Page 30 - ITA Journal 3-2018

Technical Papers
Huntingdon Fusion Techniques
Welding reactive metals
The reactive metals by classi cation are zirconium, titanium and beryl- lium. We also include here tantalum and columbium (niobium), being from the refractory class and which also present similar challenges to the welding engineer.
Aerospace, automotive, medical and military industries are increasingly using all these materials. They have many technological attractions being durable, low density, bio-compatible and offering high corro- sion resistance but they are expensive. Welding procedures need to be carefully developed and stringently applied to avoid expensive waste, rework or risk of service failure.
Successful fusion joining techniques have evolved1 since the alloys were rst used in engineering applications. The majority of metallur- gical problems, even considering dissimilar metal welding, have been resolved and ller materials are readily available. However, their reac- tive nature make it essential to continue to address the requirement for thorough pre-cleaning and particularly oxidation at the high tempera- tures involved in arc welding.
Cleanliness
Weld repair of plant that has been in service presents the most dif cult situation for welding reactive and refractory alloys. The equipment is usually dirty and may have process scale on the surface. In most cases, the repairs must be done outdoors where the work area and conditions are not optimal for welding reactive metals - the environment usually is dirty. High humidity and wind also interfere with the welding process.
Furthermore, plant shutdowns that require equipment repair usually result in pressure to complete the work quickly. There is little merit however in cutting corners. Repairs that must be made to partial-pen- etration welds or where crevices exist pose a special problem because of contaminants in crevices.
Poorly prepared surfaces can result in weld contamination and lead to a premature failure and a repeat of the weld repair. Overall, expect to spend more time preparing to weld than actually welding when work is undertaken on site.
Even under the cleaner conditions prevailing in most factory environ- ments however there is still a need to prepare surfaces by removing all contamination prior to welding. Published information on cleaning techniques is available 2 - 8 but the basic principles are to abrade sur- faces to remove any corrosion products and other debris then degrease and nally dry. Prepared surfaces should then be protected by covering until any joints are to be made.
Protection Against Oxidation
Reactive metals have a strong af nity for oxygen at the temperatures prevailing in fusion welding and when combined form very stable com- pounds. Refractory metals exhibit many of the same characteristics.
ITAtube Journal No3/October 2018
30