Keyhole Gas Tungsten Arc Welding

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Keyhole Gas Tungsten Arc Welding

• Keyhole GTA Welding of 8 mm wall thickness AISI
  304 pipe

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Overview What is Keyhole GTAW

• Keyhole GTAW (TIG) is a novel TIG process that
  takes advantage of arc forces and surface tension
  to create a robust, full-penetration keyhole.
• It is generally operated with welding currents
  above 300 amps and applied to materials between 3
  and 12 mm thickness.

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User benefits

• Very fast, single pass welds
• Quality of GTAW
• Square-edge preparations
• Conventional power sources
• Energy efficient
• Minimum consumables
• Cost-effective
• Builds on operator experience with GTAW

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Typical performance figures

• 3mm C-Mn Steel, Stainless Steels and Titanium at
  750mm/min
• 4mm 3CR12 ferritic Stainless Steels at 600 mm/min
• 6 mm SAF 2205 duplex Stainless Steel at 500
  mm/min
• 8 mm C-Mn Steel at 400 mm/min
• 10 mm AISI 304 austenitic Stainless Steel at 300
  mm/min
• 14 mm Grade 2 Titanium at 250 mm/min
• Applications to plate and pipe.

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Example Bead on pipe (5 mm wall.)
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The process in more detail
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A brief history of Keyhole GTAW
This variant of the GTA welding process was first
discovered at the CSIRO1 in 1996. First
successful generation of a GTA keyhole was
achieved on 5 mm duplex stainless steel. The
first industrial application began late 1997 for
the welding of rail wagons The process is now
being used by various industries in Australia,
USA, Europe and Korea, and subject of a major EU
project (HIPROTIG) In 2007 CSIRO assigned the IP
to Ultratig2 for commercialisation
1CSIRO is the principle publicly funded research
organisation in Australia 2Ultratig was formed by
Dr Laurie Jarvis, principle inventor of the
process while at CSIRO
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The problem with high current TIG (GTAW)

• The images show a high current weld pool and a
  soap film analog of its surface.
• As might be inferred from the soap film analog,
  the surface geometry results from a dynamic
  balance between surface tension and arc pressure.
  Arc pressure is required to inflate the surface,
  but the stability is low.
• High current GTAW research has focused on
  increasing stability through reduction of the arc
  pressure.

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The solution Alternate surface geometries can be
stable
If the arc can open a hole through the plate the
surface of the pool can become anchored to both
top and bottom surfaces (see soap film below) to
form a stable structure. This is achieved in
keyhole GTAW.
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Basic process constraints

• Considerations of the need to maintain the weld
  pool geometry lead to several important
  conclusions
• Electrode geometry is critical
• The process is not suited to highly (thermally)
  conductive materials such as aluminium because
  the root bead becomes very wide
• As materials become thicker the welding speed
  must be reduced otherwise the molten root bead
  becomes too long.
• When the material becomes too thick surface
  tension will not support the pool and it will
  fall.
• Out of position welding presents very significant
  challenges although progress has been made.
• On the other hand, the process robustness means
  that
• There is plenty of scope for further development.

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Physical illustrations of the keyhole formation

• Clockwise
• crater of an abruptly terminated weld,
• longitudinal cross section, and
• macro showing interrupted solidification.

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Features of Keyhole GTAW
High quality of GTAW Full penetration keyhole
mode High welding speeds Square-edge
preparations Conventional power sources Minimum
handling Minimum consumables Very robust
operating characteristics Single pass joint
completion
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Essential and optional process requirements

• Essential
• GTAW torch designed for keyhole mode operation
• 600 to 1000 amp constant current power source
• Water cooling for the welding torch
• An arc starting unit compatible with the
  anticipated welding currents
• Process mechanisation
• Operator control console or pendent
• Clean, squared edges with good fit-up (typically
  lt 0.5 mm gaps)

Optional Trailing and backing shrouds Wire
feeder Back purge pressure control Programmable
controller Remote viewing (e.g. CCT) Mechanical
or other device to control electrode-to-work-piece
stand-off
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Comparing keyhole with conventional GTAW
Keyhole GTAW AISI 304 10.5 mm thick Closed square
butt 50 g/m filler addition 1 pass at 300
mm/min. Arc-on time 3 min 20 sec /m.
Conventional GTAW AISI 304 10.5 mm thick Single V
preparation 1000 g/m filler addition 7 passes at
200 mm/min Arc-on time 35 min/m.
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Performance guide

• 3mm C-Mn steel, stainless steels and titanium at
  750mm/min
• 4mm 3CR12 ferritic stainless steels at 600 mm/min
• 6 mm SAF 2205 duplex stainless steel at 500
  mm/min
• 8 mm C-Mn steel at 400 mm/min
• 12 mm AISI 304 austenitic stainless steel at 250
  mm/min
• 14 mm Grade 2 titanium at 250 mm/min
• Thicker materials can be welded using multi-pass
  techniques with a keyhole GTAW root pass.

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Example of a weld in AISI 304 stainless steel

• Macrograph of a keyhole GTA weld in 10.5 mm thick
  AISI 304 stainless steel plate (as welded) and
  micrograph of the root region

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Example of a weld in ferritic stainless steel
Macrograph of a keyhole GTA weld in 6.5 mm thick
3Cr12 (12 chromium) steel plate (as welded).
Root region is shown at higher magnification
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Phase balance in duplex stainless steels can be
controlled

• Significant work has been carried out with duplex
  stainless steels, where nitrogen additions were
  made to the shielding gas to control the phase
  balance.

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The process has been applied to nickel alloys

• Macrograph of a keyhole GTA weld in 10 mm thick
  Inconel 718 plate (as-welded). Berahas tint
  etchant. Higher magnification micrographs showing
  absence of cracks in weld centreline and the HAZ,
  respectively.

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Keyhole GTAW has great potential for titanium

• Macrographs of keyhole GTA weld (top) and
  conventional GTA weld (bottom) in 13 mm thick
  ASTM B265 Grade 2-95a (CP titanium) plate. The
  conventional GTA weld was made using matching
  filler material, a double-V edge preparation and
  6 welding passes.

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Zirconium has also been welded

• This macrograph is of a keyhole GTA weld in 6.35
  mm thick commercially pure zirconium, ZR 702
  plate (as-welded). An additional dressing pass
  has been laid for improved bead profile.

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Contact Details

• General enquiries enquiries_at_ultratig.com
• Technical laurie_at_ultratig.com
• Sales and marketing agrant_at_ultratig.com