Characterization of a new type of inclusion in an aluminum high pressure diecasting alloy

Abstract

Inclusions in metal materials are often problematic. For instance, the presence of hard non-metallic inclusions in aluminum alloys can not only degrade the mechanical properties of the final product but also create a number of other processing-related problems in diecasting such as excessive tool wear [1], increased sensitivity to porosity formation [2], poor surface finish and lack of pressure tightness. A great deal of effort has been expended over the years on the identification, quantification and removal of these inclusions [3–5] and significant progress has been made. Recently, Nissan Casting Australia Pty Ltd (NCAP) sponsored a research project to examine the effects of molten metal quality on the quality of high pressure diecastings. Experimental results showed that the porosity level in the castings increased as the number of inclusions in the melts increased [6]. These inclusions were found to consist predominately of an unknown type of aluminum oxide. This letter documents the results of the characterization of this aluminum oxide. Metal used in the high pressure diecasting industry is primarily recycled aluminum produced from various types of scraps. In the case of the foundry in this study, molten metal prepared by the recycling plant is delivered directly to the high pressure diecasting plant ready to be used for production. The alloy is a Japanese standard: ADC12. In order to efficiently examine inclusions present in the aluminum, it is necessary to concentrate them from a small sample of the molten metal [7]. This is realized by utilizing PoDFA (porous disk filtration apparatus) [8]. About 1 kg of molten aluminum contained in a crucible is forced to flow under pressure through a fine ceramic filter (mounted at the bottom of the crucible). Inclusions present in the molten metal are collected on the surface of the filter forming an inclusion cake (Fig. 1 is an optical micrograph showing a cross-section of a portion of a large inclusion cake). The used filter with inclusions collected on its surface forms a sample which can be analyzed using normal metallographical procedures. Samples were taken from various melts including those as-delivered, remelted, untreated and treated in various vessels such as holding furnace, transfer ladle and degassing station. Most of the inclusions found in aluminum melts have been identified and well documented [3]. BOMEM Inc. (Quebec, Canada) has compiled an inclusion library which is supplied to PoDFA users to assist in the identification of inclusions. Most types of inclusions can be readily identified under the optical microscope either by experience or by comparison with images from the inclusion library based on shape, color and other morphological characteristics. However, if the inclusions cannot be identified by these means, then other techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis and X-ray diffraction (XRD) may be necessary for their identification. After being polished, the PoDFA samples were first examined using optical microscopy. It was found that the majority of inclusions appear as dark particles under the optical microscope (see the band of dark particles in Fig. 1 and similar particles in higher magnification image Fig. 2) along with some typical oxide films and sludge particles. These particles are typically 10–50μm in size with morphology similar to spinel crystal listed in the BOMEM Inc inclusion library. The samples were further examined using SEM (Leica S440). Fig. 3 presents a secondary electron image (a) as well as a backscattered electron image (b) of the inclusions found in a PoDFA sample. The inclusions appear to be black in the backscattered electron (BSE) image indicating that they have a lower mean atomic number than the aluminum matrix (grey). The bright phase in Fig. 3b is an Febearing intermetallic compound [9, 10] (often called “sludge”). It is interesting to note that some of the dark particles are within the sludge particles, suggesting that the dark particles may have acted as nucleation sites for the formation of sludge particles. X-ray analysis using energy dispersive spectroscopy (Oxford Link ISIS equipped in Leica S440 SEM) showed that the dark particles mainly contain: O and Al with a minor amount of P, and in some samples, a minor or trace amount of Ca (see Fig. 4), indicating that they are some type of aluminum oxide. (The X-ray spectrum acquired from the PoDFA filter grains which is corundum showed no P under similar counts). Fig. 5 shows the X-ray spectrum of a sludge particle.