Detection of machined and real flaws under rough surface in laser metal deposition samples using laser ultrasonic system
Résumé
Laser metal deposition (LMD) is one of the metallic additive manufacturing (AM) processes based on making apiece layer by layer using a cad model. The process consists in focusing a laser beam onto a substrate to create a molten pool; simultaneously metallic powder particles are injected to form a layer. The quality and repeatability of metal parts produced by AM are critical issues for the widespread of the technology. Currently, LMD metal parts are often difficult to inspect because of their complex geometry. Moreover, due to production costs, there is a strong interest for reducing scrap rates and monitoring the process. Given the large number of parameters influencing the mechanical properties of the final piece, many techniques have been developed to monitor the fabrication [1].
Laser ultrasonics (LU) is a promising technology to fulfill surface and subsurface inspections downstream the melt pool during the fabrication. Most of studies, which have been carried on regarding this application, focus on the detection of machined flaws under polished surfaces. However, the surface roughness of AM samples might affect the effectiveness of interferometer used for LU and the propagation of surface wave. Moreover, the performances of LU inspection of samples containing real flaws must be evaluated to determine if an in situ control could be done thanks to this technique.
The LU system used to conduct the experiments has been specifically designed for the generation of surface waves. It is evaluated regarding the feasibility and the effectiveness of detecting machined notches and holes under polished and rough surface in 316L AM samples. Then, we study the possibility detecting real porosities in both Inconel 718 and 316L AM samples under rough surfaces. Our study shows how surface roughness makes the sensibility of LU system decreases so that sided drilled holes which diameter are lower than 300 µm diameter cannot be detected any more. In addition, a signal processing has been developed and enhances the possibility to make a difference from a porous sample to a dense sample.
[1]B. M. Sharrat, « Non-Destructive Techniques and Technologies for Qualification of Additive Manufactured Parts and Processes », no. March, 2015.
Laser ultrasonics (LU) is a promising technology to fulfill surface and subsurface inspections downstream the melt pool during the fabrication. Most of studies, which have been carried on regarding this application, focus on the detection of machined flaws under polished surfaces. However, the surface roughness of AM samples might affect the effectiveness of interferometer used for LU and the propagation of surface wave. Moreover, the performances of LU inspection of samples containing real flaws must be evaluated to determine if an in situ control could be done thanks to this technique.
The LU system used to conduct the experiments has been specifically designed for the generation of surface waves. It is evaluated regarding the feasibility and the effectiveness of detecting machined notches and holes under polished and rough surface in 316L AM samples. Then, we study the possibility detecting real porosities in both Inconel 718 and 316L AM samples under rough surfaces. Our study shows how surface roughness makes the sensibility of LU system decreases so that sided drilled holes which diameter are lower than 300 µm diameter cannot be detected any more. In addition, a signal processing has been developed and enhances the possibility to make a difference from a porous sample to a dense sample.
[1]B. M. Sharrat, « Non-Destructive Techniques and Technologies for Qualification of Additive Manufactured Parts and Processes », no. March, 2015.