B. Cantor

Isothermal grain coarsening of spray formed alloys in the semi-solid state

Grain coarsening of alloys in the semi-solid state is important in controlling grain sizes in spray forming and in other processes in which an alloy is formed or cast in the semi-solid state. Until now, this coarsening has been analysed in terms of classic LSW theory or in terms of the migration of grain boundary liquid films. These analyses suggest that the coarsening rate should increase with increasing solid fraction, fS, but this suggestion conflicts with previous experimental results which show that coarsening rate decreases with increasing fS for fS>0.7. This paper shows, for the first time, that coarsening rate does increase with fS for fS less than approximately 0.75 in agreement with the liquid film migration model, and then decreases again with further increasing fS for fS greater than approximately 0.75. A modified model of liquid film migration is proposed which takes into account the reducing area of the liquid film as fS increases for high fS. The formation of intragranular liquid droplets, and the pinning of grain boundary liquid films by dispersoids during coarsening are also discussed.

Quasi-peritectic solidification reactions in 6xxx series wrought Al alloys

Abstract Secondary intermetallic phase formation during directional solidification of two 6xxx series wrought Al alloys at low growth velocities of 5–30 mm/min has been investigated using differential scanning calorimetry, transmission electron microscopy and scanning transmission electron microscopy. Thermodynamic calculations predict that a quasi-peritectic reaction, L+Al 13 Fe 4 → α-Al+α-AlFeSi, should occur during equilibrium solidification of the alloys. However, no composite Al 13 Fe 4 /α-AlFeSi particles, but composite Al 13 Fe 4 /β-AlFeSi particles and triple phase junctions have been observed for the first time, indicating a divorced metastable β-AlFeSi quasi-peritectic reaction, L+Al 13 Fe 4 → α-Al+β-AlFeSi. More detailed analysis suggests that the metastable β-AlFeSi quasi-peritectic reaction is more favourable both at nucleation and during growth. No unique orientation relationship was found between primary Al 13 Fe 4 and peritectic β-AlFeSi. The nucleation and growth of peritectic phases and the morphology evolution of the two intermetallic phases, Al 13 Fe 4 and β-AlFeSi, are discussed.

Direct measurement of sprayform temperatures and the effect of liquid fraction on microstructure

Abstract Surface temperatures and the corresponding liquid fractions in a spray deposited commercial 2000 series Al alloy have been measured accurately by plunging thermocouples into sprayforms during growth. Two colour pyrometry was shown to be reasonably accurate for measuring surface temperature but only under constant, or preferably zero, overspray density. Yield, porosity and grain size have been shown to be strong functions of the top surface temperature and liquid fraction in the sprayform during spray deposition, over a wide range of liquid fraction. The final grain size cannot be predicted from the liquid fraction using conventional coarsening theories, because they do not take into account variations in nucleation density and are not applicable at low liquid fractions. Rapid variation of liquid fraction with top surface temperature during spray deposition implies that to maintain optimum conditions requires control of the temperature to within at least ±2°C.

Interface effects during consolidation in titanium alloy components locally reinforced with matrix-coated fibre composite

Abstract An investigation has been made of diffusion bonding at the interface between a local reinforcing metal matrix composite and a monolithic engineering material. Diffusion bonding occurs during the consolidation of the composite during component manufacture. In this study, the composite is made up from Ti–6Al–4V titanium alloy coated SiC fibres, and the monolithic engineering material is also Ti–6Al–4V, but with a different microstructure. An interface model is presented which takes account of diffusion bonding and which is able to describe the deformation behaviour at the interface between composite and monolithic material during composite consolidation. The model is developed from an existing diffusion bonding theory, and is implemented into finite element software. The finite element simulations, and results of experiments, show that diffusion bonding can lead to localised deformation, the inhibition of consolidation, and a resulting inhomogeneous distribution of consolidated and unconsolidated regions during component manufacture. A further effect of the diffusion bonding is to increase the level of component distortion which results from the constraint imposed on the consolidating composite. The interface model presented enables the simulation of practical forming processes so that process variables such as temperature and pressure can be chosen to ensure appropriate finished component properties.

Macro-segregation in aluminium alloy sprayformed billets

Abstract It is generally assumed that during sprayforming the amount of micro- and macro-segregation in the sprayforms will be limited or absent because of low liquid fractions, small grain size and high cooling rate. This paper shows that macro-segregation can occur in sprayforms of Al, even when small liquid fractions are present during growth of the sprayform. This segregation occurs by an inverse mechanism, with solute-rich liquid sucked back through a solid network by solidification shrinkage. Liquid flow outwards is further enhanced by the centrifugal force produced by sprayform rotation. The liquid sucked outwards is enriched in solute (with partition coefficients

The effect of vanadium and grain refiner additions on the nucleation of secondary phases in 1xxx Al alloys

High purity Al-0.3 wt% Fe-0.1 wt% Si alloys with different Si, V and grain refiner contents were melt spun to produce microstructures of submicron secondary phases entrained in a higher melting point Al matrix. On reheating, a dispersion of eutectic liquid droplets forms that represents an exaggerated version of the liquid puddles that solidify punched-off between Al dendrite arms during conventional casting. The subsequent resolidification of the droplets, analyzed using differential scanning calorimetry (DSC), allows the nucleation-controlled aspects of secondary phase selection to be studied. The droplets solidify as the metastable FeAl{sub m} phase in ribbons containing {approx{underscore}equal}500 ppm V or {approx{underscore}equal}100 ppm V plus Al-Ti-B, Al-Ti-C or Al-B grain refiner. This phase contributes to the fir-tree surface defect in commercial sheet products. this work suggests that the combination of V and Al-Ti-B promotes FeAl{sub m} in commercial ingots, and confirms that solidification rate and bulk Si content also influence phase content.

Effect of semisolid microstructure on solidified phase content in 1xxx Al alloys

Abstract Melt-spun high purity Al–0.3 wt% Fe–0.1 wt% Si alloys, containing V and Al–5Ti–1B grain refiner, were melted at 2 K/min from 635°C to different temperatures above the solidus in a differential scanning calorimeter (DSC). Quenched and slow cooled (at 2 K/min) microstructures were examined using scanning electron microscopy (SEM). Melting was observed in a hot-stage reflected light microscope. Al–Fe4Al13 melted first, forming cell boundary liquid films. Rapid coarsening occurred at