A miniaturized selective laser melting device for operando X-ray diffraction studies

We report on the development of a miniaturized device for operando X-ray diffraction during laser 3D printing. The printing chamber has a size of only 130 x 135 x 34 mm(3) and is optimized to be installed at synchrotron beamlines. We describe the design considerations, details on the setup and the implementation at two different beamlines of the Swiss Light Source. Its capabilities are demonstrated by ex situ printing of complex shapes and operando X-ray diffraction experiments using Ti-6Al-4V powder. It is shown that the beamline characteristics have an important influence on the X-ray footprints of the microstructural evolution during 3D printing. From the intensity of the diffraction peaks, the evolution of the different phases can be followed during printing. Furthermore, the diffuse scattering signal provides information on the precise location of the laser beam on the sample and the scanning head settling time.

Operando X-ray diffraction during laser 3D printing: from machine design to experiments and computer simulations

Samy Hocine

The importance of additive manufacturing, also known as 3D printing, has grown exponentially in the recent years thanks to its high flexibility to manufacture intricate parts. This process can be divided into many different sub-categories, depending on the feedstock form and the source used. However, the principle stays the same: building the part one layer after another. Selective Laser Melting (SLM) is one them, combining between a fine metallic powder bed and a laser beam scanning the surface to selectively fuse the particles together. SLM is already used in the medical field where custom shapes implants are needed, or in the automotive and aerospace industries where it eases the production time of certain parts. SLM exhibits very high heating and cooling rates, up to 105-106 K/s as demonstrated in this work. Therefore, it poses several challenges such as the formation of pores, the accumulation of residual stresses and the creation of metastable phases in the microstructure. Post-processing techniques such as long heat treatments are used to counter the effect of these problems. A better way to address these different issues would be to adapt the processing parameters to reduce or remove the generation of such unwanted features. To do so, it is necessary to understand the underlying causes creating these defects. Ex situ characterisation of SLM-printed parts can be done using microscopy techniques. It consists in exploring the microstructure changes for a certain range of parameter values (laser power, scanning speed, etc). However, this is very time consuming. Simulations could be used as a prediction tool, but the degree of complexity needed here is quite high, which makes them computationally expensive. This PhD thesis is part of the CCMX-Challenge AM3 (Additive Manufacturing and Metallic Microstructures) and is dedicated to the development of a new device reproducing the SLM manufacturing process to characterise the phase evolution of metallic microstructures in real time, or operando, by X-ray diffraction (XRD) techniques. A Ti-6Al-4V (wt%) alloy is used to demonstrate the capabilities of this new device, thanks to its compatibility with the X-ray energies available at the Swiss Light Source (SLS) synchrotron beamlines of the Paul Scherrer Institut (PSI). This dissertation introduces the design of the machine and its software, as well as a characterisation of the printed Ti-6Al-4V samples to determine the optimum printing parameters. To demonstrate the measurement capabilities of the machine, different operando XRD experiments will then be presented, giving new insights into the phase evolution during SLM processing. These observations will be linked to the sample microstructure after printing. Additionally, the unique properties of this setup allow to capture the rapid temperature evolution within the probed materials during the XRD measurement. The obtained temperature profile will be compared to some results derived by Finite Elements Method (FEM) analysis on already existing models established for Ti-6Al-4V.

EPFL2020

Operando X-ray diffraction during laser 3D printing

Nicola Casati, Samy Hocine, Steven Van Petegem, Helena Van Swygenhoven

Laser based additive manufacturing allows to build a designed shape layer-by-layer, offering versatility and flexibility to many metallurgical sectors. The fast cooling rates and repeated heat cycles depending on the laser and scanning parameters are not easily measurable with conventional methods. Thus, advanced predictive computational simulations, required to reduce trial and error lead time, are difficult to validate. A newly developed in operando X-ray diffraction device implemented at a synchrotron beamline, taking advantage of the high brilliance and the fast detectors available, brings the missing link with numerical methods. By performing operando experiments on Ti-6Al-4V with different printing parameters, the temporal evolution of the low and high temperature phases are followed, the heating and cooling rates are measured for the powder and the solid material; and the formation of residual stresses in the phase is demonstrated. Moreover it is shown that the parameter that has the largest influence on the evolving microstructure is the scanning strategy, introducing a size effect related to the scanning length.

2020

An effective rule for translating optimal selective laser melting processing parameters from one material to another

Eric Boillat, Rita Drissi Daoudi, Hossein Ghasemi Tabasi, Toni Ivas, Jamasp Jhabvala, Roland Logé

Selective Laser Melting (SLM) or Laser Powder Bed Fusion (L-PBF) is the most studied laser-based additive manufacturing process for metals and alloys. One important issue in SLM is the time-consuming identification of a process window leading to quasi fully dense parts (> 99.8%), usually based on trials and errors. As some metal powders may be very expensive, and therefore not suitable for a whole battery of tests, a method to infer optimal parameters from one material to another would be highly beneficial. In this study, we use bronze as a test material for optimizing SLM parameters, before translating these parameters to red gold and 316 L steel. The concept of normalized enthalpy is used to take into account the differences in thermal and optical properties among the different materials. A translation rule is derived for the prediction of optimal processing conditions, based on the ones found for the test material. One important input for this translation rule is the powder absorptivity, which is measured at the appropriate laser wavelength and at room temperature. This approach eventually leads to the highest reported density for an additive manufactured 18-carat gold alloy (99.81% relative density), to the authors' knowledge. Finite element (FEM) simulations justify the translation rule formulation by showing the importance of the laser/powder interactions during the SLM process, leading to a finite penetration depth of the laser in the powder bed due to multiple reflections. The FEM calculations indicate that a significant part of the laser energy is directly absorbed by the powder during the manufacturing process when operating in near-optimal conditions.

2020

Operando X-ray diffraction during laser 3D printing: from machine design to experiments and computer simulations

Samy Hocine

EPFL2020