Superplastic Zn-22 Research Articles

Injection Molding of Plastic Plates Combined with Boss Features on Superplastic Zn–22Al Sheet

Abstract This paper presents the development of a hybrid process combining forming of a superplastic zinc-aluminum sheet with injection molding. The product is a plastic part covered with a superplastic zinc-aluminum layer used mainly for electromagnetic interference (EMI) shielding. This paper investigates the moldability of Zn/Al sheet covered plates with various bosses. The effects on the moldability of various parameters, including: melt temperature, injection pressure, sheet thickness, various boss shapes and boss location in the mold, are investigated. It is found that bosses formed with thick Zn/Al sheet and located close to melt entrance exhibit good moldability. Among boss geometries, boss depth is the most critical parameter; a little increase in boss depth sharply lowers the mold-ability. Large angle improves moldability. To help interpret the effects of geometrical parameters, sheet deformation is observed by measuring the deformations in printed-grids and thickness. The base circle of the boss is found to be the most severely deformed region. The thickness reduction ratio in the center of base circle could help predicting moldability.

Effect of temperature on the corrosion behavior of a superplastic Zn–Al–Ag alloy in acid rain solution

Effect of temperature on the corrosion behavior of a superplastic Zn–Al–Ag alloy in acid rain solution

Micro Deep Drawability of the Superplastic Zn–22Al Alloy at a High Strain Rate and Room Temperature

Abstract This study aimed to investigate the micro deep drawability of the Zn–22Al alloy at room temperature in which it shows superplastic properties. To this goal, first the two-step equal channel angular extrusion (ECAE) process was carried out to obtain an ultra-fine-grained structure (UFG). Upon achieving the grain size of 200 nm, the formability of the alloy at room temperature and at a high strain rate was investigated both experimentally and numerically. Micro deep drawing experiments were performed at different deep drawing ratios (1.66, 1.84, 2.0, and 2.25) and for different sheet thicknesses (0.2, 0.4, and 0.6 mm). The finite element model of the micro deep drawing was also established to assess and compare the thickness variation in deep drawn parts. Results showed that the superplastic Zn–22Al alloy has a great potential in microforming applications. It was also noted that the limiting drawing ratio can be obtained as high as 2.25 in the room temperature condition.

Effects of grain size on room temperature deformation behavior of Zn–22Al alloy under uniaxial and biaxial loading conditions

Effects of grain size on room temperature (RT) deformation behavior of superplastic Zn–22Al alloy under uniaxial and biaxial loading conditions were investigated. Two–step equal channel angular pressing (ECAP) and subsequent annealing processes were applied to the alloy in order to obtain microstructures with various grain sizes ranging from submicron to micron sizes. Grain size of 200nm was achieved after ECAP of the alloy, and it was increased up to ~2.60µm by annealing at 250°C for different time periods. Changes in deformation behaviors of the alloy with various grain sizes were found to be in good agreement under the uniaxial tensile and biaxial Erichsen test conditions. Increasing grain size decreased both the maximum elongation under uniaxial deformation and limiting dome height (LDH) under biaxial deformation. The high elongation to failure and LDH values for the samples with submicron grain size were attributed to their high strain rate sensitivity.

Microstructure Development and Superplasticity in a Zn-22% Al Eutectoid Alloy Processed by Severe Plastic Deformation

Severe plastic deformation (SPD) is an attractive processing method for refining microstructures to have ultrafine grain sizes within the submicrometer or even the nanometer levels. In SPD, the most promising techniques are equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). A conventional superplastic Zn-22% Al eutectoid alloy was processed by ECAP and HPT. Experiments were conducted to demonstrate the evolution of hardness and microstructure and the enhancement of superplastic properties of the Zn-Al alloy after processing by the SPD techniques. In addition, flow mechanisms of the Zn-22% Al alloy are discussed by utilizing a deformation mechanism map.

The significance of grain boundary sliding in the superplastic Zn–22 % Al alloy processed by ECAP

A Zn–22 % Al eutectoid alloy was processed by equal-channel angular pressing (ECAP) to reduce the grain size to ~0.8 μm. Tensile testing at 473 K showed superplastic characteristics with a maximum elongation of ~2230 % at a strain rate of 1.0 × 10−2 s−1. The significance of grain boundary sliding (GBS) was evaluated by measuring sliding offsets at adjacent grains from the displacements of surface marker lines in samples pulled to elongations of 30 % at a series of different strain rates. The highest sliding contribution was recorded under testing conditions corresponding to the maximum superplastic ductility. There were relatively large offsets at the Zn–Zn and Zn–Al interfaces, but smaller offsets at the Al–Al interfaces. Analysis shows the results are affected by the presence of agglomerates of similar grains which are present after ECAP processing and specifically by the increased fraction of Al–Al boundaries. The experimental results are in excellent agreement with the predictions of a deformation mechanism map depicting the flow behavior in the Zn–22 % Al alloy, and the results confirm the importance of GBS as the dominant mechanism of flow in superplasticity after processing by ECAP.

Corrosion behaviour of superplastic Zn–Al alloys in simulated acid rain

The influence of aluminium content on the corrosion behaviour of superplastic Zinc–Aluminium alloys immersed in simulated acid rain was investigated. ZA4, ZA8, ZA12, and ZA16 alloys were used for the test. The Al-rich phase was prone to preferential attack when the superplastic Zn–Al alloy was immersed for 5d. However, with increasing ratios of the Al phase, the corrosion rates of the four samples decreased in the order ZA4>ZA8>ZA12>ZA16. The corrosion rate of the alloy decreased with increasing Al content, which may be related to the distribution of the Al-rich phase. Corrosion kinetic parameters were also calculated.

Multiple necking in superplastic Zn–22Al under uniaxial tension

The phenomenon of multiple necking in superplastic Zn–22Al was studied experimentally and analytically. Experiments revealed a proportional relation between the wavelength of multiple necking and the square root of the ratio between the strain rate and strain, which is explained on the basis of diffusion of defects under inhomogeneous strain distributions. The order of magnitude of the diffusion coefficient and the activation energy suggest that strain diffusion can result from the flow of atomic vacancies accompanied by grain boundary sliding in the superplastic state. Moreover, the analytic derivation shows the relation between the present model and second-order strain gradient theory.

Post-characteristics of Formed Zn-22 mass%Al Alloy to Seismic Damper for General Residence

It has been reported that superplastic Zn-22 mass%Al alloy has some excellent properties needed for seismic damping devices. In addition, seismic damping devices using this alloy has already been put into practical use for a high-rise building. In the present investigation, we tried to apply this alloy to seismic damping devices for a general residence and examined the formability and the characteristics after the formation. No cracks were observed in the sample formed even at room temperature. The surface roughness, R a , was improved from 0.602μm before the formation to 0.418 μm after the formation. As the results of FVM analysis, it was predicted that the formed sample could exhibit the higher stress and strain at both sides of the inferior part of the gauge section. However, the average grain size, the ratio of the major axis length to minor axis one in a grain, and the hardness in the region exhibiting high stress and strain were almost the same characteristics as those in the initial sample.

Flow behavior of a superplastic Zn–22% Al alloy processed by equal-channel angular pressing

A Zn–22% Al eutectoid alloy was processed using equal-channel angular pressing (ECAP) to reduce the grain size to ∼0.8 μm. A maximum elongation of ∼2230% was recorded in tensile testing at 473 K at a strain rate of 1.0 × 10 −2 s −1. The significance of grain boundary sliding was evaluated by measuring the offsets in surface marker lines for samples pulled to an elongation of 30% at three different strain rates. The highest sliding contribution was recorded under testing conditions corresponding to the maximum superplastic elongation. Detailed measurements showed that relatively high sliding offsets were recorded at the Zn–Zn and Zn–Al interfaces whereas the offsets were smaller at the Al–Al interfaces. The measurements show that grain boundary sliding is a major flow process during superplastic deformation in the Zn–22% Al alloy processed by ECAP.

Experimental and numerical study on deformation behavior in dieless drawing process of superplastic microtubes

Abstract Microtubes are commonly used and required for micro components in micro system technologies (MST) and micro electro-mechanical systems (MEMS), for example, micro nozzles, painless needles and micro reactors. To fabricate microtubes, superplastic dieless drawing, i.e., dieless drawing process utilizing superplastic characteristics, has been developed. Compared with conventional processes, superplastic dieless drawing has great advantages of not requiring tools such as dies, plugs and mandrels and high flexibility. Furthermore, the process has a unique deformation behavior so called “geometrical similarity law in cross section” that diameter ratio d/D maintains a constant value in the process. In this study, its mechanism for satisfying the geometrical similarity law in cross section is focused on experimentally and numerically. In the experiment, superplastic Zn–22%Al and AZ31 magnesium alloys tubes with an outer diameter of 2 mm and a wall thickness of 0.5 mm were used as tubular workpieces. A high-frequency induction heating apparatus with an air-cooling nozzle was used for dieless drawing. The deformation behavior of a microtube in superplastic dieless drawing was investigated using finite element method (FEM) with coupled thermo-mechanical analysis. The validity of FE modeling was verified by a comparison with the experimental results. The experimental dieless drawing results imply that the geometrical similarity law in cross section with the minimization of dimensions is satisfied by this process. Furthermore, it was clarified from the FEM results and analytical results that uniaxial stress state and isotropic material are essential factors for satisfying the geometrical similarity law in cross section in the process. From these results, the mechanism of the geometrical similarity law in cross section in this process was clarified, and the effectiveness of this process was demonstrated for microtube fabrication.

Experimental study on multi-pass dieless drawing process of superplastic Zn–22%Al alloy microtubes

Abstract The fabrication process of superplastic microtubes using multi-pass dieless drawing is studied experimentally. The superplastic material used in this experiment is a Zn–22%Al alloy tube with an outer diameter of 2 mm and wall thickness of 0.5 mm. A high-frequency induction heating apparatus with an air-cooling nozzle is used for the dieless drawing. As a result, after four-pass dieless drawing, a microtube with outer and inner diameters of 190 and 91 μm, respectively, is fabricated successfully. It is confirmed that the ratio of inner to outer tube diameters remains a certain constant value in multi-pass dieless drawing. In addition, microtubes fabricated by this process are evaluated by their microstructure and surface roughness. Consequently, it is found that the manufacture of initial tubes with fine grains and high accuracy is essential for fabricating superior microtubes.

Relationship between deformation behavior and microstructural evolution of friction stir processed Zn–22 wt.% Al alloy

Zn–22 wt.% Al alloy, which exhibited room-temperature superplasticity, was refined by friction stir processing (FSP) and its deformation behavior was investigated and compared with that of superplastic Zn–22 wt.% Al alloy. The deformation mechanism at room temperature was the same in both alloys. However, at a high strain rate the elongation-to-failure of the friction stir processed material was significantly larger than that of the base material. It was considered that the microstructural evolution due to FSP influenced the deformation behavior of the friction stir processed material.

On applying superplastic Zn-22 wt.% Al alloy with nanocrystalline grains to general residential seismic dampers

Recently, a superplastic damping device, which is using Zn-22 wt.% Al alloys with an ultra-fine (nanometer order) grain size, has been put into practical use for a high-rise building. For the purpose of the development and mass-production of the damping devices for a general residence, we carried out an investigation about the design of the most appropriate damper's shape and the capability of superplastic forming. According to the results of a finite volume method analysis, the higher stress and strain at both sides of the inferior part of the gauge section were observed, but the strain was relatively small. No deterioration of mechanical properties due to the grain growth and cavitation during the forging was observed after the forging.

The significance of grain boundary sliding in the superplastic Zn–22% Al alloy after processing by ECAP

Specimens of the superplastic Zn–22% Al eutectoid alloy were processed by equal-channel angular pressing (ECAP) through a total of eight passes at 473 K. Tensile specimens were cut from the as-pressed billets and tested at two different strain rates at 473 K under conditions where there is superplastic flow. Marker lines were placed on the polished surfaces of the specimens prior to testing and these lines were used to take measurements of the grain boundary sliding offsets at elongations of ∼30%. The measurements are used to estimate the contribution of sliding to the total strain and they show that grain boundary sliding is the dominant deformation mechanism under these conditions.

Low cycle fatigue behavior of Zn–22mass%Al alloy exhibiting high-strain-rate superplasticity at room temperature

Low cycle fatigue tests on rolled and extruded superplastic Zn–22mass%Al alloy under large plastic strain were carried out at room temperature, with a view to its application as a seismic damping material. Crack initiation and propagation were strongly affected by the microstructure. The extruded alloy exhibited longer fatigue life, extensive enough for use in seismic damping materials.

Study on practical application of a new seismic damper using a Zn–Al alloy with a nanocrystalline microstructure

Superplastic Zn–Al alloy has some excellent properties needed for a seismic damper, such as high ductility, low work hardening and no harmful metal to human health. In addition, superplastic materials with ultra-fined grains exhibit the high-strain-rate superplasticity and/or low-temperature superplasticity. Some investigations were carried out for nano-sized Zn–Al alloy in order to develop a high-performance seismic damper capable of replacing conventional dampers, such as low-yield-point steel. First, the design for appropriate structure as a damping device using superplastic Zn–Al alloy was discussed by FEM analysis. Secondly, a bulk Zn–Al alloy with nanocrystalline microstructure was manufactured by thermo-mechanical controlling process (TMCP) technology, and the tensile properties of this alloy were compared with that of low-yield-point steel. Then, the full-size cyclic testing of the damping device was carried out in order to evaluate the response for earthquake. As a result, an ecological and high performance seismic damper, the so-called “maintenance-free seismic damper”, has been successfully developed and put into actual use in a high-rise building.

Analysis on the anelasticity of a superplastic Zn–22% Al eutectoid

The kinetics of anelastic deformation of a superplastic Zn–22%Al was examined to provide the systematic information on the anelastic behavior of fine grained superplastic materials showing the extraordinary high relaxation strength compared to large grained materials. For the present analysis, the strain transient data of a superplastic Zn–22%Al after unloading, which was reported in the literature, were reappraised by taking the instantaneous remaining anelastic strain as a driving force causing anelastic deformation. The results showed that anelastic behavior of the alloy could be described by the empirical description similar to that of forward superplastic deformation. In addition, anelastic strain rate was found to be insensitive to the forward applied stress prior to unloading. The anelastic behavior of the alloy was manifested by the presence of three distinct regions in the double logarithmic plot of anelastic strain rate against remaining anelastic strain according to the values of the dependency of anelastic strain rate on remaining anelastic strain, r, the grain size sensitivity, s, and activation energy, Q. Region I (long time after unloading) was characterized by r≈1, s≈5 and Q≈76 kJ mol −1. In region II (intermediate time after unloading), the values of s and Q were similar with those of region I, but r was about 3. The results of the analysis indicated that region III (short time after unloading) could be attributed to the athermal process. Activation energy close to that for forward superplastic deformation and the high value of grain size sensitivity in region I and II are indicative of the important role of grain boundary diffusion on anelastic deformation of fine grained superplastic materials. In addition, with the aid of the grain boundary back tension model for anelastic deformation, a relationship between remaining anelastic strain and the back stress resulting from grain boundary back tension was derived.

FRACTURE TYPES AND THICKNESS DISTRIBUTION IN SUPERPLASTIC SHEETS FORMED WITH PLASTIC INJECTION MOLDING

This paper investigated the fracture types and thickness ratio distribution in superplastic Zn–22% Al sheets formed during a hybrid process combining superplastic forming with plastic injection molding. Three types of sheet fractures (edge crack, central crack, and combined crack) were observed. The effects of using this approach on sheet molding and fracture window for various parameters, including melt temperature, injection pressure, and mold temperature, were investigated. They are presented and discussed as they relate to molding area and various fracture types. Central cracks occurred when superplastic sheets were formed by injection molding at higher melt temperature, whereas edge cracks occurred at higher injection pressure. When melt flow was parallel to the sheet rolling direction, areas of edge crack were enlarged. The sheet thickness ratio distribution was obtained for various injection parameters and rib depths. Observation of sheet thickness distribution for variation parameters, and the tendency for fracture can be generalized.

Injection molding of ribbed plastic plates with a superplastic Zn–22% Al sheet

AbstractThis article presents the development of a hybrid process combining the forming of a superplastic zinc–aluminum sheet with injection molding. The product is a plastic part covered with a superplastic zinc–aluminum layer used mainly for electromagnetic interference (EMI) shielding. This hybrid process involves not only molding a plastic plate, but also forming a superplastic zinc–aluminum sheet with features such as ribs. This research observes the process of filling the cavity and forming the superplastic sheet with the aid of short‐shot experiments. By understanding the molding and forming process, guidelines for process control will be derived. This research further investigates the moldability of SIM systems. To make satisfactory parts, sheet thickness, melt temperature, injection parameters, and other process parameters determine the “moldability” of a specific system. The first part of the moldability study is to identify the relative importance of these process parameters. Results show that melt temperature and injection pressure are the two most critical parameters. The second part of this moldability study is to develop the moldability diagrams based on the molding area on the plane of these two key parameters. Moldability diagrams are employed to evaluate the effects of sheet thicknesses and melt inlets. It is found that a thick sheet, high melt temperature, and an inlet providing short flow length improve moldability. © 2001 John Wiley & Sons, Inc. Adv Polym Techn 20: 216–225, 2001