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Lasers Utilized to Perform Controlled Mechanical Bending
Researched, compiled and written by World Lasers, Inc. © 2007
Laser Forming (LF) involves the non-contact controlled bending
of a metallic component with a laser. Over the
past 30 years, laser technology has rapidly developed and diversified,
replacing or enhancing many conventional manufacturing processes. Laser Forming
is a flexible forming technique that uses a (defocused) laser beam to form sheet
metal by thermal stresses alone. This forming technique
has emerged as the key process for sheet metal
rapid prototyping contributing to low lead time manufacturing. In contrast
to conventional metal forming methods, hard tooling is
not required in this process eliminating
expensive component specific tooling. The size and power of the laser beam
can be precisely manipulated, enabling accurate control
of the forming process. Laser Forming of tubes
and sheet metal (requiring no hard tooling or external forces) is therefore
suited for rapid prototyping. It has potential applications in aerospace,
shipbuilding, automobile, and other industries. Laser
tube bending does not result in wall thinning
and it does not suffer from any annealing effects making it easier to work on
materials such as titanium and nickel super-alloys. LF offers the only promising
die-less method for sheet metal and tubes.
The Laser forming process introduces thermal stress into the
surface of the object being worked on. These
internal stresses induce plastic strains bending the material or it results in
local buckling. For example after laser treatment and subsequent cooling, the
material at the surface is shorter than the
material below thus bending the object towards the laser beam.
The laser beam path depends on the desired ‘forming’ result. In the simplest
case it may be a point, in other cases it may be
a straight line across the whole part. For extrusions the paths would be very
sophisticated radial and tangential lines. Extensive
research has been carried out by Hom-Shen Hsieh and Jehnming Lin at the
Department of Mechanical Engineering, National Cheng Kung University, Tainan,
Taiwan on the study of buckling mechanism in laser tube
forming. Basic focus was on the influence of
material properties, the beam modes and the heating paths of the laser, through
lab experiments, and also through numerical analysis using 3-D meshed domain
modelling, and comparing the results. Metal tubes made
of 304 grade stainless steel were used in these
experiments. These tubes were heated by the commercially available (CO2) laser
beam that induced the buckling phenomenon on the tube surface due to elastic
plastic deformation. The buckling in thin tubes during
the Laser Forming process was found to be
inconsistent, unstable and uncertain with the direction of bending almost
random. Tackling this problem was an urgent need as
dynamic characteristics of the thin metal tube
are finding increasing importance in modern manufacturing technologies due
to expanding use of laser energy. The above research
solved this thermal-mechanical problem through
detailed investigation of the influence of temperature and the stress effects
on bending angles on buckling of the specimen. In pure technical terms this
problem was assumed to be 3-D uncoupled thermo elasto-plastic
and was solved deploying FEM methodology using
ABAQUS Software.
Some of the important findings from
this research effort were:
- By specifically selecting process parameters the sheet metal
could be formed in concave, convex or shortening shapes.
- The plastic deformation is clearly enhanced by the input
energy of the laser beam; however, the residual
stress and strain during the cooling stage shows a similar trend
under different laser powers.
- Reduction in the heat affected zone along the axial direction
of the tube causes the tube to bend towards the
laser beam. This is because of the compressive thermal stresses induced, both
axial and circumferential. The axial stress is responsible for bending
while the vertical circumferential stress forces the surface upwards.
- Surface buckling generates a longitudinal elongation in the
heat affected zone during laser heating which
causes the tube to bend in the direction of the moving laser.
- Height of the bulged surface increases with the laser power
for the same scanned speed.
- Bending angle oscillated significantly with laser interaction
on thin tubes.
- The peak height of the bulged surface during the buckling
increased with the increasing laser power.
- Buckling in laser forming in thin tubes was dependent upon
several operating factors like the laser power,
the heating time, the clamping arrangement, the thickness,
the thermal properties and even the original stress states of the specimen.
The experimental results were compared to the numerical
evaluation and a great deal of consistency in
the two was found, both for angular and longitudinal displacements.
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