Types of Aluminum Casting Processes
Aluminum alloys are cast from the following types of aluminum casting processes, with the most common listed first:
#1 High Pressure Die Casting (HPDC) Process
Aluminum HPDC (High Pressure Die Casting) is a metal casting process that is characterized by forcing molten aluminum metal alloy under high pressure into a mold cavity. The mold cavity is created using two hardened tool steel dies which have been machined into shape and work similar to a conventional plastic injection mold during the process. Most Aluminum HPDC castings are produced from 380-series alloys, as defined by the Aluminum Association numbering system.
Aluminum HPDC Machine
While there are 2 basic types of casting machines in use: Hot Chamber and Cold Chamber, the aluminum alloys are always cast using the Cold Chamber machine and process. The term “cold chamber” refers to how the metal receiving sleeve (the chamber) is retained in open air and is not placed in the molten metal (as the Hot Chamber machine system does). The “cold chamber machine is the most common type of casting machine used in industry, primarily because of the adaptability for molten aluminum. These casting machines are often referred to as “Presses” and are described by the clamping capability of the press. For instance, a press may be rated at 900 tons of locking force (1,800,000 lb-force), which is the total clamping force. With 4 tie-bars used to offset the clamping force of 900 tons, each tie bar will have a tension of 225 tons of force (450,000 lb-force) when the clamping system is set at the maximum. For this reason, most modern die casting machines have strain sensing gauges on each tie bar to monitor the amount of tension during the process.
The dies to cast aluminum are very expensive to produce because of the high quality tool-steel materials and the time required to design, machine and build these dies. These metal casting dies represent large capital costs proportional to the complexity and the labor costs associated with building them. It isn’t uncommon for a transmission casting die to cost over US$ 1,000,000. The most complex casting dies are associated with engine blocks and can approx US$ 2,000,000 due to higher design costs. With the high cost of tooling, the HPDC process is generally associated with high-volume production quantities. The economics of decision making is relatively easy if HPDC casting is compared to the other methods listed in this article.
Die Casting Presses
These presses illustrate a range between 720 tons (Petransa cell shown at right) and 3500 tons (shown below, photographed prior to shipment to FCA’s Kokomo Casting Plant). As of this writing, casting presses range between 250 Tons up to almost 6000 Tons. The size of the press needed for any particular work is predicated on the projected surface area of the casting and the desired metal pressure needed to produce a sound casting
Simulation Modeling for HPDC
In this simulation model the filling sequence is used to assess both the wave front in the shot tube during the slow-shot phase and the metal filling pattern. Temperature scale is used to confirm that the molten aluminum is capable of filling the cavity before it is below the liquidus temperature. In addition, the time scale is useful for assessing the cavity fill time.
#2 Vertical Low Pressure Die Casting (LPDC) Process
Vertical Low Pressure Die Casting (LPDC) is performed using a vertical process that injects molten aluminum against gravity, in an upward direction. In a typical process used to produce aluminum road wheels, steering knuckles or control arms, the die is filled with metal from a pressurized furnace, with an an inert gas at about 0.7 Bar (10 psi). The pressurized holding furnace is positioned in the lower part of the vertical die casting machine, with the molten metal injected upwards directly into the bottom of the mold through a ceramic tube that is submerged below the metal surface. The casting process starts when the furnace chamber is pressurized on top of the molten metal surface with careful time-based pressure regulation. The gas pressure on the larger area of the molten surface fills the part by forcing molten metal up through the smaller diameter ceramic tube (similar physics to how can drink beverages through a straw). At the end of the actual filling process, the gas pressure is maintained on the the metal surface until a small section in the sprue solidifies. After the pressure is dropped, the metal in the ceramic tube will recede back into the furnace bath. Most LPDC castings are produced from A356 or A319 alloys, as defined by the Aluminum Association numbering system.
Advantages of the Process
One of the main advantages of this process is the precise control of die cavity filling and the ability to produce thick wall sections with high quality. It is a clean process because there is no die lubrication applied to the cavity surface. Depending on the cavity volume, the inert gas pressure is regulated so that the molten metal flows quickly and smoothly through the feeding conduits, reducing oxide formation and preventing porosity. The low pressure process is capable of casting thick walled parts (greater than 5 mm thick) and is well suited for casting structural parts such as road wheels, knuckles and control arms. Another advantage is that the process yields low porosity parts that can be solution heat treated for higher strength mechanical properties and improved impact toughness.
The dies used on LPDC casting presses are significantly less expensive than HPDC dies. The dies are smaller and can sometimes be cast from steel alloys that are lower in cost that forged tool steel. In the road wheel application, the dies are most commonly a standardized 4-slide design that varies in construction very little between mold sets. This level of standardization and the relatively easy geometries associated with a road wheel provides an OEM customer with many, many options for spoke shapes. For chassis parts, the die complexity is greater because independently controlled cooling channels can be added to the die segments. However, due to concerns with plant safety, the VLP process operations would generally prefer air cooling systems in lieu of having water lines in a mold that is directly on top of the melt bath furnace.
Wheel Casting Mold
The image to the right is a cross section of a typical casting mold. The bottom die forms the outside surface of a cast aluminum wheel, so it is very carefully treated so that no damage occur to the cavity surface. The side sections slide in and out, While the top die travels vertically and is used to pull the casting away from the bottom die.
Low Pressure Casting Process
The low pressure process is filled verically by using a pressurizing gas such as Nitrogen or even dry air. The gas pressure forces the meal up the stalk tube and into the mold cavity. The pressurization is carefully regulated so that the metal filling is smooth and generates no oxides during filling. With a heated die, the wall thickness can be as low as 3 mm,. With most applications the cycle time is about 4-6 minutes.
Simulation Modeling for LPDC
In the video above, Flow-3D software is used to analytically simulate the vertical LPDC process for a sample wheel casting process. The example simulation has the direction of gravity in the vertical direction, relative to the stalk tube, however the simulation views have been oriented for better visual analysis of the animation results. The objective of the simulation is to develop a process model that will perform to the following parameters:
- In-Mold Velocity < 0.5 m/s
- Temperature at end of file > Liquidus Temp (615 deg.C)
- Entrained Air < 10% (with air pressure at end of fill < 1 Bar, predicated on venting)
- Oxide Defects < 1000, but this should be relative to the empirical casting result
Prior correlation studies conducted over a 20 year period indicate predicted locations of the defects due to trapped air and oxides indeed correlate well with the actual occurrences of porosity defects in most parts. During the same 20 period, some researchers in the field of metal casting were able to measure the in-mold velocities and correlate results to both simulation modeling and real-life casting quality. Most of these studies concluded that in-mold velocities of 0.4 to 0.7 m/s yielded the better quality results, so the general simulation rule was established at 0.5 m/s. Some authors have indicate that this velocity will prevent excessive entrapment of oxide and air, while also be effective at preventing large oxide films from forming. While simulation models are capable if time-base analysis, the foundry engineer generally relies on being able to monitor the total fill time in a production die.
#3 Gravity Permanent Mold (PM) & Semi-Permanent Mold (SPM)
Gravity PM an SPM aluminum casting is a process that uses a mold that is machined from high grade tool steel material. The term “Permanent Mold” (PM) casting is used to describe how molten metal is poured from a vessel or ladle into the mold, without any type of removable core material (sand or salt) in the mold cavity. In the situation where a sand or salt core is mounted in the mold prior to metal pouring, the process is referred to as “Semi-Permanent Mold” (SPM) casting.
Advantages of the PM or SPM Process
The primary advantage of this process is simplicity in both the die tooling and the machines needed to move the die and pour the aluminum. The mold cavity fills with no force other than gravity of the metal flowing into the mold opening. While ladle pouring is common, Mold filling can also be performed by tilting the die at a control angular velocity. Complex internal cavity shapes are often incorporated into the casting by using simple sand cores that can be set into the open mold, and held in place when the die closes prior to metal pouring.
In many instances the PM and SPM processes are capable of achieving he same material properties as the LPDC casting process using the same A356 or A319 aluminum alloys. The optimization of these mechanical properties is achieved by engineering the dies with carefully controlled internal cooling features using water and air to transport the heat away from critical regions of the casting. Using the same mold filling parameters as described with the LPDC process, PM and SPM casting systems can achieve similar results related to minimizing internal porosity, preventing inclusions and promoting fine grain sizes.
With the use of steel mold materials, the cavity surfaces in a PM or SPM die must be coated with a non-metallic refractory barrier coating. The most common of these coatings is a Boron Nitride powder that is mixed with water and some alcohol so it can be sprayed onto the die surface. By using multiple coatings, the Boron Nitride layer can be built up to desired thickness between 2-6 mils (0.002-0.006 inches). By limiting the Boron Nitride layer to 6 mils in thickness, the coating will resist flaking off or cracking due to the differences in expansion coefficients between the coating and the steel substrate.
When Boron Nitride coatings are applied to die cavity surfaces within the 6 mil recommendation, the PM and SPM casting processes are known to provide a smoother surface finish than traditional sand casting. In addition, with in-mold cooling from water or forced air flow, the resulting internal integrity and aluminum alloy mechanical properties in the casting are similar to LPDC results. Therefore, PM and SPM casting systems are frequently used for chassis and brake system castings.
Compared to sand casting, this process requires less time for finishing and fettling.
Gravity Permanent Mold (PM)
The image to the right illustrates a typical PM gravity casting set up, where the molten aluminum is poured into the mold with a hand ladle. This is the most simple representation of this process. Actual product dies are more complicated and frequently use a cooling media.
Simulation Modeling for Gravity PM and SPM Casting
In this simulation model, the metal dynamics are modeled in the die cavity. In this sample animation there the output of the model will give the analyst the opportunity to assess how well this 2-Cavity mold will fill each cavity. While the cavities are filling at approximately the same amount of time, it looks like a simple “choke” method might help reduce the amount of turbulence at the beginning of the filling process. Also, we will need to further analyze the in-mold velocities before cutting the steel die cavities. As mentioned with the LPDC application, the in-mold velocity should be in the range of 0.4-0.7 m/s (nominally 0.5 m/s).
Gravity PM & SPM Tilt Pour Casting
One of the most icon Aluminum PM and SPM casting systems is the tilt pouring process. With the tilt pour process, molten metal is transferred to a “trough”, but the mold cavity will only fill when the tilting machine rotates the die so that gravity allows the metal to flow. The tilt pour casting process is very commonly applied when casting A356 control arms and brake calipers, because it is very repeatable and doesn’t rely on operator pouring skill. Once the tilting speed and cycle times are established, the reliability of the tilt system yields very consistent quality, as long as the metal quality, temperatures and other external factors are also capable, stable and in control.
Key Process Steps
The tilt pouring process is pretty simple by nature. The most basic steps for the tilting machine are:
- Mold Preparation – As previously described in this article, the steel cavity surfaces must be coated with a suitable Boron Nitride releasing agent and barrier.
- Mold Preheat – Prior to starting the process after mold prep, the die must be preheated to a range between 250 – 450 deg.F, and is highly dependent on whether the 1st casting will not fill out or stick in the die. To avoid over heating any local area, thereby annealing the die, the pre-heat is applied slowly and monitored for complete soaking. Click HERE for die preheating systems and temperature monitoring thermocouples.
- If Cores are going to be used, the die must be designed so the core can be easily set into the mold with print areas that will retain the core in position after the top plate is lowered prior to tilting.
- Sometimes a filter is inserted into the mold prior to lowering the top plate. This filter can be a ceramic block, carbon fiber mesh or a stainless steel screen.
- With the aluminum alloy already heated above its melting temperature, it is poured into the attached pouring cup on the die.
- After clearing the area, the operator will initiate the tilting machine motion, which will allow the molten alloy to fill the mold cavity at a controlled rate.
- After the alloy has been allowed to solidify, metal cores and other loose mold members are withdrawn and the mold is opened, and the casting is ejected (usually with the casting machine returned to the normal “up” position).
- Once the full casting is extracted from the machine, it is cooled and post-process to remove gates and sand, trim the part and inspect.