Fig. 6. Temperature fields and isotherms on Ti–6Al–4V (2.5 mm thick) plate (Ref. 16),
welding parameters: 200A, 12V, 12m/h; a, c — conventional GTA welding on copper
backing bar; b, d — welding with trailing spot heat sink
Obviously, the localized thermal tensioning effect is acting only within a
limited zone behind the weld pool.
The compressive plastic strains formed before in the just solidified weld
zone can be compensated properly by the inherent tensile plastic strains in
the area of temperature valley.
In welding with trailing spot heat sink, both the value of inherent plastic
strains and the width of its distribution can be controlled quantitatively by
selecting the proper technological parameters: the distance
D
between the
welding heat source and the heat sink (see Fig. 7) as well as the intensity
of the heat sink.
For a selected intensity of heat sink, the closer the heat sink to the heat
source (the shorter the distance
D
)
, the stronger is the localized thermal
tensioning effect. For example, at the distance
D
= 25
, the residual plastic
inherent strain
ε
p
x
on the weld centerline even changes its sign from negative
to positive, and the residual stress on the weld centerline changes from
tensile to compressive correspondingly.
Fig. 8 gives some typical examples from the systematic investigation
program. As shown in Fig. 8, a, the peak tensile stress in weld on mild steel
plate welded using conventional GTAW reaches 300MPa (curve 1) and
the maximum compressive stress in the peripheral area is about 90 MPa
which causes buckling with deflections more than 20 mm in the center of
specimen of 500 mm long. In the case of welding with the trailing heat sink
the patterns of residual stress distribution (curves 2,3,4) alter dramatically
with different technological parameters, even with the compressive residual
stresses in the centerline of the weld. The reason is that the shrinkage
induced by the great temperature gradient between the arc and the cooling
ISSN 0236-3941. Вестник МГТУ им. Н.Э. Баумана. Сер. “Машиностроение”. 2005. № 4 99
