Hu-Tian Li

Shear enhanced heterogeneous nucleation in some Mg- and Al-alloys

Abstract Intensive shearing was applied to alloy melts at temperatures above their liquidus by using a twin-screw mechanism. The sheared melt was then cast into a TP1 mould for microstructural examination. Alloy melts with or without shearing were also filtered using the Prefil technique developed by N-Tech Ltd in order to analyse oxides and other second phase particles. The experimental results showed a significant grain refinement through enhancement of heterogeneous nucleation. The intensive melt shearing converted oxide films and agglomerates into well dispersed fine particles with a narrow size distribution. It was confirmed that the fine oxide particles can act as potent sites for nucleation during the solidification of the sheared melt. This paper presents the experimental results and theoretical analysis of shear enhanced heterogeneous nucleation during solidification of Mg- and Al-alloys. A multi-step heterogeneous nucleation mechanism has been proposed and discussed.

Melt Conditioned Direct Chill Casting (MC-DC) Process for Production of High Quality Aluminium Alloy Billets

A novel direct chill (DC) casting process, melt conditioned direct chill (MC-DC) casting process, has been developed for production of high quality aluminium alloy billets. In the MC-DC casting process, a high shear device is submerged in the sump of the DC mould to provide intensive melt shearing, which in turn, disperses potential nucleating particles, creates a macroscopic melt flow to uniformly distribute the dispersed particles, and maintains a uniform temperature and chemical composition throughout the melt in the sump. Experimental results have demonstrated that, the MC-DC casting process can produce aluminium alloy billets with significantly refined microstructure and reduced cast defects. In this paper, we give an overview of the MC-DC casting process and report on results obtained from an industrial scale trial.

Characterisation of Oxide Films in Al-Mg Alloy Melts

Oxide films in Al-0.7Mg and Al-9.4Si-2.3Cu-1.0Zn-0.49Mg (in wt%) alloy melts were characterised using advanced analytical electron microscopy. The oxides were collected by pressurised melt filtration for direct examination by SEM and TEM. The results showed that the oxide films consisted of numerous sub-micron sized oxide particles, rather than continuous solid films. The oxide particles formed in the two Mg-containing alloys were identified as MgAl2O4 spinel by selected area electron diffraction (SAED) and high resolution TEM combined with EDS analysis. The low level of Mg in the melt resulted in the change of the oxide from alumina to MgAl2O4 spinel. The MgAl2O4 crystals were typically faceted with their {1 1 1} crystal planes and were about 0.2-1.2 μm in size. High resolution TEM examination of the MgAl2O4 / a-Al interfaces revealed that there was a cube-on-cube orientation relationship between the two crystals. The possibility of MgAl2O4 particles to act as nucleation sites for α-Al grains during solidification is discussed in terms of the lattice matching at the MgAl2O4 / α-Al interfaces along the specific crystallographic orientation relationship.

Enhanced heterogeneous nucleation on oxides in Al alloys by intensive shearing

Oxides, in liquid aluminium alloys, can cause severe difficulties during casting, contribute to the formation of cast defects and degrade the mechanical properties of cast components. In this paper, microstructural characteristics of naturally occurring oxides in the melts of commercial purity aluminium and Al-Mg binary alloys have been investigated. They are characterised by densely populated oxide particles within liquid oxide films. With intensive shearing, the particle agglomerates are dispersed into uniformly distributed individual particles. It was found that with intensive melt shearing, grain refinement of α-Al can be achieved by the dispersed oxide particles. The smaller lattice misfit between the oxide particles and the α-Al phase is characterised by a well defined crystallographic orientation relationship. And the mechanisms of grain refinement are discussed.

Effect of intensive melt shearing on the formation of Fe-containing intermetallics in LM24 Al-alloy

Fe is one of the inevitable and detrimental impurities in aluminium alloys that degrade the mechanical performance of castings. In the present work, intensive melt shearing has been demonstrated to modify the morphology of Fe-containing intermetallic compounds by promoting the formation of compact α-Al(Fe,Mn)Si at the expense of needle-shaped β-AlFeSi, leading to an improved mechanical properties of LM24 alloy processed by MC-HPDC process. The promotion of the formation of α -Al(Fe, Mn)Si phase is resulted from the enhanced nucleation on the well dispersed MgAl2O4 particles in the melt. The Fe tolerance of LM24 alloy can be effectively improved by combining Mn alloying and intensive melt shearing.

Refinement of the Microstructure of an Al-Mg2Si Hypereutectic Alloy by Intensive Melt Shearing

Attention has been given to the microstructural refinement by high shear processing of hypereutectic Al-Mg2Si alloys as a potential in-situ composite for high-performance structural applications. The results demonstrate that intensive melt shearing promotes the formation of a refined compact polyhedral morphology of primary Mg2Si and refines both the eutectic cell size and the lamellar spacing of Al-Mg2Si pseudo eutectic structures. The refinement mechanisms are discussed.

Melt Conditioned Casting of Aluminum Alloys

High shear melt conditioning of aluminum alloy melts disperses oxide films and provides potent nuclei to promote non-dendritic solidification leading to refined as cast microstructures for shape castings, semis or continuously cast product forms. A new generation of high shear melt conditioning equipment has been developed based on a dispersive mixer that can condition either a batch melt or can provide a continuous melt feed. Most significantly the melt conditioner can be used directly in the sump of a DC caster where it has a dramatic effect on the cast microstructure. The present goals are to expand the castable alloy range and to increase the tolerance of alloys used in transport applications to impurities to increase the use of recycled metal. The paper will review the current status of the melt conditioning technology across the range of casting options and will highlight development opportunities.

Controlling the formation of iron-bearing intermetallics in wrought al alloys by melt conditioned dc (MC-Dc) casting technology

With the increasing use of recycled aluminium alloys from the end-of-life products more and more iron is accumulated into the compositions of alloys. Sometimes, recycling causes the iron levels to increase beyond the set target levels for down-stream processing. The only way to deal with this impurity currently in industry is to increase the primary aluminium added to the furnace to dilute the melt and re-add all other elements or cast it for re-melting or extrude it for products that is not surface finish critical or required higher corrosion resistance. Formation of small well dispersed spherical a- or small b- Fe-bearing intermetallics, which can be homogenised for shorter times and has no negative effect on downstream processing, would be promising even if the iron levels are above the targeted compositional limits. In the present paper, fine and dispersed Fe-bearing intermetallics have been achieved by Melt Conditioned DC (MC-DC) casting technology, instead of coarser Fe-bearing intermetallics forming network like morphology in the DC castings with grain refiner additions (DC-GR). This suggests feasibility of an increased tolerance of iron levels by melt conditioned DC casting technology.

Advanced Casting Technologies Using High Shear Melt Conditioning

Abstract In recent years BCAST has developed the high shear melt conditioning (HSMC) process for controlling solidification. HSMC is now an emerging technology to manipulate the solidification process by intensive shearing to control both the cast microstructure and defect formation. The high shear is applied to the melt using a simple rotor stator which has been demonstrated to disperse oxide films to provide a high density of oxide particles to enhance nucleation. The high-shear device has been successfully applied directly in the sump during DC casting up to the industrial scale to provide both significant grain refinement and uniform microstructure with reduced segregation and defects. It produces a DC casting sump with uniform chemical composition, uniform temperature, and a significantly increased temperature gradient at the solidification front. The sump temperature can be controlled by appropriate selection of the rotation speed. This provides the opportunity to control the cast microstructure, for example, to produce fine and uniformly dispersed second phase particles to improve mechanical properties. The high-shear device can also be used to degas alloys to unprecedentedly low hydrogen levels and has also been successfully applied to remove iron from casting alloys in which the iron containing particles are the primary solidification phase.

Refinement of Solidification Microstructures by the MCAST Process

MCAST (melt conditioning by advanced shear technology) is a novel processing technology developed recently for conditioning liquid metal under intensive forced convection prior to solidification processing. The MCAST process uses a twin screw mechanism to impose a high shear rate and a high intensity of turbulence to the liquid metal, so that the conditioned liquid metal has uniform temperature, uniform chemical composition and well-dispersed and completely wetted oxide particles with a fine size and a narrow size distribution. The microstructural refinement is achieved through an enhanced heterogeneous nucleation rate and an increased nuclei survival rate during the subsequent solidification processing. In this paper we present the MCAST process and its applications for microstructural refinement in both shape casting and continuous casting of light alloys. Discussions will be made on the effect of intensive forced convection on the enhanced heterogeneous nucleation. The concept of physical grain refinement will be proposed and discussed in contrast to the conventional chemical grain refinement by addition of grain refiners.