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It is essential that the joint and surrounding surface areas are cleaned by


first degreasing either by steam, solvent, alkaline or vapour. 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 from weld metal contamination by either gas absorption or by dissolving contaminants such as dust (iron particles) on the surface. At temperatures of above 500°C, 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 show a higher, usually 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 piece of titanium, termed ‘titanium-getter’, 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 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 onto 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, if there is sufficiently high iron content, cause embrittlement. Iron particles are equally detrimental in the HAZ where local melting of the particles form pockets of titanium-iron eutectic. Micro-cracking 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 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 fabricate titanium in a clean area. ●


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