Special IssueLaser Processing for Copper

The Features of the Blue-IR Hybrid Laser andthe Copper Processing Technique– High Quality Copper Welding by Using the Blue-IR Hybrid Laser –Masamitsu Kaneko*1, Nobuyasu Matsumoto*1, Takashi Shigematsu*2

In the laser welding of copper materials, we have developed a Blue-IR hybrid
laser that can greatly improve the processing quality compared to the processing with only a near-infrared fiber laser. By combining an efficient heat input to the coppermaterial with the blue laser and a local heat input with the fiber laser, the molten pool can bestabilized, which is difficult with only the fiber laser, and while suppressing the generation ofprocessing defects such as spatters and blowholes, a welding method which can obtain deep
penetration has been achieved.

ABSTRACT

Momentum for vehicle electrification, such as increasing
the ratio of Electrified Vehicles (xEV) in new vehicle sales
worldwide and establishing laws and regulations to accelerate the transition from internal combustion engine vehicles to xEV, is increasing in order to achieve the CO2
reduction targets set forth in the Sustainable Development
Goals (SDGs), and efforts by each original equipment
manufacturer (OEM) are accelerating. Along with this,demand for the products that make heavy use of coppermaterials, such as motors and batteries, which are electromechanical components of xEV, is increasing, andinnovation in the manufacturing process is also required.
In addition, in consideration of the declining birthrate andaging population especially in developed countries, there
is an increasing desire for labor saving and automation infactories. Therefore, Laser processing, which has features
such as non-contact, local heating, high speed, high precision and low distortion, is attracting attention as a solution that achieves high production efficiency.
In this respect, expectations for the laser welding ofpure copper are increasing, but there were problems withthe stability of penetration welding and the welding quality because the absorption ratio of copper near 1000 nm,
which is the fundamental wave of fiber lasers, is quite
low, and because the heat is difficult to accumulate locally due to its high thermal conductivity. To address theseissues, we have developed a Blue-IR hybrid laser thatcombines a visible light semiconductor laser and a nearAdvanced Laser Development Project Team,		 Incubator Department, Research & Development Division*2Industrial Laser System Department,		 FITEL Products Division,Communications Solutions Division*1

infrared fiber laser in order to achieve a high-quality welding of pure copper, and have established a welding technology that reduces spatter generation and provides stable and deep penetration compared to that of the conventional welding using only a fiber laser.

LASER
WELDING
COPPERMATERI2.2.LASERWELDING
OFOF
COPPERMATERIALS
ALS
2.1

Copper Processing With a Near-Infrared Fiber
Laser and Its Problems
Figure 1 shows the wavelength dependence of theabsorption ratio of copper. As can be seen from this figure, the absorption ratio of a copper material in the nearinfrared region is very low at 4 to 5%. Therefore, since
100
90Blue laser

Near infrared laser
Green laser

80
Absorption ratio (%)

INTRODUCTION
1.1. INTRODUCTION

70
Absorption ratio for copper

60
50403020100300

500

7009001100

Wavelength (nm)Figure 1 Wavelength dependence of the optical absorption ofcopper.

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The Features of the Blue-IR Hybrid Laser and the Copper Processing Technique – High Quality Copper Welding by Using the Blue-IR Hybrid Laser –most of the irradiated light is reflected, it is necessary toirradiate light with a very high-power density so that alarge amount of heat input can be obtained in order toobtain a molten state. On the other hand, once melted, its
absorption ratio rises to 10 to 20%1), 2), and the light energy used for melting becomes excessive energy and isincident on the processing point. Due to this excessive
energy input, the temperature of the molten metal rises
sharply, copper evaporates, and the keyhole is inducedby the evaporation recoil pressure. At this time, it isthought that spatters are generated because the shape of
the keyhole is disturbed by the vapor pressure of themetal vapor.
In addition, since the thermal conductivity of copper isvery high at 400 W/mK, the amount of heat input by laser
irradiation instantly diffuses to the surroundings. For thatreason, the temperature at the processing point repeatedly fluctuates, and as a result, the molten state becomes
unstable. Therefore, when a copper material is being processed with only a fiber laser, deep penetration can beobtained, but the bead on the surface is disturbed asshown in Figure 2 (a), and the melting depth are not uniform as shown in Figure 2 (b).
Laser light(a)(b)

300 µm300 µm

Bottom ofmelting part

Processing direction
Figure 2 Processing quality of the IR laser welding.
(a) surface, (b) cross-section.

mal energy tends to spread to the entire workpiece, andthe problem of the penetration depth becomes more significant. Figure 3 shows a schematic drawing of the thermal conduction type melting and the keyhole type melting
in the copper processing. As shown here, in the processing with a visible laser, it is stable but the melting depth isshallow, and in the processing with a fiber laser, the melting depth is deep but defects such as spatters and blow
holes are present.
(a)(b)
Laser light

Laser light(ex. ﬁber laser)

Spatter(ex. blue semiconductor laser)

Melting part
Processing target
ing
Processnotic
dire

Melting
partKeyhole

Blowhole
Processing targeting
Processdirection

Figure 3 Schematic drawings of the copper welding.
(a) thermal conduction type, (b) keyhole type.

2.3 Copper Processing With a Blue-IR Hybrid Laser
We have developed the Blue-IR hybrid laser BRACE,
which combines a blue laser and a fiber laser in order toperform high-quality copper welding while obtaining deep
penetration (Figure 4). As a blue light source, it isequipped with a direct diode laser module jointly developed with Nichia Corporation. The concept is to form astable molten pool by an efficient heat input to a wide
area with a blue laser, and to achieve deep penetration
with a fiber laser that is good at local heat input. This
Blue-IR hybrid laser is combined with an optical head or agalvanometer scanner to build a processing optical sys-

2.2

Copper Processing With a Blue Semiconductor
Laser
As is clear from Figure 1, the absorption ratio of copperfor visible light lasers such as a blue laser is 55 to 60%,
which is extremely high compared to an Infra Red (IR)laser. Therefore, more efficient heat input than with an IR
laser is expected. However, at present, it is difficult to enter
light into a minute fiber core of about several tens of μm
like a fiber laser, and as a result, the beam spot expands
and the heat conduction type melting mode is used. Inorder to obtain keyhole-type melting with a visible lightlaser, the power density of about 2 to 2.3 MW/cm 2 isneeded.
In the case of the heat conduction type melting, theheat absorbed on the surface is isotropically conducted,
resulting in a wide and shallow penetration, and it is difficult to obtain deep penetration as in the case of processing with a fiber laser. Especially in a material with a highthermal conductivity such as copper, the absorbed ther-Figure 4 Blue-IR hybrid laser, BRACE.

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The Features of the Blue-IR Hybrid Laser and the Copper Processing Technique – High Quality Copper Welding by Using the Blue-IR Hybrid Laser –tem, then, actual processing is performed. Figure 5
shows the photographs of the setup of optical system forprocessing.

(a)

(b)

shown in Figure 9 (b), the tail of the molten pool was longer than that formed by the processing with a fiber laser,
and the front part of the molten pool was also slightly
overhanging to the forward, resulting in the enlargementof the entire molten pool. By expanding the molten pool

300 µmFigure 5 Optical setup for laser welding combination with(a) an optical head (made by Laserx Inc.) and (b) agalvanometer scanner (made by YASKAWA ElectricCorporation).

Figure 6 Welding bead on a copper plate processed with theBlue-IR hybrid laser.

Laser light

Figure 6 shows the processed surface when a copper
bead-on-plate processing is performed with the Blue-IR
hybrid laser. As it is clear from the comparison withFigure 2 (a), a very beautiful bead with no turbulence,
spatter marks, or blowholes was obtained. Figure 7 is asnapshot of the results of the in-situ observation of themolten metal cross section during processing by applying
glass to the end face of the copper plate and processing
its vicinity the copper side. As shown in Figure 2 (b), themelting depth was non-uniform when processing with
only a fiber laser, but stable penetration was obtainedwith the Blue-IR hybrid laser, and the bottom depth of themolten pool was almost uniform.
Figure 8 is snapshots of the surface melting state during processing taken with a high-speed camera. It can beseen that in the processing with the Blue-IR hybrid laser,
the size of the molten part increased as the melting width
increased, compared to the processing with a fiber laser.
Looking at the keyhole aperture, the area of the reflected
light image from the aperture was large, and the keyhole
aperture diameter was widened during the processingwith the Blue-IR hybrid laser. It is considered that the widening of the keyhole aperture makes it easier for the thermally evaporated copper vapor to escape, which helps
reduce spatters.
Figure 9 shows the processed images of the in-situ
observation of the molten metal cross section duringlaser processing taken with a high-speed camera. From
this figure, the shapes of the molten pool during processing with a fiber laser and processing with the Blue-IR
hybrid laser can be estimated. As can be seen from
Figure 9 (a), in the processing with a fiber laser, the lengthfrom the processing front to the rear end was short, and a
sharp and deep molten pool was formed. On the otherhand, in the processing using the Blue-IR hybrid laser

Bottom ofmelting part

300 µmProcessing direction

Figure 7 Snapshot of the in-situ observation of the welding
cross section processed with the Blue-IR hybridlaser.

(a)

(b)

200 µm200 µmFigure 8 Snapshot of the in-situ weld surface observation witha high-speed camera.
(a) fiber laser, (b) Blue-IR hybrid laser.

(a)

(b)

Figure 9 Cross-sectional shape of the molten pool.
(a) fiber laser, (b) Blue-IR hybrid laser.

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The Features of the Blue-IR Hybrid Laser and the Copper Processing Technique – High Quality Copper Welding by Using the Blue-IR Hybrid Laser –in this way, the turbulence of the molten metal flowcaused by the evaporation recoil pressure generated on
the inner wall of the keyhole can be relaxed, which leads
to the stable behavior of the molten metal and theimprovement in the uniformity of the melting depth.
As described above, the effect of the coaxially irradiated blue laser expands the molten pool and stabilizes theinternal heat convection, and the expansion of the keyhole aperture allows the metal vapor to be discharged
well to the outside of the keyhole, and the vapor pressure
inside the keyhole can be reduced. Therefore, the generation of spatters can be suppressed and a stable molten
pool can be obtained in the processing with the Blue-IR
hybrid laser, resulting in a high-quality processing.
Figure 10 shows photographs that visualizes the trajectories of the spatters generated when a bead on plateprocessing with a length of 44 mm was performed on an
oxygen-free copper plate with a thickness of 2 mm usinga fiber laser and the Blue-IR hybrid laser. The processing
conditions were a fiber laser output of 1 kW, a blue laser
output of 150 W, and a processing speed of 200 mm/s.
As it is clear from the comparison between Figure 10 (a)and Figure 10 (b), the generation of spatters was significantly reduced in the processing with the Blue/IR hybrid
laser, which was one of several compared to the processing with a fiber laser.
(a)

CONCLUSION
3.3. CONCLUSIONThis paper briefly explains laser welding of copper materials, and reports that the Blue-IR hybrid laser stabilizes
the molten pool, suppresses welding defects such asspatters that occur during processing with a near-infraredfiber laser, and provides a deep welding depth.
In the future, we will deepen our understanding of themelt-solidification phenomenon in laser processing andpromote its elucidation, and also develop welding technologies for various components such as batteries andmotors to provide optimal processing solutions based on
physical explanations of the phenomenon to our customers.

REFERENCES
1) Miller, J.C.: “Optical Proper ties of Liquid Metals at High
Temperatures”, Phil. Mag., 20-168, (1969), 1115.
2) Yilbas, B.S., Danisman, K., Yilbas, Z.: “Measurement of temperature-dependent reflectivity of Cu and Al in the range 30-1000°C”,
Meas. Sci. Technol., 2, (1991), 668.

(b)

Figure 10 Trajectories of the spatters generated in copper
welding. (a) fiber laser, (b)Blue-IR hybrid laser.

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