Page 101 - Ship Construction.DJ Eyres 6Ed

Ch09-H8070.fm Page 90 Wednesday, October 18, 2006 7:36 AM
90 Ship Construction
welding fabricated units of complex shape. Since the beam can burn the
skin or severely damage the eyes Nd.YAG lasers require enclosures
within the fabrication shop which are fully opaque to the Nd.YAG laser
wavelength.
Hybrid laser-arc welding is also used in the shipbuilding industry. This
being a combination of laser and arc welding which produces deep penetration
welds with good tolerance to poor joint fit-up. The Nd.YAG laser is com-
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bined with a metal arc welding gas (LASER-MAG).
THERMIT WELDING This is a very useful method of welding which may
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
be used to weld together large steel sections, for example parts of a stern
frame. It is in fact often used to repair castings or forgings of this nature.
Thermit welding is basically a fusion process, the required heat being
evolved from a mixture of powdered aluminium and iron oxide. The ends of
the part to be welded are initially built into a sand or graphite mould, whilst
the mixture is poured into a refractory lined crucible. Ignition of this
mixture is obtained with the aid of a highly inflammable powder consisting
mostly of barium peroxide. During the subsequent reaction within the cru-
cible the oxygen leaves the iron oxide and combines with the aluminium
producing aluminium oxide, or slag, and superheated thermit steel. This
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steel is run into the mould where it preheats and eventually fuses and mixes
with the ends of the parts to be joined. On cooling a continuous joint is
formed and the mould is removed.

FRICTION STIR WELDING* Friction stir welding is a relatively new
materials joining process which has been used in the shipbuilding industry
and is likely to be more widely used.
Friction stir welding is a solid state process that offers advantages
over fusion welding for certain applications. In producing butt joints it
uses a non-consumable rotating tool the profiled pin of which is plunged
into the butted joint of two plates and then moves along the joint. The
plate material is softened in both plates and forced around the rotating
profiled pin resulting in a solid state bond between the two plates
(see Figure 9.8, Friction stir welding). To contain the softened material
in the line of the joint a backing bar is used and the tool shoulder under
pressure retains material at the upper surface. Both plates and the back-
ing bar require substantial clamping because of the forces involved.
Plates of different thickness may be butt welded by inclining the rotating
tool (Figure 9.9).
The process is currently used for welding aluminium alloy plates and such
plates to aluminium alloy extrusions or castings. Dissimilar aluminium

*Friction stir welding was invented and patented by TWI Ltd., Cambridge, UK.