One of the most significant early breakthoughs occurred in 1912 in Sheffield when chromium/iron alloys were found to be corrosion resistant. Since then we have witnessed the introduction of low alloy creep-resistant steels, nickel-based alloys with elevated temperature properties and, more recently, the development of lightweight titanium alloys offering high strength-to-weight characteristics.
Optimum properties of all these materials is only achieved by precisely controlling the balance of elements. The ideal composition for every application has only been realised thanks to intensive research work by metallurgists but if elements are lost during subsequent manufacturing processes such as welding or other elevated temperature excursions, the corrosion and mechanical properties can be affected significantly.
Fusion welding of stainless steels provides a good example where loss of corrosion resistance can be significant. If welding is carried out in air and even where oxygen levels are as low as 50 ppm, the effective chromium content can be reduced and since this is the principle element added for corrosion resistance, it is a major consideration.
Another consequence of chromium loss during welding is the effect on mechanical properties. In the chromium/molybdenum/vanadium materials for example, developed for their high temperature creep resistance, enhanced hardenability, wear resistance, impact resistance and machinability, any reduction in chromium content can affect these properties. Furthermore, the sensitivity of these materials to contaminating products such as hydrogen in the shield gases needs to be considered. Care needs to be taken in selection of consumables and it is essential that any shield gases are of high purity
The thermal cycles along with any local contamination involved in fusion welding titanium alloys can give rise to embrittlement of the alloy. Their reactive nature makes it essential to address the requirement for thorough pre-cleaning and particularly oxidation at the high temperatures involved in arc welding.
All in all, then, there is a strong material case for eliminating oxygen and other contaminants from the locality of the weld by purging with inert gas such as argon. A wide variety of purging solutions have been developed to combat the problem, including pipe welding systems and trailing shields but there is increasing demand for complex three-dimensional components using alloys that are sensitive to oxidation and contamination. These are best fabricated in sealed enclosures where the entire welding operation is carried out in an inert atmosphere where contamination can be eliminated and oxygen levels reduced to well below 10 ppm.
Where quality and freedom from oxidation and contamination is crucial, total protection is afforded by using weld enclosures. Metal chambers and glove boxes have been in use for decades, and these are effective in providing a totally inert atmosphere during fusion welding.
Although a traditional metal glove box can provide adequate protection it has a series of limitations. These have now been addressed successfully with currently available flexible alternatives.
There have been considerable advances in enclosure development since the concept was introduced over two decades ago. For example, Huntingdon Fusion Techniques in the UK has spearheaded a drive to design systems specifically for the welding industry. The company has been at the forefront in developing these enclosures and has exploited the opportunities offered by advanced engineering polymers.
These innovative products offer significant attractions over metal glove box alternatives; a significant reduction in cost, very small floor footprint and availability of a very wide range of sizes. The HFT product has rapidly become the preferred alternative enclosure globally. The flexible option has played a significant part in 3-D production and additive manufacture using arc welding is now being undertaken with CNC or robot systems, together with welding plant, all accommodated inside enclosures, some the size of small rooms.
A commercial spin-off from Cranfield University in the UK uses flexible enclosures to produce aerospace parts with the Wire Arc Additive Manufacturing (WAAM) process.
MONITORING THE PURGE GAS OXYGEN CONTENT
The fact that even very small amounts of oxygen in the purge gas can cause discolouration around the weld underbead makes it desirable that sensitive instruments be employed to measure residual oxygen.
Two essential characteristics of a suitable instrument are that it must have an adequate measuring range and it must sample the gas for oxygen content inside the purge volume.
Although many commercial monitoring systems are available these are generally not sensitive enough to meet the requirements for quality welding of alloys such as some stainless steels and most titanium alloys where the presence of oxygen levels as low as 20 ppm are essential if loss of corrosion resistance and reduction in mechanical properties are to be avoided.
Typical of advanced monitoring systems is the PurgEye family of instruments from Huntingdon Fusion Techniques in the UK of which the recently introduced Argweld PurgEye 500 Desk is totally compatible with the requirements to continuously monitor oxygen levels in flexible enclosures.
The instrument is fitted with an integral pump to deliver a regular flow of exhausting weld purge gas to the oxygen sensor to ensure consistent measurements and readings. Advanced software is used for control and communication purposes.