Zach Murphree Zach Murphree
VP of Technology Partnerships

Automated Calibration Optimizes Multi-Laser System Performance

October 15, 2019


Manufacturers often experience an inverse relationship between speed and quality, but when making parts for aerospace, medical devices and other complex applications, quality is fundamental. Any gains in output that compromise the safety or utility of the part aren’t gains at all. 

In the interest of faster production, additive manufacturing (AM)-equipment makers have been moving toward two-, and sometimes even four-laser setups. However, adding lasers can introduce complications quantitatively and qualitatively distinct from those of printing with a single laser.

Laser Alignment

The addition of a second (or more) laser to an AM setup can raise concerns with scan-field alignment, particularly with aerospace components and other large parts. While one laser can individually print many types of smaller medical parts, such as orthopedic replacements, printing larger parts that take up most of the build plate often requires multiple lasers to work in concert. Any slight laser misalignment can have big consequences. In engineering terms, that means that each scanning system must accurately align to the other within less than 50 microns difference throughout the build plate.

In most systems, precisely aligning two laser spots (Fig. 1) requires external hardware. The alignment procedure often consists of burning marks on a substrate and measuring the outcome, a tedious process. In addition, AM shops must perform the calibration at the build plane, since any change in the distance between the scan head and the measurement plane will cause a corresponding alignment shift.

The System’s Thermal State 

…represents another important variable 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 position, 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 pointing. On a typical job, this type of AM system 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 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—good enough for a child’s block structure, but not for an aerospace application. With in situ monitoring, the piece is indistinguishable from one printed with a single laser (Fig. 4). Manufacturers 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 if one laser sees 100-percent utilization and the other sees less, say 50 percent. Shops must therefore pay close attention to laser scheduling—this means knowing the path of the lasers. In addition, engineers must examine how laser soot affects the scheduling, and how well the lasers coordinate their efforts when printing the contour and core of larger parts.

Several related issues arise when considering soot management, gas flow and laser scheduling. A quick way to derail a build: have one laser shoot through the soot cloud created by the other. Therefore, given the tight space in which the lasers operate, each laser’s path must be carefully planned. Shooting through the soot cloud reduces beam intensity when it hits the metal powder, which can lead to poor mechanical properties. 

To ensure that neither laser gets in the other’s way, AM manufacturers can adopt the “swim lane” method. Here, each laser stays within one of two parallel tracks (Fig. 5), with all track switches synchronized. This programming method allows both lasers to run at all times during a build, taking full advantage of the two-laser system. 

Dividing the Work

A crucial engineering question (with more than one answer) is how to subdivide the work between the lasers when each is building a portion of a solid part. Consider, for example, printing a cylinder, where one laser prints the entire contour while multiple lasers work on the core. Parts printed in this way will look good on the surface due to the seamless outer layer, however this approach can cause sub-contour porosity, with any structural issues hidden by the clean outer surface.

To make overlay issues highly obvious, AM shops can task each laser with printing the core and contour for the structure, with the two sides connected by a zig-zag line (Fig. 6). The jagged connection will improve structural integrity and, like a wall of offset bricks, the connection line moves with each layer, further improving structure strength. When using this approach, any laser misalignment, which will diminish the surface finish of the part, also will make any issues with overlay immediately obvious and not hidden beneath a smooth surface. Automated calibration ensures that the laser overlay meets tolerance requirements and that the surface finish remains unchanged in the overlay region. It also ensures high quality, non-porous metal for the core of the part. 3DMP

Industry-Related Terms: Additive manufacturing, Build plate, Metal powder, Porosity, Surface finish
View Glossary of 3D Metal Printing Terms

 

See also: Velo3D

Technologies: Powder-Bed Systems

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