Eutectic solidification of Al-Cu alloys influenced by convection

Directional solidification of eutectic Al-Cu alloys has been studied to demonstrate the influence of both impurity concentration and convective conditions. The λ/R relations show that higher iron concentration and intensive stirring coarsen the microstructure. The formation and growth of new iron-containing phases at the solidification front restrict impurity influence. Kinetic data follow a dependence λ.Rⁿ=const with n-value close to 0,5.     

Melting and solidification of metallic composites in space

Metallic matrix composites are a relatively new type of strengthened metals. Other properties such as wear resistance or thermal and electrical conductivity can also be modified. One can distinguish solid phase and liquid phase fabrication methods. The latter need only a relatively simple equipment and are particularly suited for complex shapes. The interfacial phenomena, the arrangement of the dispersed particles or fibres and the solidification behaviour of the matrix have to be understood in order to enhance the properties of the composite. The microgravity environment of space drastically influences several phenomena occurring during fabrication such as the fluid motion in the liquid matrix and the transport of the solid particles or fibres. The results of our space experiments in SL1, D1, TEXUS 6,7 and 9 on copper and aluminium based composites are summarized in this context. Two topics are treated more in detail, namely the role of interfacial energies and the expulsion of particles by the solidification front. Further, the relevance of space processing is illustrated for oxide dispersion strengthened metals. Finally the constraints of previous experiments and suggestions for future research are mentioned.     

Forces acting on a foreign particle near a solidification front

The interaction of inert foreign particles and a solid-liquid interface during solidification is considered. The viscous drag on the particle which produces a force on the interface is calculated under general assumptions as well as the disjoining force for small particles and the buoyancy force for large ones.     

Melt temperature fluctuations: Causes and response of the solidification front

It is well understood that temperature oscillations and fluctuations during melt growth give rise to fluctuating concentration of solute in the crystal — so-called solute (or impurity) striations. A major source of the temperature fluctuations is the transition to turbulence in the melt resulting from strong buoyancy and/or surface-driven flows. The interaction of these natural convective flows with any imposed rotation is of significance. The major factors causing flow transitions and particularly those which lead to time-dependent flows in Czochralski and float-zone melts will be briefly reviewed and the likely influence of a low-g environment considered. Theoretical analysis and experimental study of the response of the crystal-melt interface to such temperature fluctuations is reviewed and criteria for maximising crystal homogeneity discussed.     

Transient growth effects during space solidification experiments in a Bridgman configuration

This paper reviews the different mechanisms involved during a Bridgman solidification. It is demonstrated that during a space experiment, due to the lack of time it is not possible to wait for steady states for all the phenomena considered. So a non steady state analysis is needed. Mass and heat transfers are considered as well as kinetics effects. Different coupling effects are emphasized. It is shown that fine measurements during space experiments are needed in order to verify these important transient stages. One kind of possible experiments is proposed to enable us a better understanding of the solidification laws.     

金属材料快速凝固激光加工与成形

报道近年来在先进金属材料快速凝固激光表面改性、金属间化合物高温耐磨耐蚀涂层新材料快速凝固激光熔覆制备技术、钛合金及高温合金等高性能金属零件快速凝固激光成形技术等领域的研究进展, 主要内容包括:钛合金激光表面合金化及激光熔覆表面改性、激光熔覆高温耐磨耐蚀多功能金属间化合物涂层、小面相非平衡凝固液固界面结构及生长机制、钛合金及高温合金高性能零部件激光快速成形、难熔高活性金属材料及定向生长柱状晶钛合金激光约束熔铸成形技术.      

Effect of gravity on coupled convective and interfacial instabilities during directional solidification

We investigate the role of gravity in a linear stability analysis of the onset of coupled convective and morphological instabilities during directional solidification at constant velocity of a dilute alloy of tin in lead. For solidification vertically upwards, the temperature gradient alone would cause a negative density gradient and the solute gradient alone would cause a positive density gradient. Two types of instability are found, a convective type that occurs for long wavelengths and a morphological type that occurs for short wavelengths. In general, these are coupled but the morphological instabilities are practically independent of gravity and thus correspond to the predictions of previous morphological stability theory in which density changes and convection are neglected. The convective instabilities depend strongly on gravity; for a growth velocity of V = 1 μm/s and a temperature gradient in the liquid of G_L = 200 K/cm, the critical concentrations for convective instabilities are 3.1 × 10^?4, 3.1 × 10^?2 and 2.39 wt. % for g_e = 980 cm/s², 10^?4 g_e and 10^?6 g_e, respectively. For low velocities, the convective instabilities occur at much lower solute concentrations than the morphological instabilities whereas at high velocities, the reverse is true. At intermediate velocities where the changeover takes place, there are oscillitory instabilities of mixed character whose periods range from 60 s at g_e to 6 × 10⁴ s at 10^?6 g_e.     

Mass transport in the near vicinity of solidification fronts under conditions of microgravity

Cellular solidification has been known and discussed since a long time. The appearance of cellular and dendritic microstructure closely resembles Benard cells known from fluid physics. Similar generation mechanismus may possibly be assumed. Both g-dependent and g-independent convective phenomena may probably be linked to the occurrence of instabilities at solidification fronts. It is to be expected that normal freezing of model alloys (advantageously such with no volume change at the freezing point) in a defined temperature gradient (gradient furnace) and quenching them may help to quantity g-influence on solidification.     

Thermal and thermosolutal convective effects in bridgman solidification: Effect of orientation relative to gravity

A concentrated binary system (Ge-Si) and a dilute one (Ge-Ga) have been solidified at 1 g, in a new vertical Bridgman furnace where radial thermal gradients have been minimized. Very different solutal boundary layer extents (δ_Ge-Si = 3 cm, δ_Ge-Ga = 2.5 mm) obtained for both systems solidified in the same thermal conditions are explained by simple analytical hydrodynamic models. It is demonstrated that :1. The convective transport of Ga in the Ge-Ga system is due to the unavoidable residual horizontal thermal gradients associated with discontinuity of thermal properties at the solid-liquid interface.2. The larger boundary layer extent observed for Ge-Si and the corresponding pure diffusive transport is the result of the stabilizing effect of the longitudinal solutal gradient in this system.Thus, in the case of dilute systems, convective effects can only be cancelled through a reduction of the gravity level. On the opposite, pure diffusive solute transport can be achieved in the vertical configuration a 1 g in the case of concentrated systems where a stabilizing solutal effect is operating.     

Directional solidification of complex-forming eutectic melt of the PbCl2-AgCl dielectric system under conditions of zero, normal and increased gravity

First results in the area of studying the influence of gravity field on the degree of ordering of directionally solified eutectict are presented. As the model complex-forming melts, PbCl₂ and its eutectic with AgCl was chosen. Directional solidification of the mentioned eutectic was realised under conditions of zero, normal and increased gravity. It was shown that increasing level of gravity results in a marked improvement of the degree of spacial ordering of the lamelar structure.     

Comparative study of the influence of natural convection on directional solidification of Al–3.5 wt% Ni and Al–7 wt% Si alloys

We present numerical simulations of thermosolutal convection for directional solidification of Al–3.5 wt% Ni and Al–7 wt% Si. Numerical results predict that fragmentation of dendrite arms resulting from dissolution could be favored in Al–7 wt% Si, but not in Al–3.5 wt% Ni. Corresponding experiments are in qualitative agreement with the numerical predictions. Distinguishing the two fragmentation mechanisms, namely dissolution and remelting, is critical during experiments on earth, when fluid flow is dominant.