Page 100 - Ship Construction.DJ Eyres 6Ed

Ch09-H8070.fm Page 89 Wednesday, October 18, 2006 7:36 AM
Welding and Cutting Processes used in Shipbuilding 89
ELECTRO-GAS WELDING Of greater interest to the shipbuilder is a
further development, electro-gas welding. This is in fact an arc welding
process which combines features of gas shielded welding with those of
electro-slag welding. Water-cooled copper shoes similar to those for the
electro-slag welding process are used, but a flux-cored wire rather than a
bare wire is fed into the weld pool. Fusion is obtained by means of an arc
established between the surface of the weld pool and the wire, and the CO 2
--- ใช้เพื่อการศึกษาเท่านั้น---
or CO with Argon mixture gas shield is supplied from separate nozzles
2
or holes located centrally near the top of the copper shoes. The system is
mechanized utilizing an automatic vertical-up welding machine fed by a
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
power source and having a closed loop cooling circuit and a level sensor
which automatically adjusts the vertical travel speed.
The process is more suitable for welding plates in the thickness range of
13 to 50mm with square or vee edge preparations and is therefore used for
shipbuilding purposes in the welding of vertical butts when erecting side
shell panels or for the vertical shell butt joints when joining building blocks
on the berth or dock. For this purpose the use of a single or double vee butt
with the electro-gas process is preferable since this permits completion of
the weld manually if any breakdown occurs. A square butt with appreciable
gap would be almost impossible to bridge manually.
ึ
LASER WELDING Laser welding is being used in the shipbuilding indus-
try and shows much promise as a welding process that offers low heat input
and therefore minimum distortion of welded plates and stiffeners.
For shipbuilding welding applications the laser source is either CO2
(see Laser Cutting) or Nd:YAG (neodimium-yttrium-aluminium-garnet)
crystals. Because of the wide range of applied powers and power densities
available from Nd:YAG lasers, different welding methods are possible.
If the laser is in pulsed mode and the surface temperature is below boiling
point, heat transfer is predominantly by conduction and a conduction
limited weld is produced. If the applied power is higher (for a given
speed), boiling begins in the weld pool and a deep penetration weld can
be formed. After the pulse, the material flows back into the cavity and
solidifies. Both these methods can be used to produce spot welds or
‘stake’ welds. Laser ‘stake welding’ has been used in shipbuilding, stiff-
ening members being welded to plate panels from the plate side only. A
seam weld is produced by a sequence of overlapping deep penetration
‘spot’ welds or by the formation of a continuous molten weld pool.
Pulsed laser welding is normally used at thicknesses below about 3 mm.
Higher power (4 to 10 Kw) continuous wave Nd:YAG lasers are capable
of keyhole type welding materials from 0.8 to 15 mm thickness. Nd:YAG
lasers can be used on a wide range of steels and aluminium alloys. Also
because of the possibility of using fibre optic beam delivery Nd.YAG
lasers are often used in conjunction with articulated arm robots for