Trailing Shield Information
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I have worked in the welding industry for almost
30 years. During that time I held a variety of positions from welder, welding
technician, welding engineer, sales engineer, consultant. 20 of those years
was/is dedicated to the fluid mixing industry where exotic alloys like titanium
and zirconium are used daily. This is where I gained my knowledge and experience
welding outside the bubble.
Through many years of trial and error I have developed a user friendly trailing
shield. Please take some time and view the welds below that were accomplished by
using trailing shields. As you can see, the weld quality is excellent. Click on
the trailing shield thumbnails to enlarge the pictures.
Important differences between titanium and steel
or nickel-base alloys need to be recognized. These are:
►Titanium’s lower density
►Titanium’s lower modulus of elasticity
►Titanium’s higher melting point
►Titanium’s lower ductility
►Titanium’s propensity to gall
►Titanium’s sensitivity toward contamination during
Titanium and its alloys are most often welded
with the gas tungsten-arc (GTA or TIG) and gas metal-arc (GMA or MIG) welding
processes. Resistance, plasma arc, electron beam and friction welding are also
used on titanium to a limited extent.
Protection needs to be provided to titanium weldments on cooling down to about
800°F (427°C) as well as to the molten weld puddle in order to prevent
contamination by air. During GTA and GMA welding, argon or helium shielding
gases of welding grade with dewpoint of -50°F (-46°C) or lower are used to
provide the necessary protection. Separate gas supplies are needed for:
►Primary shielding of the molten weld puddle.
►Secondary shielding of cooling weld deposit and
associated heat affected zones.
►Backup shielding of the backside of weld and
associated heat affected zones.
Primary shielding of the molten weld puddle is
provided by proper selection of the welding torch. Standard water-cooled welding
torches equipped with large (3/4 or 1-inch) ceramic cups and gas lenses, are
suitable for titanium. The large cup is necessary to provide adequate shielding
for the entire molten weld puddle. The gas lens provides uniform, nonturbulent
inert gas flow.
Argon is generally used in preference to helium for primary shielding at the
torch because of better arc stability characteristics. Argon-helium mixtures can
be used if higher voltage, hotter arc and greater penetration are desired.
Manufacturer’s recommended gas flow rates to the torch should be used. Flow
rates in the vicinity of 20 cfh have proven satisfactory in practice. Excess
flow to the torch may cause turbulence and loss of shielding. The effectiveness
of primary shielding should be evaluated prior to production welding. An arc can
be struck on a scrap piece of titanium with the torch held still and with
shielding gas only on the torch. The shielding gas should be continued after a
molten puddle forms and the arc is extinguished, until the weld cools.
Uncontaminated, i.e., properly shielded, welds will be bright and silvery in
Secondary shielding is most commonly provided by trailing shields. The function
of the trailing shield is to protect the solidified titanium weld metal and
associated heat-affected zones until temperature reaches 800°F (427°C) or lower.
Trailing shields are generally custom-made to fit a particular torch and a
particular welding operation. Design of the trailing shield should be compact
and allow for uniform distribution of inert gas within the device. The possible
need for water-cooling should also be considered, particularly for large
shields. Porous bronze diffusers have provided even and non-turbulent flow of
inert gas from the shield to the weld.
The prime purpose of backup devices is to provide inert gas shielding to the
root side of welds and their heat-affected zones. Such devices often look much
like trailing shields and may be hand-held, or clamped or taped into position.
Water-cooled copper backup bars (or massive metal bars) may also be used as heat
sinks to chill the welds. These bars are grooved, with the groove located
directly below (or above) the weld joint. About 10 cfh of inert gas flow per
linear foot of groove is required for adequate shielding.
Makeshift shielding devices are often employed very effectively with titanium
welds under shop or field conditions. These include use of plastic to completely
enclose the workpiece and flood it with inert gas. Likewise, aluminum or
stainless steel foil “tents,” taped over welds and flooded with inert gas, are
used as backup shields. When such techniques are used, it is important that all
air, which will contaminate welds, be purged from the system. An inert gas purge
equal to ten times the volume of the air removed is a good rule-of-thumb for
irregular spaces. A moderate rate of inert gas should be maintained until the
weld is completed.
Argon is generally selected in preference to helium for use in trailing shields
and backup devices, primarily because of cost but also because it is more dense.
Helium, with its lower density, is sometimes used for trailing or backup
shielding when the weld is above the device. It is important that separate flow
controls are available for primary, secondary and backup shielding devices.
Timer controlled pre-purge and post-purge of torch shielding, and solenoid
valves with manual switches interlocked with the welding current for secondary
and backup shielding are also useful.
Bend tests evaluate ductility. For
this reason, the bend test made on
preproduction trial welds or on
extensions of production welds made
for that purpose, provides a good
evaluation of weld quality. A bend
sample in which the weld is
positioned perpendicular to the bend
axis assures uniform straining of
weld metal and heat-affected zones,
thereby giving more meaningful
results. Table 1 lists weld bend
radii for various titanium alloys.
Table 1. Bend radii for
Good quality welds should be capable
of being bent to the indicated radii
without cracking. Problems with
titanium welds are generally a
result of contamination due to
inadequate shielding. The color of
welds can be used as an indicator of
shielding effectiveness and,
indirectly, weld quality. Thus, any
indication of the quality level of a
single pass titanium weld is readily
apparent to the welder and any
inspector. Weld colors reflect the
degree to which the weld was exposed
to oxygen (air) at elevated
temperature. A bright silvery
metallic luster generally can be
taken as an indication of a good
weld, provided the weld joint was
clean and good techniques were
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