Microstructural Characterization of As-Quenched and Heat Treated Al-Si-Mg Melt-Spun Ribbons

Al-Si based alloys are the most important and widely used among the family of aluminum cast alloys. With small additions of magnesium these alloys become heat treatable, increasing the mechanical strength with toughness. In this work, the commercial A359 alloy was rapidly quenched by using melt spinning process, using rotating speeds of 22 and 56 m/s. The asmelt spun ribbons were characterized by a combination of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The microhardness of the ribbons was also measured. It was found that the quenched of the ribbon (wheel side) presents a featurless zone and a second zone composed by dendritic-cellular grains. Silicon particles, with nanometer size were found to preferentially precipitate along the grain boundaries. During heat treatment the over-saturated silicon precipitated with morphology spherical. The microstructures changes as well as the increasing of hardness with the increasing of cooling rate might be attributed to the supersaturated solid solutions of Si in Al matrix and to the refinement of the microstructure. Introduction Aluminum-Silicon alloys are widely used in the aluminium foundry due to their excellent castability and high strength-to-weight ratio. Magnesium is frequently added to these alloys to promote an increase of the strength and hardenabilty of the cast part [1]. On the other hand, due to the low solid solubility of Fe, Si and Mg in Al, during conventional casting, precipitates of Al5FeSi, Al8Si6Mg3Fe, Mg2Si and others with more complex structures are formed. However, a substantial increase of solid solubility of those elements in aluminum might be achieved by non-conventional techniques such as the rapid solidification, and consequently to suppress the formation of those precipitates. The objective of the present study is to analyze the influence of rapid solidification process on the microstructure in melt-spun of the Al-9Si-0.7Mg (A359 type) alloy. Experimental Procedure The chemical composition of the Al-9Si-0.7Mg (A359-type) as-cast ingots is shown in Table 1. Approximately 4 g of the molten alloy with 250°C above the liquidus temperature was ejected with an overpressure of 200 mbar onto a rotating Cu-Be wheel. For rotating speed of 56 m/s, the melt-spun ribbon was produced with 2.7 mm in width and 28-34 μm of thickness. On the other hand, for 22 m/s the ribbon was obtained with 3.4 mm in width and 45-55 μm of thickness. The samples have been characterized by using a combination of optic, MO, scanning, SEM, and transmission electron microscopy, TEM, energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and by Vickers microhardness testing. The samples were prepared by standard metallographic procedures. The XRD measurements were carried out in a Siemens D5005 using CuKα radiation. Samples for TEM (M120-Philips with EDS) were thinned by ion milling using dual gun system operating at 5kV with 800 μA per gun. The ribbons were heat treated at three temperatures: 200°C, 350°C and 450°C for 12 hours in air. After each heat treatment, measurements of Vickers hardness were performed with a load of 10gf. Journal of Metastable and Nanocrystalline Materials Online: 2004-08-01 ISSN: 2297-6620, Vol. 22, pp 103-108 doi:10.4028/www.scientific. net/JMNM.22. 103 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific. net. (Semanticscholar.org-19/03/20,16:20:54) Table 1. Nominal chemical composition (wt.%) Results and Discussion The conventionally as-cast alloy presented a typical microstructure (Fig. 1), which consists of α-Al equiaxed dendrites with interdendritic region containing acicular Si and various intermetallic phases. The intermetallic compounds were identified as Mg2Si phase with Chinese-script morphology; the Al8Si6Mg3Fe phase with both Chinese-script morphology and with indefiniteshaped; the Al5FeSi phase with needle-like morphology. The main reactions expected to occur during solidification are [2,3]: Liq. Al + Si at 577 °C, Liq. Al + Mg2Si at ~595°C, Liq. Al + Si + Al5FeSi at 576°C, Liq. + Al5FeSi Al + Si + Al8Si6Mg3Fe at 567°C, and Liq Al + Mg2Si + Si at 555 °C. Fig. 2 presents the XRD patterns of the conventionally cast, 22m/s and 56m/s melt-spun ribbons. The effect of the increase in cooling rate, which is associated with the decrease of the ribbon thickness (produced when the wheel rotation speed is increased), can be observed in the patterns. The diffraction pattern of as-cast sample reveals the presence of the Al4Cu2Mg8Si7 phase, which was not detected by the SEM observations. For the as-quenched ribbons, the intermetallic phases were not detected; only the Al and Si phases were observed. The intensities of silicon diffraction peaks significantly decreased. However, these peaks broadened as the cooling rate increased. It was also observed a slight shifted of the silicon and aluminium peaks for the ribbons when compared with those as-cast sample. The decrease of intensity and broadening of the silicon peaks in the as-quenched ribbons (especially for the 56 m/s ribbon) indicates the increase of solubility of silicon in Al matrix but there is remaining not dissolved silicon in all samples. However, within the limits of XRD detection (typically < 5 vol.%), the absence diffraction peaks of intermetallic phases in the ribbons cannot be considered as sufficient evidence of its complete dissolution in the Al matrix. Fig.1. Microstructure of as-cast (conventionally cast) sample. Fig. 2. XRD patterns taken from: as-cast (conventionally cast) sample and as-quenched ribbons produced by different rotating speeds. Figure 3a shows optical cross-section micrograph of the as-quenched ribbon processed by wheel rotating speed of 56 m/s The micrograph shows two well defined zones: (i) featureless zone Alloy Si Fe Cu Mn Mg Zn Sn Others A359 9.45 0.11 <0.006 <0.05 0.67 <0.05 0.02 0.2