SUDAREA WIG (eng. TIG)

Arcul electric arde intre un electrod de wolfram (nefuzibil) si piesa, intr-o atmosfera protejata (uzual argon, dar si gaze mixte).

Caldura transferata materialului de arcul electric produce topirea locala a metalului de baza si eventual al celui de adaos (se poate suda si fara material adaos).

Gazul de protectie izoleaza baia de sudura si zonele fierbinti ale materialului de aer, respectiv protejeaza electrodul de wolfram impotriva oxidarii.

Conditii de pregatire imbinare sudata:1. curatarea perfecta a suprafetelor rostului de sudare, piesei si a

materialului adaos2. eliminarea oricaror urme de oxizi si grasime3. preincalzirea pieselor de dimensiuni mari (in special cele pe baza de

cupru)4. materialele puternic reactive (afinitate mare fata de oxigen) : titan,

tantal, zirconium necesita protectie suplimentara cu gaz la radacina

Temperatura maxima este in vecinatatea catodului de wolfram si nu poate fi exploatata.

Este interzisa scurtcircuitarea electrodului de wolfram de piesa!

In functie de solicitarea termica a pistoletelor, acestea pot fi racite fortat cu aer sau apa pentru curent de sudare peste 100 A.

Se racesc atat pistoletul cat si cablul de curent. Sistemul este prevazut cu protectie impotriva supraincalzirii (ex. debit insuficient de apa) si decupleza curentul la sursa.

Duza ce formeaza coloana de gaz este confectionata din ceramica si electrodul de wolfram este protejat de gazul de protectie.

Electrodul de wolfram are in partea superioara o teaca de protectie impotriva atingerii accidentale la masa.

Polaritate inversa (cc+).Incalzirea excesiva a electrodului scurteaza durata de viata a acestuia. Risc de topire electrod. Se utilizeaza la sudarea pieselor subtiri cu curenti mici.

ioni gaz

electroni

Se sudeaza cu: curent continuu (cc-) – electrodul de W este catodul (-) iar metalul de baza este anodul (+). Se pot suda: oteluri carbon, oteluri inoxidabile, Ti, aliaje pe baza de Ni, inclusiv aliaje de Al cu protectie de Heliu. curent pulsat: - polaritate directa (cc-), se utilizeaza la sudarea in pozitii diferite (reduce volumul baii de sudare prin controlul energiei termice) si se pot suda table subtiri de metal curent alternativ – se utilizeaza la sudarea aliajelor usoare (aluminiu). Se controleaza mai bine alternanta sudare/sablare a materialului prin ajustarea independenta a perioadelor si amplitudinii curentului

In timpul sudarii cu c.a. amplitudinea tensiunii pe alternante este diferita, datorita diferentei de emisie dintre catod (electrodul de W) si material. Au coeficient de termoemisie diferit (materiale diferite) si sunt la temperaturi diferite!

Emisivitatea electrodului incandescent de wolfram este mult mai mare decat cea a suprafetei “reci” a baii de sudare , asa ca amplitudinea alternantei negative este mult mai mare.

Efectul este o diminuare a efectului de curatire si afecteaza negativ stabilitatea arcului electric.

Acest fapt cauzeaza o dezechilibrare a c.a. si este necesar o filtrare a curentului pentru a stabiliza arcul electric.

Atat intensitatea cat si tensiunea curentului trec prin “zona zero” producand stingerea periodica a arcului electric.

Impulsurile de tensiune sunt necesare pe ambele alternante de curent pentru a amorsa arcul electric dupa fiecare traversare a zonei zero.

Sursele moderne de curent pot modula curentul de sudare si permit setarea duratei impulsului, pauza precum si amplitudinea impulsului in mod independent.

Alternanta pozitiva este redusa sever in amplitudine si perioada, serveste numai ca efect de curatare a piesei (si racire a electrodului) iar alternanta negativa este folosita exclusiv pentru topirea materialului.

Frecventa 60 Hz Frecventa 200 Hz

Echipamentele automate de sudare WIG pot fi echipate cu un pistolet special ce produce o deflectie axiala a arcului electric cu ajutorul unui camp magnetic generat printr-o bobina prin inductie.

Efectul este de alungire a zonei preincalzite in directia de sudare ceea ce permite marirea vitezei de sudare cu cca. 30% la o grosime a materialului mai mica de 2 mm.Aplicatii: sudarea tuburilor subtiri produse din banda

In cazul sudarii unor table lungi (3-4 metri) se poate mari productivitatea prin combinarea procedeului WIG cu sudarea cu plasma. Sudare tandem cu dispozitiv special.

Se pastreaza calitatea specifica sudarii cu plasma (penetrare mare) si se castiga in viteza cu cca. 30-40% asigurand aspectul neted si plat specific sudarii WIG.

APLICATII

APLICATII

Licences for the Australian-invented Keyhole TIG welding technique have been granted to two of the largest stainless steel pipe producers in the United States and Europe, with strong interest from other European, North American and Asian companies.

Until now, the advantages of keyhole welding, with its deep penetration through the thickness of the material, were only available using plasma arc, laser or electron beam facilities - which are high-energy processes, very expensive.

The secret of the Australian keyhole welding method lies in an understanding of the fundamental physics of the welding arc, which allows a balancing of the surface tension of the molten metal against gravity, and the gas and arc pressure of the torch.

This can be carried out in a single pass, instead of the six or seven passes previously required - which represents a dramatic gain in productivity. Welding stainless steel and titanium previously took hours. It can now be automated and completed in minutes."

• Keyhole GTAW

• AISI 304 10.5 mm thick

• Closed square butt

• 50 g/m filler addition

• 1 pass at 300 mm/min.

• Arc-on time 3 min 20 sec /m.

• Conventional GTAW

• AISI 304 10.5 mm thick

• Single V preparation

• 1000 g/m filler addition

• 7 passes at 200 mm/min

• Arc-on time 35 min/m.

Comparing keyhole with conventional GTAW

Keyhole TIG welding technique

• was first discovered at the CSIRO1 in 1996• First successful generation of a GTA keyhole was achieved on 5 mm duplex stainless steel• The first industrial application began late 1997 for the welding of rail wagons• The process is now being used by various industries in Australia, USA, Europe and Korea

• the surface geometry of the high current weld pool results from a dynamic balance between surface tension and arc pressure. Arc pressure is required to inflate the surface, but the stability is low.• High current GTAW research has focused on increasing stability through reduction of the arc pressure• If the arc can open a hole through the plate the surface of the pool can become anchored to both top and bottom surfaces (see soap film below) to form a stable structure. This is achieved in keyhole GTAW.

+ =

Considerations of the need to maintain the weld pool geometry lead to several important conclusions:

– Electrode geometry is critical

– The process is not suited to highly (thermally) conductive materials such as aluminium because the root bead becomes very wide

– As materials become thicker the welding speed must be reduced – otherwise the molten root bead becomes too long.

– When the material becomes too thick surface tension will not support the pool and it will fall.

– Out of position welding presents very significant challenges – although progress has been made.

On the other hand, the process robustness means that:

– There is plenty of scope for further development.

It is generally operated with welding currents above 300 amps and applied to materials between 3 and 12 mm thickness.

Physical illustrations of the keyhole formation

•crater of an abruptly terminated weld,

•longitudinal cross section, and

•macro showing interrupted solidification.

• High quality of GTAW

• Full penetration keyhole mode

• High welding speeds

• Square-edge preparations

• Conventional power sources

• Minimum handling

• Minimum consumables

• Very robust operating characteristics

• Single pass joint completion

Essential•GTAW torch designed for keyhole mode operation•600 to 1000 amp constant current power source •Water cooling for the welding torch•An arc starting unit compatible with the anticipated welding currents•Process mechanisation •Operator control console or pendent•Clean, squared edges with good fit-up (typically < 0.5 mm gaps)

2 mm

0.2 mm

Macrograph of a keyhole GTA weld in 10.5 mm thick AISI 304 stainless

steel plate (as welded) and micrograph of the root region

Macrograph of a keyhole GTA weld in 6.5 mm thick 3Cr12 (12% chromium) steel plate

(as welded). Root region is shown at higher magnification

1 mm

0.5 mm

Macrographs of keyhole GTA weld (top) and conventional GTA weld (bottom)

in 13 mm thick ASTM B265 Grade 2-95a (CP titanium) plate. The

conventional GTA weld was made using matching filler material, a double-V

edge preparation and 6 welding passes.

2 mm2 mm