Solid State Phenomena Vols. 116-117

Abstract: Semi-Solid Al-5%Mg-3%Zn-1%Mn alloys with or without 0.2%Zr were prepared by the isothermal heating of either cast ingot or extruded billet. The microstructural and tensile properties variations during the isothermal heating, depending on alloy composition and slurry preparation method, were investigated. The grain (globule) size of semi-solid microstructure was not remarkably changed by the slurry preparation method, however it was significantly reduced by the Zr additions. The room temperature tensile elongation of the semi-solid formed alloys was generally higher in case of the extruded billet. T6 heat treatment was not found effective in improving the strength of the semi-solid alloys, unlike the casting alloys.

Abstract: Various Mg-Al-Zn alloys with different Al and Zn contents were fabricated by conventional casting and semi-solid forming process. And the microstructure and mechanical properties of the alloys were investigated. In Mg-4%Al-(5~7)%Zn alloys most of the grain boundary phases were found to be Mg-Al-Zn while in Mg-6%Al or Mg-8%Al based alloys the Mg- Al-Zn phase coexisted with Mg17Al12 at grain boundaries. At room temperature the semi-solid formed alloys showed significantly higher tensile properties, especially elongation, than the conventionally cast alloys.

Abstract: Microstructures and damping properties of semi-solid AM50 (Mg-5%Al-0.3%Mn) alloy were investigated and compared with those of die-cast AM50 alloy, based on experimental results of X-ray diffractometry (XRD), optical microscopy (OM), hardness tests and damping tests in a flexural mode. The semi-solid AM50 specimens show higher damping capacity than die-cast one in as-fabricated state, and the higher the fraction of solid α-(Mg), the greater the damping capacity. The annealing at 200oC deteriorates the damping properties of the semi-solid and die-cast specimens. This would be due to the segregation of solute atoms on dislocation lines, which eventually leads to lower internal friction by the restriction of dislocation movement. The peak damping values of the AM50 specimens are obtained after annealing at 400oC. The disappearance of segregates acting as pinning points of dislocations is thought to be responsible for the improvement in damping capacity. This result implies that the presence of solid α-(Mg) phase and annealing treatment at high temperature are beneficial to damping property of AM50 alloy.

Abstract: In this paper the microstructure evolution of AZ91HP magnesium alloy casts produced under different disturbing treatment conditions such as non-disturbing, argon blowing and mechanical stirring in semi-solid state after holding a short time was studied. The results show that the grains of AZ91HP alloy casts refined, the secondary dendritic arms grown and the dendrites tend to rosette shape with the decrease of holding temperature. External disturbance can accelerate the evolution process, and the spherical primary α phase formed easily under mechanical stirring treatment condition. The formation of non-dendritic structure is owing to ripening of the dendritic arms, refinement of the grains and movement of the primary formed solid phases. Disturbing treatments in semi-solid state induce more equilibrium solidification and decrease the amount of brittle β-Mg17Al12 phase. Blowing argon into the refined and modified magnesium alloy in semi-solid state can obtain homogeneous non-dendritic structure and the net shaped β phase distributed on α-Mg phase boundaries become fine and thin, this may be have a good effect on the mechanical properties of the magnesium alloy casts. Due to the low disturbing strength, argon blowing can maintain the metallurgy quality of the semi-solid slurry well, and also have high efficiency to make it. This technology need not new complex equipments and can be practiced in conventional casting conditions, so it may be used in industrialize manufacture.

Abstract: Cu-Ca alloys and the squirrel cage rotors for induction motors of small capacities were used for the development of thixoforming processes. Processing conditions, motor efficiency and forming defects in macro- and microscale for thixoforming of Cu-Ca rotors have been performed to investigate the microstructural features and the filling phenomena in slots of squirrel cage rotor. Inadequate filling due to the complexity of slot structure, separation of solidified metal from the slot, porosities and phase inhomogenitites were typical microstructural defects found in thixoformed Cu-Ca rotors. Exact flow control in terms of billet and die temperatures at slot gate especially, was necessary to prevent significant defects such as incomplete filling of slot.

Abstract: In this paper, the adhesive properties of hard coatings(TiN, TiCN) made by plasma chemical vapor depositions on non-nitrided and nitrided substrates was estimated using the scratch test, where adhesion was measured by the critical load (Lc). This value was determined as the normal force affecting the indenter and causing the coating detachment as well as the acoustic emission signal containing the information on the extent of coating damage. Results of the test showed that harder substrates and coatings give higher values of critical loads.

Abstract: The formation and character of grain boundaries in the primary solid particles observed in the metal slurries of stircast Al-4wt%Zn-1wt%Mg alloy were studied. In the semi-solid state of the solid fraction (fs) of more than 0.5, fracturing of solid particles appears to be dominated by shear force at high temperature, whilst in fs < 0.4 dendritic fragmentations are expected to occur by grain boundary melting. In case of shear rupture at fs > 0.5 coarsening of spherical solid particles takes place by Gibbs-Thompson effect of small particles remelting leading to solute redistribution.

Abstract: The structural control of Al-Si alloy, which was not studied among various electromagnetic processing of materials, was considered applying the alternating current and direct current magnetic flux density. The main aim of the present study is to investigate the effects of electromagnetic vibration on the control of the size of primary Si phase in order to develop a new process of structural control in Al-Si alloy. If the current density conducted for making high frequency electromagnetic vibration (EMV) (≥ 500Hz), the size of primary Si phase goes to small. If it conducted for making low frequency EMV (≤ 200Hz), the size of primary Si phase goes to large. This phenomenon considered to be related the collision, agglomeration and diffusion of silicon atoms.

Abstract: Grain growth in the cold rolled and subsequently recrystallized AA 5182 sheets was investigated by means of microstructure observations and texture measurements. Grain growth behavior strongly depends on the annealing temperatures. Grain growth hardly took place at temperatures lower than 470°C, which is attributed to a low mobility of grain boundaries. Abnormal grain growth occurred at temperatures ranging from 480 to 530°C. Annealing above 560°C gave rise to the dissolution of inhibitor precipitates, which led to normal grain growth.

Abstract: This work focuses on the strain states of the mid AA 3003 strip sandwiched in between either AA 3003 sheets or STS 430 sheets. For that purpose, the strain states at various through-thickness layers were analyzed by measurements of crystallographic texture and by simulations with the finite element method (FEM). During sandwich rolling, the material property of outer sheets of sandwich samples played an important role in the evolution of the strain states and the corresponding texture formation. In the mid AA 3003 sheets, rolling with the harder outer sheets gave rise to pronounced through-thickness texture gradients, whereas fairly uniform strain states prevailed during rolling with the outer sheets of the same AA 3003.