Casting answers & advice.
Q We hand ladle out of the dip-out well of our wet bath reverberatory aluminum melting furnace (7,500-lb. capacity) into tilt-pour permanent molds. We add grain refiner in the charge door with every charge addition. Is this the best practice? When should we add the grain refiner?
This is not the best practice, as grain refiner should be added to the melt closer to when the casting is poured.
Aluminum grains grow in a shape similar to the branches of a tree (dendrites) when solidifying. These dendrites continue to grow in a long, columnar fashion (Fig. 1) until growth is impeded by another dendrite or the mold wall. In simple terms, grain refining is the process of adding nucleating material to the melt to initiate grain growth. As more grains start growing, they impinge on and limit the size of each other, resulting in smaller grain structure (Fig. 2). The smaller grain structure helps reduce hot tearing and provides better feeding while the metal is solidifying. Grain refining also enhances the properties of the cast component, producing castings with increased pressure tightness and mechanical properties, better response to heat treatment, more uniform anodizing and improved machinability. The typical method for grain refining aluminum alloys is the addition of titanium and boron in the form of an aluminum master alloy.
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The metalcaster in question has defined process control. The amount of molten metal removed from the dip well (weight and volume of castings poured) is monitored, and an equal amount of charge material is added hourly to keep the metal level in the furnace consistent. Measured titanium boron additions are included with each charge, based on the weight of the charge material being added. All additions are made at the rear of the furnace through the charging door.
For maximum effectiveness, grain refining should be delayed and performed as close to the point of pour as possible. In tilt pouring, the metalcaster could experiment with putting the master alloy directly into the pour cup. When put directly into the melt in a reverberatory furnace, the grain refiner should be added into the dip well, rather than at the charging door with the charge material. When an aluminum bath is held in molten state over a period of time, as in the case of this metalcasting facility, the titanium boron can settle out and drop to the bottom of the melt, reducing its effectiveness for nucleating grain growth. Chances are that the nucleating benefits of the grain refiner are never reaching the dip well, so there can be no assurance that the castings will have a finer grain structure.
But, in addition to focusing on the process of introducing the master alloy, we also would like to know if the final casting has fine grains. This metalcaster was not performing testing to validate grain size in its castings, so there wasn't a way to know if the grain refiner addition process was producing the finer grains desired in the castings.
The best means to test for grain size is through thermal analysis or by viewing the casting microstructure (metallography). Thermal analysis measures the cooling curve of the solidifying alloy. Under-cooling will indicate that the material is less resistant to nucleation, signifying that more (thus smaller) grains should start growing. Some metalcasters measure the titanium level of the alloy as an indication that the castings have fine grains. The addition of titanium boron master alloy will increase the titanium content, which can be tested using a spectrometer. However, the presence of titanium does not indicate that it was actively nucleating new grains, so it may not be an accurate validation of grain refining effectiveness.
These measurement methods can be used to validate process controls. For example, the amount of titanium present in the spectrographic analysis can be used effectively as a quality control tool to verify that the grain refining process is consistent. Once grain size has been validated through thermal analysis or metallographic examination, it can be correlated to the titanium level measured by spectrographic analysis. If the processes for master alloy additions remain consistent, then it can be estimated that the same titanium level in future melts will continue to produce the same grain size. This correlation between grain size and titanium level then should be re-established periodically.
Recommendations are the opinion of the AFS Technical Dept. based on referenced literature and experience. If you need assistance with a technical issue, fax or email your question to: Casting Answers & Advice, c/o MODERN CASTING, at 847/824-7848 or firstname.lastname@example.org.
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|Date:||Feb 1, 2007|
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