Localized Thermal Tensioning — LSND Welding with a Trailing
Spot Heat Sink.
Over the past 10 years, progress has been made at
BAMTRI in seeking active in-process control of welding buckling to
exploit a localized thermal tensioning technique using a trailing spot heat
sink. The heat sink moving synchronously with the welding arc creates an
extremely high temperature gradient along the weld bead within a limited
area of high temperature zone close to the weld pool (Fig. 2). In this
innovative method, the preset heating (as shown in Fig. 1) is no longer
necessary. The formation of specific inverse plastically stretched inherent
strains
ε
p
x
in the near arc zone behind the welding pool is dynamically
controlled by a localized trailing thermal tensioning effect induced between
the welding heat source and the spot heat sink along the weld bead (Fig. 2).
With the specially designed device attached to the welding torch, an
atomized cooling jet of the trailing spot heat sink impinges directly on the
just solidified weld bead. Liquid coolant, such as CO
2
, Ar, N
2
or water,
could be selected for atomized cooling jet. Atomizing the liquid coolant is
essential to improve the efficiency of intensive cooling rather than using
liquid jet directly impinging the weld bead. To protect the arc from the
possible interference of the cooling media, there is a co-axial tube to draw
the vaporized media out of the zone nearby the arc. The technological
parameters for the trailing spot heat sink and all the welding procedures
are automatically synchronously-controlled with the GTAW process. The
dominating factors: the distance between the heat source and the heat sink,
the intensity of the cooling jet can be selected properly to reach a buckle-
free result.
In systematic investigations, finite element analysis with a model of
cooling jet impinging the weld bead surface is combined with a series of
experimental studies (Refs. 13–17). Comparisons between the temperature
fields on conventional GTA welded titanium plate and on plate welded
using technique with the trailing spot heat sink are given in Fig. 6.
In this case, welding with trailing spot heat sink was carried out using
the same parameters as in conventional GTA welding. The flow rate of
cooling medium (atomized water) was selected at 2.5 ml/s. The distance
between the arc and cooling jet were regulated from 80 mm to 25 mm. It
can be seen clearly (Fig. 6, b, d) that in welding with spot heat sink there
is a deep temperature valley formed by the cooling jet behind the weld
pool. An extremely high temperature gradient from the peak to the valley
is created. The 800
◦
C and 400
◦
C isotherms in front of the heat sink are
severely distorted pushing forward closer to the weld pool (see Fig. 6, d).
The abnormal thermal cycles by welding with trailing spot heat sink
produce correspondingly the abnormal thermo-elastic-plastic stress and
strain cycles in comparison with the cycles formed by conventional GTAW.
98 ISSN 0236-3941. Вестник МГТУ им. Н.Э. Баумана. Сер. “Машиностроение”. 2005. № 4
