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===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
===3.4 Evaluation of Braze or Weld Joints===