Papers by Keyword: Mushy Zone

Abstract: In this study the effect of zirconia (ZrO₂) nanoparticles (40 nm) in reinforcing A356 aluminum alloys as a base metal matrix were investigated. Zirconia nanopowders were stirred in the A356 matrix with different fraction ratios ranging from (0, 1, 2, 3, 5%) by weight at variable stirring speeds ranging from (270, 800, 1500, 2150 r.p.m) at a mushy zone (600°C) and liquid state (700°C) using a constant stirring time for one minute. The microstructure revealed the change of grains from dendritic to spherical shape with increasing stirring speed. The Scanning Electron Microscopy of the fractured surface revealed the presence of nanoparticles at the interdendritic spacing of the fracture surface and was confirmed with EDX analysis of these particles.The results of the study showed that the mechanical properties (strength, elongation and hardness) using ZrO₂ as reinforcements were increased at the following parameters: 1500 r.p.m stirring speed in semi-solid state (600oC) and adding 3 wt.% of ZrO₂.

Abstract: A model for simulating mushy zone resolidification in a temperature gradient is presented. For describing macroscopic mass transport in the liquid phase in the mushy zone, an extended diffusion equation is solved numerically using the Finite Difference Method. Temperature dependent local equilibria at each position in the mushy zone are calculated using the thermodynamic software package ChemApp. The resolidification model treats multicomponent alloying systems and accounts for multiphase equilibria. Simulation results for peritectic Cu-40wt%Al and eutectic Al-5wt%Si-1wt%Mg alloys are compared with microstructures from temperature gradient annealing experiments. It is shown that the model is well suited to predict mushy zone resolidification in multicomponent and multiphase alloys. The predicted evolution of the liquid fraction is qualitatively in full agreement with the observed microstructures, including local remelting at the peritectic temperature prior to resolidification, an effect that was first predicted by the model and confirmed by the experiments.

Abstract: Gradient annealing experiments of a near-eutectic AlCu30 alloy with artificial stirring induced by a rotating magnetic field (RMF) of 6 mT were performed. The specific surface area of the primary phase was measured on metallographic sections perpendicular to the sample axis with a fixed amount of fraction solid. The variation of the specific surface area with fluid flow is compared to flow free experiments: the specific surface area varies as the inverse cube root of annealing time if no RMF is applied, but varies as an inverse forth root at 6 mT. The experimental procedure and results are presented in detail and discussed with models of convective coarsening of dendrites.

Abstract: Continual improvement of product quality has been a long challenge to Semi-Solid Metal (SSM) technology. By conventional semi-solid processes, this might be attained at the expense of economical production. The advent of Inclined Cooling Plate (ICP) process has already realized the development of non-dendritic SSM while satisfying qualitative, quantitative and economical requirements collectively. In spite of its potential advantages, functional mechanisms of this process are not yet clearly understood that makes its optimal utilization obscured. Basically, such understanding needs a picture of the process. As the first step, this picture is pursued through physical modeling of the ICP process i.e. direct observation of an analog system by virtue of transparent character of a model alloy (succinonitrile-acetone). Based on this phenomenological model, a picture of the process is presented as follows: flowing molten alloy down ICP, multiple regions form typically on the plate i.e. a chilled layer at the vicinity of the plate surface, a two-phase mushy zone on the chilled layer and ambient liquid far from the plate surface. In this process, interaction of the liquid forced-flow with mushy zone separates solid particles from the stationary mush on the plate resulting in a two-phase mixture which is responsible for the formation of slurry i.e. SSM.

Abstract: The potentials of Mg-Al system alloys for semi-solid forming have been extensively evaluated these years. In the present studies, mushy zone properties of the Mg-Zn system alloys, i.e. ZWx2 (x=2, 4, 6), are characterized and analyzed by using cooling curve thermal analysis method and continuous torque measurement technique. The results show that Tn1 (first characteristic temperature of primary crystal nucleation), Tn2 (second characteristic temperature of primary crystal nucleation) and Tch (temperature of dendritic coherency) decrease with the increasing of zinc content, and the temperature difference of (Tn1-Tn2) almost remains constant, while the temperature difference of (Tn2-Tch) changes dramatically when Zn content increases to 6wt.%; The results also show that the fs-cc (solid fraction of dendritic coherency) of ZWx2 alloys ranges from 0.4 to 0.51 while ZW22 and ZW62 alloys having higher fs-ch and ZW42 alloy having the lowest fs-ch; also, strength of alloys in mushy zone does not develop until a certain fraction solid( corresponding to the dendrite coherency point) is reached. The results obtained by cooling curve thermal analysis method and continuous torque measurement techniques are comparable to each other.

Abstract: An ensemble averaging technique is used to obtain both macroscopic equations of transport phenomena in a two-phase region and new models of permeability and thermal conductivity of the columnar mushy zone through the analysis of most likely configurations of the local microstructure. The obtained formulae are incorporated into a FEM computer code for the macroscopic analysis of binary mixture solidification with convection. The influence of various models of mushy zone permeability on temporal shapes of the two-phase region as well as on velocity and temperature fields is studied for a test case of solidification of a dilute solution of ammonium chloride and water in a square cavity. For this case, relatively small differences in liquid flow patterns and temporal shapes of the two-phase region are observed for significantly different models of mushy zone permeability.

Abstract: A constitutive equation for thermal strain in the mushy zone has recently been established [1]. The parameters in this constitutive relation are in the present study determined for the commercial alloys A356, AA2024, AA6061 and AA7075 in addition to an Al-4 wt% Cu alloy by combining experimentally measured contraction of a cast sample with thermomechanical simulations. The constitutive equation for thermal strain in the mushy zone reflects that there is no thermal strain in the solid part of the mushy zone at low solid fractions and that the thermal strain in the mushy zone approaches thermal contraction in fully solid as the solid fraction increases towards one. Experiments were performed at cooling rates in the range from 2 to 5.5 °C/s. The solid fractions when the tested alloys start to contract, gs^th , are in the range from 0.63 to 0.94. Grain refinement increases gs^th for all the tested alloys.