Die-cast aluminum
Under die-cast aluminum is meant a pressure casting process , in which aluminum is pressed in the liquid or pasty state under high pressure in a preheated steel mold. It displaces the air present in the mold and is kept under pressure during the solidification process. The permanent form made of steel is the exact negative of the cast to be made, enlarged by the amount of shrinkage . The mold consists of two halves (possibly additional slides) so that it can be opened after the filling process and the solidification of the metal and the cast can be removed from it. Aluminum die casting can only be produced on cold chamber die casting machines.
Preheating the mold
Heating and cooling devices are used for preheating. Gas burners are unsuitable because they heat up protruding moldings and cores more. Here there is a risk of local overheating of the heat-treated structural steel, which can reduce the strength.
Course of the casting process
- Pouring the metal into the filling chamber without pressure using an automatic dispenser or by hand using a ladle
- Filling the mold by the flow pressure
- During the mold filling, the liquid metal fills the mold precisely due to the hydrodynamic pressure. This is a characteristic of die casting.
- After filling, the hydrostatic pressure acts on the cast
The dosage of the amount of metal is very important so that the residue is neither too small nor too large. If the residue is too small, there will not be enough liquid metal to properly fill the mold. If the residue is too large, it will burst when the mold is opened. In addition, the switching point of the 2nd phase shifts, which leads to rejects.
Flow processes
The actual casting process begins with the advance of the piston, which initially moves slowly so that no liquid metal is thrown out of the press chamber opening (1st phase). This slow movement should continue until the pressing chamber is filled and the metal has reached the gate. The air present in the press chamber is displaced into the mold cavity. The bale chamber should always be 50 to 60% full so that there is no overturning wave and not too much air has to be displaced.
A rollover shock wave would already lead to air inclusions in the press chamber, which enter the mold cavity and can no longer be removed by ventilation measures because the air is already enclosed in the liquid metal. From the press chamber, the metal enters the barrel (2nd phase), which should either be constant in its cross-section or taper towards the cutout. This means that a constant or slightly increasing flow velocity should be achieved. If there are several gates, the cross-section of the run must be narrowed behind each branch. The actual gate should then be brought up to the barrel under a slope. Shortly before the casting, the gate cross-section should be plane-parallel so that the metal does not enter the mold cavity like a nozzle.
In most cases, the gates are defined according to the flow path in the form. The gate must be placed in such a way that the metal can fill the mold cavity without turbulence, if possible as a free jet. At the same time, care should be taken that it only flows in one direction and that good heat exchange and even heat distribution are created.
The static holding pressure (3rd phase), which is greater than the flow pressure, is intended to compress the metal of the entire casting during solidification and to improve the filling of the mold. This means that the casting pressure must suddenly increase rapidly. A gradual increase in pressure during the filling of the mold can hardly be effective, as the mold is filled in hundredths and often thousandths of a second.
Filling time of the mold
The filling time is closely related to the heat and temperature conditions in the mold. The longest filling time with which a mold can be filled without beginning to solidify is the best for the casting. On the one hand, the filling time should be so short that no solidification occurs during casting. On the other hand, it should be as long as possible in order to remove the release agent vapors via the ventilation ducts.
The prerequisite for the production of good die-cast parts is the correct arrangement of a mold in terms of divisions, slides, etc., correct dimensioning of the sprue system and ventilation and optimal setting of the machine.
Cast aluminum alloys
Pure aluminum is rarely used as a cast material , since its strength properties are insufficient for many purposes. It is only through its alloys that aluminum has achieved special technical importance as a casting material, since the addition of only a few percent of alloying elements causes a significant improvement in the casting and strength properties in many cases.
The following aluminum casting alloys are generally used for die casting:
- AlSi9Cu3 (Fe)
- AlSi12Cu1 (Fe)
- AlSi12 (Fe)
- AlSi10Mg (Cu)
Hypereutectic and hypereutectic Al-Si alloys with different mechanical properties are also processed. Alloys with cold and hot aging are also increasingly being cast, with the hot aging alloys requiring vacuum extraction from the casting tool.