Product miniaturization is an emerging trend for facilitating product usage, enabling unique product functions to be implemented in micro-scaled geometries and features, and further reducing product weight and volume. Recently, a demand for microparts increased significantly in many industry clusters. Development of the advanced micromanufacturing technologies for fabrication of such microparts has thus become a critical issue. Microforming, which offers attractive characteristics including high productivity, low cost and good quality of the formed parts, provides a promising approach to fabricating metallic microparts. In the last two decades, a lot of effort has been made to the researches on size effect related deformation behaviors in microforming process and the development of the process. Having a panorama of these researches is necessary to support micropart design and development via microforming, and further advance this micromanufacturing process. In this paper, an intensive review on the latest development of microforming technologies is presented. First of all, the paper is focused on the review of the size effect-affected deformation behaviors and the mechanisms of the changes of flow stress, flow behavior, fracture behavior, elastic recovery, tooling–workpiece interfacial friction and the surface finish of the formed parts. The state-of-the-art microforming processes, including micro deep drawing, microembossing, micropunching, microcoining, microextrusion, microheading, and micro progressive forming are then presented. Finally, some research issues from the implementation of mass production perspective are also discussed.

A review on the state-of-the-art microforming technologies

Plastic deformation at the macroscopic scale has been widely exploited in industrial practice in order to obtain desired shape and control the requested properties of metallic alloy parts and components. The knowledge of deformation mechanics involved in various forming processes has been systematically advanced over at least two centuries, and is now well established and widely used in manufacturing. However, the situation is different when the physical size of the workpiece is scaled down to the micro-scale (µ-scale). In such cases the data, information and insights from the macro-scale (m-scale) deformation mechanics are no longer entirely valid and fully relevant to µ-scale deformation behavior. One important reason for the observed deviation from m-scale rules is the ubiquitous phenomenon of Size Effect (SE). It has been found that the geometrical size of workpiece, the microstructural length scale of deforming materials and their interaction significantly affect the deformation response of µ-scale objects. This observation gives rise to a great deal of research interest in academia and industry, causing significant recent effort directed at exploring the range of related phenomena. The present paper summarizes the current state-of-the-art in understanding the geometrical and microstructural SEs and their interaction in deformation processing of µ-scale components. The geometrical and grain SEs in µ-scale deformation are identified and articulated, the manifestations of the SE are illustrated and the affected phenomena are enumerated, with particular attention devoted to pointing out the differences from those in the corresponding m-scale domain. We elaborate further the description of the physical mechanisms underlying the phenomena of interest, viz., SE-affected deformation behavior and phenomena, and the currently available explanations and modeling approaches are reviewed and discussed. Not only do the SEs and their interaction affect the deformation-related phenomena, but they also induce considerable scatter in properties and process performance measures, which in turn affects the repeatability and reliability of deformation processing. This important issue has become a bottleneck to the more widespread application of µ-scale deformation processing for mass production of µ-scale parts. What emerges is a panoramic view of the SE and related phenomena in µ-scale deformation processing. Furthermore, thereby the outstanding issues are identified to be addressed to benefit and promote practical applications.

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A review of geometrical and microstructural size effects in micro-scale deformation processing of metallic alloy components

Assessment of local deformation using EBSD: Quantification of local damage at grain boundaries

The current work investigated the influence of heat treatment (T4, T5 and T6) on high cycle fatigue (HCF) behaviour of the as-extruded Mg–10Gd–3Y (GW103, wt.%) alloy. The five heat treatment conditions applied to the as-extruded GW103 alloy were under-aging (T5, 225 °C × 4 h), peak-aging (T5, 225 °C × 10 h), over-aging (T5, 225 °C × 250 h), solution treatment (T4, 500 °C × 4 h), solution treatment plus artificial aging (T6, 500 °C × 4 h + 225 °C × 10 h), respectively. The results showed that T5 heat treatment can improve the fatigue behaviour of the GW103 alloy, while T4 and T6 heat treatments are detrimental to the fatigue behaviour of the GW103 alloy. The 10 7 cycles fatigue strength of the GW103 alloy made by peak-aging was improved by approximately 10%, compared with the as-extruded. While, the fatigue strength of the under-aged and over-aged GW103 alloys have no obvious difference, although fatigue life of the under-aged GW103 alloy is slightly higher than that of the over-aged GW103 alloy at high stress amplitude. The extruded-T4 and extruded-T6 GW103 alloys exhibit similar fatigue strengths of 110 MPa. The influence mechanism of heat treatment on HCP behaviour of the as-extruded GW103 alloy was discussed.

Influence of heat treatment on fatigue behaviour of high-strength Mg―10Gd―3Y alloy

With increasing industrial interest in high alloyed multicomponent and High Entropy Alloy (HEA) systems the integration of solid solution strengthening in the ICME framework for efficient Materials Design becomes an important translator tool A general model is proposed that performs as the framework for an extensive assessment of solid solution strengthening coefficients The model assumes the concentration dependence of x 2 / 3 as proposed by Labusch but gives a non-linear composition dependence to the strengthening parameter yielding a better description for concentrated alloys To calibrate the model, 895 alloy systems, including a wide range of elements, have been used giving a good agreement between calculated and experimental values Additionally, a promising method is proposed to represent the temperature related softening in the investigated systems

Modelling of solid solution strengthening in multicomponent alloys

A study of the surface deformation behaviour at grain boundaries in an ultra-low-carbon steel

An attempt to experimentally study deformation characteristics around grain boundaries and to analyze the presence of strain gradients is presented. The evolution of surface profiles is studied by atomic force microscopy (AFM) at relatively small strains. The results indicate that this method can be used to draw conclusions about the deformation characteristics, e.g. in large grains the surface profile seems to vary within a grain. This latter effect can be seen as an indication of the inhomogeneous deformation occurring within large grains. The results are also compared with FEM calculations using a non-local crystal plasticity theory that incorporates strain gradients in the hardening moduli.

Comparison of surface displacement measurements in a ferritic steel using AFM and non-local plasticity

In this paper, a discrete-obstacle and a collective model of solution hardening are compared. In the former model, interaction between a dislocation and a single solute is considered, while in the latter, the dislocation line interacts collectively with a row of solutes. Model predictions are compared with experimental data for CuMn and NbMo single crystal systems and a NiC polycrystal system. The collective model gives better results for the two fcc systems while the bcc system cannot be explained well by either. The limitations and modelling capability of the two approaches are discussed with respect to the experimental information.

Solid solution hardening — a comparison of two models

A two-dimensional micromechanical model based on the finite element method is presented to model two-phase ferritic/pearlitic steels, by aid of generalised plane strain elements. A periodic represe ...

Micromechanical Modeling of Two-Phase Steels

The strengthening effect of grain boundaries is well established and observed experimentally as the Hall-Petch relationship. In this paper different mechanisms proposed in the literature to explain the observed Hall-Petch effect are reviewed critically. The fundamental implications of the different approaches are discussed with reference to experimental data for two different classes of materials; -Materials with locked dislocations, i.e. with a sharp yield point behaviour. -Materials without locked dislocations, i.e. with a smooth yielding behaviour. It is shown that a simple model (Bergstrom) can be used to understand the grain size strengthening in the latter class of materials while more work is needed to quantitatively understand the behaviour of materials showing a sharp yield point.

Dilip Chandrasekaran

Papers

A study of the surface deformation behaviour at grain boundaries in an ultra-low-carbon steel

Comparison of surface displacement measurements in a ferritic steel using AFM and non-local plasticity

Solid solution hardening — a comparison of two models

Micromechanical Modeling of Two-Phase Steels

Aspects of Grain Size Strengthening in Polycrystals