W.D. Huang

History-dependent selection of primary cellular/dendritic spacing during unidirectional solidification in aluminum alloys

History-dependent selection of primary cellular/dendritic spacing is investigated systematically during unidirectional solidification of a series of aluminum alloys. A single crystal is formed in the sample before each experimental run, so that the influence of grain boundary on the primary spacing is avoided. The experimental results are compared with those of the two-dimensional crystal growth in the same alloy system and transparent model alloys. It is found that the primary cellular/dendritic spacing is remarkably history dependent. The average primary spacing is dependent not only on the current growth conditions, but also remarkably on the way those conditions were achieved. There exists a wide allowable range of primary spacings for a given growth condition. Experimental results are also compared with the Hunt-Lu model, which shows excellent fit between them, especially on the selection of cellular spacing. By comparing the three-dimensional experiments with the two-dimensional ones, it is also found that the allowable range of the primary spacing for the three-dimensional growth is wider than that for the two-dimensional growth.

Primary spacing selection of CuMn alloy under laser rapid solidification condition

In order to investigate the primary spacing selection under rapid solidification conditions, laser surface rapid resolidification experiments have been performed on Cu-26.6 wt.% Mn alloy. By taking transverse and then longitudinal sections of the directional growth dendrites/cells in the laser traces, the average primary spacing, spacing distribution range as well as the corresponding growth rate are quantitatively measured by SEM and image analyser. It has been found that there exists a wide distribution range in primary spacing under rapid solidification condition. As the growth rate increases, both the distribution range and the average spacing decrease. The maximum, λ1max, minimum, λ1min, and average primary spacing, gl1, as functions of growth rate, Vs, can be given by λ1max = 10.68V− 0.28s, λ1min = 2.42V− 0.22s, and gl1 = 510V− 0.33s, respectively. Our experimental results are compared with the current KGT model for rapid dendritic growth, and a good agreement is found.

Primary cellular/dendritic spacing selection of Al–Zn alloy during unidirectional solidification

Abstract Al–4.95xa0wt% Zn alloy is directionally solidified with Bridgman apparatus to investigate response of cellular/dendritic microstructures and primary spacing to the variation of growth velocity. The results show that, with increasing growth rate, there exist a transition from dendrite to cell and a wide distribution range in primary cellular/dendritic spacing. The maximum, λ max , minimum, λ min , and average primary spacing, λ , as functions of growth rate, V , can be given by λ max =4578 V −0.697 , λ min =1315.7 V −0.6543 , λ =3084.5 V −0.6982 , respectively. The experimental results are compared with the Hunt–Lu model, and a reasonable agreement is found.

In Situ Observation of SSM Microstructure Formation during Cooling Slope Casting of an NH4Cl-H2O Alloy with Vibrating the Slope

Vibrating the slope during cooling slope casting of a metallic alloy has shown helpful for causing the microstructure more spherical and thinner in a previous research by the present authors. This paper gives results from a direct observation of the SSM microstructure formation during cooling slope casting of an NH4Cl-H2O alloy with vibrating the slope. It is found that the vibration exerted to the slope can break down the solidification shell formed at the surface of the slope through rapid chilling and produce effectively fine globular grains in the microstructures. The main factors affecting the fine grain formation include the chilling extent, frequency and amplitude of the vibration of the slope, and the superheat of the alloy melt, respectively. The higher vibration frequency and amplitude, the lower superheat of the alloy melt, and the lower chilling temperature will help the formation of finer globular grains.

Effects of organic solvents on the morphology of the meta-nitroaniline crystal

The theoretical morphology of a meta-nitroaniline (mNA) crystal and the solvent effects on the growth morphology were analysed. The morphology of an mNA crystal grown in a nonpolar solvent such as toluene is consistent with that predicted by the BFDH model and that by the AE model which only took van der Waals interaction into consideration. This shows that the nonpolar solvent could eliminate the electrostatic interactions between solute molecules in the solution and on the crystal surface. It also means that the BFDH model could be taken as the AE model only taking van der Waals interaction into account. For solvents with strong polarity, such as ethanol and acetone, the morphologies of the crystal grown in them were far from that predicted by the AE or the Ising model.

Influence of Rapid Shear-Rate Changes on Microstructure Formation during Semi-Solid Processing

The solidification behavior of the semi-solid slurry with rapid shear-rate changes was investigated by using succinonitrile-5at.%water transparent alloy and Sn-15wt.%Pb alloy on a self-made apparatus. The results showed that the rapid changes of shear-rate improved the nucleation and promoted the globular growth of solidification microstructure in the stirred melt. The increase of shear-rate was in favor of promoting the grain growth with globular shape to a certain extent, however, when the shear-rate was beyond a critical value, the further increase of the shear-rate will lead the disappearance of the globular crystals. Thus, the microstructure of semi-solid processing could be refined by a controlled stirring process.

History-dependence of dendritic tip radius and secondary arm spacing during unidirectional solidification

Abstracts are not published in this journal

Researches on the Nucleation Behaviors of NH4Cl Crystal on Coarse Aluminum Surfaces

Coarse aluminum surfaces were prepared by etching method, the wettability of NH4Cl -70wt%H2O solution on the surfaces was investigated, and then the substrates were undercooled and immerged into NH4Cl-70wt%H2O solution to trigger nucleation of NH4Cl crystals. Experimental results indicate that the aluminum surface with step-like micro-morphology shows a transformation from strong wettability to non-wettability during its exposure in the atmosphere, in the same time, nucleation density on the strongly wetting surface is significantly higher than that of the non-wetting surface. A hypothesis is proposed to explain the relationship between the wettability and the nucleation density: the capillary effect helps the solution infiltrate into the structures of the surface with strong wettability and drive the air out of the surface structures, so nucleation is effectively promoted by the sharp corners and hollows of the coarse surface; conversely, the air bubbles trapped in the non-wetting surface structures decrease the number of the effective nucleation sites, and leads to a higher nucleation energy barrier, thus the nucleation density is decreased.

A Cellular Automaton Model with the Lower Mesh-Induced Anisotropy for Dendritic Solidification of Pure Substance

A modified cellular automaton model for describing the dendritic solidification of pure substance was developed. Instead of using the high mesh-induced anisotropy capture rules, such as Von Neumann’s and Moore’s method, a new capture rule---random zigzag method was developed, which greatly reduced the mesh-induced anisotropy in crystallographic orientation. The calculation method for the solid/liquid interface curvature was also improved. The effect of interfacial energy anisotropy on the dendritic growth behavior was analyzed.

Laser Rapid Forming of Ti-Ni Functionally Graded Alloy

Ti-Ni based functionally graded alloy is a kind of the promising material, which has potential to be used in aero engines. Using laser rapid forming, a Ti-Ni graded alloy with a continuous compositional gradient from pure Ti to Ti-50wt.%Ni were fabricated. On comparison with the graded alloy, a series of homogenous deposits with the typical composition between pure Ti to Ti-50wt.%Ni were also laser rapid formed. The phase evolution along the compositional gradient direction in the graded alloy is: α+β→β+Ti2Ni →(TiNi +Ti2Ni)+ TiNi; and the phase evolution in the corresponding compositional homogeneous deposit is: α+β→ β+(β+Ti2Ni)→ β+Ti2Ni+(β+Ti2Ni)→ (TiNi+Ti2Ni)+TiNi. The phase transformation and microstructural evolution along the compositional gradient were analyzed by using the microstructure selection map.