Poppet valve manufacture

Poppet valves are manufactured by rotating a metal spray deposition nozzle relative to a rod or tube about the longitudinal axis of the rod or tube to deposit metal at a localized region intermediate the ends of the rod or tube so as to form a flange-like accretion on the rod or tube, and subsequently cutting through the rod or tube and the accretion at the center of the localized region to form two poppet valve preforms which are then subjected to finishing treatments. Metals having differing properties of heat, wear and corrosion resistance may be coated upon one another in forming said accretion.

BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
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, 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 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 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. 
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 with reference by way of 
example to the accompanying diagrammatic drawings in which:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 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. 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 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 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 FIG. 4 which may then be transferred while still hot 
to a forging 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 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. 
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 torch gun. 
In a modified method according to the invention, the rod 10 is of 
indefinite length, and spray deposition of metal is carried out as 
described in relation to FIGS. 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 FIGS. 1 and 2, but moving in a plane containing the axis of the rod, as 
shown diagrammatically in FIG. 5 of the drawings, or may oscillate 
axially, or axially and angularly about the said position. 
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 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 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.