Copper Welding With Industrial Robots

Cop­per and cop­per alloys offer a unique com­bi­na­tion of mate­r­i­al prop­er­ties that make them ide­al for many man­u­fac­tur­ing envi­ron­ments. They are wide­ly used because of their excel­lent elec­tri­cal and ther­mal con­duc­tiv­i­ties, out­stand­ing resis­tance to cor­ro­sion, ease of fab­ri­ca­tion, good strength and fatigue resis­tance. Oth­er use­ful char­ac­ter­is­tics include spark resis­tance, met­al-to-met­al wear resis­tance, low-per­me­abil­i­ty prop­er­ties, and dis­tinc­tive color.

Cop­per Weld­ing Processes

Cop­per is often joined by weld­ing. The arc weld­ing process­es are of pri­ma­ry con­cern. Arc weld­ing can be per­formed using shield­ed met­al arc weld­ing (SMAW), gas-tung­sten arc weld­ing (GTAW), gas-met­al arc weld­ing (GMAW), plas­ma arc weld­ing (PAW), and sub­merged arc weld­ing (SAW).

Weld­ing process­es that use gas shield­ing are gen­er­al­ly pre­ferred, although SMAW can be used for many non-crit­i­cal appli­ca­tions. Argon, heli­um, or mix­tures of the two are used as shield­ing gas­es for GTAW, PAW, and GMAW. Gen­er­al­ly, argon is used when man­u­al­ly weld­ing mate­r­i­al is less than 3 mm thick, has low ther­mal con­duc­tiv­i­ty, or both. Heli­um or a mix­ture of 75% heli­um and 25% argon is rec­om­mend­ed for machine weld­ing of thin sec­tions and for man­u­al weld­ing of thick­er sec­tions of alloys that have high ther­mal con­duc­tiv­i­ty. Small amounts of nitro­gen can be added to the argon shield­ing gas to increase the effec­tive heat input. Shield­ed met­al arc weld­ing can be used to weld a wide range of cop­per alloy thick­ness­es. Cov­ered elec­trodes for sub­merged arc weld­ing (SAW) of cop­per alloys are avail­able in stan­dard sizes rang­ing from 2.4 to 4.8 mm.

Gas-Tung­sten Arc Welding

Gas-tung­sten arc weld­ing is well suit­ed for cop­per and cop­per alloys because of its intense arc, which pro­duces an extreme­ly high tem­per­a­ture at the joint and a nar­row heat-affect­ed zone (HAZ).

In weld­ing cop­per and the more ther­mal­ly con­duc­tive cop­per alloys, the inten­si­ty of the arc is impor­tant in com­plet­ing fusion with min­i­mum heat­ing of the sur­round­ing, high­ly con­duc­tive base met­al. A nar­row HAZ is par­tic­u­lar­ly desir­able in the weld­ing of cop­per alloys that have been pre­cip­i­ta­tion hard­ened.

Many of the stan­dard tung­sten or alloyed tung­sten elec­trodes can be used in GTAW of cop­per and cop­per alloys. The selec­tion fac­tors nor­mal­ly con­sid­ered for tung­sten elec­trodes apply in gen­er­al to the cop­per and cop­per alloys. Except for the spe­cif­ic class­es of cop­per alloys, tho­ri­at­ed tung­sten (usu­al­ly EWTh‑2) is pre­ferred for its bet­ter per­for­mance, longer life, and greater resis­tance to contamination.

Gas-Met­al Arc Welding

Gas-met­al arc weld­ing is used for join­ing cop­per and cop­per alloys for thick­ness less than 3 mm, while GMAW is pre­ferred for sec­tion thick­ness above 3 mm and for join­ing alu­minum bronzes, sil­i­con bronzes and cop­per-nick­el alloys.

Plas­ma Arc Welding

The weld­ing of cop­pers and cop­per alloys using PAW is com­pa­ra­ble to GTAW of these alloys. Argon, heli­um, or mix­tures of the two are used for the weld­ing of all alloys. Hydro­gen gas should nev­er be used when weld­ing coppers.

Plas­ma arc weld­ing has two dis­tinct advan­tages over GTAW: 

1. Tung­sten is con­cealed and entire­ly shield­ed, which great­ly reduces con­t­a­m­i­na­tion of the elec­trode, par­tic­u­lar­ly for alloys with low-boil­ing-tem­per­a­ture con­stituents such as brass­es, bronzes, phos­phor bronzes, and alu­minum bronzes.
2. Con­struct­ed arc plume gives rise to high­er arc ener­gies while min­i­miz­ing the growth of the HAZ. As with GTAW, cur­rent pul­sa­tion and cur­rent ramp­ing may also be used. Plas­ma arc weld­ing equip­ment has been minia­tur­ized for intri­cate work, known as microplas­ma welding.

Plas­ma arc weld­ing of cop­pers and cop­per alloys may be per­formed either auto­ge­nous­ly or with filler met­al. Filler met­al selec­tion is iden­ti­cal to that out­lined for GTAW. Automa­tion and mech­a­niza­tion of this process is read­i­ly per­formed and is prefer­able to GTAW where con­t­a­m­i­na­tion can restrict pro­duc­tion effi­cien­cies. Weld­ing posi­tions for PAW are iden­ti­cal to those for GTAW. How­ev­er, the plas­ma key­hole mode has been eval­u­at­ed for thick­er sec­tions in a ver­ti­cal-up posi­tion. Gen­er­al­ly, all infor­ma­tion pre­sent­ed for GTAW is applic­a­ble to PAW.

Sub­merged Arc Welding 

The weld­ing of thick gage mate­r­i­al, such as pipe formed from heavy plate, can be achieved by con­tin­u­ous met­al-arc oper­a­tion under a gran­u­lar flux. Effec­tive de-oxi­da­tion and slag-met­al reac­tions to form the required weld-met­al com­po­si­tion are crit­i­cal and the SAW process is still under devel­op­ment for cop­per-base mate­ri­als. A vari­a­tion on this process can be used for weld cladding or hard­fac­ing. Com­mer­cial­ly avail­able flux­es should be used for the cop­per-nick­el alloys.

Alloy Met­al­lur­gy and Weldability

Many com­mon met­als are alloyed with cop­per to pro­duce the var­i­ous cop­per alloys. The most com­mon alloy­ing ele­ments are alu­minum, nick­el, sil­i­con, tin, and zinc. Oth­er ele­ments and met­als are alloyed in small quan­ti­ties to improve cer­tain mate­r­i­al char­ac­ter­is­tics, such as cor­ro­sion resis­tance or machinability.

Nine Cop­per and Cop­per Alloy Groups:

1. Cop­pers, which con­tain a min­i­mum of 99.3% Cu
2. High-cop­per alloys, which con­tain up to 5% alloy­ing elements
3. Cop­per-zinc alloys (brass­es), which con­tain up to 40% Zn
4. Cop­per-tin alloys (phos­phor bronzes), which con­tain up to 10% Sn and 0.2% P
5. Cop­per-alu­minum alloys (alu­minum bronzes), which con­tain up to 10% Al
6. Cop­per-sil­i­con alloys (sil­i­con bronzes), which con­tain up to 3% Si
7. Cop­per-nick­el alloys, which con­tain up to 30% Ni
8. Cop­per-zinc-nick­el alloys (nick­el sil­vers), which con­tain up to 7% Zn and 18% Ni
9. Spe­cial alloys, which con­tain alloy­ing ele­ments to enhance a spe­cif­ic prop­er­ty or char­ac­ter­is­tic, for exam­ple machinability.

Many cop­per alloys have com­mon names, such as oxy­gen-free cop­per (99.95% Cu min), beryl­li­um cop­per (0.02 to 0.2% Be), Muntz met­al (Cu40Zn), Naval brass (Cu-39.5Zn‑0.75Sn), and com­mer­cial bronze (Cu-10Zn).

Prop­er­ties

Many of the phys­i­cal prop­er­ties of cop­per alloys are impor­tant to the weld­ing process­es, includ­ing melt­ing tem­per­a­ture, coef­fi­cient of ther­mal expan­sion, and elec­tri­cal and ther­mal con­duc­tiv­i­ty. Cer­tain alloy­ing ele­ments decrease the elec­tri­cal and ther­mal con­duc­tiv­i­ties of cop­per and cop­per alloys.

Weld­abil­i­ty

Sev­er­al alloy­ing ele­ments have pro­nounced effects on the weld­abil­i­ty of cop­per and cop­per alloys. Small amounts of volatile, tox­ic alloy­ing ele­ments are often present in cop­per and its alloys. As a result, the require­ment of an effec­tive ven­ti­la­tion sys­tem to pro­tect the welder and/​or the weld­ing machine oper­a­tor is more crit­i­cal then when weld­ing fer­rous metals.

Zinc reduces the weld­abil­i­ty of all brass­es in rel­a­tive pro­por­tion to the per­cent of zinc in the alloy. Zinc has a low boil­ing tem­per­a­ture, which results in the pro­duc­tion of tox­ic vapors when weld­ing cop­per-zinc alloys.

Sil­i­con has a ben­e­fi­cial effect on the weld­abil­i­ty of cop­per-sil­i­con alloys because of its deox­i­diz­ing and flux­ing actions.

Tin

Tin increas­es the hot-crack sus­cep­ti­bil­i­ty dur­ing weld­ing when present in amounts from 1 to 10%. Tin, when com­pared with zinc, is far less volatile and tox­ic. Dur­ing weld­ing, tin may pref­er­en­tial­ly oxi­dize rel­a­tive to cop­per. The results will be an oxide entrap­ment, which may reduce the strength of the weldment.

Tena­cious Oxides

Beryl­li­um, alu­minum, and nick­el form tena­cious oxides that must be removed pri­or to weld­ing. The for­ma­tion of these oxides dur­ing the weld­ing process must be pre­vent­ed by shield­ing gas or by flux­ing, in con­junc­tion with the use of the appro­pri­ate weld­ing cur­rent. The oxides of nick­el inter­fere with arc weld­ing less than those beryl­li­um or alu­minum. Con­se­quent­ly, the nick­el sil­vers and cop­per-nick­el alloys are less sen­si­tive to the type of weld­ing cur­rent used dur­ing the process. Beryl­li­um con­tain­ing alloys also pro­duce tox­ic fumes dur­ing the welding.

Oxy­gen

Oxy­gen can cause poros­i­ty and reduce the strength of welds made in cer­tain cop­per alloys that do not con­tain suf­fi­cient quan­ti­ties of phos­pho­rus or oth­er de-oxi­dizes. Oxy­gen may be found as a free gas or as cuprous oxide. Most com­mon­ly weld­ed cop­per alloys con­tain deox­i­diz­ing ele­ment, usu­al­ly phos­pho­rus, sil­i­con, alu­minum, iron, or manganese.

Iron and man­ganese do not sig­nif­i­cant­ly affect the weld­abil­i­ty of the alloys that con­tain them. Iron is typ­i­cal­ly present in some spe­cial brass­es, alu­minum bronzes, and cop­per-nick­el alloys in amounts of 1.4 to 3.5%. Man­ganese is com­mon­ly used in these same alloys, but at low­er con­cen­tra­tions than iron.

Free-Machin­ing Additives 

Lead, sele­ni­um, tel­luri­um and sul­fur are added to cop­per alloys to improve machin­abil­i­ty. Bis­muth is begin­ning to be used for this pur­pose as well when lead-free alloys are desired. These minor alloy­ing agents, while improv­ing machin­abil­i­ty, sig­nif­i­cant­ly affect the weld­abil­i­ty of cop­per alloys by ren­der­ing the alloys hot-crack sus­cep­ti­ble. The adverse effect on weld­abil­i­ty is evi­dent with about 0.05% of the addi­tive and is more severe with larg­er con­cen­tra­tions. Lead is the most harm­ful of the alloy­ing agents with respect to hot-crack susceptibility.

Fac­tors Affect­ing Weldability

Besides the alloy­ing ele­ments that com­prise a spe­cif­ic cop­per alloy, sev­er­al oth­er fac­tors affect weld­abil­i­ty. These fac­tors are the ther­mal con­duc­tiv­i­ty of the alloy being weld­ed, the shield­ing gas, the type of cur­rent used dur­ing weld­ing, the joint design, the weld­ing posi­tion, and the sur­face con­di­tion and cleanliness.

Effect of Ther­mal Conductivity

The behav­ior of cop­per and cop­per alloys dur­ing weld­ing is strong­ly influ­enced by the ther­mal con­duc­tiv­i­ty of the alloy. When weld­ing com­mer­cial cop­pers and light­ly alloyed cop­per mate­ri­als with high ther­mal con­duc­tiv­i­ties, the type of cur­rent and shield­ing gas must be select­ed to pro­vide max­i­mum heat input to the joint. This high heat input coun­ter­acts the rapid head dis­si­pa­tion away from the local­ized weld zone.

Depend­ing on sec­tion thick­ness, pre­heat­ing may be required for cop­per alloys with low­er ther­mal con­duc­tiv­i­ties. The inter­pass tem­per­a­ture should be the same as for pre­heat­ing. Cop­per alloys are not post-weld head treat­ed as fre­quent­ly as steels, but some alloys may require con­trolled cool­ing rates to min­i­mize resid­ual stress­es and hot shortness.

Weld­ing Position

Due to the high­ly flu­id nature of cop­per and its alloys, the flat posi­tion is used when­ev­er pos­si­ble for weld­ing. The hor­i­zon­tal posi­tion is used in some fil­let weld­ing of com­er joints and T‑joints.

Pre­cip­i­ta­tion-Hard­en­able Alloys

The most impor­tant pre­cip­i­ta­tion-hard­en­ing reac­tions are obtained with beryl­li­um, chromi­um, boron, nick­el, sil­i­con, and zir­co­ni­um. Care must be tak­en when weld­ing pre­cip­i­ta­tion-hard­en­able cop­per alloys to avoid oxi­da­tion and incom­plete fusion. When­ev­er pos­si­ble, the com­po­nents should be weld­ed in the annealed con­di­tion, and then the weld­ment should be giv­en a pre­cip­i­ta­tion-hard­en­ing heat treatment.

Hot Crack­ing

Cop­per alloys, such as cop­per-tin and cop­per-nick­el, are sus­cep­ti­ble to hot crack­ing at solid­i­fi­ca­tion tem­per­a­tures. This char­ac­ter­is­tic is exhib­it­ed in all cop­per alloys with a wide liq­uidus-to-solidus tem­per­a­ture range. Severe shrink­age stress­es pro­duce inter­den­drit­ic sep­a­ra­tion dur­ing met­al solid­i­fi­ca­tion. Hot crack­ing can be min­i­mized by reduc­ing restraint dur­ing weld­ing, pre­heat­ing to slow the cool­ing rate and reduce the mag­ni­tude of weld­ing stress­es, and reduc­ing the size of the root open­ing and increas­ing the size of the root pass.

Poros­i­ty

Cer­tain ele­ments (for exam­ple, zinc, cad­mi­um, and phos­pho­rus) have low boil­ing points. Vapor­iza­tion of these ele­ments dur­ing weld­ing may result in poros­i­ty. When weld­ing cop­per alloys con­tain­ing these ele­ments, poros­i­ty can be min­i­mized by high­er weld speeds and a filler met­al low in these elements.

Sur­face Condition

Grease and oxide on work sur­faces should be removed before weld­ing. Wire brush­ing or bright dip­ping can be used. Milis­cale on the sur­faces of alu­minum bronzes and sil­i­con bronzes is removed for a dis­tance from the weld region of at least 13 mm, usu­al­ly by mechan­i­cal means. Grease, paint, cray­on marks, shop dirt, and sim­i­lar con­t­a­m­i­nants on cop­per-nick­el alloys may cause embrit­tle­ment and should be removed before weld­ing. Milis­cale on cop­per-nick­el alloys must be removed by grind­ing or pick­ling; wire brush­ing is not effective.

Cop­per Weld­ing Alloys

The ide­al elec­trode mate­r­i­al would have the com­pres­sive strength of tool steel and the con­duc­tiv­i­ty of sil­ver. Unfor­tu­nate­ly, no such mate­r­i­al exists. So sev­er­al dif­fer­ent cop­per alloys have been devel­oped. All RWMA rec­om­mend­ed mate­ri­als have high­er anneal­ing or soft­en­ing tem­per­a­tures than pure cop­per, togeth­er with improved com­pres­sive strength and wear resis­tance. Because the cop­per has been alloyed to achieve high­er strength and wear prop­er­ties, there is some sac­ri­fice in conductivity.

Cop­per Alloys Classes:

Class 1: This class is most often spec­i­fied for weld­ing Alu­minum and oth­er high­ly con­duc­tive mate­ri­als. This is the most con­duc­tive of the RWMA Alloys. It is also the soft­est (and has the low­est strength and wear characteristics).

Class 2: This class of cop­per alloy is the most wide­ly used and rec­om­mend­ed cop­per alloy. It is rec­om­mend­ed for a wide range of steel alloys. The mate­r­i­al is rec­om­mend­ed for spot, seam, pro­jec­tion weld­ing, and cross wire weld­ing. It has slight­ly low­er con­duc­tiv­i­ty than class 1, and has high­er strength and wear characteristics.

Class 3: This one has the low­est con­duc­tiv­i­ty, but the high­est strength prop­er­ties of the main three grades of cop­per elec­trode mate­r­i­al. It is rec­om­mend­ed for most appli­ca­tions where high strength and wear resis­tance are imperative.

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