Papers by Author: Hong Min Guo

Abstract: The experiment of the roll-casting for semi-solid AZ91D magnesium was carried out on the equipment made by ourselves in our laboratory. Parameters of annealing process and the effect of annealing temperature on structure and hardness of semi-solid magnesium were investigated. It was shown that annealing process can improve the structure and properties of roll-casting strips for semi-solid magnesium. In addition, it can relieve the eutectic structure segregation and make β-Mg17Al12 phase precipitate from primary α-Mg grain, which can cause dispersion strengthening process when the annealing temperature is about 420°C. With the increasing of annealing temperature, semi-solid primary α-Mg grain boundary become obscure gradually, grain growth is indistinct and β-Mg17Al12 phase is precipitated from the inner α-Mg grain. However, the aberrated eutectic α-Mg grain for non-equilibrium solidification increase continuously and separate gradually from β-Mg17Al12 phase, which makes the β-Mg17Al12 phase connect together and distribute on the α-Mg grain boundary closely approaching the equilibrium solidification structure. Hardness in eutectic structure area is higher than primary α-Mg grain. With the increasing of annealing temperature, hardness in eutectic structure area will increase and then decrease, when the temperature is over 400°C, the hardness will increase again, but hardness of primary α-Mg grain nearly unchanged. Keywords: casting-rolling; anneal; semi-solid; magnesium alloy; microstructure

Abstract: Rotating tube method for preparing Cu-Ca alloy semi-solid slurry was investigated. Compared the microstructure of slurry prepared through the tube and of untreated original sample, discussed evolution characteristics of microstructure flowing through the rotation tube, and analyzed effect of process parameter on microstructure. The results show that the semi-solid slurry of Cu-Ca alloy could be prepared through rotating tube process; the melt were hardly affected in the initial stage. The tube began to play a major part in the next stage and the melt were stirred effectively in the last stage; Spherical microstructures of Cu-Ca alloy slurry were obtained through controlling or adjusting the process parameters to control solidification conditions.

Abstract: Heterogeneous nucleation is a critical factor for grain refinement magnesium alloys. The paper reports the Al-9wt.%Mg-10wt.%SiCp master alloy were prepared by adding SiCp into the Al-Mg molten with stirring in 610-595°C, which were introduced into AZ91. Experimental results indicated Al4C3 were in situ synthesis on the surface of SiCp by the reaction between SiCp and Al in 850°C, which could be as nucleant substrate for primary Mg during the solildification of magnesium alloys. This investigation provided a novel idea for grain refinement Mg-Al alloy.

Abstract: By the finite element method, the N-S momentum, energy equations and continuum equations, etc. which describe the fluid flow and heat transfer in the slope of the rheocasting-rolling for Semi-solid Magnesium Alloy were computed. The influence of the different slope parameters( pouring temperature, length of slope, lean angle)on the metallographic structure of semi-solid magnesium alloy was analyzed. The simulative results can provide effective data to confirm optimized slope size and technical parameter.

Abstract: Rheoforming is becoming the choice of the casting industry which relies on the semi-solid slurry for high integrity structural parts. The potential of rheoforming with LSPSF (Low superheat pouring with a shear field) for aluminum alloys was investigated in the present work. High quality semi-solid slurries of a series of aluminum alloys were manufactured by LSPSF process, such as casting alloy A356, high strength alloy 201, secondary die casting alloy A380 and wrought alloy 2024, 6082 and 7075, in which the primary α-Al presented spherical, small and homogeneous distribution, especially with zero-entrapped liquid. Applications of LSPSF in high pressure die casting process and squeeze casting process were presented. Results showed that LSPSF rheoforming could improve microstructures and increase mechanical properties.

Abstract: Wrought aluminum alloys have wide variety applications in aerospace and automobile industries, due to their superior properties compared to casting aluminum alloys. Rheoforging is a modification of thixoforging, which starts directly from the liquid phase instead of reheating of a billet. In the present process, suitable semi-solid slurries of wrought aluminum alloys are prepared by the LSPSF (low superheat pouring with a shear field) rheocasting process within 25s. The effects of processing conditions on the degree of spherical grain refinement in 2024, 6082 and 7075 alloys are discussed. 2024 and 7075 alloys have been used in order to investigate rheoformability of high performance aluminum alloy. Experimental results show that rheoforging based on LSPSF process can produce relatively homogeneous microstructure throughout the cup-shaped component. However, high solid fraction of semi-solid slurry promotes metal flow and results in solid/liquid segregation. Subsequent optimized heat treatments raise significantly the mechanical properties. Future potentials and challenges to be solved are discussed.

Abstract: Rheo-die casting (RDC) based on LSPSF (low superheat pouring with a shear field) rheocasting process has been exploited. In case of secondary die casting aluminum alloy YL112, LSPSF allowed for preparation of sound semi-solid slurry in 15-20s that fully meet the production rate of HPDC, the primary α-Al exhibiting a mean equivalent diameter of 70 μm and shape factor of 0.93, without any entrapped eutectic. Compared to conventional HPDC, RDC improves microstructures in castings. Secondary solidification of semi-solid slurry takes place uniformly throughout the entire cavity, producing an extremely fine and uniform microstructure. The experimental results show the RDC 380 alloy has much improved integrity and mechanical properties, particularly elongation, and heat treatment can be used to enhance the mechanical properties.

Abstract: An alternative method has been proposed for the continuous and sample production of SSM slurry for the rheo-forming process. The process named “Low Superheat Pouring with a Shear Field (LSPSF)” dose not use the conventional stirring process, instead, it uses solidification conditions to control nucleation, nuclei survival and grain growth by means of low superheat pouring, vigorous mixing and rapid cooling during the initial stage of solidification combined with thereafter a much slower cooling. The method has been applied to A356, 201 and A380 Al-alloys. The primary phases present in average equivalent diameter of 40-70μm, 35-50μm and 50-70μm for A356, 201 and A380, respectively. The morphology of primary phases is nearly spherical with shape factor of 0.78-0.86, 0.71-0.83 and 0.85-0.96 for A356, 201 and A380, respectively. For each of those alloys, there is no eutectic entrapped within the primary phase. The advantages of the LSPSF include process simplicity with high efficiency, easy incorporation into existing metal forming installation without infrastructure changes and a wide process window for pouring temperature.