Page 27 - 3D Metal Printing Fall 2019

3D
able when printing with multiple lasers. For example, consider a build chamber made from stainless steel, with the scan heads located 500 mm from the build plane. During a build, the chamber heats up; even a 15-deg. change in chamber temperature can cause the distance from the scan head to the build plane to change by more than 100 microns. Depending on the angle of incidence in the laser-overlap region, this temperature change alone can invalidate the calibration.
Tall parts, therefore, can be a cause for concern if building them using two lasers on a single part. Galvanometers tend to have some amount of drift in their posi- tion, and this drift accumulates over time. This means that a calibration that proved valid on the first layer, or even the first 100 layers, may not be valid at layer 8000 (Fig. 2). The quality of the overlay may change within a single build.
As an engineered solution to these challenges, consider systems with in-situ laser calibration. By frequently comparing the optics of the two lasers, autonomous calibration ensures that they remain in agreement on where each other is point- ing. On a typical job, this type of AM sys- tem will automatically check its laser alignment every few layers, which undoes any drift before it becomes a problem. We can test the effectiveness of this system in numerous ways.
Fig. 3 highlights the advantage of auto- mated recalibration by printing a rectan- gular block where one laser builds the top and bottom sections, and the second laser builds the middle. Without ongoing laser calibration, a visible offset in the middle section results—good enough for a child’s block structure, but not for an aerospace application. With in situ monitoring, the piece is indistinguishable from one print- ed with a single laser (Fig. 4). Manufac- turers should strive to meet this standard when using multi-laser metal-AM machines in production applications.
Laser Utilization and the ‘Swim Lane’
The increased production rate realized from multi-laser systems can be mitigated
Fig. 2—Building a tall part can be challenging when using two lasers. Galvanometers tend to have some amount of drift in their position, which accumulates over time. So a calibration that proved valid on the first layer, or even the first 100 layers, may not be valid at layer 8000, thus overlay quality may change within a single build.
Fig. 3—This graph highlights the advantage of automated recalibration by printing a rectangular block where one laser builds the top and bottom sections, and the second laser builds the middle. Without ongoing laser calibration, a visible offset in the middle section results.
Fig. 4—With in situ monitoring turned on, as is the case with the sample on the right, a piece printed with two lasers is indistinguishable from one printed with a single laser. The sample on the left, without such monitoring, reveals multiple- laser use.
3DMPmag.com
FALL 2019 • 3D METAL PRINTING | 25