A study set out to show how properties can vary throughout a casting and why that’s ok.
The bulk of A356 castings are poured in permanent molds with solidification times generally less than 10 minutes. This process normally provides the best metallurgical properties when compared to the other common casting processes such as high pressure die casting (traditional or vacuum), sand casting, plaster molding and investment casting. Because of these properties, the tilt poured permanent mold process is used often for high integrity parts.
Tensile strength and ductility are closely tied to metallurgical properties such as secondary dendrite spacing, the level of microporosity, and the metal cleanliness which impacts the level of inclusions.
Permanent molds are often gravity poured down a sprue, which results in a fair amount of turbulence. In this process, the hotter metal will often end up at the bottom of the mold, while the feeding devices (risers) are located on top. This disserves directional solidification, which requires the hotter metal be near the risers. In the tilt pour process, the mold cavity, starting in a horizontal position,
is slowly brought to an upright position, reducing turbulence and providing the hotter metal at the top of the mold.
To examine this phenomenon, an “autopsy” was performed on a swivel guide plate submitted to extreme tensile and bending loads, the failure of which would have serious consequences. Unlike wrought components, castings cannot always provide handbook tensile properties evenly throughout the part for any foundry alloy. This leads to confusion among casting users, as they are given a range of properties for the supposedly same alloy depending on the suppliers, academics or handbooks they may consult. They will often conclude that a design value one can trust does not exist.
The truth is, mechanical properties in castings not only change with the process but may vary within the same casting. For instance, in the swivel guide plate, elongation varied from 2.1-0.8% depending on the solidification conditions at a particular location (solidification time and temperature gradient). Metalcasters can reliably predict properties at various locations to ensure the appropriate properties are present at critical areas. Presenting the potential user with an analysis such as the one developed in this study may go a long way to convince non-metallurgists that the mechanical properties of structural castings can be reliably evaluated.
What We Now Know
The metallurgical study of 13 excisions in a tilt poured permanent mold aluminum A356 casting, with local solidification times comprised between 0.5 and 2.5 minutes lead to the following conclusions:
- Local tensile properties vary widely with solidification conditions, the quality index Q spanning from 308 to 363MPa. These values are expectedly lower than the minimum Q required for the standard ASTM B108 separately cast test bars (Q > 367MPa), the local solidification time of which is less than 20 seconds.
- The variations in the microporosity and tensile elongation can be reasonably predicted when the local value of the solidification time and solidus velocity are known via solidification modeling. The measured values of the elongation (2.1-8%) vary in a much wider range than the predicted ones (2.4-4.6%). This discrepancy is due to the fact only one tensile test was performed per location, the confidence interval on the elongation of cast aluminum being typically 25% of the average value obtained on a very large number of tests.
- The secondary dendrite arm spacing (DAS) can be accurately predicted when the time between the beginning and end of solidification is determined by solidification modeling.
- In addition to its conventional use to predict macroshrinkage in castings (i.e. “hot spots”), it was shown that solidification modeling could be used to evaluate the variations in microporosity and tensile elongation inside a cast part. This tool should however be used with some caution and plenty of discernment. In the realm of solidification, a prediction should be considered as a “relatively” faithful caricature of reality. For a given alloy, the trust it may inspire should be built after a long process of experimenting on a wide range of geometrical configurations and process conditions. MC
By Franco Chiesa, Jeremy Carignan, David Levasseur and Gheorghe Marin, Centre de Metallurgie du Quebec (Trois-Rvieres, Quebec, Canada); and Michael Jutras, Powercast Manufacturing Inc. (St.-Eustache, Quebec, Canada) as published in August 2018 MODERN CASTING Magazine. Read the full article here.
This article is based on the paper “Metallurgical Properties inside a Tilt Poured Permanent Mold Structural Aluminum AlSi7Mg03 (A356) Casting” (Paper 18-011) originally presented at the 122nd Metalcasting Congress.
Metal Foundry News - From Modern Casting Magazine
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