A spark plug for an internal combustion engine

Abstract: In a spark plug (100) having a noble metal layer (5) which is laser welded to a firing end (31,33,34,42,43) of an electrode (3,4), heat treatment is used to increase the gain size of the noble metal layer (5) after welding.

Description: This invention relates to a spark plug in which a noble metal layer is laser welded to the firing end of a centre or outer electrode in order to increase its spark erosion durability. In a spark plug used hitherto, there is provided a centre or outer electrode which has a heat-resistant metal (nickel or the like) as a base metal. A noble metal is welded to an outer surface of the electrode in order to decrease its spark erosion tendency. If laser beam welding is employed to weld the noble metal to the electrode, instead of electrical resistance welding which has usually been used, the noble metal layer is thermally bonded to the electrode strongly enough to remove the boundary therebetween so as to increase significantly its spark erosion durability. A spark plug comprising a noble metal layer laser welded to the firing end of an electrode is disclosed in JP-A-5082236. Upon thermally bonding the noble metal layer to the electrode by means of laser beam welding, the welding causes a local increase in the temperature of the portions to which the laser beams are applied so as to melt instantly the noble metal and the outer surface of the electrode, while the remaining portion of the electrode is left cold. The heat-drawing action of the electrode causes the molten noble metal to cool rapidly so that it solidifies in a short period of time. This rapid cooling effect induces a dendriform crystallization in the region in which the noble metal and the outer surface of the electrode are fused together. Due to the crystallized grains of the dendriform crystallization being minute, and the grain boundaries being relatively fragile and susceptible to cleavage, there is a problem that small clefts may occur in the noble metal layer which may develop into cracks when the spark plug is in use mounted on an internal combustion engine. With prolonged use of the internal combustion engine, it is possible that oxygen gas or combustion gas may permeate into the clefts or the cracks so as to induce oxidation-corrosion of the base metal of the electrode beneath the noble metal layer. If the situation is aggravated, oxidation-corrosion may exfoliate the noble metal layer from the outer surface of the electrode so that its spark erosion durability deteriorates. Therefore, it is an object of the invention to provide a spark plug in which the noble metal layer is protected against the occurrences of clefts and cracks into which corrosive matter may permeate, in order to prevent effectively the noble metal layer from flaking off the electrode so as to improve the spark erosion durability. According to one aspect of the invention, there is provided a spark plug having a noble metal layer laser welded to a firing end of an electrode characterised in that by heat treatment of the electrode after welding the grain size of the noble metal layer has been increased. According to another aspect of the invention, there is provided a method of making an electrode for a spark plug by laser welding a noble metal layer to a firing end of the electrode, characterised by the step of heat treating the electrode after welding to increase the grain size of the noble metal. After the noble metal layer is laser welded to the electrode, the noble metal layer cools rapidly, which causes a minute dendriform structure to develop in the noble metal layer which is subject to a multitude of clefts and cracks. By heat-treating (annealing) the noble metal layer, it is possible to recrystallize the dendriform structure so as to eliminate the clefts and cracks together with the intergranular space. With the elimination of the clefts and cracks, the noble metal layer is protected against the penetration of corrosive matter to the base metal underneath which effectively prevents the noble metal layer from flaking off the electrode so as to ensure an extended life of the spark plug. Preferably the noble metal layer is principally of platinum, iridium, platinum-iridium alloy or platinum-nickel alloy. Preferably the average grain size of the recrystallized dendriform structure is 10 microns or more when the annealing treatment is finished. When thermally bonding the noble metal layer to the electrode, use of pulse-type laser beam welding enhances the efficiency of the welding operation, while use of continuous-type laser beam welding makes the electrode red-hot and fuses the electrode base metal more into the noble metal layer so as to deteriorate its spark erosion resistance. Specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 is an enlarged perspective view of an end portion of a spark plug according to an embodiment of the invention; Figs. 2a - 2c are sequential views showing how a noble metal layer is laser welded to a fringe end of a centre electrode according to an embodiment of the invention; Figs. 3a - 3d are magnified views of the metallic structure of a prior noble metal layer thermally bonded to a centre electrode; Figs. 4a - 4c are magnified views of the metallic structure of the noble metal layer thermally bonded to the centre electrode according to an embodiment of the invention; and Figs. 5a - 5f are perspective views of the end portion of spark plugs according to other embodiments of the invention. Referring to Fig. 1, which shows an end portion of a dual-polarity type spark plug 100, the spark plug 100 has a cylindrical metallic shell 1 and an elongated insulator 2 placed within the metallic shell 1. The insulator 2 has an axial bore 21, circular in cross section, whose front end 22 has a tapered portion. Within the axial bore 21 of the insulator 2, a columnar centre electrode 3 is placed with its front end 31 extending slightly beyond the front end 22 of the insulator 2. On an outer surface of the metallic shell 1, is a male thread 11 provided to mount the spark plug 100 on an internal combustion engine. Diametrically opposed outer electrodes 4,4 are thermally bonded to the annular front end 12 of the metallic shell 1. The outer electrodes 4,4 are rectangular in cross section and one end of each is welded to the annular front end 12 of the metallic shell 1. A support portion 41 of each of the outer electrodes 4,4, is extended to tilt somewhat toward the centre electrode 3. The front end of each support portion 41 is bent to form a firing portion 42 which opposes the front end 31 of the centre electrode 3. The front end surface 43 of the firing portion 42 of each outer electrode 4 is concave so as to form an approximately constant spark gap G1 between the front end surface 43 and the cylindrical outer surface of the front end 31 of the centre electrode 3. A noble metal layer 5 is laser welded to the cylindrical outer surface of the front end 31 of the centre electrode 3. Referring to Fig. 2, the noble metal layer 5 is provided as follows: Firstly, a noble metal wire 50 is prepared which is made of platinum, iridium, platinum-iridium alloy or platinum-nickel alloy. The noble metal wire 50 is wound around a groove 32 provided in the front end 31 of the electrode metal 30 of the centre electrode 3 as shown in Fig. 2a. Then, four pulsed laser beams (R) with a pulse rate of 12 pulses/millisecond are shot at the noble metal wire 50 in a burst lasting 2 milliseconds, while the electrode metal 30 is continuously revolved at a predetermined rate. This operation makes it possible to melt the entire piece of noble metal wire 50 and the groove 32 of the electrode metal 30 so as to weld the noble metal layer 5 on to the front end 31 of the electrode metal 30. Upon shooting the laser beams (R), the noble metal wire 50 and front end of the electrode metal 30 are instantaneously fused together to form an alloy and the noble metal layer 5 is thereby laser welded to the front end of the centre electrode 3 as shown in Fig. 2b. In this instance, the laser beams (R) and the revolution of the electrode metal 30 may be applied intermittently or continuously. After the completion of the laser welding, the noble metal wire 50 and the front end of the electrode metal 30 are rapidly cooled (quenched) by the heat-drawing action of the other portion of the electrode metal 30 which is left cold. This causes the temperature of the molten metal to fall quickly below the solidification point. The alloy of the noble metal layer 5 and the electrode metal 30 penetrates deep into the electrode metal 30 and is strongly bonded to the electrode metal 30 when laser welding is used. This makes it possible favourably to prevent the noble metal layer 5 from accidentally detaching from the electrode metal 30, as against the case in which the noble metal layer is provided by means of electrical resistance welding, cold forging or inert gas shield welding. Upon applying laser welding, a rapid local temperature rise is observed in the portions of the electrode metal 30 and the noble metal wire 50 at which the laser beams are shot which causes them to be instantaneously fused together to form an alloy. This is followed by rapid cooling and solidification by the heat-drawing action of the other portion of the electrode metal 30 which is left cold. This rapid cooling makes it possible to finish the welding operation swiftly so as to improve productivity, while on the other hand, rendering the noble metal layer 5 into a dendriform structure in which the crystallized grains are approximately 1 micron in diameter and 10 microns in length as shown in Figs. 3a - 3d. In the dendriform structure, the orientation in which the dendrites grow is not fixed, being partly in vertical direction, and partly in lateral direction. At the boundary between a series of laterally extended dendrites and a series of vertically grown dendrites, minute clefts and cracks 51 (Fig. 2b) tend to appear at the central portion upon laser welding the noble metal layer 5, or in use of the spark plug 100, as in Fig. 3a which shows a photograph of the front end 31 of the centre electrode 3 magnified 35 times. Fig. 3b shows a photograph of the noble metal layer 5 which is magnified 1000 times. Fig. 3c shows a magnified photograph of the outer surface of the noble metal layer 5. Fig. 3d shows a photograph of the central portion of Fig. 3b and has a magnification of 3500 times, and suggests that the cracks 51 have permeated deeply. Referring back to Fig. 2c, the centre electrode 3 is placed in a vacuum kiln (A) to anneal the electrode 3 at 800°C to 1000°C for 1 to 10 hours under a pressure of between 1.33 Pa and 1.33 x 10 -6 Pa (10 -2 to 10 -8 Torr). This tempering treatment produces the dendriform structure as shown in Figs. 4a - 4c which correspond to Figs. 3a - 3c respectively. It is apparent from Figs. 4a - 4c that the annealing procedure develops large recrystallized grains which substantially eliminate the minute clefts and greater cracks 51, and thus conceals the boundary between the noble metal layer 5 and the electrode metal 30 of the centre electrode 3. In this instance, it is possible to select the annealing time period, temperature and the ambient atmosphere as desired depending upon the material of the electrode metal 30 and the thickness of the noble metal layer 5. A dual polarity type spark plug was prepared in which a noble metal, platinum (Pt), layer was pulse-laser welded to the electrode metal, and at the same time, the type of spark plug 100 in which the noble metal (Pt) layer 5 is in addition annealed was prepared. A durability test, in which these two types of spark plug are respectively mounted on a six-cylinder gasoline engine, was carried out. After operating the engine for 50000 km, it was found in the former spark plug that oxidation-corrosion of 10% of the boundary between the noble metal layer and the electrode metal had occurred. In contrast, substantially no oxidation-corrosion was found in the latter spark plug 100 after investigating the experimental test results. Figs. 5a - 5f show spark plugs with a noble metal layer according to other embodiments of the invention. A noble metal layer 5 may be laser welded to a portion 33 of the centre electrode penetrating into the front open end of the insulator 2, in addition to the noble metal layer 5 already welded to the front end of the electrode metal 30 as shown in Fig. 5a. This noble metal layer is effectively employed in a multi-polarity type spark plug in which more than two outer electrodes are provided. The noble metal layer 5 does not have to be provided around the entire circumference of the front end 31 of the electrode metal 30, but may be welded to only part of the circumference as shown in Fig. 5b. The noble metal layer 5 may be laser welded to a front end surface 34 of the centre electrode 3 as shown in Fig. 5c. As shown in Fig. 5d, the noble metal layer 5 may be laser welded to the front end surface 43 of the or each outer electrode 4. Fig. 5e shows a semi-creeping type spark plug in which the noble metal layer is laser welded to the portion 33 of the centre electrode penetrating into the front open end of the insulator 2. Fig. 5f shows another semi-creeping type spark plug in which the outer electrode 4 is integrally formed with the front end of the metallic shell 1, in an annular configuration and a noble metal layer 5 may be laser welded to the portion 33 of the centre electrode penetrating into the front open end of the insulator 2, in addition to the noble metal layer 5 already welded to the front end of the electrode metal 30. It is to be understood that types of spark plugs other than the above ones may be employed in which a noble metal layer 5 is laser welded to an electrode. It is noted that a C0 2 laser or an eximer (excited dimer) laser may be used as well as a YAG laser. It is also appreciated that the noble metal layer may be annealed in an inert gas atmosphere, nitrogen atmosphere, hydrogen atmosphere or the like when carrying out the heat treatment.

Claims: A spark plug (100) having a noble metal layer (5) laser welded to a firing end (31,33,34,42,43) of an electrode (3,4), characterised in that by heat treatment of the electrode after welding the grain size of the noble metal layer (5) has been increased. A spark plug according to claim 1, wherein the noble metal layer (5) is principally of platinum, iridium, platinum-iridium alloy or platinum-nickel alloy. A spark plug according to claim 1 or 2, wherein the recrystallized grain size of the noble metal layer (5) is 10 microns or more on average. A method of making an electrode (3,4) for a spark plug (100) by laser welding a noble metal layer (5) to a firing end (31,33,34,42,43) of the electrode (3,4), characterised by the step of heat treating the electrode (3,4) after welding to increase the grain size of the noble metal layer (5). A method according to claim 4, wherein the noble metal layer (5) is principally of platinum, iridium, platinum-iridium alloy or platinum-nickel alloy. A method according to claim 4 or 5, wherein the recrystallized grain size of the noble metal layer (5) is 10 microns or more on average. A method according to any one of claims 4-6, wherein the heat treatment includes annealing the electrode having the noble metal layer (5) at a temperature between 800°C and 1000°C. A method according to any one of claims 4-7, wherein the heat treatment includes annealing the electrode having the noble metal layer (5) for between 1 and 10 hours. A method according to any one of claims 4-8, wherein a pulsed laser is used to perform the laser welding. A method of making a spark plug, wherein an electrode of the spark plug is made by a method according to any one of claims 4-9.

Inventors: AMANO KOZO, MATSUTANI WATARU, AMANO, KOZO, MATSUTANI, WATARU, AMANO, KOZO, C/O NGK SPARK PLUG CO., LTD., MATSUTANI, WATARU, C/O NGK SPARK PLUG CO., LTD.
Applicants: NGK SPARK PLUG CO, NGK SPARK PLUG CO., LTD
Application year: 1995
Main classification: H01T13
IPC codes: H01T21, H01T13