The present invention relates to the production of engineering components
by casting and particularly to such components for example having reinforcing
inserts such as those made of fibres or whiskers.
Much research has been, carried out over recent years to produce
stronger, more wear-resistant components such as pistons, for example, for use in
internal combustion engines or compressors.
One route which has been followed by many researchers to produce better
components is that of incorporating inserts into the components. Such inserts
may, for example, comprise shaped preforms of either metallic or non-metallic particles,
fibres or whiskers. Examples of the former are stainless steel and nickel-based
alloy wires, fibres or powder metallurgy components and examples of the latter
are alumina, silica, zirconia, silicon carbide and silicon nitride. Generally
speaking the insert should be porous or at least have a porous or fibrous surface
into which the matrix metal of the component may penetrate in order to achieve
a strong bond between metal and insert. Where the insert is porous throughout its
bulk the matrix metal of the component should ideally completely impregnate the
A well known process for the production of pistons is gravity-die
casting. This technique, however, is not only unsuitable where porous inserts have
to be incorporated but also cannot be relied upon to achieve absolute soundness
even in a non-fibre reinforced casting. Gravity-die casting is unsuitable for
incoporating porous inserts into castings because only minimal or at best incomplete
impregnation of the insert is achieved. The effect of unsoundness or porosity
in piston castings is to produce a wide spread of fatigue strengths at the piston
operating temperature. A wide spread of fatigue strengths means that the average
fatigue strength is correspondingly lower than that obtainable from completely
sound material and that gravity-die cast pistons may be unsuitable for the more
To overcome the problems both of incomplete impregnation and unsoundness
other casting techniques have been developed in recent years. One such technique
now widely used is squeeze-casting wherein molten metal is poured into a female
die cavity, the die cavity then being closed with a male die member and the molten
metal allowed to solidify under a pressure, often of many kg/mm². Where the female
die cavity also contains an insert to be impregnated squeeze-casting physically
forces the liquid metal into the porous structure of the insert and because pressure
on the metal is maintained during solidification porosity is prevented from forming.
Thus sound material and where inserts are included, full impregnation thereof
may be achieved.
Squeeze-casting usually requires the use of a hydraulic press which
is both physically large and expensive. The cost of a press used in a squeeze-casting
installation for the manufacture of diesel engine pistons of about 130mm diameter
is high. A characteristic of squeeze-casting is that there is slight, though significant,
relative movement between the male and female die members during solidification
and cooling of the squeeze-cast material. The effect of this is to make the incorporation
in castings of features such as gudgeon pin holes in pistons, for example, difficult.
It has now been discovered that components may be produced with material
mechanical properties at least equivalent to the best gravity-die cast material
and approaching the properties achieved by squeeze-casting on apparatus costing
much less than that of apparatus required for the production of comparable sized
According to a first aspect of the present invention there is provided
a process for the production of an engineering component, the process comprising
filling a die cavity within a die assembly with molten metal by utilising centrifugal
force, the die cavity being rotated about an axis remote from the die cavity at
a rotational velocity sufficient to produce an acceleration of at least 200'g'
on the molten metal in the die cavity.
There is provided according to a second aspect of the present invention
an engineering component when made by the first aspect of the present invention.
It has been found that piston castings produced by the process of
the invention do not possess the porosity seen in gravity die castings.
In a preferred embodiment of the present invention the engineering
component further comprises a reinforcing insert.
Preferably the rotational velocity is sufficient to produce an acceleration
on the molten metal of 250 to 450'g'.
In one embodiment of the present invention which is a piston for
an internal combustion engine it has been found that complete impregnation of a
fibre insert having about 80% porosity may be achieved together with very high
material mechanical properties compared with those obtained from similar gravity
die-cast parts. Typically the improvements in alloy material properties have been
Preferably a three-piece die comprising a split two-piece female
die member and a single piece male die member may be used. The type of die described
is typical of that used in a squeeze-casting installation but has the advantage
in centrifugal-casting in that because the male die member is in fixed relationship
to the female die member such features as gudgeon pin holes in a piston may be
cast-in using core-pins. Because the core-pins used for producing such features
may be metallic and may have a quenching effect on the cast metal the grain structure
produced is very fine and again has superior properties in a region where it is
most needed. However, because the only forces acting on the die are those due to
centrifugal forces generated by die rotation the male die member and other core
pins etc. may comprise ceramic materials such as, for example, silicon nitride
to inhibit the premature freezing of particular regions of the casting by use
of the insulating effect of the ceramic.
An additional advantage realised with centrifugal-casting as distinct
from squeeze-casting is that with centrifugal-casting, there is the capability
that, provided that the casting machine and die are made adequately strong, more
components may be produced per machine cycle If, for example, a two cavity squeeze-casting
die were envisaged then twice the force would be required to produce the components.
The number of castings per cycle is thus clearly press capacity limited This is
not so in centrifugal-casting where the force on the molten metal is generated by
the rotational velocity and is the same for a given die cavity geometry and radial
location regardless of the number of die cavities. There is naturally, however,
a physical restriction on the number of die cavities which may be incorporated
into a casting machine of a given size.
In order that the invention may be more fully understood an example
will now be described by way of illustration only with reference to the accompanying
drawings, of which:
- Figure 1 shows in elevation a section through a die cavity for producing a
piston by the process according to the invention;
- Figure 2 shows in plan view a section through the line XX¹ of Figure 1
of half of a die assembly for producing a piston by the process of the invention;
- Figure 3 shows in elevation a section through the line YY¹ of Figure 2
of half of a die assembly for producing a piston by the process of the invention;
- Figure 4 shows a modification of the die cavity of Figure 1 to incorporate
an alumina fibre insert into the piston crown region;
- Figure 5 is a photomicrograph at X200 magnification showing the interface region
between piston alloy and impregnated insert of a piston having an alumina fibre
Referring now to the drawings and where the same or similar features
are identified by common reference numerals.
Figures 1 to 3 show various sections through a centrifugal-casting
die assembly having a piston blank cast therein. The embodiment shown in these
figures does not include inserts of any kind. The die assembly is shown generally
at 10 and comprises a base-plate 11 affixable to which is a female die member
being split in two halves 12 and 13. The die halves 12 and 13 are held together
by clamping means 14 and to the base-plate 11 by further clamping means 15 (not
shown). Passing up through the base-plate 11 is a male die member 16 having no re-entrant
angles and which may be easily withdrawn from a solidified piston casting. Passing
through holes in the die halves 12 and 13 are core-pins 17 for producing in-situ
gudgeon pin holes 18 in the piston casting 19. Included in the die halves 12 and
13 are channels forming the molten metal feeds 20 and 21 and a distribution chamber
22. A second die cavity 25 (not shown) is incorporated into the die assembly 10
the geometry of which is essentially symmetrical about the axis 24. The die assembly
10 is fixed to a rotatable bed 23 (not shown) and is rotatable about the axis
24. The die 10 and rotatable bed 23 are enclosed in suitable safety guards 26 (not
shown) to protect an operator in the event of a die burst or metal leakage. The
rotatable bed 23 is connected to suitable drive means 27 (not shown) and speed
control means 28 (not shown) which are known in the art. A filling tube 29 (not
shown) co-operating with the feed channel 20 is provided through the safety guards
26 and coincident with the axis 24 for filling the die 10 with molten metal from
an external source.
In operation the die assembly 10 is pre-heated to a temperature dependent
upon the metal to be cast and is rotated about the axis 24 at a rotational velocity
such as to produce an acceleration within the range 250 to 450'g' in the region
of the die cavity. Molten metal is poured via the filling tube 29 (not shown)
into the feed channel 20. The molten metal is then thrown by centrifugal action
from the distribution chamber 22 into the channels 21 and thence into the die
cavity formed between the die members 11, 12, 13, 16 and 17. As a result of the
centrifugal force developed by die rotation on the molten metal air is expelled
radially inwards in the opposite direction to metal flow. By suitable die design
which may include preferential heating of particular die regions or insulation,
for example, of feed channels solidification may be controlled such that the last
metal to solidify is the feeder of the casting 19. Thus liquid metal is always
present to feed developing shinkage porosity. Normal die design considerations
such as the provision of air bleed channels etc. apply to the design of dies for
In a die of the type described above where the diameter of the piston
casting cavity is approximately 76mm and the distance of the inner radial edge
of the die cavity from the axis of rotation 24 is approximately 127mm an acceleration
of approximately 318'g' will be generated at the centre of the die cavity at a
rotational velocity of 1500 rev/min.
Heat-treated material samples from pistons cast at 318'g' having
the chemical composition in wt%; Cu/0.89-Mg/0.87-Si/11.16-Fe/0.37-Mn/0.11-Ni/0.99-Al
remainder have given tensile strengths of between 18.4 and 19.5 t.s.i. Gravity-cast
alloy of the same nominal composition gave strengths in the range 13.5 to 16 t.s.i.
Furthermore, centrifugally cast material gave consistently higher fatigue strengths
with little variation, similar in fact to the variation of results in tests for
Referring now to Figure 4 which is similar to Figure 1 but shows
a die modified to allow incorporation of an alumina fibre insert into the crown
region of the piston.
The die halves 12 and 13 are modified by inclusion of a hole 40 to
receive a locator pin 41. The locator pin 41 has a spigot 42 on its lower end which
is received into a recess 43 in an alumina fibre insert 44. The piston 19 was
cast by the method described above. The aluminium-based piston alloy completely
impregnated the fibre insert under the influence of the high 'g' accelerations
generated. Figure 5 shows a photomicrograph of a section taken from a piston made
in a die according to Figure 4. Piston alloy 50 known as Lo-Ex (trade mark) appears
on the left of the photomicrograph whilst the fibre insert 51 appears on the right
fully impregnated with Lo-Ex. The interface 52 between the Lo-Ex 50 and impregnated
insert 51 may be seen to be fully continuous with no areas or regions of discontinuity.
Although the invention has been described showing fibre reinforcement
of the crown area of a piston it is also envisaged that the piston-ring groove
region and pin boss regions may also be so reinforced. The pin bosses may be reinforced
by the provision of fibre preform annuli which may be placed on the pin boss core
pins 17 for positioning purposes. The die halves 12, 13 may also incorporate location
means for the positioning of piston-ring groove reinforcements. Such positioning
means may comprise a groove or grooves around the die body cavity into which the
fibre ring preform or preforms may be placed before closure of the die.
It is also envisaged that the process may also include the provision
in the cast body of features having re-entrant angles such as, for example, combustion
chamber bowls. Such features may be achieved by the use of salt cores in known
manner. The fibre insert 44 of Figure 4 may alternatively be considerd as a salt
core having a re-entrant form at the surface of the casting.
In a die assembly of the size described above it is possible to incorporate
up to about four die cavities radially disposed about a centre of rotation.
It will be appreciated by those skilled in the art that modifications
to the process described may be made. For example, relative orientations of components
within the die may be altered and the die may be made to allow incorporation of
Al-fin (trade mark) type piston-ring groove reinforcement inserts.
Although the process of the invention has been described with respect
to the production of pistons having improved properties over gravity cast material
whether with or without reinforcement inserts the invention is clearly not limited
to such. The production of other engineering components is also envisaged. Examples
of such components include, connecting rods for internal combustion engines, blades
for compressors and turbines, suspension components for motor vehicles etc. Such
components may of course be produced having fibre reinforcement.