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 adaos
2.
eliminarea oricaror urme de oxizi si grasime
3.
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.
electroni
ioni gaz
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
APLICATII
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
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."
Comparing keyhole with conventional GTAW
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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.
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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.
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.
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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.
Essential
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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
•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)
0.2 mm
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
1 mm
0.5 mm
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
2 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 mm