Unit 215 – Welding by TIG Process TIG Welding

Process Variables The variables associated with TIG welding are: Shielding gas selection Gas flow Tungsten electrode type Filler wire Electrical polarity Amperage

Shielding Gas Argon and helium are both inert gases. Hydrogen is an active gas. Hydrogen and helium increase the heat input, producing a hotter weld with deeper penetration. Helium is a light gas, so flow rates will need to be increased. Shielding gases Type Uses Pure Argon LCS, Stainless steel, aluminium Argon/Hydrogen Austenitic stainless steel Argon/Helium Aluminium Helium

Shielding Gas Flow Rate Flow rates are measured in litres per minute (L/pm) Too high a flow rate will cause turbulence and draw surrounding air into the weld pool causing defects. Too low a flow rate will not protect the molten pool from the surrounding atmosphere. The diameter of the ceramic shroud influences the flow. The correct diameter shroud should be selected to suit. Ceramic shrouds are often identified by a number. This number should be multiplied by 1.6 to give the outlet diameter in mm.

Tungsten Electrodes The diameter of the electrode will limit its current (amps) carrying capacity. Too small an electrode will overheat. Too large an electrode will limit arc heat and lead to poor penetration.

Current Capacity Current is measured in amps Amperage ranges shown are only a guide Electrode dia. (mm) Shroud bore (mm) Amperage range DC AC Thoriated Zirconiated 1.6 10 45 - 65 45 - 70 2.4 90 - 120 110 - 150 3.2 13 100 - 160 100 - 140

Electrode Sharpening It is important to sharpen tungsten electrodes correctly. Incorrectly ground electrodes can cause arc wander. Machines are available to ensure accurate grinding.

TIG Torch

TIG Wire Selection TIG electrode wire is selected to suit the joint Material of the wire matches the material being welded Various diameters are available Ø1, Ø1.6, Ø2.4, Ø3.2 As a guide, the diameter of the wire should be slightly smaller than the metal thickness being welded.

Distortion As a weld cools it can cause distortion. Being aware of distortion and trying to reduce it can save a lot of work. One way to reduce the effects of distortion is called pre-setting. Before welding After welding

Pre-setting The amount of pre-set can’t be calculated. Before welding After welding The amount of pre-set can’t be calculated. It comes from experience.

Distortion Control Distortion can also be controlled by restraint. Restraint means holding the metal and not letting it move. Clamping is a simple way to restrain distortion

Distortion Control Jigs and fixtures not only hold the pieces to be welded in the correct position they also help restrain distortion During welding this bar will try and distort in the direct of the arrows. Jig holds this bar at 900 during welding.

Distortion Control Other methods of reducing distortion include: Tacks: Run on/run off plates Sufficient tacks also helps reduce distortion

Weld Positions Weld positions are identified by a standard code. 2 others are used: PF – vertical up PG – vertical down

Angle between filler wire and torch should be 900. Torch Angles Tilt angle = 450 Slope angle = 75 - 800 Tilt angle = 900 Slope angle = 75 - 800 Angle between filler wire and torch should be 900.

Arc Striking Striking the arc can be achieved in 3 ways: Scratch start: The electrode is scratched against the surface of the plate and then pulled away to form the arc. This method can contaminate the weld and damage the electrode. Lift start: The electrode is placed against the plate, power button pressed and then lifted to start the arc. No contamination of the weld metal occurs. High frequency (HF) start: When the power button is pressed a high frequency spark ionises the air gap. Ionisation causes the air to conduct electricity and the arc is formed. No contamination to the weld metal occurs.

Welding Strike arc at start of joint. Welding direction Stand off approx. 6mm Strike arc at start of joint. Crater fill: at the end of the weld just back up slightly to fill the end crater Stop/starts: restart arc here

Welding

Post Weld Visually check welds for any defects Welds should be wire brushed on completion. The weld itself shouldn’t be cleaned up using a grinder. Spatter needs to be removed, usually with a hammer and chisel.

Hot Metal Hot metal is a common hazard in the welding workshop. To protect yourself and others make sure: All hot metal is marked as “HOT”, with date and time included. Only carry metal using tongs. Quench (dip in water) to cool down unless the weld is going to be tested. Good practice is to treat every piece of metal as hot, don’t pick it up unless you are sure its cool!

Good Housekeeping At the end of the shift or lesson it is important to leave the work area in a safe state. This includes: isolating the welding set turning the shielding gas off returning all tools to storage area dispose of any scrap material generally tidy up the area.