Integration of a Chromatic Aberration Technique for In Situ Working Distance Measurement During Powder Blown Laser Directed Energy Deposition Additive Manufacturing
Author(s)
Venturiello, Matteo
Advisor(s)
Editor(s)
Collections
Supplementary to:
Permanent Link
Abstract
Powder blown laser Directed Energy Deposition (DED) is an additive manufacturing process that is increasingly being adopted in industry for its capability of realizing complex parts with several metal alloys. However, because of the nature of the process, the height of the deposited material can slightly deviate from the intended height, which affects the working distance (the distance between the deposition nozzle and the material deposited). This issue leads to inconsistency in deposited layer thickness and also to inferior mechanical properties. Such errors, if being accumulated, can generate significant discrepancies between the desired model and the printed object, resulting in a waste of time and material. Therefore, controlling the working distance is critical in optimizing the process.
To address this issue, this research develops a sensor that can in-situ measure the working distance and can ultimately be used for real-time control of the DED process. The sensor takes the radiation from the molten metal in the melt pool as a light source, which eliminates the needs for any additional space-taking light sources. The sensing mechanism is based on the optical phenomenon called the chromatic aberration – wavelength-dependent focal distance.
A lens is added to the existing optical system inside the printing head to collect lights radiating from the melt pool which are then delivered to a detecting optics through a fiber optic cable. The detecting optical system filter and process the lights in two wavelength bands. Changing the working distance changes the focal lengths of these two wavelength bands and accordingly their signal intensities. Therefore, these relative intensity changes are directly related to the changes in the working distance.
The sensor is calibrated for two different materials using a series of single-track prints; the heights of these single-tracks are then measured using a profilometer. Since the radiation spectrum depends on the temperature of the emitting surface; the calibration curve is unique to each material with its specific melting temperature. For this reason, two materials with very much different thermal and physical properties are tested. A steel alloy is chosen for its widespread industrial use, while an aluminum alloy is chosen to take advantage of the feature of the printer that allows to operate in a fully inert atmosphere. A procedure to process the acquired data and calibrate the sensor is proposed. The raw signal acquired is first cleaned from outliers that can negatively influence the measurement. Then, the resulting voltages (one for each wavelength band) are combined into a single-value error metric that directly correlates with a working distance. The results demonstrate that the developed sensor can precisely estimate the working distance and so be used as feedback control for the printing process. The sensor is completely passive, not requiring additional laser sources, and its sampling frequency is higher than those of existing camera-based techniques. Moreover, the sensor is designed as an add-on to the existing print head and does not interfere with the workspace of the DED system. Its compact design facilitates its applicability into powder-blown DED systems. Possible future work and optimization are also discussed.
Sponsor
Date
2025-07-28
Extent
Resource Type
Text
Resource Subtype
Thesis