Effects of Centrifuging Force and Melt Treatment on the Microstructure and Wear Resistance Property of A390 hyper-eutectic Al-Si Alloy
Date Issued
2016
Date
2016
Author(s)
Lai, Yi-Chen
Abstract
While casting the A390 hypereutectic alloy, primary Si particles tend to float to the upper part of the casting due to the difference in specific weight between primary Si particles and the aluminum bulk liquid, the so-called gravity segregation. As a result, the microstructures and the mechanical properties in different parts of the casting vary. This research utilized the above-mentioned phenomenon of the primary Si gravity segregation, and further to enhance this segregation nature by employing centrifuging casting process to achieve better wear resistance in specific area of the castings. A centrifuging casting apparatus was constructed which allows the casting to be solidified under the centrifugal force. Under the centrifuging casting condition, the primary Si particles which have low specific gravity and high hardness tend to move toward the rotational axis and aggregate at the inner portion of the castings, and thus to enhance the wear resistance of the A390 alloy. In addition, P refinement (for primary Si), Sr modification (for eutectic Si) and P + Cr refinement (for primary Si) of A390 alloy were performed to examine the effects of these different melt treatments on the microstructures and wear resistance property of the centrifuging castings at the rotational speed of 150 rpm (~10G). Further, the relationship between the microstructure features obtained and the wear resistance property was evaluated. The microstructure of the A390 alloy without any treatments exhibits somewhat large primary Si particles with an average size of around 96μm, and acicular type eutectic Si of class 1. When the alloy was refined with an appropriate amount of P, i.e., 0.02%P, the size of the primary Si was substantially reduced to around 43μm, while the eutectic Si remained more or less unchanged. In addition, when the alloy was refined with various combinations of P and Cr, no beneficial effect in reducing the size of the primary Si particle was found. Instead, a slight increase in primary Si size was obtained, with the average size being increased to around 65μm. On the other hand, when the alloy was modified by 0.02%Sr, even though the eutectic Si was improved to class 4-5, the primary Si was coarsened significantly to around 250μm. Furthermore, the effects of the microstructure features on the wear resistance property of A390 alloy was studied, and regression analyses were performed by looking into the correlations between the four main dependent variables, namely, the area ratio of the primary Si (X, %), the average primary Si size (Y, μm), degree of eutectic Si modification (Z, class), and the % porosity (C, %), and the wear rate (W, mm3/m). The obtained regression equation is: W = 0.029245 – 0.00098X + 0.000045Y – 0.00347Z + 0.001448C. The analysis results indicate that the wear rate can be reduced by increasing area ratio of the primary Si, decreasing primary Si size, increasing degree (class) of eutectic Si modification, and reducing % porosity.
Subjects
Centrifuging force
Primary Si segregation
Refinement
Eutectic Si modification
Wear resistance property
Type
thesis