Effects Of Rheocasting And Heat Treatment On Microstructure And Mechanical Properties Of A356 Alloy

As a king of hypoeutectic Al-Si alloy, A356 alloy (Al-7Si-Mg) draws more and more attention because of its high specific strength and dimensional stability. Also A356 alloy has got the extensive application in automotive industry and aerospace industry. On the other hand, rheocasting technology possesses wide prospects as a new forming technology. In this paper, investigation of improvement effects of rheocasting process and T5 heat treatment on A356 alloy was carried out. And the effects were studied through the microstructures and mechanical properties.First of all, the temperature range of solids-liquids states was obtained through solidification curve of A356 alloy. It's about 560℃~630℃. DSC technique was utilized for studying thermal properties of A356 alloy. The result shows that there are four exothermic peaks during solids-liquids temperature. And for the purpose of ascertaining solid fraction of A356 alloy slurry, microstructures of alloy ingots quenching at different temperature(560, 570, 580,590,600,610,620, 630℃) were observed. It shows that solid fraction increases from 0% to 100% with increasing temperature between 560℃~630℃. What's more, the temperature of 50% solids fraction was probably between 600℃and 610℃. Accordingly, parameters of semi-solid slurry making and rheocasting were determined.Liquids casting method was used to prepare semi-solid slurry, and rheocasting experiments were carried out at different temperatures (580,590,600,610,620℃) and different pressure mode (stationary, rotation once and rotation thrice). Rheo-casting specimens and liquid-casting specimens were obtained accordingly. For the sake of contrast, as-casting specimens were prepared. Optical microscope (OM) and Scanning electron microscope (SEM) were performed to observe the microstructural variation, and EDS was utilized to measure elemental composition. The microstructures of rheocasting are finer and closer comparing to as-casting because the pressure can break the dendrite branch. Meanwhile, the microstructures of rheocasting specimens around 600℃~610℃were more round and homogeneous than rheocasting specimens at 620℃. Besides, rotation pressure can crush the dendrite branch completely, especially rotation thrice.After T5 heat treatment, hardness tests, tensile tests and compressive test for all specimens were utilized to evaluate the mechanical properties, and they prove that rheocasting specimens have better mechanical properties than as-casting specimen, especially rheocasting specimens around 600℃~610℃. XRD analysis was used to identify the phase and measure the change of lattice parameter. Furthermore, the result shows that T5 heat treatment can strengthen A356 alloy. However, the plasticity reduces at the same time. And then the mechanism of strengthening for T5 heat treatment was investigated.