-
60
AbstractFor laser forming of sheet metal to become a practical
pro-
duction or rapid prototyping tool, multiple scans of the
work-piece with the laser are necessary to achieve the
requiredmagnitude of deformation. Between consecutive scans,
sub-stantial waiting time is normally necessary for the workpiece
tocool down so that a steep temperature gradient can
bereestablished in the next scan. This paper first
experimentallyand numerically examines the effect of forced cooling
on sin-gle-scan laser forming processes. Cooling effects under
vari-ous conditions, including different laser power,
scanningspeed, nozzle offset, and cooling air pressure, are
investigat-ed. Cooling effects on microstructure change and
othermechanical properties, including strength, ductility, and
hard-ness, are also examined. A focus is to investigate the
coolingeffect on the deformation mechanism, including
competingeffects on temperature and flow stress. The investigation
onmultiscan laser forming shows forced cooling has the potentialto
significantly reduce the total forming time while having
noundesirable effects on microstructure change and othermechanical
behavior. Cooling in the buckling mechanism dom-inated laser
forming process is also considered. The estab-lished numerical
model for laser forming with forced coolingprovides greater
insights into the cooling effects on the defor-mation mechanism,
helps predict such effects on final dimen-sional accuracy and
mechanical properties, and can beextended to optimize the multiscan
laser forming process.
Keywords: Laser Forming, Forced Cooling, Finite ElementMethod,
Microstructure, Rapid Prototyping
IntroductionLaser forming involves a laser-induced thermal
distortion to shape sheet metal without hard toolingor external
forces. Compared with the traditionalmetal forming technologies,
laser forming has manyadvantages: the cost of the forming process
is great-ly reduced because no tools or external forces
areinvolved, and laser forming is useful for small batchproduction
and a high variety of sheet metal compo-nents. With the high
flexibility in the laser beam’sdelivery and power regulating
systems, it is easy toincorporate laser forming into an automatic
flexiblemanufacturing system. Laser forming uses localizedheating
to induce controlled deformation and there-fore has the advantage
of energy efficiency as com-
pared to other thermal processes where an entireworkpiece
normally needs to be heated.
Despite of these advantages, progress needs to bemade for laser
forming to become a practical pro-cessing technology. For instance,
to create morepractical, three-dimensional shapes, multiple
laserscans are necessary to obtain the required magnitudeof
deformation; a single scan only gives a limitedamount of
deformation. If the workpiece is notcooled sufficiently close to
room temperature after ascan, the temperature gradient required in
laserforming may not be readily reestablished in the nextscan. In
addition, the aggregated heat may result insurface melting.
However, the waiting time betweenscans can be significant if there
is no forced cooling.
Sprenger et al.1 extensively investigated multiscanlaser
forming. They showed that the decrease ofabsorption coefficient,
the increase of the sheetthickness, and work hardening of the
material affectthe degressive courses of bending angle
withincreasing number of irradiations. However,Sprenger et al. did
not address the cooling issue.Odumodu and Das2 suggested to use
forced coolingin multiscan laser forming but did not investigate
itseffects. Hennige and Geiger3 have experimentallyinvestigated
cooling in multiscan laser forming ofaluminum sheets. They
suggested high-pressure aircooling and passive and active water
cooling andshowed that, compared with air cooling, active
watercooling reduces the entire processing time byincreasing the
bend angle per scan. Hennige andGeiger also showed that both air
cooling and watercooling have no detrimental effects on
themicrostructure of the formed parts. Although it wasshown that
active water cooling is more efficient, itis inconvenient in
industrial settings. Moreover,water cooling complicates the process
by introduc-ing laser-liquid-solid interactions and requires
anadditional liquid container and circulation system.
Although only limited investigations have beenconducted in the
experimental and computer model-ing of the laser forming process
with forced cooling,
Journal of Manufacturing ProcessesVol. 3/No. 12001
Cooling Effects in Multiscan Laser FormingJin Cheng and Y.
Lawrence Yao, Dept. of Mechanical Engineering, Columbia University,
New York, New York, USA
-
cooling has long been used in other thermal manu-facturing
processes for various reasons, and a greatdeal of experience and
research results have beenaccumulated, from which the present work
draws.For example, after hot rolling, deformation problemscaused by
nonuniform temperature distribution inasymmetrical ship profiles
can be solved by selec-tive water spray cooling. Olden, Smabrekke,
andRaudensky4 carried out experiments to obtain thecorrect heat
conditions and applied them to a 2-Dnumerical model to simulate the
selective air/waterspray cooling effect. Debray, Teracher, and
Jonas5
investigated the microstructure and mechanicalproperties of C-Mn
ferrite-bainite steel over a widerange of cooling rates and cooling
temperatures inhot rolling. The advantages of air-cooled
steelsinclude the elimination of heat treatment, reduceddistortion,
improved machinability, and more con-sistent properties. In forging
operations, cooling hasbeen controlled accurately and reliably,
enabling therequired mechanical properties to be imparted with-out
additional heat treatment.6
This paper presents experimental and numericalinvestigations
aimed at gaining better understand-ing of cooling effects in single
and multiscan laserforming. Experimentally validated
numericalresults provide greater insights into cooling effectson
deformation characteristics, temperature profile,stress
distribution, and process efficiency. Coolingeffects on
microstructure and other mechanicalproperties are also examined.
These results helppredict such effects on final dimensional
accuracyand mechanical properties.
Forming Process with Forced CoolingThe following general
assumptions have been
made. The workpiece material is isotropic, has con-stant
density, and is opaque; that is, the laser beamdoes not penetrate
appreciably into the solid. Thepower density distribution of the
laser beam followsa Gaussian function. The laser operates in
continu-ous wave (CW) mode. Material properties such asthe modulus
of elasticity, heat transfer properties,thermal conductivity,
specific heat, and flow stressare temperature dependent. The rate
of deformationis the total strain rate that is the sum of the
viscous,elastic, and plastic strain rate. A
strain-hardeningcoefficient, which is also temperature dependent,
isdefined to consider strain hardening of the material.
Energy dissipated through plastic deformation isnegligible
compared with the intensive laser energyinvolved. No melting is
involved in the formingprocess. Finally, no external forces are
applied to thesolid. Stresses occur only due to thermal expansionor
contraction.
Heat Transfer. The transient conduction for asolid workpiece of
dimension L by W by d (Figure1), radiated by a laser beam can be
expressed interms of temperature as:
(1)
subject to the boundary conditions: y = 0: T � T�; y= W: T � T�;
z = 0: , Qconv = hz=0(T � T�), and Qrad = � � (T4 � T4�); z = d:
Qconv =hz=H (T � T�), and appropriate initial conditions suchas t =
0: T (x, y, z, 0) = T� where �, cp, k, and �abs arethe density,
specific heat, thermal conductivity, andthe material’s absorbency,
respectively. The symbolsx, y, and z are the Cartesian coordinates
(Figure 1);
is the unit vector normal to the surface pointingn̂
( )ˆ ˆabs F n n k Tα ⋅ = − ⋅ ∇
( )pT
c k Tt
∂ρ = ∇ ⋅ ∇∂
61
Journal of Manufacturing ProcessesVol. 3/No. 1
2001
Figure 1Laser Forming System with Cooling Jets
Lens
Laser beam
Upper nozzle
D
Lower nozzle
L
H
RZ
Y Xd
r
�
Moving
direction
Width ofworkpiece:W (along Y-
axis)
Workpiece
-
(8)
where
(9)
where µ is the shear modulus, � is the viscosity con-stant, and
sij the deviatoric stress component. ePij isassumed to be governed
by flow rules associatedwith perfectly plastic behavior and the von
Misesyield criterion, that is,
where . svij is computed as though noplastic flow has occurred,
although account is takenof the viscous flow. k(T) is the von Mises
yield stressas a function of temperature. A function g(x,t)
isintroduced, which is zero in the plastic state andunity
elsewhere, that is,
Therefore, the combined stress-strain relations fromEqs. (8) to
(10) can be written as9
(12)
Numerical Simulation. Because the heat transferand
viscoelastic/plastic deformation are symmetric
about the vertical plane containing the scanningpath, only half
of the plate is modeled in the numer-ical simulation. The symmetric
plane is assumed tobe adiabatic. The same mesh model is used for
theheat transfer analysis and structural analysis. Twoadjacent
points in the middle of the symmetric planeare assumed to be fixed
to remove the rigid bodymotion. All other points within the
symmetric planeare assumed to move only within the symmetricplane
throughout the deformation process. A com-mercial code, ABAQUS, is
used to solve the heattransfer and structural problem. In
structural analy-sis, the 20-node element, C3D20, has no shear
lock-ing, no hourglass effect, and is thus suitable for
abending-deformation-dominated process such aslaser forming. On the
other hand, the eight-node ele-ment suffers from “shear locking”
and is thereforenot suitable for such a process. To remain
compati-ble with the structural analysis, a 20-node element,DC3D20,
is used in heat transfer analysis.
Experiment Straight-line laser forming with forced air cooling
is
schematically shown in Figure 1. The scanning path isalong the
x-axis, and the direction perpendicular to thescanning path and
within the plate is defined as the y-axis. The upper cooling nozzle
and lower cooling noz-zle were considered, although most
experiments weredone with the lower nozzle only. They had no
relativemotion with respect to the laser beam. Simple con-verging
nozzles with circular cross section were used.The angle of
incidence of the impinging jet relative tothe sheet surface, , was
kept close to 90 degrees tocreate higher convection heat transfer
at the stagnationpoint. The diameter of the nozzle, D, is 1.6 mm,
andthe standoff distance, H, is 8 mm. Most experimentswere done
with zero offset between the impinging jetstagnation point and the
laser beam center, althoughthe effect of the offset distance � was
also investigat-ed. The material is low carbon steel, AISI1010, and
80mm by 80 mm by 0.89 mm in size. To enhance laserabsorption by the
workpiece, graphite coating isapplied to the surface exposed to the
laser.
Most experiments use laser power of 400 or 800W, except one uses
varying power from 400 to 800W. Most experiments use scanning
velocity of 25 or50 mm/s, except one uses velocity varying from
25to 80 mm/s. Most experiments use air pressure of 80psi, except
one uses pressure varying from 0 to 80
( )
1
12 1
2 3
ij ij ij KK
kkij ij ij
v v
E E
g T
+ε = σ − δ σ
σ + + − λ σ − δ + δ α η
& & &
&
( )
( ) ( )
( )
( )
2 2
2
, 1
1 1if , or if and
2 22 0
, 0
1if and 2 0
2
ij ij
ij ij
ij ij ij
g x t
s k T s k T
s s kk T
g x t
s k T s s kk T
=
< =
′− ≤
=
′= − ≥
&&
&&
( )2v vij ij ijs e e= µ −& & &
( ) ( )
( )
2 2
2
0
1 1if , or if and
2 22 0
1if and 2 0
2
Pij
ij ij
ij ij
Pij ij
ij ij ij
e
s k T s k T
s s kk T
e s
s k T s s kk T
=
< =
′− ≤
= λ
′= − ≥
&&
&&
1 1and
2 2E vij ij ij ije s e s= =µ η& & &
E V Pij ij ij ije e e e= + +& & & &
63
Journal of Manufacturing ProcessesVol. 3/No. 1
2001
(10)
(11)
-
psi. Most experiments use laser beam size of 4 mm,except one
uses 8 mm to induce the buckling mech-anism (BM). Most experiments
use zero offset,except one uses offset � varying from 0 to 18
mm.Most experiments use bottom surface cooling only,except one uses
top surface cooling and both bottomand top surface cooling. The
exact experimentalconditions are noted in the figures and their
legends.
The laser system used is a PRC-1500 CO2 laser,which has a
maximum output power of 1500 W. Acoordinate measuring machine (CMM)
is used tomeasure the bending angle of the formed parts.
Tocalculate the Reynolds number of the air at the noz-zle exit that
is applied in the numerical simulation, avelocimeter is used to
measure the velocity of the airat the nozzle exit. Scanning
electronic microscopy isused to investigate the microstructure of
the materi-al after laser forming. Tensile test samples aremachined
by CNC along the scanning path, and ten-sile tests are conducted on
a MTS. A Rockwell hard-ness tester is used to measure the hardness
of thematerial after laser forming.
Results and Discussion
Simulation Validation and Cooling Effect on Deformation
Figures 2a and 2b compare simulation andexperimental results
under a wide range of condi-
tions, and reasonable agreements are seen. Figure2a shows the
variation of the bending angle vs.scanning speed with and without
cooling. As seen,the bending angle with cooling could be smaller
orlarger than that without cooling at laser power of400 W, while
there is no appreciable differencebetween the two at 800 W. To help
explain the phe-nomenon, peak temperature reached at the work-piece
top and bottom surfaces as well as corre-sponding flow stress at
these locations are plotted inFigures 3a and 3b. Temperature, work
hardening,and strain rate effects were considered in determin-ing
the flow stress.
At the lower power of 400 W and lower velocityof 25 mm/s
(Figures 2a and 3a), the bending anglewith cooling is lower than
that without cooling bythe greatest margin. Although the
bottom-only cool-ing lowers the bottom temperature
significantly(from about 850 K to 700 K) and therefore increas-es
the temperature gradient between the top and bot-tom surface, which
seems to favor more deforma-tion, the cooling at the same time
increases the flowstress at the bottom surface quite significantly
(fromabout 120 to 170 MPa). The net result is a decreasedbending
angle. At the same power but higher veloc-ity of 50 mm/s, the net
result is opposite. This isbecause heat dissipation at this
condition is lowerdue to the shorter cooling time, and the
temperaturedrop at the sample’s surfaces is not as great as withthe
lower scanning speed. As a result, the cooling
64
Journal of Manufacturing ProcessesVol. 3/No. 12001
Figure 2(a) Numerical and experimental bend angle vs. scanning
velocity, (b) Numerical and experimental bend angle vs. laser
power.
(Dbeam = 4 mm, Pair = 80 psi, � = 0, bottom cooling only)
2.8
2.4
2.0
1.6
1.2
0.8
0.4
0. 0
(a) (b)Scanning velocity (mm/s) Laser power (W)
Bend
ing
angl
e (d
egre
e)
Bend
ing
angl
e (d
egre
e)
4.0
3.6
3.2
2.8
2.4
2.0
1.6
1.2
0.8
0.4
no cooling, numericalcooling, numerical nocooling,
experimentalcooling, experimental
cooling, numerical no cooling, numericalcooling, experimental no
cooling, experimental
no cooling, numericalcooling, numerical nocooling,
experimentalcooling, experimental
cooling, numerical no cooling, numericalcooling, experimental no
cooling, experimental
P = 800 W
P = 400 W
30 40 50 60 70 80 400 500 600 700 800
V = 25 mm/s
V = 50 mm/s
-
effect on temperature gradient increase begins tooutweigh the
cooling effect on flow stress increase.
At the higher power level of 800 W (Figures 2aand 3b), the
cooling effect on bending angle is notsignificant. A reason is that
heat input from the laseris greater and the heat dissipation is
relatively lowerdue to relatively higher velocities. This causes
thetemperature drop due to the cooling to be lower. Asa result, its
effect on the flow stress is also lower andthe deformation remains
largely the same as in thecase without cooling. Similar results are
obtainedwhen variations of the bending angle are examinedagainst
varying laser power while the scanningvelocity is kept at constant
levels of 25 mm/s and 50mm/s (Figures 2b and 3b).
Effect of Air PressureFigure 4a shows the relationship between
the air-
cooling pressure and the bending angle under twoconditions. When
the higher power of 800 W andhigher velocity of 50 mm/s is applied,
the bendingangle increases moderately with air-cooling
pressure.This is because under this condition the net heatinput
into the workpiece is higher, the temperaturedrop due to the
cooling is lower, and therefore theincrease in flow stress is also
lower. As a result, thetemperature gradient increase between the
top andbottom surfaces due to air-pressure increase is slight-ly
more dominant. Figure 4b shows that the y-axis
compressive plastic strain on the top surface onlyincreases
slightly when cooling (80 psi) is applied.
When a lower power of 400 W and lower velocityof 25 mm/s is
applied, the bending angle decreasesas the air-cooling pressure
increases. This is becauseunder the condition the net heat input is
lower, thetemperature drop due to the air cooling is higher,and the
increase in flow stress is higher. Althoughthe temperature gradient
between the top and bottomsurfaces still increases with air
pressure, the coolingeffect on the flow stress increase becomes
dominantwhen the air pressure increases. This is evident inFigure
4b where not only the y-axis compressiveplastic strain but also the
region of such plasticstrain decrease as cooling (80 psi) is
applied due toincreased flow stress.
Effect of Cooling Nozzle OffsetThere exists a time delay between
when the top
surface reaches its peak temperature and when thebottom surface
does due to heat conduction time.Therefore, if the impinging jet is
placed coaxiallywith the laser beam, the cooling may not be themost
efficient.
Figure 5 shows the effect of cooling nozzle offset� (Figure 1)
on bending angle, where the cases of nocooling and cooling with
zero offset are also shown.For the higher power of 800 W and higher
velocity of50 mm/s, the maximal bending angle is obtained
65
Journal of Manufacturing ProcessesVol. 3/No. 1
2001
Figure 3Peak Temperature and Flow Stress
(a) P = 400 W and V = 25 mm/s, (b) P = 800 W and V = 50 mm/s
(Dbeam = 4 mm, Pair = 80 psi, � = 0, bottom cooling only)
1400
1200
1000
800
600
400
(a) (b)Scanning velocity (mm/s) Scanning velocity (mm/s)
Tem
pera
ture
(K)
Tem
pera
ture
(K)
cooling, bottom flow stress,no cooling, bottom flow
stresscooling, top flow stressno cooling, top flow stress
no cooling, top temp.cooling, top temp.no cooling, bottom
temp.cooling, bottom temp
cooling, bottom flow stressno cooling, bottom flow
stresscooling, top flow stressno cooling, top flow stress
no cooling, top temp.cooling, top temp. nocooling, bottom
temp.cooling, bottom temp
P = 400 W, V = 25 mm/sP = 400 W
25 30 35 40 45 50 50 60 70 80
P = 800 W, V = 50 mm/s210
180
150
120
90
60
30
Flow stress (M
Pa)
1800
1600
1400
1200
1000
800
600
Flow stress (M
Pa)
250
200
150
100
50
-
when the offset is approximately 5 mm. For the lowerpower of 400
W and lower velocity of 25 mm/s, theminimal angle (under this
condition, cooling causesthe bending to decrease, as seen in Figure
2a) isobtained at offset about 2 mm. The differencebetween the two
offset values can be easily explainedbecause under the first
condition the velocity is twiceas that under the second condition;
therefore, there isa larger time delay between the top and bottom
peaktemperature under the first condition.
MicrostructureGrain Structure. Figure 6a shows the SEM
micrograph of the grain structure of AISI 1010 steelbefore laser
forming. Figures 7a and 7b show theSEM micrographs of the grain
structure near the topsurface after laser forming with and without
cooling.It can be seen that the grain structure is refined
afterlaser forming in both cases. The case with cooling(Figure 7a)
exhibits a finer grain structure than theone without cooling
(Figure 7b) obviously becauseof the higher cooling rate, although
in both cases thedeformation and peak temperature reached are
aboutthe same (Figures 2b and 3b). This is because thenucleation
rate of new grains under cooling is high-er than that under no
cooling condition.
During high-temperature deformation, it is possi-ble to have
dynamic recovery and dynamic recrystal-
lization. The ability for a material to do so dependson the
stacking fault energy of the material.10
Materials, such as aluminum alloys and steels, thathave a high
stacking fault energy do not dynamical-ly recrystallize in a
significant way. Therefore, thegrain refinement observed above is
primarily due tostatic recovery and static recrystallization
takingplace after the material is plastically deformed andwhile it
cools down to room temperature. The static
66
Journal of Manufacturing ProcessesVol. 3/No. 12001
Figure 4(a) Bend angle vs. air pressure, (b) distribution of
Y-axis plastic deformation along Y direction after laser forming
with cooling and without cooling
(Dbeam = 4 mm, � = 0, bottom cooling only)
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
(a) (b)Air pressure (psi) Position in Y direction (mm)
Bend
ing
angl
e (d
egre
e)
Y-ax
is p
last
ic d
efor
mat
ion
(%)
numericalexperimental
P = 800 W, V = 50 mm/s
P = 400 W, V = 25 mm/s
0 20 40 60 80
Pair = 80 psi
0.0
-0.4
-0.8
-1.2
-1.6
-2.0
-2.4
-2.8
-4.0 -2.0 0.0 2.0 4.0
numericalexperimental
P = 800 W, V = 50 mm/s
coolingno cooling
P = 400 W, V = 25 mm/s
coolingno cooling
Figure 5Bend Angle vs. Offset �
(Dbeam = 4 mm, Pair = 80 psi, � = 0, bottom cooling only)
Bend
ing
angl
e (d
egre
e)
cooling, numericalno cooling, experimentalcooling,
experimental
P = 400 W, V = 25 mm/s
Offset (mm)
cooling, numericalno cooling, experimentalcooling,
experimental
P = 800 W, V = 50 mm/s
-5 0 5 10 15 20
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
-
recrystallization is affected by strain. The greater thestrain
is, the faster the recrystallization process, andthe finer the
grain size. Our results show that thegrain size at higher laser
power (P = 800 W) is finerthan at lower laser power (P = 400 W)
because theformer undergoes larger plastic strain.
Phase Transformation. Figure 6b shows the SEMmicrograph of the
microstructure of the same as-
received hypereutectoid steel at a higher magnifica-tion (x
4000), under which pearlite with ferrite back-ground can be seen.
Figures 8a to 8d show the SEMmicrographs near the top and bottom
surface afterlaser forming with and without cooling using thesame
high magnification. During the transient heat-ing to peak
temperature, the material is heated toaustenite region. After the
material cools down, the
67
Journal of Manufacturing ProcessesVol. 3/No. 1
2001
Figure 6SEM Micrographs of the Raw Material (AISI1010 steel)
(a) low magnification (x700) and (b) high magnification (x4000)
showing the phases (ferrite and pearlite)
(a) (b)
Figure 7SEM Micrographs of Grain Structure Near Top Surface
After Laser Forming
(a) with cooling and (b) without cooling. The grain with cooling
is finer. (P = 800 W, V = 50 mm/s,Dbeam = 4 mm, Pair = 80 psi, � =
0, bottom cooling only)
(a) (b)
-
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Journal of Manufacturing ProcessesVol. 3/No. 12001
(a) Cooling, top surface (b) No cooling, top surface
(d) No cooling, bottom surface(c) Cooling, bottom surface
(f) 10 scans, cooling, bottom surface(e) 10 scans, cooling, top
surface
Figure 8SEM Micrographs Showing Microstructure of AISI1010 After
Laser Forming Under High Magnification (x4000). (a) and (b) after
one scan,near top surface with cooling and without cooling, the
bainite phase under cooling is finer; (c) and (d) after one scan,
near bottom surface
with cooling and without cooling, the bainite phase under
cooling is finer and volume fraction is higher; (e) and (f) after
10 scans under coolingnear top (e) and bottom (f) surfaces. (P =
800 W, V = 50 mm/s, Dbeam = 4 mm, Pair = 80 psi, � = 0, bottom
cooling only)
-
final phases in the materials contain ferrite, bainite,and
perhaps a small amount of pearlite. Figures 8aand 8b show that the
microstructure near the top sur-face after laser forming with
cooling contains amore refined bainite phase than that after
laserforming without cooling. The bainite nucleates onthe ferrite
matrix, and the nucleation rate is higherwith the high cooling
rate.11 The same phenomenonis observed near the bottom surface, as
shown inFigures 8c and 8d.
Mechanical PropertiesFigure 9 shows the tensile test results of
a laser-
formed specimen with and without cooling as com-pared with raw
material. The test length of the spec-imen is 12 mm and test width
is 2 mm, with thelaser-scanned region along its axis. It is seen
that theyield strength of the material after laser forming ishigher
than that of the as-received material, the yieldstrength of the
material under laser forming withcooling is higher than that under
laser forming with-out cooling, and the yield strength of the
material athigher laser power is higher than that at lower
laserpower. The reverse can be said about elongationbefore rupture.
As discussed in the previous section,after laser forming the
material’s grain structure isrefined, the material’s phase change
is from pearliteto refined bainite, and therefore the material
isstrengthened. The microstructure of materials undercooling shows
a finer grain size and a finer bainitephase as compared with no
cooling. Therefore, after
laser forming, the strength of the material with cool-ing is
higher than without cooling. Comparing theconditions at higher and
lower laser power, the for-mer has more plastic deformation and a
higher cool-ing rate and, therefore, a finer grain size.
Figure 10c shows the hardness change due to laserforming and
cooling. The hardness of the raw mater-ial is about 45 HR. After
laser forming, it increasesdue to strain hardening and grain
refinement. Withcooling, the hardness increases slightly more.
MultiscanFigure 10a shows the experimental results of the
development of the bend angle as a function of num-ber of scans.
The results show approximately linearpatterns and indicate that the
work hardening effectis offset by the softening effect of the
repeated laserscans. Although the resultant bending angles
withcooling and without cooling only differ moderately,the total
times it takes for the 10 scans to completeare vastly different.
Without cooling, a substantialamount of time (in this case, about
150 seconds) hasto be waited for the workpiece to cool down to
nearroom temperature to be able to reestablish a steeptemperature
gradient during the scan that immedi-ately follows. With forced
cooling, such a waitingperiod was reduced to about 10 seconds
because thecooling rate with forced cooling is much higher, asseen
from the time history of temperature shown inFigure 10b. As a
result, the total forming time forthe 10-pass processes reduced
from about 23 min-utes without cooling to less than 2 minutes
withcooling. This shows cooling has the potential togreatly speed
up multiscan operations, which arenecessary if laser forming is to
become a practicalproduction tool. At the same time, it has been
shownthat cooling does not have major detrimental effectson
deformation efficiency, microstructure, andmechanical
properties.
Figure 10c shows the Rockwell hardness testresults after
multiscan under two different condi-tions. While the multiscan
proceeds, strain harden-ing and recovery/recrystallization-induced
softeningcoexist and compete with each other. At the higherpower of
800 W and higher velocity of 50 mm/s,softening is more dominant due
to higher tempera-ture although the deformation is also larger. As
aresult, the hardness decreases somewhat when morescans are carried
out. At the lower power of 400 Wand lower velocity of 25 mm/s, the
hardening and
69
Journal of Manufacturing ProcessesVol. 3/No. 1
2001
Figure 9Tensile Stress-Strain Curves of Workpiece Before and
After Laser Forming With and Without Cooling (Dbeam = 4 mm,
Pair = 80 psi, � = 0, bottom cooling only).
Stre
ss (M
Pa)
Strain0.00 0.05 0.10 0.15 0.20 0.25
600
550
500
450
400
350
300
250
200
150
100
50
0
AISI 1010
Scanningdirection
12 mm 2 mm
raw material
P = 800 W, V = 50 mm/s, coolingP = 800 W, V = 50 mm/s, no
cooling
P = 400 W, V = 25 mm/s, coolingP = 400 W, V = 25 mm/s, no
cooling
-
softening are about the same and as a result the hard-ness
remains more or less constant while the numberof scans
increases.
Figures 8e and 8f show the microstructure nearthe top and bottom
surfaces after 10 passes of laserforming with cooling. For each
pass in multiscanlaser forming, the material experiences the
phasetransformation to austenite, then to ferrite, bainite,and a
small amount of pearlite. The phase structureafter 10 passes shows
no visible difference from thephase structure after one pass.
Edge EffectFigure 11a shows the numerical results of the
bend angle variation along the scanning path undertwo conditions
with and without cooling. The differ-ence between the maximal and
minimal bendingangles along the scanning path is referred to as
edgeeffect. As seen, the edge effect is reduced by about54% and 17%
for the two conditions, respectively,after cooling is applied.
Similar results are alsofound under other conditions in this paper.
This phe-nomenon can be explained by the numerical resultsshown in
Figure 11b. The variation in the differenceof y-axis plastic strain
between the top and bottomsurfaces is reduced with cooling. This is
becausecooling can reduce the heat-affected zone and theplastic
zone, and therefore the accumulated plasticstrain due to the
bending taking place at the preced-ing regions along the scanning
path is reduced.
The curvature of the bending edge is alsodecreased after the
cooling bottom scheme is applied.The cause of the curved bending
edge is due to thedifference in x-axis contraction between the top
andbottom surfaces.12 Figure 11c shows that the differ-ence in
x-axis contraction between the top and bottomsurfaces with cooling
is lower than without cooling.
Cooling SchemeFigure 12 compares the bending angle with
cool-
ing the bottom surface, cooling the top surface, andcooling both
the top and bottom surfaces. The no-cooling case is also plotted as
a benchmark. Thetotal air pressure is 80 psi for all the cooling
condi-tions. When both the top and bottom surfaces arecooled
simultaneously, 40 psi of air pressure is usedfor each surface.
Among the three cooling schemes, the bottom-cooling scheme
produces the largest bending anglewhile the top-cooling scheme
produces the smallest.
70
Journal of Manufacturing ProcessesVol. 3/No. 12001
Figure 10(a) Development of bend angle vs. number of scans, (b)
temperature
history of a single scan with cooling and without cooling, (c)
Rockwellhardness vs. number of scans with cooling and without
cooling
(Dbeam = 4 mm, Pair = 80 psi, � = 0, bottom cooling only)
Bend
ing
angl
e (d
egre
e)
P = 800 W, V = 50 mm/s, no cooling P = 800 W, V = 50 mm/s,
cooling
(a)
2 4 6 8 10
26
24
22
20
18
16
14
12
10
8
6
4
2
P = 400 W, V = 25 mm/s, no cooling P = 400 W, V = 25 mm/s,
cooling
Number of scans
Tem
pera
ture
(K)
(b)
1 2
2000
1800
1600
1400
1200
1000
800
600
400
Time (s)
P = 800 W, V = 50 mm/s P = 400 W, V = 25 mm/s
cooling, top temp.no cooling, top temp.cooling, bottom temp.no
cooling, bottom temp.
cooling, top temp.no cooling, top temp.cooling, bottom temp.no
cooling, bottom temp.
Pair = 80 psi
Rock
wel
l har
dnes
s (H
R)
70
65
60
55
50
45
(c)
0 2 4 6 8 10Number of scans
no cooling, P = 400 W, V = 25 mm/s cooling, P = 400 W, V = 25
mm/s no cooling, P = 800 W, V = 50 mm/s cooling, P = 800 W, V = 50
mm/s
-
The bottom-cooling scheme reduces the temperatureat the bottom
surface and therefore increases thetemperature gradient between the
top and bottomsurfaces. As a result, the bending angle is
larger.Contrarily, the top-cooling scheme reduces the top-surface
temperature and therefore reduces the tem-perature gradient between
the top and bottom sur-faces. As a result, the bending angle is
smaller. Thetop and bottom cooling scheme is somewhere inbetween.
In summary, the bottom-cooling scheme ismost effective in terms of
bending. But in terms ofremoving heat, the other two schemes should
befavored because they cool the top surfaces where thetemperature
is the highest.
Buckling Mechanism So far, the discussion has been focused on
the
cooling effects in laser forming where the tempera-ture gradient
mechanism (TGM) dominates. Laserforming can also be affected by the
so-called buck-ling mechanism (BM), where the laser beam diame-ter
is large compared with workpiece thickness.Figure 13 shows the
experimental results of BM-dominated laser forming using various
coolingschemes. The experimental conditions are the sameas in
Figure 12 except the beam diameter increasesfrom 4 to 8 mm
here.
First of all, the bending angle under any coolingscheme is
smaller than that without cooling. This isunderstandable because in
a BM-dominated laserforming process, the temperature gradient in
thedepth direction is small and the forming is achieved
71
Journal of Manufacturing ProcessesVol. 3/No. 1
2001
Figure 11(a) Bend angle variation along scanning path, (b)
Y-axis plastic strainalong the scanning path, (c) Time history of
X-axis contraction (Dbeam
= 4 mm, Pair = 80 psi, � = 0, bottom cooling only)
Bend
ing
angl
e (d
egre
e)
P = 400 W, V = 25 mm/s
(a)
0 20 40 60 80
3.0
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
X-axis position (mm)
Y-ax
is p
last
ic s
train
(%)
(b)
0.008
0.004
0.000
-0.004
-0.008
-0.012
-0.016
-0.020
X-axis position (mm)
P = 400 W, V = 25 mm/s
cooling, top surface
cooling, bottom surface
no cooling, top surface
no cooling, bottom surface
Cont
radi
ctio
n in
X-d
irect
ion
(%)
0.1
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
(c)
0.1 1 10 100 1000Time (s)
no cooling, top surfaceno cooling, bottom surfacecooling, top
surfacecooling, bottom surface
no coolingcooling
no coolingcooling
P = 800 W, V = 50 mm/s
0 20 40 60 80
P = 400 W, V = 25 mm/s
Figure 12Comparison of Bend Angle Under Different Cooling
Schemes (no
cooling also included) (Dbeam = 4 mm, � = 0)
Bend
ang
le (d
egre
e)
Cooling conditions
1 & 7—no cooling2 & 8—bottom cooling (80 psi)3 &
9—top & bottom cooling(40 + 40 psi) 4 & 10—top cooling (80
psi)
1 2 3 4 5 6 7 8 9 10 11 12
2.8
2.4
2.0
1.6
1.2
0.8
0.4
0.0
P = 800 W, V = 50 mm/s
P = 400 W, V = 25 mm/s
-
through buckling induced by compressive strain thatis almost
uniform throughout the depth direction.Any efforts to reduce this
uniformity, or in otherwords to increase the temperature gradient
throughcooling, are against the buckling mechanism.
It is seen that among the three cooling schemes,the
bottom-cooling scheme continues to produce thelargest bending
angle, while the top-cooling schemeproduces the smallest. The heat
removal rate withbottom cooling is less than with top
cooling.Therefore, the average temperature with bottomcooling is
higher than with top cooling. When BMdominates, the bend angle
increases as the averagetemperature increases. In addition, the
cooling effecton flow stress increase is the largest with top
cooling.Therefore, of the three cooling schemes, the bendangle with
cooling the bottom surface is the largest,while cooling the top
surface gives the smallest.
ConclusionsThe cooling effect on forming efficiency varies
with process conditions, and the variation is dis-cussed in
terms of the competing effect on tempera-ture and flow stress by
the cooling. The formingefficiency is also experimentally and
numericallyinvestigated in terms of nozzle offset and cooling
airpressure, and numerical results agree with experi-mental
results.
Cooling significantly reduces the total formingtime in multiscan
laser forming by greatly reducingthe need for waiting time between
consecutive scans.Multiscan is necessary if laser forming is to
becomea practical production process. Cooling only moder-ately
decreases material ductility even after multi-scan because the
repeated work hardening is offsetby repeated softening. The
softening is obtainedthrough recovery and recrystallization
accompany-ing each scan. Grain refinement and partial
phasetransformation to bainite are also observed.
AcknowledgmentSupport for this project under a NSF grant
(DMI-
0000081) is gratefully acknowledged. Dr. WayneLi’s assistance in
numerical study and experimentsin particular is also greatly
appreciated.
References1. A. Sprenger, F. Vollertsen, W.M. Steen, and K.
Watkins, “Influence of
Strain Hardening on Laser Bending,” Proc. of Laser Assisted Net
ShapeEngg. (LANE ‘94) (v1, 1994), pp361-370.
2. K.U. Odumodu and S. Das, “Forceless Forming with Laser,”
ASMEMaterials Div. publication MD v74, 1996, pp169-170.
3. T.D. Hennige and M. Geiger, “Cooling Effects in Laser
Forming,”Technical Papers of NAMRI/SME (1999), pp25-30.
4. V. Olden, A. Smabrekke, and M. Raudensky, “Numerical
Simulationof Transient Cooling of Hot Rolled Asymmetrical Ship
Profile (HollandProfile),” Ironmaking and Steelmaking (v23, n1,
1996), pp88-91.
5. B. Debray, P. Teracher, and J.J. Jonas, “Simulation of the
Hot Rolling andAccelerated Cooling of a C-Mn Ferrite-Bainite Strip
Steel,” Metal. and Mat.Trans. A: Physical Metallurgy and Materials
Science (v26A, 1995), pp99-111.
6. M. Cristinacce and P.E. Reynolds, “Current Status of
theDevelopment and Use of Air Cooled Steels for the Automotive
Industry,”Proc. of 1996 Symp. on Fundamentals and Applications of
MicroalloyingForging Steels, 1996, pp29-43.
7. H. Martin, “Heat and Mass Transfer Between Impinging Gas Jets
andSolid Surface,” Advances in Heat Transfer, J.P Harnett and T.F.
Irvine, eds.,Vol. 13 (New York: Academic Press, 1977), pp1-60.
8. K.P. Perry, “Heat Transfer by Convention from a Hot Gas Jet
to aPlane Surface,” Proc. of the Institution of Mechanical
Engineers (v168,1972), p775.
9. B.A. Boley and J.H. Weiner, Theory of Thermal Stresses
(DoverPublications, 1997). 10. T.H. Courtney, 1990, Mechanical
Behavior of Materials, ch. 7 (NewYork: McGraw-Hill Series in
Materials Science and Engg., 1990).11. R.E. Reed-Hill, Physical
Metallurgy Principles, ch. 17 and 18, PWS-KENT Series in Mechanical
Engineering (1973).12. J. Bao and Y.L. Yao, “Analysis and
Predication of Edge Effects inLaser Bending,” Proc. of 18th Int’l
Congress on Applications of Lasers andElectro-Optics (ICALEO ‘99):
Conf. on Laser Materials Processing, SanDiego, CA, Section C, 1999,
pp186-195.
Authors’ BiographiesMr. Jin Cheng is a PhD candidate and Dr. Y.
Lawrence Yao is an asso-
ciate professor at Columbia University. Dr. Yao’s research
interests are inmanufacturing and design, laser machining, laser
forming, and laser shockprocessing.
72
Journal of Manufacturing ProcessesVol. 3/No. 12001
Figure 13Comparison of Bend Angle by Different Cooling
Schemes
When Buckling Mechanism Dominates (no cooling also
included)(Dbeam = 8 mm, Pair = 80 psi, � = 0)
Bend
ang
le (d
egre
e)
Cooling conditions
1 & 7—no cooling2 & 8—bottom cooling (80 psi)3 &
9—top & bottom cooling (40 + 40 psi)4 & 10—top cooling (80
psi)
1 2 3 4 5 6 7 8 9 10
5
4
3
2
1
0
P = 800 W, V = 50 mm/s
P = 400 W, V = 25 mm/s
