Pressure die-casting

The process of die-casting

With die-casting, the fused material is injected into the mould at high pressure and with high speed. Appropriate for die casting are primarily metallic materials with a low fusing point like aluminium alloys (in the first place Al-Si, Al-Si-Cu und Al-Mg) and zinc or magnesium.

The mould comprises two halves, with one half attached to the moveable plate of the die-casting machine and the other to the fixed die platen located on the plunger-side. The mould is usually sprayed with a releasing agent before casting. Large forces hold the two halves of the mould together during the casting process.

The pressure produced during casting is maintained until the end of solidification. The mould is then opened and the part is ejected. The remains, sprues and overflows are remolten again.

The mould comprises two halves, with one half attached to the moveable plate of the die-casting machine and the other to the fixed die platen located on the plunger-side. The mould is usually sprayed with a releasing agent before casting. Large forces hold the two halves of the mould together during the casting process.

The pressure produced during casting is maintained until the end of solidification. The mould is then opened and the part is ejected. The remains, sprues and overflows are remolten again.

The moulds are made of hot-working steel (e. g. X37CrMoV5-1, X40CrMoV5-1) and have a high service life (for aluminium over 100,000 castings). One can achieve outputs of up to 1000 castings an hour depending on the geometry of the part. The process is thus suitable for large-scale production runs.

In addition to the high number of pieces, the process offers a wide range of other benefits:

  • Due to the low fill times, thin-walled (for aluminium to a minimal wall thickness of 1.4 mm), are possible
  • dimensionally accurate castings with a smooth surface finish
  • Little finishing required; the parts are often ready for installation. With the trend towards near-net-shape-technologies more and more finishing steps get superfluous.
  • The possibility of adopting a composite construction by casting in bolts, stampings, bearing bushes and the like made of other, possibly non-metallic, materials.

A process-related disadvantage of conventional die-casting is the relatively high gas content (so called microporosity) and the relatively low ductility associated with die-castings. Likewise, one can hardly avoid a certain tendency for localised shrinkage cavitation.

Therefore, a few years ago, die-castings were said to be neither weldable nor heat treatable. In the meantime those features have become possible thanks to technological advancements like Vacural-, vacuum- and low-pressure casting. Thus the potential of aluminium die-castings has risen considerably. To obtain such material features it is nevertheless necessary to consider the specific requirements of the procedure early in the planning and construction process already.

The principal customers of aluminium die-castings are the automobile and the machine building industries. But the die casting industry supplies also a growing spectrum of other areas. Due to the improving material properties it becomes increasingly interesting for the production of highly resilient leightweight construction parts (e. g. space frame, crankcases).

Vacuum die-casting

In contrast to conventional pressure die-casting the injection chamber and the cavity are evacuated before casting in this refined procedure. During the injection, the air and the evolving gases are drawn off actively. The resulting "forced degassing" will significantly minimize or even eliminate the gas inclusions in the cast part. A variant of the vacuum die casting is known as vacural die-casting process.

Vacuraldruckguss

Developed by Ritter-Aluminium, VAW and Müller- Weingarten , the Vacural die-casting process is a variant of the vacuum die-casting process. Shortly before and during the filling of the mould, a vacuum is created in the mould cavity. At the same time the vacuum sucks the molten metal from the holding furnace and from below the surface of the molten bath via a heated riser into the shot chamber. Thus, practically no oxide layer is formed. The gases formed when the melt comes into contact with mould wall are evacuated at the same time. A well-coordinated valve-controlled system prevents liquid metal flowing into the extraction mechanism.

The gas content of the castings is limited to a fraction of that of products made using conventional die-casting. The shaped parts can be heat treated, welded and soldered or brazed. As a result of the high cooling rate and a very marked absence of pores, one can achieve excellent values for toughness, stiffness, formability and ductility.

There are certain limitations with respect to castings produced by conventional die-casting particularly in the fields of thin wall thicknesses and dimensional accuracy. Likewise, there are large difficulties associated with the use of slides in the mould.

Thixocasting

In thixocasting, a heated semi-fluid/semi-solid slug is forced under pressure into the mould chamber. Thixocasting is thus a special form of pressure die-casting.

Thixotropy is the property of certain metals in the semi-fluid (or semi-solidified) state to behave in a viscous manner in the absence of external stresses but to adopt a lower viscosity when an external stress is applied.

Forming takes place within the so-called solidus-liquidus interval. A prerequisite for perfect processing is a feedstock that has a fine-grained equiaxed microstructure. The range of alloys that can be used is limited at the moment almost exclusively to the AlSi7Mg alloy series (EN AC 42 100 and EN AC 42 200) because with other alloys it is (still) not possible to obtain the necessary fraction of solid material in the melt (approx. 40–60%) because most of them have a narrow solidification range. (The production of the feedstock is presented in the thixoforming section.)

The filling of the mould takes place as a result of the semi-solid state of the feedstock and because of the lower casting temperature (below 600 °C instead of 700 °C) flow is almost laminar; turbulences and thus the formation of gas bubbles in the casting can be prevented to the greatest possible extent. The casting can thus be heat treated and welded without difficulty. Thanks to the very low gas porosity, the parts are also pressure tight. Solidification shrinkage is also low, making high (final) dimensional accuracy, abrupt changes in cross-section and smaller draft angles possible. The superior values of the elongation to fracture compared with the values for products made by other, conventional casting processes coupled with the dynamic and static strength values predestine thixocastings for use as safety parts e.g. in automotive applications.

The quality of the components is close to that of comparable forgings; by contrast, the costs are significantly lower. This is despite the fact that the feedstock is more expensive than that used in conventional pressure die-casting. An added benefit is the longer mould service-life that results from the use of lower metal temperatures and lower gate speeds with a low-iron feedstock.

Pore-free casting process

In this variant of die-casting, air is forced out of the mould by introducing pure oxygen. The aim is to produce a shaped part that is as low in pores as possible. The oxides formed by the natural reaction of the molten aluminium with oxygen are evenly distributed throughout the microstructure of the casting. This process is used primarily in North America and Japan.