AWS PRGT

Introduction

Titanium need not be ail that hard to weld! Inindustrial sectors the common opinion is that titanium alloys aredifficult to weld. While it is true that titanium alloys can beembrittled by careless welding techniques, it is equally true thatthese materials are much more weldable than their reputationsuggests. Difficulties in welding titanium and titanium alloysoriginate from several basic sources. The high reactivity oftitanium with other materials, poor cleaning of parts beforejoining, and inadequate protection during welding can lead tocontamination, porosity and embrittlement of the completedjoints.

Titanium is one of the most common metals occurring in theearth’s crust. Particularly in North America, there is an abundanceof titanium ores available for commercial exploitation. Puretitanium is a silvery-colored metal that melts at approximately3035°F. It is as strong as steel, but half its weight withexcellent corrosion resistance. Traditional applications are in theaerospace and chemical industries.

Titanium and titanium alloys have a number of desirableproperties and, when suitability combined, these properties makethe metal the best material for a variety of service applications.These properties include:

• Excellent fatigue resistance.

• Good notch toughness.

• Stability over a wide temperature range.

• Low coefficient of thermal expansion.

• Low thermal conductivity

• corrosion characteristics for some of the most troublesomeindustrial chemicals.

• Excellent resistance to erosion and cavitation from highvelocity fluid flow.

• No scaling below SOOOF, although discoloration of the metalmay occur.

• Inert in electrochemical operations, when charged as an anodein an electrochemical circuit.

Titanium has a strong affinity for oxygen, and it forms a tightmicroscopic oxide film on freshly prepared surfaces at roomtemperature. Titanium tends to oxidize rapidly when heated in airabove 1200°F. At elevated temperatures it has the propensitypropensity for dissolving discrete amounts of its own oxide intosolution. For these reasons, the welding of titanium requires theuse of protective shielding, such as an inert gas atmosphere, toprevent contamination and embrittlement from oxygen and nitrogen,Titanium reacts with air to form oxides, and at elevatedtemperatures it will readily oxidize and discolor. The color of thewelds can be used as an indication of the effectiveness of theshielding and resulting weld quality. Good shielding and cleaningwill produce bright metallic, silvery welds, while the presence ofstraw, blue, gray, and white surface colors indicate increasingamounts of weld contamination. Weld contamination is usually theresult of faulty or inadequate trailing or back up shielding,excessive heat input, or too high a rate of travel whenwelding.

Titanium’s relatively low coefficients of thermal expansion andconductivity minimize the possibility of distortion duringwelding.

Pure titanium is quite ductile (15 to 25% elongation), and has arelatively low ultimate tensile strength (approximately 30 ksi) atroom temperature. Adding limited amounts of oxygen and nitrogen insolid solution will strengthen titanium markedly, but it will alsoembrittle the metal if present in excessive quantities.

The sensitivity of titanium and titanium alloys to embrittlementimposes limitations on the joining processes that may be used.Small amounts of carbon, oxygen, nitrogen, or hydrogen impairductility and toughness of titanium joints. As little as 5000 partsper million of these elements will embrittle a weld beyond thepoint of usefulness. Titanium has a high affinity for theseelements at elevated temperatures and must be shielded from normalair atmospheres during joining. Consequently, joining processes andprocedures that minimize joint contamination must be used. Dust,dirt, grease, fingerprints, and a wide variety of othercontaminants also can lead to embrittlement and porosity when thetitanium or filler metal is not properly cleaned prior tojoining.

When heated to joining temperatures, titanium and titaniumalloys react with air and most elements and compounds, includingmost refractories. Therefore titanium and titanium alloys arewelded with the inert gas shielded processes. See Table 1.

There are basically three types of alloys distinguished by theirmicrostructure.

(1) Titanium. Commercially pure (98 to 99.5%Ti) or strengthened by small additions of oxygen, nitrogen, carbon,and iron. These alloys are readily weldable.

(2) Alpha Alloys. These are largelysingle-phase alloys containing up to 7% aluminum and a small amount(<0.3%of) oxygen, nitrogen, and carbon. The alloys are welded inthe annealed condition.

(3) Alpha-Beta Alloys. These have acharacteristic two-phase microstructure formed by the addition ofup to 6% aluminum and varying amounts of betaformingconstituents-vanadium, chromium, and molybdenum. The alloys arereadily welded in the annealed condition.

Edition:

99

Published:

01/01/1999

Number of Pages:

16

File Size:

1 file , 980 KB