ELECTRICAL CONTACTS BY POWDER METALLURGY. Submitted by Priyanshu Jain (12518)

Transcription

1 ELECTRICAL CONTACTS BY POWDER METALLURGY Submitted by Priyanshu Jain (12518) 1

2 OUTLINE Electrical contacts: Introduction Desirable Properties of contact material Contact material selection Manufacturing of Contact material Examples (application and microstructure) References 2

3 WHAT ARE ELECTRICAL CONTACTS? An electrical contact is an electrical circuit component composed of two pieces of electrically conductive metal that pass electrical current or insulate when the gap between them is closed or open. These are found in electrical switches, voltage regulators, arcing tips, switch gears, contactors, relays and breakers. The gap must be an insulating medium of air, vacuum, oil, SF 6 or other electrically insulating fluid. Contacts may be operated by humans in pushbuttons and switches, by mechanical pressure in sensors or machine cams, and electromechanically in relays. Light Switch with open contact 3

4 DESIRABLE PROPERTIES OF ELECTRICAL CONTACTS? High electrical and thermal conductivities Good fusion resistance High wear resistance Good Corrosion resistance Good mechanical workability 4

5 High electrical and thermal conductivities: Thermal capacity of contact support are small, the area of heat dissipation is quite limited; thus the electric contact material must have high to ensure allowable temperature rise during operating. This will reduce the possibility of any kind of damage to insulation without affecting electrical performance. Good fusion resistance: When two contacts fuse together, the heat produced by contact resistance and electric arc will rise the surface temperature of electric contact reaching its melting point and subsequently cause sticking. Therefore to resist fusion we must have high melting point and low contact resistance material. 5

6 High wear resistance: Electric contact material should have appropriate hardness since under strong current, electric heat and electric arc may cause electric contact material to melt, spatter and even evaporate. Good Corrosion resistance: Contact material must be stable so that even at higher temperatures, they won t react with ambient agents such as Oxygen and Sulphur nor forming a thin layer of such compounds. Good mechanical workability: Contact material must be easy to be riveted and welded. 6

7 Pure Metals: CLASSIFICATION based on their composition and metallurgical structure Contact Material Pure Metals Alloys Composites silver has the greatest importance for switching devices in the higher energy technology gold and platinum are used in applications for the information technology in the form of thin surface layers. tungsten is used for some special applications e.g. automotive horn contacts. rarely pure copper is used but mainly paired to a silver-based contact material. 7

8 Alloys: made by melt technology improve the properties of one material at the cost of changing them for the second material E.g. AgCu, AgW Composites (pseudoalloys): properties are of great importance for electrical contacts formed by components exhibiting no intermiscibility in either the liquid or solid state, and heterogeneous systems composed of a matrix with fine inclusions of another phase which are firmly embedded in the matrix but do not react with it e.g., Ag--CdO, Ag--C, and W--ThO2 8

9 Importance of Composites: their heterogeneous composition ensures an additive combination of properties imparted to the materials by their individual phase constituents e.g., W-Cu composite imparts good melting point and arc erosion resistance along with good electrical conductivity of copper e.g., Ag-CdO composite, cadmium oxide as a disperse phase in a silver matrix, adds dispersion-hardening effect and an arcsuppressing abilities to the electrical and thermal conductivities of silver 9

10 CONTACT MATERIAL SELECTION There should be a proper balance in material selection in selecting contact material for specific application. Adjusting material particle sizes, choosing additives, and altering furnace temperatures plays a vital role in the final properties of the selected contact material. Following general guidelines in material selection and contact design should be considered: Contact Resistance is the resistance across a pair of closed contacts which is in series with the load of the device. High contact resistance in a device can result in nuisance tripping, overheating, and even welding of the contacts, therefore it should be avoided Contact Erosion is the loss of material due to electrical or mechanical conditions experienced at the contact-operating surface during the operation of the device. Excessive contact erosion is detrimental to the performance of the device. Contact Welding is the bonding of the stationary and moving contacts in a device. It should be avoided to continue operation. 10

11 MANUFACTURING TECHNIQUE Why Powder Metallurgy? It is possible to combine phase components of any nature and structure in a single dense material by utilizing: intermolecular bonding forces in the powder pressing stage and viscous flow phenomena and capillary effects diffusion in the liquid- or solid-phase sintering and infiltration stages suitable choice of starting structure of powder particles Thus, the nature of composite materials and flexibility of processing operations in powder metallurgy open up great possibilities for the synthesis of electrical contact composites, possessing predetermined combinations of properties. 11

12 1. Powder and Mixing: PROCEDURE Preparation of powders having the required composition, structure, and particle-size, distribution, capillary characteristics and flow characteristics; mixing is based on engineering specifications for a given material composition 2. Pressing: Pre mixed powder are fed into a die cavity, punch presses the powders into compacts of the desired size and shape 12

13 3. Sintering: Conversion of the compacts into dense parts of controlled porosity or nonporous parts at high temperatures in protective environments It allows the creation of metallurgical bonds between the powder particles and the creation of a "refractory skeleton" that can be infiltrated or repressed It also eliminates the need for expensive machining since contacts are at final dimensional requirement Sintering Techniques Press-Sinter- Infiltrate (PSI) Press-Sinter- Repress (PSR) Press-Sinter (PS) {Based on required Composition} 13

14 3a. Press Sinter Infiltrate: The powder of the higher melting component (sometimes mixed with a small amount of the second material) is pressed into parts and after sintering the porous skeleton is infiltrated with liquid metal of the second material. The filling up of the pores happens through capillary forces and composite reaches after the infiltration near-theoretical density without subsequent pressing. Widely used for Ag- and Cu-refractory contacts. For Ag/W or Ag/WC contacts, controlling the amount or excess on the bottom side of the contact of the infiltration metal Ag results in contact tips that can be easily attached to their carriers by resistance welding. 14

15 3b. Press Sinter Repress (PSR): The powder mix is first densified by pressing, then undergoes a heat treatment (sintering), and eventually is re-pressed again to further increase the density. The sintering atmosphere depends on the material components and later application; a vacuum is used for example for the low gas content material Cu/Cr. This process is used for individual contact parts. For materials with high silver content the starting point at pressing is most a larger block (or billet) which is then after sintering hot extruded into wire, rod, or strip form. The extruded product is further densified which contributes to higher arc erosion resistance. Materials such as Ag/Ni, Ag/MeO, and Ag/C are typically produced by this process. 15

16 3c. Sintering with liquid phase: Sintering with liquid phase has the advantage of shorter process times due to the accelerated diffusion and also results in neartheoretical densities of the composite material. To ensure the shape stability during the sintering process it is however necessary to limit the volume content of the liquid phase material. 4. Post Sintering operations (Surface preparation): Machining: To design product with "molded-to-size" configuration Refractory Etching: Light etch is applied where a highly conductive surface is desirable, such as in circuit breaker switching duty requirements. Etching is also helpful in creating a surface that is easy to attach to when welding or brazing. Solder Flushing: applying an appropriate braze alloy material to the backside of the contact so as to easily attach the contact to their base 16

17 Silver Enrichment: Accomplishes many of the same things as refractory etching but is employed where contact materials typically contain graphite. Through a carefully controlled thermal operation, the graphite is removed from the targeted surfaces, leaving behind a layer of high conductivity material which is easily attached to contact supports. Cleaning/Tumbling: Finally, contacts receive a cleaning prior to use. Cleaning is accomplished ultrasonically or more typically by tumbling in a specific soap and water solution with hardened media. 17

18 SOME IMPORTANT CONTACT MATERIALS AND THEIR PROPERTIES Silver Tungsten (AgW): The W-Ag and Mo-Ag electrical contact materials utilized in heavyduty applications are produced exclusively by means of powder metallurgy (PM). Composites containing wt-% Ag resist arc erosion and possess good welding resistance and current carrying capacity. W-Ag composite materials can be produced by two techniques: mixing elemental powders, cold pressing and solid state sintering. At silver contents exceeding 30% the as-sintered parts can be subjected to final processing by rolling or re-pressing infiltrating tungsten, or molybdenum, porous skeletons with liquid silver. 18

19 Infiltrated contacts predominate because infiltration process is suitable only for materials with tungsten content higher than 70%, which lead to their excellent application properties like high burn resistant Microstructure of as-infiltrated W+30%Ag composites W-dark Ag-Gray Uniform particle size and distribution of the refractory phase, homogeneity of microstructure and amount of porosity lead to excellent electrical, mechanical, and thermo-physical properties of the composite material 19

20 AgW is the most popular of the contact material families used in circuit breakers and other power switching devices Tungsten is preferred in car electronics, as contact-breaker points, for horns, etc Mo has lower density than tungsten therefore sometime used in place of W, when the mass of moving parts of the contact must be minimized. Molybdenum-base contacts are used for breaking lower currents and in high voltage electronics 20

21 Silver Tungsten Carbide (AgWC): excellent material because of its ability to resist contact sticking or welding and also because of its resistance to oxidation Since harder material than AgW, more resistant to arc erosion and contact wear 21

22 Copper Tungsten (CuW): CuW and CuWC offer low-cost alternatives to AgW and AgWC when used in non-oxidizing conditions. Apart from food electrical and thermal properties it has excellent arc resistance, high strength, good machinability, low thermal expansion Frequently used for arcing contact where contact force is sufficient to break through the oxides that are inevitably formed Used as circuit breakers as well 22

23 Silver Tungsten Carbide Graphite (AgWC): Used in many applications as a replacement for silver graphite. Less costly than comparable silver graphite materials and exhibits the positive characteristics of both silver graphite and silver tungsten carbide. When used as a stationary contact, AgWC has good thermal and electrical properties and has minimal contact erosion with low chopping current. 23

24 Copper Chromium (CuCr): Copper ensures the excellent electrical and thermal conductivity of the contact material Combination has Good mechanical strength, Low gas content, Excellent arc erosion resistance, Very good dissipation of short circuit currents and Low welding tendency Economical production of near net shapes 24

25 Fine grained and homogeneous microstructure lead to high ignition voltages and uniform arc distribution over the smooth surfaces of these materials during the switching operation By repressing, the electrical and thermal conductivity can be increased and reduction in porosity also leads to an improvement in breaking capacity. Silver Graphite (AgC): Commonly used as stationary contact material and is typically paired with AgW or AgWC. Mostly used in circuit breaker and switch grades Has superior anti-welding characteristics and therefore is a good choice when tack welding is an issue Excellent electrical conductivity but higher erosion rate. 25

26 SUMMERY Powder Mixing Pressing Sintering Extruding Repressing Infiltrating Drawing Solder Backing Solder Backing Wire Contact tips Contact tips Contact Rivets 26

27 REFERENCES ELECTRICAL CONTACTS PRODUCED BY POWDER METALLURGY METHODS, I. N. Frantsevich SILVER BASED INFILTRATED COMPOSITES, M. MADEJ Electrical contact materials based on silver, NADEZDA M. TALIJAN Contact technologies, Inc

28 Thank You 28