Papers by Keyword: Unidirectional Solidification

Abstract: In this paper, we discuss our previous experimental results and recent considerations on the preparation of solidified oxide eutectic coating on non-oxide ceramics by unidirectional solidification method using light focusing apparatus. As an example, an explanation is given here with regard to the formation mechanism of Al₂O₃-HfO₂ eutectic coatings on SiC substrate. The Al₂O₃ component in oxide melt easily reacts with SiC and primary HfO₂ phase solidified on SiC substrate. However, the solidified HfO₂ phase also react with SiC substrate and HfC-HfO₂ graded functional layer is formed on SiC substrate and Al₂O₃-HfO₂ eutectic microstructure is formed on the graded functional layer

Abstract: Ordered porous copper with elongated pores has been fabricated by a continuous unidirectional solidification method in a hydrogen gas atmosphere with high pressure. The porosity of the ordered porous copper is significantly affected by the pressure of hydrogen. A theoretical model is developed to get the relation between the porosity and the processing parameters. The calculated values are in good agreement with the experimental results. Key words: Unidirectional solidification; Ordered porous copper; Porosity; Modeling.

Abstract: In this article, a three-dimensional time-dependent model describing the evolution of single pore during the solid/gas eutectic unidirectional solidification process (also called gasar process) was established. The mass transfer, bubble nucleation, pore growth and interruption were all considered in this model. The pore structure of lotus-type porous copper and aluminum were simulated under different solidification velocities. The results indicate that: coupled growth of both solid and gas phases can be achieved in a proper range of solidification velocities. The solidification velocity for Cu-H₂ system is dozens of that for Al-H₂ system when the pore diameter is similar to each other. The differences of the solute distribution coefficient (k₀), diffusion coefficient (D_L) and the constant of solubility of hydrogen (ξ(T_m)) in the melt are regarded as the main reasons of the big discrepancy of solidification velocity between Cu-H₂ and Al-H₂ systems.

Abstract: To strengthen the bottom cooling is one of the key technologies of directionally solidified ingot process. Stool cooling scheme has a significant impact on the solidification process of the ingot. The study optimizes chassis cooling scheme according to the air gap between the ingot and the stool. With multiple sets of cooling system, water-cooled stool makes the basal water cooling adapt to air gap distribution through subregional cooling of different intensity control, and be uniform with it in the longitudinal direction solidification. The paper establishes mathematical model of unidirectional solidified ingot on temperature field of conventional water-cooled stool and improved one respectively. By the aid of finite element analysis method, numerical simulation of 45t ingot with algebraic model is carried out. In order to optimize parameters of ingot unidirectional solidification, temperature fields influenced by conventional water-cooled stool and improved one in the process of unidirectional solidification are simulated. The numerical results show that the optimized chassis cooling can result in the ingot in a longitudinal uniform solidification. The numerical simulation results can provide important reference for the optimization of unidirectional solidification process.

Abstract: The effect of superheating and cooling rate on primary precipitation of Si was studied in hypereutectic Al-Si alloys. Alloys with compositions of 15, 18 and 20 wt% Si were solidified in unidirectional solidification equipment from different temperatures and drawing speeds. The fraction of primary silicon was measured over a certain distance of the sample in the steady state region. Results show a large variation in primary silicon fraction along the sample length and with varying cooling rate. The fraction of primary silicon and primary aluminium around silicon increases with increasing superheat. These fractions decrease with increasing cooling rate and the structure changes to a more refined dendritic-like primary silicon. Different morphologies of Si and their transformation during solidification can be seen over the sample length. An analysis of the quenched solidification front shows the possibility of strong convection ahead of the solidification front. The convection can be caused by density variation in the liquid due to the cluster structure of the melt which changes the microstructure.

Abstract: Al–7wt.-% Si–1wt.-% Fe alloy was solidified unidirectionally in the Crystallizer with High Rotating Magnetic Field (CHRMF). The diameter of sample was 8 mm and its length was 120 mm. The parameters of solidification were as follows: solid/liquid interface velocity ~0.082 mm/s, temperature gradient 7+/-1 K/mm, magnetic induction 0 and 150 mT, frequency of magnetic field 0 and 50 Hz. The structure solidified without rotating magnetic field (RMF) showed a homogeneous, columnar dendritic one. The structure solidified by using magnetic stirring showed a dual periodicity. On the one hand, the branches of the “Christmas tree”-like structure known from the earlier experiments contained Al+Si binary eutectic. On the other hand, bands with higher Fe- and Si-content formed in the sample, which were at a larger distance from each other than the branches of the “Christmas tree” structure. The developed microstructure was analyzed by SEM with EDS. The average Si- and Fe-concentrations were measured on the longitudinal section at given places along the length of sample. Furthermore the Si- and the Fe-concentrations close to the bands and among the bands as well as the composition of the compound phases were determined.

Abstract: The unidirectional solidification process of magnesium alloy needs to establish a specific temperature gradient in casting mold, the direction of crystal growth and heat flow are in the opposite direction in the unidirectional solidification. The process can better control the grain orientation, and eliminate the horizontal grain boundary, so to attain columnar grain structure and excellent performance of magnesium alloy. In this paper, Numerical simulation is carried out by orthogonal experiments in order to obtain the optimal process parameters according to the actual experimental device. Different process parameters are taken into account, including pulling speed, cooling time and cooling intensity. FEM (finite element method) is employed to calculate the temperature field and reached a straight shape of temperature gradient distribution which is conductive to achieve unidirectional solidification microstructure. PFM(phase field method) is adopted into the microstructure calculation. The microstructure obtained by PFM is in agreement with the actual pattern by the optical microscope observation.

Abstract: Based on the general mathematical formulation under unidirectional solidification with mushy layer developed in Ref.[13], leading approximation of solution in mushy layer is obtained for the case that the inverse Lewis number is small parameter. For the steady state of solidification, concentration field in the mushy region is given as function of a solid volume fraction. Mushy layer width and solid volume fraction at the solid-mush interface are obtained. Effects of temperature gradient on mushy layer width and solid volume fraction at solid-mush interface are presented.

Abstract: Lotus-type porous copper materials were fabricated by continuous casting technique. The relationship between porosity and external pressure were developed based on the solute mass conservation law. Theoretical values of porosity are in agreement with our experimental results. Results show that porosity decreases when the partial pressure of hydrogen increases for cases of no pressure of argon above melt, while porosity first increases and then decreases as the partial pressure of hydrogen increases for case of nonzero pressure of argon above melt.

Abstract: The general mathematical formulation under unidirectional solidification with mushy layer is developed. For the steady state of solidification, concentration and temperature fields in the mushy region are given as functions of a solid volume fraction. The nonlinear model of solidification in mushy zone is changed into system of the second order ordinary differential equation with free boundary conditions, which is solved using numerical calculating method. Solid volume fraction at the solid-mush interface is also obtained. For fixed control conditions, mushy layer width of Fe-Ni alloy in unidirectional solidification is predicted.