Society (AWS)1 said it, and I agree. Well-done titanium welds look
like frozen mercury: shiny and reflective. A couple of basic factors
make titanium different from other metals. These unique
characteristics, and a lack of understanding about them, can lead to
the commonly held opinion that titanium is just too hard to weld.
Where do you begin to make good gas tungsten arc welds? Titanium is a
reactive metal that forms compounds with less than optimum properties.
Heated in air, the part surface contains brittle carbides, nitrides,
and oxides, each of which can reduce the fatigue resistance and notch
toughness of the weld and heat-affected zone (HAZ). Not only do you
need to protect the surface being welded, you also need to protect the
back side of the weld, which is just as sensitive.
Copper purge fixtures direct the argon over the surface of the part.
Copper or aluminum purges that have not been hard-nickel-plated to
prevent metal scuffing can allow localized alloying on the surface of
titanium parts.
Chlorine from the perspiration on your hands can create localized
corrosion. "White-glove" treatments are not just a sign of quality in
welding titanium; using lint-free gloves after the final cleaning
before welding may be necessary for the highest-quality welds.
Any time the metal reaches a temperature of 900 to 1,000 degrees F,
brittle oxygen-stabilized alpha phase (or α-case ) can form not only
on the weld surface and its back side, but also on grinding tools2.
Frictional heat, especially from aluminum oxide (Al2O3) wheels, can
create high enough temperatures to embrittle the surface. Carbide-grit
wheels are better because they have no aluminum to contaminate the
weld. A gentle touch is best, because titanium has a low thermal
conductivity and needs to be kept below the 500 degree F mark, where
scaling begins.
Weld Preparation
Weld preparation should include removing any oil, grease, dirt, or
grinding dust from the surfaces to be joined. Steam cleaning or an
alkali dip in a dilute solution of sodium hydroxide can remove most of
these contaminants. To remove the last remaining organic compounds
just before welding, use a lint-free glove and methyl alcohol,
acetone, or other chlorine-free solvent. Because most of these
solvents have a low flash point, be sure they have fully evaporated
before striking an arc.
On the most critical parts, using a small hot-air blower (hair dryer-
style) to warm the part slightly ensures no moisture has condensed on
the surface to be welded. Don't overlook the fact that rubber gloves
may contain chlorine as part of a vulcanizing process. Plastic gloves
are recommended.
Pure Argon Applied Correctly
The argon must be 99.999 percent pure. Even if the argon is as pure as
the 50 parts-per-million (PPM) range (99.995 percent), some yellow-
straw discoloration can result. Many shops strive to maintain a 10-PPM
contamination level during welding. If the color begins to mottle, or
if it exhibits any hint of blue, the argon isn't pure enough, or
you're not applying it correctly. Start the argon gas flowing for
several seconds before using the high-frequency start. If you have
enough shielding and the argon is being dispersed evenly over the
part, you should see a uniform color.
Mottled or swirl patterns usually indicate too much argon is flowing
(see Figure 1 ). Argon's density is greater than air, so it tends to
flow over the surface of a part in the same way water does. Where
eddies occur, air can become mixed with the argon cover gas and create
swirl patterns.
Figure 1
What really separates titanium welding from most other types of GTAW
is the need for an argon cover on the weld's back side. Wherever the
titanium is heated, brittle alpha-case can form. For very complex
parts with interior passages or parts that require a lot of welding
repairs, glove boxes may offer an economical answer. For parts too
large to fit through the glove box, special flexible polyethylene
plastic bags, complete with attached gloves, can be used. Use a purge
monitor to see when the bag contains clean-enough argon, strike an
arc, and weld away. Working in airtight gloves, especially for
extended periods, can be hot, but doing so is part of the challenge of
working with titanium.
Finishing Up
The end of the weld is equally important. The titanium is hot, and the
protective argon flow is still needed until the metal has cooled below
about 500 degrees F. Color can be your best indicator of sufficient
argon use. Some discoloration may occur beyond the HAZ and, depending
on the criticalness of the weld, may be acceptable.
Unwelcome Fireworks
Be certain to contain the titanium dust from grinding operations.
Titanium flake and powder are used in fireworks and should not be
allowed to accumulate. Many times the white sparkling effect in aerial
shells or pyrotechnic fountains comes from the burning reaction of
titanium. The reaction can be similar to the thermite welding reaction
if mixed dust is allowed to react. While fireworks are fun on the
Fourth of July, an unexpected release of this kind of energy can be
hazardous. Waterfall filters can be effective in controlling titanium
dust.
Why use GTAW for welding titanium and not gas metal arc welding? Many
times, it's easier to adjust the temperature of the weld pool, ensure
there is just enough metal to prevent undercuts, or add a filler metal
that is not the same as the base metal or metals with GTAW. By adding
the metal separately from the electrode maintaining the weld pool, you
have more options and more control.
Blueprint for Success
High-purity argon; clean work areas free of combustible grinding
debris; the white-glove treatment after thorough cleaning; well-
designed and -maintained purges on both sides of the part to
distribute the argon evenly; and the technique of holding the torch in
place until the metal has cooled below 500 degrees F should produce a
clean, silver-colored titanium weld every time.
How to weld titanium
IT will come as no surprise to fabricators with experience in handling
titanium that special consideration needs to be taken during
preparation and welding.
Titanium and its alloys are reactive and can become contaminated by
atmosphere gases. Adequate shielding using inert gas is essential if
repeatable, high- quality joints are to be made.
For many years it was believed that welding-titanium could only be
performed satisfactorily in sealed chambers equipped with glove ports
and viewing ports and filled with high-purity, inert gas such as
argon.
A better understanding of the mechanism of metal/gas interaction has
shown that for some applications, as long as adequate local shielding
is maintained during welding, the use of sealed chambers with or
without vacuum pumping systems may not be absolutely necessary.
This understanding has afforded possibilities for welding large
titanium fabrications and significantly extended the application areas
for the metal and its alloys into major structural forms, especially
in the aerospace industry, the motor sport industry and latterly in
the petrochemical industry. The primary requirement then is to provide
a protective gas atmosphere to the joint area during welding and
subsequent cooling.
Basic guidelines
The weld purge gas must be used correctly. It should be 99.999% pure,
even 99.995% gas can lead to some yellow discoloration.
If the colour of the welded metal begins to mottle, or if it exhibits
any hint of blue, the gas isn't pure enough, or it isn't being applied
correctly. The gas flow should be started several seconds before
starting to weld. If there is enough shielding and the argon is being
dispersed evenly over the part, a uniform colour will be seen.
Protective gas flow is still needed after welding until the metal has
cooled below about 250°C to prevent discolouration.
Good housekeeping
High-purity gas must be used. All work areas must be thoroughly
cleaned. All work components should be considered for cleaning,
depending upon the welding method.
Any titanium filler rod material should be thoroughly checked for
cleanliness.
The welding tungsten should be clean and new and not be contaminated
by the welding of other materials on previous work. Welding torches
and their associated hoses will occasionally have leaks that will draw
in atmospheric gases to the shielding gas, so they should be leak
tested prior to use. Cotton gloves should be worn for handling the
parts after they have been cleaned.
Accessories for titanium welding
Speciality products are now available to provide quality gas coverage,
including flexible enclosures for smaller components, weld purge
monitors and trailing shields.
Additionally there are "clean" expandable stoppers available for use
in sealing tubular components during gas purging.
The principle advantage of flexible welding enclosures, over the more
traditional solid metal versions is a significantly lower cost.
Additional benefits include minimal weight, easy movement and storage
when not in use and the relatively fast times to achieve a
satisfactory weld purge.
Flexible enclosures might be regarded as insufficiently robust for
welding operations, but they are made of tough PVC plastic and the
cold purging gas helps to quickly dissipate the heat that is generated
by welding. Early flexible enclosures were circular, but they are now
also produced in square and rectangular versions. Enclosures are
available up to 2m long and 1 x 1m in section.
Low-cost weld purge monitors are available for measuring oxygen levels
in any purge gas during the welding of titanium and its alloys. They
show oxygen levels down to 10ppm using an alphanumeric display and
incorporate automatic fault finding diagnostics, data acquisition
software and push-button operated menu.
Devices called trailing shields will fit any make of GTAW (Tig) or PAW
(Plasma) welding torch. They are available in flat format for sheet
metal welding and in radiused format to match any diameter of pipe or
vessel.
The radiused versions are manufactured for welding on inside and
outside radii. Smaller versions are manufactured to suit manual
welding.
Titanium Design and Fabrication Handbook for Industrial Applications
WELDING TITANIUM
Titanium and most titanium alloys are readily weldable, using several
welding processes. Properly made welds in the as-welded condition are
ductile and, in most environments, are as corrosion-resistant as base
metal. Improper welds, on the other hand, might be embrittled and less
corrosion-resistant compared to base metal.
The techniques and equipment used in welding titanium are similar to
those required for other high-performance materials, such as stainless
steels or nickel-base alloys. Titanium, however, demands greater
attention to cleanliness and to the use of auxiliary inert gas
shielding than these materials. Molten titanium weld metal must be
totally protected from contamination by air. Also, hot heat-affected
zones and root side of titanium welds must be shielded until
temperatures drop below 800 degrees F.
Titanium reacts readily with air, moisture, grease, dirt,
refractories, and most other metals to form brittle compounds.
Reaction of titanium with gases and fluxes makes common welding
processes such as gas welding, shielded metal arc, flux cored arc, and
submerged arc welding unsuitable. Likewise, welding titanium to most
dissimilar metals is not feasible, because titanium forms brittle
compounds with most other metals; however, titanium can be welded to
zirconium, tantalum and niobium.
In spite of the precautions which need to be taken, many fabricators
are routinely and economically welding titanium, making sound, ductile
welds at comparable rates to many other high performance materials.
One of the important benefits of welding the commercially pure grades
of titanium (i.e., TIMETAL 35A and 50A) is that they are over 99% pure
titanium and there is no concern for segregation. The same is true of
weld wire or rod in commercially pure grades.
WELDING ENVIRONMENT
Most titanium welding today is done in the open fabrication shop,
although chamber welding is still practiced on a limited basis. Field
welding is common. Wherever the welding is done, a clean environment
is necessary in which to weld titanium. A separate area, specifically
set aside for the welding of titanium, aids in making quality welds.
This area should be kept clean and should be isolated from dirt-
producing operations such as grinding, torch cutting and painting. In
addition, the welding area should be free of air drafts and humidity
should be controlled.
WELDING PROCESSES
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. All of these processes offer
advantages for specific situations. However, the following discussion
will be concerned primarily with GTA and GMA welding. Many of the
principles discussed are applicable to all processes.
GAS TUNGSTEN-ARC (GTA) AND GAS METAL-ARC (GMA) WELDING
The GTA process can be used to make butt joints without filler metal
in titanium base sheet of up to about 1/8-inch thickness. Heavier
sections generally require the use of filler metal and grooved joints.
Either the GTA or GMA welding process can be used, although GMA
welding is more economical for sections heavier than about one-half
inch. If the GTA process is used, care should be exercised to prevent
contact of the tungsten electrode with the molten puddle, thereby
preventing tungsten pickup.
POWER SUPPLY
A conventional power supply, connected d.c. straight polarity (DCSP),
is used for GTA welding of titanium. Reverse polarity (DCRP) is used
for GMA welding of titanium. A remote controlled contactor allows the
arc to be broken without removal of the torch from the cooling weld
metal, thereby maintaining inert gas shielding. Foot operated current
and contactor control, high frequency arc starting and shielding gas
timers are other desirable features.
WELDING TORCH
A water-cooled welding torch, equipped with a 3/4-inch ceramic cup and
a gas lens, is recommended for GTA welding of titanium. A one-inch cup
may be required for GMA welding.
Thoriated tungsten electrodes (usually 2% thoria) are recommended for
GTA welding of titanium. Pointed electrodes (end blunted) help to
control arc characteristics. The smallest diameter electrode which can
carry the required current should be used.
INERT GAS SHIELDING
Protection needs to be provided to titanium weldments on cooling down
to about 800 degrees F 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 degrees
F. or lower are used to provide the necessary protection. Separate gas
supplies are needed for:
1. Primary shielding of the molten weld puddle.
2. Secondary shielding of cooling weld deposit and associated heat-
affected zones.
3. Backup shielding of the backside of weld and associated heat-
affected zones.
PRIMARY SHIELDING
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 appearance.
SECONDARY SHIELDING
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 degrees F or lower.
Trailing shields are generally custom-made to fit a particular torch
and a particular welding operation. A schematic of a trailing shield,
useful for flat sheet or plate welding of titanium, is shown in Figure
12. 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 nonturbulent
flow of inert gas from the shield to the weld.
BACKUP SHIELDING
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.
How to weld titanium
2 October 2007
It will come as no surprise to fabricators with experience in handling
titanium that special consideration needs to be taken during
preparation and welding.
Titanium and its alloys are reactive and can become contaminated by
atmosphere gases. Adequate shielding using inert gas is essential if
repeatable, high quality joints are to be made.
For many years it was believed that welding-titanium could only be
performed satisfactorily in sealed chambers equipped with glove ports
and viewing ports and filled with high purity inert gas such as
argon.
A better understanding of the mechanism of metal/gas interaction has
shown that for some applications, as long as adequate local shielding
is provided and maintained during welding, the use of sealed chambers
with or without vacuum pumping systems may not be absolutely
necessary.
This understanding has afforded possibilities for welding large
titanium fabrications and significantly extended the application areas
for the metal and its alloys into major structural forms, especially
in the aerospace industry, the motor sport industry and latterly in
the petrochemical industry.
The primary requirement then is to provide a protective gas atmosphere
to the joint area during welding and subsequent cooling.
BASIC GUIDELINES
The weld purge gas must be used correctly. It should be 99.999 percent
pure - even 99.995 percent gas can lead to some yellow-straw
discoloration. If the colour of the welded metal begins to mottle, or
if it exhibits any hint of blue, the gas isn't pure enough, or it
isn't being applied correctly.
The gas flow should be started several seconds before starting to
weld. If there is enough shielding and the argon is being dispersed
evenly over the part, a uniform colour will be seen.
After Welding
Protective gas flow is still needed until the metal has cooled below
about 250°C to prevent discolouration.
Good Housekeeping
As mentioned above, high-purity gas must be used. All work areas must
be thoroughly cleaned according to recommended procedures that are
widely available.
All work components should be considered for cleaning, depending upon
the welding method. For example, any titanium filler rod material
should be thoroughly checked for cleanliness.
The welding tungsten should be clean and new and not be contaminated
by the welding of other materials on previous work.
Welding torches and their associated hoses will occasionally have
leaks that will draw in atmospheric are to the shielding gas, so they
should be leak tested prior to use.
Cotton gloves should be worn for handling the parts after they have
been cleaned.
The welded joint must be allowed to cool before gas coverage is
removed.
Accessories for Titanium Welding
Speciality products are now available to provide quality gas coverage.
Particularly low cost flexible enclosures for smaller components, weld
purge monitors and trailing shields.
Additionally there are 'clean' expandable stoppers available, made
from clinically clean materials for use in sealing tubular components
during gas purging.
Flexible Welding Enclosures
The principle advantage of flexible welding enclosures over the more
traditional solid metal versions is significantly lower cost.
Additional benefits also include the flexibility of minimal weight,
the ability to move and store when not in use and the relatively fast
times to achieve a satisfactory weld purge.
Flexible enclosures might be regarded as insufficiently robust for
welding operations, however they are made of tough PVC plastic and the
cold purging gas helps to quickly dissipate the heat that is generated
by welding. Early flexible enclosures were circular in section but
developments in manufacturing technology have now afforded
opportunities to produce square and rectangular enclosures as well.
Rectangular cross sections and the opportunity to produce to a length
to suit the customer's requirements have made the new additions
attractive to a wider range of end-users.
The rectilinear concept offers users the opportunity to match the
shape to meet specific requirements.
Such enclosures are available up to 2m long and 1m x 1m in section.
Weld Purge Monitors
There are now low cost instruments designed specifically for Weld
Purging, measuring oxygen levels in any purge gas during the welding
of titanium and its alloys.
They show oxygen levels down to 10 ppm using an alphanumeric display
and incorporate an impressive array of features, which include
automatic fault finding diagnostics, data acquisition software, and
push-button operated menu.
Trailing Shields
Devices called 'Trailing Shields' are available and they will fit any
make of GTAW (Tig) or PAW (Plasma) welding torch.
They are available in flat format for sheet metal welding and in
radiused format to match any diameter of pipe or vessel.
The radiused versions are manufactured for welding on inside radii as
well as outside.
Smaller versions are manufactured to suit manual welding where their
size and light weight do not restrict the manual welding operation.
For mechanised welding machines, where weight and size considerations
are less constraining, the Trailing Shields are larger giving greater
gas coverage and allowing the machines to weld much faster than they
would be able to without trailing shields.
Tube, Pipe and Tank end blocks
In recognition of the cleanliness required for reactive metal welding,
clinically prepared Expandable plugs are also available and widely
used to seal pipes so that good gas coverage is maintained.
Such plugs are available from 12 to 160 mm (0.5 to 6 in) diameter.
On Sep 22, 4:13 pm, suneel gore <novatekc...@yahoo.co.in> wrote:
> I am looking for informationon welding of Titanium , any latest development in welding of Titanium & alloys by TIG process , & general prcautions in welding of same
> Thanks
> Suneel Gore
>
>
> Keep up with people you care about with Yahoo! India Mail.. Learn how.http://in.overview.mail.yahoo.com/connectmore
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