Gas Tungsten Arc Welding

Objectives Describe the gas tungsten arc welding process List other terms used to describe it What makes tungsten a good electrode Eliminate tungsten erosion Shape and clean a tungsten electrode Grind a point on a tungsten electrode

Objectives (continued) Remove a contaminated tungsten end Melt the end of the tungsten electrode into the desired shape Compare water-cooled GTA welding torches to air-cooled torches The purpose of the three hoses connecting a water-cooled torch to the welding machine

Objectives (continued) Choose an appropriate nozzle How to get an accurate reading on a flowmeter Compare the three types of welding current used for GTA welding Shielding gases used in the GTA welding process

Objectives (continued) Define preflow and postflow Problems resulting from an incorrect gas flow rate Properly set up a GTA welder Establish a GTA welding arc

Introduction The Gas Tungsten Arc Welding (GTAW) process is sometimes referred to as a TIG or Heliarc TIG is short for tungsten inert gas An arc is established between a non-consumable tungsten electrode (heating element) and the base metal The inert gas provides the needed arc characteristics and protects the molten weld pool When Argon became plentiful, the GTA process became more common

Power Source Basics

Tungsten Electrodes

Tungsten Tungsten has the following properties: High tensile strength Hardness High melting temperature High boiling temperature Good electrical conductivity

Tungsten (continued) Tungsten is the best choice for a non consumable electrode High melting temperature Good electrical conductivity As the tungsten electrode becomes hot the arc between the electrode and the work stabilizes But a clean and correctly ground tungsten is needed Because of the intense heat some erosion of the electrode will occur

Figure 15-1 Some tungsten will erode and be transferred across the arc.

Tungsten (continued) Ways to limit erosion: Good mechanical and electrical contact Use as low a current as possible Use a water-cooled torch Use as large a tungsten electrode as possible Use DCEN current Use as short an electrode extension as possible Use the proper shape electrode Use an alloyed tungsten electrode

Torch Build Out

Torch Build Out

Tungsten (continued) The collet is the cone-shaped sleeve that holds the electrode in the torch Large-diameter electrodes conduct more current The current-carrying capacity at DCEN is about ten times greater than at DCEP The preferred electrode shape impacts the temperature and erosion of the tungsten With alternating current, the tip is subjected to more heat than with DCEN

Figure 15-3 The smooth surface of a centerless ground tungsten electrode. Courtesy of Larry Jeffus.

Types of Tungsten Electrodes Pure tungsten is an excellent nonconsumable electrode Pure tungsten can be improved by adding: Cerium Lanthanum Thorium Zirconium

Tungsten Electrodes

Table 15-1 Tungsten Electrode Types and Identification.

Shaping 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)

Grinding Often used to clean a contaminated tungsten or to point the end Should have a fine, hard stone A coarse grinding stone with result in more tungsten breakage Should be used for grinding tungsten only Metal particles will quickly break free when the arc is started, causing contamination

Figure 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.

Breaking and Remelting Tungsten is hard but brittle If struck sharply, it will break without bending Try not to do this because of $$$$$$$$ Holding against a sharp corner and hitting results in a square break After breaking squarely, melt back the end

Chemical Cleaning and Pointing Tungsten can be cleaned and pointed using one of several compounds Heated by shorting it against the work Dipped in the compound When the tungsten is removed, cooled, and cleaned, the end will be tapered to a fine point The chemical compound will dissolve the tungsten, allowing the contamination to fall free

Pointing and Remelting Tapered tungsten with a balled end is made by first grinding or chemically pointing The ball should be made large enough so that the color of the end stays dull red and bright red Increase ball size by applying more current Surface tension pulls the molten tungsten up onto the tapered end

Figure 15-14 Melting the tungsten end shape.

GTAW Equipment

GTA Welding Equipment “Cadillac Stick Welder” GTA welding torches are water- or air-cooled Water-cooled GTA welding torch is more efficient Water-cooled torch has three hoses connecting it to the welding machine Nozzle directs the shielding gas directly on the welding zone Flowmeter regulates the rate of gas flow

Figure 15-21 Schematic of a GTA welding setup with a water-cooled torch.

Types of Welding Current DCEN concentrates about 2/3 of its welding heat on the work Max penetration High Freq. – start only DCEP concentrates about 1/3 of its welding heat on the work Max cleaning action 2/3 of heat at tungsten – primarily used for balling tungsten for aluminium welding

Types of Welding Current AC concentrates its heat at 50/50 Sign 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.

Figure 15-29 Electrons collect under the oxide layer during the DCEP portion of the cycle.

Figure 15-30 Sine wave of alternating current at 60 cycle.

Shielding Gas

Shielding Gases Shielding gases used for GTA welding process: Argon (Ar) Helium (He) Or a mixture of two or more gases

Shielding Gases (continued) Argon effectively shields welds in deep grooves in flat positions Helium offers the advantage of deeper penetration

Shielding Gases (continued) Hot start allows a surge of welding current Preflow is the time gas flows to clear out air in the nozzle Some machines do not have preflow Postflow is the time the gas continues flowing after the welding current has stopped

Shielding Gases (continued) Ionization Potential Amount of voltage needed to “kick start” the arc The 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. Argon 15.7 electron volts Helium 24.4 electron volts More penetration

Figure 15-35 Too steep an angle between the torch and work may draw in air.

Remote Controls Foot or Finger

Remote Controls Can be used to: Start the weld Increase the current Decrease the current Stop the weld Remote can be foot-operated or hand-operated device

Welding Techniques

Objectives Applications using the gas tungsten arc welding process Effects on the weld of varying torch angles Why and how the filler rod is kept inside the protective zone of the shielding gas How tungsten contamination occurs and what to do Causes of change in welding amperage Correct settings for the minimum and maximum welding current

Objectives (continued) Types and sizes of tungsten and metal Factors affecting gas preflow and postflow times Minimum and maximum gas flow settings: Nozzle size Tungsten size Amperage setting Characteristics of low carbon and mild steels, stainless steel, and aluminum Metal preparation for GTA welding Make GTA welds in all positions

Introduction Gas tungsten arc is also called GTA welding GTA welding can be used to for nearly all types and thicknesses of metal GTA welding is fluxless, slagless, and smokeless Welders have fine control of the welding process GTA welding is ideal for close-tolerance welds Some GTA welds make the critical root pass GTA used when appearance is important

Introduction (continued) Setup of GTA equipment affects weld quality Charts give correct settings Field conditions affect the variables in the charts Experiments designed to evaluate the appearance of a weld After welding in the lab, troubleshooting field welding problems is easier To make a weld is good: to solve a welding problem is better

Torch Angle As close to perpendicular as possible May be angled 0-15 degrees from perpendicular for better visibility As the gas flows out it forms a protective zone around the weld Too much tilt distorts protective shielding gas zone

Figure 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.

Torch Angle (continued) Velocity of shielding gas affects protective zone Low-pressure area develops behind the cup when velocity increases Sharper angle and higher flow rate increases contamination

Filler Rod Manipulation Filler rod must be kept inside the protective zone If filler rod is removed from the gas protection, it oxidizes rapidly Oxide is added to the molten weld pool When a weld is temporarily stopped, the shielding gas must be kept flowing

Filler Rod Manipulation (continued) If the rod tip becomes oxidized, if should be cut off before restarting The rod should enter the shielding gas as close to the base metal as possible An angle less than 15 degrees prevents air from being pulled in the welding zone

Figure 16-2 The hot filler rod end is well within the protective gas envelope. Courtesy of Larry Jeffus.

Figure 16-7 Too much filler rod angle has caused oxides to be formed on the filler rod end. Courtesy of Larry Jeffus.

Tungsten Contamination Most frequent problem is tungsten contamination Tungsten becomes contaminated if it touches: Molten weld pool Filler metal Surface tension pulls the contamination up onto the hot tungsten Extreme heat causes some of the metal to vaporize and form a large oxide layer

Tungsten Contamination (continued) Contamination caused by the tungsten touching the molten pool or filler metal forms a weak weld The weld and tungsten must be cleaned before any more welding can be done Tiny tungsten particles will show up if the weld is x-rayed Contamination can be knocked off quickly by flipping the torch head This procedure should never be used with heavy contamination or in the field

Figure 16-8 Contaminated tungsten. Courtesy of Larry Jeffus.

Current Setting Amperage on the machine's control is the same at the arc when: Power to the machine is exactly correct Lead length is very short All cable connections are perfect Arc length is exactly right Remote current control is in the full on position

Figure 16-10 Melting first occurring. Courtesy of Larry Jeffus.

Figure 16-12 Oxides forming due to inadequate gas shielding Figure 16-12 Oxides forming due to inadequate gas shielding. Courtesy of Larry Jeffus.

Gas Flow Gas preflow and postflow times depend upon: Wind or draft speed Tungsten size used Amperage Joint design Welding position Type of metal welded Maximum flow rates must never be exceeded Air can be sucked into the weld zone

Practice Welds Practice welds are grouped according to the weld position and type of joint Mild steel is inexpensive and requires the least amount of cleaning With aluminum, cleanliness is a critical factor Try each weld with each metal to determine which metal will be easier to master

Low Carbon and Mild Steels Low carbon and mild steel are two basic steel classifications Small pockets of primary carbon dioxide gas become trapped Porosity most likely when not using a filler metal Most filler metals have some alloys, called deoxidizers

Stainless Steel Setup and manipulation are nearly the same as for low carbon and mild steels Most welds on stainless steels show effects of contamination Most common problem is the bead color after the weld Using a low arc current with faster travel speeds is important

Aluminum Molten aluminum weld pool has high surface tension Preheat the base metal in thick sections Preheat temperature is around 300° Fahrenheit Cleaning and keeping the metal clean is time consuming Aluminum rapidly oxidizes at welding temperatures

Metal Preparation Base and filler metals must be thoroughly cleaned Contamination will be deposited into the weld Oxides, oil, and dirt are the most common Contaminants can be removed mechanically or chemically

Figure 16-15 Aluminum filler being correctly added to the molten weld pool. Courtesy of Larry Jeffus.

Figure 16-16 Filler rod being melted before it is added to the molten pool. Courtesy of Larry Jeffus.

Figure 16-18 Surfacing weld. Courtesy of Larry Jeffus.

Figure 16-20 Establish a molten weld pool and dip the filler rod into it. Courtesy of Larry Jeffus.

Figure 16-21 Note the difference in the weld produced when different size filler rods are used. Courtesy of Larry Jeffus.

Figure 16-22 Move the electrode back as the filler rod is added Figure 16-22 Move the electrode back as the filler rod is added. Courtesy of Larry Jeffus.

Figure 16-34 Be sure both the top and bottom pieces are melted Figure 16-34 Be sure both the top and bottom pieces are melted. Courtesy of Larry Jeffus.

Figure 16-35 Oxides form during tack welding. Courtesy of Larry Jeffus.

Figure 16-36 A notch indicates the root was not properly melted and fused. Courtesy of Larry Jeffus.

Figure 16-37 Watch the leading edge of the molten weld pool Figure 16-37 Watch the leading edge of the molten weld pool. Courtesy of Larry Jeffus.

Summary Positioning yourself to control the electrode filler metal and to see the joint is critical Experienced welders realize they need to see only the leading edge of the weld pool Good idea to gradually reduce your need for seeing 100% of the weld pool Increasing this skill is significant advantage in the field Welding in the field may have to be done out of position