Solid State Phenomena Vols. 141-143

Abstract: The growing importance of Al-Sn based alloys for producing self lubricating bearing materials in automotive industries necessitates the development of new alloys for improved performance. The recent thermodynamic work by Schmid-Fetzer at Clausthal University of Technology predicted that the addition of Cu to the binary Al-Sn system makes the miscibility gap stable. With the guidance of the calculated phase diagram, experimental investigations were conducted on the ternary Al-Sn-Cu system to understand the solidification behaviour of this immiscible system under intensive forced convection. The experimental focus was to investigate the effects of shear rate, shearing temperature and cooling rate on the solidified microstructure.

Abstract: Semi-solid AZ91D magnesium slurry was prepared by mechanical stirring method and the evolution of microstructure under different technological parameters such as casting temperature, standing time, stirring time and stirring speed et al were studied. The results show that the technological parameters have great influence on the microstructure of the semi-solid magnesium slurry. Neither too high nor too low stirring speed is a benefit to form the semi-solid state; stirring time is also not too long; higher casting temperature is unfavorable; with the increasing of standing time, the steady-state apparent viscosity of the slurry increases rapidly, the number of nucleation increases and the crystalline structure becomes finer.

Abstract: Semi-solid M2 steel casting ingots were prepared using inclined slope pre-crystallization method, and the microstructure at different slope length and thermal plasticity of ingots were investigated in this paper. It was clarified that semi-solid processing technology can solve the serious problem of eutectic carbides segregation in casting microstructure. The dendritic primary austenite grains are changed into non-dentritic or equiaxed grains, and the eutectic ledeburite carbide networks are fine. So, it improves the thermal plasticity of casting ingots, and the carbide distribution of the semi-solid casting ingots is better than conventional casting after being forged.

Abstract: The microstructural evolution of SiCp/AZ61 composites during partial remelting was investigated. The results indicated that the suitable technological parameters of SiCp/AZ61 composites were the reheating temperature of 595°C ~ 600°C and isothermal holding time of 30min ~ 60min. The separation of microstructure of AZ61 matrix was faster than that of SiCp/AZ61 composites during the initial stage. In the meantime, SiCp/AZ61composites hold high stability during remelting process and a fine semi-solid thixoforming microstructure can be obtained with increases in temperature and holding time. In addition, the samples of SiCp/AZ61composites were susceptible to serious deformation and running out above 610°C.

Abstract: SiCp/AZ61 composites were fabricated by a semi-solid stirring method. The effects of volume fraction of SiC particles, stirring temperature and stirring time on the mechanical properties of SiCp/AZ61composites were studied with an orthogonal experimental method. The results indicate that volume fraction of SiC particles has a remarkable influence on the mechanical properties of SiCp/AZ61 composites, the second were stirring time and stirring temperature. In this experiment condition, the optimum technological parameters of SiCp/AZ61 composites fabricated by semi-solid stirring method were volume fraction of SiC particles 6%, stirring temperature 595°C and stirring time 5 min. Fractographies show that the fracture morphology of SiCp/AZ61 composites with volume fraction of SiC particles 6% is brittle.

Abstract: Using equal channel angular extrusion (ECAE) process, which combine the advantages of grain refinement and induced strain, for preparing semisolid billets is a relatively new Strain Induced Melt Activation(SIMA)method. This paper investigates the remelting and semisolid isothermal holding behavior of Mg-9Al alloy after ECAE processing at 350°C. It has been found that 2 passes ECAE-ed billets are qualified for obtaining spherical grains after remelting and isothermal holding treatment. Increasing the ECAE process from 2 passes to 8 passes has no further significant effects on the size and roundness of the solid particles at the semisolid state. The effects of temperature and isothermal holding time on microstructural evolution have also been investigated. In addition, the solid solution treatment before the ECAE processing affected greatly on microstructural evolution of the alloy during ECAE processing and thus the following remelting and isothermal holding behavior. The Mg17Al12 precipitates were uniformly decomposed from the saturated solid solution in the solutionized samples during ECAE processing. The coalescence of grains together with the self-blocking effect generated more entrapped liquid in the solutionized sample at semisolid state. Moreover, the solid particles of solutionized samples have bigger size and grow faster, which are detrimental to SSM processability.

Abstract: In the present work, a 2-pole linear electromagnetic stirrer (LEMS) is developed to study the effect of stirring during solidification of aluminium alloys. The stirrer design entails the placement of a stack of coils around the mold to generate a primary motion that recirculates along the longitudinal direction. The stirrer is first tested and validated by measuring the electromagnetic forces on solid aluminum cylinders of different diameters as a function of excitation current. The alloy to be stirred and solidified is placed in a cylindrical graphite mould located in the annulus of the LEMS. A suitable cooling arrangement is provided at the bottom of the mould to extract heat from the melt, in order to produce a rheocast billet inside the mould. Rheocasting experiments with A356 aluminium-silicon alloy are performed using a stirring current of 250A, in order to assess the effect of electromagnetic stirring on microstructure formation. The resulting microstructures and cooling curves with stirring are compared with those obtained without stirring.

Abstract: The study on rheocasting has been conducted based on examination for destruction, growth, ripening of solid state in accordance with various changes in cooling. And flow characteristics of slurry for high liquid range rather than practical use has gotten attention. However, the characteristics of existing rheocasting and thixocasting are better mechanical property than competing process die casting but it’s competitiveness as automotive or electric electronic parts which cost of product is important. And productivity also has been decreased. Because die-casting techniques has been advanced such as vacuum, mold auto temperature control etc. Most of the work reported at this conference concerned the semi-solid processing of low melting point alloys, and in particular light alloys of aluminum and magnesium. Therefore, the aim of this study is to devise the original strength of semi-solid process by sequential semi-solid process. The size of primary solid particle and the degree of sphericity as a function of the variation in cooling rate, stirring speed, and holding time were observed. By applying electromagnetic stirring, primary solid particles became finer and rounder relative to as-cast sample. With this background, this research conducted electromagnetic stirring through slurry deliverance of sequential process through A356 alloy electromagnetic stirring pump (30Hz, 130A) with Sequential Semi-Solid Process device to assess the microstructures, primary particle size, degree of sphericity change in A356 alloy in accordance of solid-fraction, stirring speed, and stirring maintenance time.

Abstract: Owing to its superior flow and mould-filling capability, a fully globular structure is essential for semisolid processing technologies. The present work was undertaken to identify the cooling slope casting process parameters that, upon heating to the semisolid state, gives the required globular structure for the ETIAL 160 alloy. Of the two pouring temperatures investigated, 605 °C and 615 °C, the lower pouring temperature was found to provide more globular grains surrounded by liquid phases.

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.