jet tends not only to compensate the welding compressive plastic strains
but also to alter the sign of residual strain to its opposite. Results show
that the distance
D
has more significant influence on both
ε
p
x
and
σ
x
in
controlling buckling on thin materials. After welding with the trailing spot
heat sink, the specimens are completely buckle-free and as flat as original
before welding. Similar results were obtained as shown in Fig. 8, b, c on
stainless steel and aluminum plates.
Metallurgical and mechanical examinations show that the cooling jet
medium gives no noticeable influence on the titanium weld joint properties.
Actually the cooling jet is impinging directly on the solidified weld bead at
a temperature less than 400
◦
C as shown by the distorted abnormal isotherm
of 400
◦
C in front of the heat sink (Fig. 7, c).
Recent progress in numerical simulation of welding phenomena offers
researchers powerful tools for studying in more detail of welding thermal
and mechanical behaviors. These tools allow for the prediction of precise
control of the abnormal temperature fields and therefore the abnormal
thermal elastic-plastic cycles created by the possible variable combinations
of the heat source–heat sink welding techniques. It is expected that a variety
of coupled heat source-heat sink processes are feasible for not only welding
distortion controlling but also defect-free welds. For example, the device
for trailing spot heat sink can be attached not only to the GTAW torch but
also could be coupled to other heat sources like laser beam or friction stir
welding tool to control distortion, and to improve joint performances as
well.
Conclusions.
1) Low Stress No Distortion (LSND) welding techniques
for thin materials can be implemented using either the whole cross-section
thermal tensioning effect or the localized thermal tensioning effect.
2) Basic principles and mechanism of LSND welding techniques are
clarified through experimental studies and theoretical analyses with FEA.
3) For LSND welding using the whole cross-section thermal tensioning,
the necessary condition is to create an adequate preset temperature profile
coupled to the welding temperature field whereas its sufficient condition is
to keep the thin plate elements in a plane position without any transient
loss of stability during welding.
4) In executing DC-LSND welding technique using localized thermal
tensioning, the dominating technological parameters are: the distance
between the heat source and the heat sink and the intensity of the heat sink.
For engineering solution and industrial application, optimized technological
parameters are recommended based on FEA results.
5) Both LSND welding techniques have been applied successfully
in sheet metal industries to satisfy the stringent geometrical integrity
requirements especially to ensure dimensional consistent fabrication of
aerospace components.
ISSN 0236-3941. Вестник МГТУ им. Н.Э. Баумана. Сер. “Машиностроение”. 2005. № 4 101
