Welding and Joining of NiTi Shape Memory Alloys: A Review

NiTi is an increasingly applied material in industrial applications. However, the difficulties faced when welding and joining is required, limits its broader use in the production of complex shaped components. The main weldability problems associated with NiTi are: strength reduction, formation of intermetallic compounds, modification of phase transformation and transformation temperatures, as well as, changes in both superelastic and shape memory effects. Additionally, NiTi is envisaged to be joined to other materials, in dissimilar joints with more complex problems depending on the other base material. Thus, intensive research in welding and its effects on the joints performance has been conducted since the early stages of NiTi. This paper presents a detailed review of welding and joining processes applied to NiTi, in similar and dissimilar combinations considering both fusion and solid-state processes. Since laser is the most studied and applied welding process, a special section is devoted to this technique.     

燃料气换热器管束鼓包开裂分析

某燃料气换热器在运行过程中发现其换热管束出现多处鼓包开裂现象,为查明鼓包开裂原因,对换热管束进行了宏观和微观检验、化学成分分析、金相检验和扫描电镜分析。结果表明:换热管内存在压力波动、换热管壁厚不均匀、壳程介质H2S含量较高以及换热管材料中存在,夹杂物都是导致其鼓包开裂的原因;换热管壁厚较薄处容易形成应力集中,使该处硫化氢腐蚀严重,壁厚进一步减薄,当壁厚减薄达到一定值时,在管内压力波动作用下形成鼓包,随着腐蚀的加剧和压力波动的继续作用,最终导致换热管于鼓包处产生裂纹,进而发生泄漏。     

Failure Investigation of Boiler Water Wall Tubes of a Thermal Power Station

Failure investigation was carried out on boiler water wall tubes of a thermal power plant through visual inspection, chemical analysis, and metallurgical analysis. Failure was in the form of thin/micro cracks along the length of the tubes which were located at the girth welding joint of tubes. Experimental results revealed that the cracking was from inward to outward of the tube thickness. Discontinuities/cavities were observed in the welded region which might have occurred due to lack of fusion of base metal and the weld metal. Cracks were initiated from the sharp corner/crack tip of the cavities/discontinuities present at the welded region under the action of hoop/thermal stress existed during the operation. Nature of the crack propagation indicates the case of typical hydrogen-induced cracking. Moreover, the presence of the cavities/discontinuities reduced the cross-sectional area of tubes resulting increased stress intensity. Increased stress beyond the flow stress of the material assisted by hydrogen-induced effect resulted the cracking of the tubes. In order to mitigate the problem, proper welding of tubes joints should be carried out followed by proper inspection after weld. Secondly, hydrogen dissolution during welding should be prevented and treatment for its removal after welding should be carried out.     

Ultrasonic Torsion Welding of Sheet Metals to Cellular Metallic Materials

One of the most important requirements for finding new applications for cellular metals is to integrate them in complex technical structures. The metal foams have to be joined to each other, or to sheet materials, by suitable joining techniques. The main topics of this paper are the ultrasonic torsion welding of cellular metallic materials to sheet metals and the investigation of the mechanical properties of the joints. The basic materials of foams and sheet metals were different aluminum and iron alloys. Depending on the materials used, weldings with tensile shear strengths of up to 25 MPa were realized. Using aluminum foam sandwich (AFS) and sheet metals, successful weldings were performed before and after the foaming process. Furthermore, it was possible to perform a successful foaming process with the unfoamed AFS/sheet metal joints. Microscopic investigations showed that the ultrasonic welding technique allows the joining of the metal foams with sheet metals without significant deformation of the joining partners. The temperatures during the welding process in the interface were below the melting point of the foams and the sheet metals.     

Recent developments in explosive welding

Explosion welding (EXW) is one of the joining methods consisting of a solid state welding process in which controlled explosive detonation on the surface of a metal. During the collision, a high velocity jet is produced to remove away the impurities on the metal surfaces. Flyer plate collides with base plate resulting in a bonding at the interface of metals. The metal plates are joined at an internal point under the influence of a very high pressure and causes considerable local plastic deformation at the interface in which metallurgical bonding occurs in nature and even stronger than the parent metals. Similar and dissimilar materials can be joined by explosive welding. In this paper, after detection the theories of welding and wave formation, experimental research and numerical studies on explosive welding are reviewed for the last four decades. Also, future developments in explosive welding are predicted and criticized in an outlook.     

Prediction and optimization of friction welding parameters for super duplex stainless steel (UNS S32760) joints

Friction welding finds widespread industrial use as a mass production process for joining materials. Friction welding process allows welding of several materials that are extremely difficult to fusion weld. Friction welding process parameters play a significant role in making good quality joints. To produce a good quality joint it is important to set up proper welding process parameters. This can be done by employing optimization techniques. This paper presents a multi objective optimization method for optimizing the process parameters during friction welding process. The proposed method combines the response surface methodology (RSM) with an intelligent optimization algorithm, i.e. genetic algorithm (GA). Corrosion resistance and impact strength of friction welded super duplex stainless steel (SDSS) (UNS S32760) joints were investigated considering three process parameters: friction force (F), upset force (U) and burn off length (B). Mathematical models were developed and the responses were adequately predicted. Direct and interaction effects of process parameters on responses were studied by plotting graphs. Burn off length has high significance on corrosion current followed by upset force and friction force. In the case of impact strength, friction force has high significance followed by upset force and burn off length. Multi objective optimization for maximizing the impact strength and minimizing the corrosion current (maximizing corrosion resistance) was carried out using GA with the RSM model. The optimization procedure resulted in the creation of nondominated optimal points which can aid the process operator to fix the input control variables. The selection of a point from the Pareto front will always be a trade-off between the corrosion resistance and impact strength of the weld depending on the application.     

Butt-welding technology for double walled Polyethylene pipe

In this study, mechanical analyses of a butt welding technology for joining Polyethylene pipe are presented. The pipe had unique structure with double wall, and its section topology was not flat. For an effective repair of leakage and replacements of the pipe, the butt welding technology was developed and tested. For the material characterizations, thermodynamic analyses such as thermal gravimetric analysis and differential scanning calorimetry were performed. Based on the test results, the process temperature and time were determined to ensure safe joining of the pipes using a hot plate apparatus. The welding process was carefully monitored by measuring the temperature. Then, the joined pipes were tested by various methods to evaluate the quality. The analyses results showed the detail process mechanism during the joining process, and the test results demonstrated the successful application of the technology to the sewage pipe repairs.     

Failure of Heat-Exchanger Tubes due to Localized Scaling and Microstructural Transformation

Failure of tubes in a bayonet-type high-pressure heat exchanger was caused by water interruption. The failures were accompanied by excessive plastic deformation, and cracks were found in the thick inner scale on the failed tubes. In the fractured regions, the tube inner surfaces and microstructures showed excessively thick scales and carbide networks, while the locations opposite fractures were scale free. These observations suggest the occurrence of localized heating that subsequently caused bulging and failures. Further inspection of the tube bundles also showed augmented scale thickness in sound tubes. It was recommended that the entire tube bundle be replaced.     

Process boundaries of collision welding at low energies

Collison welding is a promising material‐closed joining process that enables bonds with various advantages. It is already used as explosion welding to produce clad materials that cannot be joined otherwise. Other collision welding processes as electromagnetic pulse welding do not contain that amount of energy, but they can be used in mass production. In order to achieve a high process and product reliability, the process has to be designed accurately. But the process boundaries are not yet completely understood. In this paper, process windows for aluminium and copper joints, produced by a model test rig, are compared. Additionally, high speed observation and micro sections are used to enhance the knowledge about process boundaries and the influence of the jet.     

Investigation on a new process chain of deposition or friction welding and subsequent hot forging

According to the state of the art most current forging parts and technical components are made of mono‐materials. Nevertheless, parts consisting of only one material increasingly reach their specific material and constructive limits in the established production processes. Through use of previously joined raw parts consisting of different materials, it is possible to produce application‐optimized hybrid parts. This paper describes the production chain of hybrid parts produced by combining two different joining processes with subsequent hot compression tests. The joining of various materials is realized by a deposition welding with a laser‐stabilized gas‐metal‐arc deposition welding (LGD) process and a conventional friction welding process. Subsequently, the hybrid samples are compressed under varying forming parameters such as temperature and deformation degrees. In order to characterize the joining zone, metallurgical investigations are carried out.     

Experimental assessment of thermal grain boundary embrittlement after tubeplate failure in a petrochemical heat exchanger

The analysis of multiple cracks in a heat exchanger tubeplate at a petrochemical plant led to experimental replication of in-service damage. High lifting stresses created a first leak that was repaired. Extended branched intergranular cracking developed from the tubeplate surface around the repair weld after few weeks of renewed operation. Chemical analyses failed to detect Na on crack surfaces, while operative conditions also allowed discarding a typical SCC cracking mechanism. Lack of radial interference between tubes and plate, lack of penetration and lack of fusion at tube to tubeplate weld roots were also thought as contributors to cracking. Microstructural analyses revealed martensite clusters in welds and HAZ, and austenite grain boundary precipitates in the tubeplate base material. Thermal cycles were applied to ex-service samples to replicate the conditions for these brittle micro constituents, which were found to be unstable at operating temperatures. Mechanical testing also replicated grain boundary weakness. This inadequate structure was related to welding without proper thermal cycles and heat treatments.     

Rubber tubes in the sea

A long tube with elastic walls containing water is immersed in the sea aligned in the direction of wave travel. The waves generate bulges that propagate at a speed determined by the distensibility of the tube. If the bulge speed is close to the phase velocity of the waves, there is a resonant transfer of energy from the sea wave to the bulge. At the end of the tube, useful energy can be extracted. This paper sets out the theory of bulge tubes in the sea, and describes some experiments on the model scale and practical problems. The potential of a full-scale device is assessed.     

Ermittlung von Verfahrensgrenzen für das Fügen durch Knickbauchen anhand des Werkstoffes E235+N

Through a systematic approach and the consistent comparison of the results between experimental and numerical investigations, a deep understanding of the bulging mechanisms has been first developed. From these investigations process limitations were derived and presented in the form of a working diagram for the material E235+N. With regard to a wide industrial use of upset bulging as a joining technology, a technologically, productively as well as economically appropriate method could be developed and validated by means of practical and numerical experiments.     

A study on localized expansion defects in tube hydroforming

This study investigated localized expansion in the tube hydroforming process, which may induce cracks in the tube material. First, a model of a trailing arm in the front subframe was built using the finite-element method; it was compared with the actual component to verify the accuracy of the model. Subsequently, a localized expansion model was constructed to analyze the effects of the tube geometry parameters and material-hardening parameters on localized expansion and the fracture location. The simulation results indicated that localized expansion can be ameliorated by changing the material distribution of the tube using a preform or an inner die; these methods substantially reduced the maximum material thinning. However, these methods can only be applied to the local bulging area that occurs when both sides of the tube are constrained. When both sides are not constrained, the material bulges back to the original state, and localized expansion occurs. In this study, both sides of the localized expansion area of the trailing arm in the front subframe were constrained, and the material distribution of the fracture zone could be changed by moving the mandrel to improve the localized expansion-induced fracture characteristics. Experimental results successfully validated the proposed method and analysis.     

低碳钢薄壁焊管液压胀形行为

为了研究焊管液压胀形过程的变形行为,在管材胀形性能测试系统上进行了不同长径比条件下低碳钢(STKM11A)薄壁焊管的胀形实验,获得了焊管的壁厚分布规律、胀形区轮廓形状、极限膨胀率和应变分布规律。结果表明:管材焊缝区的减薄率仅为2.4%~5.5%,等效应变仅为0.05~0.10,变形程度相对母材区较小,主要发生几何位置移动。环向壁厚的最薄点位于以焊缝为中心的对称两侧士30°位置处。随着胀形区长度增大,管材破裂压力、减薄量、极限膨胀率均发生减小,胀形区轮廓逐渐偏离椭圆形,当长径比达到2.0时,已不再适合用椭圆函数描述。此外,胀形区长度增大过程中,管材从双拉向平面应变状态发生转变,在此基础上建立了焊管的成形极限图。     

Joining of Si-Ti-C-O fibre-assembled ceramic composites with 72Ag-26Cu-2Ti filler metal

Si-Ti-C-O fibre-assembled ceramic composites were joined with 72Ag-26Cu-2Ti filler metal at 1123 K and 1223 K in vacuum. The composites consisted of Si-Ti-C-O fibres, which were assembled unidirectionally, and oxide material filling the spaces between the fibres. During the joining process, frothing occurred at the joining interfaces. Joining interfaces were observed by SEM and analysed by electron probe microanalysis and X-ray diffraction. The strength of the joints was evaluated by four-point bending tests. Most of Si-Ti-C-O fibre/filler metal interfaces and the oxide material/filler metal interfaces were firm without cracking and separation. At the fibre/metal interfaces, a high concentration of titanium was confirmed. Among the specimens joined at 1123 K, the average strength, measured by the bending test, was 96 MPa. It was inferred that the defects at the joining interfaces formed by frothing had decreased the strength of the joints. Metallizing of the surfaces to be joined with the same filler metal as a pretreatment before joining, was effective in preventing frothing during joining and improving the joining strength. The average strength of the joints with pretreatment was 211 MPa.     

Effect of jig constraint state during welding process on fatigue properties of Al/CFRP dissimilar welds and fatigue life evaluation based on singular stress

Manufacturers have been promoting multimaterial designs. So the dissimilar material welding methods are being developed. Especially, heat welding technology, which is a direct joining by local heating, has been focussed. Therefore, this study used a heat welding technology for friction stir spot welding of aluminium alloy and carbon fibre‐reinforced plastic. This study investigated the effect of changes to jig constraint of joined members on the fatigue properties of joints. Also, the fatigue life estimation was carried out by considering the singular stress at the welding joint interface. As a result, the fatigue strength of joint in a less constrained state is higher than that in a more constrained state. The singular stress intensity at the slit tip was uniformly predicted by the differences in welding parameters of joints.     

20#钢连续驱动摩擦焊大变形及传热行为的有限元模拟研究

目的研究20~#钢连续驱动摩擦焊接过程工艺参数对焊接过程温度场和变形行为的变化规律。方法基于ABAQUS有限元软件二次开发环境,建立了20~#钢连续驱动摩擦焊接过程中的完全热-结构耦合模型。通过对比模拟和实验获得的焊接温度场、轴向缩短量和飞边形貌,对模型进行了验证。研究了工艺参数对摩擦焊接过程温度场与大变形行为和接头组织与性能的影响规律。结果在不稳定摩擦阶段,峰值温度出现在外表面附近。在稳定摩擦阶段,峰值温度稳定在距焊缝中心约2/3半径位置。接头温度的升高速度随着摩擦压力和转速的增大而增大,摩擦压力和转速对稳定阶段温度场的影响很有限;经过顶锻阶段之后摩擦面温度分布更加均匀,顶锻力越大在接头相同的位置温度越低,顶锻力越大轴向缩短量越大。结论所建立的完全热-结构耦合模型可以模拟20~#钢连续驱动摩擦焊接过程的塑性变形过程,在不稳定摩擦阶段,摩擦压力和转速对温度场和变形的影响较大。在稳定摩擦阶段,摩擦压力和转速对温度场的影响不显著。顶锻阶段轴向缩短量随着摩擦压力、转速和顶锻压力的增大而增加。     

A572 Gr.50厚板对接焊缝断裂性能研究

A572 Gr.50厚板常用于锅炉钢结构大板梁的加工制作,其对接焊缝易存在焊接裂纹缺陷。为评定含缺陷的锅炉钢结构大板梁的安全性,通过焊接接头模拟制作实际大板梁下翼缘的A572 Gr.50厚板对接焊缝,分别对其母材、焊缝金属及热影响区材料进行了系列单轴静力拉伸试验、冲击韧性试验和直三点弯曲断裂韧度试验,并结合有限元分析对A572 Gr.50厚板对接焊缝存在裂纹缺陷时的断裂性能进行了研究。研究结果表明:厚板对接焊缝母材、焊缝和热影响区材料基本力学性能均能满足规范要求,均具有良好的塑性变形能力;随温度的降低,母材、热影响区及焊缝处的夏比冲击功减少,但均具有良好的冲击韧性;比较而言,母材的抗低温冷脆性能最好,焊缝最差;母材、焊缝和热影响区3个区域中焊缝的断裂韧度最差;厚板对接焊缝接头的焊缝区是大板梁焊接缺陷安全性评估的重点控制区域;基于断裂力学,可以运用有限元软件方便的对带裂缝的工作状态下工作的钢结构构件的断裂性能进行分析,保证其安全性。     

Mechanical experiments on the superplastic material ALNOVI-1, including leak information

In subatomic particle physics, unstable particles can be detected with a so-called vertex detector, placed inside a particle accelerator. A detecting unit close to the accelerator bunch of charged particles must be separated from the accelerator vacuum. A thin sheet with a complex 3D shape prevents the detector vacuum from polluting the accelerator vacuum. Therefore, this sheet has to be completely leak tight. However, this can conflict with restrictions concerning maximum sheet thickness of the product. To produce such a complex thin sheet, superplastic forming can be very attractive in cases where a small number of products is needed. In order to predict gas permeability of these formed sheets, many mechanical experiments are necessary, where the gas leak has to be measured. To obtain insight in the mechanical behaviour of the used material, ALNOVI-1, tensile experiments were performed to describe the uniaxial stress-strain behaviour. From these experiments, a high strain rate sensitivity was measured. The flow stress of this material under superplastic conditions was low and the material behaved in an isotropic manner upon large plastic strains. The results of these experiments were used to predict the forming pressure as a function of time in a free bulge experiment, such that a predefined target strain rate will not be exceeded in the material. An extra parameter within these bulging experiments is the application of a hydrostatic pressure during the forming process. Such a pressure postpones the nucleation and growth of internal cavities, which means that higher plastic strains can be reached before failure. Results from these experiments showed that at higher hydrostatic pressures, higher bulges were made. All these bulges were leak tested, showing also that higher hydrostatic pressures lead to a lower void volume fraction at higher hydrostatic pressures, since these bulges were more leak tight at the same bulge height than bulges made without the application of this pressure. This article describes the setup and results of the uniaxial (tensile) and biaxial (bulging) experiments on the superplastic aluminium ALNOVI-1.