High-Velocity Impact Welding Process: A Review

Manganese, Iron, Lead, and Zinc Levels and Haematological Profile among Welders in Bibiani Anhwiaso Bekwai District, Ghana

Welders are exposed to metal ions or oxides through direct contact at occupational sites or indirectly through uptake from contaminated dust or air. This study was a case-control study designed to assess the levels of some heavy metals and the hematological profile of welders (cases) as compared to nonwelders (controls) from Bibiani Anhwiaso Bekwai District of Ghana, comparatively to determine whether their values are within acceptable international range. A quantitative-based survey using structured questionnaires was used to collect demographic data from purposively selected welders (n = 40) and nonwelders (n = 40) from the study area. Five (5 mL) blood samples were collected from the study participants and analyzed for blood cell count as well as levels of Mn, Fe, Pb, and Zn. There were no significant differences in the Mn, Zn, and Fe levels between the welders and nonwelders ( $p=0.431$ , 0.53 vs. 0.23 mg/L, $p=0.05$ , 0.41 vs. 0.15, $p=0.886$ , 1.82 vs. 1.11). The level of Pb was, however, significantly lower among welders compared to the nonwelders ( $p=0.016$ , 0.09 < 0.3 mg/L). The total white blood cell count did not differ significantly between welders and nonwelders ( $p=0.365$ , 5.16 vs. 4.85 × 109/L). However, the mixed cell fraction was significantly higher among welders compared to nonwelders ( $p=0.027$ , 0.34 × 109/L > 0.28 × 109/L). Red blood cell count and indices showed no significant differences between the welders and nonwelders. Hemoglobin levels in welders were, however, higher (14.47 g/dL) but this was not statistically significant compared to their nonwelder counterparts (13.85 g/dL). It was concluded from the study that welders in Bibiani Anhwiaso Bekwai District of the Western Region of Ghana had elevated levels of Pb in their bodies. This was associated with an increase in mixed white blood cell fraction platelets. However, the recorded levels were within the accepted physiological limits suggesting that the heavy metal exposure of welders had no clinically pathological significance.

Analytical Estimation of Electromagnetic Pressure, Flyer Impact Velocity, and Welded Joint Length in Magnetic Pulse Welding

Magnetic pulse welding involves the joining of two metallic parts in a solid state by the use of a short and intense electromagnetic impulses and the resulting impact between the parts. The coalesced interface undergoes visco-plastic deformation at a high strain rate and exhibits a wavy shape at a microscopic scale. A practical estimation of the electromagnetic pressure, impact velocity and welded joint length as a function of the process conditions and the electromagnetic coil geometry is required but currently not available. Three novel analytical relations for the estimation of the electromagnetic pressure, impact velocity, and welded joint length for magnetic pulse welding of tubes and sheets, are presented. These relations were developed systematically, following a dimensional analysis, and validated for a wide range of conditions from independent literature. The comparison of the analytically computed results and the corresponding values reported in the literature has illustrated that the proposed analytical relations can be used for the estimation of the electromagnetic pressure and impact velocity for the magnetic pulse welding of tubes and sheets with a good level of confidence. The analytically calculated results for the welded joint length show a little discrepancy with the corresponding experimentally measured values. Further investigations and more experimentally measured results are required to arrive at a more comprehensive analytical relation for the prediction of welded joint length.

Metallic Materials and Their Applications in Aerospace and Advanced Technologies

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On the Disintegration of A1050/Ni201 Explosively Welded Clads Induced by Long-Term Annealing

The paper presents the microstructure and phase composition of the interface zone formed in the explosive welding process between technically pure aluminum and nickel. Low and high detonation velocities of 2000 and 2800 m/s were applied to expose the differences of the welded zone directly after the joining as well as subsequent long-term annealing. The large amount of the melted areas was observed composed of a variety of Al-Ni type intermetallics; however, the morphology varied from nearly flat to wavy with increasing detonation velocity. The applied heat treatment at 500 °C has resulted in the formation of Al₃Ni and Al₃Ni₂ layers, which in the first stages of growth preserved the initial interface morphology. Due to the large differences in Al and Ni diffusivities, the porosity formation occurred for both types of clads. Faster consumption of Al₃Ni phase at the expense of the growing Al₃Ni₂ phase, characterized by strong crystallographic texture, has been observed only for the weld obtained at low detonation velocity. As a result of the extended annealing time, the disintegration of the bond occurred due to crack propagation located at the A1050/Al₃Ni₂ interface.

Research Status and Prospect of Laser Impact Welding

The demands for the connection between thin dissimilar and similar materials in the fields of microelectronics and medical devices has promoted the development of laser impact welding. It is a new solid-state metallurgical bonding technology developed in recent years. This paper reviews the research progress of the laser impact welding in many aspects, including welding principle, welding process, weld interface microstructure and performance. The theoretical welding principle is the atomic force between materials. However, the metallurgical combination of two materials in the solid state by atomic force but almost no diffusion has not been confirmed by microstructure observation. The main theories used to explain the wave formation in impact welding were compared to conclude that caved mechanism and the Helmholz instability mechanism were accepted by researchers. The rebound of the flyer is still a critical problem for its application. With proper control of the welding parameters, the weld failure occurs on the base materials, indicating that the weld strength is higher than that of the base materials. Laser impact welding has been successfully applied in joining many dissimilar materials. There are issues still remained unresolved, such as surface damage of the flyer. The problems faced by laser impact welding were summaried, and its future applications were proposed. This review will provide a reference for the studies in laser impact welding, aiming process optimization and industrial application.

Interface Formation during Collision Welding of Aluminum

Collision welding is a high-speed joining technology based on the plastic deformation of at least one of the joining partners. During the process, several phenomena like the formation of a so-called jet and a cloud of particles occur and enable bond formation. However, the interaction of these phenomena and how they are influenced by the amount of kinetic energy is still unclear. In this paper, the results of three series of experiments with two different setups to determine the influence of the process parameters on the fundamental phenomena and relevant mechanisms of bond formation are presented. The welding processes are monitored by different methods, like high-speed imaging, photonic Doppler velocimetry and light emission measurements. The weld interfaces are analyzed by ultrasonic investigations, metallographic analyses by optical and scanning electron microscopy, and characterized by tensile shear tests. The results provide detailed information on the influence of the different process parameters on the classical welding window and allow a prediction of the different bond mechanisms. They show that during a single magnetic pulse welding process aluminum both fusion-like and solid-state welding can occur. Furthermore, the findings allow predicting the formation of the weld interface with respect to location and shape as well as its mechanical strength.

Particle Ejection by Jetting and Related Effects in Impact Welding Processes

Collision welding processes are accompanied by the ejection of a metal jet, a cloud of particles (CoP), or both phenomena, respectively. The purpose of this study is to investigate the formation, the characteristics as well as the influence of the CoP on weld formation. Impact welding experiments on three different setups in normal ambient atmosphere and under vacuum-like conditions are performed and monitored using a high-speed camera, accompanied by long-term exposures, recordings of the emission spectrum, and an evaluation of the CoP interaction with witness pins made of different materials. It was found that the CoP formed during the collision of the joining partners is compressed by the closing joining gap and particularly at small collision angles it can reach temperatures sufficient to melt the surfaces to be joined. This effect was proved using a tracer material that is detectable on the witness pins after welding. The formation of the CoP is reduced with increasing yield strength of the material and the escape of the CoP is hindered with increasing surface roughness. Both effects make welding with low-impact velocities difficult, whereas weld formation is facilitated using smooth surfaces and a reduced ambient pressure under vacuum-like conditions. Furthermore, the absence of surrounding air eases the process observation since exothermic oxidation reactions and shock compression of the gas are avoided. This also enables an estimation of the temperature in the joining gap, which was found to be more than 5600 K under normal ambient pressure.

Influence of Surface Preparation on the Interface of Al-Cu Joints Produced by Magnetic Pulse Welding

Magnetic pulse welding can be considered as an advanced joining technique because it does not require any shielding atmosphere and input heat similar to conventional welding techniques. However, it requires comprehensive evaluations for bonding dissimilar materials. In addition to processing parameters, the surface preparation of the components, such as target material, needs to be evaluated. Different surface conditions were tested (machined, sand-blasted, polished, lubricated, chemically attacked, and threaded) using a fixed gap and standoff distance for welding. Microstructural observations and tensile testing revealed that the weld quality is dependent on surface preparation. The formation of waviness microstructure and intermetallic compounds were verified at the interface of some joints. However, these conditions did not guarantee the strength.

Influence of Impact Velocity on the Residual Stress, Tensile Strength, and Structural Properties of an Explosively Welded Composite Plate

This study aimed to analyze the effect of the impact velocity of a Zr 700 flyer plate explosively welded to a Ti Gr. 1/P265GH bimetallic composite on the residual stress formation, structural properties, and tensile strength. The residual stresses were determined by the orbital hole-drilling strain-gauge method in a surface layer of Zr 700 in as-received and as-welded conditions. The analysis of the tensile test results based on a force parallel to interfaces was used to propose a model for predicting the yield force of composite plates. Compressive residual stresses found in the initial state of the Zr 700 plate were transformed to tensile stresses on the surface layer of the welded Zr 700 plate. A higher impact velocity resulted in higher tensile stresses in the Zr 700 surface layer. To increase the resistance of the composite plate to stress-based corrosion cracking, a lower value of impact velocity is recommended in the welding process.

Fracture Characteristics and Analysis in Dissimilar Cu-Al Alloy Joints Formed via Electromagnetic Pulse Welding

The aim of this study was to identify and analyze the fatigue fracture characteristics of dissimilar Al 6061 to Cu (UNS C11000) lap joints made with ultrafast electromagnetic pulse welding (EMPW) via fractography, stress analysis and finite element simulation. It was observed that EMPW generated an annular (or ring-shaped) bonding area, with weld zones and a central non-weld zone when viewed from the cross section. Two types of failure modes occurred in relation to the cyclic loading levels: base metal fracture or transverse through-thickness (TTT) crack growth at a higher loading level, and joint interfacial failure at a lower loading level. In the interfacial failure, fatigue crack initiated from the outer edge of annular welding area, and propagated to form an approximate elliptical boundary. Fatigue crack propagation was characterized by fatigue striations existing in discrete areas on the fracture surface. This was attributed to a coupled role of shear and normal stresses present in a tensile lap shear sample due to the bending moment caused by the inherent misalignment. The final rapid fracture started from elliptical boundary with elongated shear dimples. Both theoretical stress analysis and finite element model revealed the maximum stress and stress concentration along the outer edge, where fatigue crack initiation occurred.

Thermal Effects in Dissimilar Magnetic Pulse Welding

Magnetic pulse welding (MPW) is often categorized as a cold welding technology, whereas latest studies evidence melted and rapidly cooled regions within the joining interface. These phenomena already occur at very low impact velocities, when the heat input due to plastic deformation is comparatively low and where jetting in the kind of a distinct material flow is not initiated. As another heat source, this study investigates the cloud of particles (CoP), which is ejected as a result of the high speed impact. MPW experiments with different collision conditions are carried out in vacuum to suppress the interaction with the surrounding air for an improved process monitoring. Long time exposures and flash measurements indicate a higher temperature in the joining gap for smaller collision angles. Furthermore, the CoP becomes a finely dispersed metal vapor because of the higher degree of compression and the increased temperature. These conditions are beneficial for the surface activation of both joining partners. A numerical temperature model based on the theory of liquid state bonding is developed and considers the heating due to the CoP as well as the enthalpy of fusion and crystallization, respectively. The time offset between the heat input and the contact is identified as an important factor for a successful weld formation. Low values are beneficial to ensure high surface temperatures at the time of contact, which corresponds to the experimental results at small collision angles.