Reference for Designers & Welders
Close-up of a typical Atlas Technologies UHV aluminum
weld joint.
Aluminum UHV Weld Design
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Aluminum UHV Weld Design
Most facilities familiar with ultrahigh vacuum
(UHV) have experience welding stainless
steel to UHV standards, however many have
not had occasion to weld aluminum to meet
these standards. This brief guide is offered
for designing and welding aluminum and Atlas’ aluminum-to-stainless bimetallic flanges,
fittings and transitions.
Someone who has been welding stainless
steel must learn an entirely new set of rules
for welding aluminum.
Each welder will ultimately develop their
own methods and techniques for welding
aluminum. The following are some basic
guidelines for getting started in this process.
Mass
Design aluminum weld geometries that have
similar masses whenever possible. If a large
mass differential exist between the com-
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Weld Grooves
Machining 'V' shaped weld grooves into
aluminum weld joints permits the liberal use
of filler rod to reduce cracking. In the case
of tube weld joints, 'V' grooves should be
machined on a tube’s inner diameter (ID) and
penetrate about 2/3 of its wall thickness.
Gas Lens
Optional
Electrode Material
2% Ceriated Tungsten (W)
Electrode Tip Geometry
3/32” (2.38mm)
(Sharpen electrode tip to short, blunt taper.
Filler rod
4043 1/16” (1.58mm) Diameter
Filler rod 5356 if parts are to be anodized
Weld Appearance Bright metallic with no soot (Yellow
discoloration results from Oxygen contaminated Helium)
Maximum Temperature at EXW Bond
300°C
(Keep damp rag on EXW bimetallic bond joint to prevent
overheating during welding)
Porosity Tungsten sputter or dip into the weld joint may
result in undesirable weld porosity.
Atlas Technical Reference
Aluminum Thickness Under 1/4” (6mm)
ponents being welded, say a chamber and
a flange, the more massive of the two will
Ideal for tube to tube and Atlas CF Flanges fitted with
be cooler leading the other to preferentially
weld-neck (WN) geometries.
melt. If similar masses are not possible, thermal chokes can be machined into the more
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massive part to constrain heat loss. Burying a Current
75-185A (foot-pedal control)
less massive component into its larger mate Gas
Thin sections, use 100% Argon
will also distribute heat in a more uniform
Thick sections, use 25% Helium / 75% Argon
manner and facilitate the welding process.
Gas Flow
15 to 25 cfm
Cleanliness
Cleanliness is crucial to aluminum welding.
Make sure that weld surfaces, including filler
rods, are freshly cleaned and completely dry.
Even though aluminum instantaneously forms
an oxide in air, a freshly abraded surface can
reduce thickness and contaminants present
in that oxide. This is important, as aluminum
melts between 580-650°C and its oxide
around 1760°C. Its worth noting that this oxide can sink into a molten weld and interfere Aluminum Thickness Over 1/4” (6mm)
with its integrity.
Ideal for flange to chamber and Atlas CF Flanges with
flush-mount (FM) geometries.
Preheating
Because of aluminum’s excellent thermal
conductivity (5x that of stainless steel), vast
amounts of heat are required when beginning
a weld. Heat typically spreads ahead of a
weld bead and begins to accumulate in a part
reducing the amount of heat required during
welding. Rather than stopping and starting a
weld, which often lead to leaks, we recommend using a foot pedal current controller.
During welding, stainless steel glows red at
welding temperature, aluminum however will
not, it just melts. A welder must observe the
molten aluminum weld puddle and use the
TIG torch to control the melt and solidification process; thus controlling size, shape and
porosity of the weld. Once a weld puddle
has been established, welding must move
along quickly otherwise the weld puddle will
spread or melt through the weldment.
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