Vaporizing Foil Actuator Welding Research Articles

Effect of interlayer thickness and gap distance on vaporizing foil actuator welding of 5A06 aluminium alloy and 321 stainless steel

The composite structure of aluminium alloy and stainless steel provides a wide range of comprehensive advantages, encompassing properties such as lightweight, high strength, and corrosion resistance. These advantages make composite structure particularly suitable for various applications in industries such as transportation and chemicals. One innovative solid-phase welding technology that is well suited for joining dissimilar materials is vaporizing foil actuator welding. This technology allows for the welding of composite structures made of aluminium and stainless steel, despite the significant differences in physical and chemical properties. To enhance the vaporizing welding process, this paper proposes the introduction of an interlayer between the dissimilar materials. The interlayer consists of a third material that is added to bridge the gap between materials with differing hardness and plasticity. The main objective of introducing the interlayer is to minimise performance disparities and reduce the formation of intermetallic compounds at the interface. By examining the vaporizing foil actuator welding process of aluminium alloy and stainless steel with the interlayer, it aims to analyse the characteristics of the interface morphology. Additionally, this study investigates the energy conversion mechanism of the aluminium foil gasification process and explore the influence of the interlayer on the microstructure and mechanical properties of the interface between aluminium alloy and stainless-steel joints.

Buharlaştırılmış Folyo Aktüatör Kaynağı ile Alüminyum Şekillendirme

Aluminum, has a wide range of uses, from industry to medical industrial products such as automotive, defense-aviation, and space. Aluminium has become a preferred material in applications where weight needs to be reduced. It is also used as a material in metal cladding and shaping applications. However, the high thermal conductivity coefficient of aluminum may cause some difficulties in the production process. These difficulties due to thermal conductivity may not allow to manufacture of desired products. In this study formability of aluminum with vaporizing foil actuator welding (VFAW) has been investigated as an alternative process to the traditional and solid state welding processes. By using different voltage values and separation distance parameters, the effects of these parameters on the formability of aluminum and copper sheet metal pairs with VFAW were examined experimentally. Experimental data show that voltage value is an important parameter in the VFAW process where a DC capacitor is used. In the VFAW process, where a 375 V voltage value is used, it has been determined that welding occurs in processes performed with three different separation distances (0.4 mm- 0.9 mm -1.5 mm). It was determined that 0.4 mm and 0.9 mm separation distances were effective in joining Al/Cu metal sheets with 0.1 mm thickness.

Acceleration of metal plates by hybridization of electrical and chemical energy for potential application in high-velocity impact welding

Vaporizing foil actuator welding is employed to connect dissimilar metals by utilizing the energy of an electrical explosion as the driving force for flyer plate acceleration. Herein, the electrical and chemical energies of explosives were combined to increase flyer plate velocity. A metal foil or wire mesh was used as an actuator, and a small amount of explosives was placed on the foil. Uniform acceleration of the flyer plate on the actuator was recorded using a high-speed video camera. The electrical energy applied to the actuator was measured synchronously by filming. The explosion of chemical explosives was successfully initiated owing to the electrical explosion of the actuator. In all experiments, the combination of the electrical explosion and chemical reaction improved the acceleration of the samples, facilitating higher velocities. The impact of the actuator type (mesh or foil) and the electric discharge parameters on the performance was studied. While foil actuators were more efficient in experiments on electric explosions alone, mesh actuators were superior when the chemical explosive was added. Explosions of the mesh actuator could initiate low density pentaerythritol tetranitrate (PETN) (the space-filling rate of PETN was approximately 30%). An explosive force of 0.3 g of PETN was converted into a driving force, imparting the flying plate with a velocity greater than 210 m/s. Electrical energy consumption for PETN initiation may be assumed to have an insignificant impact on flyer plate acceleration. In future, the proposed method may be used to connect heavier and thicker materials without expensive high-performance capacitors.

Water-augmented vaporizing foil actuator welding: Process performance and mechanisms clarifications

This paper proposes a novel impact welding method named water-augmented vaporizing foil actuator welding, in which liquid water is introduced to alter the foil’s vaporization process, thus altering the induced driving pressure and the final welding performance. A series of comparative experiments were performed to confirm the effectiveness of the process and to understand the underlying mechanism. The proposed process decreased the lower-bound required discharge energy by 58–69%, increased the peel strength by 30–60%, and significantly reduced the variability in the critical standoff distance at the lower bound of the weld window. In addition, the proposed process resolved the robustness issue by producing a much more regular and repeatable shape of the weld area compared to conventional VFAW. Furthermore, three potential mechanisms have been proposed to explain the process improvement: the added water 1) increased the electrical energy deposition, 2) introduced additional energy from the aluminum-water chemical reaction, and 3) provided a more efficient force-transfer medium. In summary, this paper introduces a novel impact welding method with much improved process capability and robustness compared to conventional vaporizing foil actuator welding. In addition, due to its simplicity, this method may be easily extended for other forms of impulse metalworking, which may stimulate a wide interest for the community of high velocity metalworking.

Buharlaştırılmış folyo aktüatör kaynağı prototip imalatı ve uygulamaları risk analizi

This study was conducted to examine risk levels for Vaporizing Foil Actuator Welding (VFAW) prototype manufacturing and applications. A risk analysis was carried out by using the L-type matrix method to detect hazards and risks in newly developing VFAW applications. 60 risks were identified for VFAW, 37 of which were high and 23 were moderate. In order to reduce the risk and occupational accidents to the lowest possible level; a measuring instrument that can continuously monitor current and voltage values, a remote detonation system and a discharge mechanism have been added to the prototype system. Thanks to the measures taken the situations that may result in serious accidents, fire, explosion, loss of limb and even death that may occur during the experiment have been brought under control and VFAW applications have been made safer.

Study on interfacial characteristics and properties of NiTi/Al–Mg joint by vaporizing foil actuator welding

As a new type of impact welding technology, Vaporizing Foil Actuator Welding (VFAW) has the advantages of high welding efficiency and small heat affected zone, which could avoid the decline of shape memory properties of the NiTi alloy in the NiTi/metals joint of fusion welding. Therefore, NiTi/Al–Mg VFAW experiments under different discharge voltages and standoffs were carried out to analyze the effects of process parameters on interface morphology, mechanical and functional properties. The results showed that high strength welding between NiTi and Al–Mg alloys were realized. With the increase of discharge voltage and standoff, the interface morphology changed to flat, a small amount of melting layer, wave, continuous melting layer, and the joint performance increased first and then decreased. The difference in properties between NiTi and Al–Mg alloys led to the change of microstructure and nano-hardness at the interface. Equiaxed recrystallized grains and small slender grains appeared on Al–Mg and NiTi alloy respectively. The nano-hardness of both sides increased, but the hardening range of the Al–Mg alloy side was smaller. The changes of phase transformation temperatures of the NiTi/Al–Mg joints were small compared to NiTi base, indicating the VFAWed NiTi alloy still maintained the original shape memory function. This work deepens the understanding about the properties of NiTi/Al–Mg VFAW, and is helpful for expanding the application of NiTi shape memory alloy.

Dissimilar Materials Welding with a Standoff-Free Vaporizing Foil Actuator between TRIP 1180 Steel Sheets and AA5052 Alloy.

This paper mainly demonstrates an advanced type of the vaporizing foil actuator welding (VFAW) process between GPa-grade steel (TRIP1180) and aluminum alloy (AA5052-H32) without applying standoff. To secure a flying distance during the VFAW process, the preformed target sheet shaped like a circular indentation has been utilized. It is necessary to optimize process parameters integrated with geometrical design of the preform since the welding strength can be decreased beyond the optimum input energy in the standoff-free VFAW process. The welded surface was evaluated by SEM-EDS, XRD, EBDS, and TEM to analyze the welding mechanism and composition at the welding interface. The diffusion zone including the AlFe3 phase was observed at the welded interface which has high grain density due to the high-speed impact by increasing the welding strength, which leads to the perfect welding between the dissimilar materials.

High strength impact welding of 7075 Al to a SiC-reinforced aluminum metal matrix composite

Vaporizing foil actuator welding (VFAW), an impact welding method, was used for the first time to produce high strength welds between SiC-reinforced 8009 aluminum metal matrix composite (MMC) (8009/SiC/11p) and 7075 aluminum alloy (AA7075). Microscopic characterization of the dissimilar impact weld revealed multiple heterogenous microstructural zones including an unbonded zone at the center, a flat interface, a wavy interface, and a trapped jet zone at the corner. SiC-depleted and SiC-rich bands were alternately distributed with consistent plastic flow patterns along the wavy interface. Microhardness tests prove that little or no heat or mechanically affected zones were formed in the dissimilar impact weld. Peel test yields a nugget pullout on the AA7075-T6 indicating high strength of the weld. VFAW process provides a solution to join the Al MMC with dissimilar metals in the applications of automotive and aerospace products.

Solid state welding of medium-entropy CrCoNi with heterogeneous, partially recrystallized microstructures

Heterogeneous, partially recrystallized (PRX) microstructures have recently been used to improve strength-ductility combinations in high-entropy alloys. However, these microstructures are incompatible with conventional joining processes that require melting or prolonged exposure to elevated temperatures. This work presents an initial exploration of solid state joining in this challenging condition using vaporizing foil actuator welding (VFAW) applied to PRX equiatomic alloy CrCoNi.

Microstructure and Fracture Toughness of an Aluminum-Steel Impact Weld and Effect of Thermal Exposure

Welding of aluminum alloys to steel is increasingly important in manufacturing; however, the use of fusion welding is difficult because of disparate melting points and the possibility of intermetallic compound (IMC) formation. Here, an impact welding technique, vaporizing foil actuator welding, was utilized to produce solid-state joints between AA1100-O and 1018 mild steel. The relationship between weld processing conditions, microstructure, and mechanical properties was investigated. For this purpose, the welds were annealed between 300 °C and 600 °C and a combination of optical and scanning electron microscopy, along with image analysis was performed to characterize the weld microstructure and monitor IMC growth. Wedge testing was applied to understand the effect of annealing on the weld fracture toughness. A numerical model incorporating the Fick’s laws of diffusion, grain boundary diffusion, and grain growth kinetics was also developed to simulate the IMC growth. The heterogeneity in the original microstructure caused persistent differences in IMC growth, as initial IMC seemed to increase nucleation and growth. Simulation results indicated short circuit diffusion to be the major contributor to IMC growth since it is consistently faster then experimental IMC growths compared with the computational results that used lattice diffusion only. Wedge testing reveals increased weld toughness for modest anneals of 300 °C, possibly due to homogeneity at the weld interface while avoiding IMC growth.

High strength welds in titanium & nickel based alloys by impact welding – A practical method

Joining titanium-based alloys to nickel-based superalloys is of great interest for applications in aerospace propulsion. Direct welding of these materials using conventional welding methods is very difficult as they are prone to heat affected zone and formation of strain-aged cracking. The purpose of this study is to produce multiple weld combinations of Ni-based superalloys (Ni-230, Ni-718, and Ni-625) to a near-alpha Ti-based alloy (6242), Ti to Ti, and Ni to Ni welds using Vaporizing foil actuator welding (VFAW) technique. The bonding quality of the joints was evaluated by mechanical lap shear and peel tests. The microstructural characterization of the weld interface was analyzed by scanning electron microscopy and energy dispersive spectroscopy (EDS). The VFAW technique was successfully used to create 6 dissimilar and 4 similar combinations presented at various levels in this work. The Ni-Ti joints displayed mixed failure modes in both lap shear and peel tests, whereas nugget failure was observed for similar weld and dissimilar Ni-Ni joints. EDS maps of the peel surfaces showed both ductile and brittle facets that signified the material transfer between both parent materials. Irrespective of weld combinations, the microstructure showed a transition from a smooth bonding interface at the weld center to a wave profile showing a natural spatial progression as the weld forms. The VFAW process can be of interest in aerospace industries to join different exotic grade alloys, difficult to weld using conventional welding methods.

Numerical simulation of the joining interface of dissimilar metals in vaporizing foil actuator welding: Forming mechanism and factors

Abstract Predicting and controlling the joining interface are the main difficulties in impact welding dissimilar metal. In this study, the smoothed particle hydrodynamics (SPH) method was used to simulate the Cu/Ti wave interface forming process during high-rate impact welding. This process simulation was verified by vaporizing foil actuator welding experiments. The forming mechanism and factors influencing the characteristics of joining interface were simultaneously investigated. Results showed that, joining interface wave forming depended on the density ratio of the flyer sheet to the target sheet. Wave formation distance was mainly related to materials yield stress. The wavelength and amplitude of wavy characteristic increased with the impact angle and impact speed. Temperature distribution analyses proved that the average temperature of the vortex at the welding interface was higher than that in the other areas, indicating that the inter-metallic distribution was mainly in the vortex of the wave interface. Investigation of the moving trail of metal particles verified that the interaction of interface materials was caused by embedded deformation.

High strength impact welding of NiTi and stainless steel wires

This work demonstrates the use of a solid-state microwelding process to create very high strength welds between superelastic NiTi and themselves as well as stainless steel (SS) wires. Vaporizing foil actuator welding (VFAW) uses the pressure from a vaporized metal foil to impact weld the wires by driving them into one another. The small scale nature of this explosive-like process allows its use in factories. Advanced techniques were used to examine the structures and properties of the welds that were created. The NiTi/SS welds had the typical wavy interface of ideal impact welds, with no thermally induced defects such as chemical segregation near the interface or brittle intermetallic formation. These latter two are significant issues in traditional fusion based methods. Microhardness tests show that the NiTi/NiTi and NiTi/SS weld interfaces were locally strengthened compared to the softening of other welding processes. Lap shear tests show that NiTi/SS welds made by VFAW present much higher joint efficiency (near 100%) compared to less than 60% for other traditional joining technologies. Tensile cycling of both the NiTi/NiTi and the NiTi/SS welds shows similar stabilization of the pseudoelastic curves as the NiTi base metal. The irrecoverable strains of the NiTi/NiTi welds and NiTi/SS welds after cycling were comparable to that in the NiTi base metal. The unparalleled performance of these high-strength solid-state similar and dissimilar welds of nitinol is a solution to the increasing demands of applications in various industrial fields including the medical, aerospace and electronic sectors.

Molecular dynamics study on interface formation and bond strength of impact-welded Mg-steel joints

It was recently demonstrated that the vaporizing foil actuator welding (VFAW) method can directly join immiscible magnesium and steel alloys without coating or a third chemical element based intermetallic compound layer. The VFAW Mg/steel joint exhibits a mixed interface layer of up to 200μm thickness consisting of Mg matrix and Fe particles. Multi-scale process simulations have suggested the formation of the interlayer is from the high-velocity frictional shearing between the Mg/steel substrates during the oblique impact in the VFAW process. This paper investigates the formation of Mg-Fe interlayer under VFAW condition with different shearing velocities using molecular dynamics (MD) models, and studies the bonding strength under different scenarios. The results elucidate the critical role of shearing velocity and surface roughness in achieving desirable Mg/Fe joint.

Joining aluminum alloy to ultrahigh-strength boron steel through an impact welding approach

Direct welding of ultrahigh-strength boron steels to aluminum alloys is challenging but of great practical importance. The present study reports a new approach in vaporizing foil actuator welding to produce direct spot welds between 6022-T4 Al alloy and Usibor 1500®, a representative boron steel. Mechanical testing results revealed a strong joint with failure occurring at the Al base metal outside the weld nugget. Comparison of joint strengths with other joining techniques revealed performance on par if not better, opening prospects for development of more robust dissimilar welding pairs in the future.

High strength welding of NiTi and stainless steel by impact: Process, structure and properties

High strength solid-state welds are demonstrated between NiTi shape memory alloy (SMA) and 436 stainless steel (SS) through Vaporizing Foil Actuator Welding (VFAW) method. This process uses the rapid ablation of a foil in a lab-scale process that is similar to explosive welding. The welding process characteristics including collision speed and angle were estimated using an optical method, Photon Doppler Velocimetry (PDV). The local microstructure had strong spatial variation along the weld. The first area to impact, the center of the structure does not weld, as impact is at a normal inclination. This center region has the highest collision speed, but lowest angle; moving outward collision speed decreases and impact angle increases. This causes a progression of varied weld morphologies. Transmission Electron Microscopy shows that some regions that locally melted solidified as amorphous zones. This corresponds to the Thermo-Calc calculations which also predict a high glass forming potential in the resulting multi-component alloy. While the local structure is heterogeneous from varied features across the entire span of the weld (over millimeters) to thin amorphous layers, they exhibit much higher joint efficiency than traditional welding technologies. This work sets a foundation to design processes to harness this high joint strength available through impact welding with other weld morphologies.

Unveiling non-equilibrium metallurgical phases in dissimilar Al-Cu joints processed by vaporizing foil actuator welding

An integrated experimental and computational approach has been adopted to investigate the formation mechanism of intermetallic compounds (IMCs) in dissimilar Al-Cu joints processed by vaporizing foil actuator welding (VFAW). Aiming to understand the formation of IMCs and their transition, the present work employed first-principles calculations of thermodynamic properties as a function of temperature and pressure together with diffusivities of Al and Cu from the literature. Agreeing with experimental observations, the predicted IMC formation sequence in the low-energy welding region is as follows. θ-Al2Cu forms first due to the fast diffusivity of Cu in the Al-matrix. θ-Al2Cu then transforms to η2-AlCu because of the fast diffusivities of Al/Cu in θ-Al2Cu. In the high-energy welding region, the stable γ1-Al4Cu9 and the metastable θ’’-Al3Cu are also formed in addition to the aforementioned θ-Al2Cu and η2-AlCu. η2-AlCu and considerable γ1-Al4Cu9 are the major phases in the high-energy welding region. The present approach, combining experiments and calculations, is proven to be a practical way to understand non-equilibrium metallurgical processes, as demonstrated through the study of VFAW Al-Cu joints.

Vaporizing foil actuator welding

Abstract

Joining Performance and Microstructure of the 2024/7075 Aluminium Alloys Welded Joints by Vaporizing Foil Actuator Welding

Vaporizing foil actuator welding (VFAW) was used for joining 2024-T3 and 7075-T6 aluminum alloy sheets, and the resulting joint microstructure was analyzed. 2024/7075 aluminum alloy pairs with suitable processing parameters can be prepared by using VFAW. Dynamic preform addresses the poor formability problem of target material and advantage of VFAW on dissimilar materials in some conditions. But with standoff sheet inserting in the flyer and target, 2024/7075 welded pairs gets the better weld strength, compared with flyer preformed method. The microstructure of the circular weld area of the welded joint showed a wave interface, in which a thin melt layer formed at the center and edge parts. The crystal grains near the bonding interface were remarkably elongated and refined. Therefore, the joining of the 2024/7075 pairs was facilitated through plastic forming and melting.

Cascading microstructures in aluminum-steel interfaces created by impact welding

With the growing need to develop light weight structures joining dissimilar metals is inevitable. Hence a fundamental understanding of the microstructure evolution and bond formation mechanism is necessary. In this work we probe the welded interface in aluminum and steel joined using vaporizing foil actuator welding (VFAW) a novel impact welding process. The goal of such a detailed characterization campaign is to understand the bond formation mechanism. The study shows that the interfacial microstructure has a pronounced hierarchical nature. At the macro scale, the interface is characterized by a wavy interface which is a result of extensive plastic deformation that accompanies the process. Detailed transmission electron microscopy shows evidence for the formation of a liquid film at the interface resulting in significant inter diffusion of Fe. The solidified structure at the interface consisted of crystalline α-Al and an amorphous metastable Al-Fe intermetallic compound similar to the observations in rapid solidification of Al-Fe alloys. The sub-nm scale characterization of the interface using atom probe tomography showed two distinct zones1.A magnesium (5 at.%) and oxygen (15 at.%) rich (not detectable using energy dispersive spectroscopy) which was probably from the decomposition of the surface oxide film2.The rapidly solidified reaction zone consisting of primary crystalline α-Al and an amorphous Al-Fe (15 at.%)-O (0.5 at.%)-Si (1.5 at.%).Based on these observations a mechanism for the bond formation is presented.