ANALYSIS: ADDITIVE MANUFACTURING
Laser triangulation for coaxial height measurement in laser metal deposition
Ali Gökhan Demir, of the Politecnico di Milano, reports on the use of laser triangulation to address a significant challenge being faced by laser metal deposition users
Figure 1: examples of height mismatch errors leading to part failure in complex trajectories
This article was co-authored by Simone Donadello and Barbara Previtali of the Politecnico di Milano
Metal additive manufacturing (MAM) has gained an important pace during the last few years. A key contributing factor to this progress is the role of lasers in MAM processes. Indeed, the last two decades have seen the rise of industrial high-power solid state lasers – namely, fibre, disc and diode lasers. The robust architecture of these systems, the flexible control of the optoelectronic device, the high beam quality – as well as an improved availability of service and reduced costs – have been some of the important advances that lead to such industrial acceptance. Concerning MAM processes, a highly
flexible and robust digitally-controlled energetic beam appears a natural match. Most laser powder bed fusion (LPBF) systems operate with high-brilliance fibre lasers for process durations measured by days. On the other hand, market share of directed energy deposition (DED) processes is limited. Laser metal deposition (LMD), which unites
the laser beam with blown powder feedstock, is arguably the most industrially mature DED process. LMD could be seen as a natural derivation of the laser cladding process, which has long been used in industry. In the most common form, the process uses powder feedstock blown from coaxial or multijet nozzles encased in a processing head that projects the laser beam to the deposition area. Several industrial solutions are present where cartesian axes and robotic systems are employed for beam positioning and workpiece handling.
32 LASER SYSTEMS EUROPE SUMMER 2019
phenomenon has been reported, where a stable standoff distance is reached over a certain time1
. However, this self-regulation
of the standoff distance is less likely when complex parts with variable sections are manufactured with variable sections and inclination angles – see Figure 1. In industrial practice, human operators often intervene with the process parameters when such geometrical derivations are observed. This requirement of a skilled operator to maintain process stability over prolonged process durations is not a feasible option, however.
Limits of existing methods The measurement of the workpiece dimensions in LMD has been an object of scientific research as well as industrial practice2
. Several options based on tactile
measurements, projection techniques, as well as tomographical techniques have been tested and employed. While these techniques have particular strengths in terms of the temporal or spatial resolutions provided, they also have disadvantages. Tactile measurements, projection techniques and x-ray tomography commonly require the LMD process to stop in order for the measurement to be taken. Interferometric measurements can be employed online and coaxially, but these require expensive equipment. Optical pyrometers can be used to provide an indirect measurement through the average temperature of the melt pool area.
Inline height measurements required in LMD Similar to other MAM processes, LMD suffers from a lack of design rules, industrial standards and stable process parameters for new materials. In addition, process robustness and dimensional accuracy can be compromised, especially in long depositions. One of the biggest differences of LMD
compared to the other MAM processes is related to the build rate variability. Such variability in long process runs manifests a height mismatch between the commanded height increment of the deposition head and the deposited layer thickness. The workpiece can grow faster or slower than expected, changing the standoff distance between the nozzle and the deposition region. Consequently, the workpiece geometry can be wrong, or in some cases the process can fail completely. The height mismatch can be related mainly to heat accumulation and geometrical changes, whereas issues related to the nozzle or the powder feeder can also be the underlying cause. In scientific literature, a self-regulation
A novel solution Laser triangulation is a well-known method for dimensional measurements which has been employed in LMD3, as well as in 3D scanners and camera vision systems – however, their typical off-axis configuration limits their applicability to real-world cases. At the Politecnico di Milano, the triangulation method has been developed and demonstrated on an existing Additube LMD system from BLM Group4. The triangulation measurement has been implemented in an innovative coaxial configuration, allowing for an omnidirectional height measurements with big advantages in terms of flexibility and robustness. Such implementation provides an important tool for real-time process monitoring. Figure 2 shows the schematic functioning principle of the coaxial laser triangulation system launched into an industrial LMD head (MWO-I, Kuka Reis) mounted on a six-degrees of freedom industrial robot (IRB 4600-45, ABB) and a two-axis rotary table, employing a 3kW multimode active fibre laser beam source (YLP-3000, IPG Photonics). The probe beam shares the process laser optical path, and it is tilted with an angle with respect to the normal of the substrate.
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