**1. Introduction**

Trend of miniaturization of everyday devices increases industry demand for efficient produc‐ tion of miniature parts. Machining production technologies of small-dimension elements by turning, milling, and polishing are well known for a long time. However, these methods are not efficient enough for the great demand for small and handy devices. This makes engineers to search for new methods of microelements manufacturing or to adapt traditional ones for the requirements of miniaturization.

Microforming is an adopted technology of production of small parts by metal forming. This process is characterized by good productivity, high dimensions accuracy, proper surface smoothness, high material usage, and good mechanical properties of manufactured items,

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which makes it a good alternative to machining. Excepting benefits, microforming also brings some limitations, for example, limited forming possibilities of deformed materials or narrow shapes of obtained elements. Approaches of microforming methods are presented in **Figure 1**.

**Figure 1.** Microforming methods [1].

Comparing microforming to traditional forming process, it is obvious that while going to a microscale some process parameters, such as grain size or surface structure, keep constant [2]. Relationships between the dimensions of treated items and morfometric parameters of their microstructure and surface geometry, in billets as well as in tools, are different in macro- and microscale. This leads to the formation of the size effect phenomenon. In the available technological knowledge relating to conventional macroscale forming methods, presence of size effects does not permit direct application into microforming of metals [3–6]. Microforming is defined as the forming of the part features with at least two dimensions in the submillimeter range [3]. **Figure 2** presents some microparts made by microforming processes.

**Figure 2.** Mechanical microparts formed by microforming [7, 8].

**Figure 3** presents all the issues which need to be considered in microforming system. There are four factors influencing the material deformation: tool-workpiece interface condition, grain size, workpiece size, and element feature size [7]. These factors further affect the efficiency of microforming system and the quality of manufactured items influencing on such a process parameters as: deformation load, forming stability (scatter of the process variables), defects of deformation, dimensional accuracy, mechanical properties, and the quality of achieved surface.

**Figure 3.** Issues related to size effect in microforming system [7].

The primary problem connected with microforming is the so-called "size effect" ensuing from same miniaturization. The occurrence of unpredictable changes in process parameters, while treating similar scaled workpieces, is called size effect [9]. These effects distinguish described process from conventional methods of metal forming, and significantly influence on the possibilities and limitations of this technology. Sources of size effect formation can be divided into two groups [2, 6]: physical—related to the workpiece size and the forces affecting the process; structural—induced by the microstructure of the material.

Physical sources:

which makes it a good alternative to machining. Excepting benefits, microforming also brings some limitations, for example, limited forming possibilities of deformed materials or narrow shapes of obtained elements. Approaches of microforming methods are presented in **Figure 1**.

Comparing microforming to traditional forming process, it is obvious that while going to a microscale some process parameters, such as grain size or surface structure, keep constant [2]. Relationships between the dimensions of treated items and morfometric parameters of their microstructure and surface geometry, in billets as well as in tools, are different in macro- and microscale. This leads to the formation of the size effect phenomenon. In the available technological knowledge relating to conventional macroscale forming methods, presence of size effects does not permit direct application into microforming of metals [3–6]. Microforming is defined as the forming of the part features with at least two dimensions in the submillimeter

range [3]. **Figure 2** presents some microparts made by microforming processes.

**Figure 2.** Mechanical microparts formed by microforming [7, 8].

**Figure 1.** Microforming methods [1].

4 Modeling and Simulation in Engineering Sciences


Structural sources:

**•** Grain size to element thickness size effect—the grain size of metallic materials results from the material properties and determined by the casting condition, the thermal and mechan‐ ical treatments. It is impossible to obtain each material with each grain size, thus the grain size cannot be scaled down in parallel to the element dimension. **Figure 4** schematically shows microformability of polycrystalline and amorphous materials.

**Figure 4.** Microformability of polycrystalline and amorphous material [10].

In conventional metal forming operations all treated materials are considered as homogene‐ ous. In microforming processes they are heterogeneous, because of relatively large grains size to the billet volume. As the grain size increases, the grain structure of the billet becomes to be more heterogeneous and the material shows anisotropic behavior in the submillimeter dimensional range. The anisotropic behavior of the workpiece material can directly change local deformation mechanisms as shown in **Figure 5**.

**•** Surface structure scalability (SSS) size effect—similarly to grain size, it is often not possible to reduce the die and billet surface roughness with the element dimension, due to that the surface structure scalability is the source of size effects. In lubrication as well as in dry conditions, SSS leads to a size-dependent friction behavior.

**Figure 5.** Backward extrusion of a billet composed of heterogeneous grain structure [3].

Microforming is a relatively young technology, developed in the last two decades, but more and more research centers in the world deal with it. Together with reports of experimental work results, attempts of identified numerical simulating phenomena are undertaken and presented.
