A Case for Low Pressure Sand Casting of Aluminum
Less common than gravity sand casting and low pressure permanent mold casting, low pressure sand molding holds a distinct combination of advantages for large aluminum castings.
Franco Chiesa, Centre de Métallurgie du Québec, Trois-Rivières, Québec, Canada, and Jocelyn Baril, Technology Magnesium & Aluminium, Trois-Rivières, Québec, Canada
(Click here to see the story as it appears in the January/February issue of Metal Casting Design & Purchasing.)
A majority of aluminum castings are produced via sand or permanent mold casting, but for large precision components, another viable option is low pressure sand casting, which uses principles from both low pressure permanent mold (LPPM) and gravity pour sand casting.
Low pressure sand casting marries the use of bottom pouring for tranquil filling of the mold (which avoids metal oxidation) with the flexibility to make larger parts. The capable process can be ideal when producing large “top quality” aluminum castings. The process also can be considered when walls are too thin (such as 0.1 in. [2.5 mm]) to be obtained by gravity casting.
LPPM casting is a common process producing high quality castings due to tranquil filling of the mold and the application of pressure to fill the mold efficiently and cleanly.
The two main characteristics of the LPPM process are:
1. The filling from the bottom of the mold is perfectly controlled compared to the turbulent flow associated with gravity casting. Also, the liquid metal is drawn from under the melt surface, preventing dross entrainment into the mold cavity.
2. Efficient feeding from the bottom injection pipe occurs through pressure applied to the melt during solidification, eliminating the need for risers. The resulting yield is high: typically 80-90% versus 50-60% for gravity permanent mold casting. However, not all casting geometries are amenable to the LPPM process.
In low pressure sand casting, the sand mold rests on top of a pressurized enclosure as shown on Figures 1 and 2. The similarity between LPPM and low pressure sand casting is in the controlled tranquil filling of the mold with a dross-free melt. Both processes also share the ability to produce thinner walls than gravity pouring would.
However, in contrast with the LPPM process, in low pressure sand casting no excess pressure is applied at the end of filling. Feeding from the bottom is interrupted early and long before the casting is fully solidified, so risers are necessary, just as in gravity casting.
Low pressure sand casting eliminates liquid metal handling, so the process is also advantageous over gravity sand casting when pouring large parts.
Size, quality and wall thickness will be the primary considerations when deciding between LPSM and gravity sand casting.
Compared to gravity sand casting, the low pressure sand molding process simplifies the filling of the mold. A single operator can repeatedly fill the mold for a 600-lb. casting at the push of a button, compared to the manpower necessary to fill the mold by gravity through multiple sprues. The filling metal is also cleaner.
Solidification times are typically five times longer in sand casting than in permanent mold. This is why the low pressure sand molding is no match to LPPM when castings are small enough to be produced on a LPPM press. Since the majority of aluminum castings are relatively small, the LPPM process is much more widely used than low pressure sand casting. But when the dimensions are too large for LPPM, low pressure sand casting is a viable option. Dozens of casting facilities are capable of this process and often cater to the aerospace industry.
A good candidate is illustrated through the following case study of a cast A356 aluminum mold used to make plastic parts for the food container industry.
The overall dimensions of the casting are 32 x 18 x 66 in. (800 x 460 x 1700 mm). Its inner surface will be polished to a 60 grit finish, so the as-cast surface roughness must be less than 250 RMS. For the same reason, subsurface porosities greater than 150 µm are not acceptable.
In Figure 3, the quiescent filling is illustrated by showing the melt temperature at three seconds, 10 seconds, 20 seconds and 40 seconds after the start of filling. This rate of filling was obtained by applying a rise in pressure of 10 mB per second inside the crucible enclosure.
Figure 4 presents a map of the metal front temperature anywhere in the casting. It indicates no risk of cold shuts (seams in the casting) exists because the liquid metal front temperature never drops below 1,159 F (626 C). (Alloy A356 begins to solidify at 1,135 F [613 C].)
The molten aluminum is fed from the furnace to the runners by thin gage 1.5-in. (38-mm) diameter steel tubes. Given the great propensity of aluminum to dissolve iron, the composition of the A356 alloy after a run was measured in a runner and in the steel tube and then compared to that of the furnace melt. The results, shown in Table 1, indicate that only a small amount of iron (up to ~0.02%) was picked up when the melt remained fully liquid and still inside the steel tube for several minutes. Because the transit time of the aluminum in the tube during filling is of the order of one second, the iron pick-up is negligible.
Thermal Analysis
In Figure 5, the three green dots indicate the locations of three thermocouples which were inserted inside the mold cavity during molding.
The responses of the thermocouples are shown in Figure 6. The arrival time of the liquid metal and the start and finish of solidification are listed in Table 2.
The measured solidification times are reasonably close to the predicted values shown in Figure 7. It is predicted the solidification progresses from the mid-height of the casting, down to the feeding gates maintained under pressure until their complete solidification (10 minutes), and from the mid-height up toward the top risers in the other direction. This ensures a directionally solidified shrink-free casting.
Because the casting will be submitted to service temperatures up to 482 F (250 C), any hardening via heat treatment would be lost after a few hours of operation. Consequently, the mold will be used in the as-cast condition. The inner surface was polished to a pit-free finish shown in Figure 8.
To ensure the lowest porosity level, the melt was degassed to a Reduce Pressure Test sample density of 2.63. The porosity level was related to the local solidification time and temperature gradient. Because these thermal parameters were readily available from solidification modeling, it was possible to predict the distribution of the porosity (Fig. 9).
Samples were cut out at two locations where thermocouples had been inserted, i.e. in the feeder tube and in a gate. The measured porosity in the gate was 0.8%, in reasonable agreement with the predicted results. The actual porosity level of the aluminum solidified inside the steel tube was 0.4%, much lower than predicted. The long solidification time (19.2 minutes) of the quiescent liquid melt inside the steel tube is believed to have allowed natural degassing to take place. Figure 10 shows the metallographic aspect of both samples at low magnification.
Due to the longer solidification time inside the tube, the secondary dendrite arm spacing was larger than in the gate (90µm vs 71µm).
While low pressure sand casting is not as common as gravity sand casting or LPPM, it holds a distinct combination of advantages when pouring large castings, including tranquil filling, obtaining metal from underneath the oxidized surface of molten aluminum for higher metallurgical quality, thin wall capability, easy metal handling, and cost efficiency. ■
In August, Lightweight Innovations for Tomorrow (LIFT) announced a new project intended to advance technologies for diecasting and heat treating aluminum parts, primarily for aerospace, defense and automotive applications.
“If we can reduce just a few ounces of metal from automobile engine mounting cradles or the housings that hold transmissions, we can deliver an impact that is multiplied by the millions,” said Larry Brown, executive director, LIFT. “In aerospace, an added benefit might lower manufacturing costs as well as increase fuel savings from the lighter weight designs.”
LIFT is operated by the American Lightweight Materials Manufacturing Innovation Institute and is one of the founding institutes in the National Network for Manufacturing Innovation, which is a federal initiative to create regional hubs to accelerate the development and adoption of cutting edge manufacturing technologies. It was formed in 2014, and the vacuum diecasting project is one of the first two started. The other project focuses on thin-walled gray iron parts.
Lead partners for the project are Boeing and The Ohio State University. The focus is to develop key process technologies (super vacuum diecasting and a shortened heat treatment) and computer engineering tools for 300 series aluminum diecasting alloys to improve mechanical properties and reduce the minimum wall thickness (up to 40%) and weight (up to 20%). According to Alan Luo, professor of materials science and engineering and integrated systems engineering, The Ohio State University, the project will reduce the variability in quality and improve the mechanical properties of high pressure die castings. The project also will explore new design methods of lightweight castings using local mechanical properties predicted by the new computer engineering tools, as opposed to the current casting design using minimum properties of cast alloys.
“If you can take a common part, such as an access panel you see on the wing of an airplane and use high integrity die castings, it could reduce weight and manufacturing costs,” said Russ Cochran, associate technical fellow, Boeing. “We hope to demonstrate that advances in high vacuum diecasting will produce parts that meet all the rigorous performance specifications we require, while realizing weight and efficiency goals.”
In current high-speed aluminum diecasting, microscopic air bubbles can form inside the part as the molten metal races through the mold. These tiny bubbles are not an issue for most diecast parts in typical applications, and engineers allow for them by using more metal and making parts thicker to meet strength and other performance requirements. These aluminum parts can achieve tensile strengths up to 47 ksi and minimum wall thicknesses of 0.04 in.
For this project, however, researchers are looking at methods to cast thinner walls with increased strength for structural applications, and for that, the bubbles can be detrimental. By applying a vacuum to the mold, diecasters remove air from the environment. Air is the culprit for porosity.
“We know in the laboratory that if we pull all the air out of the mold just before the molten metal flows in, we can eliminate the bubbles,” Luo said. “Without bubbles, we can design thinner parts that are just as strong and durable, but with less metal and lighter weight. There are other benefits, as well, because the new process allows us to heat treat parts after they are cast, which will improve their performance in service.”
The group also will be working on a shortened solution heat treatment to improve mechanical properties cost efficiently. A simple T5 heat treatment (where castings are cooled from an elevated temperature and artificially aged) has shown in preliminary work to increase yield strength by 40% for E380-type alloys. Now researchers want to see if a shortened T6 heat treatment (where castings are solution heat treated and artificially aged) can be developed to achieve even better properties in 300 series aluminum.
An important part of the two-year project will be enhancing the ability of computer models to predict the performance of aluminum diecast parts by combining information about the microstructure of the metal with a host of design and production parameters. The process, called integrated computational materials engineering (ICME), has great potential for reducing the time it takes to design and qualify new components for vehicles and will address some of the key challenges in implementing thin-wall diecasting technologies: die design, process control, casting design and process simulation.
Currently, castings are designed using the minimum properties of alloys as a baseline for the entire part. The ICME approach will allow designers to pinpoint higher or lower minimum properties to localized regions.
The aim is to connect the thermodynamic prediction of alloy composition and heat treatments to process modeling, which will enable designers to locate specific properties in specific areas of a part to meet service loading conditions. When load paths are clearly defined, the research group also plans on establishing topology optimization techniques to enhance design optimization.
In the first three months of the project, the group has selected the baseline alloy (A380) and identified an experimental high strength aluminum alloy for structural casting development for aerospace and automotive applications. A concept design on a thin-wall casting die also has been started. The die is based on a thin-walled zinc die casting, which is being redesigned for aluminum. Researchers have begun mechanical property evaluation and ICME model validation in thin-wall casting development. Key real-world applications, such as an aerospace wing fuel door, to demonstrate the benefits of vacuum diecasting and ICME have been identified.
“What we are doing here is bridging the gap between great research in laboratories and great manufacturing skills in private industry,” Brown said. “Once you bring these innovations into production, the results just multiply.”
Lightweight aluminum diecast components have a significant market to fill, particularly in the transportation industries, including aerospace, automotive, military and marine. Industry partners from these markets include Eaton Corp., Comau and Nemak. On the research side, Worcester Polytechnic Institute, Southwest Research Institute, the University of Michigan and Massachusetts Institute for Technology have joined the effort, while the American Foundry Society and North American Die Casting Association are assisting with modeling, technology oversight and dissemination of knowledge on how to manage the new thin-wall aluminum diecasting process in a production environment.
LIFT is operated by the American Lightweight Materials Manufacturing Institute and was selected through a competitive process led by the U.S. Department of Defense. It is one of the founding institutes in the National Network for Manufacturing Innovation, a federal initiative to create regional hubs to accelerate the development and adoption of cutting-edge manufacturing technologies.
After the two-year project is concluded, the group plans to deliver design guidelines and property specifications for thin-walled aluminum diecasting and ICME models for thin-wall casting design with location-specific properties. As these technologies and guidelines are incorporated into production diecasting operations, the options for aluminum components in structural applications will expand. ■