Part 3 Welding Enclosures and Trailing Shields
In Parts 1 and 2 the author outlined the problems in selecting the optimum gas for use during weld purging and then analysed the options available to engineers when choosing a purging technique for tube and pipe welding.
This article examines protective enclosures and trailing shields as potential solutions to the need to prevent contamination during welding.
Traditionally, permanent metal enclosures have been used when welding materials that are sensitive to contamination from atmospheric gases. Such enclosures may be evacuated using vacuum pumps or purged with inert gas.
These solutions are expensive, time consuming and occupy a large permanent footprint – clearly quite unsuitable for routine welding applications. A typical steel GTAW vacuum enclosure with a volume of 1 cu m costs in the region of £11,000 1 and a simple inert gas version about £5000 2. A high vacuum electron beam welding plant of similar size is likely to cost upwards of £60,000.
Conscious of the need for an economic solution, engineers developed flexible enclosures that exploited the opportunities offered by advanced engineering polymers. These innovative products offered attractive advantages over both vacuum and glove box alternatives; a significant reduction in cost, very small and not necessarily permanent floor footprint and availability of a range of sizes from stock.
Flexible Enclosures are now widely used within the aerospace, medical, auto sport and pharmaceutical industries. Size for size these can cost less than 10% of a metal glove box and only 2% that of a vacuum system.
Additionally the biochemical, medical, food and beverage, semiconductor and nuclear sectors all take advantage of the low cost and ease of use.
Fig 1. Large flexible enclosure. This is used for training purposes in addition to providing opportunities for multiple access for welding of large components.
Fig 2. Robot welding cell contained within custom-built flexible enclosure. This system is used by Cranfield University 4 during weld additive manufacturing operations for the aerospace industry. The leak-tight enclosure is filled with argon and can provide protection during welding of sensitive materials such as titanium alloys.
With standard enclosures the vertical sides are made from translucent material and the top is constructed using optically clear sheet. Robot welding cells employ ‘soft glass clear’ pvc.
Ultra violet stabilized engineering polymers are used throughout during manufacture. Material thickness is nominally 0.5 mm (480 microns).
Standard products are fitted with leak-tight principle access zips 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, electrical leads and cooling water supplies. A purge gas entry port and an exhaust valve to vent displaced gas to atmosphere are incorporated into each enclosure.
In the event of accidental damage a complete repair kit is provided with each enclosure.
Recent developments have led to the introduction of connections for some of the ¬¬exhaust gas to be vented through a Weld Purge Monitor® and Dew Point Meter so that the operators can be assured that their working environment will satisfy the prescribed oxygen and any specified dew point level.
Size and shape can be made to meet customer requirements. Standard models from 0.3 to 3.0 cubic metres are generally available from stock. Weight is very low – the collapsed volume of a 1.25 metre diameter system is less than 0.2 cubic metres and weighs only 8 kg. They can thus be moved easily and stored efficiently so floor footprint is minimised.
Large viewing area
The entire upper section is manufactured from optically transparent
ultra violet stabilized engineering polymer. This offers the opportunity for use by several operators at the same time – ideal for training purposes.
Multiple access points
Systems can be manufactured with numerous access locations for personnel gloves and gas/electrical entries. Large leak-tight zips afford easy access for components.
The oxides formed when some metal alloys are exposed to high temperatures can cause undesirable discolouration and loss of mechanical properties. For these reasons, all parts of the weld and heat-affected zone must be shielded from the atmosphere until the temperature drops below around 400°C.
Primary shielding of the molten weld can be provided by careful design of the welding torch. Standard water-cooled welding torches equipped with large ceramic cups and gas lenses can be suitable, but a large ceramic cup is necessary to provide adequate shielding for the entire molten weld. Secondary shielding is commonly provided by trailing shields, the function of these is to protect the solidified weld metal and associated heat-affected zones until temperature falls below a critical value, typically 400°C. Trailing shields are generally custom-made by welders to fit a particular torch and a particular welding operation.
Trailing Shield technology
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.
Suppliers such as Huntingdon Fusion Techniques Ltd 5 design and manufacture trailing shields to interface with mechanised and manual gas tungsten arc welding torches. In addition to ensuring total inert gas coverage of the fusion and heat affected zones these devices also give optimised and smooth distribution of gas, thus avoiding turbulence.
Gas shielding to the root side of fusion zone may also be necessary for complete protection against oxidation. Makeshift shielding devices are often employed including the use of plastic, aluminium or stainless steel foil to completely enclose the work piece and protect the work piece by admitting inert gas. Local inert gas coverage using small temporary shields can also be used for this purpose. Another alternative is to use copper backup bars but these can be expensive and clumsy. Trailing shields can be made to offer weld root protection.
Fig 3 Trailing Shield concept showing how the gas shield protects the cooling metal following welding.
The range of shields available is extensive and can accommodate internal and external curved surfaces in addition to linear weld joints. Suppliers also have expertise available to provide mechanised welding shields. Modifications can be made to provide increased shielding for extra protection if required. This may arise from a need to cover hot metal on each side of the fusion zone or behind the fusion zone when heat input during welding is particularly high.
Choosing a shield
Selection is dependent upon a number of factors, but the following need to be considered;
- the extent to which oxidation is to be limited along the weld length.
- thickness of material being welded (thicker sections will remain hotter longer).
- sensitivity of the material to oxidation (titanium alloys and some stainless steels are particularly sensitive to contamination by residual oxygen).
- welding process being used (PAW has a different heat input than GTAW).
- welding speed and current
Fig 4. Selection of Argweld® 6 shields. The product range covers all applications between flat and 25 mm diameter for external and internal weld configurations.
Fig 5. Internal design showing welding torch clamped in position.
Fig 6. External weld design showing robust silicone skirt to ensure efficient gas sealing.
- TFS Technologies, Inc. Albuquerque, USA
- Precision Technologies Inc. Enfield CT, USA
- Wire and Arc Additive Layer Manufacture. J. Mehnen et al Manufacturing Department, Cranfield University, UK
- Argweld® is a registered product of Huntingdon Fusion Techniques
By Dr. Michael J. Fletcher M.Sc. Metallurgy
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