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Welding Reactive Metals
 

Welding is a process by which two or more metals are fused together using heat. In its simplest form a relatively easy task, but for advanced forms of structure a task beset with quality problems.

Metallurgy has advanced the understanding of metal fusion and identified the known and probable causes of structural failure and contamination. Yet eliminating all contributory factors to quality issues is not practical, economical or necessary, and a pragmatic selective approach is usually taken with controlled degrees of imperfection measured and accepted.

This section highlights some quality aspects of metal fusion and the glove box weld chamber equipment we offer which has been specifically designed to minimise or eradicate the selected areas of concern.

Metals in their pure state are by definition without contaminants. It is gas or vapour contaminants that are adsorbed as a film on the surface of the base material or absorbed within the base material, that are contributory factors in weld defects.

Other contributory factors are consumable materials, which have the same potential for contamination as the base materials, and the environment within which the weld process takes place, which can be contaminated with oxygen, moisture and other elements.

Some of these other elements can be a by-product of the fusion process when the applied heat changes the material composition of the base materials, the consumables or the contaminants. These by-products are produced in gaseous or solid form and should be removed from the weld chamber environment.

Contamination can therefore be looked at separately in terms of base materials, consumables and the weld chamber environment, and can be addressed as such.

Base materials or weld components, ideally need to be free of gas and vapour prior to entry into the weld chamber. Surface contamination should be removed using an appropriate method which, could be any process from a simple degrease and wash-down to the more elaborate ultrasonic bath treatment. The remaining contamination, which will be gaseous and vapour, is readily removed by a combination of heat and vacuum or by vacuum only over a longer period. An antechamber can performs this function in either mode in a process termed "out-gassing". The total preparation process ensures the weld component will not contribute to any contamination issues by importation.

Consumables, like weld components, should be free of gas, vapour and solid matter. Reputable suppliers of consumables normally pack the supplies in protective barrier material to minimise contamination, but routine testing should be taken, along with ongoing precautions, for verifying quality of consumables. It should not be necessary to degrease consumables, but it may be desirable to outgas them.

The weld chamber environment needs to be established and maintained oxygen and moisture free. This can be accomplished to differing degrees of purity by two methods, the first being a gas purge process in which Argon is released into an enclosed chamber and vented, in what is termed an open loop or total loss system, and the second commences with the first gas purge process and then changes over to a closed loop or gas re-circulated process, only venting excess gas and admitting make up, torch gas or supplementary purge gas.

The open loop system is reliant upon the supply gas quality, flow rate, factors related to the leak rate of the enclosure, and factors pertinent to the level of contaminants introduced with weld components and consumables. These will all combine to reach a state of equilibrium for oxygen and moisture in the contained gas environment. A change in any contributory factor will change the state of equilibrium of oxygen and moisture in the contained gas. An increase in gas flow will have a marked effect on gas purity, particularly oxygen in the shorter time and to a less extent moisture over a longer period. The open loop system is therefore reliant upon high gas consumption to maintain low levels of oxygen and moisture. For high specification stainless steel enclosures this is in the order of 10 to 20 parts per million (ppm) oxygen and 50 to 60 ppm moisture, using high purity argon at a flow rate between 10 and 20 litres per minute. Polymers have a high leak rate at a molecular level and require considerably higher gas flow rates to maintain desirable oxygen levels.

The closed loop system uses a blower to re-circulate the argon gas through the enclosure and a chemical bed, which removes oxygen and moisture to well below 1 ppm. The blower circulates the gas at a high flow rate thereby maintaining a much lower state of equilibrium, usually less than 5 ppm for both oxygen and moisture under working conditions. However, re-circulated systems are not without their own problems, for the chemical bed is vulnerable to the weld process by-products.

The weld process produces soot as a by-product of introduced contaminants and gases/vapours as a by-product of contaminants and the fusion of metals. These by- products need to be removed prior to the gas entering the gas purifier to avoid or minimise the poisoning or de-activation of the chemical bed. In-line high efficiency particulate filters and activated carbon traps need to be employed to remove these undesirable elements. Whilst these devices protect the gas purifier by removing the targeted by-products, other gaseous elements not harmful to the chemical bed remain, and these can only be removed by further expensive filtration processes, or by gas purge dilution to avoid a build up to undesirable levels.

Given the two choices, the open loop, which has a high cost of consumables with gas quality parameters just within acceptable criteria for reactive materials, and the closed loop with high capital cost and very good criteria for reactive materials, it can be seen why a pragmatic and selective approach is usually taken. Reactive metals require an argon environment of less than 30 ppm oxygen to maintain good quality welds free of oxidation, and for this the open loop system is technically suitable, but for weld processes that demand the best attainable, a gas re-circulated weld enclosure must be employed.

Welding enclosures can be manufactured in any non-porous material, but permeability needs to be taken into account when considering technical parameters. Oxygen, as noted above, needs to be maintained at below 30 ppm for satisfactory weld quality and oxygen being a very small molecule will permeate or diffuse through many materials, some of which are typically used in the manufacture of glove boxes, as they are less costly to produce. To maintain required weld quality gas in containments of materials that have high rates of diffusion, will require a higher rates of gas flow in open loop systems and more frequent regeneration of the gas purifier chemical bed in the closed loop system.

Other considerations that need to be made concern the durability of the materials of construction and their ability to maintain the gas environment under arduous working conditions, and the risk of cross contamination where weld components are in contact with the containment material. Weld chambers manufactured from carbon steel present a very high risk of cross contamination with weld components made from reactive materials, and painting the surface only masks the potential until chipped and the paint then becomes an added contaminant for inclusion.

The specification for a weld chamber can be made from a wide choice of available options. Making those decisions often requires a balance between technical desirability and commercial reality, factors not readily available to most and upon which the success or failure of a capital purchase relies. We hope this overview of points will help those challenged with the responsibility of drawing up a specification (or evaluating one), make decisions from an informed base. Should readers have need to know more or would like specific points expanded upon we would welcome your enquiries, and they may well influence our review of future updates of this presentation.

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