Note: Descriptions are shown in the official language in which they were submitted.
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FBS/ll
POPPET VALVE MANUFACTURE
This invention relates to the manufacture of poppet
valves and has a particularly useful but not exclusive
application in the manufacture of poppet valves for use in
high performance petrol and diesel engines.
The advent of the high performance petrol and diesel
engine has made severe demands on the poppet valve
manufacturer. Good performance at ever increasing
temperatures coupled with the need for keeping costs to
the lowest level have resulted in most valves being
composite in nature. Often the head is made of a high
performance steel friction welded to a lower cost steel
stem or the valve is faced with an expensive oxidation
resistant high temperature alloy.
The present invention provides a new approach to the
manufacture of poppet valves using metal spray forming
techniques.
According to this invention there is provided a
method of manufacturing poppet valves including the step
of spray depositing metal on to a localized region of a
workpiece in the form of a rod or tube while rotating
the workpiece and the spray gun relative to one another
about the lengthwise axis of the workpiece, and
subsequently cutting through the rod and deposited metal
in a direction normal to said axis at the mid-length of
said localized region.
The invention also provides a poppet valve when made
by a method as described in the preceding paragraph.
It is usual but not essential for the spray deposited
metal to be different from the metal of the rod forming
the stem of the valve and to be a metal having a higher
resistance to temperature and/or wear and/or corrosion.
It may be desirable to spray deposit two or more
different metals at the localized region, the first
being for example a lower cost metal or a lighter metal,
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and the subsequent coating or coating thin coatings of
more expensive metals having higher resistance to
temperature and/or wear and/or corrosion, for example
Stellite. The final coating material will ultimately
form the area in contact with the valve seat, where
operating conditions are most severe.
Where the rod or tube is twice the predetermined
length of the poppet valve, the above method provides two
poppet valve preforms, each of which can be forged and/or
0 machined and/or ground and/or otherwise processed to form
the finished valve. Where the workpiece is tubular, the
ends will be suitably plugged.
Where the rod or tube is effectively continuous, the
said localized region may be spaced from an end of the rod
or tube by the predetermined length of a valve, and
subsequent localized regions spaced from the first said
localized regions and from each other by twice the said
length, the workpiece being cut though at the middle of
each localized region and mid-way between adjacent
localized regions to form a series of poppet valve
preforms.
The workpiece is preferably preheated at the or each
localized region. The localized region may also be
placed under axial compression to cause a bulge to form
at the localized region prior to the commencement of
spray deposition, so that the amount of metal which has
to be sprayed is reduced.
To obtain some forms of distribution of the sprayed
metal on the localized region the nozzle means is
3o oscillated angularly and/or axially in a plane containing
the longitudinal axis of the rod or tube about a position
radially aligned with the middle of said localized region
and directed towards said axis.
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The invention further provides poppet valve
manufacturing apparatus including means for supporting a
rod or tube, spray deposition means including a spray
gun directed towards the axis of such rod or tube means
for rotating the spray gun and the rod or tube relative
to each other about said axis, and a saw blade mounted
for movement at right angles to said axis to cut through
the rod or tube and the metal sprayed thereon.
The invention will now be described in more detail
0 with reference by way of example to the accompanying
diagrammatic drawings in which:
Figures 1 to 4 illustrate stages in the manufacture
of poppet valves in a method according to the invention,
and
Figure 5 illustrates a modification.
Referring to Figures 1 to 4, a rod 10 made from a
medium alloy steel suitable for the valve stem of a poppet
valve is mounted to be driven in rotation about its
longitudinal axis at approximately 300 rpm. The length
of the rod is twice the length of the poppet valves which
are to be made. The localized region 12 of the rod at
its mid-length, where the spray deposit is to be formed,
is preheated in order to ensure a good bond. The bond
can alternatively be improved by using a bond coat and/or
by minimizing oxidation of the rod, for example by
carrying out the process in an inert atmosphere. An
alloy steel of good heat and corrosion resistant
properties is sprayed from a stationary wire-fed arc spray
gun 11 on to the rod at a localized region 12 at its
mid-length to form a deposit 13. The spray is of
conical formation and the resultant distribution of the
sprayed metal is Gaussian, the deposit being a maximum
at the middle 14 of the localized region and falling off
sharply away from the middle in both axial directions.
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When the deposit has built up to a desired radial
thickness at the middle of the localized region, spraying
from the gun 11 is stopped and spraying of an alloy
steel with high heat and corrosion resistance, e.g.
Stellite, is commenced from a second metal spray gun 15
while rotation of the rod with the first deposit 13
continues. The surface produced by the spray
deposition is relatively rough and consequently the
surface of the first deposit 13 provides an excellent
10 key for the second deposit 16. When the deposit 16 of
alloy steel sprayed from the second spray gun reaches
the required radial dimension at the middle 14 of the
localized region, spraying from the second spray gun is
stopped, and the rod and deposits are cut into two
15 substantially identical parts by means of a narrow
slitting saw 18 traversed in a plane normal to the axis
of the rod at the middle of the rod. Two poppet valve
preforms 20 are thus produced as shown in Figure 4 which
may then be transferred while still hot to a forging
20 machine for forging to an accurate size preparatory to
being machined and ground to form the required profile
and seating face.
The speed of the rotation of the rod is not critical
but will normally be in the range 50 - 1000 rpm so as to
25 provide uniform deposition of the metal in a
circumferential direction without causing liquid metal to
be centrifuged off the periphery by the use of too high a
rotational speed.
In a similar manner any required number of thin
layers of hard refractory alloy metals and metal matrix
composites can be formed in a manner ultimately to provide
a surface on which the sealing face of the valve can be
ground.
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Although any metallic material can in principle, be
used, aluminides of nickel, titanium and ion, metal
matrix composite materials, stainless steel and
nickel/chrome alloys are particularly advantageous.
The type of spray gun used will normally be
selected according to the metal to be sprayed. Thus
certain metals are available only in powder form
necessitating the use of a plasma ~orch gun.
In a modified method according to the invention, the
0 rod 10 is of indefinite length, and spray deposition of
metal is carried out as described in relation to Figures 1
to 4 at a localized region at a distance equal
substantially to the length of the required poppet valve
from one end of the rod. After the saw has cut through
the deposit and rod as described above, the saw is used to
cut through the rod again to complete the second pre-form.
The rod is then moved forward and a fresh spray deposit is
formed at a localized region spaced from the new free end
of the rod equal to the desired length of the poppet
valve, and so on. Where the rod is very long, it may be
preferable to fix the rod against rotation and to rotate
the metal spray gun or guns about the axis of the rod.
Where it is desired to lighten the valve, the rod 10
may be replaced by a tube, but it will of course be
necessary to plug or otherwise seal the head end of the
tube.
In order to provide the sprayed metal with a desired
profile, the metal spray guns may be arranged to
oscillate angularly about the position shown in Figures
1 and 2, but moving in a plane containing the axis of
the rod, as shown diagrammatically in Figure 5 of the
drawings, or may oscillate axially, or axially and
angularly about the said position.
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The preforms may be subjected to forging or hot
isostatic pressing before or as an alternative to
machining and grinding to form the finished valve, and
5 may also be heat treated.
The resulting valves were found to be highly
resistant to mechanical shock, and microscopic
examination showed them to be 98% or more dense.
The method is highly advantageous over the usual
10 method of manufacture which involves friction welding a
forged valve head on to a stem, optionally coating
vulnerable surfaces with wear/heat/corrosion resistant
materials, and machining and/or grinding the valve head
to the required shape.
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