The metallurgy,mechanics, modelling and assessment of dissimilar material brazed joints

At the heart of any procedure for modelling and assessing the design or failure of dissimilar material brazed joints there must be a basic understanding of the metallurgy and mechanics of the joint. This paper is about developing this understanding and addressing the issues faced with modelling and predicting failure in real dissimilar material brazed joints and the challenges still to be overcome in many cases. An understanding of the key metallurgical features of such joints in relation to finite element modelling is presented in addition to a study of the mechanics and stress state at an abrupt interface between two materials. A discussion is also presented on why elastic singularities do not exist based on a consideration of the assumption of an abrupt change in material properties and plasticity in the vicinity of the joint. In terms of modelling real dissimilar material brazed joints; there are several barriers to accurately capturing the stress state in the region of the joint and across the brazed layer and these are discussed in relation to a metallurgical study of a real dissimilar material brazed joint. However, this does not preclude using a simplified modelling approach with a representative braze layer in design and failure assessment away from the interface. In addition modelling strategies and techniques for assessing the various failure mechanisms of dissimilar material brazed joints are discussed. The findings from this paper are applicable to dissimilar material brazed joints found in a range of applications; however the references listed are primarily focussed on work in fusion research and development.     

Corrosion of Ti-STS dissimilar joints brazed by a Ag interlayer and Ag-Cu-(Pd) alloy fillers

Corrosion behavior of dissimilar brazed joints between titanium Gr. 2 (Ti) and S31254 stainless steel (STS) was investigated. For the study, a Ag interlayer and two Ag-base eutectic alloys, 72Ag-28Cu and 66.2Ag-25.8Cu-8Pd (wt.%), were introduced as a diffusion control layer and fillers, respectively, between the base materials. The joints commonly had a layered structure of Ti(base)/TiAg/Ag solid solution/STS(base), but the one brazed by the Ag-Cu-Pd filler was slightly alloyed with the noble Pd elements over the Ag-rich solid solution region. A series of corrosion test experiments in a sea water revealed that a corrosion of TiAg layer and a stress-induced cracking at the TiAg/Ag solid solution interface were dominant due to a galvanic attack, but notably the Ti-STS dissimilar joint’s resistance to corrosion was significantly improved by alloying the Pd in the joint. The corrosion behavior of such dissimilar metal joints was discussed based on galvanic corrosion effect.     

Pre-qualification of brazed plasma facing components of divertor target elements for ITER like tokamak application

Qualification of tungsten (W) and graphite (C) based brazed plasma facing components (PFCs) is an important R&D area in fusion research. Pre-qualification tests for brazed joints between W–CuCrZr and C–CuCrZr using NDT (IR thermography and ultrasonic test) and thermal fatigue test are attempted. Mockups having good quality brazed joints of W and C based PFCs were identified using NDT. Subsequently, thermal fatigue test was performed on the identified mockups. All brazed tiles of W based PFC mockups could withstand thermal fatigue test, however, few tiles of C based PFC mockup were found detached. Thermal analyses of mockups are performed using finite element analysis (ANSYS) software to simulate the thermal hydraulic condition with 10 MW/m² uniform heat flux. Details about experimental and computational work are presented here.     

核级304L不锈钢钎焊接头组织及耐腐蚀性能研究

核级304L不锈钢与BNi 7钎料真空钎焊接头存在晶间腐蚀行为,但工艺与钎缝耐腐蚀性能的关系尚未得到充分研究。为充分评估压水堆燃料组件结构件中不锈钢真空钎焊接头的晶间腐蚀和应力腐蚀敏感性,降低腐蚀失效风险,采用定量金相方法分析了钎缝中的化合物相含量,采用硫酸 硫酸铁法和双环动电位再活化（DL EPR）法评价了钎缝耐晶间腐蚀性能,并采用高温高压水应力腐蚀裂纹扩展试验评价了钎缝的耐应力腐蚀性能。结果表明,钎缝中化合物相含量越高,耐晶间腐蚀性能越好。且钎缝在高温高压水中存在明显的应力腐蚀开裂行为,但其与钎焊工艺的关系尚需进一步试验研究。     

Technological parameters of fine tube-sheet joints brazed by scanning electron beam heat

The present paper proposes the vacuum scanning EB brazing (SEBB) technology. Investigation on SEBB processing features of stainless steel tube-sheet structure was performed, the effect of SEBB technological factors (EB current, heating time, scanning area, EB current rising slope, filler metal mass preset, joint fit-up gap) on the brazed joints quality was analyzed, and the resulting optimal parameters array is presented in the paper.     

Fatigue strength of dissimilar welded joints for the main vessel of an LMFBR

The employment of welded joints composed of dissimilar metals is one simple and inexpensive way to connect a main vessel made of austenitic stainless steel and a roof slab constructed of ferritic steel in the design of liquid metal fast reactors. Since dissimilar-metal welded joints have not been used for such large structures so far in Japan, the structural integrity of this type of joint should be carefully examined for such a design option to be selected. Here various kinds of tests were conducted for eleven types of welded joints of 50 mm thickness to obtain this fundamental strength characteristics. Type 304 stainless steel was used as one of the parent metals in all the joints. They differ from each other in regard to the type of ferritic steel, welding metal and welding procedure. Low-cycle fatigue tests were conducted for round-bar specimens made from these welded joints at room temperature. Fatigue crack-propagation tests were also conducted for some of the joints. Tests after manufacturing a large-scale shell model were also conducted. The results of these tests demonstrated that the present manufacturing technique can, produce welded joints of high quality and reliability. A trial calculation for actual design conditions showed the existence of large margins against fatigue failure or fatigue crack-propagation of a significant amount.     

Predicting welding residual stresses in a dissimilar metal girth welded pipe using 3D finite element model with a simplified heat source

Dissimilar metal welds are commonly used in nuclear power plants to connect low alloy steel components and austenitic stainless steel piping systems. The integrity assessment and life estimation for such welded structures require consideration of residual stresses induced by manufacturing processes. Because the fabrication process of dissimilar metal weld joints is considerably complex, it is very difficult to accurately predict residual stresses. In this study, both numerical simulation technology and experimental method were used to investigate welding residual stress distribution in a dissimilar metal pipe joint with a medium diameter, which were performed by a multi-pass welding process. Firstly, an experimental mock-up was fabricated to measure the residual stress distributions on the inside and the outside surfaces. Then, a time-effective 3-D finite element model was developed to simulate welding residual stresses through using a simplified moving heat source. The simplified heat source method could complete the thermo-mechanical analysis in an acceptable time, and the simulation results generally matched the measured data near the weld zone. Through comparing the simulation results and the experimental measurements, we can infer that besides the multi-pass welding process other key manufacturing processes such as cladding, buttering and heat treatment should also be taken into account to accurately predict residual stresses in the whole range of the dissimilar metal pipe.     

Vacuum joints of dispersion-strengthened copper and its applications in synchrotron radiation front end

Masks are critical elements of synchrotron radiation front end that are exposed to high temperature and stress.The absorber material is typically comprised of dispersion-strengthened copper,which can retain high performance at elevated temperature.Joining processes under vacuum,including brazing and electron beam welding,are novel approaches for prolonging the absorber and for reducing power densities.The mechanical properties of brazed joints and electron beam welded joints of dispersion-strengthened copper workpieces are evaluated by tensile testing at 20,100,and 200 C.The testing results indicate that the tensile strength and elongation of both vacuum joints decrease with increasing temperature.Compared to brazed joints,electron beam welded joints have higher tensile strength,better ductility,and more stable performance.A novel welded mask with a total length of 600 mm is presented and shown to be practical for use in the highest heat load front end in the Shanghai synchrotron radiation facility phase-Ⅱ beamline project.     

Tensile tests and metallography of brazed AISI 316L specimens after irradiation

Stainless steel type 316L tensile specimens were vacuum brazed with three kinds of alloys: BNi-5, BNi-6, and BNi-7. The specimens were irradiated up to 0.7 dpa at 353 K in the High Flux Reactor at JRC Petten, the Netherlands. Tensile tests were performed at a constant displacement rate of 10⁻³ s⁻¹ at room temperature in the ECN hot cell facility. BNi-5 brazed specimens showed ductile behaviour. Necking and fractures were localized in the plate material. BNi-6 and BNi-7 brazed specimens failed brittle in the brazed zone. This was preceded by uniform deformation of the plate material. Tensile test results of irradiated specimens showed higher stresses due to radiation hardening and a reduction of the elongation of the plate material compared to the reference. SEM examination of the irradiated BNi-6 and BNi-7 fracture surfaces showed nonmetallic phases. These phases were not found in the reference specimens.     

Effect of geometric conditions on residual stress of brazed stainless steel plate-fin structure

Plate-fin heat exchanger (PFHE) is the delegate of the miniaturization of heat exchanging equipment. As the core of PFHE, plate-fin structure is fabricated by vacuum brazing. Owing to the material mismatching between filler metal and base metal, residual stresses can arise during the brazing process and decrease the structure strength greatly. Therefore, estimating the magnitude and distribution of residual stress and discussing its influence factors for the early design and fabrication of plate-fin structure are deemed necessary. This paper presented a finite element analysis (FEA) of brazed residual stresses for a counter-flow stainless steel plate-fin structure. The effects of geometric conditions including brazing gap, plate thickness, fin thickness, fin pitch, fin height and fin layers on residual stress were investigated. This work provides a reference for design and decreasing residual stress for stainless steel plate-fin structure.     

Silicon doped carbon/Cu joints based on amorphous alloy brazing for first wall application

Amorphous ribbon-type filler-metals represent a promising selection for joining heterogeneous materials together. In this work, rapidly solidified ribbon-type Ti based amorphous filler with a melting temperature of 850 °C and a thickness up to 20 μm is used to join silicon doped carbon to pure copper. SEM examinations demonstrate that a high quality brazed joints could be acquired. The brazed seam has a uniform structure and pore free along its entire length. TiC and ZrC are formed near the interface of carbon and filler-metal when the brazing holds enough time. Using very thin Mo and Cu foil (0.2 mm in thickness) as multiple interlayer are very effective to mitigate the thermal stress occurred in the interface between carbon and copper. The shear strength of this carbon-multiple interlayer-copper joint is more than 20 MPa, and the rupture is mainly occurred on the carbon side.     

Thermomechanical behavior of actively cooled, brazed divertor components under cyclic high heat flux loads

Actively cooled divertor mock-ups consisting of various low-Z armor tiles brazed to refractory metal heat sinks were tested in the electron beam test facility at Jülich. Screening and thermal cycling tests were perfomed on the mock-ups to estimate the overall thermal performance under cyclic high heat flux (HHF) loadings. By detecting the temperature of the armor surface and the braze layer, it was possible to assess the heat removal capability and the accumulation of interfacial damage. Microstructures were investigated to elucidate the degradation of the joints. Finite element analyses are carried out for the simulated HHF test conditions. Temperature fields and thermal stresses are calculated for a typical divertor module. The nature of thermomechanical behavior of the divertor mock-ups under cyclic HHF loadings is discussed.     

Thermal stress analysis for fatigue damage evaluation at a mixing tee

Fatigue cracks have been found at mixing tees where fluids of different temperature flow in. In this study, the thermal stress at a mixing tee was calculated by the finite element method using temperature transients obtained by a fluid dynamics simulation. The simulation target was an experiment for a mixing tee, in which cold water flowed into the main pipe from a branch pipe. The cold water flowed along the main pipe wall and caused a cold spot, at which the membrane stress was relatively large. Based on the evaluated thermal stress, the magnitude of the fatigue damage was assessed according to the linear damage accumulation rule and the rain-flow procedure. Precise distributions of the thermal stress and fatigue damage could be identified. Relatively large axial stress occurred downstream from the branch pipe due to the cold spot. The variation ranges of thermal stress and fatigue damage became large near the position 20° from the symmetry line in the circumferential direction. The position of the cold spot changed slowly in the circumferential direction, and this was the main cause of the fatigue damage. The fatigue damage was investigated for various differences in the temperature between the main and branch pipes. Since the magnitude of accumulated damage increased abruptly when the temperature difference exceeded the value corresponding to the fatigue limit, it was suggested that the stress amplitude should be suppressed less than the fatigue limit. In the thermal stress analysis for fatigue damage assessment, it was found that the detailed three-dimensional structural analysis was not required. Namely, for the current case, a one-dimensional simplified analysis could be used for evaluating the fatigue damage without adopting the stress enhancement factor K_t quoted in the JSME guideline. The results also suggested that, for a precise assessment of the fatigue damage at a mixing tee, the effect of multi-axial stress on the fatigue life together with the mean stress effect should be taken into account.     

Fabrication and characterization of tungsten and graphite based PFC for divertor target elements of ITER like tokamak application

The development of the fabrication technology of macro-brush configuration of tungsten (W) and carbon (graphite and CFC) plasma facing components (PFCs) for ITER like tokamak application is presented. The fabrication of qualified joint of PFC is a requirement for fusion tokamak. Vacuum brazing method has been employed for joining of W/CuCrZr and C/CuCrZr. Oxygen free high conductivity (OFHC) copper casting on W tiles was performed followed by machining, polishing and ultrasonic cleaning of the samples prior to vacuum brazing. The W/CuCrZr and graphite/CuCrZr based test mockups were vacuum brazed using silver free alloys. The mechanical shear and tensile strengths were evaluated for the W/CuCrZr and graphite/CuCrZr brazed joint samples. The micro-structural examination of the joints showed smooth interface. The details of fabrication and characterization procedure for macro-brush tungsten and carbon based PFC test mockups are presented.     

A comparison between Japanese and French A16 defect assessment procedures for thermal fatigue crack growth

This paper presents the results of a benchmark on thermal fatigue crack growth evaluation for a thick-wall cylinder subjected to cyclic thermal transients. The simplified crack growth evaluation methods of both JNC in JAPAN and A16 procedures proposed by CEA in France are presented. The predictions obtained using both methods are compared with the experimental data. The JNC method, which accounts for the non-linear stress component provides predictions of crack advance in a good agreement with the experimental data. In contrast, significant differences are observed between the A16 predictions and the experimental data. The discrepancies are mainly due to the non-linear stress component which is not accounted for in the A16 method. When using the JNC stress intensity factor solution determined by finite element analysis to account for the non-linear stress component, the A16 method well predicts the thermal fatigue crack growth behavior.     

Behavior of Brazed W/Cu Mockup Under High Heat Flux Loads

In order to transfer the heat from the armor to the coolant, tungsten has to be connected with a copper heat sink. The joint technology is the most critical issue for manufacturing plasma facing components. Consequently, the reliability of the joints should be verified by a great number of high-heat-flux （HHF） tests to simulate the real load conditions. W/Cu brazed joint technology with sliver free filler metal CuMnNi has been developed at Southwestern Institute of Physics （SWIP）. Screening and thermal fatigue tests of one small-scale fiat tile W/CuCrZr mockup were performed on a 60 kW electron-beam Material testing scenario （EMS-60） constructed recently at SWIP. The module successfully survived screening test with the absorbed power density （Pabs） of 2 MW/m2 to 10 MW/m2 and the following 1000 cycles at Pabs of 7.2 MW/m2 without hot spots and overheating zones during the whole test campaign. Metallurgy and SEM observations did not find any cracks at both sides and the interface, indicating a good bonding of W and CuCrZr alloy. In addition, finite element simulations by ANSYS 12.0 under experimental load conditions were performed and compared with experimental results.     

Environmentally-assisted cracking behaviour in the transition region of an Alloy182/SA 508 Cl.2 dissimilar metal weld joint in simulated boiling water reactor normal water chemistry environment

The stress corrosion cracking (SCC) and corrosion fatigue behaviour perpendicular and parallel to the fusion line in the transition region between the Alloy 182 Nickel-base weld metal and the adjacent SA 508 Cl.2 low-alloy reactor pressure vessel (RPV) steel of a simulated dissimilar metal weld joint was investigated under boiling water reactor normal water chemistry conditions. A special emphasis was placed to the question whether a fast growing interdendritic SCC crack in the highly susceptible Alloy 182 weld metal can easily cross the fusion line and significantly propagate into the adjacent low-alloy RPV steel. Cessation of interdendritic SCC crack growth was observed in high-purity or sulphate-containing oxygenated water under constant or periodical partial unloading conditions for those parts of the crack front, which reached the fusion line. In chloride containing water, on the other hand, the interdendritic SCC crack in the Alloy 182 weld metal very easily crossed the fusion line and further propagated with a very high rate as a transgranular crack into the heat-affected zone and base metal of the adjacent low-alloy steel. The observed SCC cracking behaviour at the interface correlates excellently with the field experience of such dissimilar metal weld joints, where SCC cracking was usually confined to the Alloy 182 weld metal.     

Corrosion and microstructural aspects of dissimilar joints of titanium and type 304L stainless steel

To link titanium and zirconium metal based (Ti, Zr-2, Ti-5%Ta, Ti-5%Ta-1.8Nb) dissolver vessels containing highly radioactive and concentrated corrosive nitric acid solution to other nuclear fuel reprocessing plant components made of AISI type 304L stainless steel (SS), high integrity and corrosion resistant dissimilar joints between them are necessary. Fusion welding processes produce secondary precipitates which dissolve in nitric acid, and hence solid-state processes are proposed. In this work, various dissimilar joining processes available for producing titanium-304L SS joints with adequate strength, ductility and corrosion resistance for this critical application are highlighted. Developmental efforts made at IGCAR, Kalpakkam are outlined. The possible methods and the microstructural-metallurgical properties of the joints along with corrosion results obtained with three phase (liquid, vapour, condensate) corrosion testing are discussed. Based on the results, dissimilar joint produced by the explosive joining process was adopted for plant application.     

Probability-based assessment and maintenance of welded joints damaged by fatigue

This paper presents a probabilistic reliability assessment procedure for steel components damaged by fatigue. The study combines the structural reliability theory with a maintenance strategy. The fatigue assessment model is based on a modelisation of the fatigue phenomenon issued from the principles of fracture mechanics theory. The safety margin includes the crack growth propagation and allows to treat fatigue damage in a general manner. Damaging cycles and non damaging cycles are distinguished. The sensitivity study of the different parameters shows that some variables can be taken as deterministic. Applications are made on a welded joint ‘stiffener/bottom-plate' of a typical steel bridge. The model is then used for taking into account inspection results. Non destructive inspection (NDI) techniques are also used for updating failure probabilities. The results show their ability to be inserted in a maintenance strategy for optimizing the next inspection time. This has led to define cost functions related to the total maintenance cost; this cost is then minimized for determining the optimal next inspection time. An example of welded joint cracked by fatigue highlights the different concepts. The approach presented in the paper is not only restrained to fatigue problems, but can be applied to a wide variety of degrading phenomena.     

反应堆压力容器内部大尺寸环形异种金属焊缝残余应力分布研究

获得反应堆压力容器内部大尺寸环形异种金属焊缝残余应力分布可为反应堆压力容器结构设计和制造工艺优化提供指导,通过设计和制造能够代表产品焊接结构形式的镍基合金和低合金钢异种金属焊接结构模拟件,采用轮廓法测试焊接结构模拟件内部纵向残余应力,采用有限元法模拟计算焊接结构模拟件横向和纵向残余应力,获得了整个异种金属焊接接头残余应力分布特征。结果表明:焊缝区域内部纵向残余应力为拉伸应力,峰值应力达到500 MPa左右,并且表层应力大于内部应力,峰值应力出现在距下表面3 mm和24 mm位置;横向残余应力在焊缝区域从上表面到下表面的分布为拉应力-压应力-拉应力,压缩横向残余应力峰值达到?300 MPa,出现在距下表面约18 mm位置。本文研究可为焊接结构设计提供理论指导。