Laser Shock Peening (micro-LSP) and Micro-scale Laser Peen Forming
Micro-scale Laser Shock Peening (Micro-LSP)
and Micro-scale Laser Peen Forming (Micro-LPF)
Laser shock processing (LSP) involves
laser-induced, medium-confined plasma sending strong shock waves into target
and thus imparting compressive residual stress into surface layer to improve
fatigue performance. Compared with mechanical shot peening, LSP offers a
deeper layer of compressive residual stress, and is more flexible especially
for irregular shapes. It has been shown that LSP can improve fatigue life
by 5 to 6 times. LSP was proposed by Dr. Allen Clauer of Battelle Institute
in 1960s but requires powerful lasers to general several to tens GW/cm2
laser intensity. Prof. Fabbro’s group in France was responsible for
many advances in LSP.
We at Columbia University have been carrying out
investigations on micro-scale LSP, where the laser beam spot size is in the
order of tens of microns instead of in mm scale as in the previous studies.
The motivation is (1) to enable less powerful and thus less expensive laser
to be used, and (2) to enable micro-devices to have the same benefits as
their macro counterparts. Since the beam spot size now is in the same order
of magnitude as the grain size of the material to be peened, the material
behaves anisotropically. Our strategy has been to use single crystal
materials for our experimental and numerical investigations. Single crystal
plasticity and rate-dependent flow stress models are implemented in the
numerical models. We also carried out an extensive investigation of
spatially resolved residual stress characterization using the micro X-ray
diffraction technique. This is done in collaboration with the National
Synchrotron Light Source at Brookhaven National Laboratory.
More recently, LSP has been extended to LPF
(laser peen forming), in which the target is shaped (say, bent) and at the
same time imparted desirable residual stress on BOTH sides of the target.
In mechanical bending, there is compressive residual stress on one side of
the specimen only. Lawrence Livermore National Lab has reported progress in
LPF and our work focuses on using a micron-sized laser beam to affect LPF.