© 2012 Delmar, Cengage Learning Chapter 16 Gas Tungsten Arc Welding of Plate
Mar 31, 2015
© 2012 Delmar, Cengage Learning
Chapter 16
Gas Tungsten Arc Welding of Plate
© 2012 Delmar, Cengage Learning
Objectives
• Name the applications for which the gas tungsten arc welding process is more commonly used
• Discuss the effects on the weld of varying torch angles
• Explain why the filler rod end must be kept inside the protective zone of the shielding gas and how to accomplish this
• Tell how tungsten contamination occurs and what should be done when it happens
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Objectives (cont'd.)
• Explain what can cause the actual welding amperage to change
• Determine the correct machine settings for the minimum and maximum welding current for the machine used, the types and sizes of tungsten, and the metal types and thicknesses
• List factors that affect the gas preflow and postflow times required to protect the tungsten and the weld
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Objectives (cont'd.)
• Determine the minimum and maximum gas flow settings for each nozzle size, tungsten size, and amperage setting
• Compare the characteristics of low carbon and mild steels, stainless steel, and aluminum in respect to GTA welding
• Describe the metal preparation needed before GTA welding
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Objectives (cont'd.)
• Demonstrate how to properly make GTA welds in butt joints, lap joints, and tee joints in all positions that can pass the specified standard
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Introduction
• Gas tungsten arc welding– Also called GTA welding
– Can be used to for nearly all types and thicknesses of metal
– Fluxless, slagless, and smokeless
– Welders have fine control of the welding process
– Used when appearance is important
– Setup of equipment affects weld quality• Charts give correct settings• Field conditions affect the variables
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Torch Angle
• Key points– Torch should be held as close to perpendicular as
possible
– May be angled zero to fifteen degrees from perpendicular for better visibility
– As the gas flows out it must form a protective zone around the weld
– Too much tilt distorts protective shielding gas zone
– Velocity of shielding gas affects protective zone as torch angle changes
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Filler Rod Manipulation
• Filler rod must be kept inside the protective zone– If removed from the gas protection
• Oxidizes rapidly: oxide is added to weld pool• Rod tip becomes oxidized: cut it off
• Weld is temporarily stopped– Shielding gas must be kept flowing
• Rod should enter shielding gas as close to base metal as possible– Angles under 15 degrees prevent air from being
pulled in welding zone
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FIGURE 16-2 The hot filler rod end is well within the protective gas envelope. Larry Jeffus
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FIGURE 16-5 Filler being remelted as the weld is continued. Larry Jeffus
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Tungsten Contamination
• Most frequent problem– Tungsten becomes contaminated if it touches
molten weld pool or filler metal
– Surface tension pulls contamination up onto the hot tungsten
– Extreme heat causes some of the metal to vaporize and form a large oxide layer
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FIGURE 16-8 Contaminated tungsten. Larry Jeffus
© 2012 Delmar, Cengage Learning
Tungsten Contamination (cont'd.)
• Contamination forms a weak weld– 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
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FIGURE 16-8 Contaminated tungsten. Larry Jeffus
© 2012 Delmar, Cengage Learning
Current Setting
• Amperage on machine's control is the same at the arc when:– Power to machine is exactly correct
– Lead length is very short
– All cable connections are perfect
– Arc length is exactly right
– Remote current control is in full on position
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Experiments
• Designed to help new welders learn basic skills– Help troubleshoot welding problems
• Learn more– Subtle changes will become more noticeable
– Even experienced welders make changes
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Figure 16-10 Melting first occurring. Larry Jeffus
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Gas Flow
• Gas preflow and postflow times depend upon:– Wind or draft speed
– Nozzle size
– Tungsten size
– Amperage
– Joint design
– Welding position
– Type of metal welded
• Maximum flow rates must never be exceeded
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Practice Welds
• Grouped according to weld position and type of joint– Mild steel
• Inexpensive • Requires the least amount of cleaning
– Aluminum• Cleanliness is a critical factor
– Try each weld with each metal • Determine which metal will be easier to master
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Low Carbon and Mild Steels
• Two basic steel classifications– Most common
– During manufacturing small pockets of primary carbon dioxide gas become trapped
• Do not affect strength
– Porosity: likely when not using a filler metal
– Most filler metals have some alloys (i.e., deoxidizers)
• Prevent porosity caused by gases trapped in base metal
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Stainless Steel
• Setup and manipulation– Nearly the same as for low carbon and mild steels
• Welds show effects of contamination– Precleaning is important
• Most common problem – Bead color after the weld
• Using a low arc current with faster travel speeds is important– Carbide precipitation
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Aluminum
• Molten aluminum weld pool – High surface tension
• Preheat base metal in thick sections– Preheat temperature is around 300 degrees
Fahrenheit
• Cleaning and keeping the metal clean – Time consuming
• Aluminum resists oxidation at room temperature– Rapidly oxidizes at welding temperatures
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FIGURE 16-15 Aluminum filler being correctly added to the molten weld pool. Larry Jeffus
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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
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Summary
• Position yourself to control the electrode filler metal and to see the joint – 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
• Welding in the field – May have to be done out of position