Vaporizing foil actuator used for impulse forming and embossing of titanium and aluminum alloys

Electrically driven rapid vaporization of thin conductors is known to produce short-duration pressure pulses of high magnitude. This impulse can be used for applications such as high strain rate forming, shearing, collision welding, and springback calibration. Mechanical impulse was developed from aluminum foils of various thicknesses, which were vaporized using a capacitor bank discharge with a maximum charging voltage of 8.6 kV. Peak current was delivered on the order of 100 kA with rise times of about 12 μs. In this work, polyurethane was used as a medium to transfer pressure from the aluminum foil vaporization zone to the workpiece. Fundamental experiments, where AA 3003-H14 aluminum alloy was formed into perforated plates, show that for a given foil thickness, a limit existed over which supplying higher electrical energy from a given capacitor bank did not necessarily result in higher pressure. The magnitude of generated pressure was proportional to the excess Joule heat deposited into the foil before it burst. Although the polyurethane layer helped spread the pressure pulse over a larger area, the resulting pressure distribution remained heterogeneous. Practical applications, such as forming into cavities and embossing into shallow dies, were possible with this method. Sheets of 0.508 mm thick commercially pure titanium were nearly fully formed into a cellphone case die using a hybrid process that combined a quasistatic pre-forming step with a vaporizing foil forming step. Sheets of 0.508 mm thick AA 2024-T3 aluminum alloy were embossed into a die with features of varying depths. Aluminum foils with straight and curved active sections were used as actuators. The curved-section foils resulted in higher conformation of the workpiece to the die in the center region, while the straight-section foils produced better conformity to the die features on the ends.     

On the limit drawing ratio of magnetron sputtered aluminium–scandium foils within micro deep drawing

Two different foils out of the alloy aluminium-scandium with a thickness of about 15 μm were produced by the d.c. magnetron-sputtering process applying different substrate temperatures, i.e. S37 at the substrate temperature of 37°C and S160 at the substrate temperature of 160°C. They show different forming properties, e.g. flow stress. In this work these two different foils were used as blank material in micro deep drawing with a punch diameter of 0.75 mm to investigate the formability of these foils. A limit drawing ratio of 1.6 was achieved for both foils. Using the strip drawing test the friction coefficients between the foils and the tools were acquired experimentally, i.e. μ = 0.12 on the smooth side and μ = 0.16 on the rough side for the foil of S37 and μ = 0.15 on the smooth side and μ = 0.17 on the rough side for the foil of S160.     

Mechanism investigation for the influence of tool rotation and laser surface texturing (LST) on formability in single point incremental forming

Single point incremental forming (SPIF) is a new sheet metal forming process which achieves higher formability, greater process flexibility and reduced forming force compared to conventional sheet forming operations due to its characteristic of localized deformation. In recent years, a novel SPIF process assisted by localized friction heat is developed to further improve the material formability. Physically, the frictional heat is generated by the high relative motion at tool–workpiece interface resulted from tool rotation. However, the mechanisms behind formability difference induced by tool rotation at both low and high speed ranges are required to investigate in detail. In this paper, a series of experiments with an increase of tool rotation speeds ranging from 0 to 7000 rpm are conducted to form AA5052-H32 aluminum alloy sheets into a truncated funnel. Additionally, the obtained results are analyzed in terms of formability, forming forces and temperature trends to find out the different roles of friction and heat during the forming process. As a result, the formability behaviors at varying tool rotation speeds can be categorized into four stages according to different reasons. It indicates that friction is the dominant factor in low tool rotation speed range (0–1000 rpm) but will be substituted by thermal effect and potential dynamic recrystallization in high tool rotation speed range (2000–7000 rpm). Furthermore, due to the proved lubrication enhancement and hydrodynamic enhancement generated by surface textures, a laser surface textured forming tool is also utilized to show its influence on forming forces, measured temperatures and the corresponding formability. Finally, it demonstrates that the fabricated laser surface texturing (LST) is capable to reduce the friction at tool–workpiece interface and change the magnitude of heat generation.     

铝箔制品模具设计

通过介绍铝箔制品模具结构特点 ,即在1副立式级进模上、下模垂直方向上的不同位置分别完成落料、拉伸、成形、卷边等工序 ,解决了0.065～0.085mm厚铝箔制品在拉伸、卷边时的起皱及冲件的失稳问题 ,实现了在国产普通开式自动压力机上的自动生产 ,提高了生产效率。     

Forming of Micro Channels with Ultra Thin Metal Foils

The objective of this paper is to investigate the feasibility of producing micro scale structures by forming ultra thin metal foils. During this investigation, flat rolled foils of AISI 304 stainless steel (2.5μm in thickness) and pure copper (3.0μm in thickness) were formed into channels of varying shapes. The shapes of these channels were straight lines, concentric circles, crosses, and other curved shapes. The cross sections of the channels ranged from 10∼20μm wide and 5-10μm deep. Forming was done by cold isostatic pressing. Two types of micro dies were used. One was made of SU-8 photo resist on a Si wafer, the other of dry etched (DRIE) Si wafer. The die and metal foil were vacuum packed in a bag made of multilayered film. The forming was conducted with a 240MPa cold hydrostatic press. The formed structures were examined in terms of their dimensions, surface qualities, and potential for defect. The fabrication results show that the sheet metal forming process can be applied to the manufacturing of micro scale structures.     

Laser keyhole micro welding of aluminum foils to lap joints even with large gap sizes

The increasing miniaturization of components leads to increasing demands on manufacturing processes. High-brilliance lasers allow fast, low-heat deep penetration laser welding to produce narrow seams even in metal foils. In micro-joining, adjusting comparatively small gaps is a major challenge. Here, the welding of 100 µm thick aluminum foils in lap joints is investigated. Gaps of > 80% of the foil thickness were bridged. The results show that the seam width can be used to detect not only the gap size but also inner connection defects for quality control purposes. The explanation for this is the increasing gap-bridging keyhole depth, and thus heat input with increasing gap size.     

Electromagnetic reduction of a pre-formed radius on AA 5754 sheet

Electromagnetic (EM) forming is a high-speed forming process that uses the forces induced on a conductive workpiece by a transient high frequency current to form the workpiece into a desired shape. This paper presents the results of an experimental and numerical study carried out to determine whether EM forming techniques could be used to obtain sharper radii in aluminum alloy AA 5754 compared to that attained using conventional stamping process alone. AA 5754 1 mm sheet was formed into a v-shape with a 20 mm outer radius and then the radius was reduced or “sharpened” to 5 mm using EM forming. This “hybrid” process was modeled numerically to gain insight into the process and the challenges involved in the numerical simulation of the physical phenomena that are present in this process. As with any novel process, there are limitations and issues that must be addressed if this technique is to be implemented commercially; however; the research indicates that features that are not achievable using traditional stamping techniques can be obtained using EM forming.     

Phasengrenzreaktionen bei der Oxydation von Metallen. Hochtemperaturoxydation Dünner Nickelfolien

During the oxidation of nickel foils 5 μm thick in oxygen (1000–1200°C, oxygen partial pressure about 0·2–0·8 torr) a logarithmic rate law was observed. The apparent activation energy of the oxidation and the oxidation rate at the beginning of the reaction agree with the apparent activation energy and the double, extrapolated rate of the homomolecular oxygen exchange on NiO. Therefore the reaction of O₂ molecules with electrons at the interface may be discussed as the rate determining step of the high temperature oxidation of thin nickel foils.     

Electrically driven plasma via vaporization of metallic conductors: A tool for impulse metal working

Forming, cutting and welding of metal by impulse has significant advantages, in that short time scales change the fundamental nature of the forming process and short duration impulses can enable much lighter and more agile equipment because large static forces do not need to be resisted. Impulse forming is most commonly executed using electromagnetic forming. However, the application of electromagnetic forming is limited at high energies and large numbers of operations by the availability of long-lived electromagnetic coils (or actuators, as they are sometimes referred to). Low-cost, disposable actuators have been suggested as one method to counteract this issue. Here we propose the use of low-cost foils or wires that are intentionally vaporized by a pulsed electric current, in order to create an intense mechanical impulse. Applications including cutting, forming, and dimensional calibration are demonstrated and discussed. The available literature that could provide design guidance is reviewed. A simple cutting and welding experiment using a vaporizing aluminum foil is demonstrated. Further experiments study the expansion of simple copper tubes using the impulse developed from copper and aluminum wires that are vaporized using capacitor bank discharge with nominal charged voltages between 3.4 and 6.7 kV, and peak currents between 60 and 150 kA delivered with rise times on the order of 20 μs. This gives some guidance on how forming operations may be designed and, opens possible areas for further research.     

Factors affecting the oxidation behaviour of thin Fe-Cr-Al foils Part I: Effect of foil dimensions

The effect of foil dimensions on the life time of thin Fe-20Cr-5Al foils with a thickness ranging from 0.049 to 0.25 mm was investigated in air at 1100°C. It was found that the life time of the foils depends sensitively on the foil thickness. Specimens with a foil thickness of 0.091 mm and less suffered high oxidation rates until the planned test time of 3072 hours had expired. This so-called “Breakaway Corrosion” was caused by aluminium consumption, aluminium depletion of the foil and subsequent formation of non-protective, rapidly growing chromium and iron-rich oxides. The “Time to Breakaway” was calculated as a function of the foil thickness. The agreement between measured and calculated values is very good.     

Vaporizing foil actuator: A tool for collision welding

A method for implementing collision welding at moderate to small length scales has been developed. The flyer, instead of being driven by chemical explosives (explosive welding) or magnetic forces (magnetic pulse welding), is launched toward the target by the pressure created from the electrically driven rapid vaporization of a thin metallic conductor. Mechanical impulse is developed from 0.0762 mm thick aluminum foils, which are vaporized using capacitor bank discharge with nominal charging voltage of 5.5 kV and peak current on the order of 100 kA delivered with rise times of about 12 μs. Welding couples of copper–titanium, copper–steel, aluminum–copper, aluminum–magnesium and titanium–steel have been successfully created with the same input parameters such as foil geometry, input energy and standoff distance. Instrumented peel tests, lap shear tests and optical and scanning electron microscopy reveal a wide spectrum of both strengths and interface structures. Copper–titanium and copper–steel welds are strong and have characteristic wavy interfaces with little intermetallics or void formation. The other combinations are seen to have brittle interfaces with intermetallics and defects, with the collision welding parameters used presently. For the titanium–steel system, a thin nickel interlayer is introduced and all the layers are welded in a single experiment. Peel strength of the weld was observed to be quadrupled. Peak velocities of up to 560 m/s were obtained for titanium flyer sheets.     

Forming Limit Curves in Single Point Incremental Forming

New experimental data is presented on Forming Limits in Single Point Incremental Forming (SPIF), which is a sheet metal forming process which does not require dies. A Box-Behnken Design of Experiment is used to develop the experimental plan and analyze data. In former work, the most critical factors affecting Single Point Incremental Forming were found to be material type, material thickness, formed shape, tool size, and incremental step size. In this experimental work, new results are presented as graphical response surfaces which show the forming limit for all the critical factors listed previously. In addition, forming limits are presented in terms of Forming Limit Diagrams.     

成形参数对AZ31B镁合金板料渐进成形表面质量及温度的影响

介绍了镁合金摩擦生热渐进成形原理,并研究了在工具头行进速度为1000 mm·min^-1下,工具头主轴转速(1000~6000 r·min^-1)、轴向进给量(0. 5~3 mm)、成形角度、工具头半径、环境温度对厚度为2 mm的镁合金板料圆锥台零件成形性的影响。实验结果表明:随着主轴转速的增加,零件表面质量先升高后降低;零件表面质量随着轴向进给量的增加而降低;在成形极限角内,零件表面质量随着成形角度增加而提高;零件表面质量随着工具头半径增加先升高后降低;加工时环境温度对成形结果有影响。通过摩擦生热的方式对AZ31B镁合金板料加热,板料温度会随着主轴转速、轴向进给量和工具头半径递增而增加,成形角度对板料温度的影响不大。     

The coupling influence of size effects and strain rates on the formability of austenitic stainless steel 304 foil

In order to understand the coupling influence of size effects and strain rates on the formability of the austenitic stainless steel 304 foils in micro scale, a series of micro scale limited dome height (LDH) tests were designed and conducted in three different speeds without lubricant on the annealed and as-received austenitic stainless steel 304 foils. In this study, a technique was developed to coat a layer of pure chromium (≈0.3 μm thick) on the foils and by using the etching process to make the micro square grids (50 μm × 50 μm) on the foils. Then, the foils were annealed at different temperatures for obtaining different microstructures. A set of the forming limit curves (FLC) of the foils were obtained and they can be used by industry right away for product design, process design and development, die design, and simulations, etc. Besides, the coupling influence of the size effects and the strain rates on the formability of the austenitic stainless steel 304 foils has been studied, observed and understood.     

Structure formation and properties of rapidly quenched aluminum alloys

Aluminum alloys in a rapidly quenched state are studied by four technological methods, namely, fabricating foils and wire with unique properties, fabricating granulated alloys, surface strengthening and alloying articles by a laser or an electron beam treatment, and realizing superplasticity. In rapid quenching of alloys, crystallization occurs with cooling the melt at a rate of at least 10³–10⁴ K/sec. Such cooling can be realized in practice by several methods, for example, by drop crystallization on a rotating heat-conducting substrate, spraying of the melt in a cooling medium, quenching a thin foil on cooled rolls in liquid rolling, or melting a thin surface zone of a quite massive part. The present paper describes a study of flakes of aluminum alloys obtained by the method of drop crystallization. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 31–34, May, 1997.     

Non-contact estimation of thickness and elastic properties of metallic foils by laser-generated Lamb waves

Non-contact estimation of the thickness and elastic properties of metallic foils was attempted by quantitative analysis of velocity dispersion of laser-generated Lamb waves. Lamb waves were generated in stainless steel (AISI304) foils with a thickness of less than 40 μm by a Q-switched Nd:YAG laser. Both the zeroth order symmetric S₀ and anti-symmetric A₀ waves were monitored using a heterodyne-type laser interferometer. Dispersion of group velocity of the A₀ mode was obtained by the wavelet transformation, and was found to agree well with the numerical solution of the Rayleigh–Lamb equation. A modified method to estimate both the thickness and acoustic (or elastic) properties from the sheet wave velocity and the group velocity dispersion of the A₀ mode was proposed. The modified method was found to provide a correct estimate for stainless steel foils thinner than 30 μm.     

Microstructure and mechanical properties of ODS Ni-based superalloy foil produced by EB-PVD

Y₂O₃ dispersion strengthened Ni-based superalloy foil 0.1 mm thick was deposited by EB-PVD technology and followed by hot isostatic pressing (HIP) treatment. The phase composition and microstructure investigations on as-deposited and HIPed superalloy foils were performed by XRD, SEM and TEM. Columnar crystals formed on the evaporation side and equiaxed grains formed on the substrate side. Cross-section observation showed 150–300 nm grains of matrix with 10–25 nm particles of Y₂O₃ homogeneously dispersed in them. After HIP treatment, columnar crystals broke and turned into equiaxed grains. Little growth of oxide dispersoids was observed. The results of room temperature tensile tests indicated that the tensile and plastic properties of the foil after HIP treatment are both promoted with the ultimate tensile strength and elongation percentage reached the values of 1230 MPa and 0.92 in contrast with 725 MPa and 0.49 that of the as-deposited foil, respectively.     

Fundamental studies on the incremental sheet metal forming technique

The idea of incremental forming technique has been investigated for production of sheet metal components. With this technique, the forming limit curve (FLC) appears in a different pattern, revealing an enhanced formability, compared to conventional forming techniques. In the present study, the formability of an aluminum sheet under various forming conditions was assessed and difficult-to-form shapes were produced with the technique. By utilizing knowledge and experience obtained during the present study, it became possible to produce some free surfaces.     

Autonomous on-line system for fracture identification at incremental sheet forming

Before starting, the production forming processes require real experiments in order to accurately define forming limits. For this reason and because incremental sheet metal forming technology requires a relatively long production time, an autonomous on-line system for fracture identification has been developed. The system is a versatile tool for the identification of the location and time of the occurrence of the fracture, without human influences or oversight. The system is based on an investigation of the forming forces, responsive to very small variations, appearing during the forming process, and works effectively with different material types, material thicknesses and product shapes.     

Effect of annealing temperature on microstructure,texture and properties of molybdenum foil

The microstructure, texture and mechanical properties of 0.06 mm thick molybdenum foil after annealing at different temperatures were studied by micrograph and EBSD analysis. The results show that the grains of the cold-rolled molybdenum foil along the rolling direction are fibrous. As the annealing temperature increasing from 750 °C to 1000 °C, the average layer thickness of these fibrous grains gradually grows. And the relative frequency of low-angle grain boundaries increases, while the relative frequency of high-angle grain boundaries decreases. Also, the main texture of the unidirectional-rolled and annealed molybdenum foils is {112}〈110〉 which content continuously increase to 62% till 1000 °C. Therefore, the strength of RD, TD and 45°RD direction decreases but the elongation increases. During these processes, the mechanism includes classical nucleation and coarsening of subgrain and with annealing temperature rising, the latter dominates the recrystallization process. After annealing at 1050 °C, the molybdenum foil undergoes secondary recrystallization. The grain boundary distribution becomes diffused and the main texture changes into {001}〈110〉, which the fraction is as high as 96%, thus, the mechanical properties of RD, TD and 45°RD exhibit sharp decrease simultaneously.