PSI - Issue 34

Zoé Jardon et al. / Procedia Structural Integrity 34 (2021) 32–38 Zoé Jardon/ Structural Integrity Procedia 00 (2019) 000 – 000

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process, metal powder particles are carried by means of a carrier gas through inner channels of the nozzle. The high power laser heat source goes through the inner nozzle cone and locally initiates a liquid metal melt pool on the workpiece surface in which the metal powder particles are melted and subsequently fused with the base material. The shielding gas going through the inner nozzle cone is used to minimize oxidation of the workpiece surface with melt pool and to protect the optical system in the nozzle head from any possible damage during the printing process.

Fig. 2 : Schematic of MiCLAD hybrid DED machine (left : additive process, right : subtractive process).

The metal powder particles used for the experiments are commercially available gas atomized 316L stainless steel powders from Carpenter Additive, known to be near-spherical. A powder sieve analysis realized by Additive Carpenter indicates a particle size range of [45-106µm]. A Flowmotion DUO powder feeder unit from Medicoat is used. The powder feeder is composed of a powder container and a vibrating channel feeder that vibrates at a chosen frequency to control the powder mass flow rate and guarantee a stable powder flow. After that, the powder particles are coaxially fed through the nozzle. The nozzle is a Harald Dickler HighNo 4.0 coaxial powder nozzle. Argon is used both as carrier and shielding gas to protect the melt pool from oxidation. The laser-based DED printer is equipped with an invisible (infrared) continuous wave redPOWER® QUBE fiber laser of 1064nm with power of 2kW that coaxially passes through the nozzle and melts the metal particles in the melt pool. The theoretical laser beam diameter is chosen at ~1.66mm to maximize the efficiency (considering the [45-106µm] particle range) and follows a near flat top intensity profile.

2. Results 2.1. Proposed printing strategies

The generation of the machining trajectories and associated G-codes is performed using Matlab Mathworks®. The code asks for the sample geometry, capillary diameter and capillary position as input and generates the corresponding trajectory depending on the chosen print parameters (melt pool diameter, overlap, layer thickness). A vertical build direction of the sample is adopted to allow the integration of the capillary. The capillary needs to be integrated in the tensile stress zone of the sample to optimize the crack detection during the fatigue test. Moreover, to not induce too high tensile stresses at the capillary surface and therefore initiate fatigue cracks on that surface, the capillary is not located too close to the sample walls. Hence, the final capillary position is chosen at distances = 6.32 , = 5.67 from the sample edge (0,0). Three different hybrid machining strategies are considered and presented in Fig. 3. The first strategy consists of subsequently printing the sample as a full solid (without capillary) over a number of layers, mill the top surface of the -layer sample to equalize its height, mill the sample contours and drill a hole through the -layers. After having identified the location of the milled sample edge via a coaxial camera through the nozzle and adapting the spacing of the trajectories to match the new sample dimensions, the sequence of operations is repeated till the desired sample height is reached. The print trajectory starts with the sample contour and then proceeds with the infill. To minimize the introduction of thermal stresses due to non-uniform cooling rates and to enhance the geometrical precision, the contour starting point of layer + 1 is chosen at the next clockwise sample edge with