TIG Welding

97 flashcards by MentisLibrary

GTAW for welders moving up to precision work: tungsten selection and grinding, AC and DC polarity, balance and frequency, shielding gas and back purging, filler choice, torch technique, and welding aluminum, stainless and titanium.

What does GTAW stand for, and what else is the process called?
Gas Tungsten Arc Welding. The shop names are TIG, from tungsten inert gas, and heliarc, from the original helium shielded version.
What makes TIG fundamentally different from MIG and stick?
The electrode is not consumed and carries no filler. Heat and filler are controlled independently, so the welder sets exactly how much of each the joint gets.
Why does TIG run on a constant current power source?
Constant current holds amperage nearly steady even as the welder's arc length varies, so a shaky hand changes the arc length without running away with the heat.
Why does TIG demand cleaner joints than any other arc process?
There is no flux and no slag. Nothing in the process absorbs oil, oxide, paint or moisture, so every contaminant ends up in the weld as porosity or an inclusion.
When is TIG the right process and when is it the wrong one?
Right for thin material, precision, cosmetic welds, aluminum, stainless, titanium and pipe root passes. Wrong when deposition rate matters, on dirty or rusty steel, or outdoors in wind.
What is an autogenous weld?
A weld made by fusing the base metal alone, with no filler added. Common on tight fitting thin joints, tube to tube joints and orbital welding.
Name the three ways a TIG arc is started, and rank them.
High frequency start, which ionizes the gap so the arc jumps without touching. Lift arc, which touches at very low current and lifts to establish the arc. Scratch start, which drags the tungsten across the work.
What does a foot pedal or torch amptrol give the welder?
Live control of amperage during the weld, so heat can be raised to start the puddle, held through the middle, and tapered off to fill the crater.
Air-cooled or water-cooled torch: how do you choose?
Air-cooled torches are lighter and simpler and are typically rated to around 150 to 200 A. Water-cooled torches handle roughly 250 to 400 A and stay comfortable on long high current work.
Name the parts inside a TIG torch that hold the tungsten.
The collet grips the electrode, the collet body seats the collet and directs gas, the back cap seals the tail end and applies clamping pressure, and the cup or nozzle shapes the shielding gas around the arc.
What is a gas lens, and what does it buy you?
A collet body containing layered stainless mesh screens that converts turbulent gas flow into smooth laminar flow.
How are TIG cup sizes numbered?
By the orifice diameter in sixteenths of an inch. A number 6 cup is 6 sixteenths, or 3/8 in. A number 8 cup is 1/2 in.
What determines how far the tungsten can stick out past the cup?
The gas coverage. With a standard collet body, roughly the electrode diameter of stickout is typical. A gas lens supports considerably more because the flow stays laminar.
What does the back cap length change?
Nothing electrically. A long back cap holds a full length electrode, a medium or short cap lets the torch fit into tight spaces but requires the tungsten be cut short to match.
Why is tungsten used for the electrode?
It has the highest melting point of any metal, about 6,192 F, so it carries the arc without melting away into the weld.
What is pure tungsten, and where is it still used?
EWP, banded green. It balls easily and gives a stable arc on AC with older transformer machines, but has poorer current carrying capacity and harder starts than alloyed types.
What is 2 percent thoriated tungsten, and what is the concern with it?
EWTh-2, banded red. An excellent DC electrode with easy starts and long life. The concern is that thorium dioxide is a low level radioactive material, and grinding produces inhalable dust.
What is 2 percent ceriated tungsten used for?
EWCe-2, banded orange. It starts easily at low amperage and is a common choice for thin material, small parts and orbital welding on DC, and it also works on AC.
What is lanthanated tungsten, and why has it become the shop default?
EWLa-1.5, banded gold, and EWLa-2, banded blue. It starts easily, holds a point, carries high current, and works on both AC and DC, so one electrode covers most of a shop's work.
What is zirconiated tungsten used for?
EWZr-1, banded brown. It is an AC electrode that balls readily and resists tungsten spitting into the weld, used where weld contamination cannot be tolerated on AC work.
Which way do you grind a tungsten electrode, and why?
Lengthwise, with the grind marks running parallel to the electrode axis. Grinding across the point leaves circumferential ridges that make the arc wander and dance.
Sharp point, truncated tip or balled end: when do you use each?
A sharp point for low current DC and precise arc placement. A truncated flat on the tip for higher DC current, so the tip does not melt off. A balled end for AC on transformer machines with pure or zirconiated tungsten.
How do tungsten diameter and amperage relate?
Roughly, on DCEN a 0.040 in electrode covers about 15 to 80 A, a 1/16 in covers about 70 to 150 A, a 3/32 in about 150 to 250 A, and a 1/8 in about 250 to 400 A. AC ranges are lower for the same diameter.
What happens when the tungsten touches the puddle or the filler rod?
It is contaminated. The contaminated end must be broken off and the electrode reground before welding continues, and grinding it spreads that contamination to the wheel.
What are the visible symptoms of a contaminated tungsten?
A wandering arc that will not stay under the torch, a hissing or sputtering sound, a dull or blackened electrode tip, and a dirty gray weld with visible dark specks.
Why is a tungsten inclusion a serious defect?
It is a hard, brittle, foreign particle locked in the weld metal. It is a rejectable discontinuity under welding codes and shows on a radiograph as a bright spot, because tungsten is far denser than steel.
Why is 100 percent argon the default TIG shielding gas?
It is inert, dense enough to blanket the puddle, cheap relative to helium, easy to ionize so the arc starts and holds well, and it works on every base metal TIG is used on.
What does adding helium to the shielding gas do?
Helium raises the arc voltage and therefore the heat delivered at a given amperage, which helps on thick aluminum and copper and increases travel speed.
When is an argon hydrogen mix used, and when is it forbidden?
A small hydrogen addition, commonly 2 to 5 percent, increases heat and travel speed and gives a cleaner weld face on austenitic stainless. It must never be used on carbon steel, ferritic or martensitic stainless, aluminum, titanium or copper.
Why can TIG never use carbon dioxide or oxygen bearing shielding gas?
Both are oxidizers. They attack the hot tungsten electrode directly, eroding it and contaminating the weld.