross-sectional scanning electron micrographs, fractographs, electron dispersive spectroscopy, X-ray diffraction, mechanical strength tests, and finite element analysis are used to investigate the physics of the process and to study the effect of process conditions on resulting microstructures, phase formations and mechanical properties of the joint. The diffusion layer at the interface is predominantly composed of lamella with alternating phases of Al and Al2Cu at homogeneous eutectic composition. Very thin Cu rich transition layers are formed towards Cu side. X-ray diffraction and SEM images of fracture surfaces on Al- and Cu-side, from peel tests, show that fracture occurs in the interface layer towards the Cu side. Finite element analysis (FEA), using a fully coupled thermo-mechanical model, is used to understand the temperature evolution in the weld zone for different plunge rates. The method holds great promise for joining dissimilar metal sheets of a wide range of thicknesses.

Effects of laser oscillation on metal mixing, microstructure, and mechanical property of Aluminum–Copper welds

Laser welding of dissimilar metals is important in many industrial applications. However, as dissimilar metals get mixed during the melting process, intermetallic compounds are often formed in the welds which can significantly undermine the electrical and mechanical properties of the welds. This poses a critical challenge to the widespread utilization of this welding technique. Compared with conventional line-scan laser welding, oscillating laser welding offers additional processing parameters to control the welding process. Although some work has been reported on oscillating laser welding of dissimilar metals, a mechanistic understanding of this process was still missing. The research objective of this work was to reveal the physical mechanisms and evaluate their relative significance to the fluid flow, metal mixing, and microstructure evolution in the molten pool in oscillating laser welding of dissimilar metals. A combination of experiments and simulations was leveraged to achieve the objective. Four fluid flows have been found to determine the metal mixing in the molten pool, and their dependences on the laser oscillating parameters were discussed. In addition, the thermo-solutal conditions of the molten pool solidification were quantified as functions of the laser oscillating parameters, and the effects of the thermo-solutal conditions on the final weld microstructures were analyzed.

Irradiation-assisted cracking of SA508-304L weldments with 308L groove filler and 309L butter in hot water immersion constant rate extension tests

Low alloy SA508 bainitic steel is often welded to 300 series austenitic stainless steel in nuclear power plants. These dissimilar metal weldments use 309L butter applied to the low alloy steel and 308L groove filler material to join the 309L to austenite. High alloy steels (such as the 308L and 309L material used here) have been identified as having greater susceptibility to irradiation-assisted stress corrosion cracking (IASCC). Evidence is presented here of cracking under tensile loads in 309L weldment butter in LWR hydrothermal conditions that correlates irradiation induced changes at δ ferrite−γ austenite phase boundaries and γ−γ grain boundaries; specifically intergranular Cr23C6 precipitates. Cracking is not observed in irradiated 308L groove filler under the same conditions. Both the 309L butter and 308L filler material have a duplex or mixed δ−γ phase microstructure due to solidification during the welding procedure. We have performed constant extension rate autoclave immersion tests in BWR normal water chemistry hot water (288 °C, 10 MPa, 2000 wt. ppb dissolved oxygen, <100 nS/cm conductivity, neutral pH) of proton irradiated (2 MeV, approximately 3 displacements per atom at a depth of 10 μm) tensile specimens. Proton irradiation to the dpa levels used here induced Cr23C6 carbide precipitation at γ−γ grain boundaries and δ−γ phase boundaries in the 309L butter but not the 308L filler. Cracking was observed in the proton-irradiated volume of 309L at these interfaces in the vicinity of the Cr23C6 precipitates. We hypothesis the SA508 is the source of carbon in the 309L butter. Cr depletion at γ−γ grain boundaries was also observed and may also be a root cause of cracking. The 309L butter is under significant tensile residual stress in the weldment that adds to applied tensile loads and this too may be a contributing factor.

Effect of Alloying Elements on Intermetallic Formation during Friction Stir Welding of Dissimilar Metals: A Critical Review on Aluminum/Steel

The main drawback of friction stir welding (FSW) dissimilar metals is the formation of intermetallic compounds (IMCs), which are brittle and affect the strength of the joint. The formation of these compounds is inevitable due to their low enthalpy of formation; however, their emergence is an indication of metallurgical bonding between dissimilar metals. This means that the determining factors of intermetallics should be optimal to ensure the formation of the joint and, at the same time, the performance of the joint. It is known that various parameters such as welding parameters, joint configuration, and tool geometry have an influence on the formation of these compounds. However, the influence of the base metal is not adequately addressed in the literature. The current review paper focuses on intermetallic formation during the friction stir welding of aluminum/steel (Al/St) alloys to explore how the types of alloys affect the thicknesses and morphologies of the intermetallics. Different structural steels and stainless steels were considered to see how they affect intermetallic formation when welded to different types of aluminum alloys. The thicknesses of the IMCs in the FSW of different aluminum/steel alloys were taken from the literature and averaged to provide insight into the contribution of the elements to IMC formation. Thermodynamic and kinetic analyses were used to explain this effect. Finally, the mechanism of intermetallic formation is explained to provide a useful guide for selecting dissimilar metals for welding using friction stir welding.

Microstructure and creep properties of graded transition joints between nickel-based alloys and martensitic heat-resistant steels

Interfacial failure can significantly reduce creep lifetime of dissimilar metal welds (DMWs), bringing great economic losses and safety issues to power plants. The fundamental reason for interfacial failure is steep gradient of chemical compositions between different metals. Many researchers have made great efforts to develop graded transition joints (GTJs) aiming to eliminate the steep gradient, but the results of creep tests were rarely seen. The goal of the present work was to determine whether GTJs could effectively prolong creep life compared with conventional DMWs, and further to clarify the reason. In our study, a GTJ between creep-resistant steels and nickel-based alloys was fabricated by a dual-wire TIG method. It was shown that the gradients of chemical compositions and hardness were significantly reduced over a longer distance of ∼17 mm than that of ∼100 μm in conventional DMWs, and microstructures varied from full martensite to mixed martensite + austenite, and finally to full austenite. The formation of dual-phase region composed of martensite and austenite was due to the segregation behavior of Cr, Ni, and Mo to inter-dendritic regions during solidification. Creep tests showed that creep life of the GTJ was much shorter than conventional DMWs, and two different failure modes were found, one of which failed in COST E HAZ (creep life 790 h), and the other in the nickel-rich martensite zone of transition region (creep life 435 h). The failure in COST E HAZ was the combined results of creep, plastic deformation and oxidation, and the failure in nickel-rich martensite zone was due to localized austenization induced shrinkage during creep tests. Nickel-rich martensite was not observed after post weld heat treatment (PWHT) due to the sluggish kinetics of nickel atoms diffusion, while creep tests provided enough time for nickel atoms to diffuse into austenite, which transformed into martensite with nickel enriched after creep tests. Based on the results, it is concluded that the GTJ might not be a feasible solution to extend creep life of DMWs at least in the current situation, due to the inevitable occurrence of nickel-rich martensite.

Role of Surface-Active Element Sulfur on Thermal Behavior, Driving Forces, Fluid Flow and Solute Dilution in Laser Linear Welding of Dissimilar Metals

Understanding heat and mass transfer and fluid flow in the molten pool is very helpful in the selection and optimization of processing parameters, and the surface-active element has an important effect on the heat and mass transfer in laser welding of dissimilar metals. A three-dimensional (3D) numerical model coupled with a sub-model of surface tension, which considers the influence of local temperature and the concentration of surface-active element sulfur at the gas/liquid surface, is used to analyze the thermal behavior, driving forces, fluid flow, and solute dilution during laser linear welding of 304SS and Ni. The relationship between surface tension, driving forces, and the temperature coefficient of surface tension with the spatial distribution of temperature and the surface-active element sulfur is quantitatively analyzed. The simulation results show that the molten pool is fully developed at 45 ms, and the collision of inward and outward convection, with the maximum velocity reaching 1.7 m/s, occurs at the isotherm with a temperature between 2200 K and 2500 K. The temperature-gradient term and concentration-gradient term of surface shear stress play different roles in different positions of the free surface. The local sulfur concentration changes the temperature sensitivity of the surface tension at different sides of the free surface and further determines the transition of convection. Complex fluid flow promotes solute dilution, and the distribution of solute becomes uniform from the front to the rear of the molten pool. The Ni element is transferred to 304SS mainly at the rear side. The work provides theoretical support for the control of joint quality by changing the content of surface-active elements in dissimilar welding.

Studies on Metallic Glasses in Utsunomiya University

Studies on dissimilar metal welding and mechanical fastening using metallic glasses in Utsunomiya University have been introduced. Metallic glass ribbons placed between the two dissimilar metal plates become supercooled liquid with low viscosity by resistance welding and promote supercooling at the nuggets. As a consequence, the residual stress at the welding boundary and the formation of brittle intermetallic compounds are suppressed, resulting in excellent dissimilar resistance welding with high joint strength and high reproducibility. Mechanical fastening of two metal plates is also demonstrated by resistance heating and viscous flow forming of metallic glass rivets. Producing metallic glass rivets using bulk metallic glasses by a resistance heating is now under examination.