Effects of Si Content and Forging Pressure on the Microstructural and Mechanical Characteristics in Semi-solid Forging of Al-Si-Mg Alloys

In this study, semi-solid forging (SSF) using electromagnetic stirring of Al-Si-Mg alloys was carried out. To evaluate the acceptability for producing a semi-solid slurry of an alloy with various Si contents, the temperature sensitivity of the solid fraction and solidification range were calculated by Thermo-Calc. The effects of the Si content and forging pressure on the microstructural and mechanical properties, such as tensile strength and elongation, were ascertained. Specimens were taken from two typical positions on the semi-solid-forged products, where one position corresponds to a region of directly applied forging pressure and the other region corresponds to the position of slurry being squeezed and extruded. Microstructural characteristics such as average grain size, volume fraction, and form factor of the alpha-Al were evaluated as the Si content was changed from 5 to 7 wt pct at 0.5 wt pct intervals. As the Si content increased, the particle size of alpha-Al and tensile strength increased, while the volume fraction of alpha-Al and elongation decreased. At lower Si contents, solidification shrinkage was observed, resulting in a decrease in the elongation. At 5 and 6 wt pct Si, alpha-Al particles were agglomerated, and shrinkage was observed. Uniform, fine, and globular microstructures were obtained at 6 wt pct Si. At 6 and 7 wt pct Si, alpha-Al particles were coarsened. The forging pressure was changed from 100 to 250 MPa, and at a forging pressure of 250 MPa, shrinkage was observed. As the forging pressure was increased from 100 to 200 MPa, the tensile strength and elongation increased, and the particle size of alpha-Al decreased. At a forging pressure of 250 MPa, alpha-Al particles were deformed and agglomerated, which decreased the strength.