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===3.3 Attachment of Single Contact Parts===
The following segments give an overview of the usually applied attachment
technologies for contact parts to carrier components. They include mechanical
as well as brazing and welding methods used for electrical contact assemblies.
===3.3.1 Mechanical Attachment Processes===
Rivet staking and the insertion and forming of wire segments into pre-stamped
carrier parts or strips with punched holes are the most commonly used methods
for the mechanical attachment of contact materials.
Riveting (or staking) for smaller volumes of assemblies is mostly done on
mechanical, pneumatic or magnetically operated presses. For larger volumes
the staking process is integrated into a progressive die for fully automated
assembly. Rivets are fed in the correct orientation through special feeding
tracks into the staking station of the tool. To ensure a mechanically secure
attachment the rivet shank must be dimensioned correctly. As a general rule the
shank length of the rivet should be about 1/3 longer than the thickness of the
carrier material.
For switch-over contacts part of the rivet shank is formed into the secondary
rivet head. To minimize deformation of the contact blade carriers, especially thin
ones, this head forming is often performed by orbital riveting.
The insertion and forming of wire segments can be easily integrated into stamp
and bending multi-slide tooling ''(Fig. 3.7)''. Compared to the use of composite
rivets this process uses more precious contact material but for silver based
contact materials these costs or often offset by higher and more efficient
manufacturing speeds. For the more brittle Ag/SnO<sub>2</sub> materials however close
attention must be paid to the danger of crack formation.
Fig. 3.7: Direct press-insertion of wire segments
===3.3.2 Brazing Processes===
Brazing is a thermal process for the metallurgical bonding of metallic materials in
which a third metal component (brazing alloy or solder) is added. In addition a
flux or processing in a protective atmosphere is applied to eliminate oxidation of
the non-precious carrier. The melting range of the brazing alloy starts at the
beginning of the melting (solidus temperature) all the way to complete liquid
phase (liquidus temperature). This range always is below the melting points of
the two materials to be joined. During the brazing process with solubility of the
materials in each other diffusion processes are thermally activated by which
elements of the base material diffuse into the brazing alloy and elements of the
braze alloy diffuse into brazing alloy. This increases the bond strength and
therefore the mechanical stability of the brazed joint.
For attachment of contact parts to carrier base materials only brazing alloys (as
opposed to solders) are used. The reason is the higher softening temperature
and melting point as well as higher mechanical strength and electrical
conductivity of these alloys. The brazing alloys and fluxes used for electrical
contact attachment are listed in Chapter 4 in more detail. Following the most
frequently used brazing methods are described.
References to the bond quality are given according to the test methods
described in Chapter 3.4.
===3.3.2.1 Flame (or Torch) Brazing===
The simplest way to produce braze joints is the use of a gas torch fueled by a
burning gas and air or oxygen containing gas mixes. For higher production
volumes partial automation is applied. The parts to be assembled are
transported after adding the suitable amounts of brazing alloy and flux through a
series 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 move
the contact tips forth and back (also known as puddeling) as soon as the
brazing alloy is liquefied. The bonded area achieved in torch brazing is typically
65 – 90% of the contact foot print depending on the size and geometry of the
contact tip.
===3.3.2.2 Furnace Brazing===
Furnace brazing is usually defined as brazing in a protective atmosphere or in
vacuum. Both processes do not require the use of fluxes.
The protective atmosphere brazing is conducted in batch operation in either
muffle or pot furnaces or as a continuous process in belt furnaces using a
reducing 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 vacuum
furnaces is more complicated and rather inefficient. Therefore this process is
only used for materials and assemblies that are sensitive to oxygen, nitrogen, or
hydrogen impurities. Not suitable for vacuum brazing are materials which
contain components with a high vapor pressure.
Parts with oxygen containing copper supports should not be brazed in reducing
atmosphere because of their susceptibility to hydrogen embrittlement. Similarly
contact tips containing silver–metal oxide should not be exposed to protective
atmospheres because a reduction of the metal oxide even in a thin contact
surface layer changes the contact properties of these materials.
===3.3.2.3 Resistance Brazing===
In this process the resistive heating under electric currents is the source of
thermal energy. For contact applications two methods are used for resistance
brazing
''(Fig. 3.8)''.
During Direct Resistance Brazing the electric current flows straight through the
joint area composed of the contact tip, brazing alloy, flux, and the contact
carrier. These components are secured between the electrodes of a resistance
brazing machine and heated by an electrical current until the brazing alloy
liquefies.
In Indirect Resistance Brazing the current flows only through one of the
components to be joined (usually the non-precious contact carrier). This
process allows to move the contact tip (“puddeling”) when the brazing alloy is in
its liquid stage and this way remove residue bubbles from the heated and boiling
flux 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)
The heat is created in the electrodes and thermally conducted into the
joint area
*Electrodes from higher conductive and thermally stable metallic materials
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.
Graphite electrodes are mainly used for indirect resistance brazing and for joint
2 2 area > 100 mm . For contacts tips with a bottom area < 100 mm which are
already coated with a phosphorous containing brazing alloy the heating time can
be reduced to a degree that the softening of the contact carrier occurs only very
closely to the joint area. For this “short-time brazing” specially designed metal
electrodes with compositions selected for the specific assembly component
pairings are used.
The bond quality for normal resistance brazing with the application of flux ranges
from 70 to 90% of contact size, for short-time welding these values can be
exceeded significantly.
Fig. 3.8: Resistance brazing (schematic)
===3.3.2.4 Induction Brazing===
During induction brazing the heat energy is produced by an induction coil fed by
a medium or high frequency generator. This creates an electromagnetic alternating
field in the braze joint components which in turn generated eddy currents
in the work piece. Because of the skin–effect these currents and their resulting
heat are created mainly on the surface of the assembly components. The
distance of the inductor must be chosen in a way that the working temperature
is generated almost simultaneously in the full joint area. For different contact
shapes the geometry of the induction coil can be optimized to obtain short
working cycles. One of the advantages of this method is the short heating time
which limits the softening of the material components to be joined.
Typical bond qualities of > 80% can be reached with this method also for larger
contact assemblies. The widely varying working times needed for the different
brazing methods are given in Table 3.1.
Table 3.1: Brazing Times for Different Brazing Methods
*Examples of brazed contact assemblies
bild
*Contact materials
als bild?
Ag, Ag-Alloys., Ag/Ni (SINIDUR), Ag/CdO (DODURIT CdO),
Ag/SnO (SISTADOX), Ag/ZnO (DODURIT ZnO) and Ag/C (GRAPHOR D) with 2
brazable backing, refractory materials on W -, WC- and Mo-basis
*Brazing alloys
L-Ag 15P, L-Ag 55Sn et.al. als Bild?
*Carrier materials
Cu, Cu-Alloys. et al. als Bild?
*Dimensions
Brazing area > 10 mm²
*Quality criteria
The testing of the braze joint quality is specified in agreements between the
manufacturer and the user.
===3.3.3 Welding Processes===
Welding of contact assemblies has both technological and economic
importance. Because of the short heating times during welding the carrier
materials retain their hardness except for a very small heat affected area. Of the
methods described below, resistance welding is the most widely utilized
process.
Because of miniaturization of electromechanical components laser welding has
gained some application more recently. Friction welding is mainly used for
bonding (see Chapter 9). Other welding methods such as ball (spheres) welding
and ultrasonic welding are today used in only limited volume and therefore not
covered in detail here.
Special methods such as electron beam welding and cast-on attachment of
contact materials to carrier components are mainly used for contact assemblies
for medium and high voltage switchgear.
===3.3.3.1 Resistance Welding===
Resistance welding is the process of electrically joining work pieces by creating
the required welding energy through current flow directly through the
components without additional intermediate materials. For contact applications
the most frequently used method is that of projection welding. Differently
shaped weld projections are used on one of the two components to be joined
(usually the contact). They reduce the area in which the two touch creating a
high electrical resistance and high current density which heats the constriction
area to the melting point of the projections. Simultaneously exerted pressure
from the electrodes further spreads out the liquefied metal over the weld joints
area. The welding current and electrode force are controlling parameters for the
resulting weld joint quality. The electrodes themselves are carefully designed
and selected for material composition to best suit the weld requirements.
The waveform of the weld current has a significant influence on the weld quality.
Besides 50 or 60 Hz AC current with phase angle control, also DC (6-phase
from 3-phase rectified AC) and medium frequency (MF) weld generators are
used for contact welding. In the latter the regular AC supply voltage is first
rectified and then supplied back through a controlled DC/AC inverter as pulsed
DC fed to a weld transformer. Medium frequency welding equipment usually
works at frequencies between 1kHz to 10kHz. The critical parameters of
current, voltage, and weld energy are electronically monitored and allow through
closed loop controls to monitor and adjust the weld quality continuously. The
very short welding times needed with these MF welding machines result in very
limited thermal stresses on the base material and also allow the reliable joining
of otherwise difficult material combinations.
===3.3.3.1.1 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
''(Fig. 3.9)''. With one or more weld pulses the roof shaped wire end – from the
previous cut-off operation – is welded to the base material strip while exerting
pressure by the clamp-electrode. Under optimum weld conditions the welded
area can reach up to 120% of the original cross-sectional area of the contact
wire. After welding the wire is cut off by wedge shaped knives forming again a
roof shaped weld projection. The welded wire segment is subsequently formed
into the desired contact shape by stamping or orbital forming. This welding
process can easily be integrated into automated production lines. The contact
material must however be directly weldable, meaning that it cannot contain
graphite or metal oxides.
Fig. 3.9: Vertical wire welding (schematic)
===3.3.3.1.2 Horizontal Wire or Profile Welding===
During horizontal welding the wire or profile contact material is fed at a shallow
angle to the carrier strip material ''(Fig. 3.10)''. The cut-off from the wire or profile
is performed either directly by the electrode or in a separate cutting station. This
horizontal feeding is suitable for welding single or multiple layer weld profiles.
The profile construction allows to custom tailor the contact layer shape and
thickness to the electrical load and required number of electrical switching
operations. By choosing a two-layer contact configuration multiple switching
duty ranges can be satisfied. The following triple-layer profile is a good example
for such a development: The top 5.0 μm AuAg8 layer is suitable to switch dry
circuit electronic signals, the second or middle layer of 100 μm Ag/Ni 90/10 is
used to switch relative high electrical loads and the bottom layer consists of an
easily weldable alloy such as CuNi44 or CuNi9Sn2. The configuration of the
bottom weld projections, i.e. size, shape, and number of welding nibs or weld
rails are critically important for the final weld quality.
Because of high production speeds (approx. 700 welds per min) and the
possibility to closely match the amount of precious contact material to the
required need for specific switching applications, this joining process has
gained great economical importance.
Fig. 3.10: Horizontal profile cut-off welding (schematic)
===3.3.3.1.3 Tip Welding===
Contact tips or formed contact parts produced by processes as described in
chapter 3.1.2 are mainly attached by tip welding to their respective contact
supports. In this process smaller contact parts such as Ag/C or Ag/W tips with
good weldable backings are welded directly to the carrier parts. To improve the
welding process 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 on
the carrier parts. Larger contact tips usually have an additional brazing alloy
layer bonded to the bottom weld surface.
Tip welding is also used for the attachment of weld buttons (see chapter 3.1.3).
The welding is performed mostly semi or fully automated with the buttons
oriented a specific way and fed into a welding station by suitably designed
feeding mechanisms.
===3.3.3.2 Percussion Welding===
This process of high current arc discharge welding required the contact material
and carrier to have two flat surfaces with one having a protruding nib. This nib
acts as the igniter point for the high current arc ''(Fig. 3.11)''. The electric arc
produces a molten layer of metallic material in the interface zone of the contact
tip and carrier. Immediately afterwards the two components are pushed together
with substantial impact and speed causing the liquid metal to form a strong joint
across the whole interface area.
Because of the very short duration of the whole melt and bonding process the
two components, contact tip and carrier, retain their mechanical hardness and
strength almost completely except for the immediate thin joint area. The
unavoidable weld splatter around the periphery of the joint must be
mechanically removed in a secondary operation.
The percussion welding process is mainly applied in the production of rod
assemblies for high voltage switchgear.
Fig. 3.11: Percussion welding (schematic)
===3.3.3.3 Laser Welding===
This contact attachment process is also one of the liquid phase welding
methods. Solid phase lasers are predominantly used for welding and brazing.
The exact guiding and focusing of the laser beam from the source to the joint
location is most important to ensure the most efficient energy absorption in the
joint where the light energy is converted to heat. Advantages of the method are
the touch-less energy transport which avoids any possible contamination of
contact surfaces, the very well defined weld effected zone, the exact
positioning of the weld spot and the precise control of weld energy.
Laser welding is mostly applied for rather small contact parts to thin carrier
materials. To avoid any defects in the contact portion, the welding is usually
performed through the carrier material. Using a higher power laser and beam
splitting allows high production speeds with weld joints created at multiple
spots at the same time.
===3.3.3.4 Special Welding and Attachment Processes===
In high voltage switchgear the contact parts are exposed to high mechanical
and thermal stresses. This requires mechanically strong and 100%
metallurgically bonded joints between the contacts and their carrier supports
which cannot be achieved by the traditional attachment methods. The two
processes of electron beam welding and the cast-on with copper can however
used to solve this problem.
===3.3.3.4.1 Electron Beam Welding===
The electron beam welding is a joining process which has shown its suitability
for high voltage contact assemblies. A sharply focused electron beam has
sufficient energy to penetrate the mostly thicker parts and generate a locally
defined molten area so that the carrier component is only softened in a narrow
zone (1 – 4 mm). This allows the attachment of Cu/W contacts to hard
and thermally stable copper alloys as for example CuCrZr for spring hard
contact tulips ''(Fig. 3.12)''.
Fig. 3.12:
Examples of contact tulips with Cu/W
contacts electron beam welded
to CuCrZr carriers.
===3.3.3.4.2 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.
*Examples of Wire Welding
bild
===Vertical Wire Welding===
*Contact materials
Ag, Ag-Alloys, Au- and Pd-Alloys, Ag/Ni (SINIDUR) als bild?
*Carrier materials
Cu, Cu-Alloys, Cu clad Steel, et.al. als bild?
*Dimensions
bild
Functional quality criteria such as bonded area percentage or shear force are
usually agreed upon between the supplier and user and defined in delivery
specifications.
===Horizontal Wire Welding===
*Contact materials
Au-Alloys, Pd-Alloys, Ag-Alloys, Ag/Ni (SINIDUR), Ag/CdO (DODURIT CdO),
Ag/SnO (SISTADOX), Ag/ZnO (DODURIT ZnO), and Ag/C (GRAPHOR D)
*Carrier materials
(weldable backing of multi-layer profiles)
Ni, CuNi, CuNiFe, CuNiZn, CuSn, CuNiSn, and others.
*Braze alloy layer
L-Ag 15P (CP 102 or BCUP-5)
*Dimensions
bild
*Quality criteria
Functional quality criteria such as bonded area percentage or shear force are
usually agreed upon between the supplier and user and defined in delivery
specifications.
===Percussion Welding===
*Contact materials
W/Cu, W/Ag, others
*Carrier materials
Cu, Cu-Alloys, others
*Dimensions
Weld surface area (flat) 6.0 to 25 mm diameter
Rectangular areas with up to 25 mm diagonals
*Quality criteria
Test methods for bond quality are agreed upon between supplier and user
Fig. 3.13: Examples for percussion welded contact parts
The following segments give an overview of the usually applied attachment
technologies for contact parts to carrier components. They include mechanical
as well as brazing and welding methods used for electrical contact assemblies.
===3.3.1 Mechanical Attachment Processes===
Rivet staking and the insertion and forming of wire segments into pre-stamped
carrier parts or strips with punched holes are the most commonly used methods
for the mechanical attachment of contact materials.
Riveting (or staking) for smaller volumes of assemblies is mostly done on
mechanical, pneumatic or magnetically operated presses. For larger volumes
the staking process is integrated into a progressive die for fully automated
assembly. Rivets are fed in the correct orientation through special feeding
tracks into the staking station of the tool. To ensure a mechanically secure
attachment the rivet shank must be dimensioned correctly. As a general rule the
shank length of the rivet should be about 1/3 longer than the thickness of the
carrier material.
For switch-over contacts part of the rivet shank is formed into the secondary
rivet head. To minimize deformation of the contact blade carriers, especially thin
ones, this head forming is often performed by orbital riveting.
The insertion and forming of wire segments can be easily integrated into stamp
and bending multi-slide tooling ''(Fig. 3.7)''. Compared to the use of composite
rivets this process uses more precious contact material but for silver based
contact materials these costs or often offset by higher and more efficient
manufacturing speeds. For the more brittle Ag/SnO<sub>2</sub> materials however close
attention must be paid to the danger of crack formation.
Fig. 3.7: Direct press-insertion of wire segments
===3.3.2 Brazing Processes===
Brazing is a thermal process for the metallurgical bonding of metallic materials in
which a third metal component (brazing alloy or solder) is added. In addition a
flux or processing in a protective atmosphere is applied to eliminate oxidation of
the non-precious carrier. The melting range of the brazing alloy starts at the
beginning of the melting (solidus temperature) all the way to complete liquid
phase (liquidus temperature). This range always is below the melting points of
the two materials to be joined. During the brazing process with solubility of the
materials in each other diffusion processes are thermally activated by which
elements of the base material diffuse into the brazing alloy and elements of the
braze alloy diffuse into brazing alloy. This increases the bond strength and
therefore the mechanical stability of the brazed joint.
For attachment of contact parts to carrier base materials only brazing alloys (as
opposed to solders) are used. The reason is the higher softening temperature
and melting point as well as higher mechanical strength and electrical
conductivity of these alloys. The brazing alloys and fluxes used for electrical
contact attachment are listed in Chapter 4 in more detail. Following the most
frequently used brazing methods are described.
References to the bond quality are given according to the test methods
described in Chapter 3.4.
===3.3.2.1 Flame (or Torch) Brazing===
The simplest way to produce braze joints is the use of a gas torch fueled by a
burning gas and air or oxygen containing gas mixes. For higher production
volumes partial automation is applied. The parts to be assembled are
transported after adding the suitable amounts of brazing alloy and flux through a
series 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 move
the contact tips forth and back (also known as puddeling) as soon as the
brazing alloy is liquefied. The bonded area achieved in torch brazing is typically
65 – 90% of the contact foot print depending on the size and geometry of the
contact tip.
===3.3.2.2 Furnace Brazing===
Furnace brazing is usually defined as brazing in a protective atmosphere or in
vacuum. Both processes do not require the use of fluxes.
The protective atmosphere brazing is conducted in batch operation in either
muffle or pot furnaces or as a continuous process in belt furnaces using a
reducing 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 vacuum
furnaces is more complicated and rather inefficient. Therefore this process is
only used for materials and assemblies that are sensitive to oxygen, nitrogen, or
hydrogen impurities. Not suitable for vacuum brazing are materials which
contain components with a high vapor pressure.
Parts with oxygen containing copper supports should not be brazed in reducing
atmosphere because of their susceptibility to hydrogen embrittlement. Similarly
contact tips containing silver–metal oxide should not be exposed to protective
atmospheres because a reduction of the metal oxide even in a thin contact
surface layer changes the contact properties of these materials.
===3.3.2.3 Resistance Brazing===
In this process the resistive heating under electric currents is the source of
thermal energy. For contact applications two methods are used for resistance
brazing
''(Fig. 3.8)''.
During Direct Resistance Brazing the electric current flows straight through the
joint area composed of the contact tip, brazing alloy, flux, and the contact
carrier. These components are secured between the electrodes of a resistance
brazing machine and heated by an electrical current until the brazing alloy
liquefies.
In Indirect Resistance Brazing the current flows only through one of the
components to be joined (usually the non-precious contact carrier). This
process allows to move the contact tip (“puddeling”) when the brazing alloy is in
its liquid stage and this way remove residue bubbles from the heated and boiling
flux 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)
The heat is created in the electrodes and thermally conducted into the
joint area
*Electrodes from higher conductive and thermally stable metallic materials
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.
Graphite electrodes are mainly used for indirect resistance brazing and for joint
2 2 area > 100 mm . For contacts tips with a bottom area < 100 mm which are
already coated with a phosphorous containing brazing alloy the heating time can
be reduced to a degree that the softening of the contact carrier occurs only very
closely to the joint area. For this “short-time brazing” specially designed metal
electrodes with compositions selected for the specific assembly component
pairings are used.
The bond quality for normal resistance brazing with the application of flux ranges
from 70 to 90% of contact size, for short-time welding these values can be
exceeded significantly.
Fig. 3.8: Resistance brazing (schematic)
===3.3.2.4 Induction Brazing===
During induction brazing the heat energy is produced by an induction coil fed by
a medium or high frequency generator. This creates an electromagnetic alternating
field in the braze joint components which in turn generated eddy currents
in the work piece. Because of the skin–effect these currents and their resulting
heat are created mainly on the surface of the assembly components. The
distance of the inductor must be chosen in a way that the working temperature
is generated almost simultaneously in the full joint area. For different contact
shapes the geometry of the induction coil can be optimized to obtain short
working cycles. One of the advantages of this method is the short heating time
which limits the softening of the material components to be joined.
Typical bond qualities of > 80% can be reached with this method also for larger
contact assemblies. The widely varying working times needed for the different
brazing methods are given in Table 3.1.
Table 3.1: Brazing Times for Different Brazing Methods
*Examples of brazed contact assemblies
bild
*Contact materials
als bild?
Ag, Ag-Alloys., Ag/Ni (SINIDUR), Ag/CdO (DODURIT CdO),
Ag/SnO (SISTADOX), Ag/ZnO (DODURIT ZnO) and Ag/C (GRAPHOR D) with 2
brazable backing, refractory materials on W -, WC- and Mo-basis
*Brazing alloys
L-Ag 15P, L-Ag 55Sn et.al. als Bild?
*Carrier materials
Cu, Cu-Alloys. et al. als Bild?
*Dimensions
Brazing area > 10 mm²
*Quality criteria
The testing of the braze joint quality is specified in agreements between the
manufacturer and the user.
===3.3.3 Welding Processes===
Welding of contact assemblies has both technological and economic
importance. Because of the short heating times during welding the carrier
materials retain their hardness except for a very small heat affected area. Of the
methods described below, resistance welding is the most widely utilized
process.
Because of miniaturization of electromechanical components laser welding has
gained some application more recently. Friction welding is mainly used for
bonding (see Chapter 9). Other welding methods such as ball (spheres) welding
and ultrasonic welding are today used in only limited volume and therefore not
covered in detail here.
Special methods such as electron beam welding and cast-on attachment of
contact materials to carrier components are mainly used for contact assemblies
for medium and high voltage switchgear.
===3.3.3.1 Resistance Welding===
Resistance welding is the process of electrically joining work pieces by creating
the required welding energy through current flow directly through the
components without additional intermediate materials. For contact applications
the most frequently used method is that of projection welding. Differently
shaped weld projections are used on one of the two components to be joined
(usually the contact). They reduce the area in which the two touch creating a
high electrical resistance and high current density which heats the constriction
area to the melting point of the projections. Simultaneously exerted pressure
from the electrodes further spreads out the liquefied metal over the weld joints
area. The welding current and electrode force are controlling parameters for the
resulting weld joint quality. The electrodes themselves are carefully designed
and selected for material composition to best suit the weld requirements.
The waveform of the weld current has a significant influence on the weld quality.
Besides 50 or 60 Hz AC current with phase angle control, also DC (6-phase
from 3-phase rectified AC) and medium frequency (MF) weld generators are
used for contact welding. In the latter the regular AC supply voltage is first
rectified and then supplied back through a controlled DC/AC inverter as pulsed
DC fed to a weld transformer. Medium frequency welding equipment usually
works at frequencies between 1kHz to 10kHz. The critical parameters of
current, voltage, and weld energy are electronically monitored and allow through
closed loop controls to monitor and adjust the weld quality continuously. The
very short welding times needed with these MF welding machines result in very
limited thermal stresses on the base material and also allow the reliable joining
of otherwise difficult material combinations.
===3.3.3.1.1 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
''(Fig. 3.9)''. With one or more weld pulses the roof shaped wire end – from the
previous cut-off operation – is welded to the base material strip while exerting
pressure by the clamp-electrode. Under optimum weld conditions the welded
area can reach up to 120% of the original cross-sectional area of the contact
wire. After welding the wire is cut off by wedge shaped knives forming again a
roof shaped weld projection. The welded wire segment is subsequently formed
into the desired contact shape by stamping or orbital forming. This welding
process can easily be integrated into automated production lines. The contact
material must however be directly weldable, meaning that it cannot contain
graphite or metal oxides.
Fig. 3.9: Vertical wire welding (schematic)
===3.3.3.1.2 Horizontal Wire or Profile Welding===
During horizontal welding the wire or profile contact material is fed at a shallow
angle to the carrier strip material ''(Fig. 3.10)''. The cut-off from the wire or profile
is performed either directly by the electrode or in a separate cutting station. This
horizontal feeding is suitable for welding single or multiple layer weld profiles.
The profile construction allows to custom tailor the contact layer shape and
thickness to the electrical load and required number of electrical switching
operations. By choosing a two-layer contact configuration multiple switching
duty ranges can be satisfied. The following triple-layer profile is a good example
for such a development: The top 5.0 μm AuAg8 layer is suitable to switch dry
circuit electronic signals, the second or middle layer of 100 μm Ag/Ni 90/10 is
used to switch relative high electrical loads and the bottom layer consists of an
easily weldable alloy such as CuNi44 or CuNi9Sn2. The configuration of the
bottom weld projections, i.e. size, shape, and number of welding nibs or weld
rails are critically important for the final weld quality.
Because of high production speeds (approx. 700 welds per min) and the
possibility to closely match the amount of precious contact material to the
required need for specific switching applications, this joining process has
gained great economical importance.
Fig. 3.10: Horizontal profile cut-off welding (schematic)
===3.3.3.1.3 Tip Welding===
Contact tips or formed contact parts produced by processes as described in
chapter 3.1.2 are mainly attached by tip welding to their respective contact
supports. In this process smaller contact parts such as Ag/C or Ag/W tips with
good weldable backings are welded directly to the carrier parts. To improve the
welding process 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 on
the carrier parts. Larger contact tips usually have an additional brazing alloy
layer bonded to the bottom weld surface.
Tip welding is also used for the attachment of weld buttons (see chapter 3.1.3).
The welding is performed mostly semi or fully automated with the buttons
oriented a specific way and fed into a welding station by suitably designed
feeding mechanisms.
===3.3.3.2 Percussion Welding===
This process of high current arc discharge welding required the contact material
and carrier to have two flat surfaces with one having a protruding nib. This nib
acts as the igniter point for the high current arc ''(Fig. 3.11)''. The electric arc
produces a molten layer of metallic material in the interface zone of the contact
tip and carrier. Immediately afterwards the two components are pushed together
with substantial impact and speed causing the liquid metal to form a strong joint
across the whole interface area.
Because of the very short duration of the whole melt and bonding process the
two components, contact tip and carrier, retain their mechanical hardness and
strength almost completely except for the immediate thin joint area. The
unavoidable weld splatter around the periphery of the joint must be
mechanically removed in a secondary operation.
The percussion welding process is mainly applied in the production of rod
assemblies for high voltage switchgear.
Fig. 3.11: Percussion welding (schematic)
===3.3.3.3 Laser Welding===
This contact attachment process is also one of the liquid phase welding
methods. Solid phase lasers are predominantly used for welding and brazing.
The exact guiding and focusing of the laser beam from the source to the joint
location is most important to ensure the most efficient energy absorption in the
joint where the light energy is converted to heat. Advantages of the method are
the touch-less energy transport which avoids any possible contamination of
contact surfaces, the very well defined weld effected zone, the exact
positioning of the weld spot and the precise control of weld energy.
Laser welding is mostly applied for rather small contact parts to thin carrier
materials. To avoid any defects in the contact portion, the welding is usually
performed through the carrier material. Using a higher power laser and beam
splitting allows high production speeds with weld joints created at multiple
spots at the same time.
===3.3.3.4 Special Welding and Attachment Processes===
In high voltage switchgear the contact parts are exposed to high mechanical
and thermal stresses. This requires mechanically strong and 100%
metallurgically bonded joints between the contacts and their carrier supports
which cannot be achieved by the traditional attachment methods. The two
processes of electron beam welding and the cast-on with copper can however
used to solve this problem.
===3.3.3.4.1 Electron Beam Welding===
The electron beam welding is a joining process which has shown its suitability
for high voltage contact assemblies. A sharply focused electron beam has
sufficient energy to penetrate the mostly thicker parts and generate a locally
defined molten area so that the carrier component is only softened in a narrow
zone (1 – 4 mm). This allows the attachment of Cu/W contacts to hard
and thermally stable copper alloys as for example CuCrZr for spring hard
contact tulips ''(Fig. 3.12)''.
Fig. 3.12:
Examples of contact tulips with Cu/W
contacts electron beam welded
to CuCrZr carriers.
===3.3.3.4.2 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.
*Examples of Wire Welding
bild
===Vertical Wire Welding===
*Contact materials
Ag, Ag-Alloys, Au- and Pd-Alloys, Ag/Ni (SINIDUR) als bild?
*Carrier materials
Cu, Cu-Alloys, Cu clad Steel, et.al. als bild?
*Dimensions
bild
Functional quality criteria such as bonded area percentage or shear force are
usually agreed upon between the supplier and user and defined in delivery
specifications.
===Horizontal Wire Welding===
*Contact materials
Au-Alloys, Pd-Alloys, Ag-Alloys, Ag/Ni (SINIDUR), Ag/CdO (DODURIT CdO),
Ag/SnO (SISTADOX), Ag/ZnO (DODURIT ZnO), and Ag/C (GRAPHOR D)
*Carrier materials
(weldable backing of multi-layer profiles)
Ni, CuNi, CuNiFe, CuNiZn, CuSn, CuNiSn, and others.
*Braze alloy layer
L-Ag 15P (CP 102 or BCUP-5)
*Dimensions
bild
*Quality criteria
Functional quality criteria such as bonded area percentage or shear force are
usually agreed upon between the supplier and user and defined in delivery
specifications.
===Percussion Welding===
*Contact materials
W/Cu, W/Ag, others
*Carrier materials
Cu, Cu-Alloys, others
*Dimensions
Weld surface area (flat) 6.0 to 25 mm diameter
Rectangular areas with up to 25 mm diagonals
*Quality criteria
Test methods for bond quality are agreed upon between supplier and user
Fig. 3.13: Examples for percussion welded contact parts
