Effects of grain refining and modification on the microstructural evolution of semi-solid 356 alloy

Semi-solid metal (SSM) processing is gaining momentum as an effective alternative to the classical manufacturing processes of casting and forging. This category takes advantages of both liquid and solid forming processes. In the absence of shear force, semi-solid slug is similar to a solid, self standing, while with the application of shear force, the viscosity is reduced appreciably and the material flows like a liquid; Thixotropic-"dual characteristics where the SSM billet has solid-like characteristics, but flows like a liquid under shear stress."; For SSM processing of Al-Si alloys, there are two important microstructural features which influence the properties of finished products. These are the size and morphology of the primary alpha-Al phase and the eutectic mixture which mainly consists of silicon.; The aim of the current study was to investigate the effects of inoculation and modification, individually and combined, on the microstructural evolution of Al-7%Si group of alloys with special emphasis on the 356 alloys in two different processes, i.e. conventional and rheocasting using the ALCAN patented process, Swirled Enthalpy Equilibration Device, SEED technology.; One binary and two commercial Al7% Si alloys have been used for the entire research plan. After melt preparation, different master alloys, depending on the objective, were added to the melt and the prepared alloy was poured and examined in two different setups to correlate a relation and highlight the differences between the conventional and SSM processes. Several techniques were used in order to characterize the obtained material. These techniques include thermal analysis, optical microscopy, quantitative metallography using image analysis technique, scanning electron microscopy, electron probe microanalysis, x-ray mapping, and rheological tests with a parallel plates compression machine.; Based on the SSM processing as a focal point, it is proposed that partitioning of the solidification range is also essential. Therefore the solidification range could be divided into two main intervals; the temperature range for the formation and growth of alpha-Al dendrites (DeltaTalpha) and/or iron intermetallics and the temperature range for the main Al-Si eutectic and secondary-eutectic reactions including the formation of M9ZS1 and/or iron compounds. The main emphasis in this context is the alpha-Al solidification range since it has the paramount importance in the science of SSM processing.; The grain refiner results have shown that generally refiner increases the alpha-Al percentage due to the shifting of the cooling curve up and consequently a wider alpha-Al solidification range would be obtained. From SSM standpoint, larger alpha-Al mushy zone provides more flexibility for slurry production and control however side issues such as the risk of hot tearing, porosity formation, and segregation should be considered. Quantitative metallography on the samples prepared from billets reveals that by refining, the alpha-Al percentage increases and due to the higher number of efficient nuclei per unit area the globule size decreases. Improvement of sphericity and size reduction of globules are the main advantages of refiner addition. Limited rheological tests proved that smaller, and rounder particles give better slurry flowability.; It has been shown that small addition of Sr in the range of 150-200ppm is quite adequate to change the silicon morphology from flakes to fibrous in both processes. By modification, alpha-Al solidification range increases due to the lowering of the main eutectic reaction temperature and interestingly by considering a constant total solidification range, eutectic solidification range decreases. In contrast to the SEED refined samples, by modification, the average globule diameter increases however the higher sphericity percentage is one of the main advantages of the Sr addition. It has been proposed that lowering the surface tension by Sr is the m...