Molten aluminum flow control via foam filtration.
* Splashing, air entrainment and surface turbulence have a strong influence on the formation and severity of defects. While these problems are magnified when dealing with molten aluminum, they can be reduced with the addition of filters.
* Detailed within is how ceramic foam filters can serve a dual purpose to not only keep out unwanted material, but also modify the flow of molten aluminum through the gating system.
Aluminum is well known for its propensity for hydrogen gas, its high shrinkage factor and its oxide-forming tendencies. Significant efforts are made to clean, degas and feed the metal because these characteristics can lead to quality problems.
The high sensitivity of aluminum and its ability to react with moisture in the atmosphere is arguably the single biggest cause of defects in the industry. Today, even more importance is being placed on aluminum's high sensitivity because research has shown that some defects that would have been attributed to poor melting practices in the past are now blamed on the re-oxidation of the molten metal stream in the mold.
Because of that, metalcasting facilities work hard to clean and degas, keep melt temperature down and avoid air contact with the melt as much as possible. However, all of these efforts are of little value if splashing occurs. Real time X-ray video and modern flow simulations are confirming the strong influence that splashing, air entrainments, surface turbulence and other flow characteristics have on the formation and severity of defects. These same tools also are providing insight into the use of filters to prevent defects in aluminum castings.
The aluminum oxides that are formed have densities close to that of molten aluminum and are slow to float out of the molten metal as it moves through the runner system. Aluminum oxide inclusions reduce the fluidity of molten aluminum, diminish its mechanical properties and cause poor machinability. Oxide stringers and films are the most common forms of nonmetallic inclusions (Fig. 1) and contribute to defects that typically result in a lack of pressure tightness.
[FIGURE 1 OMITTED]
Air entrainment occurs when bubbles of air passing through the metal form trails or strings of oxides that remain trapped below the surface of the casting (Fig. 2), causing weakness after solidification. Entrained air may be carried into the runner system or vesting cavity during filling, or when surface turbulence and splashing is generated into the mold cavity.
[FIGURE 2 OMITTED]
Aluminum gating systems are non-pressurized so that metal is not squired into the mold through the ingates. For example, a ratio of 1:2:4 or 1:4:4 would be an unpressurized system. This means that after the choke, the runners and gates increase in size. The velocity in unpressurized systems must decrease as the molten metal goes through the system.
The effect of drawing air into the mold cavity and of splashing is often greatly underestimated by the industry. Aluminum reacts rapidly with water in the atmosphere, so there will always be an oxide layer on the advancing metal front as the mold fills. This layer will remain intact (even with a certain amount of bulk turbulence), unless the filling velocity is great enough to create surface turbulence, in which case the film will be broken and the resulting fragments will be distributed throughout the casting. Furthermore, if the oxide layer becomes folded, not only will the thickness of the oxide increase, but there will be an increased probability of a crack or trapped air between the two surfaces.
A gating and risering system contributes greatly to the cost of a cast metal component. Not only is it never sold to an end-user, but before being remelted as returns, it must be redesigned and modified. It is then fabricated and added to the pattern, molded into the sand or cut into the die and finally separated from the casting.
The function of a gating and risering system is to fill the mold cavity and promote directional solidification while reducing turbulence and avoiding air entrainment. It also must clean the metal and promote consistent pouring. However, other factors are often built into the system that sometimes compromise the basic principles and can undermine the integrity of what the system was de signed to do.
For example, it is not uncommon for the system to be designed to be easy to remove and fit into the contours of a molding box or permanent mold set up. It also may be designed to aid with shakeout and machining operations and be simple and consistent with other castings.
These compromises are demanded for good economic reasons. However the gating system efficiency has a major impact on the financial aspect. A system that allows a 20% increase in scrap but is easy to remove does not make good economic sense. So why does it happen?
The answer is that we do not have enough knowledge to predict what any particular parameter change will do to the overall system. And we certainly do not know how two interacting parameters will affect the efficiency of the rigging. All gating components have a part to play in the design, but it is not known what impact any design changes will have on the overall system.
X-ray examinations of molten metal flowing through simple gating systems into plate castings allow the effects of the various gating designs and filtration devices to be studied. When metal is poured without any filtering device in the runner, a turbulent flow of metal reaches the gate before the runner is completely filled, entraining air in the process. The metal jets violently into the mold cavity, continuing to entrain air bubbles that contribute to the formation of re-oxidation inclusions (Fig. 3).
[FIGURE 3 OMITTED]
Computer casting simulations can be used to develop gating designs as well as predict fill times and gating-related casting defects. As research and development tools, they are used to establish the feasibility of new casting techniques. They also can be used to model molten metal flow to predict the level of turbulence and splashing. However, like any computerized activity, the output is only as good as the input. A small deviation can create completely different flow characteristics.
Enhancing Gating Systems
Recently, the flow modification properties of foam filters have been uncovered, leading to the simplification of gating because the so-called "filter" serves a similar purpose. In the late 1980s it was discovered that some filters could modify metal flow by reducing its velocity and turbulence as it entered the casting cavity. However, not all filtration devices were equally effective. Filters used in aluminum casting include 2-D screens, 3-D strainer cores and extruded ceramic filters as well as reticulated foam filter/flow modifiers. A reticulated foam filter is considered to be the most effective for aluminum because it is a 3-D network of special pores connected by windows. It minimizes turbulence and prevents entrained air from passing through. The network of pores in the foam filter creates a tortuous path through which the molten metal must flow. Because of the dry nature of aluminum oxides, they do not readily attach to refractory walls. This makes the torturous path essential to separate small particles. They impact the walls of the filter where they become attached through mechanical entrapment and frictional forces (Fig. 4).
[FIGURE 4 OMITTED]
The addition of a foam filter in the runner bar allows it to fill completely with no entrained air before the metal enters the mold cavity. When the runner bar is completely full, metal enters the mold cavity without splashing or turbulence and evenly fills the cavity. The flow benefits of foam filters are significantly greater than the benefits of filtration alone. Screens, strainer cores and extruded cellular filters have some filtration ability but do little to reduce turbulence. Reticulated foam filters, when properly applied, provide the best filtration, flow modification and protection from air entrainment.
So, if foam filters reduce defect rates so effectively, why aren't all aluminum castings filtered this way? Among other concerns, there are application obstacles that must be overcome that are unique to aluminum casting processes.
For example, will the filter float in the remelt of returns? Traditional ceramic foam filters do not generally float in the melt, leading to processing problems, secondary reactions with the filter body and metal bath contamination problems.
Phosphate-bonded alumina or silicon carbide ceramic loam filters also have been used. But because of their abrasive and brittle qualities, they can wreak havoc in sand molds and cause even bigger problems in permanent mold casting due to crushing. A non-ceramic, non-phosphate-bonded foam filter has now been developed that readily primes, floats and remains intact during remelt for easy removal. It does not contain chemical elements that can potentially contaminate the remelt.
The primary purpose of runners and gating systems is to deliver metal to the casting cavity in a non-turbulent fashion. Can the foam filter, due to its inherent flow smoothing properties, simplify this system? By pouring directly through a filtered sprue that is attached to the top of the casting cavity or mounted on a side entry close to the casting, many conventional gating system costs can be eliminated. This direct pouring will reduce poured weight and increase yield while promoting smooth flow into the cavity and improved thermal gradients. A direct pouring unit for sand or permanent mold casting has been developed that incorporates an insulating pouring sleeve (that may also act as a riser) with a foam filter to trap inclusions and modify the flow of the metal as it enters the casting cavity.
The Economics of Oxides
Accurate filter flow coefficients developed through precise water flow modeling allow aluminum foundries to more accurately predict fill times
and simulate metal flow profiles. Since the economic health of a foundry depends on properly assessing the costs and benefits of each pall of the operation, it is important to understand how oxide defects cost the foundry money and what value there is in reducing those defects.
The cost of producing scrap is the stun of the costs of all the foundry processes through which the casting has passed before it is identified as scrap. Although misruns are usually identified early on, porosity often is not detected until machining is completed. Even though the metal remains with the foundry, the cost to produce the useless casting is absorbed by those components that are shipped. If the casting is not scrapped, defects can cause additional processing, such as 100% x-ray inspection, impregnation, rework or re-inspection.
To manage turbulent flow and splashing, and to ultimately prevent defects, foundries develop highly engineered gating systems. In addition to their design costs, these systems are usually larger and heavier than they might otherwise be. This diminishes the foundry's yield and increases its melt losses, energy costs, labor costs and consumable costs. These incremental costs can be reduced if the splashing and air entrainment can be managed without the extra metal.
Because filters simplify and minimize the size of gating systems, they increase metal yield. Reduced cycle time to pour, solidify or eject castings in a permanent mold operation offers the possibility that additional castings can be poured in the same amount of time, with the same equipment and without melting any additional metal.
More Than a Filter
A properly applied foam filter, in addition to trapping undesirable inclusions, becomes a flow modification device with far-reaching benefits of scrap reduction, yield improvement, physical property improvements and greater efficiency. Filtration/flow modification is valuable for permanent molding processes, not only because it leads to quality benefits, but also because it offers the potential for increased productivity, faster pouring, lower pouring temperatures and faster solidification.
In fact, the introduction of a foam filter/flow modifier to an aluminum foundry can be comparable to the addition of new machinery designed to increase productivity--but without the capital expenditure. The use of filtration/flow modification offers opportunities beyond simplification of gating designs. It can be an opportunity to change the entire casting process.
Casting a Pressure Cooker at Wisconsin Aluminum
Pressure cookers are unique to the industry because of the metal-to-metal seal that is used. They are subjected to constant heating and cooling during normal operation, seeing temperatures in excess of 250F (121C) and pressures of 15-20 psi. In addition, flawless cosmetic requirements are demanded in order to present a high-quality product to the marketplace.
The bottom portions of the six different sizes of models produced at Wisconsin Aluminum Foundry, Manitowoc, Wis., range in weight form 10 lbs. to more than 41 lbs. The parts are poured using A356 in a gravity-cast permanent mold process.
The production of the castings remained relatively constant for many years with a significant amount of fallout accepted based on the design constraints. Because the side walls of the component are approximately 30% thinner than the bottom, directional solidification was extremely difficult to consistently achieve. Defects included shrink porosity in the upper portion of the bottom wall and oxide inclusions in several areas as a result of a turbulent fill process and entrained air. Some of the larger models had production runs with defects exceeding 50%.
In 1996, a radical new approach was attempted. A new permanent mold was manufactured that completely changed the orientation of the part. The casting was placed upside down to allow the thicker bottom wall to assist in feeding the side walls. A concern about the distance the metal would be dropped down the runner system was compensated for by modifying the gating system.
After adjusting the riser size and configuration, shrink defects were essentially eliminated. But the occurrence of oxide defects did not improve because the molten metal now had to be dropped more than 18 in. in the runners. Fortunately, at this same time, there had been success with using foam filters in a number of applications in the firm's sand foundry. Armed with this success pattern, it was decided to try filters in the cooker mold. After overcoming issues with designing a cavity that allowed the filter to perform its function and prevent molten metal from passing around the filter, the results were finally acceptable. The filters effectively removed the majority of oxide inclusions and helped maintain a consistent fill rate.
In reviewing the overall impact of the changes, the number of rejected castings was reduced by 50%. On the largest model, a 75% reduction in defects was realized. From a financial standpoint, defects were costing Wisconsin Aluminum in excess of $200,000/yr. The firm is now saving more than $100,000/yr.
For More Information
"The Use of Foam Filters to Improve Production Control in the Aluminum Automotive Foundry," R.S. Kendrick, First International Conference on Gating, Filling and Feeding of Aluminum Castings.
Bob Braun, Wisconsin Aluminum Foundry, Manitowoc, Wis.
Bob Braun is the vice president of engineering at Wisconsin Aluminum Foundry, Manitowoc, Wis., and is a member of the AFS Board of Directors.
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|Date:||Mar 1, 2004|
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