Scan Heads for Micro Laser Welding: “Ham n’ Eggs” of Laser Industry

What do you get when you pair a non-contact, high intensity heat source with a compact, relatively inexpensive high speed motion system? A perfect match! The “ham n’ eggs” of laser industry: a micro laser welding system that can push productivity to the max with three key features:

1. Point to point positioning in milliseconds
2. On the fly customizable weld sizes and
3. Very small footprint for integration

(See the last section of this post if you need a quick guide to how a scan head works.)

Maximizing production by minimizing point to point positioning time

Scan heads are ideal for use in volume manufacturing, where they can shave seconds off cycle times on parts with multiple weld locations by minimizing point-to-point positioning times. Typically, a 1-inch move will take 5 milliseconds to move, settle and be ready for welding.

Disk drive armature manufacturing is a great example of how a laser can be used with a scan head in a mass-production, high-quality driven industry. Each armature assembly has 15-30 spot welds between materials that measure just 0.001 to 0.004-inch thick. 7-8 years ago a transition was made from the use of high speed linear stages to scan heads – essentially doubling the productivity of each machine. Each set of spot welds is made in less than a second, and in some places the proximity of the weld spots pushes the weld pace beyond 85 welds per second. Figure 1 shows disk drive armatures welded using scan heads, each armature completed in less than 1 second. The small dots indicate the locations of the spot welds.

Figure 1: disk drive armatures
welded using scan heads

Customizable weld sizes

Single mode fiber lasers offer incredible welding performance in terms of power vs penetration. A 500W laser is capable of penetration beyond 0.04-inch in steel and aluminum. However, with an optical focus spot size of only 0.001-inch, the welds are sometimes too narrow to meet weld strength requirements and part fit-up tolerances. The scan head, however, can rapidly dither or “wobble” the laser perpendicular to the weld direction so the weld may be widened according to requirements. The amplitude and frequency of the wobbling is programmed via software that controls the scan head. A file can contain multiple welds, each with different wobble dimensions. If you have a high mix of parts, you may build a library of files and load them into the machine via remote commands or I/O. Figure 2 shows a schematic of this technique and figure 3 is an image of the welding results.

Figure 2: schematic of single mode fiber laser and scan head to
to enable tailored weld profiles

Figure 3: cross section of bead on plate welds using a single
mode fiber laser and scan head. Top: without wobble. Bottom: with wobble.
Note: linear welding speed is the same.

Small footprint for ease of integration

The scan head and connection optics for the fiber that delivers the laser are about the size of large shoe box. With the scan head providing the XY laser motion there is generally no requirement for additional part motion which is highly beneficial for inline volume processing. Fiber-delivered lasers offer an additional benefit – the laser can be positioned 20 or 30 feet from the scan head, adding the convenience of routing the fiber delivery cable as needed through the production line or machine.

How a scan head works

A scan head consists of two mirrors, each mounted on very small rotary motors, called galvanometers. These two mirrors and motors are mounted orthogonally to each other such that the rotary motion of the two mirrors translates to linear motion in the X- and 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 though the scan head by two mirrors to what is known as an “F-theta lens,” which 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.

Figure 4 is a schematic of the laser and scan head system. The laser is moved by high speed, low inertia galvanometer motors mounted with mirrors through a lens producing a focal XY plane for welding. Also shown is the scan head, the laser delivered to the beam expander/collimator. The size of the entire assembly is about 20-inches x 6-inches x 6-inches.

Figure 4: schematic of scan head and image of actual hardware