Shouxun Ji

Semi-solid processing of engineering alloys by a twin-screw rheomoulding process

Abstract Based on the extensive experience in injection moulding of polymeric materials, a twin-screw rheomoulding process has been developed in our laboratory for near net-shape production of engineering components. The rheomoulding equipment consists of a liquid metal feeder, a twin-screw extruder with closely intermeshing, self-wiping and co-rotating screws, a shot assembly and a central control unit. The fluid flow in the twin-screw rheomoulding process is characterised by high shear rate and high intensity of turbulence. The experimental results of rheomoulded Sn–15wt.% Pb and Mg–30wt.% Zn alloys have demonstrated that the twin-screw rheomoulding process is capable of producing small and near mono-sized solid particles distributed uniformly in a fine-grained eutectic matrix. Compared with other existing semi-solid metal processing techniques, the twin-screw rheomoulding process has the following advantages: small and spherical solid particles of near mono-size, chemical and microstructural uniformity throughout the component, accurate control over a large range of solid volume fractions, lower overall component cost due to low cost of feedstock materials, and shorter cycle time.

Morphological development of solidification structures under forced fluid flow: a Monte-Carlo simulation

Abstract A Monte-Carlo simulation of microstructural development under forced convection is presented. The model takes into account both diffusive and forced fluid flow, kinetics of atomic attachment at the solid–liquid interface and structural modification under the influence of capillary forces. It has been shown that the nature of fluid flow has a very significant influence on the morphology of the solidification structure. A laminar type flow is shown to destabilize the solid–liquid interface promoting dendritic growth for solid growing from fixed substrate. Particle rotation under streamlined flow, or a periodic change in the fluid flow direction around the growing solid is, however, shown to produce the rosette type solidification morphology. A turbulent type flow penetrating into the interdendritic region produces fine and compact solidification structures with or without liquid entrapment.

THE TOXIC COMPOUNDS AND LEACHING CHARACTERISTICS OF SPENT FOUNDRY SANDS

In foundry industry, the millions tones of spent sands weresuccessfully used as landfill materials for many years. Butthis practise is becoming a problem as the disposal costsincrease rapidly and legislation gets tighter. However, thereis not much experimental data to describe their chemicalcharacteristics, especially for the leaching behaviour andtoxic compounds of the different kinds of spent sands. Thisarticle aims to present the analysis of organic compounds andleaching characteristics of the spent foundry sands. Based onthe evaluation of the chemistry of spent sands, the contentsof the selected 32 polyaronmatic hydrocarbon (PAH) and otherorganic compounds in 11 different spent foundry sands wereanalysed. The concentrations of As, Ba, Cd, Cr, Pb, Hg, Se,Ag, Cu and Zn in 11 different spent foundry sands were alsomeasured with regard to leaching characteristics. ThepH-dependent leaching characteristics of chromium were furtherinvestigated. It was found that all spent foundry sandscontain PAHs in which naphthalene is about 30%. The PAHs ingreen sands are much higher than those in chemical binderspent sands, even though phenolic/ester sands have higher PAHsthan furan/acid and silicate sands. The leaching metals arevery low in all spent foundry sands. The leached Cr increaseswith increasing pH of the eluted solution, which can be usedin practise for shortening the leaching time of the spentfoundry sands.

Effects of rheo-die casting process on the microstructure and mechanical properties of AM50 magnesium alloy

Abstract AM50 Mg alloy is usually processed by high-pressure die casting (HPDC) for applications demanding high ductility. However, this alloy has a high tendency of hot tearing, and is difficult to cast because of its large freezing range and low final solidification temperature. In this investigation, the rheo-die casting (RDC) process, a new semi-solid processing technology developed by BCAST at Brunel University, was used to overcome such casting difficulties and to improve mechanical properties. It was found that the RDC AM50 alloy exhibited much improved mechanical properties, especially for the elongation, over those of the same alloy produced by HPDC and other semi-solid processing technologies. The improved mechanical properties can be attributed to the elimination of large gas pores as a result of gas entrapment, substantial reduction of hot tearing, and fine, uniform microstructure, as a consequence of solidification under high shear rate and high intensity of turbulence in the RDC process.

Processing of immiscible metallic alloys by rheomixing process

Abstract Mixing immiscible alloys has been a long standing challenge to both materials scientists and processing engineers. Despite great efforts made worldwide, including extensive space experiments, no casting techniques so far can produce the desirable fine and uniform dispersed microstructure. Based on extensive experience in mixing immiscible organic liquids offered by the polymer processing community, the authors have successfully developed a rheomixing process for mixing immiscible alloys. The rheomixing process utilises first the intensive shear stress-strain field offered by a twin screw extruder to create a fine homogeneous liquid dispersion within the miscibility gap and then the viscous force offered by a semisolid slurry at a temperature below the monotectic temperature is used to counterbalance the gravitational force and the Marangoni effect. A laboratory scale rheomixer has been designed and constructed to realise this two step mixing strategy. The Ga–Pb and Zn–Pb systems were selected to demonstrate the principles of rheomixing. The experimental results showed that the rheomixing process developed is capable of creating a fine and uniform microstructure from immiscible alloys. This paper describes the rheomixing process in detail and the preliminary experimental results on rheomixing in the Ga–Pb and Zn–Pb systems are discussed.

Development of the Rheo-Diecasting Process for Mg-Alloys

Research on rheology of semisolid slurries suggests that an ideal semisolid slurry for semisolid metal (SSM) processing is one in which a suitable volume fraction of fine and spherical particles dispersed uniformly in a liquid matrix. Such ideal semisolid slurry can be obtained by enhancing the effective nucleation and promoting spherical growth during solidification. Experimental investigation and theoretical analysis of solidification under forced convection allowed us to identify the conditions for achieving enhanced effective nucleation and promotion of spherical growth. Such conditions can be summarised as uniform temperature and chemistry throughout the whole volume of liquid alloy, high shear rate and high intensity of turbulence during the solidification process. Based on such understandings, a new SSM processing technology, rheodiecasting (RDC), has been developed for the production of components with high integrity. AZ91D Mg-alloy was used to optimise the rheo-diecasting process and component production trials. The experimental results indicate that the rheo-diecast samples have close-to-zero porosity, fine and uniform microstructure and much improved strength and ductility. Rheo-diecasting process is particularly suitable for production of high-safety, airtight and highly stressed components in the automotive industry.

Solidification behavior of the remnant liquid in the sheared semisolid slurry of Sn-15 wt.%Pb alloy

Abstract Solidification characteristics of the remnant liquid in a sheared semisolid slurry of Sn–15 wt.%Pb alloy is reported. A high shear rate and shear duration combination may promote fine spherical morphology of the secondary solidification product. Resting prior to solidification following shearing appears to wear off the effects of shearing.

Effect of shot peening on fatigue performance of ductile iron castings

Abstract Ductile iron is a commonly used structural material. However the unsatisfactory fatigue performance has limited its application for some dynamic loads. Shot peening is a mechanical surface modification process to extend the fatigue life of materials. Results of the influence of the shot peening treatment on ductile iron castings with as-cast surface and machined surface are presented. The results showed that shot peening ductile iron castings could double the fatigue life for an as-cast surface and quadruple the fatigue life for a machined surface. It is believed that shot peening affects fatigue life through the retardation of crack nucleation and growth as a result of the introduction of work hardening, the existence of compressive stresses on the surface layer, and the removal of the surface irregularities of the ductile iron castings.

Melt Conditioned Twin Roll Casting (MC-TRC) of Thin Mg-Alloy Strips for Direct Stamping of Mg Components

In this paper we introduce a novel process for the production of thin-walled magnesium components by direct stamping of twin roll cast thin Mg strips. In this process, the melt conditioned twin roll casting (MC-TRC) process is used to produce thin Mg strips (thickness <2 mm) which have a fine equiaxed grain structure and little basal texture and, more importantly, are free from centreline segregation. Such thin Mg strips can be used for thin-walled component production by direct stamping without any rolling. A major advantage of this process is that it circumvents the low formability problem inherently associated with Mg based alloys. In this paper, AZ31 alloy is used to demonstrate this new process. For both TRC and MC-TRC strips, we will analyze the microstructures, assess the mechanical performance at elevated temperatures and conduct hot stamping in the as-cast condition without any prior rolling.

Melt superheating on the microstructure and mechanical properties of diecast Al-Mg-Si-Mn alloy

The application of aluminium alloys in automotive body structure is one of the main developments in recent years. The increase of the mechanical properties of the ductile die-casting is one of the most critical topics for the application. In this work, the effect of melt superheating on the morphology, size and distribution of α-Al phase and Fe-rich intermetallics, and on the mechanical properties of the Al-Mg-Si-Mn diecast alloy was investigated. The results showed that the refined microstructure could be obtained through melt superheating. The volume fraction of dendritic α-Al phase and the Fe-rich intermetallic phase formed in the shot sleeve was significantly reduced, resulting in the refined microstructure. Overall, the melt superheating could improve the mechanical properties of the yield strength, ultimate tensile strength, and elongation of the diecast Al-Mg-Si-Mn alloy. And, the Fe-rich intermetallic phase formed in the shot sleeve with the coarse compact morphology and formed in the die cavity with the fine compact particles were identified as the same α-Al12(Fe,Mn)3Si composition in the present experimental conditions, which was not affected by the melt superheating.