3Objectives Describe the gas tungsten arc welding process List other terms used to describe itWhat makes tungsten a good electrodeEliminate tungsten erosionShape and clean a tungsten electrodeGrind a point on a tungsten electrode
4Objectives (continued) Remove a contaminated tungsten endMelt the end of the tungsten electrode into the desired shapeCompare water-cooled GTA welding torches to air-cooled torchesThe purpose of the three hoses connecting a water-cooled torch to the welding machine
5Objectives (continued) Choose an appropriate nozzleHow to get an accurate reading on a flowmeterCompare the three types of welding current used for GTA weldingShielding gases used in the GTA welding process
6Objectives (continued) Define preflow and postflowProblems resulting from an incorrect gas flow rateProperly set up a GTA welderEstablish a GTA welding arc
7IntroductionThe Gas Tungsten Arc Welding (GTAW) process is sometimes referred to as a TIG or HeliarcTIG is short for tungsten inert gasAn arc is established between a non-consumable tungsten electrode (heating element) and the base metalThe inert gas provides the needed arc characteristics and protects the molten weld poolWhen Argon became plentiful, the GTA process became more common
11Tungsten Tungsten has the following properties: High tensile strength HardnessHigh melting temperatureHigh boiling temperatureGood electrical conductivity
12Tungsten (continued)Tungsten is the best choice for a non consumable electrodeHigh melting temperatureGood electrical conductivityAs the tungsten electrode becomes hot the arc between the electrode and the work stabilizesBut a clean and correctly ground tungsten is neededBecause of the intense heat some erosion of the electrode will occur
13Figure 15-1 Some tungsten will erode and be transferred across the arc.
14Tungsten (continued) Ways to limit erosion: Good mechanical and electrical contactUse as low a current as possibleUse a water-cooled torchUse as large a tungsten electrode as possibleUse DCEN currentUse as short an electrode extension as possibleUse the proper shape electrodeUse an alloyed tungsten electrode
17Tungsten (continued)The collet is the cone-shaped sleeve that holds the electrode in the torchLarge-diameter electrodes conduct more currentThe current-carrying capacity at DCEN is about ten times greater than at DCEPThe preferred electrode shape impacts the temperature and erosion of the tungstenWith alternating current, the tip is subjected to more heat than with DCEN
18Figure 15-3 The smooth surface of a centerless ground tungsten electrode. Courtesy of Larry Jeffus.
19Types of Tungsten Electrodes Pure tungsten is an excellent nonconsumable electrodePure tungsten can be improved by adding:CeriumLanthanumThoriumZirconium
21Table 15-1 Tungsten Electrode Types and Identification.
22Shaping the Tungsten To obtain the desired end shape: Grinding (for MS and SS)Breaking (not recommended due to cost)Re melting the end (Aluminium welding)Using chemical compound (doesn’t work that well)
23GrindingOften used to clean a contaminated tungsten or to point the endShould have a fine, hard stoneA coarse grinding stone with result in more tungsten breakageShould be used for grinding tungsten onlyMetal particles will quickly break free when the arc is started, causing contamination
24Figure 15-8 Correct way of holding a tungsten when grinding Figure 15-8 Correct way of holding a tungsten when grinding. Courtesy of Larry Jeffus.
25Breaking and Remelting Tungsten is hard but brittleIf struck sharply, it will break without bendingTry not to do this because of $$$$$$$$Holding against a sharp corner and hitting results in a square breakAfter breaking squarely, melt back the end
26Chemical Cleaning and Pointing Tungsten can be cleaned and pointed using one of several compoundsHeated by shorting it against the workDipped in the compoundWhen the tungsten is removed, cooled, and cleaned, the end will be tapered to a fine pointThe chemical compound will dissolve the tungsten, allowing the contamination to fall free
27Pointing and Remelting Tapered tungsten with a balled end is made by first grinding or chemically pointingThe ball should be made large enough so that the color of the end stays dull red and bright redIncrease ball size by applying more currentSurface tension pulls the molten tungsten up onto the tapered end
30GTA Welding Equipment “Cadillac Stick Welder” GTA welding torches are water- or air-cooledWater-cooled GTA welding torch is more efficientWater-cooled torch has three hoses connecting it to the welding machineNozzle directs the shielding gas directly on the welding zoneFlowmeter regulates the rate of gas flow
31Figure 15-21 Schematic of a GTA welding setup with a water-cooled torch.
32Types of Welding Current DCEN concentrates about 2/3 of its welding heat on the workMax penetrationHigh Freq. – start onlyDCEP concentrates about 1/3 of its welding heat on the workMax cleaning action2/3 of heat at tungsten – primarily used for balling tungsten for aluminium welding
33Types of Welding Current AC concentrates its heat at 50/50Sign wave provides for DCRP (cleaning action) and DCSP (penetration action)Square wave technology allows for adjusting the cleaning or penetration cycle.High Freq. is on Continuous so there is equal firing of both sides of sign wave.DC Component will take place if there is no High Freq.
34Figure 15-29 Electrons collect under the oxide layer during the DCEP portion of the cycle.
35Figure 15-30 Sine wave of alternating current at 60 cycle.
37Shielding Gases Shielding gases used for GTA welding process: Argon (Ar)Helium (He)Or a mixture of two or more gases
38Shielding Gases (continued) Argon effectively shields welds in deep grooves in flat positionsHelium offers the advantage of deeper penetration
39Shielding Gases (continued) Hot start allows a surge of welding currentPreflow is the time gas flows to clear out air in the nozzleSome machines do not have preflowPostflow is the time the gas continues flowing after the welding current has stopped
40Shielding Gases (continued) Ionization PotentialAmount of voltage needed to “kick start” the arcThe ionization potential, or ionization energy, of a gas atom is the energy required to strip it of an electron. That is why a shielding gas such as helium, with only 2 electrons in its outer shell, requires more energy (higher voltage parameters) for welding. The ionization potential of a shielding gas also establishes how easily an arc will initiate and stabilize. A low ionization potential means the arc will start relatively easy and stabilize quite well. A high ionization potential has difficulty initiating and may have difficulty keeping the arc stable.Argon15.7 electron voltsHelium24.4 electron voltsMore penetration
41Figure 15-35 Too steep an angle between the torch and work may draw in air.
45Objectives Applications using the gas tungsten arc welding process Effects on the weld of varying torch anglesWhy and how the filler rod is kept inside the protective zone of the shielding gasHow tungsten contamination occurs and what to doCauses of change in welding amperageCorrect settings for the minimum and maximum welding current
46Objectives (continued) Types and sizes of tungsten and metalFactors affecting gas preflow and postflow timesMinimum and maximum gas flow settings:Nozzle sizeTungsten sizeAmperage settingCharacteristics of low carbon and mild steels, stainless steel, and aluminumMetal preparation for GTA weldingMake GTA welds in all positions
47Introduction Gas tungsten arc is also called GTA welding GTA welding can be used to for nearly all types and thicknesses of metalGTA welding is fluxless, slagless, and smokelessWelders have fine control of the welding processGTA welding is ideal for close-tolerance weldsSome GTA welds make the critical root passGTA used when appearance is important
48Introduction (continued) Setup of GTA equipment affects weld qualityCharts give correct settingsField conditions affect the variables in the chartsExperiments designed to evaluate the appearance of a weldAfter welding in the lab, troubleshooting field welding problems is easierTo make a weld is good: to solve a welding problem is better
49Torch Angle As close to perpendicular as possible May be angled 0-15 degrees from perpendicular for better visibilityAs the gas flows out it forms a protective zone around the weldToo much tilt distorts protective shielding gas zone
50Figure 16-5 Filler being remelted as the weld is continued Figure 16-5 Filler being remelted as the weld is continued. Courtesy of Larry Jeffus.
51Torch Angle (continued) Velocity of shielding gas affects protective zoneLow-pressure area develops behind the cup when velocity increasesSharper angle and higher flow rate increases contamination
52Filler Rod Manipulation Filler rod must be kept inside the protective zoneIf filler rod is removed from the gas protection, it oxidizes rapidlyOxide is added to the molten weld poolWhen a weld is temporarily stopped, the shielding gas must be kept flowing
53Filler Rod Manipulation (continued) If the rod tip becomes oxidized, if should be cut off before restartingThe rod should enter the shielding gas as close to the base metal as possibleAn angle less than 15 degrees prevents air from being pulled in the welding zone
54Figure The hot filler rod end is well within the protective gas envelope. Courtesy of Larry Jeffus.
55Figure 16-7 Too much filler rod angle has caused oxides to be formed on the filler rod end. Courtesy of Larry Jeffus.
56Tungsten Contamination Most frequent problem is tungsten contaminationTungsten becomes contaminated if it touches:Molten weld poolFiller metalSurface tension pulls the contamination up onto the hot tungstenExtreme heat causes some of the metal to vaporize and form a large oxide layer
57Tungsten Contamination (continued) Contamination caused by the tungsten touching the molten pool or filler metal forms a weak weldThe weld and tungsten must be cleaned before any more welding can be doneTiny tungsten particles will show up if the weld is x-rayedContamination can be knocked off quickly by flipping the torch headThis procedure should never be used with heavy contamination or in the field
58Figure 16-8 Contaminated tungsten. Courtesy of Larry Jeffus.
59Current SettingAmperage on the machine's control is the same at the arc when:Power to the machine is exactly correctLead length is very shortAll cable connections are perfectArc length is exactly rightRemote current control is in the full on position
60Figure 16-10 Melting first occurring. Courtesy of Larry Jeffus.
61Figure 16-12 Oxides forming due to inadequate gas shielding Figure Oxides forming due to inadequate gas shielding. Courtesy of Larry Jeffus.
62Gas Flow Gas preflow and postflow times depend upon: Wind or draft speedTungsten size usedAmperageJoint designWelding positionType of metal weldedMaximum flow rates must never be exceededAir can be sucked into the weld zone
63Practice WeldsPractice welds are grouped according to the weld position and type of jointMild steel is inexpensive and requires the least amount of cleaningWith aluminum, cleanliness is a critical factorTry each weld with each metal to determine which metal will be easier to master
64Low Carbon and Mild Steels Low carbon and mild steel are two basic steel classificationsSmall pockets of primary carbon dioxide gas become trappedPorosity most likely when not using a filler metalMost filler metals have some alloys, called deoxidizers
65Stainless SteelSetup and manipulation are nearly the same as for low carbon and mild steelsMost welds on stainless steels show effects of contaminationMost common problem is the bead color after the weldUsing a low arc current with faster travel speeds is important
66Aluminum Molten aluminum weld pool has high surface tension Preheat the base metal in thick sectionsPreheat temperature is around 300° FahrenheitCleaning and keeping the metal clean is time consumingAluminum rapidly oxidizes at welding temperatures
67Metal Preparation Base and filler metals must be thoroughly cleaned Contamination will be deposited into the weldOxides, oil, and dirt are the most commonContaminants can be removed mechanically or chemically
68Figure Aluminum filler being correctly added to the molten weld pool. Courtesy of Larry Jeffus.
69Figure Filler rod being melted before it is added to the molten pool. Courtesy of Larry Jeffus.
70Figure 16-18 Surfacing weld. Courtesy of Larry Jeffus.
71Figure 16-20 Establish a molten weld pool and dip the filler rod into it. Courtesy of Larry Jeffus.
72Figure Note the difference in the weld produced when different size filler rods are used. Courtesy of Larry Jeffus.
73Figure 16-22 Move the electrode back as the filler rod is added Figure Move the electrode back as the filler rod is added. Courtesy of Larry Jeffus.
74Figure 16-34 Be sure both the top and bottom pieces are melted Figure Be sure both the top and bottom pieces are melted. Courtesy of Larry Jeffus.
75Figure 16-35 Oxides form during tack welding. Courtesy of Larry Jeffus.
76Figure 16-36 A notch indicates the root was not properly melted and fused. Courtesy of Larry Jeffus.
77Figure 16-37 Watch the leading edge of the molten weld pool Figure Watch the leading edge of the molten weld pool. Courtesy of Larry Jeffus.
78SummaryPositioning yourself to control the electrode filler metal and to see the joint is criticalExperienced welders realize they need to see only the leading edge of the weld poolGood idea to gradually reduce your need for seeing 100% of the weld poolIncreasing this skill is significant advantage in the fieldWelding in the field may have to be done out of position