W. D. Griffiths

A method to study the history of a double oxide film defect in liquid aluminum alloys

Entrained double oxide films have been held responsible for reductions in mechanical properties in aluminum casting alloys. However, their behavior in the liquid metal, once formed, has not been studied directly. It has been proposed that the atmosphere entrapped in the double oxide film defect will continue to react with the liquid metal surrounding it, perhaps leading to its elimination as a significant defect. A silicon-nitride rod with a hole in one end was plunged into liquid aluminum to hold a known volume of air in contact with the liquid metal at a constant temperature. The change in the air volume with time was recorded by real-time X-ray radiography to determine the reaction rates of the trapped atmosphere with the liquid aluminum, creating a model for the behavior of an entrained double oxide film defect. The results from this experiment showed that first oxygen, and then nitrogen, was consumed by the aluminum alloy, to form aluminum oxide and aluminum nitride, respectively. The effect of adding different elements to the liquid aluminum and the effect of different hydrogen contents were also studied.

Metal/mould interfacial heat transfer during solidification of cast iron in sand moulds

Heat transfer at the metal-sand interface was investigated for the case of solidification of cast iron in (1) cylindrical sand moulds and (2) ceramic cylindrical moulds with sand blocks at the bottom. An inverse method of solving the one-dimensional heat conduction equation was used to determine the metal-sand interfacial heat flux transients and heat transfer coefficients with the heat conduction equation modified to take into account the packed bed nature of the sand mould, the effect of convection of the mould gases and the evolution and absorption of heat due to mould reactions. However, temperature measurements in the moulds during the experiments revealed that the heat transfer in the cylindrical sand moulds was not truly radial and could therefore not be used to obtain accurate interfacial heat transfer results. In the case of solidification of cast iron against sand blocks, mean values of the heat flux of about 50 kWm−2 were measured for green and dry clay-bonded silica sands, with and without additions of seacoal. The corresponding heat transfer coefficients were about 625Wm−2K−1. Within the scatter of results obtained there was no discernible difference in the heat flux or the heat transfer coefficients with the different sand formulations. The heat transfer mechanisms through the sand-casting interface were interpreted from an examination of the nature of the sand and casting surfaces. Heat transfer through the interface is proposed to occur by conduction through the gas forming the atmosphere of the interface and by radiation, in approximately equal amounts.

One-dimensional predictive model for estimation of interfacial heat transfer coefficient during solidification of cast iron in sand mould

Abstract A one-dimensional predictive model is proposed to estimate the interfacial heat transfer coefficients during unidirectional solidification of a cast iron alloy, vertically upwards, against a sand block. The model is based on the surface roughness characteristics of the casting and sand surfaces and the concave deformation of the initial solidified casting skin towards the sand surface. The modelled interfacial heat transfer coefficients and predicted temperatures inside the casting and the sand block showed an approximate agreement with experimentally determined values. The model showed that radiation was a significant mode of casting/sand interfacial heat transfer with the predicted contribution of radiation to the overall heat transfer being nearly 50%. The evaluation of the model in comparison to the interfacial heat transfer models proposed by Zeng and Pehlke suggested that the interfacial conditions considered in this model, namely, the mean peak to valley heights of the casting/sand mould surfaces and the gap width calculated from the deformation of the initial solid skin, gave a more accurate prediction. This predictive heat transfer model has an advantage over the inverse modelling technique as the matching of experimentally measured temperatures to determine the boundary conditions is avoided and the heat transfer coefficients can be estimated as an integral part of the casting simulation.

Hydrogen, bifilms and mechanical properties of Al castings

Abstract Recent research has suggested that H dissolved in an Al melt could diffuse into double oxide films (bifilms), increasing their size and forming oxide related hydrogen containing porosity, which was found to decrease the Weibull moduli of the tensile properties of castings. In this work, the Weibull moduli of the tensile properties of two Al castings, both expected to contain oxide films of approximately the same amount and age, were compared. The results showed that, when the H content of the castings was reduced to ∼50%, from 0·18 to 0·08 cm3/100 g Al, there was an increase in the Weibull moduli of the ultimate tensile strength (UTS) and the % elongation by ∼400% and 200% respectively. The increased Weibull modulus was thought to be brought about by holding the moulds under vacuum and thus reducing H pick-up by the metal, from the solvent and the resin in the sand moulds.

Instability of the Liquid Metal–Pattern Interface in the Lost Foam Casting of Aluminum Alloys

Abstract The nature of the liquid metal–pattern interface during mold filling in the Lost Foam casting of aluminum alloys was investigated using real-time X-ray radiography for both normal expanded polystyrene, and brominated polystyrene foam patterns. Filling the pattern under the action of gravity from above or below had little effect on properties, both cases resulting in a large scatter of tensile strength values, (quantified by their Weibull Modulus). Countergravity filling at different velocities demonstrated that the least scatter of tensile strength values (highest Weibull Modulus) was associated with the slowest filling, when a planar liquid metal–pattern interface occurred. Real-time X-ray radiography showed that the advancing liquid metal front became unstable above a certain critical velocity, leading to the entrainment of the degrading pattern material and associated defects. It has been suggested that the transition of the advancing liquid metal–pattern interface into an unstable regime may be a result of Saffman–Taylor Instability.

A Study of the Behaviour of Double Oxide Films in Al Alloy Melts

The mechanical properties of Al castings are reduced by inclusions, particularly double oxide films, or bifilms, which are formed due to surface turbulence of the liquid metal during handling and/or pouring. These defects have been reported not only to decrease the tensile and fatigue properties of Al alloy castings, but also to increase their scatter. Recent research has suggested that the nature of oxide film defects may change with time, as the air inside the bifilm would react with the surrounding melt leading to its consumption, which may enhance the mechanical properties of Al alloy castings. In order to follow changes in the composition of the internal atmosphere of a double oxide film defect within an Al melt, a series of analogue experiments were carried out to determine the changes in gas composition of an air bubble trapped in a melt of commercial purity Al, subjected to stirring. The bubble contents were analysed using a mass spectrometer to determine their change in composition with time. Also, the solid species inside the bubbles solidified in the melt were analysed. The results suggested that first oxygen and then nitrogen inside the bubble were consumed, with consumption rates of 2.5x10-6 and 1.3x10-6 mol m-2s-1, respectively. Also, hydrogen diffused into the bubble from the melt at an average rate of 3.4x10-7 mol m-2s-1, although the rate of H diffusion increased significantly after the consumption of most of the oxygen inside the bubble. Based upon these reaction rates the time required for a typical alumina bifilm to lose all its oxygen and nitrogen was determined to be just under 10 minutes.

Hydrogen pick-up during mould filling in the lost foam casting of Al alloys

In the Lost Foam casting of Al alloys the foamed polystyrene pattern is broken down by the heat of the advancing liquid metal as it fills the mould. This has led to discussion about the possibility of increased hydrogen pick-up by the liquid metal from the gaseous pattern degradation by-products accumulating at the liquid metal—foam pattern interface, leading to detrimental porosity in the final casting. The results presented here were derived from comparisons of the initial measured hydrogen content of the liquid Al alloy before mould filling, and the hydrogen content of the final castings, coupled with real-time X-ray imaging of the filling of the mould to determine whether entrainment of the foam pattern degradation by-products was occurring. This showed that increased hydrogen content in Al Lost Foam castings was attributable to the entrainment of degrading pattern material, and not due to increased absorption of hydrogen from the interfacial atmosphere.

Tracking inclusions in aluminium alloy castings using positron emission particle tracking (PEPT)

Abstract Inclusions in castings lead to the initiation of fatigue cracks and are detrimental to their mechanical properties. Computer simulations of the casting process include models of inclusion movement but there is currently no method of experimentally verifying these models. The technique described here used positron emission particle tracking (PEPT) to follow the movement of radioactive alumina particles entrained in a liquid aluminium alloy as the mould was filled, thus showing the behaviour of inclusions in castings. Five experiments were carried out with two different alumina particle sizes, inserted into a simple cast plate made in resin bonded sand moulds. Besides demonstrating the value of the experimental technique, these experiments showed how inclusion movement in castings was greatly affected by the random nature of the fluid flow during mould filling.

Comparison of Oxide Thickness of Aluminium and the Effects of Selected Alloying Additions

An experiment was undertaken to study the oxidation of liquid superpure aluminium (SP-Al) and alloys containing Mg, Si, Cu and Fe. The alloys were held at 750 °C, for a number of different holding times up to 7 hours. A comparison of the oxidation of SP-Al (superpure) held at 700 °C, 750 °C and 800 °C for 3 hours was also carried out. On observation of the samples using SEM and EDX the oxides of SP-Al and the Al-Mg alloy grew quickly (by 5.8 μm after 7 hours and 2 μm after 1 hour, respectively), in a manner reported in the literature. The other alloys had reduced rates of oxide growth, with thickness changes between 30 nm and 0.25 μm for Al-Si, Al-Cu and Al‑Fe alloys. Changes in holding temperature showed a thick oxide on samples held at 850 °C for 3 hours.

Focused Ion Beam Milling and Imaging: An Advanced Method to Detect Fine Inclusions in Cast Aluminium Alloys

The combined function of a FIB milling technique utilising beam sizes of under 10 nm coupled with a micromanipulator and FIB imaging enables analysis of the microstructure of samples and fabrication of TEM thin foils. This is accomplished at desired locations in the same chamber without moving the sample or any mechanical sawing and thinning. In this study, the FIB milling and imaging technique was used to examine the microstructure and chemical composition of fine inclusions in an Al alloy, which are generally difficult to detect by conventional optical and/or scanning electron microscopy due to their size and volume fraction. Examples are presented of fine particles in cast commercial purity aluminium and a melt conditioned AA 5754 alloy.