Foil Actuator Research Articles

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.

Microstructure Evaluation of Vaporizing Foil Actuator Welds of 1 GPa Strength Steel

Microstructure Evaluation of Vaporizing Foil Actuator Welds of 1 GPa Strength Steel

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.

A quick model for demonstrating high speed forming capabilities

High speed metal forming processes apply dynamic pressures on a sheet metal blank. To predict the forming capabilities inherent to these processes, suitable modeling approaches are of interest. Often, complex and large finite element (FE) simulations are necessary. A simplified novel mechanical approach, named chain model, is introduced and compared with experiments employing the innovative vaporizing foil actuator technology. It allows for a fast basic assessment of different material laws and dynamic loading conditions without the need of advanced soft- or hardware resources. The model consists of allocated masses connected by plastic segments. Despite its simplicity, efficiency, and some remaining challenges, the model predicts the experimental observation acceptably.For a next step, model results indicate that applying vaporizing actuators in a varied two-sided fashion might enable a toolless manufacture of some part geometries under free forming conditions.

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.

Experimental and Numerical Analysis of the Influence of Burst Pressure Distribution on Rapid Free Sheet Forming by Vaporizing Foil Actuators

Vaporizing Foil Actuators (VFA) can be employed as an innovative, extremely fast sheet metal forming method. An ultimate goal in forming technologies is generally to be flexible and rely on as few part-specific tools as possible. Therefore, various realizable VFA pressure distributions were investigated with a focus on the free forming result. Fundamental experiments including laser-based dynamic velocity measurements were conducted to discuss some key forming characteristics of the process. To compare more complex pressure distributions in a well-defined way, a numerical model was built. The strain rate dependency of the blank material was identified experimentally and incorporated in the model. It is shown that there are some VFA free forming capabilities in terms of creating certain part shapes, but only to a limited degree because relevant inertial forces can be present in regions where displacements would actually be either undesirable or wanted. Potential solutions to this are given at the end.

Multi-scale characterization and simulation of impact welding between immiscible Mg/steel alloys

Vaporizing foil actuator spot welding method is used in this paper to join magnesium alloy AZ31 and uncoated high-strength steel DP590, which are typically considered as un-weldable due to their high physical property disparities, low mutual solubility, and the lack of any intermetallic phases. Characterization results from scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) of the weld interface indicate that the impact creates an Mg nanocrystalline interlayer with abundant Fe particles. The interlayer exhibits intact bonding with both DP590 and AZ31 substrates. To investigate the fundamental bond formation mechanisms at the interface, a finite element (FE)-based process simulation is first performed to calculate the local temperature and deformation at the interface under the given macroscopic experimental condition. Taking the FE results at the interface as inputs, molecular dynamics (MD) simulations are conducted to study the interlayer formation at the Mg/Fe interface during the impact and cooling. The results found a high velocity shearing-induced mechanical mixing mechanism that mixes Mg/Fe atoms at the interface and creates the interlayer, leading to the metallurgical bond between Mg/steel alloys.

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.

High strength welding of Ti to stainless steel by spot impact: microstructure and weld performance

Vaporizing foil actuator (VFA) spot welding, a type of spot impact welding, was used to weld a titanium alloy (Ti-1.2ASN) to a stainless steel (436 SS). The interfacial microstructures and fracture surfaces were characterized using scanning electron microscopy (SEM). Lap shear tests that strained the samples to failure with digital imaging correlation (DIC) were conducted to study the mechanical performance of these welds. A mesh-insensitive structural stress method was used to understand the stress distribution and model the failure modes of VFA welds in ABAQUS. Despite experimental scatter in this developing joining method, most samples failed through the base metal, but multiple failure modes coexisted, including interface failure. These failure modes were used to classify the results. The failure process can be best understood through the lens of the spatial variation that is natural in this type of weld. The center is naturally unwelded, and there is an annulus of high strength material surrounding this unwelded zone that has a wavy morphology. The mesh-insensitive structural stress method could naturally provide a link between the joint structure and the mechanical properties of the spot impact welds. This could show that despite varied failure modes and nugget strength, strength itself is not usually affected adversely by the size of the central unbonded zone.

Process characteristics and interfacial microstructure in spot impact welding of titanium to stainless steel

Vaporizing foil actuator (VFA) spot welding was used to weld a titanium alloy (Ti-1.2ASN) to a stainless steel (436 SS) for the first time. The relation of process to microstructure was explored. Interfacial microstructure was studied through optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Chemical compositions of molten regions and jet zone were characterized by energy-dispersive X-ray spectroscopy (EDS). The microhardness across the interface was measured through Vickers and nanoindentation tests. The results show that an impact velocity of 800 m/s can be achieved at very low input energy. VFA spot welds present heterogeneity in microstructure at large and small length scales. At long length scales, several regions of weld can be seen. At the center there is no bonding; moving radially outward the collision angle increases from zero and this causes a transition from a flat area with considerable melting to a wavy region, and finally an area of accumulated jetted material in the corner. EDS analyses show that discontinuous intermetallics including TiCr2, Fe2Ti and FeTi, and Ti fragments coexisted in a mixed zone that probably has components of mechanical and liquid-state mixing. TEM results show that equiaxed nanograins exhibited in the recrystallization zone and mixed zone. There is an ‘impact affected zone’ near the interface where the hardness rises significantly. At the edges of the weld there is a mixed, and oxidized zone that supports the idea that solid-state impact jets the nascent surfaces away to allow interatomic bonding.

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.

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.

Joining Aluminium Alloy 5A06 to Stainless Steel 321 by Vaporizing Foil Actuators Welding with an Interlayer

Direct aluminium–stainless steel joints are difficult to create by the vaporized foil actuator welding (VFAW) method because brittle intermetallic compounds (IMCs) tend to form along the interface. The use of an interlayer as a transition layer between the two materials with vast difference in hardness and ductility was proposed as a solution to reduce the formation of the IMCs. In this work, VFAW was used to successfully weld sheet aluminium alloy 5A06 to stainless steel 321 with a 3003 aluminium alloy interlayer. Input energy levels of 6 kJ, 8 kJ, 10 kJ, and 12 kJ were used and as a trend, higher energy inputs resulted in higher impact velocities, larger weld area, and better mechanical properties. In lap-shear and peel testing, all samples failed at the interface of the interlayer and target. At 10 kJ energy input, flyer velocities up to 935 m/s, lap-shear peak load of 44 kN, and peel load of 2.15 kN were achieved. Microstructure characterization and element distribution were performed, and the results show a wavy pattern created between the flyer and interlayer which have similar properties, and the interface between the interlayer and target was dominated by element diffusion and IMCs identified mainly as FeAl3 and FeAl. The results demonstrate VFAW is a suitable joining method for dissimilar metals such as aluminium alloy and stainless steel, which has a broad and significant application prospect in aerospace and chemical industry.

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.