ME 410: Casting and Welding Engineering

Welding processes

Faculty of Engineering Mechanical Dept. 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 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 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 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 Allied processes

 Thermal cutting  Oxyfuel gas, plasma, laser cutting

 Gouging  Air-arc, plasma, oxyfuel gas

 Surfacing  Powder and arc spray coating  Clad welding, hardfacing Solid phase welding

 Hot processes  Forge welding  Friction welding  Diffusion bonding

 Cold processes  Ultrasonic welding  Explosive welding 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) Fusion welding heat sources

Electric resistance Chemical reaction Electric arc Power beams

Spot, seam and Oxyfuel gas MMAW Laser projection welding welding GMAW Electron beam GTAW FCAW Electroslag Thermit welding SAW The electric arc

 Electric discharge between 2 Peak electrodes through a gas - Cathode temperatures  10 to 2000 amps at 10 to 500 V arc 18,000 K drop zone voltage

 Column of ionised gas at high temperature Anode  Forces stiffen the arc column drop zone  Transfer of molten metal from electrode to workpiece +  Can have a cleaning action, breaking up oxides on workpiece 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 Arc Heat Input

EI High arc heat Q  0.06 xEfficienc y • Large weld pool size v • Low cooling rate • Increased solidification Q = arc heat input in kJ/mm cracking risk E = arc voltage • Low ductility and strength I = current in amps • Precipitation of unwanted phases v = travel speed in mm/min (corrosion and ductility)

Efficiency Low arc heat SMAW = 75% • Small weld pool size • Incomplete fusion MIG/MAG = 90% • High cooling rate SAW = 90% • Unwanted phase transformations TIG = 80% • Hydrogen cracking 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

Manual Metal Arc Welding

MMAW, SMAW, Stick welding MMAW Process

Electrode lead

Electrode Coating +

Power Source Slag DCEP Shown Core wire - Weld metal Work Lead

Base material Weld pool 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 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 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) 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) 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 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 Classification

E xx yz - nHmR

Rm/10 (xx) Hydrogen level (HmR) 60 = 60000 psi min H5 = 5 ml / 100g of WM R = low moisture pick-up

Useable positions (y) Flux type (z) Impact properties (n) 1=all positions 20 = acid (iron oxide) 0 = 47J at 0°C 2=flat + horizontal 10, 11 = cellulosic 2 = 47J at -20°C 4=vertical down 12, 13 = rutile 3 = 47J at -30°C 24 = rutile iron powder 4 = 47J at -40°C 27 = acid iron powder 16 = basic 18, 28 = basic iron powder AC vs. DC

 DC  AC  Heat Concentrated at  Heat Concentrated at Work piece Electrode

 Forceful, Digging Arc  Lower Penetration

 Medium to Deep  Increased Deposition Penetration Rates

 (used for welding thin metal) 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

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 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 Submerged Arc Welding

SAW, subarc Submerged arc welding

Electrode Wire from Reel

Flux Hopper Drive rolls Power Source Contact tip +

- Granular Unfused flux Flux Slag Work Lead

Weld metal Arc cavity

Weld pool

Workpiece travel 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) Equipment

 Power source

 Welding head and control box

 Welding head travel

 Flux recovery system (optional)

 Positioners and Fixtures 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 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 Gas shielded arc processes

Gas metal arc welding (GMAW) Gas tungsten arc welding (GTAW) 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 GTAW process outline

Inert Torch gas Tungsten lead (-) electrode Torch Collet Power source Ceramic Gas lens shroud (optional) Arc Filler

Weld metal Work Weld pool lead (+) 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 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 Characteristics of Current Types for Gas Tungsten Arc Welding 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 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 GMAW and FCAW Gas metal arc welding

(MIG, MAG, CO2 welding) Flux cored arc welding 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) GMAW and FCAW outline

Torch gas Wire feeder Wire + feed Power source

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Weld Metal Weld pool Base material Return Lead 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 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

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