Research Papers

Design and Implementation of a Multisensor Coaxial Monitoring System With Correction Strategies for Selective Laser Melting of a Maraging Steel

[+] Author and Article Information
Ali Gökhan Demir

Department of Mechanical Engineering,
Politecnico di Milano Via La Masa 1,
Milan 20156, Italy
e-mail: aligokhan.demir@polimi.it

Chiara De Giorgi

Department of Mechanical Engineering,
Politecnico di Milano Via La Masa 1,
Milan 20156, Italy
e-mail: chiara.degiorgi@polimi.it

Barbara Previtali

Department of Mechanical Engineering,
Politecnico di Milano Via La Masa 1,
Milan 20156, Italy
e-mail: barbara.previtali@polimi.it

1Corresponding author.

Manuscript received February 23, 2017; final manuscript received November 14, 2017; published online February 5, 2018. Assoc. Editor: Hongqiang Chen.

J. Manuf. Sci. Eng 140(4), 041003 (Feb 05, 2018) (14 pages) Paper No: MANU-17-1113; doi: 10.1115/1.4038568 History: Received February 23, 2017; Revised November 14, 2017

Development of monitoring devices becomes crucially important in selective laser melting (SLM) due to the high process complexity and the high value of the products obtained. This work discusses the design of a coaxial monitoring system for SLM using multiple sensors. In particular, an optical model is developed for the propagation of the process emission from the workpiece to the monitoring module. The model is used to determine the field of view (FOV) around the monitored zone. The lens arrangements and the optical filters are chosen according to the model results. They were implemented to construct a monitoring module consisting of two cameras viewing visible and near-infrared wavelength bands, as well as a photodiode viewing the back-reflected laser emission, all integrated in a coaxial configuration. The system functionality is tested with a prototype SLM machine during the processing of 18Ni300 maraging steel, a material known to be prone to porosity. In particular, different remelting strategies were employed as possible correction strategies to reduce porosity. The signals were interpreted as being indicators of the change in absorptivity of the laser light by the powder bed, of the plasma and molten pool, as well as of the evolution of the temperature field.

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Grahic Jump Location
Fig. 1

(a) Generic description of object, image and lens distances in a three-lens configuration and (b) schematic disposition of the implemented optical arrangement in the monitoring module

Grahic Jump Location
Fig. 2

(a) Schematic representation of the monitoring module and (b) the implemented system

Grahic Jump Location
Fig. 3

Photograph of the realized specimens

Grahic Jump Location
Fig. 4

Effect of process parameters on the part porosity. Error bars represent standard deviation.

Grahic Jump Location
Fig. 5

Example of signal acquired with the laser channel depicting (a) volume and remelting passes and (b) zoomed signal showing back-reflected laser pulses

Grahic Jump Location
Fig. 6

Examples of images and signal acquired with the visible channel depicting (a) melt pool collapse (ton = 70 μs, Nl = 32, volume pass), (b) signal intensity showing instances of melt pool separation and spark generation (ton = 80 μs, Nl = 32, volume pass). Images show an area of 1 × 1 mm2.

Grahic Jump Location
Fig. 7

Example of signal dynamics acquired with the NIR channel depicting change in thermal emission at the remelting pass (Nl = 32, SR)

Grahic Jump Location
Fig. 8

Mean value of back-reflected laser intensity as a function of process parameters

Grahic Jump Location
Fig. 9

Mean value of image intensity in the visible bandwidth as a function of process parameters

Grahic Jump Location
Fig. 10

Mean value of image intensity in the NIR bandwidth as a function of process parameters

Grahic Jump Location
Fig. 11

(a) Change in mean intensity of visible channel at volume pass over different layer as a function of total energy and (b) relationship between visible mean intensity and porosity. The dashed lines are for visualizing the trend only. Error bars represent standard deviation.




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