CA2610328A1 - Contact tip for arc welding - Google Patents

Abstract

The invention is a contact tip for use in Gas Metal Arc Welding having a wire feed aperture for a welding wire in the center thereof, comprising a tip body with a tip end that is permanently built up on it via the Cold Gas Dynamic Spray process. Cold Gas Dynamic Spray or CGDS is a spray deposition technique wherein the deposits form due to high-velocity impact of solid particles. CGDS produces a high density coating with fairly high bonding, while pores and oxides are minimal or even absent. This leads to excellent thermal and electrical conductivity with excellent wear resistance values of sprayed coatings.
The tip body is composed of copper or a copper alloy material. The tip end is made of a material or a mixture of materials such as silver, silver alloy, copper, copper alloy, aluminum, nickel, chromium, tungsten with aluminum oxide, zirconium oxide, metal carbides and their combinations.

Description

Title Contact tip for arc welding.
Field of the invention This invention relates to improved contact tips for use in Gas Metal Arc Welding.
Background of the invention In conventional gas metal arc welding operations, a wire of filler metal or welding wire is continuously fed and charged through a welding torch and contact tip to a target metal work piece where it is consumed and becomes the filler metal.

The contact tip has a double role; namely acting as a guide for the welding wire, while also feeding electric power to the welding wire. In order to supply welding current to the wire, the contact tip is put into contact with the welding wire. An electric arc is formed between the charged end of the welding wire and the oppositely charged metal work piece which provides heat to form a weld puddle.

The welding wire is unwound from a spool and automatically fed into the welding zone as the welding wire is consumed. Drive rollers are often used to feed the wire off the spool and into the welding torch. The welding wire has a cast that is formed when wound onto the spool. The cast helps to create electrical contact between the contact tip and the welding wire.
The contact tip wire feed aperture is subject to wear damage by abrasion and electrical erosion caused by sliding friction between the charged wire feed aperture surface and the moving welding wire.

Contact tips are traditionally fabricated as cylindrical tubes made of pure copper or high copper content alloys because copper is a very good electrical conductor, is relatively cheap, and is easy to machine.
Copper materials have excellent electrical and thermal conductivity, are readily fabricated, and have good strength and fatigue characteristics. An important disadvantage of copper is its low resistance to surface oxidation and corrosion. Also, it is not regarded as having good resistance to arcing, welding and sticking, which are needed in contact applications.

2 Silver alloy coatings are often used for elevated-temperature solid lubrication. Silver deposits prevent galling, especially during start-up. The silver serves both as a corrosion deterrent and a dry lubricant. Silver coating can be used up to 870 C, and thus is a good high-temperature lubricant.
Silver, in pure or alloyed form, is the most widely used material for a considerable range of electrical contacts up to 600 A. Silver has the highest electrical and thermal conductivity of all metals at room temperature and, as result, will carry high currents without excessive heating, even when dimensions of the contacts are only moderate. For electrical contacts, silver is used instead of copper chiefly because of its resistance to oxidation in air. All non-noble metals are subject to a phenomenon called fretting corrosion. This fretting corrosion is an accelerated corrosion that occurs at the interface of contacting metals when they are subject to certain relative motions.

During welding, the highest temperature of a welding gun is present at the tip end of the contact tip.
This is due to the tip end being the closest portion of the contact tip to the electrical arc and to the welding puddle which emanates heat, and due to the heat generated by the electrical current passing through the contact tip to the welding wire.
Under normal service conditions, contact tips may be exposed to operating temperatures well above 400 C. Operating temperature is critical in determining the performance of contact tips. Higher temperatures degrade the material properties and accelerate failure of contact tips. In consequence of the exposure to high operating temperatures, the charged wire feed aperture surface, especially the tip end, is so heavily worn that the tip is soon unable to conduct its double role. In other words, the tip fails to both guide the wire, resulting in misplaced welds, and to supply welding current.

An ideal electrical contact material would have the following: high electrical conductivity to minimize heat generated; high thermal conductivity to dissipate both the resistive and arc heat developed; high reaction resistance to all environments used, in order to avoid the formation of insulating oxides, sulfides and other compounds; and finally, immunity to arcing damage while making and breaking electrical contact. Both the force required to close a contact rnade of this material and the electrical resistance between mating members would have to be low. The melting point of the material would have to be high enough to limit arc erosion and metal transfer, but would also have to be low enough to increase resistance to re-ignition in switching.
Also, the vapour pressure would have to be low in order to minimize arc erosion and metal transfer. Hardness would have to be high enough to provide good wear resistance, while being ductile enough to ensure ease

3 of fabrication. Purity of the material would have to be maintainable at a level that ensures consistent performance of the product. Finally, the material would have to be available at a low cost in any desired form.

To achieve all these performance parameters in order to extend the contact tip life, it is necessary to use a mechanical mixture of materials and apply it at the charged wire feed aperture surface, specifically to the end of the contact tip, using a process that will not change or destroy its properties. This can be done using a new spray coating process called Cold Gas Dynamic Spray.
Cold Gas Dynamic Spray or CGDS can be used to apply a wide variety of metallic, dielectric, metallic alloys, and mixed combinations to a variety of substrate materials.
CGDS is a relatively new coating process by which coatings can be produced without significant heating of the sprayed powder. In contrast to flame, arc, and plasma spraying processes, in CGDS
there is no melting of particles prior to impact with the substrate. The adhesion of particles in this process is due solely to their kinetic energy upon impact. In this process, very high particle velocities are obtained by the acceleration of an expanding gas stream to velocities in the range of supersonic speed in a converging-diverging De Laval type nozzle. The gas and particle temperatures remain well below the melting temperature of the spray material.

Various innovations for welding torch contact tips are present in prior art, including contact tips with an insert shown in U.S. Pat. Nos.: 5,101,093 and 4,937,428, and in Canadian Pat. Nos.:
1,182,871 and 2,233,662. Typically these inserts are ceramic or of a conductive material harder than the copper, which claim to extend the contact tip life by reducing wear or temperature of the contact tip as the welding wire is fed through it. However, these inserts are much less conductive and inhibit current transfer close to the arc, and the cost of manufacturing contact tips with these inserts is high.
Summary of the invention This invention relates to electric welding torch tips for use on a consumable electrode type welder having a wire feed aperture for a welding wire in the center thereof, comprising a tiip body with a tip end that is permanently built up on it via the Cold Gas Dynamic Spray process.
Cold Gas Dynamic Spray or CGDS is a spray deposition technique wherein the deposits form due to high-velocity impacts of solid particles. This process produces a high density coating with fairly high bonding.
CGDS coatings exhibit phase purity, while inclusions such as pores, and oxides are minimal or even

4 absent. This leads to excellent thermal and electrical conductivity with excellent wear resistance values of sprayed coatings.

The tip body is composed of copper or a copper alloy material. The tip end is made of a material or a mixture of materials such as silver, silver alloy, copper, copper alloy, aluminum, nickel, chromium, tungsten with aluminum oxide, zirconium oxide, metal carbides and their combinations.
Brief description of the drawings Fig.l is a sectional view of a known welding contact tip;
Figs. 2, 3, 4, 5 are sectional views of welding contact tips embodying the present invention.
Detailed description of the preferred embodiments As shown in Fig.l, a cross-section of a conventional welding contact tip comprises a tip body 1 with an axial centered wire passage 10 to feed the welding wire through it.
Figs. 2, 3, 4, and 5, show cross-sections of a welding contact tip according to the present invention comprising a tip body 1 and a tip end 2. The welding contact tip is fabricated by building up the tip end 2 over the tip body 1 by spraying it on layer by layer using the Cold Gas Dynamic Spray process, in such a way that the two parts are integrated with each other.
The tip end 2 is sprayed over a variety of prepared shapes 21, 31, 41, and 51 of the tip body 1 as shown in Figs. 2, 3, 4, and 5. The most economical one is the tip end shown in Fig.3 and Fig.5 with shape 31 and 51 accordingly on the tip body 1.
According to the present invention, the tip body 1 is made of copper or a copper alloy, and the tip end 2 which is subjected to serious wearing by abrasion and electrical erosion during welding is made of a heat- and wear-resistant conductive material or mixture of materials suc:h as silver, silver alloy, copper, copper alloy, aluminum, nickel, tungsten with aluminum oxide, zirconium oxide, metal carbides and their combinations.
The wire passage 10 in the tip body 1 has a diameter 1.05 - 1.35 times the welding wire diameter, the wire passage 10 in the tip end 2 has a diameter 1.05 - 1.20 times the welding wire diameter. If the contact tip is manufactured using an extruded tube with an existing wire passage 10, this wire passage 10 has to be temporarily closed at the tip end 2 before applying the CGDS spray - possibly during preparation of the shapes 21, 31,41, or 51. To create a smooth coating it is necessary to prepare a uniform surface without holes or dents. For better bonding strength before applying a coating, shapes 21, 31, 41, and 51 can be sandblasted with grit 80 silicone carbide or aluminum oxide. After spraying the tip end 2, a machining operation is required to create or rework the welding wire passage 10 and create the final cut for shapes 21, 31, 41 and 51 with smooth surfaces.
The thickness of the sprayed tip end 2 after final machining should be at least 2 times the size of the welding wire, while being no less than 1 mm thick.

A variety of powder mixtures was used to extend the contact tip life as well as to obtain low manufacturing cost of such contact tips. The best results were obtained when tip end 2 was sprayed with mixture # 1- silver powder - size 1-5 m with aluminum oxide powders mesh -200 +325 at a volume ratio of 3:1; mixture #2 - silver powder - size 1-5 m with copper powder -- size -325 mesh and aluminum oxide powders -200 +325 mesh at a volume ratio of 2:1:1; and mixture #3 - silver powder - size 1-5 m with tungsten powder - size 5-10 m and aluminum oxide powders -200 +325 mesh at a volume ratio of 2:1:1. In all these cases, the tip end 2 composed of the silver powder mixture has excellent non-stick properties when faced with welding splatter.
Another advantage to using these powder mixtures is an economical one. Since silver critical impact velocities for 25 m particles are as low as -350-375m/s, and copper and tungsten critical impact velocities for the same sized particles are -475-500m/s and -480-650m/s respectively, the low pressure portable CGDS
equipment can be used for the spraying process. Other advantages to using metal powder mixtures with brittle materials such as aluminum oxide, zirconium oxide, and metal carbides in the CGDS
process include very good deposition efficiency, strength of adhesion, as well as wear resistance of the coating.
While other tested powder compositions had good results, the above presented materials were superior.

Claims