Casting of aluminum alloys- Permanent molds: high pressure die casting and related processes

Casting in permanent molds (metal, cast iron or steel) makes it possible to produce a large number of parts with a single metal mold, capable of withstanding cyclic thermal shock, erosion and chemical attack (partial dissolution or tinning) resulting from contact with liquid aluminum, without deforming. The mold must have good mechanical strength, and the casting bench or machine must be able to keep it closed during the casting process (which can be carried out either by gravity or under pressure).

Aluminum casting in permanent metal molds can be divided into two main groups of processes, which for convenience we'll call die-casting and pressure die-casting.

Generally speaking, these metal mold casting processes are superior to sand casting in the following respects:

• fine metallurgical structure ;

• static and dynamic mechanical characteristics via the cooling rate, except in conventional die casting where the turbulence of the filling considerably degrades compactness and inclusion content;

• dimensional accuracy ;

• surface finish (depending on the poteyage).

These processes require very well thought-out part layouts and high-quality molds. They call for great rigor in implementation, and require the monitoring of certain parameters such as the regularity of production cycles, mold and cast metal temperatures. Al-Si alloys are by far the most suitable for these processes. Alloys that are sensitive to hot cracking, such as AlCu5 and AIMg3 to 6, can be used in moulds with adapted geometries, as can AlMg5Si2 for die-casting.

Overall, a judicious choice of alloy/process combination can satisfy the requirements of the majority of applications, and it is estimated that over 90% (by volume) of aluminum castings are produced using these permanent mold processes.

To complete this approach, readers are invited to consult the following articles:

• [M 3 635] Aluminium alloy melting ;

• [M 3 636] Aluminum alloy casting – Sand, precision casting...