Experimental and numerical investigations on forming limit curves in micro forming

Material behavior modelling in micro/meso-scale forming process with considering size/scale effects

Abstract: Size effects make most know-how of traditional macro forming inappropriate for the micro forming process. Material behavior greatly varies in micro forming process with the decreasing of the scale. The purposes of this paper are to analyze the influence of material size effects and establish a martial model in micro/meso-scale. By combining surface model with theories of single crystal and polycrystal, a mixed material model, which contains a size dependent term and a size independent term, is proposed in this paper. It finds that the flow stress of material in micro/meso-scale is between that of single crystal model (lower bound) and polycrystal model (upper bound). Based on a mixed material model, the influence of size effects is discussed in both micro bulk forming process and micro sheet forming process. At the end, the validity of the mixed model is approved by comparing the simulations with the experimental results.

Microdeep drawability of very thin sheet steels

Abstract: The purpose of the present study is to clarify the microdeep drawing characteristics of very thin sheet steels of below 0.2 mm thickness. An experimental study was conducted with specially developed experimental apparatus. In this study, a relative punch diameter D p / t ( D p : punch diameter, t : thickness of sheet material) was chosen as an important experimental parameter and ranged from 10 to 100. Drawability was evaluated, and the similarity laws in deep drawing were examined with sheet steels of 0.05, 0.1, 0.2 and 1.0 mm thickness. For D p / t from 10 to 100, ordinary drawability was obtained for very thin sheet steels without a large amount of blank holder pressure, and a decrease in value of limiting drawing ratio was observed with an increase in D p / t . The similarity law of thin sheet metal drawing is adapted above D p / t =40 because the drawing mechanism below D p / t =20 should differ from the above D p / t =40. The differences are also revealed in the shape of drawn cups. For D p / t =10 and R d / t =5.0, bending is the dominant forming mechanism since blank holder pressure has little effect on drawability and no effect on the working material during the latter half of the drawing process. Therefore, the drawing behavior seems similar to that in drawing thick sheet metals and is characterized as microdeep drawing of very thin sheet metals.

Microforming: Experimental Investigation of the Extrusion Process for Micropins and its Numerical Simulation Using RKEM

Abstract: Microforming using a small machine (or so-called desktop machine) is an alternative new approach to those using full-size heavy equipment for manufacturing microparts. Microparts are commonly defined as parts or structures with at least two dimensions in the submillimeter range, which are used extensively in electronics and micromechanical products. However when scaling down a conventional forming process to microscale, the influence of the so-called size effect needs to be considered. The individual microstructure (size, shape, and orientation of grains) and the interfacial conditions show a significant effect on the process characteristics. In this paper, the process of extrusion is investigated to establish it as a viable process for microforming. A forming assembly is fabricated and used in conjunction with a loading substage to extrude micropins with a final diameter of I mm. The effect of grain size is investigated by using workpieces heat treated to produce grain sizes varying from 32 μm up to 211 μm. Two extrusion dies with different roughness are used to study the effect of surface finish. While experiments lead to interesting questions and new discoveries, theoretical or numerical solutions are necessary tools for process optimization. Here, knowing the limits of the current widely used numerical simulation tools [i.e., the Finite Element Method (FEM)], a new method, the Reproducing Kernel Element Method (RKEM), has recently been developed to address the limitations of the FEM (for example, remeshing issue), while maintaining FEM's advantages, e.g., the polynomial reproducing property and function interpolation property. The new RKEM method is used to simulate the microextrusion problem. Its results are compared with that obtained from the FEM and the experiment result. Satisfactory results were obtained. Future directions on the experimental and simulation work are addressed.

A Review on Micro-manufacturing, Micro-forming and Their Key Issues

Abstract: Micro-manufacturing has received good attention globally in terms of its manufacturing methods/processes. One of the most popular micro-manufacturing processes is micro-forming. Many efforts have been focused on micro-forming, mainly on the micro-stamping process due to the process itself contributing numerous products, especially in its conventional macro-process. Most every-day products are made by this process. Although there were efforts made to realize micro-forming for industrial application, the technology itself was seen as being insufficiently mature. Much development work needed to be done, specifically to develop a fully-automated high-volume production micro-forming machine, which is reliable and at all times ready for operation in terms of it processes, tooling, and material-handling to ensure the successful production of micro-products. The paper addresses key issues encountered by researchers worldwide on both micro-manufacturing, specifically micro-forming.

Experimental and numerical evaluation of forming limit diagram for Ti6Al4V titanium and Al6061-T6 aluminum alloys sheets

Abstract: The forming limit diagram (FLD) is a useful concept for characterizing the formability of sheet metal. In this work, the formability, fracture mode and strain distribution during forming of Ti6Al4V titanium alloy and Al6061-T6 aluminum alloy sheets has been investigated experimentally using a special process of hydroforming deep drawing assisted by floating disc. The selected sheet material has been photo-girded for strain measurements. The effects of process parameters on FLD have been evaluated and simulated using ABAQUS/Standard. Hill-swift and NADDRG theoretical forming limit diagram models are used to specify fracture initiation in the finite element model (FEM) and it is shown that the Hill-swift model gives a better prediction. The simulated results are in good agreement with the experiment.