ME 410: Casting and Welding Engineering Welding processes€¦ · Overview of joining methods...
Transcript of ME 410: Casting and Welding Engineering Welding processes€¦ · Overview of joining methods...
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Faculty of Engineering
Mechanical Dept.
ME 410: Casting and Welding Engineering
Welding processes
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Importance of joining
Wide use in manufacture
Occurs late in manufacturing process
Large number of practitioners
Cost is high proportion of manufactured item
Risk and cost of defective welds is high
Science is complex
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Overview of joining methods
Mechanical methods
Screwed fasteners, rivets, crimp or snap locks
Adhesive bonding
Brazing and Soldering
Base metal does not fuse.
Molten filler drawn into close-fit joints by capillary
action (surface tension forces).
Brazing filler melts >450˚C, solder <450˚C
Welding
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Weld
A joint produced by heat or pressure or both
so there is continuity of material.
Filler (if used) has a melting temperature
similar to the base material
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Welding processes
Fusion welding
Welding in the liquid state with no pressure
Union is by molten metal bridging
Solid phase welding
Carried out below the melting point without filler
additions
Pressure often used
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Allied processes
Thermal cutting
Oxyfuel gas, plasma, laser cutting
Gouging
Air-arc, plasma, oxyfuel gas
Surfacing
Powder and arc spray coating
Clad welding, hardfacing
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Solid phase welding
Hot processes
Forge welding
Friction welding
Diffusion bonding
Cold processes
Ultrasonic welding
Explosive welding
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Fusion welding
Intense energy source melts base metal locally
Energy density 0.001 W/cm2 to 1 MW/cm2
Energy source may be stationary or move at a
constant speed
Filler metal
From electrode
Independently added filler
No filler (autogenous welding)
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Fusion welding heat sources
Power beams
Laser
Electron beam
Spot, seam and
projection welding
Electroslag
Electric arc Chemical reaction Electric resistance
Oxyfuel gas
welding
Thermit welding
MMAW
GMAW
GTAW
FCAW
SAW
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The electric arc
+
- Cathode
drop zone
Anode
drop zone
Peak
temperatures
18,000 K
Electric discharge between 2
electrodes through a gas
10 to 2000 amps at 10 to 500 V arc
voltage
Column of ionised gas at high
temperature
Forces stiffen the arc column
Transfer of molten metal from
electrode to workpiece
Can have a cleaning action,
breaking up oxides on workpiece
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WELDING ARC The cathode drops the electrical connection between the
arc column and the negative pole (cathode). There is a
relatively large temperature and this is the point at
which the electrons are emitted through the arc column.
The stability of the arc depends on the smoothness of
the flow of electrons at this point.
Tungsten and carbon provide thermionic emissions
since both are good emitters of electrons.
They have high melting temperatures, are practically
nonconsumable, and are therefore used for welding
electrodes. Tungsten has the highest melting point of
any metal.
The anode drop occurs at the other end of the arc and
is the electrical connection between the positive pole
and the arc column. The temperature changes from the
arc column to the anode is considerably lower
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Arc Heat Input
yxEfficiencv
EIQ 06.0
Q = arc heat input in kJ/mm
E = arc voltage
I = current in amps
v = travel speed in mm/min
Efficiency
SMAW = 75%
MIG/MAG = 90%
SAW = 90%
TIG = 80%
High arc heat • Large weld pool size
• Low cooling rate
• Increased solidification
cracking risk
• Low ductility and strength
• Precipitation of unwanted phases
(corrosion and ductility)
Low arc heat • Small weld pool size
• Incomplete fusion
• High cooling rate
• Unwanted phase transformations
• Hydrogen cracking
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Some arc welding processes
MMAW - Manual Metal Arc Welding
SAW - Submerged Arc Welding
GTAW - Gas Tungsten Arc Welding (TIG)
GMAW - Gas-Metal Arc Welding (MIG, MAG)
FCAW - Flux Cored Arc Welding
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Manual Metal Arc Welding
MMAW, SMAW, Stick welding
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MMAW Process
Work
Lead
+
-
Electrode
lead
Power Source
DCEP Shown
Base material
Electrode
Coating
Core
wire
Weld pool
Slag
Weld metal
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Minimum equipment
Power source (ac or dc, engine driven or
mains transformer)
Electrode holder and leads
May carry up to 300 amps
Head shield with lens protects face & eyes
Chipping hammer to remove slag
Welding gloves protect hands from arc
radiation, hot material and electric shock
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Process features
Simple portable equipment
Widely practiced skills
Applicable to wide range of materials, joints,
positions
About 1kg per hour of weld deposited
Portable and versatile
Properties can be excellent
Benchmark process
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Covered electrodes
Core wire
Solid or tubular
2mm to 8mm diameter, 250
to 450mm long
Coating
Extruded as paste, dried to
strengthen
Dipped into slurry and dried
(rare)
Wound with paper or chord
(obsolete)
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Functions of coating
Slag protects weld pool from oxidation
Gas shielding also protects weld pool
Surface tension (fluxing)
Arc stabilising (ionising)
Alloying and deoxidation
Some ingredients aid manufacture (binder
and extrusion aids)
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Typical coating constituents
Organic materials (Cellulose)
Titanium dioxide (rutile)
Silica, alumino-silicates
Sodium and potassium silicate binders
Calcium carbonate and fluoride
Iron powder, ferro-alloys
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Electrodes for C-Mn Steel
E6010, E6011 - cellulosic
Punchy, penetrating arc
E6012, E6013 - rutile
Smooth arc, general purpose
E7024 - iron powder (rutile)
Thick coating, high deposition
E7016, E7018, E7028 - Basic low hydrogen
High toughness, low cracking risk
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Classification
E xx yz - nHmR
Useable positions (y) 1=all positions
2=flat + horizontal
4=vertical down
Rm/10 (xx) 60 = 60000 psi min
Flux type (z) 20 = acid (iron oxide)
10, 11 = cellulosic
12, 13 = rutile
24 = rutile iron powder
27 = acid iron powder
16 = basic
18, 28 = basic iron powder
Impact properties (n) 0 = 47J at 0°C
2 = 47J at -20°C
3 = 47J at -30°C
4 = 47J at -40°C
Hydrogen level (HmR) H5 = 5 ml / 100g of WM
R = low moisture pick-up
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AC vs. DC
DC
Heat Concentrated at
Work piece
Forceful, Digging Arc
Medium to Deep
Penetration
AC
Heat Concentrated at
Electrode
Lower Penetration
Increased Deposition
Rates
(used for welding
thin metal)
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Approximate Amperage Settings
Approximate Electrode Amperage Settings
Fast Freeze Fill Freeze Fast Fill Low Hydrogen
E6010 - E6011 E6013 - E7014 E7024 - E7028 E7018
Diameter of Current Setting Current Setting Current Setting Current Setting
Electrode
Inches(Millimeters) Amperes Amperes Amperes Amperes
3/32 in (2.4 mm) 40 - 90 75 - 105 85 - 155 70 - 140
1/8 in (3.2 mm) 75 - 130 100 - 165 100 - 175 90 - 185
5/32 in (4.0 mm) 80 - 160 135 - 225 160 - 270 140 - 230
3/16 in (4.8 mm) 110 - 225 185 - 280 220 - 330 210 - 300
7/32 in (5.6 mm) 200 - 260 235 - 340 270 - 410 230 - 380
1/4 in (6.4 mm) 220 - 325 260 - 425 315 - 520 290 - 440
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Applications
Wide range of welded products:
Handyman & light structure
Heavy steel structures, workshop and site
High integrity (nuclear reactors, pressure
equipment)
Ideal where access is difficult - construction
site, inside vessels, underwater
Joins a wide range of materials
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Limitations of MMAW
Low productivity
Low power
Low duty cycle (frequent electrode changes)
Hydrogen from flux coatings
Electrode live all the time
Arc strike, stray current and electric shock risks
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Submerged Arc Welding
SAW, subarc
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Submerged arc welding
Power
Source
Work Lead
-
+
Workpiece travel
Granular
Flux
Flux
Hopper
Unfused flux
Electrode Wire from Reel
Drive rolls
Contact tip
Slag
Weld metal
Weld pool
Arc cavity
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SAW features
High productivity
2 to 10 kg/hour
Up to 2m/min
Bulky, expensive and
heavy equipment
Flat and horizontal
positions only
Thicker sections (6mm
and above)
Mostly ferrous materials
(also Ni alloys)
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Equipment
Power source
Welding head and
control box
Welding head travel
Flux recovery system
(optional)
Positioners and Fixtures
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Consumables
Solid or cored wires
Granular fluxes
Agglomerated, fused or sintered
Alloying activity
Contribution to weld metal chemistry from flux
Basicity
Acid fluxes made from manganese oxide, silica, rutile
are easy to use
Basic fluxes (MgO, CaO, CaF2, Al2O3) provide excellent
toughness welds
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Applications of SAW
Long straight welds in heavier material
Vessel longitudinal and circumferential welds
Flange to web joints of I beams
Flat or horizontal position
Flux has to be supported
Access has to be good
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Gas shielded arc processes
Gas metal arc welding (GMAW)
Gas tungsten arc welding (GTAW)
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Gas Tungsten Arc Welding
Alternative names -
GTAW,TIG (Tungsten
Inert Gas), Argonarc
Heat source is an electric
arc between a non-
consumable electrode and
the workpiece
Filler metal is not added or
is added independently
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GTAW process outline
Ceramic
shroud
Torch
Gas lens
(optional)
Inert
gas
Power
source
Torch
lead (-)
Work
lead (+)
Filler Arc
Weld metal
Weld pool
Collet
Tungsten
electrode
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Process features
Excellent control
Stable arc at low power (80A at 11V)
Independently added filler
Ideal for intricate welds eg root runs in pipe or thin sheet
Low productivity 0.5kg/h manual
High quality
Clean process, no slag
Low oxygen and nitrogen weld metal
Defect free, excellent profile even for single sided welds
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Equipment for GTAW
Welding power source with constant current
characteristic
DC for most metals, AC for Al
Arc starting by high frequency (5000V, 0.05A)
Sequence timers for arc starting, arc finishing & gas
control
Water- or gas-cooled torch with tungsten
electrode
Electrode may contain thoria or zirconia, etc
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Characteristics of Current Types for Gas Tungsten Arc Welding
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Shielding gases
Torch is fed with an inert or reducing gas
Pure argon - widespread applications
Argon-helium - Higher arc voltage, inert
Argon-2% hydrogen - Cu alloys & austenitic steel
Torch gas must not contain oxygen or CO2
Backing (or purge) gas
Used for all single-sided welds except in carbon steel
Argon, nitrogen, former gas (N2 + H2)
Supplementary shielding
Reactive metals: Ti, etc
Gas filled chambers or additional gas supply devices
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Filler metals
Autogenous welding (no filler)
Filler wire or rod of matching composition
C-Mn & low alloy steel
Stainless Steel
Al, Mg, Ti
Cu & Ni
Consumable inserts - filler preplaced in joint
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GMAW and FCAW
Gas metal arc welding
(MIG, MAG, CO2 welding)
Flux cored arc welding
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GMAW & FCAW processes
A continuous solid wire, small diameter
GMAW uses solid wire, no flux
FCAW uses flux-filled wire
Fed through the gun to the arc by wire
feeder.
The weld pool may be protected from
oxidation by shielding gas.
High productivity 3 kg/h or more
Direct current (DCEP mostly)
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GMAW and FCAW outline
Base material Return Lead
+
_
Power
source
Torch gas
Weld Metal
Weld pool
Wire
feed
Wire feeder
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GMAW & FCAW equipment
Welding power source
Wire feeder mechanism
May be in power source cabinet
Gun with gas supply & trigger switch
Manual (semiautomatic) guns
Automatic torches available
Can be fitted to robot etc
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Consumables
Solid Wires (GMAW)
A wide variety of alloys are available
Flux cored arc welding (FCAW)
Gas shielded flux cored wires
Self-shielded flux cored wires
Used outdoors
Metal cored wires
Light flux cover
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Torch gas mixtures
Inert gases (MIG)
Argon or helium or mixtures of these
Active base metals, Al, Mg, Ti
Active gases (MAG and FCAW)
Carbon dioxide
Argon plus oxygen and/or carbon dioxide
Nitrogen, hydrogen
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