Vaporizing Foil Actuator Welding Research Articles

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.

High-Velocity Impact Welding Process: A Review

High-velocity impact welding is a kind of solid-state welding process that is one of the solutions for the joining of dissimilar materials that avoids intermetallics. Five main methods have been developed to date. These are gas gun welding (GGW), explosive welding (EXW), magnetic pulse welding (MPW), vaporizing foil actuator welding (VFAW), and laser impact welding (LIW). They all share a similar welding mechanism, but they also have different energy sources and different applications. This review mainly focuses on research related to the experimental setups of various welding methods, jet phenomenon, welding interface characteristics, and welding parameters. The introduction states the importance of high-velocity impact welding in the joining of dissimilar materials. The review of experimental setups provides the current situation and limitations of various welding processes. Jet phenomenon, welding interface characteristics, and welding parameters are all related to the welding mechanism. The conclusion and future work are summarized.

Wave formation in impact welding: Study of the Cu–Ti system

The cause of wavy interface morphology during impact welding is debated in the literature. In this paper, the effect of internal stress waves is investigated by varying the target (Cu 110) thickness during vaporizing foil actuator welding with constant thickness CP-Ti flyers. Experiments and smooth particle hydrodynamics simulations show that increasing target thickness causes the interfacial wavelength to increase, until it reaches about two times the flyer thickness. Also, to demonstrate the effects of consistency and transients of interfacial parameters, i.e., flyer velocity and wavy interface morphology, multi-probe photon Doppler velocimetry, electron microscopy and X-ray computed micro-tomography results are presented.

Spot impact welding of an age-hardening aluminum alloy: Process, structure and properties

Naturally aged AA6111 was successfully spot-welded to itself using vaporizing foil actuator welding in as-received condition as well as after three different heat treatments. Material weldability, mechanical properties, hardness distribution, and weld interface characteristics were evaluated using lap-shear tests, micro/nanoindentation, optical and scanning electron microscopy and energy dispersive spectroscopy analysis. Lap-shear tests indicated that even after aging and varied heat treatments all samples had repeatable button pullout failure, the failure being governed by the thinning and tearing of the base metal around the weld nugget. The strength of the welds and the energy absorbed corresponding to the peak load varied with the heat treatments. Microhardness measurements displayed significant hardening in the weld regions compared to base metal hardness. Optical and scanning electron microscopy revealed re-solidified molten zones in the weld interface, and the extent and size of these zones varied with the heat treatments. Nanoindentation showed softening in these zones signifying presence of weld defects in the weld interior.

Civilized explosive welding: Impact welding of thick aluminum to steel plates without explosives

In this paper, we report welding of 6.35 mm thick AA5052-H32 and AA1100-H14 plates to mild steel, which was achieved in a laboratory environment by vaporizing foil actuator welding (VFAW). An AA3003-H14 interlayer was used between the AA5052-H32 and the mild steel to improve weldability. The flyer velocity was measured by in-situ photon Doppler velocimetry, and the recorded data provides information about the impact condition and weldability of each material combination. Micro-indentation confirmed that there is no heat affected zone (HAZ) as most conventional welding techniques would produce. Mechanical testing showed that the lap shear strength is comparable to the ultimate tensile strength of the aluminum base plate. We demonstrate VFAW as a viable manufacturing technique for defense and other applications requiring thick aluminum to be joined to steel.

Flyer Thickness Effect in the Impact Welding of Aluminum to Steel

Impact welding is a material processing technology that enables metallurgical bonding in the solid state using a high-speed oblique collision. In this study, the effects of thickness of the flier and collision angle on weld interface morphology were investigated through the vaporizing foil actuator welding (VFAW) of AA1100-O to AISI 1018 Steel. The weld interfaces at various controlled conditions show wavelength increasing with the flier thickness and collision angle. The AA1100-O flier sheets ranged in thickness from 0.127 to 1.016 mm. The velocity of the fliers was directly measured by in situ photon Doppler velocimetry (PDV) and kept nearly constant at 670 m/s. The collision angles were controlled by a customized steel target with a set of various collision angles ranging from 8 deg to 28 deg. A numerical solid mechanics model was optimized for mesh sizes and provided to confirm the wavelength variation. Temperature estimates from the model were also performed to predict local melting and its complex spatial distribution near the weld interface and to compare that prediction to experiments.

A robust process-structure model for predicting the joint interface structure in impact welding

In this work, a numerical simulation framework based on the finite element method (FEM) with Eulerian formalism is developed to model the high-speed impact between metal plates. The model accounts for the thermomechanical interactions in the process and captures the complex interfacial deformations. A thorough validation of the model is achieved by comparing the wave characteristics obtained from numerical simulations with the experimental results from vaporizing foil actuator welding. The amplitude and wavelength of the interfacial waves and the resulting joint microstructure are shown to depend significantly on the process conditions and the specific material system, which is well captured by the model.

Interfacial characteristics of Ti/Al joint by vaporizing foil actuator welding

The 3003 aluminum alloy was welded to the pure titanium by a collision welding method called as vaporizing foil actuator welding (VFAW). By designing a groove die with special angles, it was found that the interfacial wave morphology transformed from irregular to regular, and recovered to irregular with increasing the angle. The best impact angle range was from 16°to 24° to generate the regular interfacial wave. By building the VFAW experiments with parallel standoff gaps, influence of standoff and input energy on interfacial morphology and mechanical properties were investigated. To generate obvious interfacial wave, the standoff was higher than 0.93 mm at constant input energy of 6.4 kJ, and the input energy was from 6.4 kJ to 8.0 kJ at constant standoff of 0.93 mm. A small amount of intermetallic compounds only appeared at troughs of the wavy interfaces with the maximum size of 3–6 μm. The interface has higher strength than the aluminum base metal in the range of the optimized processing parameters. The influence of the collision angle on the interfacial wave was clarified from the view of fluid dynamics. The relationship between the mechanical properties and the interfacial wave was discussed based on the impact velocity measured by a photonic Doppler velocimetry (PDV) method.

Dissimilar Impact Welding of 6111-T4, 5052-H32 Aluminum Alloys to 22MnB5, DP980 Steels and the Structure–Property Relationship of a Strongly Bonded Interface

The ability to weld high-strength aluminum to high-strength steel is highly desired for vehicle lightweighting but difficult to attain by conventional means. In this work, vaporizing foil actuator welding was used to successfully weld four Al/Fe combinations consisting of high-strength alloys: AA5052-H32, AA6111-T4, DP980, and 22MnB5. Flyer velocities up to 727 m/s were reached using 10 kJ input energy. In lap-shear testing, samples primarily failed in base aluminum near the aluminum’s native strength, showing that the welds were stronger than a base metal and that the base metal was not significantly weakened by the welding process. A particularly strong weld area was studied by transmission electron microscopy to shed light on the microstructural features of strong impact welds. It was found to be characterized by a continuously bonded, fully crystalline interface, extremely fine (nanoscale) grains, mesoscopic as well as microscopic wavy features, and lack of large continuous intermetallic compounds.

Benchmarking strength and fatigue properties of spot impact welds

An adaptation of vaporizing foil actuator welding (VFAW), a solid-state impact welding technique for producing similar JSC590R and 6061-T6 spot-welded joints was used in this study and the mechanical and fatigue properties of the joints were characterized using lap-shear testing, microhardness measurements, optical imaging and fatigue testing and the results compared with those of resistance spot welding (RSW) joints. Results indicated that the VFAW joints had improved strength and energy absorption compared to RSW joints for both the material combinations, however the strength of JSC590R-JSC590R VFAW welds were found to be lower than RSW when corrected for nugget size. Microhardness measurements in VFAW joints showed constant hardness values across the weld length signifying the absence of a heat affected zone, whereas softening was observed in RSW joints. Optical imaging revealed the homogeneity in microstructure of VFAW joints and the inhomogeneous distribution of the microstructure in RSW joints. For JSC590R members, fatigue resistance was equivalent for VFAW and RSW, whereas improved fatigue properties were observed for 6061-T6-6061-T6 VFAW welds. Fatigue strength scaled with static strength in a predictable way and was mostly affected by the shape and type of notch formed at the end of the nugget.

Joining sheet aluminum AA6061-T4 to cast magnesium AM60B by vaporizing foil actuator welding: Input energy, interface, and strength

Dissimilar joining of sheet aluminum AA6061-T4 to cast magnesium AM60B was achieved by vaporizing foil actuator welding (VFAW). Three input energy levels were used (6, 8, and 10kJ), and as a trend, higher input energies resulted in progressively higher flyer velocities, more pronounced interfacial wavy features, larger weld zones, higher peel strengths, and higher peel energies. In all cases, weld cross section revealed a soundly bonded interface characterized by well-developed wavy features and lack of voids and continuous layers of intermetallic compounds (IMCs). At 10kJ input energy, flyer speed of 820m/s, peel strength of 22.4N/mm, and peel energy of 5.2J were obtained. In lap-shear, failure occurred in AA6061-T4 flyer at 97% of the base material’s peak tensile load. Peel samples failed along the weld interface, and the AM60B-side of the fracture surface showed thin, evenly-spaced lines of Al residuals which had been torn out of the base AA6061-T4 in a ductile fashion and transferred over to the AM60B side, indicating very strong AA6061-T4/AM60B bond in these areas. This work demonstrates VFAW’s capability in joining dissimilar lightweight metals such as Al/Mg, which is expected to be a great enabler in the ongoing push for vehicle weight reduction.

Depiction of interfacial morphology in impact welded Ti/Cu bimetallic systems using smoothed particle hydrodynamics

Numerical simulations of high-velocity impact welding are extremely challenging due to the coupled physics and highly dynamic nature of the process. Thus, conventional mesh-based numerical methodologies are not able to accurately model the process owing to the excessive mesh distortion close to the interface of two welded materials. A simulation platform was developed using smoothed particle hydrodynamics, implemented in a parallel architecture on a supercomputer. Then, the numerical simulations were compared to experimental tests conducted by vaporizing foil actuator welding. The close correspondence of the experiment and modeling in terms of interface characteristics allows the prediction of local temperature and strain distributions, which are not easily measured.

Numerical investigation of CP-Ti & Cu110 impact welding using smoothed particle hydrodynamics and arbitrary Lagrangian-Eulerian methods

Vaporizing Foil Actuator Welding (VFAW) is a solid-state impact welding process where the material is deformed at high strain rates with severe plastic deformation occurring at the interface. Thus, conventional Lagrangian numerical simulation methods are not able to accurately model this process. In this study, two alternative numerical methods, Smoothed Particle Hydrodynamics (SPH) and Arbitrary Lagrangian-Eulerian (ALE), were utilized to investigate parameters that are difficult to measure experimentally, i.e., temperature, pressure, collision velocity, and plastic strain, during CP-Ti/Cu110 impact welding. The other parameters predicted by these two numerical methods such as wavelength and amplitude were used to validate the numerical results versus experimental observations. While both simulation methods can predict the wavy interface pattern created, the vorticities, jetting phenomenon, and molten zone can only be predicted by the SPH method, not the ALE method.

A Microsample Tensile Test Application: Local Strength of Impact Welds Between Sheet Metals

Microsample tensile testing was conducted to evaluate the quality of impact welds created by vaporizing foil actuator welding. Tensile test samples with a gauge length of 0.6 mm were electro-discharge machined out of welds created between 1-mm-thick aluminum alloy type 6061 (AA6061) sheets and 6-mm-thick copper (Cu110) plates. Aluminum sheets were used as flyers, while copper plates acted as targets. Flyer sheets in T6 as well as T4 temper conditions were utilized to create welds. Some of the welds made with T4 temper flyers were heat treated to a T6 temper. It was found that the welds made with T4 temper flyers were slightly stronger (max. of 270 MPa) than those produced with T6 temper flyers. Generally, failure propagated in a brittle manner across the weld interface; however, elemental mapping reveals material transfer on either member of the welded system. This work proves the feasibility to apply microsample tensile testing to assess impact welding, even when conducted with flyer sheets of 1 mm or less, and provides insight that is complementary to other test methods.

Interfacial microstructures and mechanical property of vaporizing foil actuator welding of aluminum alloy to steel

Different from traditional collision welding methods in driving principle, vaporizing foil actuator welding (VFAW) was developed to weld 3003 aluminum alloy (Al) to 4130 steel (Fe). The interfacial morphology and microstructures of VFAW were investigated, and the effect of the impact angle on interfacial morphology was analyzed through a groove die with a special angle. Results showed that irregular interfacial waves were formed between Al and Fe, confirming that the VFAW achieved metallurgical bonding. The range of impact angle forming interfacial irregular waves was from 8° to 24°, which meant the impact welding of 3003 Al to 4130 Fe has relatively wide weldable range of collision angle. The formation of irregular interfacial waves considered to be related to match between the density of the flyer and the ultimate static strength of the target. A very thin layer with intermetallic compounds, composed of continuous Al86Fe14 and massive Fe4Al13 and FeAl, was formed in the trough of interfacial waves. The layer with intermetallic compounds was formed by very thin liquid metal, which was the residual liquid metal of the jet flow along the interface. The highest anti-shear capacity of the joint exceeds 200N/mm.

Vaporizing foil actuator welding as a competing technology to magnetic pulse welding

Photonic Doppler velocimetry was applied to compare magnetic pulse welding and vaporizing foil actuator welding against each other in the form of lap joints made of 5000 series aluminum alloy sheets under identical experimental conditions which are: charging energies of the pulse generator, specimen geometry, initial distances between flyer and target plate. Impact velocities resulting from rapidly vaporizing aluminum foils were up to three times higher than those of purely electromagnetically accelerated flyer plates. No magnetic pulse welds were achieved, while every vaporizing foil experiment yielded a strong weld in that failure always occurred in the joining partners instead of in the weld seam during tensile tests. An analytical model to calculate the transient flyer velocity is presented and compared to the measurements. The average deviation between model and experiment is about 11% with regard to the impact velocity. Hence, the model may be used for the process design of collision welds generated by vaporizing foil actuators.

Arbitrary Lagrangian–Eulerian finite element simulation and experimental investigation of wavy interfacial morphology during high velocity impact welding

Finite element simulations of high strain rate forming processes are of significant interest to industry, but are challenging due to the coupled physics and dynamic nature of the processes. For instance, in high velocity impact welding, extremely high plastic strain regions develop. Thus, a traditional pure Lagrangian analysis is not able to accurately model the process due to excessive element distortion near the contact zone. In this study, the Arbitrary Lagrangian–Eulerian method (ALE) was used to simulate an impact welding process for two Al6061-T6 plates. The ALE method was able to numerically predict the temperature at the interface as well as the necessary process parameters to achieve a wavy morphology at the interface of the materials which is assumed to be a characteristic of a quality impact weld. This allowed the prediction of a weldability window, including separate regions for melting and purely solid state welds. To validate the proposed numerical model and wavy morphology regions, experimental tests with a Vaporizing Foil Actuator Welding (VFAW) process were conducted including Photon Doppler Velocimetry (PDV) measurements of the impact velocity. The numerical results were in good agreement with the experimental tests including the temperature prediction and interfacial melting observed through microstructural analyses.

Solid state impact welding of BMG and copper by vaporizing foil actuator welding

The objective of this study was to create impact welds between a Zr-based Bulk Metallic Glass (BMG) and copper at a laboratory scale and subsequently investigate the relationship between interfacial structure and mechanical properties. Vaporizing Foil Actuator (VFA) has recently been demonstrated as a versatile tool for metalworking applications: impact welding of dissimilar materials being one of them. Its implementation for welding is termed as VFA Welding or VFAW. With 8kJ input energy into an aluminum foil actuator, a 0.5mm thick Cu110 alloy sheet was launched toward a BMG target resulting in an impact at a velocity of nearly 600m/s. For this experiment, the welded interface was straight with a few BMG fragments embedded in the copper sheet in some regions. Hardness tests across the interface showed increase in strength on the copper side. Instrumented peel test resulted in failure in the parent copper sheet. A slower impact velocity during a separate experiment resulted in a weld, which had wavy regions along the interface and in peel failure again happened in the parent copper sheet. Some through-thickness cracks were observed in the BMG plate and there was some spall damage in the copper flyers. TEM electron diffraction on a sample, cut out from the wavy weld interface region using a focused ion beam, showed that devitrification of the BMG was completely avoided in this welding process.