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Ch7 Welding Processes.Pdf 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.
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