TIG Process (GTAW) and Techniques

The TIG welding process is similar to the braze welding process used with an oxy-fuel gas torch.

It requires more skill than other types of welding.

As a guideline, the TIG welder should be set on the smallest ampere necessary for successful completion of the project.

The process uses a small puddle and heat-affected zone.

No spattter is created since metal isn’t transferred through the arc.

Welding torch positions and how to stop the weld is described below.

TIG welding is used to weld thin metals, for exotic metals and pipeline welding.

It is seldom used for metals over ¼ inch, except for aluminum and magnesium.

When metals are less than 3 /16 inch, they may not require edge preparation.

For metals thicker than 3 /16 inch, edges should be machined or ground so full bead penetration can be achieved.

HTS-2000 brazing rods are an alternative to aluminum TIG welding.

Overview

Before TIG Process welding begins, all oil, grease, paint, rust, dirt, and other contaminants must be removed from the welded areas.

This may be accomplished by mechanical means or by the use of vapor or liquid cleaners.

Striking the arc may be done by any of the following methods:

• Touching the electrode to the work momentarily and quickly withdrawing it.
• Using an apparatus that will cause a spark to jump from the electrode to the work.
• Using an apparatus that initiates and maintains a small pilot arc, providing an ionized path for the main arc.

High Frequency Arc Stabilizers

High frequency arc stabilizers are required when alternating current (AC) is used. They provide the type of arc starting described above.

High frequency arc initiation occurs when a high frequency, high voltage signal is superimposed on the welding circuit.

High voltage (low current) ionizes the shielding gas between the electrode and the workpiece, which makes the gas conductive and initiates the arc. Inert gases are not conductive until ionized.

For DC welding, the high frequency voltage is cut off after arc initiation. However, with ac welding, it usually remains on during welding, especially when welding aluminum.

Description of Manual TIG Process

When welding manually, once the arc is started, the TIG torch is held at a travel angle of about 15 degrees. For mechanized welding, the electrode holder is positioned vertically to the surface.

To start manual welding, the arc is moved in a small circle until a pool of molten metal forms. The establishment and maintenance of a suitable weld pool is important and welding must not proceed ahead of the puddle. Once adequate fusion is obtained, a weld is made by gradually moving the electrode along the parts to be welded to melt the adjoining surfaces. Solidification of the molten metal follows progression of the arc along the joint, and completes the welding cycle.

The welding rod and torch must be moved progressively and smoothly so the weld pool, hot welding rod end, and hot solidified weld are not exposed to air that will contaminate the weld metal area or heat-affected zone. A large shielding gas cover will prevent exposure to air. Shielding gas is normally argon.

Welding Rod Angle for TIG Process

The welding rod is held at an angle of about 15 degrees to the work surface and slowly fed into the molten pool. During welding, the hot end of the welding rod must not be removed from the inert gas shield. A second method is to press the welding rod against the work, in line with the weld, and melt the rod along with the joint edges. This method is used often in multiple pass welding of V-groove joints. A third method, used frequently in weld surfacing and in making large welds, is to feed filler metal continuously into the molten weld pool by oscillating the welding rod and arc from side to side. The welding rod moves in one direction while the arc moves in the opposite direction, but the welding rod is at all times near the arc and feeding into the molten pool. When filler metal is required in automatic welding, the welding rod (wire) is fed mechanically through a guide into the molten weld pool.

TIG Process Welding Position

The selection of welding position is determined by the mobility of the weldment, the availability of tooling and fixtures, and the welding cost. The minimum time, and therefore cost, for producing a weld is usually achieved in the flat position. Maximum joint penetration and deposition rate are obtained in this position, because a large volume of molten metal can be supported. Also, an acceptably shaped reinforcement is easily obtained in this position.

Good penetration can be achieved in the vertical-up position, but the rate of welding is slower because of the effect of gravity on the molten weld metal. Penetration in vertical-down welding is poor. The molten weld metal droops, and lack of fusion occurs unless high welding speeds are used to deposit thin layers of weld metal. The welding torch is usually pointed forward at an angle of about 75 degrees from the weld surface in the vertical-up and flat positions. Too great an angle causes aspiration of air into the shielding gas and consequent oxidation of the molten weld metal.

Joints that may be welded by this process include all the standard types, such as square-groove and V-groove joints, T-joints, and lap joints. As a rule, it is not necessary to bevel the edges of base metal that is 1/8 in. (3.2 mm) or less in thickness. Thicker base metal is usually beveled and filler metal is always added.

The gas tungsten arc welding process can be used for continuous welds, intermittent welds, or for spot welds. It can be done manually or automatically by machine.

Gas Flow

Set the gas flow at the recommended volume for the gas nozzle and metal size. The same holds true for the amperage setting of the welding machine and type of current being used.

Operating Variables

The major TIG Process operating variables summarized briefly are:

1. Welding current, voltage, and power source characteristics.
2. Electrode composition, current carrying capacity, and shape.
3. Shielding gas–welding grade argon, helium, or mixtures of both.
4. Filler metals that are generally similar to the metal being joined and suitable for the intended service.

TIG Welding

TIG Technique

1. First check the recommended gas flow based on the nozzle selected and the metal size.
2. Next, adjust the gas welding machine to the suggested type of current and amperage for the TIG process being performed.
3. Turn the welder on and locate the foot control. Put the control in an area which is most comfortable.
4. Press the foot control and strike the arc.
5. After a puddle is formed, add the filler rod to the leading edge.
6. To reduce the chance that you contaminate the tungsten, move the TIG welding torch to the back of the puddle when the filler rod is added.
7. Clean all metal surfaces and joints that are to be welded using a chemical or mechanical process (scraping, grinding, brushing with a wire brush). Note that the wire brush should be made out of the same material as the material being welding. (exception is aluminum which can be cleaned with a stainless steel brush).
   Chemical solvents ranch from harsh chemicals such as acetone (used to remove oxidation and rust from steel) and the light cleaning properties of alcohol.
   The TIG process itself is the same for all types of welding.
8. TIG Welding and Metal Size
   1. With the exception of magnesium and aluminum, the TIG process should not used with metals over ¼ inch.
   2. Metals under 3/16 of an inch may not need edge prep.
   3. Metals that have a thickness over 3/16 of an inch require machined or ground edges to help bead penetration.

TIG Process Joints

Square Butt Joint

For a square butt joint, maintain the tungsten electrode in the centerline of the metals that are being joined together.

Recommended square butt joint TIG process welding angles:

• 20 to 30 degree electrode angle
• 90 degree work angle
• 60 to 70 degree dray angle

Next strike the arc. Hold tungsten about 1/8 inch above the base metal.

After the puddle forms, add filler rod at the leading edge of the puddle. Then move the bead forward as quickly as possible.

T-Joints and Lap Joints

Flat Position Welding:

• When in the flat position and joining metals using lap joints and T-joints, tack weld the base metals every 3 inches.
• Joints should be set so that the resulting welds are made in the flat position.
• The torch should be held at a:
  • 60 to 70-degree drag angle
  • 10 to 20-degree work angle
• Point the electrode toward the horizontal edge to be welding rather than the vertical edge.
• Strike the arc and allow the puddle to form.
• A “C”-shaped puddle should develop indicating that both edges of the metal are melting.
• Hold the tungsten electrode approximately 1/8 inch above the base metal.
• When the puddle forms, move the electrode toward the rear of the puddle and then add the filler rod to the front of the puddle.  Then, move the electrode back to the middle of the puddle.
• Repeat this TIG process as you move the bead forward.
• When the end is reached, move the electrode toward the rear of the puddle to fill the crater with the filler rod and then withdraw it from the weld zone.
• Raise the TIG welding torch slowly to provide a gas shield while the puddle solidifies.

Horizontal Position Welding

The drag angle of the torch should be:

• The drag angle of the torch should be 60 to 75 degrees
• The work angle should be a 15 to 30 degree angle. Maintaining a 15 to 30 degree work angle will help the force of the arc to keep the puddle from drooping.

To prevent any molten metal from sagging, maintain a smaller puddle than that used in the flat position.

Add filler rod at the upper edge of the puddle to help prevent sagging.

How to Stop Welding

Welding is stopped by shutting off the current with foot-or-hand-controlled switches that permit the welder to start, adjust, and stop the welding current.

They also allow the welder to control the welding current to obtain good fusion and penetration.

Welding may also be stopped by withdrawing the electrode from the current quickly, but this can disturb the gas shielding and expose the tungsten and weld pool to oxidation.

Shielding gas should stay on for a few seconds in order to keep the molten puddle, filler rod and tungsten electrode from becoming contaminated.

Filler Metals

The base metal thickness and joint design determine whether or not filler metal needs to be added to the joints. When filler metal is added during manual welding, it is applied by manually feeding the welding rod into the pool of molten metal ahead of the arc, but to one side of the center line.

The technique for manual TIG welding is shown in figure 10-34.