Grain Refinement Mechanism and Its Effect on Strength and Fracture Toughness Properties of Al–Zn–Mg Alloy

Abstract

Grain refinement has a significant effect on the mechanical properties. Grain refinement in cast Al–Zn–Mg alloy tends to reduce porosity, size and number of the pores. This improves the mechanical properties, specially fracture toughness. Inoculation effect is most important factor during feeding of liquid metal. For these reasons, most cast aluminium alloys are grain refined. Fluidity is another factor that increases formation of a coherent equiaxed dendritic network at the flow front. The degree of grain refinement is dependent on the level of scandium (Sc) concentration and the Zn/Mg ratio. Grain refiner namely Al–Sc master alloy is added in small amounts (>0.2 wt% Sc) to molten aluminium alloys to control the grain structure in castings. Al3Sc particles act as nucleating sites for the formation of primary α-Al dendrites and promote a uniform and fine equiaxed structure. In addition, friction stir processing (FSP) is the most innovative solid-state surface modification technology that eliminates cast heterogeneity and refines grains under the depth of the processed zone of aluminium alloys. The basic principle is based on friction stir welding (FSW). Thus, three main causes for fine grains are severe plastic deformation, heat input and dynamic recrystallization. Therefore, FSP should result in heterogeneous nucleation and growth of Al3Sc precipitates for diffusional mechanisms, which explains the rapid growth of precipitates in the processed zone. It has been experimentally proven that after as-cast (AC) + FSP condition aluminium alloy enhance mechanical properties such as 0.2% proof strength is 89.81 MPa, ultimate tensile strength is 187.1 MPa, elongation is 3.42%, and fracture toughness (K IC) is 25.10 MPa\( \surd \)m. In the present study, further evaluation has been done with optical microscopy, EPMA, FESEM, SEM, and TEM of Al–Zn–Mg–Sc alloy.