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The following segments give an overview of the usually applied attachmenttechnologies for contact parts to carrier components. They include mechanicalas well as brazing and welding methods used for electrical contact assemblies.
=== Mechanical Attachment Processes===Rivet staking and the insertion and forming of wire segments into pre-stampedcarrier parts or strips with punched holes are the most commonly used methodsfor the mechanical attachment of contact materials.
Riveting (or staking) for smaller volumes of assemblies is mostly done onmechanical, pneumatic or magnetically operated presses. For larger volumesthe staking process is integrated into a progressive die for fully automatedassembly. Rivets are fed in the correct orientation through special feedingtracks into the staking station of the tool. To ensure a mechanically secureattachment , the rivet shank must be dimensioned correctly. As a general rule , theshank length of the rivet should be about 1/3 longer than the thickness of thecarrier material.For switch-over contacts part of the rivet shank is formed into the secondaryrivet head. To minimize deformation of the contact blade carriers, especially thinones, this head forming is often performed by orbital riveting.
The insertion and forming of wire segments can be easily integrated into stampand bending multi-slide tooling ''(<xr id="fig:Direct_press_insertion_of_wire_segments"/><!--(Fig. 3.7)''-->). Compared to the use of compositerivets this process uses more precious contact material but for silver basedcontact materials these costs or are often offset by higher and more efficientmanufacturing speeds. For the more brittle Ag/SnO<sub>2</sub> materials however , closeattention must be paid to the danger of crack formation.
===Brazing Processes===Brazing is a thermal process for the metallurgical bonding of metallic materials inwhich a third metal component (brazing alloy or solder) is added. In addition aflux or processing in a protective atmosphere is applied to eliminate oxidation ofthe non-precious carrier. The melting range of the brazing alloy starts at thebeginning of the melting (solidus temperature) all the way to complete liquidphase (liquidus temperature). This range always is below the melting points ofthe two materials to be joined. During the brazing process with solubility of thematerials in each other , diffusion processes are thermally activated by whichelements of the base material diffuse into the brazing alloy and elements of thebraze alloy diffuse into brazing alloy. This increases the bond strength andtherefore the mechanical stability of the brazed joint.
For attachment of contact parts to carrier base materials , only brazing alloys (asopposed to solders) are used. The reason is the higher softening temperatureand melting point as well as higher mechanical strength and electricalconductivity of these alloys. The brazing alloys and fluxes used for electricalcontact attachment are listed in Chapter 4 [[Brazing Alloys and Fluxes|Brazing Alloys and Fluxes ]] in more detail. Following the mostfrequently used brazing methods are described.References to the bond quality are given according to the test methodsdescribed in Chapter 3.4.[[Evaluation_of_Braze_or_Weld_Joints|Evaluation of Braze or Weld Joints ]]
==== Flame (or Torch) Brazing====The simplest easiest way to produce braze joints , is the use of a gas torch fueled by aburning gas and air or oxygen containing gas mixes. For higher productionvolumes , partial automation is applied. The parts to be assembled aretransported after adding the suitable amounts of brazing alloy and flux through aseries of fixed gas burners on a turntable or belt driven brazing machine.To limit the amount of flux or gas inclusions , it is recommended to slightly movethe contact tips back and forth and back (also known as puddeling) as soon as thebrazing alloy is liquefied. The bonded area achieved in torch brazing is typically65 – 90% of the contact foot print , depending on the size and geometry of thecontact tip.
====Furnace Brazing====Furnace brazing is usually defined as brazing in a protective atmosphere or invacuum. Both processes do not require the use of fluxes.
The protective atmosphere brazing is conducted in batch operation in eithermuffle or pot furnaces or as a continuous process in belt furnaces , using areducing atmosphere of pure hydrogen (H<sub>2</sub>) or dissociated ammonia (H<sub>2</sub>,N<sub>2</sub>).
A vacuum is a very efficient protective environment for brazing but using vacuumfurnaces is more complicated and rather inefficient. Therefore this process isonly used for materials and assemblies that are sensitive to oxygen, nitrogen, orhydrogen impurities. Not suitable for vacuum brazing are materials whichcontain components with a high vapor pressure.
Parts with oxygen containing copper supports should not be brazed in reducingatmosphere atmospheres because of their susceptibility to hydrogen embrittlement. Similarlycontact tips containing silver–metal oxide should not be exposed to protectiveatmospheres because a reduction of the metal oxide , even in a thin contactsurface layer changes the contact properties of these materialschange.
==== Resistance Brazing====In this process , the resistive heating under electric currents is the source ofthermal energy. For contact applications two methods are used for resistancebrazing''(<xr id="fig:Resistance brazing (schematic)"/><!--(Fig. 3.8)''-->).<figure id="fig:Resistance brazing (schematic)">[[File:Resistance brazing (schematic).jpg|right|thumb|Figure 2: Resistance brazing (schematic)]]</figure>
During Direct Resistance Brazing the electric current flows straight through thejoint area composed of the contact tip, brazing alloy, flux, and the contactcarrier. These components are secured between the electrodes of a resistancebrazing machine and heated by an electrical current until the brazing alloyliquefies.
In Indirect Resistance Brazing the current flows only through one of thecomponents to be joined (usually the non-precious contact carrier). Thisprocess allows to move the contact tip (“puddeling”"puddeling") when the brazing alloy is inits liquid stage and this way additionally remove residue bubbles from the heated and boilingflux and increase the percentage of the bonded area.Two different kinds of electrodes are used for resistance brazing:
*Electrodes from poorly conducting carbon containing materials (graphite)<br />The heat is created in the electrodes and thermally conducted into the joint area<br />
*Electrodes from higher conductive and thermally stable metallic materials<br />The heat is created by the higher resistance in the joint area which, through selected designs, creates a constriction area for the electrical current in addition to the resistance of the components to be joined.<br />
Graphite electrodes are mainly used for indirect resistance brazing and for jointarea > 100 mm<sup>2</sup>. For contacts contact tips with a bottom area < 100 mm<sup>2</sup> which arealready coated with a phosphorous containing brazing alloy , the heating time canbe reduced to a degree that the softening of the contact carrier occurs only veryclosely to the joint area. For this “short"short-time brazing” brazing" specially designed metalelectrodes with compositions selected for the specific assembly componentpairings are used.
The bond quality for normal resistance brazing with the application of flux rangesfrom 70 to 90% of contact size, for short-time welding these values can beexceeded significantly.
<figtable id===Induction "tab:Brazing Times for Different Brazing===During induction brazing the heat energy is produced by an induction coil fed bya medium or high frequency generator. This creates an electromagnetic alternatingfield in the braze joint components which in turn generated eddy currentsin the work piece. Because of the skin–effect these currents and their resultingheat are created mainly on the surface of the assembly components. Thedistance of the inductor must be chosen in a way that the working temperatureis generated almost simultaneously in the full joint area. For different contactMethods">shapes the geometry of the induction coil can be optimized to obtain shortworking cycles. One of the advantages of this method is the short heating timewhich limits the softening of the material components to be joined.Typical bond qualities of <caption> 80% can be reached with this method also for largercontact assemblies'''<!--Tab. The widely varying working times needed for the differentbrazing methods are given in Table 3.1.:-->Brazing Times for Different Brazing Methods'''</caption>
*Examples of brazed contact assemblies <xr id="fig:Examples of brazed contact assemblies"/><figure id="fig:Examples of brazed contact assemblies">[[File:Examples of brazed contact assemblies.jpg|right|thumb|Figure 3: Examples of brazed contact assemblies]]</figure>*Contact materials <br />Ag, Ag-Alloys., Ag/Ni (SINIDUR), Ag/CdO (DODURIT CdO), Ag/SnO<sub>2</sub> (SISTADOX), Ag/ZnO (DODURIT ZnO) and Ag/C (GRAPHOR D) with brazable backing, refractory materials on W -, WC- and Mo-basis<br />
*Brazing alloys <br />L-Ag 15P, L-Ag 55Sn et.al.<br />
*Quality criteria <br />The testing of the braze joint quality is specified in agreements between the manufacturer and the user.<br />
=== Welding Processes===Welding of contact assemblies has both technological and economicimportance. Because of the short heating times during welding , the carriermaterials retain their hardness except for a very small heat affected area. Of themethods described below, resistance welding is the most widely utilizedprocess.
Because of miniaturization of electromechanical components laser welding hasgained some application more recently. Friction welding is mainly used forbonding (see Chapter 9)[[Applications for Bonding Technologies|Applications for Bonding Technologies. ]] Other welding methods such as ball (spheres) weldingand ultrasonic welding are today used in only limited volume and therefore notcovered in detail here.Special methods such as electron beam welding and cast-on attachment ofcontact materials to carrier components are mainly used for contact assembliesfor medium and high voltage switchgear.
===== Vertical Wire Welding=====During vertical wire welding the contact material is vertically fed in wire form through a clamp, which at the same time acts as one of the weld electrodes (<xr id="fig:Vertical wire welding (schematic)"/>)<!--(Fig. 3.9)-->. <figure id="fig:Vertical wire welding (schematic)">[[File:Vertical wire welding (schematic).jpg|right|thumb|Figure 5: Vertical wire welding (schematic)]]</figure>
*Dimensions (<xr id="fig:Vertical Wire Welding Dimensions"/>)<figure id===Tip "fig:Vertical Wire Welding====Dimensions"> Contact tips or formed contact parts produced by processes as described in[[File:Vertical Wire Welding Dimensions.jpg|right|thumb|Figure 6: Vertical Wire Welding Dimensions]]chapter 3.1.2 are mainly attached by tip welding to their respective contact</figure>supports. In this process smaller contact parts Functional quality criteria such as Ag/C bonded area percentage or Ag/W tips withgood weldable backings shear force are welded directly to usually agreed upon between the carrier parts. To improve thewelding process supplier and user and quality the bottom side of these tips may have serrations(Ag/C) or shaped projections (Ag/W). These welding aids can also be formed onthe carrier parts. Larger contact tips usually have an additional brazing alloylayer bonded to the bottom weld surfacedefined in delivery specifications.
=== Percussion == Horizontal Wire or Profile Welding=====This process of high current arc discharge During horizontal welding required the wire or profile contact materialand carrier is fed at a shallow angle to have two flat surfaces with one having a protruding nib. This nibacts as the igniter point for the high current arc ''carrier strip material (<xr id="fig:Horizontal profile cut-off welding (schematic)"/>)<!--(Fig. 3.1110)''-->. The electric arcproduces a molten layer of metallic material in the interface zone of the contact<figure id="fig:Horizontal profile cut-off welding (schematic)">tip and carrier[[File:Horizontal profile cut-off welding (schematic). Immediately afterwards jpg|right|thumb|Figure 7: Horizontal profile cut-off welding (schematic)]]</figure>The cut-off from the two components are pushed togetherwith substantial impact and speed causing wire or profile is performed either directly by the liquid metal to form electrode or in a strong jointacross the whole interface areaseparate cutting station. This horizontal feeding is suitable for welding single or multiple layer weld profiles.Because of The profile construction allows to custom tailor the very short duration of contact layer shape and thickness to the whole melt electrical load and bonding process therequired number of electrical switching operations. By choosing a two components, -layer contact tip and carrierconfiguration, retain their mechanical hardness andstrength almost completely except multiple switching duty ranges can be satisfied. The following triple-layer profile is a good example for the immediate thin joint areasuch a development: The top 5. Theunavoidable weld splatter around 0 μm AuAg8 layer is suitable to switch dry circuit electronic signals, the periphery second or middle layer of 100 μm Ag/Ni 90/10 is used to switch relative high electrical loads and the joint must bemechanically removed in a secondary operationbottom layer consists of an easily weldable alloy such as CuNi44 or CuNi9Sn2.The percussion welding process is mainly applied in configuration of the production bottom weld projections, i.e. size, shape, and number of rodassemblies welding nibs or weld rails are critically important for high voltage switchgearthe final weld quality.
*Dimensions (<xr id="fig:Horizontal Wire Welding Dimensions"/>)<figure id=== Electron Beam "fig:Horizontal Wire Welding====Dimensions"> The electron beam welding is a joining process which has shown its suitabilityfor high voltage contact assemblies[[File:Horizontal Wire Welding Dimensions. A sharply focused electron beam hasjpg|right|thumb|Figure 8: Dimensions]]sufficient energy to penetrate the mostly thicker parts and generate a locallydefined molten area so that the carrier component is only softened in a narrowzone (1 – 4 mm). This allows the attachment of Cu</W contacts to hardand thermally stable copper alloys as for example CuCrZr for spring hardcontact tulips ''(Fig. 3.12)''.figure>
*Carrier Contact materials <br />W/Cu, Cu-Alloys, Cu clad SteelW/Ag, et.al.others<br />
*DimensionsbildFunctional quality criteria such as bonded area percentage or shear force areusually agreed upon between the supplier and user and defined in deliveryspecifications.Carrier materials <br />Cu, Cu-Alloys, others<br />
*Contact materialsAu-Alloys, Pd-Alloys, Ag-Alloys, AgQuality criteria <br />Test methods for bond quality are agreed upon between supplier and user<br /Ni > (SINIDUR<xr id="fig:Examples for percussion welded contact parts"/>), Ag/CdO <!--(DODURIT CdOFig. 3.13),-->Ag/SnO<subfigure id="fig:Examples for percussion welded contact parts">2[[File:Examples for percussion welded contact parts.jpg|right|thumb|Figure 10: Examples for percussion welded contact parts]]</subfigure> (SISTADOX), Ag/ZnO (DODURIT ZnO), and Ag/C (GRAPHOR D)
===Percussion Welding==Cast-On of Copper=====The cast-on of liquid copper to pre-fabricated W/Cu contact parts is performed in special casting molds. This results in a seamless joint between the W/Cu and the copper carrier. The hardness of the copper is then increased by a secondary forming or deep-drawing operation.
==References==
[[:Manufacturing Technologies for Contact Parts#References|References]]
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