In the machine manufacturing, generally, the concept of processing means the change of the shape, dimensions, aspect and eventually of the structure of the workpiece material, to obtain a piece part with specific properties for its usage.

For the processing methods resulting in material removal from the workpiece (subtractive processing methods), the general concept of machining is used. 

If in the case of the cutting processes, the principle of plastic deformation is considered as essential in material removal [1], in the case of nonconventional machining processes sometimes the principle of plastic deformation is also appreciated as significant, but most of them are based on local temperature increase or/and chemical reactions development in order to remove the excess material from the workpiece [2].

As a consequence of the pressure applications, of the significant temperature increase or of the chemical reactions development, major changes could arise in the polycrystalline surface layer affected by the machining process. From the manufacturer point of view, it is important that the workpiece layer affected by the processing method is as thin as possible.

In the cases of the so-called classical machining, essentially the workpiece material is pressed by the tool cutting edge up to the moment when a shear phenomenon appears and develops, determining the chips formation and respectively the material removal from the workpiece. In the case of some of the nonconventional machining processes, the transfer of an additional quantity of energy, eventually strongly focused on the work zone, determines the melting and even vaporizing the material from localized zones; thus, a process of material removal will develop.

Thus, in the machined surface layer, the polycrystalline grains whose structure or dimensions were affected by the machining could be observed. The amplitude of the changes determined in the surface layer by the effects of pressure, heating/cooling or/and chemical reactions could be highlighted by examining the surface layer. These changes are influenced by the main phenomena that occur during the machining process and by the intensity of these phenomena. For example, some of the subtractive processes able to illustrate the above-mentioned effects could be the chip cutting, the electrical discharge machining, the laser beam machining.

In order to develop a comparison among the effects of such machining processes on the polycrystalline surface layer, some processes able to generate material removal by classical and nonconventional machining methods were analyzed. One selected processes of abrasive disc machining, electrical discharge machining using plate-type tool electrodes and laser beam machining. The investigation of the polycrystalline superficial layer obtained as a result of the machining process facilitated the highlighting of the changes generated in this layer.

References

1. Haddadi, H., Bouvier, S., Banu, M., Maier, C., Teodosiu, C. Towards an accurate description of the anisotropic behaviour of sheet metals under large plastic deformations: Modelling, numerical analysis and identification. International Journal of Plasticity 22, 2006, 2226–2271

2. Slătineanu, L., Nagîţ, G., Dodun, O., Coteaţă, M., Chinesta, F., Gonçalves-Coelho, A., Pamies Teixeira J., San Juan, M., Santo, L., Santos, F. Non-traditional manufacturing processes. Chişinău: Editura Tehnica Info, 2004