Molten metal system design: the GM way: GM Powertrain found that designing a molten metal supply system cannot be done in isolation. There must be cooperation from different areas of the operation to achieve the desired results.
The start up years are spent optimizing scrap mixes, energy costs, melt loss and consistent metal quality. While this approach may work for some job shops, it drives up the cost in a high-volume production facility.
Metalcasting facilities producing high-volume, high-quality castings must adopt a more radical approach to their molten metal supply design. It is imperative to carry out a detailed systematic analysis for every melt shop expansion project.
This article describes the integrated system approach adopted to design a molten metal supply system at General Motors Powertrain, Saginaw Metal Casting Operations (SMCO), Saginaw, Mich., a lost foam engine block and head facility that GM ramped up in 2001. The approach consisted of eight steps:
* define the casting process;
* select the aluminum alloy that meets the product property needs;
* establish a site-specific business model for melting in-house vs. receiving molten metal;
* define system deliverables;
* finalize system layout:
* design system components;
* monitor system performance at full production volumes;
* capture lessons learned and create a blueprint for future projects.
Lost foam was the process of choice for GM's Inline 6 family of engines because it allowed the designers to cast-in various features into cylinder blocks and heads. However, it posed a major challenge to the melt system designers. Not only was it necessary to provide clean metal at high production volumes, but the metal had to have low gas levels at a pouring temperature of 1,450F (788C).
Aluminum alloy 319 with T5 heat treatment has been the traditional choice of material for the lost foam block and head castings. However, initial product validation trials did not deliver the properties required of the castings. The alloy was subsequently changed over to A356, which provided the required high-cycle fatigue strengths.
Molten vs. Melting
Making the right choice between melting and alloying in-house, receiving cold on-grade metal or receiving molten metal is a million dollar question for aluminum metalcasting facilities embarking on high-volume product lines. Site-specific, techno-economic feasibility studies must be prepared for new products. Melting and alloying in-house is most cost-effective when the aluminum alloy used can be processed from scrap, making scrap availability and proximity key considerations.
Typical melt facilities incorporate large furnaces designed for fast melting. Melt processing and treatment are done downstream after the metal is transferred to a homogenizing/holding furnace. These operations need to have the financial muscle to take advantage of the volatile scrap market and ride out high inventory-carrying costs during a market downturn. This option is most suitable for firms producing high-volume castings using different types of aluminum alloys and requires skilled manpower in scrap sorting, scrap handling and processing. For those producing one alloy, high-volume automotive castings, melting in-house from scrap is an expensive option.
Melting cold on-grade ingots/sows is ideally suited for operations manufacturing a variety of castings in low to medium volumes in different alloys. While this eliminates the high capital investment required in scrap processing equipment, substantial capital is still tied up in valuable real estate needed to store ingot/sow inventories. Additionally, increasing safety requirements mandate the need for costly sow/ingot preheating and charging equipments. Medium-size pressure diecasting, semi-permanent mold and sand casting operations find this option suited for changing product and alloy mix. It also provides flexibility to the metal purchaser in sourcing standard chemistry ingots/sows from multiple sources. However, the cost of molten metal could be higher due to the cost incurred on melting twice--once from initial scrap to ingots and again from ingots/sows to molten metal.
Molten metal delivery provides the least capital cost option for high-volume aluminum metalcasting facilities and lends itself to just-in-time metal delivery. Typical melt facilities include receiving/holding liquid bath furnaces with low thermal inputs.
Metal processing and treatment are done downstream prior to the heated dipwell or the ladling furnaces. Typical heat input for holding aluminum at 1,350F (732C) is 25 Btus/lb./hr. compared to 1,500-2,000 Btus for melting.
The first step in system design is identifying customer needs--in this case, the castline--as well as lbs./hr and the metal quality requirement. A problem that hinders a good metal system is lack of real-time metal quality assessment. A good practical measurement tool for defining the quality of metal is metal density. The Reduced Pressure Test (RPT) can be used as a quantitative tool to monitor hydrogen and measure porosity levels. It also can be used as a reliable technique to control the process. At GM Powertrain it was found that the greatest variability in the day-to-day test results was due to daily changes in melt cleanliness. A minimum molten density of 2.60 was established as a requirement based on extensive trials during the initial development.
The following system deliverables were defined:
* metal temperature across the entire system must be controlled within [+ or -] 5F;
* metal density (as measured by the RPT) must meet a minimum of 2.60 at the dipwell;
* metal system draw down across the entire system shall not exceed 3.5 in.;
* molten metal shall never be a constraint for castline operations.
One important consideration while finalizing the melt system layout is the operator's access for cleaning the furnaces. A minimum distance of 30-40 ft. must be provided between the receiving and holding furnace to operate a welwalker/forklift and scrape the wall of the furnace. A modular system layout embraces the lean principles of just-in-time metal, visual management. standardized work, one piece flow design, pull system and 5-S while providing backup capability using a bypass launder during major furnace relines. The following additional guidelines were defined:
* metal level control shall be maintained using gravity to ensure quiescent flow of molten metal between system components;
* melt equipment and other system components shall be installed at floor level with no floor pits;
* a 4-in. freeboard shall be provided across the entire system;
* optional degass and metal recirculation capability shall be provided at every furnace;
* forced air-cooling for furnace bottom shells shall be provided on all furnaces;
* a common furnace design shall be used wherever possible;
* system backup capability shall be provided for extended periods of furnace relines.
In designing the ladling system, the dipwell, energy choice, temperature stratification, degass well, furnace size, thermal input, refractory and forced air-cooling was scrutinized.
Dipwell--The size of the dipwell is governed by the clearances required by the pouring robot fixture and ceramic ladle. It is desirable to restrict the metal level variation to as low as possible. This offers two advantages--low erosion of the refractories at the bellyband area and repeatable dosing accuracy as the ceramic ladle seeks a fixed point at the dipwell. The depth of the furnace is 28 in. including a 4-in. freeboard. The width and length of the dipwell is designed at 36 x 48 in. to provide for adequate cleaning access and to accommodate larger ladles.
Energy Choice--Electric furnaces provide effective control of metal temperature through proportional control using SCRs compared to gas-fired furnaces and are a natural choice for critical applications. Moreover, the molten aluminum is not exposed to products of combustion. The furnace is powered by 15 silicon carbide glo-bars mounted on the furnace roof at a distance of 16 in. from the metal level.
Temperature Stratification--A recirculation pump well size of 24 x 28 in. is required to house a recirculation pump. A gentle recirculation is required to prevent temperature stratification along the depth of the furnace. A temperature difference of 20-30F is typically observed without metal circulation. The recirculation pump is programmed to run at slow speeds to eliminate temperature stratification without causing undue turbulence.
Degass Well--A degass well of 20 x 24 in. is required to house a rotary degasser, which is designed to run continuously, using argon as the inert gas.
Furnace Size--The furnace size is determined by the minimum size required to accommodate the dipwell, recirculation pump well, degass well and main bath. The furnace size is 24,000 lbs. and based on a sloping hearth design with adequate surface area to accommodate the silicon carbide glo-bars.
Thermal Input--The connected heat input for the furnace was set at 1,385,272 Btus/hr. to compensate for heat loss and temperature rise. A proportional control system constantly modulates the power drawn to control the metal temperature within limits. In a steady-state idle condition, the furnace consumes 480,000 Btus/hr.
Refractory--The high pouring temperatures required for lost foam castings can drive the chamber temperatures above 1,600F (871C). At these temperatures, filler material in refractory oozes out of pores and increases the risk of metal penetration. A high-alumina (90%) refractory using phosphoric acid as a liquid activator was used as the hot face refractory for the ladling furnace. The furnace roof is lined with an insulated castable refractory.
Forced Air-Cooling--A centralized blower provides forced air-cooling for the bottom shell, recirculation pump motor and rotary degasser motor. Air-flow is adjusted to keep shell temperatures below 200F (93C).
Molten Metal Transfer--Heated launders and ladle transfers are common methods of molten metal transfer. Ladle transfer is ideally suited for operations that have a centralized melt department supplying molten metal to different remote casting product lines. In these cases, long lengths of heated launders do not justify the high capital costs required.
Transfer of rectal using a ladle exposes the aluminum for oxidation. Ladle transfer also has significant safety risks whereas launders are simple to operate and maintain, incur low metal loss and provide for quiescent metal transfer. Unheated launders utilizing a molten metal transfer pump can be used for metal transfer over short distances. However, gravity transfer using heated launders provides the best quality molten metal transfer.
The following design guidelines were used for the launder system:
* launder depth of 12 in.;
* heat input of 2 KW per linear foot must be connected. Heating is provided by 6 U-shaped heating elements/section;
* individual launder sections shall not exceed 5 ft. for manual lift covers or 10 ft. for air-operated activated covers;
* launder covers shall provide an effective seal to prevent air ingress, eliminating the need for inert nitrogen/argon covers along the entire length of launders;
* refractory weirs will be used along the length of the launder;
* pre-formed refractory sections with a phosphate-bonded, high-alumina castable shall be used;
* thermocouples for temperature monitoring shall be provided at every 10-15 ft. of launder section;
* temperatures shall be controlled with on-off or proportional controls.
Degassing Needs--Degassing needs should be defined based on metal throughput and quality requirements at the ladling furnace. There are two types of aluminum degassing systems--static and dynamic.
Static systems involve bubbling an inert gas through the metal using a lance or a refractory porous plug. In dynamic degassing systems, argon, nitrogen or chlorine gas is injected into the molten metal through a rotating graphite shaft and impeller.
Degassing times are a function of metal temperature, residence time and the inert gas used (argon and nitrogen are common). Argon is expensive but provides shorter degassing times compared to nitrogen. Nitrogen produces a thick, wet dross layer at the surface that is difficult to clean while argon produces a dry, easy to skim dross.
Large melt systems for high-volume automotive castings must be designed with continuous in-line degassing. In melt systems requiring high-quality metal, multi-point degassing has been proven to be effective.
For the lost foam melt system, degassing was identified at the outlet of the receiving and holding furnaces and at the ladling furnace adjacent to the dipwell. Each of these degass chamber volumes was designed to provide a minimum residence time of 3 min. using argon gas in a rotary degasser.
Metal Filtration--Ceramic foam filters are used extensively in primary aluminum industries and are ideal for batch operation. Silicon carbide-bonded grit filters have adequate strength and can be easily inserted vertically as a wall separating the hearth from the dipwell. The flow rate capacity for these filters is 2 lbs./min./[in.sup.2].
The system incorporates an eight-grit silicon carbide-bonded, vertical gate filter at the inlet of the ladling furnace to filter inclusions above 30 microns.
Main Receiving/Holding Furnace--The types of furnaces used for melting and holding are dry hearth, liquid bath and stack/jet melters. Dry hearth and stack/jet melters are ideal for melting solids. While stack/jet melters cannot be used for melting sows and large scrap, they are cost-efficient for melting ingots and small scrap. Liquid bath furnaces with side charge wells are used for receiving metal and melting fine scrap through the charge well for high recoveries.
It was decided to go with liquid bath furnaces with a side charge well as the main receiving and holding furnace. A sow preheat ledge on one side of the furnaces was designed-in to provide some melting insurance.
The size of the furnace was primarily derived from the total surface required to provide 2.5-3.5 in. of draw down across the entire system. Two identical furnaces were chosen instead of one large furnace to provide backup capability during major furnace relines.
Once the hearth area had been finalized, the depth of the furnace was determined based on the sill height of 48 in. from the floor. The furnace depth was set at 26 in., which includes a 4-in. draw-down, representing a furnace capacity of 75,000 lbs. A side charge well, recirculation pump well and discharge well were added to the main hearth.
Metal is circulated from the hearth well into the side charge well to maintain temperature uniformity. The furnace opening to the well needs to be large enough to allow improved conductive heat transfer to the charge well. The recirculation pump is typically sized to circulate 3-4 times the furnace hold capacity- every hour. The discharge well must be sized to accommodate a rotary degasser and hold enough volume of metal to provide a residence time of 3 min. at rated metal flow rates.
Having finalized the size of the furnace, the next step was selection of the burner configuration. There are three types of burner configurations commonly used--W-fired firing, dual side burners and direct pass through. W-fired burners provide the most efficient heat transfer by enabling products of combustion to travel the entire length of the furnace before returning to the flue. This arrangement is most efficiently used in dry hearth furnaces requiring high melt rates and cannot be used for long furnaces due to burner flame length limitations.
A dual-sided burner arrangement has two burners mounted on the sidewalls along the width of the furnace with a central flue positioned hallway along the length of the side wall. Each flame travels only half the length of the furnace and exits through the central flue.
Large liquid bath furnaces with high headroom and double-ended, full-width doors normally employ the direct pass through burner configuration. In this arrangement, two burners are mounted on one sidewall along the length of the furnace. The flame travels down the width of the furnace, transferring heat to the molten bath by radiation. Heat is convected and radiated to the roof and side walls and is reradiated back from the refractory walls to the bath.
Burner sizing is dependent on melt rate, emissions requirement, furnace geometry and a number of other factors. High-velocity burners are used in furnaces that require high melt rates. In most other applications, medium velocity burners are used. The burner input at low fire must compensate for heat losses. Two baffle burners with a total capacity of 9.6 M Btu were used in the main receiving and holding furnaces.
Flue Pressurization--Any cold air infiltration into the furnace reduces its thermal efficiency. This occurs mainly through door openings. Controlling furnace pressure to slightly positive (+0.020 in. w.c at the hearth) prevents infiltration of cold air. This is achieved by using an air curtain damper at the flue opening.
Refractory--Monolithic linings are the natural choice for liquid bath melting/ holding furnaces. Aluminum is a strong reductor, so reduces the Si[O.sub.2] in the refractory to silicon metal, converting the aluminum into an aluminum oxide.
It is necessary to use refractories with a low amount of free Si[O.sub.2] or silica glass (preferably with a phosphate based bond), which effectively closes the pores to prevent penetration of molten aluminum into the refractory pores. High-alumina refractories made from sintered bauxite with a low content of free Si[O.sub.2] or mullite-based alumina must be used.
Hearth--The hearth floor, ramps and charging sills must be resistant to molten metal penetration, chemical flux attack and take the physical abuse during charging and cleaning. Refractories used in the furnace floor should contain a minimum of 60% alumina. High-purity mullite alumina or phosphate-bonded, zircon-bonded bricks also have been successful. The ramps and sills also must withstand the physical abuse of solid metal charging and furnace cleaning. High alumina (80-85%) phosphate-bonded castables have been successfully used in these areas.
Upper Sidewalls and Roof--The roof and upper walls act as a heavily insulated barrier to keep heat inside the furnace chamber. Refractories used in these areas must withstand the temperature of the combustion gases. They should possess high refractoriness and resistance to thermal shocks.
Insulation--Special care must be taken in choosing backup insulation for the floor. It is desirable to keep the furnace bottom steel shell below, 200F (93C). Although this may mean sacrificing marginal energy savings, it contributes to longer service life. Backup insulation for the side walls and dipwell must be chosen to give a shell temperature of 160F (70C) or less to reduce a potential thermal hazard for operating personnel.
Doors--Ceramic fibers are used for the charging and dross cleanout doors. These have extremely high insulation values and provide major energy savings.
Five molten metal supply systems that deliver metal to nine castlines producing lost foam engine block and head castings are operating success fully at SMCO and GM's Defiance. Ohio, plant. The first system was commissioned at SMCO in 1999 and has been operating efficiently ever since with no down time and excellent refractory performance.
Through this latest project, GM learned that designing molten metal systems for producing high-volume automotive castings requires a paradigm shift. System designers need to steer away from designing individual system components and progress toward a total integrated system approach that starts with the raw material. Every new melt system expansion or rebuild must be treated as a new business opportunity.
For More Information
Visit www.moderncasting.com to view "Aluminum Melting and Melt Quality Processing Technology for Continuous High-Quality Castings," S. Kennedy, Proceedings from the 6th International Conference on Molten Aluminum Processing, 2001.
"Effective Furnace Design for Receiving Molten Aluminum Delivery," B. Guthrie, North American Die Casting Assn. Transactions p. 251-255, 1995.
About the Author
Venky Srinivasan is a senior manufacturing project engineer for General Motors Powertrain, Saginaw Metal Castings Operations, Saginaw, Mich.
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|Date:||Jul 1, 2004|
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