Friction stir processing (FSP) was used to modify the surface layer of the AZ91 magnesium alloy. The treatment was carried out using a jet cooling nozzle, generating a stream of cold air and enabling intensive cooling of the friction stir processed (FSPed) zone. Single-pass FSP was carried out using a tool rotational speed of 500 rpm and travel speed of 30 mm/min. The treatment was conducted using a truncated cone-shaped tool with a threaded side surface. Strong grain refinement and microstructural changes typical for FSP were found in all the samples. Very fine, equiaxed recrystallized grains dominated in the stirring zone. In the samples modified with the jet cooling nozzle, greater grain refinement was obtained than in the case of naturally cooled material. The average grain size in the surface part of the stirring zone was 1.4 μm and 9 μm in the samples with air-cooling and with natural cooling, respectively. Both the naturally cooled specimen and air-cooled specimen were characterized by a distinctly higher hardness than the base material. The average Vickers hardness in the stirring zone was 91 HV0.1 in the FSPed sample with the air-cooling system and 85.5 HV0.1 with natural cooling, respectively. The average Vickers hardness of the as-cast alloy was 64 HV0.1. Slightly higher wear resistance of the FSPed samples using a jet cooling nozzle was found in relation to the naturally cooled sample. Based on the conducted research, high efficiency of the jet cooling nozzle in cooling the modified zone during friction stir processing was found.

FSP (friction stir processing) technology is a modern grain refinement method that is setting new trends in surface engineering. This technology is used not only to modify the microstructure of the surface layer of engineering materials, but increasingly more often also to produce surface composites. The application potential of FSP technology lies in its simplicity and speed of processing and in the wide range of materials that can be used as reinforcement in the composite. There are a number of solutions enabling the effective and controlled introduction of the reinforcing phase into the plasticized matrix and the production of the composite microstructure in it. The most important of them are the groove and hole methods, as well as direct friction stir processing. This review article discusses the main and less frequently used methods of producing surface composites using friction stir processing, indicates the main advantages, disadvantages and application limitations of the individual solutions, in addition to potential difficulties in effective processing. This information can be helpful in choosing a solution for a specific application.

The samples of the as-cast AM60 magnesium alloy were subjected to Friction Stir Processing (FSP). The effect of FSP on the microstructure of AM60 magnesium alloy was analyzed using optical microscopy and X-ray analysis. Besides, the investigation of selected properties, i.e. hardness and resistance to abrasion wear, were carried out. The carried out investigations showed that FSP leads to more homogeneous microstructure and significant grain refinement. The average grain size in the stirred zone (SZ) was about 6-9 μm. in the thermomechanically affected zone (TMAZ), the elongated and deformed grains distributed along flow line were observed. The structural changes caused by FSP lead to an increase in microhardness and wear resistance of AM60 alloy in comparison to their non-treated equivalents. Preliminary results show that friction stir processing is a promising and an effective grain refinement technique.