John F. Grandfield

New apparatus for characterising tensile strength development and hot cracking in the mushy zone

A novel apparatus for solidification research has been developed which utilises a modified tensile testing machine and customised mould to control thermal and mechanical parameters during solidification. The test provides constraint during solidification and generates information about strength development, strain accommodation and hot cracking behaviour of the mushy zone material. The equipment has been used to determine the effect of grain refinement and composition variations on these parameters. This paper describes the test apparatus and the type of information it generates. It includes a comparison of hot cracking produced in the rig with that obtained from direct chill cast product. The development of strength of a relatively pure material, alloy AA194, and a highly alloyed, extended freezing range alloy, AA7075 is presented. In the case of alloy AA7075 strength development began at a solid fraction of approximately 0.7 and continued to increase as solidification progressed. The development of strength in alloy AA194 did not occur until a fraction solid of 0.9. Under tensile loading conditions, the development of strength appears to occur at higher solid fractions than for rheological test methods.

The Effect of Grain Refinement and Cooling Rate on the Hot Tearing of Wrought Aluminium Alloys

Using modifications to the Rappaz-Drezet-Gremaud hot tearing model, and using empirical equations developed for grain size and dendrite arm spacing (DAS) on the addition of grain refiner for a range of cooling rates, the effect of grain refinement and cooling rate on hot tearing susceptibility has been analysed. It was found that grain refinement decreased the grain size and made the grain morphology more globular. Therefore refining the grain size of an equiaxed dendritic grain decreased the hot tearing susceptibility. However, when the alloy was grain refined such that globular grain morphologies where obtained, further grain refinement increased the hot tearing susceptibility. Increasing the cooling decreased the grain size and made the grain morphology more dendritic and therefore increased the likelihood of hot tearing. The effect was particularly strong for equiaxed dendritic grain morphologies; hence grain refinement is increasingly important at high cooling rates to obtain more globular grain morphologies to reduce the hot tearing susceptibility.

Observation and Prediction of the Hot Tear Susceptibility of Ternary Al-Si-Mg Alloys

An investigation into the hot tear susceptibility of ternary Al-Si-Mg alloys has been made using direct crack observation, measurement of load response, and predictions made by a modified Rappaz-Drezet-Gremaud (RDG) hot tearing model. A peak in both the hot tear susceptibility and the load at solidus occurred at approximately 0.2Si and 0.15Mg, and then the hot tear susceptibility decreased as the total solute content increased. In general, a good correlation was found among the observation of cracks, the load at solidus, and the predictions of the RDG hot tearing model, although it was shown that correlation with the RDG model depended critically on the fraction solid at which solid coalescence was assumed to occur. A combination of these approaches indicated that when the total Si+Mg content and the Si:Mg ratio increased toward four, a decrease occurred in the hot tear susceptibility because of an increase in the amount of final eutectic formed. At the lowest Si:Mg ratio of 0.25, the RDG model also predicted a lower relative hot tear susceptibility than that measured by the load at solidus. In these alloys, the final stages of solidification are predicted to occur over a large temperature range, and hence, both the predictions of the RDG model and the measurement of the load were dependent on which fraction solid was chosen for grain coalescence. In the alloys studied in this article, the formation of small amounts of the ternary eutectic Al+Mg2Si+Si caused the highest hot tear susceptibility.

The Impact of Rising Ni and V Impurity Levels in Smelter Grade Aluminium and Potential Control Strategies

The technology for controlling smelter metal impurities post reduction has steadily improved. For example, control of sodium has seen the reduction and, in some plants, the elimination of chlorine gas from the casthouse. However, changes in the purity of cell feed materials such as anodes are giving rise to new challenges in impurity control; vanadium and nickel levels are an emerging problem. This paper briefly reviews the important impurities and their effects on downstream casting, forming and final application properties. Particular emphasis is given to nickel and vanadium. Strategies for controlling these impurities are also discussed and areas where new technology is needed are also highlighted. In some cases it is not known what the tolerable limits of impurities are. There are a plethora of metal refining techniques used in the extraction of other metals which could be investigated for the control of impurities in smelter grade aluminium.

Greenhouse Emissions in Primary Aluminium Smelter Cast Houses - A Life Cycle Analysis

The aluminium industry is reducing its carbon dioxide emissions and environmental footprint. In order to identify and prioritise areas in the cast house where greenhouse gas emissions can be reduced it is necessary to quantify CO2e (CO2 equivalent tonnes) emissions for the various cast house operations. In this study two typical cast house layouts are examined. In one case, 22kg 99.85% aluminium remelt ingots are produced using chain conveyor ingot casting machines. In the second case, wrought alloy extrusion and rolling slab direct chill cast products are made. Both plants are sized at 500ktpa. The various process inputs in terms of energy and materials were identified and typical usage rates assigned. The results show that general electricity consumption, dross generation and furnace energy consumption are the three biggest areas of CO2e and should be targeted for improvement. Magnesium consumption also has a large effect in the case of the

Thermodynamics and kinetics analyses of ZrB2 formation in molten aluminium alloys

Smelter grade aluminium can be used as a source for electrical conductor grade aluminium after the transition metal impurities such as zirconium (Zr), vanadium (V), titanium (Ti) and chromium (Cr) have been removed. Zirconium (Zr), in particular, has a significant effect on the electrical conductivity of aluminium. In practice, the transition metal impurities are removed by adding boron-containing substances into the melt in the casthouse. This step is called boron treatment. The work presented in this paper, which focuses on the thermodynamics and kinetics of Zr removal from molten Al–1 wt-%Zr–0.23 wt-%B alloy, is part of a broader systematic study on the removal of V, Ti, Cr and Zr from Al melt through boron treatment carried out by the authors. The thermodynamic analyses of Zr removal through the formation of ZrB2 were carried out in the temperature range of 675–900°C using the thermochemical package FactSage. It was predicted that ZrB2 is stable compared to Al–borides (AlB12, AlB2) hence would form during boron treatment of molten Al–Zr–B alloys. Al–Zr–B alloys were reacted at 750 ± 10°C for 60 minutes, and the change in the chemistry and microstructure were tracked and analysed at particular reaction times. The results showed that the reaction between Zr and AlB12/B was fast as revealed by the formation of boride ring at the early minutes of reaction. The presence of black phase (AlB12), i.e. the original source of B, after holding the melt for 60 minutes advocated that the reaction between Zr and AlB12/B was incomplete, hence still not reached the equilibrium state. The kinetics data suggested a higher reaction rate at the early minutes (2 minutes) of reaction compared to at a later stage (2–60 minutes). Nevertheless, a simple single-stage liquid mass transfer controlled kinetic model can be used to describe the overall process kinetic. The analysis of integrated rate law versus reaction time revealed that the mass transfer coefficient (km) of Zr in molten alloy is 9.5 × 10−4 m s−1, which is within a typical range (10−3 to 10−4 m s−1) observed in other metallurgical solid–liquid reactions. This study suggests that the overall kinetics of reaction was predominantly controlled by the mass transfer of Zr through the liquid aluminium phase.

Analysis of Boron Treatment for V Removal Using AlB 2 and AlB 12 Based Master Alloys

Boron treatment is a widely used practice in industry for removing transition metals such as Ti, V, and Zr. Mostly, Al-B master alloys containing AlB2 and AlB12 are used for the treatment. This paper describes the analysis and comparison of the boron treatment using these two types of Al-B master alloys. Kinetic experiments of V removal using the two alloys were carried out for alloy Al-1wt %V at 750°C in a resistance furnace. Samples were taken at regular interval and characterized using scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The change of V concentration, analyzed using ICP (inductively coupled plasma) analysis, was also tracked with reaction time. It has been shown that the kinetics of boron treatment using AlB2 based alloy was faster compared to those using AlB12, which was related to the particle size and available interfacial area for reaction. Boride particles settling was also analyzed in the present study using multi-locations sampling technique that revealed faster settling for AlB12 based master alloys compared to AlB2.