US2406966A - Spark plug electrode - Google Patents

Description

Sept. 3, 1946. B. PFEH.

SPARK PLUG ELECTRODE Filed May 5, 1945 y HG2..

8 Inventor F/GS.-

Patented Sept. 3, 1946 SPARK PLUG ELECTRODE Leonard Bessemer Pfeil, Edgbaston, Birmingham,

England, assignor to International Nickel Company Inc., New York, N. Y., a corporation of Delaware Application May 5, 1943, Serial No. 485,704 In Great Britain May 15, 1942 (Cl. 12S-169) Claims. 1

This invention relates to spark plug electrodes made of precious metal alloys.

The electrodes used in high duty plugs, such as aircraft engine spark plugs, are commonly made of precious metal alloys, of which those containing 96% platinum and 4% tungsten, or 80% platinum and 20% iridium, or 90% platinum and rhodium are examples. Other a1- loys sometimes used are platinum-molybdenum and platinum-tin alloys. Particularly when the service conditions are very severe, as is the case in aircraft engines, these electrodes tend to deteriorate in use as a result of intergranular corrosion.

The primary object of this invention is to prevent or minimise such deterioration.

Another object of the invention is to provide a spark plug ready for use that will have an increased service life.

I have observed that one of the most important causes of deterioration is attack by lead, and I have discovered that precious metal alloys having a fibrous microstructure are particularly resistant to attack by lead.

Accordingly I provide electrodes of which at least the unsupported parts have a fibrous microstructure when they are assembled in spark plugs and are ready for service. By a fibrous microstructure I mean one in which the crystals are considerably elongated so that they resemble fibres with their axes all substantially parallel to one another. When a Wire is drawn down the crystals are always distorted and elongated, but mere distortion so that each crystal has one axis longer than another is not enough; rather it is essential that each crystal should be drawn down to a length that is very much 'greater than its width, such as results from reductions in crosssectional area exceeding '75%. A fibrous structure of this kind is removed by recrystallisation when the electrode, or the wire in the course of its fabrication into an electrode, is heated to a high temperature.

As a result of a number of experiments I have found that even a few crystals having longitudinal axes of the same order of magnitude as their transverse axes cause a considerable reduction inv the resistance of the electrode to attack by lead. Accordingly it is important that the structure should be uniformly fibrous and that there should be substantially no such equi-axial crystals.

My invention will be more readily understood with the aid of the annexed drawing, in which Figure 1 shows a non-brous microstructure;

Figure 2 shows a fully fibrous microstructure;

Figure 3 shows a partially recrystallised microstructure; and

Figure 4 is a central longitudinal section through a spark plug.

Referring now to Figures 1 to 3, a microstructure composed essentially of equi-axial nuclei is shown in Figure l, which is a microphotograph, taken with 200 magnification, of a piece of wire made from an alloy containing 96% platinum and 4% tungsten. When this wire had been cold drawn so that its cross-sectional area was reduced by 98.6%, its microstructure became uniformly fibrous, as shown in Figure 2. When the wire was subsequently exposed to a temperature of 1,000 C. for 5 minutes the fibrous microstructure partially recrystallised, the resultant structure being shown in Figure 3.

Although a fibrous microstructure may be produced by severe cold working, the mere fact that cold work has been applied does not suffice to ensure the existence of a brous structure when the electrode is assembled in the spark plug and ready for service; it is also necessary to ensure that at least the unsupported part of the electrode is not subjected to temperatures high enough to cause recrystallisation, either in the course of annealing operations that form stages in the production of the wire or after the cold working has been completed. Thus at least during the later stages of cold Working the alloy must not be annealed at temperatures high enough to bring about recrystallisation, although some recrystallisation may be brought about by early annealing operations provided that severe cold-drawing follows. However, annealing of the kind known as stress-relief annealing and used to reduce hardness and increase ductility without causing visible alteration in the structure may be applied at intermediate drawing stages without preventing the electrode from having a brous structure at the time when it is ready to be assembled into a spark plug.

As indicated above, it is not only in the course of annealing operations during the production of the wire that heating to temperatures high enough to cause recrystallisation must be avoided. In spark plugs used in aircraft engines at the present time, the central or insulated electrode is commonly set into silver or copper, which then occupies part of the central cavity in the insulator. As a rule, the short length of precious metal Wire that constitutes the electrode, such as that shown at I in Figure 4, is fitted into a hole at the bottom of the insulator 3 and temporarily held in position by cement. A piece of silver or copper wire is dropped in and the whole assembly is passed through a furnace to melt the silver or copper Iwire and cause it to fuse around the wire I as shown at 4 and subsequently' to solidify in position. A metal wire 5 is set into the silver or copper during this process, in order to provide the necessary electrical connection to the plug terminal 6. In order to make the interior of the insulator gas-tight, a glass seal I is formed around the wire 5 by applying molten. glass to the top of the silver or copper. vIn the furnace the temperature must exceed 961 C. in the case of silver and 1,083 C. in the case of copper. The insulator, which is usually made of sintered alumina, must not be heated too suddenly, and the melting point of the silver or copper must be sufliciently exceeded to ensure that the silver or copper melts. The precious metal must therefore be subjected to a temperature exceeding 961 or 1,083o C., and the part of it that protrudes from the insulator may remain at a high temperature ior a longer time than that immersed in and shielded by `the insulator. Temperatures over 950 C. are likely to cause recrystallisation of the alloys usually em* ployed. Cold-working lowers the recrystallisation temperature, so that the more the alloy is cold-worked, that is to say, the more iibrous the structure, the lower is the temperature at which the fibrous structure is lost by recrystallisation. Of course, the time during which the alloy is heated in the recrystallisation temperature range is also a factor of importance. Accordingly in carrying the invention into `effect it is necessary to correlate the degree of cold 'work with the duration of the heating and the temperature to which the electrode is heated during any coresetting, glass-sealing or similar process that is employed. In addition, the composition of the alloy must also be taken into account and if necessary adjusted, since alteration in the recrystalli-sation temperature may be effected by variation in the proportion of an element of the alloy.

As indicated above, it is in the unsupportedfpart of the electrode that it is essential to maintain the brous structure. The unsupported part of the central electrode I is that which projects from the insulator 3. the silver or copper is protected from attack by "lead and may recrystallise without causing deterioration to any substantial extent.

The earth electrodes of the standard form of sparking plug now used in aircraft'engines do not offer the same diiliculties as Vthe central electrode so far as the maintenance of the brous structure is concerned. They are usually spot- Welded Ito the nose of the plug. Figure 4 shows one earth electrode 8 spot-welded at 9 to the 4% tungsten alloy as recrystallisation is least marked in this alloy. As an example, thi-5 alloy may be prepared by melting in a high-frequency furnace and cast into an ingot, which may then be reduced hot to a rod 0.2 inch square. The rod may then be annealed at l,050 C. for 15 'The part that is embedded in .w

minutes. At this stage its microstructure is substantially that shown in Figure l. Next the rod is reduced by cold rolling and swaging to the desired size, usually 0.046 inch round wire for the central electrode, and, if drawing difficulties are encountered, the alloy may be held at a temperature of 800 C. for a period of l0 to 15 minutes between stages, e. g. at 0.09 inch a 15 minute stress annealing heat treatment at 800 C. may `be effected, prior to further drawing down to the finished size. The total reduction in area is about 92% and the microstructure is substantially that shown in Figure 2.

Comparative tests made on the one hand with electrodes of -such a cold-drawn wire and on the other hand of a wire of the same composition produced in the normal Way, i. e. by a process of cold drawing with intermediate anneals of 1 hour at 1,050 C., until the nal size is reached, show that the brous electrode has much the better properties. Thus 'the .tensile strength of the non-fibrous wire after exposure to lead contamination in a reducing atmosphere for 36 hours, fell from 95,000 lbs. per vsquare inch to 42,000 lbs. per square inch, but that of the brous wire fell only from 121,000 lbs. per square inch to 118,000 lbs. per square inch after similar exposure. The depth to which the lead penetrated was 10 thousandths of an inch in the case of the non-fibrous wire but only 4 thousandths of an inch in the case of the fibrous wire.

In general, in order to produce the -brous microstructure severe controlled cold drawing must be used, so that the reduction in cro-ss-sectional area after the last annealing is `more than '75%. Below this reduction the microstructure is not sufficiently fibrous.

A metal with a melting point no higher than that oi silver is preferably employed as the material for setting the electrode in the plug so that the temperature to which the alloy is subjected during assembly of .the .plug does .not greatly exceed the melting point of silver. The time'for which the alloy is exposed to this temperature should be restricted to the minimum necessary to cause fusion of the metal.

'The maintenance of a brous microstructure can also be assisted by varying the composition of the alloys so as to raise the recrystallisation temperature. For example, in the usual platinum-tungsten alloy the tungsten contentl maybe raised from 4 to '5% or ruthenium may beintroduced into the alloy. ,Another measure'that maybe taken is to shield the exposed part ofthe electrode by a material of high specific heat or low thermal conductivity so as to reduce the' time 4during which the electrode'is exposed "tohig'h temperatures during the manufacture of thev plug. Againthe silver or copper may be replaced 'by a metal or alloy or lower meltingpoint, e. g. by 'the silver-copper eutectic that melts at 778'c C. In do-ing'this, however, care .mustbe taken not to use ametal or alloy having such' apoor thermal conductivity and low melting point'` that the `electrode becomes over-heated in service andthe setting material melted. Such a eutectic maybe used, for examplewith alloys consistingpredominatingly of platinum and containing vmolybdenum, e. g. 98% platinum-2% molybdenum -alloys, which recrystallise well below r961" C.

I claim:

l. A spark plug ready for service and having an electrode-made from a rprecious-metal-alloy, at least the unsupported `part of said electrode having a non-recrystallized fibrous microstructure made up of crystals having longitudinal lengths much greater than their widths.

2. A spark plug ready for service and having a central insulated electrode made from a precious metal alloy, at least the unsupported part of said electrode having a non-recrystallized brous microstructure resulting from a cold reduction in cross-sectional area of at least 75%.

3. A spark plug ready for service and having all its electrodes made from a precious metal alloy, at least the unsupported part of each of said electrodes having a non-recrystallized iibrous microstructure made up of crystals having much greater lengths than widths, said fibrous structure resulting from a cold reduction in cross-sectional area of at least 75%.

4. A spark plug ready for service and having an electrode made from a precious metal alloy, at least the unsupported part of said electrode having a iibrous microstructure resulting from cold drawing to bring about a cold reduction in cross-sectional area of at least 75% Without any intermediate and subsequent recrystallizations.

5. In a spark plug, an electrode Consisting of about 96% platinum and 4% tungsten and having at least in its unsupported part a brous grain structure resulting from cold reductions in crosssectional area of at least 75% Without any intermediate and subsequent recrystallizations.

LEONARD BESSEMER PFEIL.

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