Aluminium casting processes are classified as Ingot casting or Mould casting.
During Ingot casting, primary or secondary aluminium is cast into rolling ingot (slab), extrusion ingot (billet) and wire bar ingot which are subsequently transformed in semi- and finished products.
Mould Casting is used in the foundries for producing cast products. This is the oldest and simplest (in theory but not in practice) means of manufacturing shaped components.
This section describes exclusively Mould casting which can be divided into two main groups:
• Sand casting
• Die casting
Other techniques such as "lost foam" or "wax pattern" processes are also used but we will not describe as it’s a very small quantity of producers.
In sand casting, re-usable, permanent patterns are used to make the sand moulds. The preparation and the bonding of this sand mould are the critical step and very often are the rate-controlling step of this process. Two main routes are used for bonding the sand moulds:
• The "green sand" consists of mixtures of sand, clay and moisture.
• The "dry sand" consists of sand and synthetic binders cured thermally or chemically.
The sand cores used for forming the inside shape of hollow parts of the casting are made using dry sand components.
This versatile technique is generally used for high-volume production
Normally, such moulds are filled by pouring the melted metal in the filling system. Mould designing is a particularly complex art and is based on the same principle as gravity die casting.
In the "low pressure" sand casting technique, the melted metal is forced to enter the mould by low pressure difference. This more complicated process allows the production of cast products with thinner wall thickness.
In this technique, the mould is generally not destroyed at each cast but is permanent, being made of a metal such as cast iron or steel. There are several die casting processes. High pressure die casting is the most widely used, representing more than 50% of all light alloy casting production. Low pressure die casting currently accounts for about 20% of production and its use is increasing. Gravity die casting accounts for the rest, except for a small but growing contribution from the recently introduced vacuum die casting and squeeze casting process.
A schematic view in Figure 3 shows the main parts constituting a classical mould for gravity die casting. Cores (inner parts of the mould) are generally made of bonded sand. Gravity die casting is suitable for mass production and for fully mechanized casting.
High Pressure Die Casting
In this process, the liquid metal is injected at high speed and high pressure into a metal mould.
This equipment consists of two vertical platens on which bolsters are located which hold the die halves. One platen is fixed and the other can move so that the die can be opened and closed. A measured amount of metal is poured into the shot sleeve and then introduced into the mould cavity using a hydraulically-driven piston. Once the metal has solidified, the die is opened, and the casting removed.
In this process, special precautions must be taken to avoid too many gas inclusions which cause blistering during subsequent heat-treatment or welding of the casting product.
Both the machine and its dies are very expensive, and for this reason pressure die casting is economical only for high-volume production.
Low Pressure Die Casting
The die is filled from a pressurized crucible below, and pressures of up to 0.7 bar are usual. Low-pressure die casting is especially suited to the production of components that are symmetric about an axis of rotation. Light automotive wheels are normally manufactured by this technique.
Vacuum Die Casting
The principle is the same as low-pressure die casting. The pressure inside the die is decreased by a vacuum pump and the difference of pressure forces the liquid metal to enter the die. This transfer is less turbulent than by other casting techniques so that gas inclusions can be very limited. Therefore, this new technique is specially aimed to components which can subsequently be heat-treated.
Squeeze Casting or Squeeze Forming
Liquid metal is introduced into an open die, just as in a closed die forging process. The dies are then closed. During the final stages of closure, the liquid is displaced into the further parts of the die. No great fluidity requirements are demanded of the liquid, since the displacements are small. Thus, forging alloys, which generally have poor fluidities which normally precludes the casting route, can be cast by this process.
This technique is especially suited for making fiber-reinforced castings from fiber cake preform. Squeeze casting forces liquid aluminium to infiltrate the preform. In comparison with non-reinforced aluminium alloy, aluminium alloy matrix composites manufactured by this technique can double the fatigue strength at 300°C. Hence, such reinforcements are commonly used at the edges of the piston head of a diesel engine where solicitations are particularly high.
- Pre-heat oven: used to pre-heat prime (prime is 99.9% pure aluminum) before it is added to the furnace. The heat comes from the recuperator and does not require the use of additional gas. Pre-heating is done to remove water from the prime. The presence of water when the metal is placed in the furnace will cause an explosion as the water rapidly vaporizes.
- Recuperator: Hot air from furnace flows over a series of tubes where the air is heated which in turn feeds the burners for the furnace and in turn reduces gas consumption.
- Front-end Loader/ Fork Truck: the front-end loader is a lift with a large bucket that is used to add scrap to the furnace when preparing a charge. The fork truck is used to add prime ingot into the side doors of the furnaces as well as remove dross and stir the mix.
- Furnaces: used to melt down aluminum and cast logs. The furnaces are open well reverb. Open well means there is a well opening in the front of the furnace where is scrap added. The definition of reverb is when the flame does not melt the aluminum but the heat from the walls and ceiling of the furnace. The following diagram will illustrate the above definitions.
The furnaces are powered by natural gas 95-98% of the time. When there is a gas curtailment, propane gas is used. The furnaces are made and lined with 400-600mm refractory, which is a heat resistant material that comes in two forms: brick and a castable mix.
There are two types of aluminum logs, primary and secondary. Primary consists of pure aluminum and secondary consists of prime and scrap. There are three components used when charging a furnace to make secondary aluminum: prime, scrap, and hardeners. Bonnell Aluminum's Carthage, Tennessee and Newnan, Georgia casting facilities manufacture secondary logs. However, all billets/logs are cast within the Aluminum Association's specifications.
Prime: 99.9% pure aluminum. Prime comes in three forms: T-bars, tub sows, and pig.
- T-Bar and Tub Sows weigh more than fifty pounds
- Pig is prime that weighs less than 50 pounds
- Prime is also categorized by the iron content for example, 10/20 prime tells us there is a maximum of .10% silicon and .20% iron.
Scrap: material purchased from outside sources and that which is generated from within the plant.
Hardeners: Elements, which are added to a bath of aluminum to increase strength and give the final product the characteristics, desired such as finish, strength, and grain refinement. The elements are as follows: Silicon (Si), Iron (Fe), Copper (Cu), Manganese (Mn), Magnesium (Mg), Chromium (Cr), Zinc (Zn), Titanium (Ti), and Boron (Bo). Silicon, iron, copper, manganese, magnesium, chromium, and zinc are used to increase strength and to improve finish. Titanium and Boron are used for grain refinement which is a reduction in the size of the grains, creates a more consistent grain, and better extrudability.
Degassing is the removal of hydrogen from molten metal by bubbling a mixture of gasses up through the melt. Flux is a substance that promotes fusion, especially of metals or minerals. Fluxing causes impurities, such as alkaline, sodium, and lithium, (which cause the material to have a bad finish), to rise to the surface of the bath. Once degassing is complete a sample is taken and analyzed for proper chemical content.
Dross is a mixture of aluminum oxides and non-metallic material, which float to the surface of molten aluminum. Dross is produced whenever aluminum is added to the furnace that has been painted, anodized, or dirty. Dross is skimmed off of the top of the molten aluminum into dross pots. Dross is cooled with argon gas to eliminates the oxygen in the mixture and prevent thermiting. Thermiting is the temperature at which aluminum will burn up. The dross is recycled to recover the aluminum from within it.
Each aluminum casting process has characteristics that are beneficial for different applications. Following are a few guidelines when considering diecasting and permanent mold:
- Die castings can be made to closer dimensional limits with thinner sections;
- Permanent mold castings are sounder, can be produced at lower tooling costs and be made with sand cores to yield shapes not available via diecasting;
- Die castings can be produced at higher rates with less manual labor and commonly cost less per casting when the production run is high;
- Diecasting produces smoother surface finishes and smaller cored holes;
- Dies used in diecasting must be stronger and are therefore more expensive than permanent molds;
- Permanent mold castings are less porous than die castings;
- Diecasting is the least tolerant of varying alloys. Only highly castable alloys are used.
We all know about the pores on our face or the porous nature of things like wood, which leave the material open for invaders such as water and bacteria — that’s why many cooks will recommend using plastic cutting boards instead of wood ones for cutting raw chicken.
Porosity is also a big concern in die casting because it can mean that there is some kind of defect in the material you’re working with or have just created. The good news is that porosity doesn’t always mean a casting is defective and that changes need to be made. A full inspection may show that it meets your stability and structural integrity requirements.
What is Porosity in Die Casting?
This porosity is found when there are small voids, holes or pockets of air that is found within metal.
Typically, this porosity occurs when air is trapped into the metal by the die casting machinery, often leaving gaps at the top of the die or filling a mold too slowly and having some solidification occur too soon. It can also occur when the air used to force molten metal into the mold isn’t completely forced out or able to escape via vents and overflows.
Causes of Porosity in Die Casting
- The design of the mold and cast parts
- The purity of the metal or alloy being used
- Pressure and shot speed of the machines
- Shrinkage of the material wall thickness
- Too much lubricant in the die
- Sharp corners in the mold
- Low metal temperatures
- Air trapped in the metal
The most common way to check for porosity is an X-ray of the material, using computerized tomography or by cutting and polishing a section and then analyzing it under a microscope.
Porosity varies in severity. Sometimes it is acceptable, but most often it’s best to limit it as much as possible.
The most direct way to control for porosity is to create an efficient process for die casting while ensuring the material you’re working with is of a high quality. Process monitoring should focus on equipment maintenance and stability, which can help guarantee an even and adequate amount of pressure throughout the casting.
Gas porosity, which is the formation of air bubbles inside of a casting as it cools, can be avoided by melting the material in a vacuum or in an environment of low-solubility gases, including argon. This porosity occurs because liquids can naturally hold in dissolved gas. Sometimes this can be addressed by exposing the melted material to another gas — the two gases react and pull each other out of the liquid. One important factor generating porosity is the pouring from liquid metql from one to the other recipient.
If oxide formation is the cause of your porosity, then your materials will also benefit from being properly degassed after melting or filtered before using the metal for making the casting.
Hot tears and hot spots are metallurgical defects that occur because of problems during cooling. If this occurs, you’ll first need to make sure your casting is properly being cooled in that local area of the part. If hot spots persist, you will need to adjust the cooling practices by either more die spray or adding more localized cooling channels to that problem area.
General Effects of Temperature in Die Casting
In general, as temperatures increase, tensile and yield strengths decrease in die casted alloys. If you need parts with high tensile strength, this fact will be important to keep in mind. In addition, ductility increases as temperature increases, but changes depend on the type of alloy being die casted. For instance, aluminum is less ductile than zinc, but temperature must be higher for aluminum in the process.
Thermal Factors of Die Casting
The thermal factors of die casting are the temperature of the alloy used for casting, the temperature of the die itself and the temperature of the die casting machine. All of these factors must be kept in an optimum temperature range for the highest-quality parts. If the temperature of the molten metal is too hot for the mold, it could damage the mold by causing premature heat checking on the mold steel. If the mold temperature is too cold, it could drop the temperature of the metal too quickly as it flows into the mold and cause defects — especially greater porosity or misruns on the casting surface.
Temperature Considerations for Die Casting
To avoid putting a strain on the mold, you will want the alloy to be about 30 to 50 degrees °C higher than the initial crystallization temperature. The mold itself needs to be maintained at about a third of the alloy temperature. The correct temperature for the casting chamber is a complex calculation that will be determined by the casting engineer.
Effects of Temperature on Various Die Casting Metals
So how does temperature affect the different metals used for die casting? Here are a few useful examples:
Aluminum is one of the most popular choices for metal die casting. It is lightweight, durable and has good corrosion resistance. The most popular aluminum alloys for die casting are 360, 380 and 413.
Zinc is another highly popular die casting alloy. Some common and most popular zinc alloys include Zamak 3, 5 and 7.
Did you know that 90% of all finished manufactured products contain at least one metal casting?
Metal castings are very important to the global economy and while each die casting manufacturer follows strict alloy standards, alloy
names vary slightly across the world.
So whether you’re a high-tech start-up from California, an automotive OEM from China or a Major european Automotive subcontractor,
we’ve created this table to help you classify international equivalents for zinc, aluminum, and magnesium die casting alloys.
|A380 (ASTM B85)||46500 (EN1706)||LM24 (BS1490)||ADC10 (JIS5302)|
|383 (ASTM B85)||46000/46100 (EN1706)||LM2 (BS1490)||ADC12 (JIS5302)|
|390 (ASTM B85)||-||-||ADC14 (JIS5302)|
|A413 (ASTM B85)||44300 (EN1706)||LM20 (BS1490)||ADC1 (JIS5302)|
|413 (ASTM B85)||47100 (EN1706)||LM20 (BS1490)||ADC1 (JIS5302)|
|A360 (ASTM B85)||43400 (EN1706)||LM9 (BS1490)||ADC3 (JIS5302)|
|Zamak 2 (ASTM B86)||ZP2 (EN12844)||ZL2 (BS EN1774)||-|
|Zamak 3 (ASTM B86)||ZP3 (EN12844)||ZL3 (BS EN1774)||ZDC2 (JIS5301)|
|Zamak 5 (ASTM B86)||ZP5 (EN12844)||ZL5 (BS EN1774)||ZDC1 (JIS5301)|
|Zamak 7 (ASTM B86)||-||ZL7 (BS EN1774)||-|
|ZA8 (ASTM B86)||ZP8 (EN12844)||ZA8 (BS EN1774)||-|
|ZA27 (ASTM B86)||ZP27 (EN12844)||ZA27 (BS EN1774)||-|
|ACuZinc5 (ASTM B894)||-||-||-|
|AZ91D (ASTMBG4)||MC21121 (EN1753)||MDI1D (JIS2222)|
|Material||Alloy||Tensile Strength||Yield Strength (0.2%)||Impact Strength||Shear Strength||Hardness||Elongation||Process|
|MPa||MPa||J||MPa||Brinell (HB)||% in 50mm|
|Zinc||Zamak 2||359||283||47||317||100||7||Hot Chamber Die Casting|
|Zinc||Zamak 3||283||221||58||214||82||10||Hot Chamber Die Casting|
|Zinc||Zamak 5||328||228||65||262||91||7||Hot Chamber Die Casting|
|Zinc||Zamak 7||283||221||58||214||80||13||Hot Chamber Die Casting|
|Zinc||ZA 8||374||290||42||275||103||10||Hot Chamber Die Casting|
|Zinc||ZA 27 - Zinc Aluminum||425||376||12.8||325||119||3||Cold Chamber Die Casting|
|Zinc||ACuZinc5||407||338||-||-||115||5||Hot Chamber Die Casting|
|Zinc||EZAC||414||393||-||-||-||6.7||Hot Chamber Die Casting|
|Aluminum||Aluminum Alloy A380||324||160||4||190||80||3.5||Cold Chamber Die Casting|
|Aluminum||Aluminum Alloy 383||310||150||4||-||75||3.5||Cold Chamber Die Casting|
|Aluminum||B390||317||250||-||-||120||1||Cold Chamber Die Casting|
|Aluminum||A413||290||130||-||170||80||3.5||Cold Chamber Die Casting|
|Aluminum||413||295||145||-||170||80||2.5||Cold Chamber Die Casting|
|Aluminum||K-Alloy||295||172||-||-||80||5||Cold Chamber Die Casting|
|Aluminum||A360||317||170||-||180||75||3.5||Cold Chamber Die Casting|
|Aluminum||DCA1||140||69||30||-||48||3.1||Cold Chamber Die Casting|
|Magnesium||AZ91D||230||160||3||140||63||3||Hot Chamber Die Casting|
|Material||Alloy||Density||Melting Point (Average +/- 50)||Thermal Conductivity||Coefficient of Thermal Expansion||Electrical Conductivity||Process|
|Zinc||Zamak 2||6.6||385||105||27.7||25||Hot Chamber Die Casting|
|Zinc||Zamak 3||6.6||384||113||27.4||27||Hot Chamber Die Casting|
|Zinc||Zamak 5||6.6||383||109||27.4||26||Hot Chamber Die Casting|
|Zinc||Zamak 7||6.6||384||113||27.4||27||Hot Chamber Die Casting|
|Zinc||ZA 8||6.3||390||115||23.3||27.7||Hot Chamber Die Casting|
|Zinc||ZA 27 - Zinc Aluminum||5||431||123||26||29.7||Cold Chamber Die Casting|
|Zinc||ACuZinc5||6.85||452||106||24.1||26.9||Hot Chamber Die Casting|
|Zinc||EZAC||6.49||396||-||-||-||Hot Chamber Die Casting|
|Aluminum||Aluminum Alloy A380||2.71||566||96||21.8||23||Cold Chamber Die Casting|
|Aluminum||Aluminum Alloy 383||2.74||549||96||21.1||23||Cold Chamber Die Casting|
|Aluminum||B390||2.71||580||134||18||27||Cold Chamber Die Casting|
|Aluminum||A413||2.66||578||121||21.6||31||Cold Chamber Die Casting|
|Aluminum||413||2.66||578||113||20.4||31||Cold Chamber Die Casting|
|Aluminum||K-Alloy||2.63||680||113||-||32||Cold Chamber Die Casting|
|Aluminum||A360||2.63||577||113||21||29||Cold Chamber Die Casting|
|Aluminum||DCA1||2.65||585||170||-||42.1||Cold Chamber Die Casting|
|Magnesium||AZ91D||1.81||533||72||25.2||12.2||Hot Chamber Die Casting|