Materials Science Forum Vols. 561-565

Abstract: The interaction between flow and progressing solidification front is of great importance, since it occurs in all casting processes. The present paper provides a better understanding of the flow phenomena and associated complex effects on solidification in a rectangular cavity under forced flow conditions, by means of experiments and computer simulations. It is shown that the cavity-driven flow with solidification is determined by several interacting features. The variation in bulk flow velocity and initial superheat dramatically changes the macro- and microstructure, promoting grain refinement, formation of peculiar grain and dendrite morphologies, etc. In particular, twinned feathery grains are found in the structure formed under certain heat and flow conditions during solidification. Some correlations between twinned feathery morphology, flow and solidification parameters are obtained. The effect of flow vortices on progressing solidification front and their effects on structure evolution are analyzed. Finally, the quantitative correlations between microstructure, solidification and flow parameters are established.

Abstract: Hot tearing is a significant problem upon direct-chill casting of high-strength aluminum alloys. The occurrence of hot cracks is related to the thermal contraction of the solid phase and to the lack of feeding by the liquid phase during solidification. It has been identified that structure features such as grain size and amount of nonequilibrium eutectics influence both phenomena involved in hot tearing. Experimental and computer-simulation results are presented for a range of model and commercial aluminum alloys. The results are obtained both during special small-scale experiments and during industrial-scale direct-chill casting. It is shown that grain refinement reduces hot tearing susceptibility of aluminum alloys through the related decrease of the temperature of thermal contraction onset and increased permeability of the mushy zone. The effects of process parameters on hot tearing are also discussed.

Abstract: Directionally solidified Alumina-based eutectic ceramic in situ composite is a kind of promising candidate for high temperature structural material applied at elevated temperature above 1923K because of its excellent properties. With laser zone melting directional solidification, Al2O3/Y3Al5O12 (YAG) eutectic ceramics are successfully prepared. The relationship between the eutectic microstructure and the processing parameter is studied, and the mechanical property of the composite is measured. The results show that: (1) Laser power density and scanning rate strongly affect the eutectic microstructure. With proper processing parameters adjusted, the binary lamellar eutectic microstructure is obtained, in which Al2O3 and YAG phases are three-dimensionally coupled and continuously connected without grain boundaries and amorphous interface phases. (2) The eutectic spacing decreases to about 1μm with increasing scanning rate. (3) The maximum hardness of 19.5GPa and the room fracture toughness of 3.6MPa.m1/2 are obtained by Vickers indentation measurement.

Abstract: Critical experiments in Al-Cu hypo-eutectic alloys have been carried out in capillary samples in order to establish the lamellar/rod eutectic transition condition. It is shown that this transition occurs over a range of compositions. Physics of the diffuse nature of this transition is proposed and the dynamics of the transition is shown to give a specific geometric relationship between the lamellar and rod spacing at the transition. A model of lamellar/rod eutectic transition is established that incorporates the dynamic nature of this transition and the anisotropy in interfacial energies.

Abstract: This paper focuses on the determination of the heat flow density (HFD) and interfacial heat transfer coefficient (IHTC) during the high pressure die casting (HPDC) process of AM50 alloy. A specially designed “step shape” casting is used during the die casting experiment. Based on the temperature measurements inside the die, HFD and IHTC are successfully determined. Calculation results indicate that HFD and IHTC at the metal-die interface increases sharply right after the fast injection process until approaching their maximum values, and after that their values decrease to a much lower level until the dies are opened. Casting thickness has a great influence on both of the HFD and IHTC. Process parameters, such as the intensification pressure, the piston velocity, have little influences on HFD while on the other hand the die temperature has a great influence on the HFD. The IHTC seems to be independent upon all those process parameters so the IHTC peak values maintain at a particular level when the casting thickness is fixed.

Abstract: The microstructure and characteristic of fracture between TC4 titanium alloy and 1Cr18Ni9Ti stainless steel bonding joints were analyzed concretely in this paper. For direct SP/DB of TC4/1Cr18Ni9Ti, the diffusion layer joint was composed of some new phases such as β-Ti, TiFe2, TiFe, σ-FeCr and α-Fe, and the shear strength could be ensured when the thickness of TiFe/TiFe2 brittle phases was controlled within 3~5μm. In SP/DB of TC4/1Cr18Ni9Ti, the joint status of microzone in interface was asymmetric and exhibited different fractograph. The bonding area of fracture could be divided into three zones: similar original interface zone, metallurgical bonding zone and quasi-metallurgical bonding zone. The interface deficiencies were constitutive of mechanical bonding area, cavity and impurity.

Abstract: In the present investigation, serial sectioning and 3D reconstructions are made on samples quenched at selected temperatures during unconstrained solidification in order to observe the evolution in morphology of coarse dendrites in 3D. The 3D microstructure reconstruction during the solidification of an Al−7 wt.% Cu alloy allowed the identification of a complex coarse morphology of dendrites. High-ordered branches present different morphologies at different temperatures and locations in the microstructure due to coarsening and coalescence. 3D visualization of complex dendritic structures is discussed in the present investigation.

Abstract: The process of different sorts of magnesium alloys investment casting is studied using a mold materials composed of zircon and colloided silica binder. The investment shell is flushed out employing protective gas mixture of 1% HFC134a and inert gas. The castings with perfect appearance and high metallurgical quality are produced successfully for ZC62 and ME-1magnesium alloy respectively. The mechanical properties of the castings reached the ASTM standard.

Abstract: The carbide and graphite formation and redistribution of alloy elements during solidification were investigated on Ni-hard type cast iron (Fe-C-Si-Ni-Cr-Mo) to develop higher quality rolls for hot steel strip mills. By the control of Ni and Si contents of iron, eutectic graphite flakes crystallize even in cast irons containing strong carbide formers such as V, Nb and Cr. The crystallization of Ni-hard type cast iron with V and Nb proceeds in the order of primary , + MC, + M3C and + graphite eutectic. Since the influence of each alloying element on graphite formation is estimated based on the solubility of C in molten iron, the change in graphite forming tendency of residual liquid is evaluated by the parameter expressing the solubility limit of C to molten iron. The amount of graphite increases with the decreasing of solubility parameter. In addition, inoculation with ferrosilicon effectively increases the graphite flakes.

Abstract: Gold particles several tens of micrometers in size were welded onto a gold substrate. High voltages of 4 kV or more were applied to a tungsten needle in contact with the particle on the substrate at a very low contact pressure. The particle was welded to the substrate in an instant with sparks. The needle was then retracted to 20 !m above the particle and a high voltage of about 2kV was applied to the needle. An electric discharge between the needle and the substrate enveloped the particle, and the joint was strengthened. The joint strength was measured, and the fractured surface was examined by scanning electron microscopy and scanning laser microscopy. The mechanism of joint strengthening is discussed. It is clear that an inert gas flow during the electric discharge is necessary to strengthen the joints.