Liquid Pressure Forming
In the Liquid Pressure Forming (LPF) process
a pre-heated preform (an array of fibres, short fibres, porous
media or particles) is placed into a heated die, which is closed
by using a mechanical toggle system. The furnace pressure vessel,
riser tube and die are then evacuated to as low as 1 mbar. Following
evacuation molten metal is transferred from the crucible into
the die via the riser tube by the introduction of nitrogen gas
into the pressure vessel. After the molten metal has taken up
the shape of the die, which can be complex, higher nitrogen pressure
is applied (up to 22 bar) to largely infiltrate the preform. High-pressure
hydraulic pistons achieve further consolidation of the metal during
solidification. After cooling the die is then separated and the
component removed by an integral ejection mechanism.
The LPF process has the
following significant advantages over other fabrication routes
for producing light alloys and Aluminium Matrix
Composite components:
| · |
The low pressure achieved during the vacuum cycle acts to
degas the melt in the crucible and ensures an absolute minimum
of trapped gas and porosity in the die. Oxidation of the melt
is also inhibited, which is particularly important for magnesium
which is highly reactive in the molten state. |
| |
| · |
Metal is taken from the centre of the melt which substantially
minimises the amount of entrained surface oxide. |
| |
| · |
The low-pressure stage during mould filling ensures minimum
fibre displacement, whilst the second stage facilitates preform
infiltration and metal consolidation during solidification. |
| |
| · |
The hydraulic piston compaction not only ensures complete
infiltration but also can impart the metallurgical advantages
usually expected of squeeze casting such as regular, controlled
grain size, which makes the process a candidate for casting
wrought alloys. |
The LPF process is highly flexible and can be
arranged in a number of different configurations. In particular
the characteristics of the process permit it to be used for the
production of very low defect, unreinforced castings.
| LPF Process |
Stage 1
Evacuation |
Stage 2
Gas
pressurisation |
Stage 3
Piston
pressurisation |
Stage 4
Cooling and
ejection |
| |
|
 |
|
|
