Welding of Titanium and Its Alloys

7/23/2019 Welding of Titanium and Its Alloys

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Welding of titanium and its alloys - Part 1

Job Knowledge

Titanium is a reactive metal; it will burn in pure oxygen at 6!" and in nitrogen at around #!"$ %xygen and nitrogen will also diffuse into titanium attemperatures above &!" raising t'e tensile strengt' but embrittling t'e metal$ (n t'e form of a powder or metal s'avings titanium also constitutes a

fire 'a)ard$

*espite t'is reactivity titanium is used extensively in c'emical processing+ offs'ore and aerospace applications$ T'is is due to,

T'e tenacious protective oxide film t'at forms+ giving t'e alloys very good corrosion resistance+ particularly in c'loride containing

environments$

o loss of toug'ness at temperatures down to -1.6!"

/ood creep and oxidation resistance at temperatures up to almost 6!"$

0imilar strengt' to steel but at approximately 'alf t'e weig't$

ecause of t'e affinity of titanium and its alloys for oxygen+ nitrogen and 'ydrogen and t'e subse2uent embrittlement+ fluxed welding processes are not

recommended alt'oug' t'ey 'ave been used+ primarily in t'e former 3004$ 5rc welding is t'erefore restricted to t'e gas s'ielded processes T(/+ 7(/and plasma-T(/8 alt'oug' power beams+ t'e solid p'ase processes and resistance welding are also used$

Titanium is allotropic; it 'as two different crystallograp'ic forms depending on t'e temperature and c'emical composition$ elow ##!" it forms t'e

'exagonal close pac9ed alp'a p'ase+ above ##!" it exists as body centred cubic beta p'ase$

5 range of elements may be used to improve t'e mec'anical properties+ some stabilise t'e alp'a p'ase and ot'ers promote t'e formation of beta$

%xygen+ carbon+ nitrogen and aluminium promote t'e formation of t'e alp'a p'ase; c'romium+ molybdenum+ niobium+ tin and vanadium promote t'e

formation of beta$ y suitable additions of t'ese elements it is possible to produce four families of titanium alloys+ divided on t'e basis of microstructure+

into commercially pure titanium+ alp'a or near alp'a alloys+ alp'a-beta alloys and beta alloys$ 50T7 designations+ a simple numbering system+ are acommonly used s'ort'and way of identifying t'e various alloys and bot' t'ese and t'e alloy composition egTi-65l-&:+ will be used wit'in t'is article$

"ommercially pure+ unalloyed 50T7 1 - & and grades contain small amounts of contaminants suc' as oxygen+ nitrogen and carbon+ typically less t'an$


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T(/ welds in commercially pure titanium s'eet made wit' successively greater air contamination of t'e s'ielding

T'e maximum tolerable limits in weld metal 'ave been estimated as $C= oxygen+ $1A= nitrogen and 1Appm 'ydrogen so scrupulous cleanliness is

essential for bot' parent metals and filler wires$ *egreasing t'e weld preparation followed by stainless steel wire brus'ing and a furt'er degrease is

generally sufficient$ ?eavily oxidised components may need to be pic9led in a nitricE'ydrofluoric acid mixture to remove t'e oxide layer$ *egreasing of t'e

filler wire for T(/ welding s'ould be done as a matter of course and t'e cleaned wire 'andled wit' clean cotton gloves; grease and perspiration from t'efingers can cause local contamination andEor porosity$ 7(/ wire s'ould be ordered in a degreased condition+ stored in clean dry conditions and not left

unprotected on t'e s'op floor$

*uring welding t'ose parts of t'e weldment exposed to temperatures above A


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Titanium and its alloys are remar9ably resistant to t'e crac9ing problems experienced by many of t'e ot'er alloy systems$ 0olidification and li2uation

crac9ing are virtually un9nown and w'at could per'aps be called cold crac9ing+ occurs generally only because of embrittlement arising from

contamination+ as discussed in Part 1$

Porosity is t'e commonest problem+ particularly w'en close s2uare butt >oints are used$ (t is generally attributed to 'ydrogen and cleanliness is t'erefore

crucial in eliminating porosity$ T'e porosity may be of one or a mixture of two types, firstly micro-porosity formed wit'in t'e arms of t'e dendrites duringsolidification and secondly+ larger pores t'at often align t'emselves along t'e weld centre line$

5s discussed in Part 1+ cleanliness is t'e 9ey to defect free welds and t'is means t'at not only must t'e component be t'oroug'ly degreased but sos'ould t'e filler wires; weld preparation edges must be deburred and t'e 'ig'est purity s'ielding gas must be used$ (deally t'e gas s'ould 'ave a dew

point of less t'an -A!" C.ppm ?

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Welding of Titanium and Its Alloys