Monitoring Laser Melting of Powders.

Industries worldwide are now developing prototypes from 3D metal printing. Slowing the adoption of this form of additive manufacturing beyond prototyping and into larger-scale production is the high cost of system acquisition, resulting in industries that deploy 3D metal printing producing only small quantities of very expensive and complex structures with low weight and high strength, such as those increasingly required in aerospace, medical technology or auto racing.

Despite its high costs, it is difficult to overlook the value of 3D metal printing when compared to subtractive material processing, casting processes or even other types of additive manufacturing. Production of a workpiece by 3D metal printing allows for completely new designs. In a single operation, it is possible to create structures with a high degree of geometrical freedom that would otherwise have to be assembled from several individual workpieces.

For the 3D printing process, a workpiece is generated on the computer using CAD and is then optimized for printing. Based on the generated print data, the workpiece is then created in the build space of the printer from layer-by-layer laser melting using a powdery material. The powder is applied in thin layers, which are smoothed to the set layer thickness between 10 and 100 micrometers by using a variation of a doctor's blade. After the printing, the workpiece is cleaned, removed from the building platform and, if necessary, reworked. Mostly metals or metal alloys -- ranging from stainless steel, aluminum, and titanium to precious metals such as gold -- are processed as powdery pressure media. This essentially determines the properties of the product and represents a cost-intensive element of the manufacturing process.

Standard industrial CW lasers (continuous wave) are used for locally precise melting of the powder, with laser beams being controlled by powerful galvanometer scanners. The type and quality of the exposure, achieved by the laser beam and the resulting melting of the powder, have a major influence on the properties of the workpiece, such as its density and surface quality. The control parameters of the laser also affect the setup speed during 3D printing. Optimized process monitoring and control at the melting point can, therefore, have a positive influence on the quality of the process and the product.

Scientists like Tobias Kolb, Chair of Photonic Technologies of Friedrich-Alexander University in Erlangen-Nuremberg (D), devote their attention to these processes, which take place in rapid succession and in the tightest of spaces. As part of his research project, he uses coaxial process monitoring to investigate the thermal radiation generated when melting the powder.

In addition to the complex theoretical background that needs to be mastered, the instrumental use in the laboratory is considerable. There, coaxially integrated high-speed cameras capture the thermal radiation emitted during the melting of the powder by using the optics of the laser.

"With high-speed cameras we obtain a high temporal and spatial resolution and can draw conclusions about process fluctuations, surface roughness in the process or splashes in the powder bed," explains Kolb.

The studies are conducted by using EoSens® CL CMOS cameras from Mikrotron that enable the retrieving of data on the size and shape of the weld pool and on the intensity distribution of the thermal radiation. In the optical system, there is a dichroic mirror that transmits thermal radiation at a wavelength range of 700 to 950 nanometers to the camera sensor. Since the process requires a minimum of 500 mm/sec scanning speed to over 1,000 mm/sec, a recording frequency of more than 10 kHz is necessary. This is the only way to achieve the required high spatial resolution.

Kolb describes the particular requirements concerning the cameras as follows: "To obtain a resolution in the order of magnitude of the weld pool (100 micrometers), a recording frequency of 10- 15 kHz is required. With a macro-optic, we focus on the weld pool and observe the process with the sensor's recording area reduced to 100 x 100 pixels in order to achieve this high recording frequency."

The result is an enormous data volume providing information about the weld pool. This data volume must be processed in the shortest possible time. Therefore, the signals delivered by the image sensor are pre-evaluated with FPGA (Field Programmable Gate Array). A vector is generated from each camera image, which describes the properties of the image. This information is assigned to an exact spatial position based on the data from the scanner system. From this, images of the thermal radiation are generated layer by layer, before being analyzed.

"We are working on the further development of an image processing software to evaluate this data," adds Kolb. "In the future, this could result in a controlled process, where defects can be detected during printing and then compensated for in the subsequent layers through laser polishing or other methods."

Modern high-speed cameras from Mikrotron can deliver what is needed for optimized process analysis in Selective Laser Melting (SLM), Direct Metal Laser Sintering (DMLS) or Selective Laser Sintering (SLS). Thanks to the rapid process analysis, laser welding, soldering and drilling with lasers used to manufacture modern products based on innovative technologies and materials could also be improved in a comparable way.

Workpieces produced with laser-based manufacturing methods are becoming increasingly complex, light and robust. They will soon be indispensable in every automobile and aircraft, as well as in countless other products ranging from medical technology to high-quality consumer goods.

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