GB1597471A - Electric welding or metal and/or metal alloy components - Google Patents

Description

(54) ELECTRIC WELDING OR METAL AND/OR METAL ALLOY COMPONENTS (71) We, LUCAS INDUSTRIES LIMITED, a British Company of Great King Street, Birmingham B19 2XF, England, do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the electric welding of metal and/or metal alloy components.

In the joining of metal and/or metal alloy components by electric welding techniques, e.g.

resistance welding and arc welding, it is often desirable to retain the components in the required relative orientation during welding by way of locating means engaged with the component. The locating means must be formed of an insulating material capable of withstanding the substantial thermal cycling which necessary occurs during welding and in the past a problem has been encountered in finding a material having the required properties over a satisfactory working life. Various materials have been suggested for the purpose, including Tufnol (Registered Trade Mark) and pyrophyllite, and although Tufnol has generally been adopted, all the existing materials have only a short working life and so must be replaced at frequent intervals.

It has however, now been found that a locating means having the required physical properties and an excellent working life can be provided by producing the locating means, at least at the region thereof presented to the components to be welded, from sintered silicon nitride or a sintered ceramic product containing at least 80% by volume of a substituted silicon nitride. It is to be appreciated that the term "substituted silicon nitride" is used throughout the present specification to mean a single phase compound having a silicon nitride lattice in which the silicon and nitrogen atoms in the lattice have been partially replaced by other elements, particularly, but not exclusively, aluminium and oxygen respectively. Moreover, it is to be appreciated that the term "sintered" is used throughout the present specification to refer to products obtained by sintering either with or without applied pressure.

Accordingly, the invention resides in a method of joining metal and/or metal alloy components by electric welding wherein the components are retained in a required relative orientation during welding by locating means which is engaged with the components and is formed, at least at the region thereof presented to the components, of sintered silicon nitride or a sintered ceramic product containing at least 80% by volume of a substituted silicon nitride.

Preferably, said sintered silicon nitride or said sintered ceramic product is arranged to have a mean modulus of rupture value at 250C of at least 90,000 p.s.i.

Preferably, said substituted silicon nitride is a single phase silicon aluminium oxynitride obeying the formula: Si6z Alz Oz Nsz where z is greater than zero and less than or equal to 5.

Preferably, the joining operation is effected by resistance welding.

In the accompanying drawings: Figures 1 and 2 illustrate diagrammatically methods of joining metal components according to first and second example respectively of the invention.

Referring to Figure 1, in the method of the first example it is required to join two mild steel components 11, 12 by resistance welding. The components define respective halves of the pinion-actuating lever of a pre-engaged starter motor and each is formed with a centrally disposed aperture, indicated by the suffix a, about which the assembled lever is pivoted in use. Each component is stamped from a 0.04" thick mild steel sheet with the stamping of the component 11 being arranged to produce upstanding dimples 13 on its surface to be joined to the component 12.

To effect the joining operation, the component 12 is initially laid on top of the dimpled surface of the component 11. The components are then held in the required relative orientation by means of a cylindrical locating member 14 which is inserted through the apertures gila, 12a and which is formed of sintered silicon nitride or a sintered product containing at least 80% by weight of a substituted silicon nitride, prefreraby a silicon aluminium oxynitride obeying the above formula. The resultant assembly is then located between a pair of welding electrodes 15, 16 which are recessed so as to accommodate the ends of the member 14 projecting from the components. Welding current is then passed between the electrodes so as to melt the dimples 13 and provide, on cooling, the required joint between the components.

In one practical embodiment, the member 14 was produced by initially nitriding a mixture consisting of 42 parts by weight of aluminium as supplied by The Aluminium Company of America with a particle size of about 20 microns, 14 parts by weight of silicon as supplied by Union Carbide Limited with an average particle size of 3 microns and 44 parts by weight of alumina supplied by Linde as Type B and having a nominal particle size of 0.05 microns, Nitriding was effected with the mixture located in an alumia boat positioned in a nitriding furnace and with the nitriding atmosphere supplied to the furnace consisting of 64% by volume nitrogen, 6% by volume hydrogen and 30% by volume argon.

During nitriding, the temperature was carefully controlled to prevent thermal runaway means of separate thermocouples in the reaction mixture and the furnace walls respectively.

In particular, the temperatures registered by the thermocouples were compared and, if the temperature in the mixture exceeded that in the furnace walls, the supply of the nitriding atmosphere was terminated until the temperature in the mixture fell below that in the walls.

A heating schedule which allowed the nitriding to proceed with a minimum amount of interruption was as follows: (a) raising the temperature at a rate of 100"C/hour to 500 and holding for 24 hours, (b) increasing the temperature at said rate from 500"C to 6000C and holding for seven hours, (c) increasing the temperature at said rate from 600"C to 1000"C and holding fo 24 hours, (d) increasing the temperature at said rate from 1000"C to 1100 C and holding for 18 hours, (e) increasing the temperature at said rate from 1 1000C to 1200"C and holding for 5 hours, (f) further increasing the temperature at said rate to 13000C and holding for 20 hours, and (g) still further increasing the temperature at said rate to 13500C and holding for 6 hours.

After completion of the heating process, the nitrided mixture was allowed to cool in an argon atmosphere and was then removed from the furnace. The material was then jaw crushed and cone ground to a particle size below 500 microns and thereafter was cold isostatically pressed at 15000 p.s.i. into a pellet which was then introduced into a graphite pot and buried in boron nitride powder to provide a protective environment for the subsequent sintering operation. The temperture of the pot was then raised in 1l/2 hours from room temperature to 1500-2000C preferably 1800"C, and then held at this temperature for 1 hour. Apart from some unconverted oxides, the resultant sintered product consisted substantially entirely of a single phase silicon aluminium oxynitride having a rhombohedral crystal structure and obeying the approximate formula SiAl4 2 N4. Thus this material constituted a different ceramic phase from the silicon aluminium oxynitride which obeys the formula given above and which has a crystal structure based on the hexagonal phase silicon nitride lattice. For the sake of convenience, this additional ceramic phase will hereafter be referred to as the phase 15B.

After removal from the graphite pot, the sintered mass of the 15R produce was jaw crushed and then micronised to an average particle size of 7 microns. 14.25 parts by weight of the ground 15R product were then mixed with 85.75 parts by weight of silicon nitride powder having a mean particle size of 2 microns and containing 89% by weight of the a-phase material together with 4% by weight of silica as an inherent impurity, and 7 parts by weight of yttria powder as supplied by Rare Earth Products Limited with a particle size of about 1 micron were added to the mixture. The resultant mixture was colloid milled, sieved and then cold isostatically pressed in a rubber bag at 20,000 p.s.i. so as to produce a preform having a green density of 1.6.g.cm-3. The resultant preform was then provided with a protective surface coating of silica together with 50% by weight of boron nitride, the coating having a thickness of between 0.01 and 0.02 inch. The coating materials were applied to the preform as a suspension in a mixture of iso-butyl-methyl-ketone with between 5 and 10% by volume of collodion, the suspension containing 40% by weight of solids.

The coated preform was then buried in a powdered boron nitride protective medium contained in a graphite pot and was heated, without the application of pressure, to a sintering temperature of 1840"C over a period of eighty minutes. The preform was then held at this temperature for a further sixty minutes to produce a product which, after cooling and removal from the graphite pot, was found to have a mean modulus of rupture value at 25"C of 810 MNm-2 and a density of 3.208, the total weight loss during protection of this product being 2%. During the final sintering operation it was found that the 15R material and silicon nitride had been converted to the required silicon aluminium oxynitride material obeying the above formula with a z value of 0.6 so that said material constituted in excess of 90% by volume of the final sintered product. Transmission electron microscopy examination showed that the product also contained a yttrium-containing glass phase in an amount of the order of 10% by volume.

The locating member 14 produced according to the above embodiment was 1.5" long and 0.3" in diameter and, when tested in the welding pairs of components 11, 12, was found to survive 160,000 operations without significant damage. By way of contrast an equivalent member formed from Tufnol (Registered Trade Mark) required replacement after only 500 operations and after the first few welds caused the apertures 11a, 12a to become eccentric by about 0.03". Some eccentricity was also experienced when testing the member 14 produced according to the above embodiment but this was limited to about 0.004".

Comparison tests were also performed on a locating member formed of pyrophyllite but this was found to be destroyed during the first welding operation.

Referring to Figure 2, in the method of the second example it is required to provide a captive nut 21 on a body panel 22 of a road vehicle. This is effected by resistance welding the nut to the rear surface of the panel 22 so that the threaded bore in the nut communicates with an aperture 23 in the panel, the nut 21 being provided with upstanding dimples 24 which provide a high resistance contact with the panel when the nut is pressed against the panel during the resistance welding operation. As in the previous example, a locating member 25 formed of sintered silicon nitride or a sintered ceramic product containing at least 80% by volume of a substituted silicon nitride is located in the aperture 23 and the bore in the nut 21 during welding so as to maintain the nut in the required position relative to the panel. Again the locating member 25 is found to have a prolonged working life, while at the same time the use of the member 25 enables the nut 21 to be formed without the hollow, projecting spigot normally required to locate the nut relative to the aperture 33.

In the methods described above, it is desirable to ensure that the sintered ceramic material of the locating member has a mean modulus of rupture value at 250C of at least 90,000 p.s.i., and, where the ceramic material is silicon nitride, this will normally require the application of pressure to the material during sintering. However, as in the case of the first example, the application of pressure may not be necessary to achieve the desired strength where the ceramic material is a silicon aluminium oxynitride obeying the above formula, although again pressure may be used if desired.

Although the locating means in the above examples has been formed completely of sintered silicon nitride or a sintered ceramic product containing at least 80% by volume of a substituted silicon nitride, satisfactory results can be obtained when only the region of the locating means engaging the components being welded is formed of the required organic material. Thus, for example, the locating means could be defined by a single hollow tube or a plurality of rings of the requied ceramic material carried by a central metal carrier pin.

WHAT WE CLAIM IS: 1. A method of joining metal and/or metal alloy components by electric welding wherein the components are retained in a required relative orientation during welding by locating means which is engaged with the components and is formed, at least in the region thereof presented to the components, of sintered silicon nitride or a sintered ceramic product containing at least 80% by volume of a substituted silicon nitride.

2. A method as claimed in Claim 1, wherein said sintered silicon nitride or said sintered ceramic product is arranged to have a mean modulus of rupture value at 250C of at least 90,000 p.s.i.

3. A method as claimed in Claim 1 or Claim 2, wherein said substituted silicon nitride is

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. by weight of yttria powder as supplied by Rare Earth Products Limited with a particle size of about 1 micron were added to the mixture. The resultant mixture was colloid milled, sieved and then cold isostatically pressed in a rubber bag at 20,000 p.s.i. so as to produce a preform having a green density of 1.6.g.cm-3. The resultant preform was then provided with a protective surface coating of silica together with 50% by weight of boron nitride, the coating having a thickness of between 0.01 and 0.02 inch. The coating materials were applied to the preform as a suspension in a mixture of iso-butyl-methyl-ketone with between 5 and 10% by volume of collodion, the suspension containing 40% by weight of solids. The coated preform was then buried in a powdered boron nitride protective medium contained in a graphite pot and was heated, without the application of pressure, to a sintering temperature of 1840"C over a period of eighty minutes. The preform was then held at this temperature for a further sixty minutes to produce a product which, after cooling and removal from the graphite pot, was found to have a mean modulus of rupture value at 25"C of 810 MNm-2 and a density of 3.208, the total weight loss during protection of this product being 2%. During the final sintering operation it was found that the 15R material and silicon nitride had been converted to the required silicon aluminium oxynitride material obeying the above formula with a z value of 0.6 so that said material constituted in excess of 90% by volume of the final sintered product. Transmission electron microscopy examination showed that the product also contained a yttrium-containing glass phase in an amount of the order of 10% by volume. The locating member 14 produced according to the above embodiment was 1.5" long and 0.3" in diameter and, when tested in the welding pairs of components 11, 12, was found to survive 160,000 operations without significant damage. By way of contrast an equivalent member formed from Tufnol (Registered Trade Mark) required replacement after only 500 operations and after the first few welds caused the apertures 11a, 12a to become eccentric by about 0.03". Some eccentricity was also experienced when testing the member 14 produced according to the above embodiment but this was limited to about 0.004". Comparison tests were also performed on a locating member formed of pyrophyllite but this was found to be destroyed during the first welding operation. Referring to Figure 2, in the method of the second example it is required to provide a captive nut 21 on a body panel 22 of a road vehicle. This is effected by resistance welding the nut to the rear surface of the panel 22 so that the threaded bore in the nut communicates with an aperture 23 in the panel, the nut 21 being provided with upstanding dimples 24 which provide a high resistance contact with the panel when the nut is pressed against the panel during the resistance welding operation. As in the previous example, a locating member 25 formed of sintered silicon nitride or a sintered ceramic product containing at least 80% by volume of a substituted silicon nitride is located in the aperture 23 and the bore in the nut 21 during welding so as to maintain the nut in the required position relative to the panel. Again the locating member 25 is found to have a prolonged working life, while at the same time the use of the member 25 enables the nut 21 to be formed without the hollow, projecting spigot normally required to locate the nut relative to the aperture 33. In the methods described above, it is desirable to ensure that the sintered ceramic material of the locating member has a mean modulus of rupture value at 250C of at least 90,000 p.s.i., and, where the ceramic material is silicon nitride, this will normally require the application of pressure to the material during sintering. However, as in the case of the first example, the application of pressure may not be necessary to achieve the desired strength where the ceramic material is a silicon aluminium oxynitride obeying the above formula, although again pressure may be used if desired. Although the locating means in the above examples has been formed completely of sintered silicon nitride or a sintered ceramic product containing at least 80% by volume of a substituted silicon nitride, satisfactory results can be obtained when only the region of the locating means engaging the components being welded is formed of the required organic material. Thus, for example, the locating means could be defined by a single hollow tube or a plurality of rings of the requied ceramic material carried by a central metal carrier pin. WHAT WE CLAIM IS:

1. A method of joining metal and/or metal alloy components by electric welding wherein the components are retained in a required relative orientation during welding by locating means which is engaged with the components and is formed, at least in the region thereof presented to the components, of sintered silicon nitride or a sintered ceramic product containing at least 80% by volume of a substituted silicon nitride.

2. A method as claimed in Claim 1, wherein said sintered silicon nitride or said sintered ceramic product is arranged to have a mean modulus of rupture value at 250C of at least 90,000 p.s.i.

3. A method as claimed in Claim 1 or Claim 2, wherein said substituted silicon nitride is

a single phase silicon aluminium oxynitride obeying the formula: Swizz Alz Oz Ns-z where z is greater than zero and less than or equal to 5.

4. A method as claimed in any one of Claims 1 to 3, wherein the joining operation is effected by resistance welding.

5. A method as claimed in Claim 1, of joining metal and/or metal alloy components by electric welding substantially as hereinbefore described with reference to, and as shown in, Figure 1 or Figure 2 of the accompanying drawings.