Weldability of materials
Titanium and titanium alloys
Titanium and its alloys are chosen because of the following properties:
- high strength to weight ratio;
- corrosion resistance;
- mechanical properties at elevated temperatures.
Titanium is a unique material, as strong as steel but half its weight with
excellent corrosion resistance. Traditional applications are in the aerospace
and chemical industries. More recently, especially as the cost of titanium has
fallen significantly, the alloys are finding greater use in other industry
sectors, such as offshore.
The various types of titanium alloys are identified and guidance given on
welding processes and techniques employed in fabricating components without
impairing their corrosion, oxidation and mechanical properties or introducing
defects into the weld.
Material types
Alloy groupings
There are basically three types of alloys distinguished by their
microstructure:
Titanium - Commercially pure (98 to 99.5% Ti) or
strengthened by small additions of oxygen, nitrogen, carbon and iron. The alloys
are readily fusion weldable.
Alpha alloys - These are largely single-phase alloys
containing up to 7% aluminium and a small amount (< 0.3%) of oxygen, nitrogen
and carbon. The alloys are fusion welded in the annealed condition.
Alpha-beta alloys - These have a characteristic two-phase
microstructure formed by the addition of up to 6% aluminium and varying amounts
of beta forming constituents - vanadium, chromium and molybdenum. The alloys are
readily welded in the annealed condition.
Alloys which contain a large amount of the beta phase, such as chromium, are
not easily welded.
Commonly used alloys are listed in Table 1 with the appropriate ASTM grade.
The internationally recognised designation. In industry, the most widely welded
titanium alloys are the commercially pure grades and variants of the 6%Al and 4%
V alloy which is regarded as the standard aircraft alloy.
Table 1: Commonly used titanium alloys and the recommended filler material
| ASTM Grade |
Composition |
UTS (min) Mpa |
Filler |
Comments |
|
1 |
Ti-0.15O2 |
240 |
ERTi-1 |
Commercially pure |
|
2 |
Ti-0.20O2 |
340 |
ERTi-2 |
,, |
|
4 |
Ti-0.35O2 |
550 |
ERTi-4 |
,, |
|
7 |
Ti-0.20O2-0.2Pd |
340 |
ERTi-7 |
,, |
|
9 |
Ti-3Al-2.5V |
615 |
ERTi-9 |
Tube components |
|
5 |
Ti-6Al-4V |
900 |
ERTi-5ELI |
Aircraft alloy |
|
23 |
Ti-6Al-4V ELI |
900 |
ERTi-5ELI |
Low interstitials |
|
25 |
Ti-6Al-4V-0.06Pd |
900 |
Matching |
Corrosion grade |
Filler alloys
Titanium and its alloys can be welded using a matching filler composition;
compositions are given in The American Welding Society specification AWS
A5.16-90. Recommended filler wires for the commonly used titanium alloys are
also given in Table 1.
When welding higher strength grades of unalloyed titanium or titanium alloys,
fillers of a lower strength are sometimes used to achieve adequate weld metal
ductility. For example, an unalloyed filler ERTi-2 can be used to weld Ti-6Al-4V
and Ti-5Al-2.5Sn alloys in order to of balancing weldability, strength and
formability requirements.
Weld imperfections
This material and its alloys are readily fusion welded providing suitable
precautions are taken. TIG and plasma processes, with argon or argon-helium
shielding gas, are used for welding thin section components, typically <10mm.
Autogenous welding can be used for a section thickness of <3mm with TIG, or
<6mm with plasma. Pulsed MIG is preferred to dip transfer MIG because of the
lower spatter level.
The most likely imperfections in fusion welds are:
- Weld metal porosity
- Embrittlement
- Contamination cracking
Normally, there is no solidification cracking or hydrogen cracking.
Weld metal porosity
Weld metal porosity is the most frequent weld defect. As gas solubility is
significantly less in the solid phase, porosity arises when the gas is trapped
between dendrites during solidification. In titanium, hydrogen from moisture in
the arc environment or contamination on the filler and parent metal surface, is
the most likely cause of porosity.
It is essential that the joint and surrounding surface areas are cleaned by
first degreasing either by steam, solvent, alkaline or vapour degreasing. Any
surface oxide should then be removed by pickling (HF-HNO3 solution), light
grinding or scratch brushing with a clean, stainless steel wire brush. On no
account should an ordinary steel brush be used. After wiping with a lint-free
cloth, care should be taken not to touch the surface before welding. When TIG
welding thin section components, the joint area should be dry-machined to
produce a smooth surface finish.
Embrittlement
Embrittlement can be caused by weld metal contamination by either gas
absorption or by dissolving contaminants such as dust (iron particles) on the
surface. At temperatures above 5000C, titanium has a very high affinity for
oxygen, nitrogen and hydrogen. The weld pool, heat affected zone and cooling
weld bead must be protected from oxidation by an inert gas shield (argon or
helium).
When oxidation occurs, the thin layer of surface oxide generates an
interference colour. The colour can indicate whether the shielding was adequate
or an unacceptable degree of contamination has occurred. A silver or straw
colour shows satisfactory gas shielding was achieved but for certain service
conditions, dark blue may be acceptable. Light blue, grey and white shows an
unacceptable level of oxygen contamination.
For small components, an efficient gas shield can be achieved by welding in a
totally enclosed chamber, filled with the shielding gas. It is recommended that
before welding, the arc is struck on a scrap pieces of titanium, termed
'titanium-getters', to remove oxygen from the atmosphere; the oxygen level
should be reduced to approximately 40ppm before striking the arc on the scrap
titanium and <20ppm before welding the actual component.
In tube welding, a fully enclosed head is equally effective in shielding the
weld area and is be preferable to orbital welding equipment in which the gas
nozzle must be rotated around the tube.
When welding out in the open, the torch is fitted with a trailing shield to
protect the hot weld bead whilst cooling. The size and shape of the shield is
determined by the joint profile whilst its length will be influenced by welding
current and travel speed. It is essential in 'open air' welding that the
underside of the joint is protected from oxidation. For straight runs, a grooved
bar is used with argon gas blown on to the joint. In tube and pipe welding,
normal gas purging techniques are appropriate.
Contamination cracking
If iron particles are present on the component surface, they dissolve in the
weld metal reducing corrosion resistance and, at a sufficiently high iron
content, causing embrittlement. Iron particles are equally detrimental in the
HAZ where local melting of the particles form pockets of titanium - iron
eutectic. Microcracking may occur but it is more likely that the iron-rich
pockets will become preferential sites for corrosion.
Particular attention should be paid to separating titanium from steel
fabrications, preferably by designating a specially reserved clean area. Welders
should guard against embedding steel particles into the surface of the material
by:
- Avoiding steel fabrication operations near titanium components.
- Covering components to avoid airborne dust particles settling on the
surface
- Not using tools, including wire brushes, previously used for steel
- Scratch brushing the joint area immediately before welding
- Not handling the cleaned component with dirty gloves.
To avoid corrosion cracking, and minimise the risk of embrittlement through
iron contamination, it is best practice to fabrication titanium in a specially
reserved clean area.
Copyright by TWI, 1999
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