Titanium and its alloys are highly reactive metals and welding requires the use of special techniques to avoid embrittlement by contamination with oxygen and to a lesser extent, nitrogen from the atmosphere. Both gases are absorbed rapidly at temperatures prevailing in fusion welding. Gas shielding requirements are thus much more stringent than for most other materials. Complete protection for the weld pool, the underside of the joint and the region behind the weld metal as it cools is essential if successful welds are to be made.
The world leaders in welding research, TWI in the UK and AWS in the USA, are in total accord with the requirements for preparation and weld procedures. Pre-weld cleaning is crucial and inert gas quality around the weld zone has to be such that the oxygen content is below 10 ppm (0.001%).
Avoiding Defects
It’s clear from knowledge based on many years experience, particularly in the aerospace sector, that the provision of a suitable inert environment is crucial. Not only is it necessary to protect the molten weld metal, there is also a need to preclude oxygen from any hot metal during post weld cooling. As a general rule any part of the weld zone above 500°C must be protected.
The aerospace industry often produces identical parts in large enough volumes to justify the capital cost of vacuum chambers and remote handling equipment. Whilst this solution is technically very effective it is quite inappropriate when welding batch quantities of small components because of the cost penalty. The space taken up by permanent vacuum chambers is another important consideration. Vacuum Chambers are extremely expensive and are not cost effective for the welding of smaller components that may need to be manufactured for a competitive market place.
Gas purging in small glove boxes is not uncommon but where several sizes of vessel are needed the component cost can be very high. Flexible purge enclosures are a lower cost alternative but the most cost-effective solution, is the integrated titanium shield.
For over 40 years now, accessories have been developed to ensure the production of high integrity fusion joints in titanium and other reactive alloys without having to resort to expensive vacuum chamber technology in order to remove oxygen and nitrogen from the welding environment.
Preparation
Dry, not lubricated, machining is preferred for final preparation of the joint surfaces so that contamination from cutting fluids is eliminated. Following mechanical preparation the weld surfaces must be cleaned by degreasing and then dried.
Welding should then be carried out within 48 hours.
For the highest-quality welds, handling after cleaning needs to be done with care. Operators should wear lint-free gloves after the final cleaning operations - chlorine from hand perspiration can create localised contamination.
Commercially Available Equipment
As mentioned previously specialist ancilliaries have been available for many years, to resolve the difficulties in welding titanium and its alloys. A wide range of trailing shields has been introduced and these take full advantage of advances in engineering technology. Flexible purge enclosures are available in spherical and rectilinear forms up to 2 metres in size. Ensuring that the oxygen content of purge gas is low enough to permit welding without incurring defects or oxidation the use of a weld purge monitor® is essential.
Flexible Purge Enclosures
Looking at a typical flexible enclosure range of the Argweld® brand, they have ultra violet stabilised engineering polymers used throughout, during manufacture. The vertical sides are made from translucent material and the top is constructed using optically clear sheet. High integrity tapes and adhesives are employed to give permanent sealing of the various panels and leak tight zips are incorporated to facilitate entry and removal of equipment or parts for fabrication.
An integral principle access zip is provided and this has a total length typically 60% greater than the enclosure diameter ie a 900 mm enclosure will have a 1400 mm long main zip. Additional entry points provide for operators gloves. A service panel incorporates access ports for welding torches and for electrical leads and cooling water supplies. Purge gas entry ports and an exhaust valve to vent displaced gas to atmosphere are incorporated into each enclosure.
Trailing Shields
The Argweld® brand also has a range of Trailing Shields that have been designed specifically for use with GTAW (TIG) or PAW (plasma) welding torches and provide a high level of additional inert gas shielding to supplement that supplied by the basic torch.
The welding torch is mounted on the leading end of the shield and inert gas fed through one or more ports behind the fusion zone.
A seal between the shield and the work is ensured through the use of a flexible, pre-formed and easily replaceable silicone skirt. Turbulence inside the cavity is avoided by passing the gas through a mesh filter above the fusion zone.
Curved and flat variations are stocked for most common diameters of pipes and vessels. Larger, more robust, versions for attaching to automatic/mechanised welding systems are manufactured to order.
Weld Purge Monitors®
An often overlooked prerequisite when welding titanium alloys is to ensure that the oxygen content of the purge gas is very low.
Oxygen measuring devices are available from various sources, but only the Argweld® range have been designed specifically for the welding industry. The instruments are called Weld Purge Monitors® of which there is a complete family of 5. They can be used in conjunction with weld purging systems in tanks, pipes, chambers and vessels being purged completely or to measure oxygen content inside welding enclosures.
PurgEye® instruments can be used for continuous measurement with free flow of the purge exhaust gas across the sensor. They can also be employed as sampling instruments using either the hand vacuum pump or the integrally mounted electronic pump fitted in some models, extracting samples from the purged volume on a predetermined basis.
Available Instruments
Introduced in 2012 following a lengthy development period and extensive on-site testing the PurgEye® range of instruments represents the highest standards in accuracy, ease of use and flexibility.
PurgEye® 100 is a hand held or tripod mounted instrument to display from 20.9% to 100 ppm Oxygen.
PurgEye® 300 reads down to 10 ppm with extreme accuracy and is a Zirconia cell instrument, to display from 1,000 to 10 ppm. Mains driven, the instrument has integral switching software to control external devices like power supplies or alarms and software to give QC print of results for each weld when connected to a PC.
For simpler applications where switching of external devices or data acquisition are not required, the PurgEye® 300 Nano is the lowest cost instrument anywhere to be able to read very accurately down to 10ppm.
PurgEye® 500 has all of the features of the PurgEye® 300 but additionally incorporates an internal pump to extract gas samples from the enclosure at preset intervals, giving fast reaction times.
The fully integrated PurgEye® 600 incorporates large touch screen controls. It also records all purge data for each weld and makes it available for downloading via a USB port onto a memory stick or by connecting directly to a PC for subsequent evaluation as a quality control document. The ‘600’ also has on/off switching capability for alarms or welding equipment in the event of rise in oxygen levels during welding outside pre-set limits as well as an integral electronic sampling pump.
For accurately measuring oxygen content in an enclosed space inside a pipeline is difficult. The time taken for the exhaust gas to travel up the pipeline can be too extreme to be tolerated. In addition, the gas sample is prone to contamination due to the outgassing inside the gas tube during its travel.
The PurgEye® 1000 solves this problem by placing the gas sampling head next to the purge space and sending the results electronically down the pipeline to the monitor that gives instant readings.
Manufactures of the Argweld® and PurgEye® product ranges, Huntingdon Fusion Techniques Ltd, have over 40 years experience of working with the welding of Titanium and willingly provides extensive expertise to its customers to assist them to overcome some of the difficulties that occur when trying to weld this difficult material.
