Improving Melt, Casting Quality.
Van Wiggen described various experiments performed using Al-Sr rod in combination with an in-line or last minute addition concept. The experiments showed that the Al-Sr rod displayed fast modification and ready modification performance. These characteristics enable the product to be used late in the production process at an aluminum foundry. The benefit is a higher yield on the use of Sr, as savings of more than 50% Sr were shown.
M. Stucky, CTIF, and J.E. Gruzleski and J. Anson, McGill Univ., discussed the relationship between Sr and microporosity in their presentation, "Effect of Strontium Concentration on Microporosity in A356 Aluminum Alloy (01-009)." In this presentation, the effect of several different Sr levels on the amount of microporosity in A356 Al-Si foundry alloy was examined to determine whether changes in pore properties are due to chemical effects (amount of Sr in the casting) or microstructural effects (modification of the eutectic structure). In addition, nearest neighbor cluster analysis was used to distinguish between the effect on gas and shrinkage porosity.
Conclusions from their analyses were:
* the percentage porosity increases with the addition of Sr,
* the increase in percent porosity is due to an increase in gas porosity;
* the increase in gas porosity is due to the chemical effect of Sr;
* the pore density decreases with the addition of Sr;
* the decrease in pore density is due to a decrease in the number of shrinkage pores;
* the decrease in the number of shrinkage pores is due to the microstructural effect of Sr;
* the average pore size increases with the addition of Sr;
* both gas and shrinkage porosity increase in average pore size with the addition of Sr;
* the pore shape becomes dramatically more spherical with the addition of as little as 30 ppm Sr.
For the aluminum foundry, said the presenters, these conclusions mean that:
* the Sr content of the melt should be kept to the minimum value necessary to achieve the desired level of modification;
* a small amount of Sr (30 ppm) may be added to reduce shrinkage porosity;
* in fatigue applications where irregular pore morphologies are harmful, it may be beneficial to add small amounts of Sr (30 ppm) in order to "spheroidize" the pore shape.
For practical purposes, said the presenters, it may be best to always add a small amount of Sr to the casting (15-30 ppm) to produce a partially modified structure with more dispersed and spherical porosity. These low amounts of Sr do not significantly increase the amount of porosity although they do alter the pore properties.
In a look at gating system designs and their effect on casting quality, M. Divandari, Iran Univ. of Science and Technology, and J. Campbell, Univ. of Birmingham, presented "Mechanisms of Bubble Trail Formation in Castings (01-048)." Bubble trail describes the defect remaining in film-forming alloys after the passage of a bubble through the melt, said the presenters. Since air, water vapor and other core gases normally are highly oxidizing to liquid metal, a bubble of any of these gases will react aggressively and oxidize the metal as it progresses. The result is a bubble train, the collapsed tube of oxide as a crack resembling an old sack. This trail is a threat to the mechanical strength and integrity of a casting.
Through experimentation, the presenters determined the following about bubble trail formation:
* the bubble trail is a form of double oxide casting defect that is dependent on the splitting and reforming of the oxide film on the upper half of the bubble;
* the first form of the bubble trail is an open tube, which is observed in highly viscous, semi-solid metal;
* the second form of the bubble trail is a partially collapsed tube that is formed in a lower viscosity environment where the fraction of solid present is low.
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|Title Annotation:||P.C. Van Wiggen|
|Comment:||Improving Melt, Casting Quality.(P.C. Van Wiggen )|
|Article Type:||Brief Article|
|Date:||Jun 1, 2001|
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