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Laser Additive Manufacturing

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Laser Additive Manufacturing Diagnostic & Control Tools

Laser-based Additive Manufacturing

This application note focuses on using focus beam profiler for laser additive manufacturing and answering the following questions. Where is my beam focusing? How big is my spot? Is it stable in time?

Selective laser sintering (SLS) is an additive manufacturing (AM) technique that uses a laser as the power source to sinter powdered material (typically nylon or polyamide), aiming the laser automatically at points in space defined by a 3D model, binding the material together to create a solid structure. It is similar to Selective Laser Melting (SLM); the two are instantiations of the same concept but differ in technical details. Selective laser melting (SLM) uses a comparable concept, but in SLM the material is fully melted rather than sintered, allowing different properties (crystal structureporosity, and so on).

Those processes require uniform, symmetrical and stable power density distribution of the laser beam. More specifically, the focus spot size and  intensity have to be maintained within a finite acceptance range throughout each build. Because of the high power of the laser used (typically in the range of several 100s of Watts to kW, thermal lensing can occur and affect the focus position in time. Therefore, understanding the stability of this parameter is critical in order to avoid structural weakness captured stress during the building process.

The Focus Beam Profiler (FBP) is a proven solution developed by Cinogy Technologies hand-in-hand with major actors in the industry. The Focus Beam Profiler is a robust industrial system designed to directly image the high-power on a 2D focal plane array after being attenuated with passive optical components. The position of the measurement plane is calibrated and known with high accuracy, thus giving a direct overview what the beam looks like at that position.

Schematic of the Focus Beam Profiler:

After positioning the Focus Beam profiler onto the build-plate and directly under the path of the laser beam, a complete beam caustic can be acquired by changing the build-plate position. The software quickly outputs the beam parameters according to ISO standard 11146-1 (a detailed description of the method used can be found in our application page —> https://axiomoptics.com/application/laser-beam-profiling-applications/).

RayCi software:

Top-left corner: 2D profile of the beam at selected position. Dotted line delimitates the active area of the beam. Red and blue lines show the beam orientation in beam coordinates (U,V), as opposed to lab coordinates (x,y).

Bottom-left corner: 3D view of the beam caustic. Top-right corner: beam size (y-axis) along the beam path (x-axis). Red and blue lines correspond to the  beam coordinate system (U,V). The continuous vertical lines indicate the position of the beam waist (U,V). Note that in this case the beam shows some astigmatism. Bottom-right corner: Numerical data computed from the beam caustic.

Output parameters include:

  • Caustic position z0 (mm)—> position along the beam path where the focus spot size is minimum (aka waist position or focus position)
  • Beam Waist Diameter d0 (mm) —> diameter of spot size at the true focus position
  • Rayleigh Length zR (mm) —> distance from the beam waist (in the propagation direction) where the beam radius is increased by a factor of the square root of 2
  • Divergence theta (mrad) —> angular measure of the increase in beam diameter or radius with distance
  • M2 —> beam quality factor, represents the degree of variation of a beam from an ideal Gaussian beam

For measurements in the field, an alternate software solution (Focus Beam Profiler Control Tool) is available for quick step-by-step measurements: 

Overview of the Focus Beam Profiler models:

Real-time closed-loop control and monitoring of laser processing 

Coaxial imaging of the melt pool in laser processing has enabled a number of approaches to real time closed-loop control and monitoring of different laser processing applications. CMOS technology has dominated this research area with most relevant works appearing during the last decade. As a result, the few imaging commercial systems available for this purpose are mostly based on CMOS sensors. However, these sensors present a number of issues that seriously limit their performance in practical settings. Firstly, they are sensitive only to wavelengths under 1 μm hardly seeing thermal emission from bodies at temperatures under 900ºC, thus being blind to typical cooling processes (e.g. in laser cladding). Secondly, they suffer much in the presence of reflections and bright spots from projections or powder, due to their high sensitivity (in the visible range). Moreover, radiance increases much faster with body temperature in the visible range at process temperatures than in the IR. As a result, a very limited dynamic range is available for process observation. The images acquired are practically binary with little information about actual heat distribution in space. In the last years novel uncooled PbSe imagers that work in the Mid-wave infrared (MWIR) spectral range (1-5 μm wavelength) have appeared with the potential of being game changers in this field. Being sensitive in the MWIR means that these sensors can see radiance emitted at much lower temperatures -down to 100ºC- and they can make a better use of their dynamic range, even at high temperatures.

Application to additive manufacturing (AM) by Laser Metal Deposition (LMD)

Direct Energy Deposition (DED) processes are showing a growing interest in the industry as they have strong capabilities to build large-sized components, even over non-flat surfaces and with fast building rates compared to other AM processes. Among them, Laser metal deposition (LMD), also known as Direct Laser Deposition (DLD), processes are gaining importance and have been investigated heavily in the last several years as it provides the potential to rapidly prototype metallic parts, produce complex and customized parts,  clad/repair precious metallic components. Recently, different closed-loop control systems have been implemented to improve the robustness, reliability and the geometrical accuracy of components built by powder- LMD. Specifically, researchers have monitored laser parameters, melt pool metrics, part temperature, feed material, geometry, and optical emissions during processing. A common strategy is sensing and control of melt-pool size or temperature. Other efforts have attempted to maintain a constant layer build height by directly sensing build height and adjusting processing head position, processing speed, material feed rate or laser power. As a result, the exploitation of LMD processes continues to accelerate. However, work remains for AM to reach the status of a full production-ready technology. Production challenges remain such as: assurance of quality, right-first-time manufacturing capability and the complexity of AM processes involving many input parameters are technological barriers preventing the widespread deployment in manufacturing sectors at industrial level. Ensuring AM process qualification and good part quality has many different aspects, such as: part design, feedstock material, process parametrization, process planning, manufacturing strategies, inline and online monitoring and control systems, etc. Besides geometrical accuracy of the part, microstructure is a very important characteristic of the laser deposit because it has a strong impact on the mechanical properties. The two main common defects or material discontinuities that limit final part quality are porosity and cracks. Thus, the wider adoption of AM technologies require techniques that improve the quality of parts, namely, microstructure anomalies and main process defects such as porosity and cracks. CLAMIR by New Infrared Technologies is a closed loop control system based on high speed coaxial MWIR imaging. The embedded system with real time processing capabilities obtains IR images of melt pool at very high frame rates (1 KHz), extract key features of this image (such as width) and based on specific algorithms, it controls the power of the laser during process by the action of an embedded proportional-integrated (PI) controller. CLAMIR main features:                                                                         

  • Continuous monitoring and measurement of the melt pool geometry using a MWIR infrared camera (1µm – 5.0 µm)
  • Closed-loop control of the laser power during the complete process, ensuring quality and repeatability
  • Compatible with most of laser optics and powders
  • Easy mechanical integration and quick configuration
  • Consistent operation, no need of reconfiguration during the process

Principle of operation:

  • Continuous on-axis monitoring of the melt pool geometry using a high-speed MWIR infrared camera (1.1 um – 5.0 um)
  • Embedded processing electronics performs a real-time dimensional measurement of the melt pool.
  • The optimum laser power is calculated and controlled through an analog output (0 VDC – 10 VDC)

Advantages over existing CMOS-based solutions:

  • Wider range of detected temperatures (+100C) – better accuracy
  • Robustness against high-power, high-intensity signals and spatters
  • Wider dynamic range

Advantages compared to pyrometry-based solutions:

  • Image processing techniques vs single point measurement
  • 2-color pyrometer is required to achieve accurate temperature reading

System components: 1.Camera and embedded processing unit2.Connection box

3.Software (configuration, control and visualization)

  • Desktop application for configuration and data logging (not required for operation of CLAMIR)
  • Allows configuration of process parameters (range of the laser power) and closed-loop feedback control
  • Other features: selection of the operation modes, camera control, definition of ROIs (rounded, square)
  • Data files visualization and analysis
  • DLL for custom S/W development

Mechanical integration:                    

  • On-axis optical system integration to monitor melt pool geometry
  • Laser head optical path needs IR transmission (>1.1 um) Integration in the laser head using an existing optical port
  • Easy mechanical integration and quick configuration
  • Dichroic mirror for compatibility with present VIS cameras for alignment and process visualization

Results with LMD processes: