8/13/2019 Equipment for TIG Welding

1/4

Equipment for TIG Welding

Job Knowledge for Welders No. 6 describes the TIG welding process. Using aninert gas shield instead of a slag to protect the weldpool, this technology is a

highly attractive alternative to gas and an!al etal arc welding and has playeda a"or role in the acceptance of high #!ality welding in critical applications.

Essential equipment

In TIG, the arc is fored between the end of a salldiaeter t!ngsten electrode and the wor$piece. Theain e#!ipent coponents are%

power so!rce

torch

bac$ing syste

protective e#!ipent

Power source

The power so!rce for TIG welding can be either &' or (' b!t in both the o!tp!tis tered a drooping, or constant c!rrent, characteristic) the arc voltage * weldingc!rrent relationship delivers a constant c!rrent for a given power so!rce setting.If the arc voltage is slightly increased or decreased, there will be very littlechange in welding c!rrent. In an!al welding, it can accoodate the welder+snat!ral variations in arc length and, in the event of the electrode to!ching thewor$, an ecessively high c!rrent will not be drawn which co!ld f!se the

electrode to the wor$piece.

The arc is !s!ally started by - /-igh re#!ency0 spar$s which ionise the gapbetween the electrode and the wor$piece. - generates airborne and linetransitted interference, so care !st be ta$en to avoid interference with controlsystes and instr!ents near welding e#!ipent. When welding is carried o!t insensitive areas, a non1- techni#!e, to!ch starting or +lift arc+, can be !sed. Theelectrode can be short circ!ited to the wor$piece, b!t the c!rrent will only flowwhen the electrode is lifted off the s!rface. There is, therefore, little ris$ of theelectrode f!sing to the wor$piece s!rface and foring t!ngsten incl!sions in theweld etal. or high #!ality applications, !sing - is preferred.

DC power source

&' power prod!ces a concentrated arc with ost of the heat in the wor$piece,so this power so!rce is generally !sed for welding. -owever, the arc with itscathode roots on the electrode /&' electrode negative polarity0, res!lts in littlecleaning of the wor$piece s!rface. 'are !st be ta$en to clean the s!rface priorto welding and to ens!re that there is an efficient gas shield.

http://www.twi.co.uk/j32k/protected/band_3/jk17.html#tag1%23tag1http://www.twi.co.uk/j32k/protected/band_3/jk17.html#tag2%23tag2http://www.twi.co.uk/j32k/protected/band_3/jk17.html#tag3%23tag3http://www.twi.co.uk/j32k/protected/band_3/jk17.html#tag4%23tag4http://www.twi.co.uk/j32k/protected/band_3/jk17.html#tag1%23tag1http://www.twi.co.uk/j32k/protected/band_3/jk17.html#tag2%23tag2http://www.twi.co.uk/j32k/protected/band_3/jk17.html#tag3%23tag3http://www.twi.co.uk/j32k/protected/band_3/jk17.html#tag4%23tag4

8/13/2019 Equipment for TIG Welding

2/4

Transistor and inverter power so!rces are being !sedincreasingly for TIG welding. The advantages are%

the saller si2e a$es the easily transported

arc ignition is easier

special operating feat!res, e.g. c!rrent p!lsing, arereadily incl!ded

the o!tp!t can be pre1prograed for echanised

operations

The greater stability of these power so!rces allows very low c!rrents to be !sedpartic!larly for icro1TIG welding and largely replaced the plasa process foricro1welding operations.

AC power source

or aterials s!ch as al!ini!, which has a tenacio!s oide fil on thes!rface, (' power !st be eployed. 3y switching between positive andnegative polarity, the periods of electrode positive will reove the oide andclean the s!rface.The fig!re shows c!rrent and voltage wavefors for /sine wave0 (' TIG welding.

&isadvantages of conventional, sine wave (' copared with &' are%

the arc is ore diff!se

- is re#!ired to reignite the arc at each c!rrent reversal

ecessive heating of the electrode a$es it ipossible to aintain a

tapered point and the end becoes balled

4#!are wave (', or switched &', power so!rces are partic!larly attractive forwelding al!ini!.3y switching between polarities, arc reignition is ade easier so that the - canbe red!ced or eliinated. The ability to ibalance the wavefor to vary theproportion of positive to negative polarity is iportant by deterining the relativeao!nt of heat generated in the wor$piece and the electrode.

To weld the root r!n, the power so!rce is operated with the greater ao!nt ofpositive polarity to p!t the ai! heat into the wor$piece.

or filler r!ns a greater proportion of negative polarity sho!ld be !sed to iniiseheating of the electrode. 3y !sing 57 negative polarity, it is possible to aintaina pointed electrode. ( balanced position /87 electrode positive and negativepolarities0 is preferable for welding heavily oidised al!ini!.

Torch

8/13/2019 Equipment for TIG Welding

3/4

There is a wide range of torch designs for welding, according to the application.&esigns which have the on*off switch and c!rrent control in the handle are oftenpreferred to foot controls. 4pecialised torches are available for echanisedapplications, e.g. orbital and bore welding of pipes.

Electrode

or &' c!rrent, the electrode is t!ngsten with between 9 and 87 thoria to aid arcinitiation. The electrode tip is gro!nd to an angle of 6 to 5 for an!alwelding, irrespective of the electrode diaeter. or echanised applications asthe tip angle deterines the shape of the arc and infl!ences the penetrationprofile of the weld pool, attention !st be paid to consistency in grinding the tipand chec$ing its condition between welds.

or (' c!rrent, the electrode is either p!re t!ngsten or t!ngsten with a sallao!nt /!p to .870 of 2irconia to aid arc reignition and to red!ce electrode

erosion. The tip norally ass!es a spherical profile d!e to the heat generatedin the electrode d!ring the electrode positive half cycle.

Gas shielding

( gas lens sho!ld be fitted within the torch no22le, to ens!re lainar gas flow.This will iprove gas protection for sensitive welding operations li$e weldingvertical, corner and edge "oints and on c!rved s!rfaces.

Backing sstem

When welding high integrity coponents, a shielding gas is !sed to protect the!nderside of the weld pool and weld bead fro oidation. To red!ce the ao!ntof gas cons!ed, a localised gas shro!d for sheet, das or pl!gs for t!b!larcoponents is !sed. (s little as 87 air can res!lt in a poor weld bead profile anday red!ce corrosion resistance in aterials li$e stainless steel. With gasbac$ing systes in pipe welding, pre1weld p!rge tie depends on the diaeterand length of the pipe. The flow rate*p!rge tie is set to ens!re at least fivevol!e changes before welding.

4tic$ on tapes and ceraic bac$ing bars are also !sed to protect and s!pport theweld bead. In an!al stainless steel welding, a fl!1cored wire instead of a solid

wire can be !sed in the root r!n. This protects the !nderbead fro oidationwitho!t the need for gas bac$ing.

Inserts

( pre1placed insert can be !sed to iprove the !nifority of the root penetration.Its ain !se is to prevent s!c$1bac$ in an a!togeno!s weld, especially in theoverhead position. The !se of an insert does not a$e welding any easier and

8/13/2019 Equipment for TIG Welding

4/4

s$ill is still re#!ired to avoid probles of incoplete root f!sion and !neven rootpenetration.

Protecti!e equipment

( slightly dar$er glass sho!ld be !sed in the head or hand shield than that !sedfor ::( welding.

;ecoended shade n!ber of filter for TIG welding%

Shade number Welding current A

9 less than 20

10 20 to 40

11 40 to 100

12 100 to 175

13 175 to 250

14 250 to 400