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
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
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
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
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.