Aluminium Alloys 2006 - ICAA10

Abstract: In the present research the possibility of studying the solidification of aluminum alloys by using the quenching technique is analyzed. Since the quenching technique does not provide reliable information (i.e. due to an overestimation of solid fraction) when measuring the solid fraction over 2D images from samples quenched at high temperature, the overestimation problem is investigated by analyzing 3D reconstructed microstructures from quenched samples. The 3D reconstructed microstructure may provide better understanding about the cause of overestimation of solid fraction when quenching at high temperatures. Consequently, the reconstruction of the microstructure that has existed before quenching may be possible after identifying and removing the solid phase that develops during quenching. In the present research, binary aluminum alloys are solidified and quenched at different temperatures, and then 3D reconstructed images are analyzed. The possibility of reconstructing the microstructure that develops during solidification before quenching is discussed.

Abstract: The influence of different cooling regimes on the surface microstructure of strip cast AA1XXX alloys was investigated. The cooling rate was varied over a wide range using a bench scale simulation apparatus. It was found that the type and morphology of the intermetallics varied over a wide range both at the cast surface and in the near surface regions. These observations were compared to the compounds formed in the bulk of the slab. It was found that, a surface layer containing a high concentration of alloying elements, an inverse segregation layer, forms under some conditions and can result in a high density of intermetallics. By varying casting conditions, the mechanism of formation for these structures was studied. The surface microstructure of the slabs was characterized as a function of the different local cooling regimes.

Abstract: Intermetallic phases formed during directional Bridgman solidification of a 6xxx series Al alloy in the growth velocity range 5-120 mm/min have been characterised using conventional and high-resolution transmission electron microscopy. Cubic αc-AlFeSi and β-AlFeSi were always present in the alloy, but Al13Fe4 was only observed at 5mm/min and monoclinic αT-AlFeSi at 80 mm/min. Cubic αc-AlFeSi was observed to have a variety of morphologies resulting from flexibility in its growth mechanisms. β-AlFeSi with twins and faults showed strongly anisotropic growth by steps, resulting in its platelet morphology. Occasionally blocky β-AlFeSi particles were observed particularly at 30-60 mm/min, suggesting that other mechanisms could influence their morphology. Two types of composite particles, β-AlFeSi/Al13Fe4 and β-AlFeSi/cubic αc-AlFeSi, were observed in this alloy, and likely formed by two quasi-peritectic reactions.

Abstract: The potential use of an Al-9Si-1Cu alloy (W328 alloy) as a replacement for W319 alloy in engine blocks was investigated.. The ambient mechanical properties (tensile, hardness and fatigue) of sand and permanent mould cast test bars along with bars machined from a sand cast automotive component were studied. The tensile properties were evaluated in as-cast and T6 heat-treated conditions. The effects of Fe and Mn on properties were also investigated. The castings in W328 alloy exhibit lower porosity than W319 alloy, which is attributed to the smaller solidification range and feeding distance and larger volume fraction of the eutectic phase. Higher iron levels increased the level of intermetallics and reduced properties. The addition of Mn did not offset the effect of higher iron levels. The W328 alloy offers some advantages over the 319 alloy and is a potential candidate for production of automotive components such as engine blocks and cylinder heads.

Abstract: Cast plates of A356 aluminum alloy with different thickness were fabricated by rheocasting and squeeze casting. Tear tests were performed on the as-cast and heat-treated products (T5 and T6), and effects of solidified structure and heat treatment on unit crack propagation energy (UEp) were examined. Increased cooling rate (corresponding to a decrease in plate thickness) resulted in refined solidified structure and enlarged UEp values for as-cast samples. For both rheocast and squeeze-cast samples, both spheroidized eutectic Si particles and age-hardened α-Al matrix by T6 treatment were effective for increasing UEp. UEp of the squeeze-cast sample was higher than that of the rheocast sample. Observation of crack growth path and fracture surface revealed that the tear toughness of the present cast alloy was controlled by distribution of eutectic solidified region in the cast structure, which provided a preferential crack growth path. The discontinuous distribution for the squeeze-cast sample is considered to be beneficial for increasing crack growth resistance rather than the continuous arrangement of the network-like eutectic region for the rheocast sample. Relatively small UEp was obtained for the T6 treated rheocast sample collected from the 6 mm thickness plate. This is attributable to the volume fraction of the eutectic solidified region in the sample being larger than others under the present experimental conditions.

Abstract: The effect of size refinement of primary silicon and modification of eutectic silicon, separately and simultaneously, on microstructure of hypereutectic Al-20wt%Si has been investigated under different casting conditions. Results showed that number per unit volume of primary silicon particles was doubled by inoculation with 0.02% phosphorus in the alloy with 0.0015% phosphorus already in it, whereas 70% of a small ingot (40×35 mm) became modified eutectic, and the formation of primary silicon was suppressed, on adding 0.1% strontium. Simultaneous addition of phosphorus and strontium still refined the primary silicon but did not modify the eutectic. Addition of strontium alone caused roughening of the primary silicon, exemplified by increased undercooling registered on the cooling curve. Also crystallographic relationships between polyhedral primary silicon particles have been investigated by Electron Back Scattered Diffraction (EBSD).

Abstract: Solidification behaviour of 357 Al-alloy under intensive forced convection in the rheo-die-casting (RDC) process, was investigated experimentally to understand the effects of the intensity of forced convection and shearing time on the nucleation and growth behaviour. It was found that under intensive forced convection, heterogeneous nucleation occurred continuously throughout the entire volume of the solidifying melt. All the nuclei could survive due to the uniform temperature and composition fields created by the forced convection. This has been named as ‘effective and continuous nucleation’. It is also found that the nuclei grow spherically with an extremely fast growth rate. This makes the primary solidification essentially a slow coarsening process, in which Ostwald ripening takes place by dissolution of the smaller particles. In addition, it was found that intensive forced convection suppresses partially the formation of the primary phase, promote nucleation of the primary particles, and hinders the particle growth.

Abstract: A quantitative understanding of the effect of fluid flow on the microstructure of cast alloys is still lacking. The application of time dependent magnetic fields during solidification offers the possibility to create defined flow conditions in solidification processing. The effect of rotating magnetic fields (RMF) on the microstructure formation in cast Al-alloys (Al-7wt.%Si, Al-7wt.%Si- 0.6wt.Mg) is studied experimentally and numerically. The forced fluid flow conditions result in pronounced macrosegregation effects and affect microstructural parameters. With increasing fluid flow the primary dendrite spacing decreases whereas the secondary dendrite arm spacing increases. The experimental analysis is supported by a rigorous application of numerical modeling with the software package CrysVUn.

Abstract: By imposing a high frequency magnetic field, the surface quality and macro/micro structure of the direct chill cast billets are improved. The casting speed can also be improved by simultaneous applying mold and electromagnetic coil, especially for the alloys with a wide freezing range such as Al-Cu-Mg alloy. The extent of macrosegregation in the Al-Cu-Mg alloy is reduced by application of the magnetic field. Moreover, the inverse segregation layer usually observed in the direct chill cast billet is eliminated by the effect of the electromagnetic field. The microstructure of the hot-rolled and heat treated billets cast under the high frequency field also show better characteristics.