空化水射流微成形工艺的数值模拟研究

提出一种空化水射流冲击金属钛箔微成形的新工艺,该工艺利用空化水射流中空泡溃灭产生的高压冲击载荷,实现金属箔材的微成形。采用ANSYS/LS-DYNA数值模拟方法,以100 μm厚的TA2钛箔作为微成形研究对象,对空化水射流冲击微成形性能进行研究。通过模型建立、定义材料及属性、定义单元类型及接触、划分网格、边界条件及力的加载等技术处理,对金属钛箔的成形形貌、成形件中心点运动等进行数值模拟,为空化水射流冲击微成形机制和进一步试验研究提供依据。     

Microforming at elevated temperature - forming and material behaviour

Manufacturing of metallic parts by forming methods is industrially widespread due to several advantages like good surface quality, high accuracy and good efficiency at concurrent high quantity. As a result of the steady miniaturisation of products, large quantities of smallest metallic parts with the above mentioned attributes are needed. Despite the advantages of forming methods, microparts are mainly produced by machining, because of problems caused by so-called size-effects. These effects occur by scaling down geometry and process parameters, leading to the fact that the existing know-how for conventional processes cannot be transferred unrestrictedly to microscale. One reason for the difference between macro- and microscale is the number of grains within the forming area. At microscale only a small number of grains are directly involved in the forming process, so that the single grain, characterised by its individual size, orientation and position, gains influence on the process. The stochastic distribution of the grain characteristics leads to an inhomogeneous material behaviour and causes an increased scatter of the process parameters. To minimise the effect of inhomogeneous material behaviour, microforming at elevated temperature is applied. Experiments with different materials at elevated temperature show a homogenising effect which leads to a reduced process scattering. This indicates that elevated temperatures are suitable to minimise and control the size-effects at microforming processes. Additionally an enlargement of the forming limits by microforming at elevated temperature is observable.     

Metal direct nanoimprinting for photonics

In this paper metal direct nanoimprinting (embossing) for the production of metallic microparts is discussed, with a main focus on its suitability for the fabrication of metal-containing optical devices such as photonic crystals, plasmon waveguides or chiral structures. Silver and gold were chosen, since they have the lowest light absorption in the near infrared and visible range. They are also easily formable due to their good ductility, which can be further enhanced by processing at elevated temperature. The mold material, which may form part of the optical device, usually consisted of silicon, but other dielectric materials such as silicon oxide and silicon nitride were also successfully tested.Cylindrical and line-shaped holes with lateral dimensions down to 250 nm and aspect ratios of up to 5 were etched in silicon wafers. All the structures were successfully filled with silver and gold, and the filling of smaller dimensions is also deemed possible. This technique is therefore suitable for producing metal-containing optical devices working in the infrared, at least down to the standard telecom wavelength of 1.5 μm, which requires metallic dimensions of 200-300 nm.     

Key Problems in Microforming Processes of Microparts

From the viewpoint of production engineering, microforming is considered as an effective process to fabricate various microparts. Several key problems in microforming processes were investigated. A new microforming apparatus with a high stiffness piezoelectric actuator as the punch driver was developed to produce microparts.To improve the forming abilities and locate the billets, a floating microdie was designed. The size effects of the billets and die cavities on the microforming abilities were studied with upsetting and coining tests, respectively.And the isothermal microforming process of microgears was performed with the developed microforming apparatus. Several analysis methods were used to evaluate the forming quality of the microparts.     

Effects of superimposed high-frequency vibration on deformation of aluminum in micro/meso-scale upsetting

Micro/meso forming is an economically competitive process for the fabrication of miniature metallic parts. However, the size effects observed when scaling down the process leads to challenges such as tribological problems at the interface and premature tool wear due to localized stress concentration regions. In this study, a hybrid micro/meso forming assisted by high-frequency vibration and the contributing mechanisms of the high-frequency vibration were investigated. The effects of high-frequency vibration on the improvement of surface finish, decrease of the friction at the die-specimen interface, and reduction of forming stress were analyzed and discussed based on the vibration-assisted upsetting experiments. In addition, finite element analysis was conducted to help understand the significance of the vibration. Results show that a transverse vibration of 9.3 kHz led to surface roughness (Ra) reduction from 1.5 μm to 0.9 μm at the top surface, friction coefficient decrease from 0.14 to 0.07 at the punch/specimen interface, and nearly 50% reduction in the forming stress. The findings of this study confirmed the significant benefits of high-frequency vibration applied in micro/meso forming of metallic materials and provided a basis to understand the underlying mechanisms of vibration-assisted forming.     

The influence of die geometry and workpiece mechanical properties in T-Shape friction test

In this study, T-Shape friction test was redesigned to make it more suitable for application to microforming processes. Workpiece with aspect ratio (length/diameter) of 5 was proposed in order to ease workpiece handling. The die geometry was also modified from the original test to improve friction sensitivity especially within the range of friction factors commonly observed in metal forming. Geometric deviation of the die was simulated using Deform-2D to establish the acceptable tolerance for the fabrication. The effect of variation in workpiece mechanical properties on the test behavior was also investigated through Deform-2D simulation. Based on simulations on a 1 mm diameter copper workpiece, a tolerance of 0.01 mm (1% of workpiece diameter) was found to be the most suitable for the die fabrication. In addition, it was shown that variations in workpiece mechanical properties of up to 10% do not significantly influence the friction test results. Ultimately, T-Shape test experiment was conducted using copper workpieces to examine how the test complied with the friction behavior observed in the experiment.     

Friction behavior modeling and analysis in micro/meso scale metal forming process

Currently, a lot of know-how in conventional metal forming process cannot be directly applied to micro/meso forming processes due to so-called size effects. As a very important phenomenon in metal forming process, friction size effects are observed with an increasing degree of miniaturization. For microforming application, the input data of friction behaviors becomes critical to obtain accurate results for process simulation and traditional friction models are not reliable.     

FE-simulation of microforming processes applying a mesoscopic model

Basic research in the field of microforming has shown that scaling down the process from macro to microscale is subjected to so-called size effects in particular concerning flow stress and friction. As a consequence, process knowledge of conventional forming could not simply be transferred to microscale. To plan production processes with process simulation methods the knowledge of these size effects and their impact on the forming behavior is essential. For the identification of the size effects a large number of experiments have been carried out. With the detected effects a numerical mesoscopic model is being developed—based on the theory of metal physics—in order to simulate the forming behavior of microparts as well as the occurring scatter of the process factors.     

Plastic Yielding and Vibrations of Rolling Element Bearings

A high degree of resistance to plastic deformation at the contacts between rolling bodies and rings is required in bearings used for equipment that must be quiet running. It is shown that, within practical bearing design limits this resistance is primarily a function of material structure variables that affect the elastic limit.

Hardness, as measured by Rockwell or Diamond Pyramid methods is a primary variable, but in itself, an inadequate measure of this resistance. A test method is described giving good correlation with conditions causing plastic deformation of bearings in applications. The effects of residual stress, surface conditions and contact configuration are discussed.

It is shown that bearings with substantially improved resistance to plastic indentation are obtainable by the use of appropriate heat treatment.     

Micro-extrusion of ultra-fine grained aluminium

Microforming of normal, coarse grain (CG) metals leads to scale problems which originate from the fact that the grain size becomes comparable to the part size. A possible way of dealing with these problems is replacing CG metals with ultra-fine grained (UFG) metals. UFG metals can be produced in bulk by severe plastic deformation (SPD). This paper describes using UFG aluminium 1070 for preliminary trials of micro extrusion of a cylindrical cup. The process of producing bulk UFG aluminium by SPD is explained and the material obtained characterised. The preparation of micro billets for the extrusion operation is discussed. Backward extrusion is carried out for two types of material, CG and UFG. This enables a comparison of the material behaviour and product characteristics. Using UFG aluminium in microforming results in more uniform products with improved mechanical properties.