2 means to melt aluminum.
* Detailed are the melt operations of two aluminum casting facilities, including charging, fluxing and degassing.
* This overview of the operations can help guide other plants in search of a means to optimize their melt capabilities.
Have you ever wondered how well your nonferrous melting operation is optimized for your facility? Are you curious as to how other firms operate their melt facilities? A look at several facilities might answer these questions.
This article briefly outlines the melt operations of two facilities--Plant A and Plant B--from startup to casting. Both firms have found these operations have contributed to their efficient casting production. These firms' operations may contrast with how you melt your nonferrous alloys. However, taking a page from Plant A or Plant B's practices may pave the way to improving your melting methods.
Laying the Groundwork
Plant A is an aluminum sand and permanent mold casting facility that produces components for multiple industries, with aerospace being the largest customer base. The firm pours many aluminum alloys, but 319, 355 and 356 are the most common alloys cast. The facility has three automated molding machines and also several manual cope and drag and jolt-squeeze molding lines.
The firm uses slightly different melt practices for its manual and automated lines to maximize as much of the melt as possible. Plant A has four gas-fired melting furnaces: three 6,000-lb. (2,721-kg) dry hearth reverbatory furnaces and one 10,000-lb. (4,536-kg) tower melt unit.
Plant A uses these furnaces to save energy and provide a large capacity for the melts. A melt changeover for a furnace takes 3-4 hrs. to drain, clean and start up the furnace again. All four furnaces are inspected and cleaned daily. At the end of each shift, the furnaces are turned down to 1,300 F (704C).
Plant B, which is significantly larger than Plant A, has a higher number of molding lines for its sand and permanent mold cast components. This facility, like Plant A, taps into a number of industries, including automotive, military and aerospace. Plant B pours a wide range of alloys, from the aluminum alloy 300 series to specialty alloys in the F3S series. The firm melts its alloys in either 500- or 1,000-lb. (227-or 454-kg) gas-fired crucible furnaces. Because of its large quantity of components (1,620 active part numbers), Plant B possesses more than 40 of these furnaces, which it prefers due to the range of alloys poured. These furnaces sometimes are used as both a melting and holding unit.
Once a melt is poured from a furnace, an entirely different alloy can be melted, allowing for a rapid changeover. The plant operates its furnaces 24 hrs. a day, five days a week. Therefore, the firm does not have any end-of-shift practices until the end of the week, when all the furnaces are emptied and cleaned.
Startup and Charging
Plant A always has a furnace melting 356 due to demand, and 319 is on high order, as well. The facility's initial charge materials are a 50/50 combination of pure ingot and scrap. This balance allows the facility to stay within its specifications of what it is melting. Sometimes a charge of 100% scrap will be run if it becomes built up in the melt area, but the operators try to limit this to only once an hour. As the initial charge is melted, Plant A will add scrap accordingly to meet demand. For instance, if 356 is needed on several lines, the bath will be charged twice an hour, but if it is not in such demand, the bath will be charged once. The well set points for both types of furnaces are the same at 1,300-1,550F (704-843C), but this depends on the temperature needed on the pouring lines.
At 75 min. before the casting shift begins, melt operators pour a small portion of the molten bath to create a spec disc casting for the facility's metal inspection lab (Fig. 1). This disc is examined in a spectrometer, and the melt operators will be told if the alloy meets the required specifications for the casting. If not, the firm will bring the melt to the required formula and make additions from a selection of three alloying elements: magnesium, copper and silicon. The predominant element used is magnesium because of its burnout characteristic during aluminum hold and degas cycles. Plant A determines the amount of alloying elements to add to a melt based on weight by studying a proprietary chart that the company developed for its operations.
[FIGURE 1 OMITTED]
After the casting shift begins, additional spec discs from the furnace are examined at 2-hr. intervals to ensure that the melt stays adequate. The firm discovered this time frame works well for its operation, and if the melt needs to be readjusted, the operators will add the elements right then.
Plant B's operations begin with a broader range of charge ratios depending on the type of casting to be poured (Fig. 2). If the components are for low-intensity applications, an ingot-scrap ratio ranges from 50/50-70/30. This ratio is ideal for castings in which superb component design practices were followed. However, a different ratio is used if the casting design is not optimized for the casting process, and the casting is for a more critical application. This 80/20-100/0 ratio allows for a higher pure ingot makeup, which provides better mechanical properties due to fewer oxides and tramp elements. This optimizes the feeding distance in the mold during pouring.
[FIGURE 2 OMITTED]
When the first charge materials are set, the furnaces spend 1-2 hrs. (depending on heal size) creating a molten bath. Plant B's melt operators look to obtain an optimal furnace temperature of 1,350F (732C), which allows for a reduced energy cost and a degassing process with few consequences. Similar to Plant A, a sample of the melt will be obtained for spectrographic analysis to determine what additional elements will be needed to achieve the desired melt concentration. Plant B commonly uses four additives during this time: magnesium, silicon, strontium and titanium boron (Fig. 3). Magnesium and silicon are used less frequently than the other two elements; magnesium is used to make up for magnesium loss during holding times, and silicon is used if the customer requires high levels of the element.
[FIGURE 3 OMITTED]
If a melt moves out of specification, the firm often takes two approaches: bring the melt back to its specification by diluting it with a pure ingot, or using additives to correct the chemistry.
Degassing and Fluxing
At Plant A, once the molten bath meets its specifications, the top is skimmed, and the molten metal is tapped into a 600-lb. (272-kg) ladle and transferred via an overhead conveyor to the flux and degas area (adjacent to the molding line) for processing. If the melt is to be poured on a manual line, it is then poured into one of three 900-lb. (408-kg) crucible holding furnaces. Processing for the automated lines is performed in the transfer ladle because it would be less time-efficient to flux and degas the subsequent larger holding furnaces.
Plant A always fluxes its melts prior to degassing because a flux will help trap oxides during degassing and keep the metal cleaner. For the automatic line, the firm will flux and skim the melt once an hour due to the high demand of metal. Plant A has found that adding 24-40 oz. (0.68-1.13 kg) of cover flux to each melt works best for the operation (Fig. 4). As the flux covers the top of the melt, a portion of it is scraped open to allow a rotary impeller to be placed in the melt for degassing. Before the impeller is placed, 1 lb. (0.45 kg) of titanium boron is added to the melt for grain refinement.
[FIGURE 4 OMITTED]
When an impeller is lowered into a melt, it is located off-center to prevent any vortexing of the metal. Nitrogen is used as the degassing element, as the firm found it worked best after it operated with several other gases in the past (Fig. 5). Plant A only performs one 6-min. cycle for degassing, but this time can vary due to weather conditions, such as humidity. Once the degassing process is complete, the cover flux is scraped off and the melt is visually inspected for cleanliness. If there is a clear molten metal surface with no sign of sludge, then the melt is clean.
[FIGURE 5 OMITTED]
On the manual lines, after processing, the furnace rests for 3 min. and a spec disc then is poured for a reduced pressure test (RPT) that lasts 6 min. The inspectors then calculate the specific gravity of the disc to determine porosities in the melt. This is done by placing the sample under a vacuum in an RPT machine and then comparing the sample's dry weight with its weight after it is submerged in water. If the specific gravity differs from the target number, then additional degassing is performed.
Another test Plant A practices during processing is thermal analysis to examine for grain refinement, which can be done right on the shop floor. This is performed by filling a sand cup with metal at least 1,200F (649C) in a thermal analytical machine, which will determine grain size. If the machine shows inadequate grain refining levels, further modification will be performed. Once the metal is fully processed, it is scraped and ready to pour.
For the automated lines, the processed ladle is transferred to one of three crucible or two electric holding furnaces (Fig. 6). For the crucible furnaces, there is a baffle that extends 2 ft. (61 cm) into the furnace wells, and when the ladle is poured, the baffle separates the clean metal from the dross. A lance is added in these furnaces to continue processing nitrogen into the melt for cleanliness. During this time the clean metal is moved under the baffle, and the furnace is poured from the clean metal side into a ladle, which is automatically poured into the molds. The 900-lb. (408-kg) crucible holding furnaces on the manual lines keep the melt small so no metal goes unused. The metal here is hand ladled into the molds.
[FIGURE 6 OMITTED]
Plant B's degassing/fluxing methods carry both similarities and differences to Plant A. The time span between Plant B's charging and processing varies depending on when each melt is needed for pouring. The firm looks to process its melts as close to the pouring time as possible to avoid any reprocessing that might be needed if the melt is held too long. If the melt would be used for large jobs, it is transferred to a ladle for processing. If a melt were processed in a furnace and then transferred to a ladle, the transfer would damage the metal, create oxides and add gas, which is why the ladle processing is preferred. Contrarily, in lower-run jobs, which are poured manually, the metal is processed in the furnace.
Plant B combines its degassing and fluxing practices into one operation. Its primary degassing method incorporates a lance placed in the molten bath for a 30-min. cycle to distribute a chlorine nitrogen gas mixture, which is used in every melt. Plant B also has found this gas to be very effective in cleaning the molten aluminum, thus it does not utilize a separate fluxing process. The firm avoids using powdered fluxes because they tend to reduce crucible life.
During the degassing process, Plant B also will add the other alloying elements in the melt. Titanium boron (5% titanium, 1% boron and 94% aluminum) is added at the beginning of the cycle, and strontium (10% strontium and 90% aluminum) is added 10-15 min. into the cycle. These elements are used for grain refinement and structural modification, and they are added during degassing because the lance helps mix them into the melt. The firm has found it also is a good practice to add these as close to the pouring operation as possible to minimize any further processing. During this time, Plant B evaluates a gas plug sample (using a RPT) by comparing porosities with images on a proprietary chart to determine if additional processing is necessary. The gas level for critical jobs is verified using specific gravity measurements.
For critical components or if further degassing is needed, the melt operators will use one of the firm's rotary degas units providing the melt with a sulfahexafluoride nitrogen gas mixture for 15 min. If additional strontium is required, the firm will use this gas in the rotary unit for 30 min. while adding the strontium 10 min. into the cycle. It cannot add the strontium to the chlorine mix with the lance because the chlorine will strip away the strontium from the melt.
After the degassing/fluxing process is complete, the firm will let the molten bath rest for 15 minutes and then take another gas plug sample from the melt for another RPT. Like with degassing, Plant B will perform as many RPTs necessary to attain the adequate melt. After the specific gravity of the plug is measured, and if the hydrogen gas content is low enough to ensure an efficient alloy for the casting line, then the melt is approved. Another spectrometer test is conducted, as well, to examine for the proper alloy chemistry. Once this is approved, the melt is ready for pouring.
After the furnace is completely tapped out, it is scraped clean and recharged to begin a new melting cycle.
This article was adapted from presentations at the Molten Metal Treatment Workshop held at CastExpo '05 in St. Louis. Michael Koch. Bodine Aluminum, St. Louis, and Steve Evans and David Weiss, Eck Industries Inc., Manitowoc, Wis., contributed to this article.
For More Information
"The Best of Aluminum Melt Shops," A MODERN CASTING Staff Report, MODERN CASTING, September 2004, p. 30-34.
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|Date:||Sep 1, 2005|
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