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Fiber-Rich Welding: Fiber lasers are a good choice for microwelding applications

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July/August 2013 Volume 6 Issue 4

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Geoff Shannon

By Geoff Shannon

Technology Manager, Miyachi Unitek Corp.

(626) 303-5676

Geoff@muc.miyachi.com

The use of fiber lasers for microwelding is increasing. They are particularly well-suited for microwelding medical and electronic components, such as hypodermic tube assemblies and disk-drive armatures.

Fiber laser fundamentals

A fiber laser’s beam is produced within a small-core silicon fiber, typically between 9µm and 50µm in diameter, doped with the chemical element ytterbium. The laser is “pumped” using telecom-grade diodes that enable the lasing process. (See schematic of a typical fiber laser below.)

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A fiber laser equipped with a scan head makes 20 spot welds in less than half a second. All images courtesy Miyachi Unitek.

Because the laser is generated wholly within a fiber, there is no need to align the medium to cavity mirrors or maintain optics and alignment, as is the case with other lasers. A unique feature of the fiber laser is its “focusability.” For example, a 500w laser can be focused down to a 10µm spot size, which is not possible with any other laser. This enhances a fiber laser’s ability to create small welds and perform high-speed seam welding.

Fiber lasers operate in either single-mode or multimode. The single-mode laser has higher focusability, down to a 10µm-dia. spot size. The multimode laser is typically used to create focus spot sizes greater than 75µm in diameter. In most welding applications, the multimode laser is used because it produces a weld large enough to fulfill strength requirements. Also, multimode lasers’ “flat-top” power profile promotes even heating—meaning no hot spots—and weld stability.

Capability and applications

Both single-mode and multimode fiber lasers produce weld beads with varying dimensions.

The ability of a fiber laser to tailor both the depth and width of a weld allows the weld to be optimized for functionality within each application. This also includes production considerations, such as part stack-up tolerances and part fit, which must be addressed to ensure weld reliability and repeatability.

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Interaction of the pumping diode light with the active core of a fiber laser.

Fiber lasers are ideal for high-speed welding. Parts manufactured in large volumes, such as battery components for wireless devices, typically call for welding speeds greater than 1m/sec. at weld depths of 200µm—requirements fiber lasers can meet.

Another area where the use of fiber lasers is growing is disk-drive armature production. One reason is that the manufacture of hard drives for computers requires extremely tight manufacturing tolerances. The armature scans across the hard drive, reading and inscribing data. It is comprised of thin layers of stainless steel—typically two or three layers, each less than 100µm thick—that are laser-welded.

Welding thin materials is always a challenge, and that challenge is magnified by the need in this application for relatively large welds—up to 250µm in diameter. When the weld diameter is larger than the part thickness, there is an increased chance of process instability and weld defects. The stability of multimode power distribution across the focused spot helps ensure acceptable failure rates, in the range of a few parts per million.

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Spot-welded disk-drive armatures. Each armature is welded in less than 1 second.

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An example of a fine spot weld performed by a fiber laser. A cross section of a bead on a plate-weld with a width of 30µm and penetration of 200µm into steel.

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Schematic of a laser scan-head system, including high-speed, low-inertia galvanometer motors and an F-theta lens.

Pressure-sensor diaphragm welding is another common use for fiber lasers. Typically, a 100µm- to 200µm-thick metal cover is welded to a base, creating a barrier, or hermetic, seal. When welding thin materials, heat-input control is important, because minor deviations are magnified due to the material’s minimal heat-sinking capabilities. Likewise, minimizing welding heat that enters the part by using a small spot size reduces thermal distortion and the chances of the thin disk lifting from the base and interrupting the welding process.

Another laser welding application involves airbag initiators. During an automobile crash, the iniator deploys an airbag with an explosive charge. Initiators are made of two parts joined by a circumferential seam weld. The integrity of the weld is critical to the charge working properly. This makes a fiber laser a good choice for this application because it can lay down a narrow but penetrating weld—typically 0.2mm wide and 0.5mm deep—at speeds of more than 75mm/sec.

High-speed options

When paired with high-speed scan heads, fiber lasers can improve productivity and simplify system integration. This combination enables high-speed seam welding and high-speed, point-to-point positioning between spot welds.

A scan head consists of two mirrors, each mounted orthogonally on a very small rotary motor called a galvanometer. The rotary motion of the two mirrors translates to linear motion in the X-axis and the Y-axis. The small size of the motors provides high-speed positioning, short settling times, and high acceleration and deceleration, which is ideal for high-speed, short-distance motion.

The laser is directed through the scan head by two mirrors, then focused by what is known as an “F-theta lens” (see graphic). This lens focuses the laser over an X/Y area according to where the motors have positioned the laser at the input side of the lens. This contrasts with a regular lens, which focuses to a single point in X and Y. For microwelding applications, the X/Y area over which the F-theta lens operates ranges from 25mm × 25mm to 100mm × 100mm.

Scan-head performance typically provides point-to-point positioning of up to 6m/sec., which means that a distance of 25mm is covered in 5 milliseconds. The disk-drive armature application uses a scan head to make up to 20 spot welds over a 25mm × 25mm area in less than 1 second. This is not possible with any other motion system.

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The range of weld sizes possible using various single-mode and multimode fiber lasers. Any weld size is possible in the blue region. Each corner of the image has a cross section through the weld showing depth and width characteristics.

Fiber lasers are well-suited for use with scan heads because their high power densities can take advantage of the high-speed motion a scan head offers. Also, the optical characteristics of the fiber, in terms of its beam quality, allow smaller scan heads to be employed. This allows the use of less-expensive lasers with smaller footprints.

The scan head offers excellent motion performance and can be more economical than other motion solutions. Another benefit is that the small scan head can be easily fitted into an in-line manufacturing process, offering motion without the bulk of stages. Once the part is delivered to the processing area, it conveniently remains stationary during processing as the scan head moves the laser on and around the part.

Integrating a fiber laser into a production process is similar to integrating other types of lasers, but the superior beam quality of the fiber laser and efficient scan-head technology allow the creation of fast, in-line and small-footprint welding systems. µ

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Geoff Shannon is laser technology manager at Miyachi Unitek Corp., a laser manufacturer based in Monrovia, Calif. Telephone: (626) 303-5676. E-mail:  Geoff@muc.miyachi.com.