TWI JOB KNOWLEDGE


OF MATERIALS – TITANIUM AND


WELDABILITY TITANIUM ALLOYS


TWI identifies titanium and its alloys and outlines the best fabrication methods without impairing their properties or introducing defects into the weld


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 is given in this article 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:


Table 1: Commonly used titanium alloys and the recommended filler material


ASTM GRADE


1 Ti-0.15O 2 Ti-0.20O 4 Ti-0.35O


7 Ti-0.20O -0.2Pd 9 Ti-3Al-2.5V 5 Ti-6Al-4V


23 Ti-6Al-4V ELI 25 Ti-6Al-4V-0.06Pd 18


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.


COMPOSITION 240


340 550 340 615 900 900 900


UTS (MIN) MPA FILLER COMMENTS ERTi-1


ERTi-2 ERTi-4 ERTi-7 ERTi-9 ERTi-5


ERTi-5ELI ERTi-25


Commercially pure Commercially pure Commercially pure Commercially pure Tube components 'Workhorse' alloy Low interstitials


Corrosion resistant grade


The alloys are readily welded in the annealed condition. Alloys which contain a large amount of the beta


phase, stabilised by elements such as chromium, are not easily welded. Commonly used alloys are listed in Table 1 below 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.


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- 2004. Recommended filler wires for the commonly used titanium alloys are also given in Table 1 below. When welding higher strength 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 to balance 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 welding, using novel coated wires, results in very low porosity and spatter. The most likely imperfections in fusion welds are:


Weld metal porosity. Embrittlement. Contamination cracking.


Normally, there is no solidification cracking or hydrogen cracking.


Weldmetal porosity Weld metal porosity is the most frequent weld defect. Porosity arises when gas bubbles are 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.


Hard-hat worker with giant gears machinery in background, titanium and steel. Copyright: Christian Lagerek, courtesy of Shutterstock.com.


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