Coupled heat and fluid flow model and experimental verification of aluminium plate die casting

Abstract

The present work aims to forecast mould filling, void shape, location and size as well as columnar to equiaxed transition (CET) in commercial pure aluminium casting. A model coupling the momentum equations of the fluid flow and heat transfer equations is presented, in which metallostatic pressure, air gap and oxide layer are considered. Different casting parameters were investigated such as casting configuration by varying the plate thickness from 5 to 20 mm, melt superheat from 40 to 120°C, mould preheat up to 200°C and different pouring heads ranging from 0·3 to 0·6 m. Regarding the microstructure and void formation, the approach based on the Niyama criterion, was considered. The experimental verification of the model was achieved by gravity die casting in the form of a rectangular cavity. Voids inside aluminium plate were investigated by X-ray imaging. Microstructure and CET was investigated microscopically. The supposed model proves its validity for mould filling and in detecting the void features and CET.     

Thermomechanical response of 50.7 at.-%Ni-Ti alloy in the pseudoelastic regime

Abstract

The thermomechanical response of 50.7 at.-%Ni-Ti superelastic alloy has been investigated for a range of applied strains and strain rates. Parameters of interest include the critical stresses for nucleation and completion (σ _ms and σ _mf respectively) of the stress induced martensite (SIM) transformation, as well as the magnitude of the endothermal and exothermal reactions as a function of the applied strain and rate of deformation. Novel techniques including infrared thermometry and laser extensometry have been utilised, and are found to be particularly suited to experimentation with nitinol. The transformation temperatures and values of entropy and enthalpy of formation have been determined using differential scanning calorimetry.     

A study of shell growth irregularities in continuously cast 310S stainless steel

Abstract

Growth irregularities in continuous casting are believed to be associated with crack formation and breakouts. Differential thermal analysis on 310S stainless steel samples indicated primary precipitations of both austenite and ferrite during solidification. In tensile tests on solidifying samples, abrupt shrinkages in volume were detected in the peritectic range of temperatures. Micrographic and microsegregation analysis on samples extracted from a breakout shell revealed high ratios of primary-precipitated austenite in the thick sections of the shell, and high ratios of primary-precipitated ferrite in the thin sections. Alternating precipitations of austenite and ferrite are proposed to occur during solidification. Regions of the shell with high ratios of primary austenite remain in contact with the mould and exhibit high growth rates, whereas regions with high ratios of primary ferrite shrink in volume due to the ferrite to austenite transformation, which results in the formation of air gaps between the shell and the mould and reductions in growth rate.     

Dynamic strain aging effects in niobium microalloyed steel

Abstract

The dynamic strain aging behaviour of a niobium microalloyed steel has been examined. Hot tensile testing was carried out on heat treated and as received specimens. Heat treated specimens were austenitised at 1000°C for 1 h, and then cooled in air or in a stainless steel cylinder to obtain various amounts of free or uncombined interstitial solutes in solid solution, to examine the effect on the dynamic strain aging behaviour of the steel. It was found that dynamic strain aging takes place in niobium microalloyed steel during tensile testing at temperatures ranging from ambient temperature to 450°C at a crosshead displacement rate of 2 mm min^-1. As a result, the ultimate tensile strength and initial work hardening rate exhibit maximum values at temperatures between 200 and 350°C. Also, load-extension graphs for tested specimens show serrated behaviour and yield points at 200, 250, and 300°C. It is believed that dynamic strain aging in niobium steel is caused by interaction between dislocations and interstitial solutes (nitrogen and carbon) or solute pairs consisting of one interstitial and one substitutional solute atom (for example Mn-C and Mn-N).     

A Method of Magnesium Treatment for Producing Gravity-diecast Ductile Iron

Equipment for the in-stream treatment of ductile iron has been adapted for gravity-diecasting. The treatment unit consists of a pouring basin, reaction chamber, and a reservoir for holding treated iron. The device is highly mobile in so much that it can be transferred from furnace to successive dies and back again, allowing consecutive pouring. The authors have studied the effect of reaction chamber design and nodularising-alloy type on the microstructure of cast specimens, using three types of chamber design and three types of nodulariser at different temperatures. The results indicate that a carbide-free structure with a high nodule count of up to 1500 n/mm² within the as-cast structure of a diecast rod specimen of 30 mm dia. can be achieved using this treatment equipment, when employing suitable nodularising alloy and process chamber design.     

Optimal feeder design in sand casting process by growth method

Abstract

The computer aided optimal feeder design in the sand casting process is considered. The design problem is formulated as an additive evolutionary topology optimisation problem. The initial design of the presented method is a casting part which is placed in a suitable environmental mould box. The method is composed of three stages: determination of the feeder neck connection point, construction of the feeder neck and the feeder growth. During the growth stage, the feeder topology is improved gradually until satisfying some predefined criteria. The technological constraints and user contribution can be easily incorporated in the presented method. The success of the proposed method is supported by illustrative examples.     

Fluidity of aluminium die castings alloy

Abstract

The aim of the present work was to investigate the fluidity of four different high pressure die cast Al–Si alloys at different pouring temperatures. A vacuum fluidity test apparatus was employed to measure fluidity. The analysed alloys showed different flow sensitivities to casting temperatures. Furthermore, it is showed that among the considered alloying elements, magnesium and silicon affected the fluidity of the melt. One alloy was then contaminated with 50% scrap addition, increasing the amount of oxide inclusions. The fluidity of the contaminated melt has then been measured and compared with the fluidity of the clean melt. The results show that the fluidity of the alloy with scrap addition is lower than that of the clean melt. Further the fluidity linearly increases at increasing temperatures within the range between 580 and 680°C until it reaches a plateau at the highest pouring temperatures.     

Measurement of autoclave thermal profiles during high pressure steam de-waxing of investment shells Part 1 - Vessel profiles

Abstract

Investment casting research is being carried out by the University of Birmingham, sponsored by EPSRC and a consortium of industrial companies. The programme is aimed at developing a fundamental understanding of the process, with a view to routinely producing sound, net shape castings. A key stage within the investment process is that involving the removal of wax from the unfired ceramic shell. This important process is carried out within the confines of a sealed pressure vessel, more commonly referred to as a Boilerclave*, with external pressure gauges as the only indication of what is actually happening inside the cavity. To metaphorically 'see' inside the vessel would allow the factors controlling de-wax to be characterised and controlled. This paper contains results obtained at The University of Birmingham using a specially instrumented steam autoclave that allows visual data capture, thermal and steam pressure profiles within the chamber, and thermal instrumentation of waxes and shells to be obtained. Results of thermal and pressure profiles inside the vessel body are presented and implications of these results upon the mechanism of wax removal and the effect upon ceramic shells are discussed.     

The Effect of Trace Elements on the Cooling Curves,Microstructure and Mechanical Properties of Eutectic Aluminium-Silicon Alloy

Abstract

The effect of trace elements, used for modification, on the cooling curves obtained during solidification, microstructure and mechanical properties of eutectic aluminium-silicon alloy was investigated. The results of this study indicate the following: 1 The addition of sodium or sodium plus strontium or antimony modifies the eutectic silicon while the addition of sulphur does not alter the microstructure.

2 Those elements which modify the eutectic-silicon, lower the eutectic solidification temperature, while those elements which do not bring about modification, do not alter the eutectic solidification temperature.

3 The addition of those elements which modify the eutectic aluminium-silicon alloy, viz., sodium or sodium plus strontium or antimony improves the UTS and percentage elongation. The addition of titanium to eutectic aluminium-silicon alloy containing these trace elements improves the UTS and percentage elongation to a further extent. Among the various trace elements added to eutectic aluminium-silicon alloy, the addition of sodium plus titanium improves the UTS and percentage elongation to the maximum extent.

     

Compression behaviour of semisolid YL112 die casting aluminium alloy following isothermal heat treatment

Abstract

The semisolid compression deformation behaviour of YL112 die casting aluminium alloy with the non-dendritic and dendritic structures has been compared in tests using a Gleeble-1500 thermomechanical simulator. The non-dendritic structure was obtained by isothermal treatment at 570°C for 120 min. In tests up to compressive strains of 0·8, the semisolid compression stress of the alloy with non-dendritic structure initially increases rapidly with increasing strain, then decreases, before reaching a plateau value. Under different deformation temperatures and deformation rates, the maximum compressive stresses are obtained in all cases at strain values of ～0·07. The semisolid deformation stress decreases with increasing deformation temperature and increases with increasing deformation rate. The compression behaviour of the two types of alloy differs. The semisolid compression stress of the alloy with dendritic structure is higher than that of the alloy with non-dendritic structure; and for strains >0·07, the semisolid compression stress increases and decreases with increasing strain for alloys with dendritic and non-dendritic structures respectively.