Understanding Grain Refinement. (Aluminum Silver Anniversary Paper).

In his Silver Anniversary lecture tided, "Grain Refinement in Cast Aluminum (Al) Alloys" (02-194), C.R. Loper, Univ. of Wisconsin-Madison, showed how the non-stable nature of grain refinement is affected by the addition of titanium (Ti) and/or boron (B) when aluminum is held at temperature. The objective of this research was to better understand the grain refinement mechanism and to assess the differences in grain refining between ingot and casting production.

Loper pointed out that the presentation represented a portion of the graduate research studies by R. Kotschi at the Univ. of Wisconsin-Madison. The studies were conducted using alloys that did not contain silicon (Si). According to Loper, the influence of Si on the effectiveness of the grain refining mechanism is significant.

This study used two alloys--one with a high purity (99.99% Al) and one produced from 99.99% Al alloyed with 1% Magnesium (Mg). In his lecture, Loper showed that the fading of the grain refinement effect changed as a function of the equilibrium stability of [Al.sub.3]Ti particles above the peritectic temperature. Boron was shown to slow the fading rate, but B additions to aluminum pre-alloyed with Ti were ineffectual.

Loper noted that refinement of the pro-eutectic high purity aluminum grains in aluminum casting alloys has a direct relationship to more efficient feed metal transfer during solidification. Therefore, grain refining reduces microporosity by inducing more effective mass feeding in these alloys.

While Al-Ti-B master alloys were shown to be powerful grain refiners in the original study, Al-B master alloys do not grain refine at all. Loper said that these observations were limited to alloys of low Si content (usually less than 0.5%). When 5-7% Si is present in the aluminum casting alloy, Al-B master alloys have been found to be powerful grain refiners. This has led to improved grain refinement master alloys for use in the production of hypo-eutectic Al-Si casting.

This study found that the process of grain refinement is a non-equilibrium process and therefore grain refinement techniques are open to improved methods, both in terms of improved efficiency in the introduction of grain refining agents into the melt and in the generation of more effective grain refinement alloys. According to Loper, since grain refinement is typically practiced in the production of cast aluminum alloys, the grain refinement agents are pre-alloyed into ingots in amounts considerably in excess of that required.

Loper also noted that while nearly all grain refiners are based on Ti and B, this system has not been advisable for some cast aluminum alloys, such as Mg and Zirconium. This has resulted in the development of boride-free grain refiners where TiC particles are the active nucleants.

This study suggests that future developments of grain refinement technology be directed at improved efficiency in the use of grain refiners and improved melt processing procedures rather than reliance upon excessive grain refiners introduced through pre-alloying of ingot.