Ultrasonic Welding of Ni and Cu Sheets

Ultrasonic metal welding is widely used in various fields due to its ability to weld a variety of materials such as new materials and sheet structures. In this study, a special horn with four-point tips was developed for the ultrasonic welding of Ni and Cu sheets used as electrode materials of the secondary cell. The effects of welding parameters (welding time, clamping pressure, and vibrational amplitude) on weldability were investigated using the developed horn. The weldability of Ni/Cu sheet was assessed via the tensile test, scanning electron microscope observation, and EDX-ray analysis of the weld zone. Experimental results showed that the optimal welding parameters were a welding time of 0.25 s, pressure of 0.20 MPa, and vibrational amplitude of 80%, with a welding strength of 87.45 N under these conditions. It was also confirmed that solid-state diffusion by vibrational and frictional heat was clearly generated at the welding interface under good welding conditions.     

Effect of process parameters on electromagnetic impact welding of aluminum sheets

Electromagnetic forming of aluminum sheets has been studied in detail by many researchers due to its advantages of increased formability and reduced springback. The feasibility of electromagnetic impact welding of aluminum sheets has been established, but the effect of process parameters on the weld strength has not been reported. The present study investigates the effect of parameters such as energy, standoff distance and coil geometry on the shearing strength and the width of the weld achieved by electromagnetic impact welding of aluminum sheets. A study has been carried out to characterize the effectiveness of the process of welding two aluminum sheets of same chemical composition and of 1 mm thickness. The results of the microstructure and shearing strength tests are reported in this paper. The shearing strength and the width of the weld are found to increase with an increase in discharge energy. Further, when the geometry of the coil is changed from rectangular shape to tapered shape, it gives higher shearing strength. The standoff distance has an optimum value that gives maximum shearing strength and width of the weld. If the standoff distance is varied on either side of its optimum value the shearing strength and the width of the weld reduce.     

铝/钢搅拌摩擦焊对接焊缝组织及机理研究

为了提高Q235钢板和6082-T6铝合金对接的连接强度,采用搅拌摩擦焊进行对接焊接.研究了不同尺寸和形状的搅拌头、转速、焊接速度和偏移量等对铝钢对接焊缝组织的影响,进而优化了搅拌摩擦焊工艺.实验结果表明:不同形状的搅拌头影响接头"钉子"形状,接头的不同位置处由于受到不同热循环和搅拌导致晶粒尺寸不同,从而影响接头的力学性能.当搅拌针旋转速度260 r/min,焊接速度16 mm/min,针头偏向铝侧0.2 mm时,所得焊缝的拉伸强度为141.204 MPa,为最佳工艺参数.在此最优参数下获得过渡层的厚度约为8μm,界面的主要成分是Fe Al3.     

Melting of multipass surface tracks in steel incorporating titanium carbide powders

Overlapping tracks were processed by melting preplaced titanium carbide (TiC) powder on steel surfaces using a tungsten inert gas torch. The tracks produced ～1·0 mm melt depth free from cracks, but occasional pores were observed. The microstructure consisted of unmelted and partially melted TiC particulates together with reprecipitated TiC particles, which were prominent in tracks processed in the initial stage. A greater number of reprecipitated globular and cubic TiC particles were observed in tracks processed in the later stages, indicating more dissolution of TiC particulates from the overlapping operation. Those multitracks processed in the initial stage developed a maximum hardness of 850–1000 HV, which was lower in most other tracks, although comparable hardness values were recorded in the last track.     

Influences of heat source model on welding residual stress and distortion in a multi-pass J-groove joint

Welding mechanical behaviors including residual stress and distortion are highly non-linear phenomena in nature. When numerical simulation methods such as thermal elastic plastic finite element method (FEM) are used to quantitatively predict welding residual stress and distortion, a long computational time is required especially for multi-pass joints. In real engineering structures, many weldments have large dimensions and complex shapes, and they are usually assembled by a multi-pass welding process. Therefore, it is necessary to develop time-effective computational approaches for practice engineering analysis. In this study, a method based on variable length heat sources was proposed for the analysis of thermo-mechanical behaviors for multi-pass joints. The welding residual stress field in a dissimilar metal J-groove joint with axis-symmetric geometrical shape, which was performed by a semi-circle balanced welding process, was investigated using the proposed method. The simulation results were compared with the measured data as well as the simulation results computed by a moving heat source. Meanwhile, the instantaneous line heat source was also employed to estimate the welding residual stresses in the same joint in an extreme case. The influences of heat source model (type) on welding residual stress and distortion were discussed.     

Cleavage failure of transformer storage tank under dynamic rates of loading: Influence of base plate and weldment microstructure and toughness

A high capacity transformer storage tank that was in service for more than a decade ruptured due to an explosion and a visual examination showed that the fracture facets were essentially brittle in nature with little evidence of plastic deformation. Fracture occurred mainly along weld seam; however, there were sharp cracks even in the base plate. Present study is a focused effort to find reasons for this brittle behavior of normally ductile steel. This entailed an in-depth metallographic examination of samples, selectively cut out from the tank walls and fractured weldments, supplemented by characterization of hardness, tensile, ductility and toughness properties. Results indicate that base plate and weld metal meet the required specifications for static design but not for the dynamic loading experienced under this failure mode (i.e. cleavage). Under dynamic loading conditions, the weld metal is the weakest link in terms of toughness and resistance to crack propagation. The need for adopting modern strong, tough, weldable steels to the construction of these tanks are pointed out.     

Fatigue life enhancement of aluminium joints through mechanical and thermal prestressing

This paper presents some simple and flexible methods to enhance the fatigue life of welded aluminium components. Besides enhancing the fatigue life, the proposed methods can easily be implemented into manufacturing processes. The key element of the methods is to change residual stresses from tension to compression at locations vulnerable to fatigue. This is accomplished by mechanical prestressing using elastic pre-deformation or by thermal prestressing using induction heating. The specimens tested are welded aluminium rectangular hollow section T-joints. Prior to fatigue testing, welding FE-simulations were carried out to verify the magnitude and pattern of the residual stress fields (through process modeling). Fatigue testing was later carried out on four different batches. One batch was produced using elastically pre-deformed chords, two batches were treated by means of thermal prestressing (induction heating), and one batch was “as welded” representing a “reference case”. Based on statistical evaluation of S–N data, the introduction of superimposed compressive stress fields results in a significantly improved fatigue life. Among the different batches, induction heating turned out to be the most promising method with a fatigue strength improvement factor of 1.5 on stress, compared to “as welded” components.     

A notch stress intensity approach applied to fatigue life predictions of welded joints with different local toe geometry

In the Notch Stress Intensity Factor (N‐SIF) approach the weld toe region is modelled as a sharp V‐shaped corner and local stress distributions in planar problems can be expressed in closed form on the basis of the relevant mode I and mode II N‐SIFs. Initially thought of as parameters suitable for quantifying only the crack initiation life, N‐SIFs were shown able to predict also the total fatigue life, at least when a large part of the life is spent as in the propagation of small cracks in the highly stressed region close to the notch tip. While the assumption of a welded toe radius equal to zero seems to be reasonable in many cases of practical interest, it is well known that some welding procedures are able to assure the presence of a mean value of the weld toe radius substantially different from zero. Under such conditions any N‐SIF‐based prediction is expected to underestimate the fatigue life. In order to investigate the degree of conservatism, a total of 128 fillet welded specimens are re‐analysed in the present work by using an energy‐based N‐SIF approach. The local weld toe geometry, characterised by its angle and radius, has been measured with accuracy for the actual test series. The aim of the work is to determine if the N‐SIF‐based model is capable of taking into account the large variability of the toe angle, and to quantify the inaccuracy in the predictions due to the simplification of setting the toe radius equal to zero.     

Correlation of fatigue properties and microstructure in investment cast Ti-6Al-4V welds

The effect of microstructural characteristics on high-cycle fatigue properties and fatigue crack propagation behavior of welded regions of an investment cast Ti-6Al-4V were investigated. High-cycle fatigue and fatigue crack propagation tests were conducted on the welded regions, which were processed by two different welding methods: tungsten inert gas (TIG) and electron beam (EB) welding. Test data were analyzed in relation to microstructure, tensile properties, and fatigue fracture mode. The base metal was composed of an alpha plate colony structure transformed to a basket-weave structure with thin platelets after welding and annealing. High-cycle fatigue results indicated that fatigue strength of the EB weld was lower than that of the base metal or the TIG weld because of the existence of large micropores formed during welding, although it had the highest yield strength. In the case of the fatigue crack propagation, the EB weld composed of thinner platelets had a faster crack propagation rate than the base metal or the TIG weld. The effective microstructural feature determining the fatigue crack propagation rate was found to be the width of platelets because it was well matched with the reversed cyclic plastic zone size calculated in the threshold ΔK regime.     

45Mn17Al3低磁钢焊接特性及工艺研究

针对45Mn17Al3低磁钢与其焊接材料合金系性能差异较大的特点,对手工电弧焊、脉冲MIG焊、陶瓷衬垫焊焊接特性进行分析,改进了焊接工艺,为低磁钢焊接材料的工程应用提供了试验依据.