Kevin Kirkham, Senior Manager, Product Development, Ophir
Sensor: "A device that detects or measures a physical property and records, indicates, or otherwise responds to it.” A sensor is a device that detects a physical quantity and responds by transmitting a signal.
Fusion: "The process or result of joining two or more things together to form a single entity.” A blending, amalgamation, joining, marrying, bonding, merging, melding, or synthesis of materials or information to realize a sum that is greater than their parts. In this case, the data from numerous sensors are fused to create a more complete understanding of the laser-enabled process.
Most laser applications require the beam spot size and intensity to be maintained within a finite acceptance window. In many cases, laser parameters must be measured to be certain the requirement for consistent, stable, and correct laser power density is met.
But just as Icarus was warned not to fly too low should the dampness of the sea damage his wings, nor fly too high for the sun would melt the wax that secured the feathers that formed his wings, laser sintering applications must monitor the power density of the presented laser beam so the power density is neither too “hot” nor too “cold” for successful material modification.
How do we know the laser is delivering exactly the beam that's needed? Most commercially available laser power / energy measurement products offer traceability of the calibration of both the sensor and sensor interface or meter. The US-based National Institute of Standards and Technology (NIST) and the German-based Physicalisch-Technische Bundesanstalt (PTB) provide laser power sensor calibration services. These government organizations trace their calibrations to a standard based on calorimeter measurements.
A detailed description of typical laser power and energy calibration errors is available from Ophir Photonics. This paper discusses the key factors of wavelength, linearity, and uniformity, as well as how these factors affect sensor accuracy and how to minimize laser intensity measurement errors.
Commercially available products that measure some of the key SLS parameters include laser power meters, laser beam profiling systems, and products that combine both of these products into one device.
CCD and CMOS silicon array sensors are used to produce quantitative, 2D images of the energy distribution. These systems provide accurate measurements of the beam size, beam location (Centroid), and how energy is distributed within the focused spot (beam profile).
Digital camera measurement systems provide a spatially accurate intensity map of the laser’s output. By measuring the total laser power and associating that power measurement with the beam profile taken of the same beam, an accurate, cross-sectional power density map of the laser beam is possible.
The absolute accuracy of camera based beam profiling systems is limited by: detector linearity, spatial uniformity, modulation transfer function (MTF) or spatial sampling frequency of the imaging system, and temporal resolution (temporal sampling frequency). Scanning slit and other types of laser beam profile measurement devices are also used to measure the energy distribution within high power (>1MW/cm2) focused laser spots.
NIST does not currently offer calibration services of beam profile measurement equipment. While this service has been offered in the past, it is not currently available.
Most beam profile sensors can be used to help create a meaningful map of the irradiance of the working laser beam, if an accurate power or energy measurement of the beam is available. Not all focused beams are alike. Some beams focus to very clean, super-Gaussian distribution, while other focused beams relay an image of the fiber optic to the work surface. Some beams are designed to deliver a uniform or Top Hat distribution at the presentation plane of the system. Focused beams can be severely distorted to the point that the task at hand is in jeopardy.Understanding the energy distribution within your focused, working beam may be the difference between success and failure. Areas of the beam that exceed the operation power density threshold will do work. While areas of the beam that do not exceed the working power density of the application may harm the process by coupling unwanted heat into the material to be modified.
Any areas where power density greatly exceeds the working threshold may damage the material or cause weakness in the structure.
Insufficient power density may cause the joining of layers to be incomplete which can introduce weakness or even voids into the build.
The range of power densities between the minimum effective irradiance and the irradiance that causes damage to the build, or irradiance that is insufficient to bond the new layer to the previously build structures, is the operational range of the additive manufacturing system. The beam profile and delivered laser power should be measured to assure the process is robust and to avoid damaging or insufficient irradiance levels.
While in its infancy, numerous diagnostic products and procedures have been developed to help to assure all variables are optimized for the best possible additive manufacturing outcomes. Commercially available products include:
- Powder Analysis: Powder size, particle shape, particle size distribution, chemistry, and powder density all impact the integrity and metallurgical properties of the additive manufacturing build. Laser diffraction technology can be used to analyze and understand the particle size and particle size distribution of powder. Electron scanning microscopy can evaluate the surface and internal morphology. X-ray fluorescence spectroscopy can analyze the chemical composition of the powder before and after processing.
- Thermal Image Analysis: Some additive manufacturing systems offer a thermal analysis option as a way to monitor the process. There have been some efforts to use the irradiated spectrum of the process as a diagnostic indicator.
Laser beam profiling products have been modified for analyzing SLS/SLM lasers. The challenges include but are not limited to:
- The very high power density of the AM working laser beam. Power densities of greater than 2 MW/cm2 are typical.
- Rapid changes in the delivered beam require measurement update rates sufficient to capture these changes. Measurement cycle times must be 10mS or less. Otherwise, small changes in the characteristics of the delivered laser may be missed.
- The SLS/SLM environment may also provide environmental challenges such as powder contamination of the optics, incompatible purge gasses, or elevated temperature build environments.
New technologies are being used to solve the challenge of understanding the laser performance as it impacts Additive Manufacturing processes. Rayleigh scatter is one new method used to image the focused beam without interacting with or modifying the delivered laser energy in any way. In the case of the Ophir BeamWatch non-contact beam profiling system, Rayleigh scatter allows measurement of the focused region of the beam (caustic), at up to 100 times per second. This permits the additive manufacturer to understand and measure the focal shift of the delivered beam as the delivery optics thermalize and reach operating temperatures.