Transcript Tresca Yield Criterion

Joining Processes
WELDING
WELDING
Definition: Material joining process. Two
parts connected at their contacting
surfaces by suitable heat and pressure.
Many
welding
processes
are
accomplished by heat alone, some others
by heat and pressure, and some with
pressure only.
In some welding operations a filler
material is used.
Welding operation usually applied to
metals but also used for plastics.
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Joining Processes
Parts produced by any of the manufacturing processes
can be made into larger, more complex bodies via Joining
processes
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Creating a metallurgical bond by adhesion and diffusion
Joining by fusion with the use of various heat sources
Brazing or soldering with a lower-melting metal
Mechanical fastening
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Fusion Welding Processes
Sources of Energy for Fusion Welding
• Chemical reactions
– Burning gases
– GAS WELDING
• Heat from electricity
– Arc
– ARC WELDING
SYSTEMS
– Resistance welding
• Light
LASER
Three Specific Types of Welding
Modules
• In this Welding, Cutting, and Brazing
module, three specific types of welding are
covered. These are listed below:
– Oxygen-fuel gas welding and cutting
– Arc welding and cutting
– Resistance welding
Resistance Welding
• Definition:
– This is a group of fusion welding
processes that use heat and pressure to
make the coalescence.
• The heat comes from electrical
resistance to current flow at the site of
the weld.
– The processes include:
• Spot Welding
• Projection Welding
• Seam Welding
Note:
FP&M only does spot welding.
Resistance Welding
• Spot Welding
–A process typically used in highvolume, rapid welding applications.
• The pieces to be joined are
clamped between two electrodes
under force, and an electrical
current is sent through them.
Welding/Cutting Hazards
• Potential Hazards
– Fires may start by hot materials igniting
nearby combustibles.
– Burns to the operator may occur if unprotected
skin comes into contact with the extremely hot
work.
– Magnetic fields could easily
destroy/disrupt electronic
components, stored data if not
careful.
Welding/Cutting Hazards
• Potential Hazards Cont.:
– Metal fumes from vaporizing of the work with the
extremely hot arcs may be inhaled into the worker’s
lungs.
– Certain metals and metal oxide fumes, including zinc,
cadmium and beryllium, produce serious illnesses
when inhaled.
– Fluxes used with welding to create inert atmospheres
at the point of the weld also present inhalation
hazards.
– All welding and cutting must have adequate
ventilation to protect the person doing the welding
and those working around the welding area.
Weld Joint Structure
Characteristics of a typical fusion-weld zone in oxyfuel-gas and arc welding.
Microhardness (HV) profile across a
weld bead.
A fusion joint is far from homogenous. Degree of
inhomogeity increases from pure metals to
multiphase alloys.
Typical weld zone in arc and gas welds
• The base material adjacent to the melt boundary is exposed to high
temperatures, and the properties and structure are changed within the
heat-affected zone.
• Cold worked base material will show recrystallization in HAZ, with coarse
grain sizes.
• In either case, a coarse-grained structure of lower strength exits at the
melt boundary.
Melt
Weldability and Weld Quality
- Welding Defects-
Welding Defects
1. Fusion welding defects due wrong heat input, insufficient
rate of weld metal deposition, and cooling.
2. Lack of bonding or gas porosity due to surface
contaminants, including oxides, oils, etc.
3. Undesirable reactions with surface contaminants
4. Solidification cracks in the weld.
5. Solidification shrinkage coupled with solid shrinkage
imposes internal tensile stresses on the structure, may
lead to distortion.
6. Gases released or formed during welding (eg CO) can
lead to porosity which weakens the joint and acts as a
stress raiser.
Weld joint
There are 5 basic joint types in welding
• Butt joint: Two materials are in the same
plane, joined from the edges.
• Corner joint:The corners of two materials
form a right angle and joined.
• Lap joint: Two parts overlaps.
• Tee joint: One part is perpendicular to
the other, making a T shape.
• Edge joint: Edges of the two materials
joined.
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Weld Joints
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Types of weld
1. Fillet weld: Used in T joints,corner joints, lap
joints.
2. Groove weld:Used in butt joints.
3. Plug weld: Used in lap joints.
4. Slot weld: Used in lap joints.
5. Spot weld: Used in lap joints.
6. Seam weld: Used in lap joints.
7. Flange weld:Used in edge joints.
8. Surfacing weld:Not a joining process, it is used
to increase the thickness of the plate, or
provide a protective coating on the surface.
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Fillet Weld
Slot and Plug Weld
Groove weld
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Metal Treatment
1. Preheating the weld zone – reduces energy input, cooling
rates in the weld and HAZ, reduces differential shrinkage,
residual stresses, and distortion.
2. Postwelding heat treatment of the entire welded structure
a. Stress-relief anneal reduces residual stresses to
acceptable level.
b. Normalizing a steel wipes out most undesirable effects of
welding.
c. Full heat treatment (quenching and tempering of steels)
3. Peening (hammering or rolling) of weld bead improves the
strength of welds.
Oxyacetylene Gas Welding
Three basic types of oxyacetylene flames used in oxyfuel-gas welding and
cutting operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or
reducing, flame. The gas mixture in (a) is basically equal volumes of oxygen
and acetylene. (d) The principle of the oxyfuel-gas welding operation.
Oxyacetylene Torch
The acetylene valve is opened first; the
gas is lit with a spark lighter or a pilot
light; then the oxygen valve is opened
and the flame adjusted.
Basic equipment used in oxyfuel-gas welding. To ensure correct connections, all
threads on acetylene fittings are left-handed, whereas those for oxygen are righthanded. Oxygen regulators are usually painted green, and acetylene regulators red.
Oxyfuel gas welding
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Oxygen-fuel gas welding & Cutting
• The elements of Oxygen-fuel gas welding
and cutting:
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General Requirements
Cylinders
Service Pipe Systems
Pipe System Protection
Oxygen-fuel gas welding and cutting
• General Requirements
– Focuses on using Acetylene Safely
• Flammable
• Unstable
• Cannot be adjusted above 15 psi
– Safe Work Practices
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The pressure adjusting screw:
–
Turning clockwise allows the gas
allows to flow.
Turning counterclockwise reduces
or stop the gas flow.
• Blow out cylinder valve
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• Turn on cylinder valve first and
then adjust the regulator pressure screw.
• Never stand in front or behind
a regulator when opening the cylinder valve
• Open cylinder valve slowly
Oxygen-fuel gas welding and cutting
• General Requirements Cont.:
– Safe Work Practices
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Purge oxygen and acetylene passages
Light the acetylene
Never use oil or grease
Do not use oxygen as a substitute for air
Keep your work area clean
Oxygen-fuel gas welding and cutting
• Cylinders
– Cylinder approval and marking
• marked for the purpose of identifying the gas content,
with either the chemical or trade name of the gas
– Storage of cylinders
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Storage area must be well ventilated
Cylinders must be at least 20 feet from combustibles
Valves must be closed
Valve protection must be in place
Inside storage must be limited to 2,000 cubic feet.
Cylinders must be stored in upright position
Oxygen must be at least 20 feet from fuel gas
or 5 feet with a 1/2 hour fire barrier
• Separate oxygen from fuel gas
L10 : Joining processes
Oxygen-fuel gas welding and cutting
• Cylinders Cont.:
– Operating Procedures
• Operation must emphasize the absence
of oily or greasy substances. Follow these
rules of operation:
– Cylinders, cylinder valves, couplings,
regulators, hose, and apparatus shall be kept
free from oily or greasy substances.
– Oxygen cylinders or apparatus shall not be
handled with oily hands or gloves.
– A jet of oxygen must never be permitted to
strike an oily surface, greasy clothes, or enter
a fuel oil or other storage tank.
Oxygen-fuel gas welding and cutting
• Service Pipe Systems
– There are special requirements for service
pipe systems when using oxygen or
acetylene.
• Oxygen
• Acetylene or Acetylene Compounds
Oxygen-fuel gas welding and cutting
• Pipe System Protection
– The entire service pipe system must be
protected against build-up of excessive
pressure and leaks. This protection is
accomplished with:
• Protective equipment
• Regulators
• Proper hose and hose connections.
Oxygen-fuel gas welding and cutting
• Pipe System Protection Cont.:
– Protective equipment is divided into the two
categories listed here:
• Pressure Relief Devices
– The pressure relief device should discharge upwards to
a safe location.
– Pressure relief valves are required in fuel-gas piping
systems to prevent excessive pressure build up within
the system.
Oxygen-fuel gas welding and cutting
• Pipe System Protection Cont.:
– Approved protective equipment shall be
installed in fuel-gas piping to prevent:
• Backflow of oxygen into the fuel-gas supply
system
• Passage of a flash back into the fuel-gas
supply system
• Excessive back pressure of oxygen in the
fuel-gas supply system.
Arc Welding
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Arc Welding and Cutting
• Definition:
– A fusion process wherein the coalescence of
the metals is achieved from the heat of an
electric arc formed between an electrode and
the work.
• Application
• Installation
• Operation & Maintenance
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Arc Welding & Cutting
• Application
– Applies to a large and varied group of
processes that use an electric arc as the
source of heat to melt and join metals.
• Installation
– Arc welding requires proper installation of
equipment.
– A critical part of installation is ensuring that
proper grounding is completed.
Arc Welding & Cutting
• Operation & Maintenance
– All connections to the machine shall be checked
to make certain that they are properly made.
– The work lead shall be firmly attached to the
work.
– Magnetic work clamps shall be free from
adherent metal particles of spatter on contact
surfaces.
– Coiled welding cable shall be
spread out before use to avoid
serious overheating and
damage to insulation.
Arc Welding & Cutting
• Operation and Maintenance Cont.:
– During welding operations, cables with splices
within 10 feet (3m) of the holder shall not be
used.
– Welders should not coil or loop welding
electrode cable around parts of their body.
– Cables with damaged insulation or exposed
bare conductors shall be replaced.
– Joining lengths of work and electrode cables
shall be done by the use of connecting means
specifically intended for that purpose.
– The connecting means shall have insulation
adequate for the service conditions.
Methods of Arc Welding
• Three Types of Welding Methods:
– Tungsten Inert Gas Welding (TIG)
– Gas Metal Arc Welding (MIG)
– Shielded Metal Arc Welding (SMAW)/ Stick
Welding
Electric Arc Welding
• Heat is produced from electric
arc between workpiece and
electrode material for melting
the workpiece material.
• AC and DC are used
• An Inert gas shields both
electrodes
• Most heat energy is due to
electron flow to metal
Heat input H  EI
v
Direct Current Electrode Negative
(DCEN): Deeper weld.
Direct Current Electrode Positive
(DCEP): Shallower and Wider
H- heat input, E, Voltage, I, Current, and v the
velocity of the arc travels along the weld line
Electric Arc Welding
- Classification • The Electrode
– Consumable: melts and serves as a filling material
– Non-consumable: does not melt, parent metal is used, or a
separate filler rod
– Coated or Uncoated
• Coating
– Provides a gaseous shield to prevent oxidation
– Lowers the voltage needed to establish the arc
– May provide slag-blanket to protect the joint
– Add alloying elements to enhance the properties of the joint.
Non-Consumable Electrode Arc Welding
• Gas Tungsten Arc Welding ( TIG)
• Plasma arc welding
• Atomic hydrogen welding
Non-Consumable-Electrode Welding:
Gas Tungsten-Arc Welding (GTAW)
• Nonconsumable gas tungsten
Inert gas welding (TIG)
• Weld zone is protected by inert
gas
• DC with straight polarity is used
with steel, cast iron, and
stainless
• AC with Al, Mg alloys where ac
helps in stripping the oxide
• Both hand and automatic
operations are possible
• The process demands
considerable skill but produces
very high-quality welds on almost
any material
• No weld spatter or slag formation
Consumable Electrode Arc Welding
Processes
• Shielded metal arc welding
• Submerged arc welding
• Gas metal arc welding
Consumable Electrode Arc Welding
Shielded-Metal Arc Welding
Schematic illustration of the shielded metal-arc welding process.
About 50% of all large-scale industrial welding operations use this
process.
Consumable-Electrode Welding:
Gas Metal-Arc Welding (GMAW) - MIG
• Consumable gas metal-arc welding
(MIG)
• Consumable electrode is metal which
melts to become part of the weld seam.
• Weld zone is protected by a gas or a
flux
• No slag is formed
• Several layers could be build with little
or no intermediate cleaning
• It is suitable for most metals
• Wire electrode can be supplied in long,
coiled lengths which allow uninterrupted
welds in any welding position.
Consumable Electrode Arc Welding Gas
- Gas Metal-arc Welding Process-
(a) Schematic illustration of the gas metal-arc welding process,
formerly known as MIG (for metal inert gas) welding. (b) Basic
equipment used in gas metal-arc welding operations.
Other Welding Processes
High Energy Beam Welding
• Electron Beam welding (EBW)
– Heat is produced by high velocity
electron gun in a narrow beam
– No filler material
– High rate of heating results in greater
depth and heat-affected zone is very
small
– Suitable for welding refractory materials
like: molybdenum and zirconium
– Requires a vacuum (limitation)
– x-ray will be generated around the
welding gun which may be cancerous
High Energy Beam Welding
• LASER Beam welding (LBW)
– Uses a focused high power monochromatic
light beam as a source of heat to the metal
– Beam can be directed to the welding spot
with a lens
– Depth of welding similar to electron beam
welding
– Vacuum is not necessary (advantage)
– Workpiece usually needs protection by a
gas
– Process is suitable for automation
– Welding speeds can be upto 7 m/min
Laser beam welding
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Thermit Welding
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Explosion Welding
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Resistance welding
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Forge welding
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PCB – printed circuit boards
Solder Pads
Soldering
Top View
Side View
Circuit Board
Resistor
Soldering
Iron
Move soldering iron until tip is
touching wire & solder pad
Move solder to touch edge of tip.
Solder
Hold until solder melts
on tip by wire
Solder
Move solder back to touch wire only
Solder
Move solder in to form a
small pocket
Solder
Move soldering iron tip up. This
will drag solder up with it.
Solder
Look for
shinny fillets
Brazing
• Definition:
– A process which a filler metal is placed at or between
the faying surfaces, the temperature is raised high
enough to melt the filler metal but not the base metal.
• The molten metal fills the spaces by capillary attraction.
• Torch Brazing
– Oxy-fuel torch with a carburizing flame
– First heat the joint then add the filler metal
Safe Work Practices
• Electric & Gas Welding
– Safety Check:
• Ensure electrical cord, electrode holder
and cables are free from defects
– No cable splices within 10 feet of electrode holder.
• Ensure welding unit is properly grounded.
This helps to avoid over heating.
• All defective equipment shall be repaired
or replaced before using.
Safe Work Practices
• Electric & Gas Welding Cont.:
– Safety Check:
• Remove all jewelry – rings, watches, bracelets,
etc…
• Ensure PPE e.g.. welding hood, gloves, rubber
boots or safety shoes, apron are available and in
good condition.
• Ensure fire extinguisher is charged and available.
• Ensure adequate ventilation and lighting is in place.
• Set Voltage Regulator to Manufacture’s
specifications.
• Avoid electrical shock DON’T wrap cables around
any body part.
• Ensure fittings are tight.
Safe Work Practices
• Electric & Gas Welding Cont.:
– Safety Check:
• Inspect hoses for cuts and frayed areas.
• Set gauges to desired PSI.
• Ensure that sufficient PPE is made
available.
• Locate welding screens to protect
employee’s – DON’T block your exit.
• Ensure that adequate ventilation and
lighting are in place.
Fire Protection & Prevention Cont.:
• Welding areas should meet the following
requirements:
– Floors swept & cleared of combustibles 35 ft.
radius of work area.
– Flammable and combustible liquids kept 35 ft.
radius of work area.
– At least one fire extinguisher – on site
– Protective dividers to contain sparks
and
slag
• Welding curtains
• Non-combustible walls
• Fire resistant tarps & blankets
UW-Eau Claire
Proper Ventilation for Welding
• Ventilation
– Proper ventilation can be obtained either naturally or
mechanically.
• Natural Ventilation is considered sufficient for welding and
brazing operations if the present work area meets these
requirements:
– Space of more than 10,000 square feet is provided per welder
– A ceiling height of more than 16 feet.
• Mechanical ventilation options generally fall into two basic
categories.
– Low vacuum system which takes large volumes of air at low
velocities.
– High vacuum system that are captured and extracted fumes as
near to the work as possible.
Fire Protection & Prevention
• Fire hazards must be removed, or
– Guards installed, or
– Welding/cutting must NOT take place
• Hot work permit should be used
outside designated areas to ensure
that all fire hazards are controlled
• Use of fire watch
– 1/2 hour after operation ceases
Proper Ventilation for Welding
L10 : Joining processes
• Ensure protection from fumes
and gases by one or a
combination of the following:
– Good general ventilation.
– Use of a booth.
– Local exhaust ventilation
on the hand piece.
– Air supply to the helmet.
Welding Operators Protection
• Welding involves specialized personal
protection that must be worn every time
you perform welding operations. The
following is a list of basic PPE:
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Fire-resistant gloves
Aprons
Safety shoes
Helmet
Ultraviolet radiation filter plate (arc welding)
Goggles with filter lenses
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Welding, Cutting and Brazing
• Summary
– Major hazards include:
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Fire
Burns
Shock
Toxic Exposure
– Follow proper procedures to prevent fires
– Use appropriate engineering controls
– Wear appropriate PPE