Connect, no.82, January 1997 This is the first in a series of Connect articles focusing on titanium in industry - how to join it and how to use it.
General advice on titanium applications can be obtained from the Titanium Information Group (+44 1562 60276), and for any problems encountered in joining titanium, further advice can be sought from TWI (+44 1223 899000).
Introduction to Titanium and its Alloys
Why use titanium?
Titanium emerged as an engineering material in the 1950s and rapidly became established in gas turbine engines because of its unique combination of strength, low weight, corrosion resistance and elevated temperature properties.Titanium is as strong as steel, but nearly 50% lighter. It also has excellent corrosion resistance. These two attributes have led to two traditional areas of application being established; in aerospace and chemical plant.
In recent years, a number of factors have caused the price of this material to fall. This has resulted in more diverse applications.
In the offshore industry, advantage has been taken of its weight reduction properties, corrosion performance, and the increased flexibility provided by the characteristically low modulus of titanium.
Important Alloys
Although a wide range of titanium alloys has been developed, there are relatively few alloys of commercial importance. They consist of the commercially pure grades and variants of the Ti-6Al-4V alloy. (see Table)
| ASTM Grade |
Composition |
Min.Tensile Strength MPa |
Comments |
| 1 |
Ti-0·15O |
240 |
commercially pure grade |
| 2 |
Ti-0·20O |
340 |
commercially pure grade |
| 4 |
Ti-0·35O |
550 |
commercially pure grade |
| 7 |
Ti-0·20O-0·2Pd |
340 |
|
| 9 |
Ti-3Al-2·5V |
615 |
for tubes |
| 5 |
Ti-6Al-4V |
900 |
standard aircraft alloy |
| 23 |
Ti-6Al-4V ELI |
900 |
extra low interstitial variant |
| 25 |
Ti-6Al-4V-0·06Pd |
900 |
corrosion resistant grade |
Welding
Titanium can be welded by a variety of processes. The most widely used are TIG and electron beam, although friction and diffusion bonding are important in aerospace applications. More detailed description of welding process aspectswill be given in subsequent articles in this series, for example, joining titanium by arc welding, solid phase welding, with power beams and by brazing.
Process Joint types Comments
Tungsten inert gas (TIG) Close square butt autogenous <1.6mm thick. Most widely used process. Filler wire and prepared edges required for butts >1.6mm thick. Very flexible
Metal inert gas (MIG) For sections >3mm thick. Use in pulsed current or spray mode. Welds regarded as lower quality welds than TIG
Plasma arc welding One pass welding up to 19mm thick. Faster than TIG
Electron beam welding Up to 200mm (single pass). Low contamination welds due to vacuum
Laser beam welding Limited by available laser power to <13mm. Gas shielding required
Resistance welding Cleaned sheet. No shielding required
Friction welding Rotary - bars and pipes. Shielding not usually required. Linear - flat products and non-symmetrical sections
Diffusion bonding Sheets or heavy sections can be joined by this. Often employed in combination with superplastic process forming
