A laser powder bed fusion system for in situ x-ray diffraction with high-energy synchrotron radiation.
TL;DR: In situ diffraction experiments with high-energy synchrotron radiation allow an analysis of the lattice spacing during the LPBF process and provide insight into the dynamics of stress generation and texture evolution.
Abstract: In Laser Powder Bed Fusion (LPBF), the highly localized energy input by the laser leads to high-temperature gradients. Combined with the inherent cycles of re-melting and solidification of the material, they can result in high mechanical stresses. These stresses can cause distortion and cracking within the component. In situ diffraction experiments with high-energy synchrotron radiation allow an analysis of the lattice spacing during the LPBF process and provide insight into the dynamics of stress generation and texture evolution. In this work, an LPBF system for the purpose of synchrotron x-ray diffraction experiments during the manufacturing process of multi-layer components with simple geometries is described. Moreover, results from diffraction experiments at the HEMS beamline P07 at PETRA III, DESY, Hamburg, Germany, are presented. Components with a length of ls = 20 mm and a width of ws = 2.5 mm consisting of 100 layers with a layer thickness of Δz = 50 µm were produced using the nickel-base alloy Inconel 625 as the powder material. Diffraction experiments were carried out in situ at sampling rates of f = 10 Hz with a synchrotron radiation beam size of 750 × 70 µm2. The presented experimental setup allows for the observation of arbitrary measuring positions in the sample in the transmission mode while gathering full diffraction rings. Thus, new possibilities for the observation of the dynamic evolution of strains, stresses, and textures during the LPBF process are provided.
Summary (2 min read)
- Yet, those are surface related measurement techniques with limited suitability to gather information about bulk properties.
- They observed dynamic melt pool evolution, keyhole pore formation, and the motion of ejected particles in the process of melting and solidification of Ti6Al4V powder with a frame rate of f = 50 kHz.
- The setup simulates the thermal behavior of a multi-layer LPBF process but under considerably different conditions than in a common industrial system as no powder material is involved.
II. SYSTEM DESCRIPTION AND SPECIFICATIONS
- The experimental instrumentation consists of an LPBF system positioned in the x-ray beam path of a synchrotron light source such that a desired gauge volume in the sample is irradiated and the diffracted x rays are detected by a 2D detector, e.g., scintillatorbased.
- The customized LPBF system, which allows for in situ x-ray diffraction corresponding to the described experimental geometry, is presented below.
- The third module contains the components for circulation and filtration of the inert gas atmosphere in the process chamber.
- The sample holder is mounted on a rigid guide rail, which is actuated in the z-axis direction by an EMC electromechanical cylinder, BOSCH REXROTH AG, Lohr am Main, Germany.
- On its way back, the elastomeric lip attached to the funnel levels the powder layer, as depicted in Fig. 3(c), and scrapes the excess powder to the sides of the sample holder.
B. Measurement modes
- For the synchrotron x-ray diffraction experiments described here, a two-dimensional area detector is used.
- Up to this point, three different measurement modes are possible to conduct with the InSituLPBF system at PETRA III (see Fig. 4).
- In measurement mode 2 (MM2), the gauge volume corresponds to a fixed volume in the sample, which can be monitored during the melting and solidification of all the above layers.
- This measurement mode is limited to samples with scan tracks longitudinal to the incident synchrotron radiation beam.
- As a result, the process time increases significantly compared to the other measurement modes.
C. Data preparation
- The scan path of the laser is provided in a Common Layer Interface (CLI) file, where the scan vectors are listed in sequence for each layer separately.
- They are defined by their starting and ending coordinates in the working plane.
- An algorithm to generate CLI-files of simple cuboid samples was implemented in MATLAB 2017b, The MATHWORKS, INC., Natick, USA.
- The laser scan tracks were oriented either parallel or perpendicular to the incident synchrotron x-radiation beam such that either transverse or longitudinal strains relative to the scan track can be evaluated in situ.
- Feasible measurement modes with the InSituLPBF system at PETRA III.
A. Sample and system preparation
- In order to produce a new sample, a substrate plate has to be inserted into the groove of the build plate and fixed via two headless screws.
- The measuring position can then be set accordingly in relation to the first layer’s edges by adjusting (a) the position of the process chamber to set the vertical distance of the gauge volume to the top layer and (b) the position of the sample holder in the process chamber to set the lateral measuring position.
- The irradiated volumes Vs,irr and Vp,irr are calculated according to the chosen gauge volume of 750 × 70 μm2, the thickness of the sample ws, and the combined thickness of the powder gaps.
- The laser scan time tL indicates the time during which the laser emits radiation, i.e., exposes the current powder layer.
- During the in situ measurements, diffraction patterns are collected continuously by using the post-trigger function of the beamline’s control software.
IV. EVALUATION OF EXPERIMENTAL RESULTS
- The diffraction patterns are segmented into equal size cake pieces, which correspond to the principal directions of the sample geometry.
- This figure illustrates three subplots (a)–(c), which show different measuring positions according to Fig. 6(d).
- For the center and right edge gauge volume, the median strain in TD during the peak is about ε311TD = 0.8%, while in the left gauge volume, it reaches only about 0.4%.
- The material expands, but the colder material below hinders the thermal expansion due to its lower temperature.
- The energy input by the laser does still have a noticeable effect, which is indicated by the increase in the strain difference during cooling after the laser passed over the gauge volume.
- An LPBF system for the realization of in situ x-ray diffraction experiments during the buildup of multi-layer samples was presented.
- The design of the process chamber allows for observation of arbitrary measuring positions in the sample in the transmission mode while gathering full diffraction rings.
- First experiments conducted at PETRA III show promising results and give insight into the dynamics of the lattice spacing during the build-up of Inconel 625 samples consisting of 100 layers.
- The instrumentation is not limited to the single use of Inconel 625 as the powder material but allows for in situ strain analysis during the manufacturing of other commonly used metals and alloys.
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