Gas Tungsten Arc Welding TSM 233 Unit 7

What is GTAW? A welding process in which an electric arc is struck between a non- consumable tungsten electrode and the base metal. An inert gas is used to shield the weld area, filler metal may/or may not be used.

What are the basic components of the GTAW process? Welder, ac/dc with high frequency capability Shielding gas Gas Flowmeter and regulator. Leads Welding torch (Electrode holder) Ground clamp Tungsten electrode

What are some specific safety precautions for GTAW? Produces very intense ultra- violet rays, greater than other forms of arc welding. Need to use number 10 or 12 shade lens Somewhat greater chance of electrical shock, because of the high frequency electrical current used.

How does the GTAW process produce heat? Similar to SMAW, in that electrical resistance produces heat. Welder works at a low voltage, pushes large amounts of electricity, across a gap. Arc is stabilized by the inert shielding gas. Voltage drops as the arc length decreases. Uses high frequency, stabilizes arc, ionizes the shielding gas.

What are the key variables when GTAW? type of metal, kind, thickness, weld-ability type of weld, characteristics, position, types of tungsten electrodes type of shielding gas filler welding rod. types of electrical current, amperage range. welder characteristics, speed, skill, electrode angles.

What are the characteristics of the different types of electrical current used for GTAW? AC –alternating current, high frequency, used for Aluminum and Magnesium DC – direct current DCEN – electrode negative, straight polarity. generally preferred for GTAW, as electrodes would last longer. Used for ferrous metals, stainless steel, and most metals.

Alternating Current Alternating current flows from positive to negative. Rectification - the resistance of electrons to flow to point. (hence a ball shaped W electrode when using a pure tungsten electrode for AC). High Frequency Alternating Current is used to maintain a ionization of the Argon or Helium shielding gas. Generally used continuously for AC, and for starting DC. AC-HF is a rapid cycle, high voltage, low amperage electrical flow that is used maintain the ionized Argon gas during the electrode positive side of the AC wave.

Square wave forms/patterns. The AC wave pattern is “squared off”. The change from positive to negative, and from negative to positive is almost instant. This allows for: a more stable arc, better ionization of the argon gas, and better cleansing action of the oxides on the base metal.

Balanced/Unbalanced Wave Patterns Balanced/unbalanced arc waves allows for setting the AC wave patterns. A setting is provided on some GTAW machines that allows for the changing the percentage of + and - electron flows of the AC wave. More electrode negative concentrates the heat more to the base metal, provides greater penetration More electrode positive provides greater cleansing action on the base metals, but the Tungsten electrodes burn faster. Note: these actions are opposite from SMAW due to the shape of the Tungsten electrode. (Note: on the adjustable GTAW welders we use, it varies from about 30% electrode positive to about 70% electrode positive.)

Pulsed wave. Pulsed wave patterns can be created for using DC electrical current for GTAW. The current is reduced as a part of the DC pattern. Peak Current is the high part of the wave pattern, used for the actual welding. Background Current is the low part of the wave pattern, used to maintain the arc as the weld puddle cools. This allows for cooling of the weld puddle during DC GTAW and better welds especially for thin metal and out-of-position welds. Also used for stainless steel, which tends to burn through.

Upslope and Downslope Current Some GTAW machines can set the start and stopping current levels and time periods. Upslope Current is the starting current, from 0 up to the welding current over a period of time (seconds) Downslope Current is stopping current, welding current down to 0 over a period of time (seconds). (Note: the GTAW welders we use do not have this as an adjustable feature)

How are different types of electrodes? Tungsten electrodes, EWP - Use pure tungsten for Al. and Mg., green color coding, create a ball on the end, used with AC-HF current. EWTh-1, EWTh-2 - Tungsten w/ 1% or 2% Thoria (Thorium oxide ThO2) added, called “Thoriated electrodes”, used for steels and alloys, use a pointed end, use with DCEN, straight polarity, 1% yellow, 2% red color coding  EwCe-2 Tungsten w/2% Cerium, all purpose electrode, works for all materials and currents, recommended for many of the newer inverter GTAW machines, orange color coding. This electrode is replacing both EWP and EWTh electrodes. Be sure to properly prepare the electrodes, grinding from the end, creating a correct size point, then if using a EWP with AC-HF, create a desired sized ball on the end.

Types of shielding gases used? Two main gases: Argon (Ar), widely used, heavier than He, greater arc stability, generally preferred. Helium, (He), lighter, sometimes used for semi-automatic production processes. Ar and He, mixtures, used for production welding processes. Set the recommended flow rates, 15-20 cfh, not enough gas flow leaves smoke and oxide deposits, too much gas flow can cause turbulence which may produce unstable arcs. Important to get pre-flow and post-flow of the shielding gas to protect the electrode and the weld area. Some welds, particularly titanium require a gas shield on the “underside” of the weld, called “backflow”.

How to start the arc?   Important not to contaminate the tungsten electrode, start the gas flow, set the electrode holder on it side, turn over to create the arc. High frequency helps the start, use the amperage manual control, either thumb or foot pedal, starts the gas flow. Good to use a copper strike plate.

References: Modern Welding textbook: Chapters 13 and 14. On-line GTAW Manual developed by Miller Welding, pdf. File: http://www.millerwelds.com/pdf/gtawbook.pdf