Macro-Segregation Characteristics in Semi-Solid Forging of a High Strength Al-4.8Si-0.7Mg Alloy

Abstract

Semi-solid forging of a high strength Al alloy was carried out to investigate the macro-segregation characteristics in regard to the evolution of solidification microstructures. The evolution of segregation was closely affected by the feeding behavior of semi-solid slurries. Two types of macro-segregation were found in semi-solid forging: one is composed of fine dendritic ¡-Al particles caused by liquid segregation and the other is the residual eutectic segregation. The formation of these segregations was strongly affected by the quality of semi-solid slurries, such as the shape and the size of the primary ¡-Al particles. In the case of coarse dendritic microstructures, ¡-Al particles are easily interlocked with each other, resulting in the formation of macro-segregation. In the case of fine and uniform globular microstructures, primary globular ¡-Al particles can be readily filled into a die cavity during forging, leading to uniform microstructures without the formation of macro-segregation. Optimization of process parameters was carried out both for semi-solid slurry making and for semi-solid forging. The T6 heat treatment was carried out, and hardness distribution on the semi-solid forging specimens was also evaluated to investigate the effect of macro-segregation on mechanical properties.

Comparison of Microstructure, Density and Shrinkage Porosity for Casting and Rheocasting of AlSi7Mg alloy

“…The sensitivity of the solid fraction and solidification range, which are essential parameters for the usability of a semi-solid method for alloys with varied Si concentrations, were assessed using thermodynamic estimations [21,22]. The slope of the solid fraction vs temperature curve is described as temperature sensitivity, which is written as dfs/dT [23]. The solid fraction has a high-temperature sensitivity, which indicates that a slight temperature change generates a huge change in the solid fraction, making it difficult to control the solid-liquid melt and get the required microstructure [24].…”