Mags welding theory.docx

7/26/2019 Mags welding theory.docx

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Belfast Metropolitan College

MIG/MAG Training Programme

Learning & Assessment Package


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Welding terms

MAGS Metal Arc Gas Shielded (covers all shielding gases MIG & MAG)

MIG Metal Inert Gas (covers totally inert gases e.g. Argon and/ or Helium)

MAG Metal Active Gas (covers gas mixtures that are not totally inert e.g.Argon + !"# or Argon + Hydrogen or Argon + !xygen etc)

GMAWGas Metal Arc $elding (American term %or MAGS elding)

TAGS 'ungsten Arc Gas Shielded (covers all shielding gases 'IG & 'AG)

TIG 'ungsten Inert Gas (covers totally inert gases e.g. Argon and/ orHelium)

TAG 'ungsten Active Gas (covers gas mixtures that are not totally inert e.g.Argon + Hydrogen or itrogen or itrogen + Argon etc)

GTAW Gas 'ungsten Arc $elding (American term %or 'AGS elding)


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MAGS Welding

'he MAGS elding rocess as %irst develoed in *,- %or the urose o% eldingaluminium lates. 'his tye o% elding is a re%inement o% the 'AGS rocess and uses a

consumale ire electrode# hich relaces the %ixed 'ungsten electrode o% the 'AGSrocess.

In *0# the introduction o% the constant otential tye1elding machine oened urand ne elding alications %or the MAGS rocess.

'he advent o% the constant otential tye elding machines as a signi%icantadvancement %or the MAGS rocess. 2or the %irst time the amerage could echanged over ide ranges ith very little change in the arc voltage. 'his led to thedeveloment o% ire %eed e3uiment ith constant seed drive motors. Instead o% theire %eed seed varying# as it had to do ith the conventional tye elding machines.'he constant otential elding machines ma4e the amerage correction to maintain

the re1set voltage. 'he oer suly unit must e the constant otential tye.

5nits normally comrise o% a 6.. generator or A.. recti%ied trans%ormer. 'he recti%iedtrans%ormer is a device that ermits current %lo in one direction only. Its main %unctionis to change A.. to 6.. It is usual ith units to rovide a voltmeter and ammeter %orvoltage and amerage ad7ustment uroses# and voltage and ire %eed controllers.'he volt and ammeters urose is rimarily to assist the oerator in setting u thecorrect elding conditions %or the elding alications.

'he voltmeter is read in di%%erent ays deending on the conditions. 2or examle# thevoltmeter ill read 8!8 hen the elder is energised ut not eing used. It ill readoen circuit voltage (!9) hen the trigger is actuated ut no arc struc4. It ill read

the 8load voltage8 hen the arc is initiated. 'he 8arc8 voltage is only measured at thearc.

'his tye o% elding is a re%inement o% the 'AGS rocess and uses a consumale ireelectrode# hich relaces the %ixed 'ungsten electrode o% the 'AGS rocess. In theMAGS rocess# the heat generated y an arc %ormed eteen the end o% aconsumale %iller ire and the or4iece is used to %use the 7oint area. 'he %iller ire is%ed continuously through a contact tue or ti. 'he arc is %ormed in an inert gas# hichrevents oxidation o% the eld# assists in cleaning# and determines the heatingcharacteristics o% the arc and the mode o% trans%er.


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MAGS Welding Equipment

'he comonent arts o% a MAGS system are: 1

;oer suly unit.

$ire %eeder unit.

2lexile lead (tue) assemly and return cale.

$elding gun.

Gas suly system

The Poer Suppl! "nit

'he oer suly unit must e o% the onstant ;otential tye.

2or %ree %light or sray trans%er the unit should e caale o% delivering u to ,


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Wire #eeder "nit

'he electrode ire reel or coil is mounted onto a sindle or sider hu# eitherhori>ontally or vertically as re3uired. 'he hu is %ree to rotate as the ire drive unit

dras o%% the ire.An ad7ustale ra4ing device is incororated in the assemly to revent overrun o% theelectrode ire hen the motor o% the ire drive unit is sitched o%%. 'o rotect theelectrode ire %rom dust and contamination# the reel assemly is usually enclosed y acover or laced ithin the control unit cainet. A cover is essential hen usingaluminium alloy ires.

Wire #eeder

'his may either e a ush or a ull tye or comined.

Push T!pe

In the ush tye# the mechanism consists o% to or more %eed rolls here the gri orressure can e ad7usted. 'his method o% %eeding is used %or ase so%t ire or adiameter not less than *."mm and o% not less than


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Pull T!pe

'he ull tye consists o% a drive usually uilt into the handle o% the elding gun. 2eedrolls ull ire o%% a small reel attached to the gun. In the comined method# ush andull %eed units are used. !ne is mounted near the electrode ire reel assemly# andthe other in the elding gun.

Spool $n Gun


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#le%ile Lead Asseml!

'his ill deend uon the tye o% e3uiment used. It normally contains: 1

ale %or the elding current.

ontrol cale to lin4 the sitch on the gun ith the control unit.

Hose to convey shielding gas.

onduit to convey electrodes ire (excet in the case o% the reel on gunarrangement).

Hoses to convey cooling ater (i% used).

Hose to convey air to drive ire %eed turine (i% used)


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Welding Torch

2or di trans%er and ulse trans%er elding# the electrode holder normally has a curvednec4. It is 4non as a elding gun since it resemles the shae o% an oxy1acetylene

elding no>>le and incororates a utton or lever1oerated sitch. $here a ater1cooled gun or gun is re3uired# there is usually a mechanism that cuts o%% electricaloer should the ater %lo cease.

2or %ree %light trans%er elding (sray trans%er) ith %lux1cored ires *."mm diameter orlarger# or here a ull %eed unit is incororated# the e3uiment is generally istol

shaed# has a trigger tye sitch and is re%erred to as a elding gun. It can e atercooled hen re3uired %or use ith currents in excess o% 0


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The 'onstant Potential Welding Machine

'he constant otential tye1elding machine has a relatively %lat volt1amere curve andmuch loer maximum oen circuit voltage than a conventional elder. It ermits the

elding machine to reach elding voltage ithout the delay in time involved ith ahigher oen circuit voltage. 'he resonse time o% the constant otential eldingmachines ith the %lat voltage1amere curve is very %ast. 'he loer oen circuitvoltage used ith this tye o% elder reduces the ossiility o% a 8last8 o% high voltagecurrent at the eld start. Such a last o% current could cause urn ac4 o% theelectrode into the contact tue o% the gun.

onstant ;otential 'ye ;oer Source (having a %lat characteristic) used in MAGSelding

'ontactor 'ontrol

All constant otential elding machines have uilt in rimary contactors as standarde3uiment. 'he %unction o% the rimary contractor is that o% a sitch. It loc4s the %loo% current to the rimary coil o% the main elding trans%ormer hen in the 8oen8osition. $hen the contactor coil is energised it 8closes8 and comletes the circuit in

the rimary section o% the elding machine. Something must trigger the rimarycontactor so that its coil ill energise and cause the contactor to close. 'his %unction iser%ormed hen the trigger sitch is actuated on the gun.


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Wire #eed Mechanism

?ocate the reel o% ire on hu or sindle so that ire ill e dran o%% in the correctdirection. 6o not release the reel inding at this stage.

?oosen ra4ing mechanism so that reel runs %reely. 'hen tighten 7ust su%%iciently torevent overrun o% sool hen ire is dran o%%.

2it correct si>e inlet guide to ire %eed mechanism.

@elease the end o% the ire %rom the reel inding# ut do not allo ire to ecomeloose on the reel.

ut o%% the 4in4ed end o% ire cleanly# ma4ing sure that the cut end is not 7agged orurred.

'hread the ire through the oened ire %eed mechanism.

lose the ire %eed rolls so that the ire is gried and ad7ust ressure in accordanceith manu%acturers instructions.

$ith the reel1on1gun assemly# the ire can no e 8inched8 through the gun no>>lehen the current is sitched on.

A%ter ma4ing sure that all hose assemly connections other than the ire conduitconnections have een made# %it the correct si>e outlet guide to the ire conduit.

'hread the outlet guide over the electrode ire rotruding %rom the ire %eedmechanism and loc4 the guide in osition.

'he ire can no e 8inched8 through the conduit to the gun contact tue hen thecurrent is sitched on.

Ad7ust the osition o% contact tue end/ti and %it gas no>>le.


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Welder(Manipulator 'ontrolled )ariales

'he main di%%erence eteen MAGS and 'AGS elding is that a ider range o%shielding gasses is availale ith MAGS

'he main variales (alicale to MAGS elding techni3ues) are: 1

$elder controlled variales. (Seed o% travel# techni3ue and angle# 7oint setu)

Machine1controlled variales (arc voltage# elding current# ire %eed seed# sel%1ad7usting arc length)

Welder controlled *ariales

'hese include rotrusion o% electrode ire# no>>le and ire seed %eed. 'he eldercan ad7ust all o% these variales# ut in doing so# the need %or machine1controlled

variales ad7ustments occurs.

'he three main gun angles determine the enetration o% the arc column. I% signi%icantchange needs to e made in enetration characteristics# machine variales o%amerage or voltage control should e used# or the original selection o% electrode si>eshould e revieed.

In the drag attern# hich is the standard movement# the gun is moved in a straight lineithout oscillation# and ithout touching the or4iece.ote: 'his attern is mainly alicale to or4ieces in the %lat osition.

2or out1o%1osition or4 the hi attern# eaving attern# or 858 attern may ere%erred.

'he hi and 858 atterns are articularly suitale %or eld ool maniulation.

'o ma4e a cover ass# the %igure1o%1eight attern is used. 'his attern is not suitale%or tac4 elding.


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Work piece )ariales

'hese are dictated y the comosition o% the arent metal hich determines the tyeand amount o% shielding gas# the electrode ire tye and si>e# the elding osition andthe mechanical roerties o% the %inished eld.

$e can also use ac4ing ars and other aids mainly %or elding and utt 7oint.


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'ontrol o+ the Process

'he main controlling arameters in MAGS elding ith a constant otential oersource and continuous ire %eed system are: 1

$ire 2eed Seed

Arc 9oltage

Inductance

'he rate o% ire %eed must match the rate at hich the %iller is melted# and this urn o%%rate is related to current.

I% a constant otential oer source is used# the or4ing voltage is reset and thecurrent otained varies to suit the arc characteristics. I% the reset voltage is too high#

or the ire %eed seed is too lo# the arc length may ecome excessive# and the %illerire may urn ac4 onto the contact ti. I% the ire %eed seed is too high# or the arcvoltage is too lo# the unmelted %iller ire short circuits the arc ga causing an e%%ect4non as stuing.

'he inductance control enales the amount o% inductance in the outut circuit o% theoer source to e varied# to minimise satter and give a smoother arc hen eldingin the di1trans%er mode. As elding current is increased# it ill e necessary to usemore inductance.

6i trans%er# or short1circuiting arc techni3ue is a mode o% metal trans%er in hich thearcing eriod and short circuiting eriod alternate raidly. 'he rate o% this se3uence is

4non as the di trans%er rate# or di %re3uency# is normally eteen < and "ed ya high1itched u>># and minimum# light satter. 'he inductance also controls heatinut to the eld# and around the otimum setting an increase in inductance ill resultin a hotter eld# and vice versa.

'he lo inductance# characteri>ed y a criser sound arc# gives minimum heat inutand ell1de%ined riles on the cooled eld.

ontrol o% the inductance thus enales the oerator to select the est conditions %oreach alication# %rom very thin sheet here lo inductance is used to limit the heatinut to revent urning through# to heavier steel late here the high inductance canminimise satter and ensure ade3uate %usion.


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'orrect Stuing

Shielding Gasses and Metals

'here are several tyes o% shielding gases and shielding gas mixtures in general useith the MAGS rocess. Some o% the gases have a road range o% alication hileothers are restricted in their use. 'his section ill deal ith the various shielding gasesand their uses ith eldale metals.

Argon

Argon is a chemically inert gas that ill not comine ith the roducts o% the eld >one.Argon has a lo thermal conductivity. 'he arc lasma is constricted ith the result thathigh arc densities are resent. 'he high arc density ermits more o% the availaleenergy to go into the or4 as heat. 'he result is a relatively narro ead idth ith

dee aillary enetration at the eld centre. Argon causes a more concentrated arcthan any o% the other commonly used gases emloyed ith the MAGS rocess. It isthis reason that argon has a reutation %or cleaning the or4 area. Actually# it is theconcentration o% the arc lasma# and there%ore heat energy# that causes the re%ractoryoxides to e loosened. 'his henomenon is articularly noticeale hen eldingaluminium.

Argon is in aundant suly since it comrises almost *B o% the earth8s atmoshere.'he gas is otained as a y1roduct in the manu%acture o% oxygen.

Argon is used as the shielding gas hen elding many tyes o% metal. Its rimary useis in the elding o% non1%errous metals and alloys such as aluminium# magnesium#

alloys o% the to# and coer. In some metals alications# argon does not rovide theenetration characteristics re3uired %or heavier eldments.In these cases# argon1helium mixtures are sometimes used. 'his gas mixture ill ediscussed under a searate heading.


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,elium

Helium is also an inert gas and may e comared to argon in that resect. 'here thesimilarity ends. It is lighter than air and has high thermal conductivity. 'he Helium arclasma ill exand under heat (thermal ioni>ation) reducing the arc density. $ithHelium there is simultaneous change in arc voltage here the gradient o% the arc lengthis increased y the discharge o% heat %rom the arc stream or core. 'his means thatmore arc energy is lost in the arc itsel%# and is not transmitted to the or4. 'he result isthat# ith Helium there ill e a roader eld ead ith relatively shalloer enetrationthan ith argon. 'his also accounts %or the higher arc voltage# %or the same arc lengththat is otained ith helium as oosed to argon.Helium is derived %rom natural gas. 'he rocess y hich it is otained is similar tothat o% argon. 2irst the natural gas is comressed and cooled. 'he hydrocarons aredran o%%# then nitrogen# and %inally the helium. 'his is a rocess o% li3uidi%ying thevarious gases until at C"0 ing as li3uid caron dioxide exands intogas# it is necessary to %it an electric heater1vaori>er unit eteen they cylinder valveand the regulator hen using syhon tye cylinders.


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1S E2 3456(7 '!linder 'olour 'odes

ote that the ne !" ?i3uid ithdraal cylinders have a G@EF shoulder in additionto eing ainted lac4 ith a hite strie.

Argon($%!gen

Argon is an excellent shielding gas %or the MAGS rocess ecause it allos the use o%sray tye metal trans%er. $hen deositing %lat or hori>ontal %illet elds hoever# thetyical dee central enetration does not allo the eld metal to 8et out8 at the toes o%the eld. 'his is articularly noticeale hen elding steel or stainless steel. 'hesehenomena ill invarialy cause undercut at the edges o% the eld ead. 'hetendency to undercut may e revented y the addition o% *1B oxygen to the argon.'he oxygen ermits a controlled oxidation to ta4e lace# as ell as increasing thetemerature o% the molten metal trans%erred across the arc. 'he additional time at


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li3uidus allos the hot molten metal to 8et out8 at the toes o% the eld. 'his actionroduces a %eatheredge at the 7unction o% the eld and arent metal.

Argon1oxygen mixtures are very common %or elding stainless steels. 'hey may eused %or lo caron and lo alloy steels# ut the cost is usually rohiitive. Argon1oxygen shielding gases are usually urchased as re1mixed gases.

Argon( '$

2or some alications o% lo caron steel elding# elding grade ! "does not rovidethe arc characteristics needed %or the 7o. 'his ill usually mani%est itsel% heresur%ace aearance is a %actor# in the %orm o% intolerale satter in the eld area. Insuch cases a mixture o% argon1 !" has usually eliminated the rolem. Some eldingauthorities elieve that the mixture should not exceed "B !".

'he reason %or anting to use as much !" as ossile in mixture is rimarily the cost.y using a cylinder o% each tye o% gas# argon and !"# the mixture ercentages maye varied y the use o% %lo meters. 'his method recludes the ossiility o% gassearation such as may occur in re1mixed cylinders. 'he rice o% !"is *B cheaer

that that o% argon in most areas.

Argon !" shielding gas mixture are emloyed %or elding lo caron steel# lo alloysteel and in some cases %or stainless steel.

Argon(,elium( '$

'his mixture o% shielding gases is used rimarily %or elding austenitic stainless steels.'he comination o% gases rovides a uni3ue characteristic to the eld. It is ossile toma4e a eld ith very little uild u o% the to ead ro%ile. 'he result is excellent %orthose alications here a high croned eld is detrimental rather than hel. 'his gas

mixture has %ound considerale use in the elding o% stainless steel ies.


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'he shielding gas ill also have a ronounced e%%ect uon the %olloing asects o% theelding oeration and the resultant eld:

Arc characteristics

Mode o% metal trans%er

;enetration and eld ead ro%ile

Seed o% elding

5ndercutting tendency

leaning action

The Inert Shielding Gases ( Argon and ,elium

Argon and helium are inert gases. 'hese gases and mixtures o% the to arenecessarily used in the elding o% non1%errous metals and also idely used to eldstainless steel and lo alloy steels. asic di%%erences eteen argon and helium are:

6ensity

'hermal conductivity

Arc characteristics

'he density o% argon is aroximately *., times that o% air (heavier) hile the density o%helium is aroximately


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What does this mean8

'he ne system means that the shoulder colours %or all gas cylinders %rom all gascomanies in the 5 ill adot a standard colour scheme.

6uring the changeover eriod# oth old and ne cylinders ill e in circulation. So it isimportant to read the lael9

'he cylinder lael or collar should e the rimary means o% identi%ying the cylinder. 'hecolour o% the cylinder itsel% should only e the secondary means %or identi%ying thecylinder and used hen the lael or collar is not visile or accessile.

'!linder Shoulder 'olours

2lammale @E6

Inert @IGH' G@EE

!xygen $HI'E

Argon 6A@ G@EE

itrogen ?A

aron 6ioxide G@EF

Helium @!$

Acetylene MA@!!

In an emergency and hen cylinder lael is not clearly visile this ill 3uic4ly hel youidenti%y the main chemical ha>ard o% the gas.


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2e Industrial Identi+ication 'hart


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Gases used in MAGS elding

Gas '!linder used +or MAGS Welding

ylinders %or MAGS elding are %illed to a ressure o% either "0< ars or 0


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Spra! Trans+er

#lat position .utt0ecause o% the high heat inut and high elding seeds ossile hen using srayarc# it is imortant to maintain ade3uate gas coverage to the solidi%ying eld areatrailing the gun. 2orehand elding (most right handed elders move le%t to right iththe gun inclined toards the ody is re%erale and eaving is ermitted alloing aneven %urther sread o% shielding gas.

,ori:ontal/*ertical +illet2orehand elding is desirale %or good gas cover and otimum eld shae.

Techniques


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-ip Trans+er

Vertical DownAde3uate gas cover is imortant 1 stic4 out length should e 4et constant %or an eveneld contour. y controlling stic4 out# oor 8%it u8 can e tolerated y deositing amuch cooler eld. 9ertical don is not recommended %or heavier sections ecause o%the ris4 o% cold laing and lac4 o% %usion articularly in the root run.

Vertical UpGenerally gives a convex eld contour and it is suggested that a slight eavingtechni3ue e used# even on smaller asses. 'his eaving roduces a eld o% goodaearance.

?arger %illets re3uire a triangular movement# ith slight auses at either side toeliminate under cutting and to deosit enough metal at the edges to avoid a convexead shae. Illustrated are various 7oint con%igurations using di trans%er.

Overhead (thicker sections)A circular movement can e used to %latten the eld %ace and lessen the ris4 o% aconvex eld.

Hori>ontal utt 7oint 5se %orehand techni3ue %or good gas cover and use eaving asshon.


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Techniques Applications

Low Carbon Steel

It has een estimated that aroximately -


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urrent densities that are too high ill o%ten cause orosity due to excessive heat o%the molten metal %rom the electrode. In some cases alloying and deoxidising elementshave excessive urnout across this tye o% arc. It is roale that the electrode ire istoo small in diameter %or the alication and the next larger diameter should e used. I%the smaller ire si>e must e used then decrease the current value y decreasing ireseed %eed.

Shielding gases used ith the MAGS rocess must e o% the right tye %or the metaleing elded# and must have the right %lo in litres er hour (?;H)# or unsatis%actoryresults ill occur. Shielding gas %los are usually aove *< ?;H ut not more than 0

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Mags welding theory.docx