Gas Shielded Welding Processes Gas Metal Arc Welding (GMAW) Lecture 4 p1 Gas Metal Arc Welding ( Process Fundamentals • In GMAW the welding heat sour~ is an arc maintained­ between a consumable wire electrode and the workpiece • The weld is formed by melting and solidification of the joint edges together with filler material transferred from the electrode • An flow of inert gas shields the high-temperature arc and weld pool from reactions with the surrounding atmosphere 4 p2 GMAW Process Fundamentals Electrode Shielding Gas In • Direction of Travel /GaSNOZZle CONSUMABLE ELECTRODE GAS _~~~~,~/ SHIELD A~£P~ BaseMetal~ Metal Lecture 4 p: GMAW Metal Transfer Modes ( • Short Circuiting or "Dip" Transfer • Globular Transfer • Spray Transfer • Pulsed or Synergic Transfer I, p4 GMAW Short Circuiting Transfer IIZE~Rn=O==~---~:::=====~------=~-_' (.) - Time z o E ARCING PERIOD II ~ I 7ize'iiRno---rTTl'-.--.---.-i::=------ Lecture 4 p ~ GMAW Globular Transfer Mode ( GAS NOZZLE (~L - p~ELECTRODE I (I r~ I I I I I '" R (AI R-Anode reaction P-Electromagnetic "pinch" force {~(~ g I I I I \.. j V IB) ( p6 GMAW Spray Transfer Variation in volume and rate of drop transfer with welding current 300 15.10·' .. 20 "- ,•• I ~ ~ ~ § I ~ 0.. 1/16 in. (1.6 mm) 10 E 0 200 DROP I .S E II: MILD STEEL 0 I W 15 VOLUME ELECTRODE. DCRP ::E W a! I ::E w 3 ... I ARGON-I% OXYGEN 0 z TRANSITIO~ SHIELDING GAS > 10 ~ '" 1/4 in. (6.4 mm) ~ ~ 100 CURRENT I ARC LENGTH 5 a.. ~ ... 0 0.. 0 II: 0 W c 5 ~ i 0 0 0 0 100 200 300 400 600 600 CURRENT, A Lecture 4 p GMAW Spray Transfer ( - p8 ~---~- --- GMAW Pulsed Transfer PULSE PEAK CURRENT SPRAY TRANSFER 3 CURRENT RANGE - -------- -,--------PULSE 2 GLOBULAR . 4 « TRANSITION 5 TRANSFj;R. 1-' CURRENT CURRENT z RANGE a:UJ 1 a: :J 1 2 3 4 5 u BACKGROUND CURRENT ~U~~U TIME • Pulse frequency and amplitude determine wire melting rate • "Synergic" control automatically gives the optimum pulse conditions for a given wire feed rate Lecture 4 p' GMAW Welding Procedures ( • Process Variables - Welding current (electrode melting rate) - Polarity - Arc voltage (length) - Travel speed - Electrode extension - Electrode size - Shielding gas composition 4 p10 GMAW Electrode Melting Rate .-C 800 20 C E .- c 700 - E - E c 600 -15 -C LLI LLI W a.. 500 w en 0- c 400 10 en LLI c LLI 300 W U. W I&. LLI 200 5 W 0:: a:: ~ 100 -3: 0 0 50 100 150 200 250 300 350 400 450 WELDING CURRENT (A) Lecture 4 pl GMAW Torch Geometry ( Nozzle Contact Tube to Electrode Extension Work Distance I\ ,;\ , Arc Length I\ .---'-- 'rL------, \ p12 GMAW Consumables • Electrode composition is usually similar to desired weld metal composition with ad9itional deoxidizers e.g. Si, AI, Ti • Electrodes are covered by AWS and other specifications - Carbon steel electrodes AWS A 5.18. • Shielding Gases - Various shielding gases are used depending on metal being welded and desired transfer mode - Principally Ar, C02 and mixtures of Ar-G02, 02 or He - Several commercial "brand-name" compositions Lecture 4 pt GMAW TlPical Welding Procedures ( Carbon Steel 'Q R ~~- pz ?? zzJl/fl ?? zzl T ~eM+F ~R A B C p14 GMAW Welding Equipment ELECTRODE SuPPLY Shielding ~as Supply Welding Gun Workpiece (j) WOAK WoO @ CABLEA&SEMIM.Y (i) WA1eATOQU.I <i> 8HElDING OM FROM cvt.HJER () WATERFAONQUN ~ WELlllNO OOHTACl'OA CONTflOl @ GUNSWfTCH CIACUT '" PCMEA CAaE <!> SHfElDlNG GAS TO GUN ® """"""'H'UTPOWER Lecture 4 P 1! GMAW Welding Gun ( WIRE CONOUIT SHIELDING GAS PATH ELECTRODE INPUT WATER LINE Q o WATERCHAMBER~~~~'--­ CONTACT TIP CONTROL GAS NOZZLE POWER BLOCK SWITCH POWER CABlE! RETURN WATER 4 p16 GMAW Applications • Aluminum alloys, copper/bronze, magnesium. titanium, austenitic stainless steels, nickel alloys • Sheet metal fabrication (short circuit mode) - eg automotive, appliance, light structures • General structural fabrication (spray/pulsed mode) • High production, mechanised and robotic welding Lecture 4 p17 GMAW Mechanized Application ( Field welds in oil/gas transmission pipelines Weld preparation and bead sequence P 18 .;.,;,0;.......__Uk ~.U;6lJ......_--......-----.1Mf;.....Ii----------""J----------'-~~"'<.ci!CI : . GMAW: Mechanized Applications Transfer Flask Shielded Workstation r.w,:=-Welding guns & seam trackef Spent Fuel Bay Fuel cominer .J Undelwaler ~.,., loading station A/ Cover seal welded "X:.<'""'''' after loading o i i'",52 "'X>Fuel Bundles '. j-,/, . ,.- '-{". Lecture 4 p19 GMAW: Mechanized applications ( Shielded station wall Welding guns mounted on automatic seam trackers I'"IIr~~+---f:+----" TV Camera -+ ,-+1-----.!::,~J--' fl"----' 1...... 1 Wire II feeder Shielding 1 1 I", 1 Gas L---liI---+I" II Control "'-1 Panel 1 ~--, I lop I L-----1r----'IC"" \ Video mon~or L------ft----t-"...... 1 o l ~ p20 ____...._ ......... ...._ .......... nl"il"" ~__~·" Process Control Component Status Weld Cycle TIme Cycle,start Cycle,end Weld overlap SEAM Tracking TRACKER Drive In/Out Idle TURNTABLE Motion Idle waDING Welding Current EQUIPMENT Shield gas flow Idle Lecture 4 P21 GMAW Capabilities & Limitations ( + Applicable to range of - Complex equipment metals and and set up _ thicknesses • Wire feeding can + Higher production be temperamental rates than SMAW or - Less portable than GTAW SMAW + No flux or slag - Gas shield sensitive to residues air currents + Adaptable to manual or mechanized/robotic applications ( p22 WELDING PROCESSES GAS TUNGSTEN ARC WELDING (GTAW) Lecture 4 p23 GTAW: Process Fundamentals ( • In GTAW the welding heat source is an arc maintained between a non-consumable tungsten electrode and the workpiece • Inert gas shields the arc and weld zone from atmospheric contamination • Filler need not always be added (autogenous welding) • Filler if required is added to the weld pool in the form of wire or rod. 4 p24 GTAW: Process Fundamentals • DIRECTION OF WELDING SHIELDING GAS IN I NON-CONSUMABLE TUNGSTEN ELECTRODE ARC SOUDIFIED WELD METAL Lecture 4 p25, GTAW: Process Variables ( • Welding current - DC, pulsed DC, high frequency pulsation, AC, variable polarity AC - • Arc length (Voltage) • Weld travel speed • Oscillation • Filler addition • Shielding gas composition &flow rate - generally Ar or He or mixtures. P 26 GTAW: Current Polarity • DCEP is mostly used - approx 70% of heat produced at anode (workpiece) • DCEN used to disperse tenacious surface oxides when welding AI, Mg • AC also used in AI welding - variable polarity pulsed AC Lecture 4 GTAW: Effect of Polarity ( e4 p28 ----~- - .. _ - ._ _- ._ _--_.. _--------- ---------_._-----~..-, GTAW: Arc Voltage ARC LENGTH 30 TUNGSTEN ARC. ALUMINUM -- 0.08 in. (2 mm) - 0.16 in. (4 mm) - > 25 w· C) <{ 20 HELIUM ~ , ~ 0 '... !lo-"'~d" > 15 u .-- a: <{ 10 0 0 50 100 150 200 250 300 350 400 ARC CURRENT, A Lecture 4 P 2~ Pulsed GTAW (PGTAW) ( pulse duration 0.1 - 1sec ~ Peak r-- ~oo current It.< ".I'~ "O.IL I "'...... Background - '--- '-- --l::-.......J-+-'>..--L.. current TIME Pulsed DC advantages: ,f Greater penetration for given average current ,f Minimizes heat affected zone & distortion ,f Improved capability to weld in all positions ,4 P 30 GTAW: Gas backing Lecture 4 p3' GTAW: Pipe internal purge ( GAS INLET PURGE GAS OUTLET !!::~~~~====~--BAFFLE PIPE WELD JOINT ,4 P 32 GTAW Equipment Schematic TORCH ELECTRICAL CONDUCTOR POWER ARC SOURCE Lecture 4 p3:: GTAWTorch ( POWER CONDUCTOR ... COOLING WATER IN/OUT ~1Sl '" SHIELDING GAS INLET / HANDLE SHIELDING GAS OUTLET- ---....... TUNGSTEN ELECTRODE:- _ 4 p34 Mechanized GTAW Applications • Mechanization of - weld head travel motions - wire feed (if required) - process controls: start/stop sequence, weld current profile, shielding gas flow, etc • Typical applications: Pipe, tube, tube-tubesheet welding, longitudinal welds in formed tubes. • Advantages of mechanization -Increased productivity - reduced weld defect rates - shorter joint completion times - reduced need for skilled labour Lecture 4 p35 Hot Wire GTAW ( HIGH SPEED FILLER AUX. WIRE FEEDER SHIELDING "-.~ GAS • CON~\ GTAW TUBE---~ TORCH AC HOT DC WIRE GTAW POWER ~.:=-.. POWER HEATED WIRE 4 P 36 GTAW Deposition Rates 20 9 HOT WIRE WITH 18 OSCILLATION~,/ 8 .c'C / . :0 16 :::::;. , 7 14 W , 6 ~ 12 , " ,, 5 ... Z 10 , .c , HOTWlRE 4~ 0 8 , I- COLD WIRE I 3 (/) 6 ,I 0 " 2 a.. 4 \ w - 0 2 1 0 0 2-- 6 4 8 10 ARC ENERGY (kW) Lecture 4 P37 Narrow-Gap GTAW ( Special Narrow-Gap. v.::::.-..~Torch Narrow-Gap Joint ~.Lh4- Preparation 4 p38 GTAW Capabilities & Limitations + Superior quality - Low deposition rates welds free from flux - Higher welder_ skill residues or spatter required in manual + Excellent control of processes penetration - Gas shielding + Applicable to almost sensitive to air all metals currents + Adaptable to manual or precision mechanized applications Lecture 4 p39.