LASER WELDING OF THIN SHEET HEAT-TREATABLE STEEL
Slobodan Kralj, Branko Bauer, Zoran Kožuh
Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Croatia
E-mail: fsb.zk@fsb.hr
ABSTRACT
A report on the welding of heat-treatable steel 25 CrMo 4 and 42 CrMo 4, 2mm
thick sheet, with high power Nd:YAG laser is presented. Laser and process
parameters were explored. Argon and Helium were used as shielding gases.
Metalographic analysis was done to asses weld geometry on bead on plate welded
samples. Optimum focus position and full penetration welding speed range was
determined for each combination of material and shielding gas.
Keywords: Laser welding, steel, Nd:YAG laser, Parameters
1. Introduction
2.2. Experimental Procedure
Laser welding of thin sheet structures is a
joining process of materials having ever growing
application in electronics, aeronautics, automotive and
military industries. Thin sheets made of steels for heat
treatment are offering advantage of reduced weight of
structure due to their increased strength. Despite the
reduced wall thickness, stiffness of structure is
improved. Prior to any laser welding procedure to be
implemented in the production line, optimal position
of focus related to the surface of workpice to be
welded must be established. Also, welding speed
producing full penetration, type and flow of shielding
gas are determined through trials [1].
The intention of the research has been to
establish the optimum focus position and feasible
range of welding speeds providing full penetration
welds on 2 mm thick sheets made of 25 CrMo 4 i 42
CrMo 4 steel grade. Additionally, research has been
targeted to establish whether effects of material
quality and shielding gases (argon or helium) upon
the studied parameters can be noted.
When the maximum laser power is applied,
maximum welding speed is attained, this being
interest of industry. For this reason, laser power has
been preset at 1800 W, i.e. at 90 % of the unit rated
power. Welding has been performed on the samples
of 100 x 25 mm size. Prior to welding, samples have
been cleaned applying emery paper and ethanol. The
samples have been fixed in the jigging tool. To
determine welded joint geometry, macro-etches of
joint cross sections have been prepared. Following
features have been measured: penetration depth, weld
width, width of HAZ, joint cross section area.
During the research, argon and helium, the
inert shielding gases have been applied. The reactive
gases such as nitrogen and carbon dioxide have been
rejected since they have effects upon the metallurgical
processes in the weld and produce increased hardness
in the weld joint [2]. To supply the gas to the weld
location, coaxial nozzle has been used.
2.3. Determination of Focus Position
Relative to Workpiece Surface
2. Experimental Work
For each tested material and shielding gas,
series of bead-on-plate welds have been made,
applying constant power rating of 1800 W and
welding speed of 200 cm/min. Position of focus have
been changed moving the lens of the focusing
assembly in steps of 0,2 mm. Welding speed of 200
cm/min has been selected to avoid full penetration,
since the penetration depth is the feature that is to be
measured.
2.1. Equipment
Welding trials were performed using 2kW
Nd:YAG laser of “ROFIN CW 020”. 600m optic
cable is used for beam transfer, and focusing optics
120/120mm. The beam diameter in the focus is
0.6mm. Focusing optics is fixed to the robot arm
“IGM limat RT 280”.
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The Annals of “Dunarea de Jos” University of Galati
Fascicle XII, Welding Equipment and Technology, Year XVII, 2006
ISSN 1221 – 4639
2.4. Determination of Welding Speed Range
Producing Full Penetration
3. Results and Discussion
3.1. Optimum Focus Position
The highest welding speed producing full
penetration has been determined applying constant
parameters, and only welding speed has been
increased. Series of welds have been made applying
different welding speeds, and root side of weld has
been inspected to check the full penetration. For the
maximum speed, uniform appearance of root face still
must be visible. For the lowest welding speed, shape
of the weld must be of the form characteristic for the
laser welding. Welding speeds in range from 80 to
170 cm/min for each material and shielding gas have
been tested, step being 10 cm/min. Welding has been
conducted applying optimal position of focus
determined in preliminary testing, and using
parameters applicable for this condition.
Measuring penetration depth on the macroetches applying image analyzer, an interval located
from 0 mm up to 1,4 mm beneath the surface of the
plate has been revealed, in which penetration is
approximately equal, for both tested materials and
shielding gases. Medium value of this interval is
frequently used as the optimum focus position,
securing that small vertical movements of focus will
not be reflected in significant change of penetration
depth [1].
Optimum focus position is in the middle of
revealed interval and is located at 0,7 mm beneath
plate surface, Fig.1.
Penetration (p), weld width (b_weld) and width of HAZ (b_HAZ) in
relation to the focus position, steel grade
25
CrMo 4, shielding gas: He.
p, b_weld, b_HAZ mm
key hole welding technque
2
1,8
1,6
1,4
1,2
1
0,8
0,6
0,4
0,2
0
-3,4
p
b_weld
optimum focus position
z=-0,7 mm
b_HAZ
-2,8
-2,2
-1,6
-1
-0,4
+0,2
+0,8
+1,4
+2
Focus position relative to plate surface, z mm
Fig.1. Optimum focus position: steel 25CrMo4, shielding gas He
This result is in accordance with data in
references, where it is suggested that maximum
penetration is achieved if focus is located at depth of
1/3 of plate thickness beneath the surface [3]. Results
are indicating that effects of type of material and
shielding gas upon the position of focus providing
maximum penetration are very limited. When helium
is used as the shielding gas, maximum penetration is
achieved if focus is located slightly deeper in the
material.
Moreover, there are examples where location
of maximum penetration produces unsatisfactory
outer appearance of the weld. In such case, criterion
for selecting the optimal focus position is not
penetration but weld surface appearance. In welding
of certain aluminium alloys, optimal position of focus
is located above the material surface [4].
3.2. Welding speed range
In the case of bead-on-plate welding on steel
25 CrMo 4, welding speeds are ranging from 110 to
160 cm/min for argon and helium gas shielding, Fig.
2. In subsequent welding of butt joint applying
welding speed of 160 cm/min full penetration along
the length of weld has not been attained. Welding
speed range is reduced to values from 110 to 150
cm/min. This corresponds to heat input of 72 to 98
J/mm.
In the case of bead-on-plate welding on steel
42 CrMo 4, welding speeds are ranging from 90 to
150 cm/min, for argon and helium gas shielding Fig.
3. In subsequent welding of butt joint applying
welding speeds of 140 and 150 cm/min full
penetration along the length of weld has not been
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attained. Welding speed range is reduced to values
from 90 to 130 cm/min.
This corresponds to heat input of 83 to 120
J/mm.
a) Argon
b) Helium
Fig. 2. Cross-sections, material 25 CrMo 4, maximum speed 160 cm/min, focus position z=-0,7 mm
a) Argon
b) Helium
Fig. 3. Cross-sections, material 42 CrMo 4, maximum speed 150 cm/min, focus position z=-0,7 mm
The obtained results indicate existence of the
relationship between type of material and welding
speed, this being in conformance with other
investigations [5]. The reason for this phenomenon
may be attributed to size and distribution of globular
cementite within ferrite, phases making the bulk of
microstructure of base material. Influence of shielding
gas upon the welding speed is not registered.
In the industrial production, target is to apply
the maximum speed, though such approach may have
certatin drawbacks, since the more precise joint fitup
and laser beam manipulation is required. Maximum
welding speed gives minimal heat input, having
favourable effects upon distortions and negative
effects considering occurrence of increased hardness.
Therefore, welding speed range ensuring full
penetration has been investigated. In such case,
welding speed that is associated with most suitable
heat input for given application can be selected.
Geometric characteristics of welded joint,
joint cross-section area, weld width, root width and
HAZ width, measured for same welding speeds for
both material grades are slightly greater when helium
shielding is applied.
The obtained results indicate that
approximately same amount of heat input is required
to attain full penetration when applying both shielding
gases. This is a contrast to gas shielded arc welding
processes (MIG/MAG).
Effect of shielding gas upon the weld pattern
is significant in the case of lower power densities (2
to 5 x105 W/cm2), while in the case of higher power
densities (2 x106 W/cm2) it is less marked. In this
trial welding, power density was at 6,4 x105 W/cm2
level and this value is ranked into the high density
field.
Effect of shielding gas upon the joint
geometry (weld width and penetration) is less marked
[6]. According the [6] sources, it is generally accepted
that both argon and helium shielding are providing
approximately equal penetration depth and weld
width in the case of using Nd:YAG laser.
Higher ionization energy required for helium
is a higher resistance to plasma generation. As there is
no generation of plasma during Nd:YAG laser
welding [7], there is no significant difference in
effects of argon and helium upon the penetration
depth.
4.Conclusion
From the investigation of parameters for
laser welding of 2 mm thick heat-treatable steel 25
CrMo 4 and 42 CrMo 4, applying argon and helium
shielding atmosphere following conclusions can be
made:
-interval of focus position in which
maximum penetration can be achieved is within 0 to
1,4 mm below the plate surface, the optimum distance
of focus being 0,7 mm beneath the plate surface.
Effects of type of base material and shielding gas
upon the optimal focus position have not been
detected.
-the optimum range of welding speed for
steel grade 25 CrMo 4 welded applying argon and
helium shield is between 110 and 150 cm/min, wile
for steel grade 42 CrMo 4 this range is between 90
and 130 cm/min. These results suggest that there is
influence of type of material, but there are no effects
of type of shielding gas upon the welding speed.
This investigation has been targeted to
determine focus position interval and speed range
producing full penetration, but heat input was
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The Annals of “Dunarea de Jos” University of Galati
Fascicle XII, Welding Equipment and Technology, Year XVII, 2006
ISSN 1221 – 4639
[3] P. Limley, Parameter beim Laserstrahlschweißen, In:
Laserstrahlschweißen und
Prüfen und Bewerten
von
Laserstrahlschweißungen, pp 1-16, SLV München, München, May,
2000.
[4] S. Kralj, Z. Kožuh, B. Bauer, Welding of Aluminium Alloys
with a High Power Nd:YAG Laser, Proc. of 11th International
DAAAM Symposium, pp. 243-244, ISBN 3-901509-13-5, Opatija,
October, 2000.
[5] A.W.E. Nentwig, Untersuchungen zur Schweißeignung von
Stählen zum Laserstrahlschweißen mit CO2 Hochleistungslasern,
AiF-Bericht Nr. 9803, SLV München, 1997.
[6] ..., VDI-Technologiezentrum Physikalische Technologien,
Schweissen mit Festkoerperlasern, Handbuchreihe: Laser in der
Materialbearbeitung, Band 2, VDI Verlag, ISBN 3-18-401407-X,
Düsseldorf, 1995.
[7] L. Dorn, H. Grutzeck, S. Jafari, Schweißen und Löten mit
Festkörperlasern, Springer-Verlag, ISBN
3-540-55543-9,
Berlin, 1992.
different. However, exactly heat input is the major
factor affecting the hardness rise in the welded joint.
Determination of the effects of heat input upon the
hardness changes in the welded joint may be topic of
the further research, so providing a recommendation
for the most suitable combination of focus position
and welding speed taking into consideration the
maximum hardness in the welded joint.
References
[1] …, ANSI/AWS C7.2, Recommended Practices for Laser Beam
Welding, Cutting and Drilling, AWS, ISBN 0-87171-562-7, Miami,
1998.
[2] M. Faerber, Process Gases for Laser Welding, Proc. of 6e
CISFFEL, pp. 837-841, Toulon, Jun,1998
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