Extrudable Al-Si-Mg Alloys: Simulation of Microsegregation and Homogenization

Extrudable Al-alloys of the 6xxx series are subjected to a homogenization treatment prior to extrusion in order to remove inhomogeneities generated during casting. Microsegregation of elements and phases is developed as a result of the solidification process. During homogenization, several phenomena take place such as the dissolution of various phases, the transformation of iron intermetallics, spheroidization of the remaining intermetallics, and reprecipitation during cooling. All these phenomena affect the extrudability of the material. An integrated simulation of microsegregation and homogenization is described. Microsegregation is simulated with the application of the Scheil-Gulliver model, employing computational thermodynamics. A Dual Grain Model has been developed for the simulation of homogenization, taking into account the variability of the grain size in the as-cast material. In this way, it is possible to simulate the dissolution of Mg2Si and the transformation of iron intermetallics concurrently. The results of the simulations provide a deeper understanding of the effects of processing on alloy microstructure and can be used toward the design of the homogenization process of extrudable Al-alloys.

Effects of solution temperature on mechanical properties of 319.0 aluminum casting alloys containing trace beryllium

The effects of beryllium (Be) and solution temperature on the morphologies of iron intermetallics, silicon particles, and copper intermetallics, relative to the mechanical properties of 319.0 alloys, were investigated. The experimental results indicated that adding Be to the alloy can raise the Al–Al2Cu eutectic melting temperature, change some plateletlike shape (βndash;Al5FeSi) of iron intermetallics to comparatively harmless Chinese-script morphologies (α–Al8Fe2Si), and reduce the amount and average length of βndash;Al5FeSi platelets. During high-temperature solution treatment (>500 °C), the thinner and smaller the β–Al5FeSi platelets were, the faster they dissolved and fragmented. However, where the solution temperature exceeded the Al–Al2Cu eutectic melting point, the Al–Al2Cu eutectic melted and resulted in an ultrafine eutectic phase on quenching, which deteriorated mechanical properties. The fracture behavior of 319 alloy was affected by the morphologies of the iron intermetallics, silicon particles, and copper intermetallics. Fractographic analysis of tested compact tension specimens revealed that the fracture processes were mainly initiated by void nucleation at β–Al5FeSi platelets as a result of their cracking and decohesion from the matrix. Adding Be to the 319.0 alloy and optimizing the solution temperature could significantly decrease the number of fracture-initiation sites of β–Al5FeSi platelets and improve the tensile properties and fracture toughness.