Seam welding is used in the same way as spot welding, and operates on essentially the same principle. The difference is that two wheel-shaped electrodes are used, rolling along (and usually feeding) the workpiece.
The two wheels should be of the same size, in order to prevent the part from being deflected towards one of them. The actual contact profile can be designed in a number of ways, in order to suit the shape of the part to be welded. The current may flow continuously while welding is being carried out, or intermittently to produce a series of spots, so closely positioned as to produce a single, continuous weld. An unavoidable problem of seam welding is that some of the current 'leaks' through the completed weld.
As the electrode rollers rotate, they do not need to be lifted between each spot, as with spot welding. If the weld does not have to be continuous, seam welding can be used therefore to position spots some regular distance from each other, which can be carried out quicker than ordinary spot welding.
(Weman K., Welding processes handbook, p. 83)
Resistance Seam Welding(New)(Better) (Manufacturing Method)
Its
features are:
• It is a
process in which heat caused by resistance to the flow of electric current in
the work metal is combined with pressure to produce a welded seam consisting of
a series of overlapping spot welds (see Fig. 9.22).
• The disc electrodes are continuously rotated, so that the workpiece gets advanced underneath them while at the same time the pressure on the joint is maintained. The electrodes need not be separated at any time.
• The current is applied through the heavy
copper electrodes in a series of pulses at proper intervals.
• The timing is adjusted so that the pulses overlap each other and thus form a continuous scam joint as shown.
• The timing is adjusted so that the pulses overlap each other and thus form a continuous scam joint as shown.
• As the welding proceeds, the applied current
will try to pass through the already welded portion, thus reducing the heating
in the portion to be welded. One way of overcoming this difficulty is to
increase the current as the welding progresses. Sometimes, external heating
such as high frequency heat to offset the effect of reduced current due to
shunting can be adopted.
• The applied pressure in seam
welding may range from 3 MN to 8.5 MPA, depending on the thickness of the
workpiece.
• The
current density may be as high as 775 A-mm2.
• Seam welding can be carried out on steels,
aluminium, magnesium and nickel alloys. Seam welding of copper and its alloys
is not recommended.
(Mukherjee, Metal Fabricecation Techonology, pp.310-311)
5-Adhesive Bonding(Previous)
Adhesive
bonding is a joining process in which a filler material is used to hold two (or
more) closely spaced parts together by surface attachment. The filler material
that binds the parts together is the adhesive. It is a nonmetallic
substance—usually a polymer. The parts being joined are called adherends.
Adhesives of greatest interest in engineering are structural adhesives, which
are capable of forming strong, permanent joints between strong, rigid
adherends, A large number of commercially available adhesives are cured by
various mechanisms and suited to the bonding of various materials. Curing
refers to the process by which the adhesive's physical properties are changed
from a liquid to a solid, usually by chemical reaction, to accomplish the
surface attachment of the parts. The chemical reaction may involve
polymerization, condensation, or vulcanization. Curing is often motivated by
heat and/or a catalyst, and pressure is sometimes applied between the two parts
to activate the bonding process. If heat is required, the curing temperatures are
relatively low, and so the materials being joined are usually unaffected—an
advantage for adhesive bonding. The curing or hardening of the adhesive takes
time, called curing time or setting time. In some cases this time is
significant—generally a disadvantage in manufacturing.
(Mikell P.
Groover,Fundamentals of Modern Manufacturing,4th Edition,pg.756)
Adhesive Bonding(New) (Better) (Jointing Method)
Adhesive
bonding is a method of joining structure together that eliminates some, or all,
of the cost and weight of mechanical fasteners. In adhesive bonding, cured
composites or metals are adhesively bonded to other cured composites, honeycomb
core, foam core, or metallic pieces. Cocuring is a process in which uncured
composite plies are cured and bonded simultaneously during the same cure cycle
to either core materials or other composite parts. The ability to make large
bonded and cocured unitized structure can eliminate a significant portion of
the assembly costs.
Adhesive bonding is a widely used industrial joining
process in which a polymeric material (the adhesive) is used to join two
separate pieces (the adherends or substrates). There are many types of
adhesives; sonic are strong and rigid while others are weak and flexible.
Adhesives used for structural bonding are always cured at either room or
elevated temperatures and must possess adequate strength to transfer the loads
through the joint. There are many types of structural adhesives; howeve,. Epoxies,
nitrile phenolics, and bismaleimides are the most prevalent. In addition to
fabricating large bonded components, adhesive bonding is frequently used for
repairing damaged structural parts.
Bonded
joints may be preferred it thin composite sections are to he joined where
bearing stresses in bolted joints would be unacceptably high, or when the
weight penalty for mechanical fasteners is too high. In general, thin
structures with well-defined load paths are good candidates for adhesive
bonding, while thicker structures with complex load paths are better candidates
for mechanical fastening.
Advantages
of Adhesive Bonding
The advantages of adhesive bonding include:
• Bonding provides a more uniform stress distribution
than mechanical fasteners by eliminating the individual stress concentration
peaks caused by mechanical fasteners. As shown in Fig. 8.1, the stress
distribution across the joint is much more uniform for the adhesive bonded
joint than for the mechanical .joint. leading to better fatigue life than that
for mechanically fastened joint. Bonded joints also provide superior vibration
and damping capability.
• Due to the elimination of mechanical
fasteners, bonded joints are usually lighter than mechanically fastened joints
and are less expensive in some applications.
• Bonded joints enable the design of smooth
external surfaces and integrally sealed joints with minimum sensitivity to
fatigue crack propagation. Dis-similar materials can be assembled with adhesive
bonding and the joints are electrically insulating, preventing galvanic corrosion
of metal adherends.
• Bonded joints provide a stiffening effect
compared to riveted or spot welded constructions. While rivets or spot welds
provide local point stiffening. bonded joints provide stiffening
over the entire bonded area. The significance of this effect is shown in Fig.
8.1, where bonded joints may increase the buckling strength of the structure by
as much as 30-100%.
Disadvantages
of Adhesive Bonding
Adhesive bonding also has some disadvantages,
including:
• Bonded
joints should be considered permanent joints. Disassembly is not easy and often
results in damage to the adherends and surrounding structure.
• Adhesive honding is much more sensitive to
surface preparation than mechanical fastening. Proper surface preparation is
absolutely essential to producing a strong, durable bond. For field repair
applications, it can be extremely difficult to execute proper surface
preparation. For original manufacturing, adhesive bonding requires clean rooms
with temperature and humidity control.
•
Adhesively bonded joints can be non-destructively tested for the presence of
voids and tin bonds: however, at this time there is no reliable non-destructive
test method for determining the strength of a bonded joint. Therefore, traveler
or process control test specimens must be fabricated and destructively tested
using the same surface preparation, adhesive, and bond cycle as the actual
structure.
• Adhesive materials are perishable. They must
be stored according to the manufacturer's recommended procedures (often
refrigerated). Once mixed or removed from the freezer. they must be assembled
and cured within a specified time.
• Adhesives
are susceptible to environmental degradation. Most will absorb moisture and
exhibit reduced strength and durability at elevated temperature. Some are
degraded by chemicals such as paint strippers or other solvents.
(Flake C.
Campbell, Manufacturing Technology for
Aerospace Structural Materials,pp.370,372)
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