For microstructural analysis and tensile tests, prismatic specimens (10 × 10 × 15 mm
3) and cylindrical coupons 80 mm long and 11 mm in diameter were manufactured, using a DMG MORI, Lasertec 30 SLM machine (DMG MORI, Bielefeld, Germany) and IN 625 metal powder (15–45 μm particle range) produced by LPW Technology Ltd (Runcorn, UK) as feedstock. The powder’s chemical composition is presented in
Table 1, and it was characterized by the following powder size distribution: D
10 = 22 μm, D
50 = 34 μm, and D
90 = 42 μm, experimentally determined by the authors [42 (
link)].
All specimens were manufactured on a heated building plate (80 °C) using the same process parameters: 250 W laser power, 750 mm/s laser speed, 40 μm layer thickness, 0.11 mm hatch distance, and 70 µm laser focus. All specimens, for both microstructural analysis and tensile testing, were manufactured using three different scanning strategies, respectively a 45°, 67°, and 90° scanning strategy, rotated by 90° between two successive layers, as schematically shown in
Figure 1. The difference between the three scanning strategies used (45°, 67°, and 90°, all with a 90° change of the scanning direction between successive layers) is represented by the angle of the laser path.
The coupons intended for machining tensile test pieces were manufactured on four different building orientations: along
X, Y, Z-axis, and tilted at 45° in the
XZ plane, as depicted in
Figure 2, while the prismatic specimens used for microstructural analysis were built in a vertical position (
Figure 3). In all cases, and henceforth in this study, the
X-axis is parallel to the front of the machine, while the
Z-axis designates the vertical direction.
Two sets, of seven cylindrical coupons each, were manufactured for each building orientation and each scanning strategy. Two prismatic specimens were manufactured using each of the three scanning strategies. One of the specimens was used for microstructural analysis in the as-built condition, while the second was heat treated together with the coupons.
All specimens and coupons were subjected to heat treatment in air using an electrical Nabertherm LH 30/14 chamber furnace (Nabertherm GmbH, Lilienthal, Germany). The heat treatment consisted of stress relieve heat treatment (heating from room temperature until 870 °C, holding for 1 h, and cooling in air to room temperature), and annealing heat treatment (heating from room temperature until 1000 °C, holding for 1 h followed by fast cooling (oil quenching)). The heat treatment regimen used was adapted for AMed IN 625; starting from the typical heat treatment of conventionally manufactured IN 625, the same stress relieving temperature was used, but the temperature for the annealing heat treatment was increased by 20 °C for the AMed IN 625, and the cooling was realized in oil not in water [43 ]. Standard round tensile test pieces were machined from annealed coupons, according to the geometry and dimensions presented in
Figure 4.
Monotonic tensile tests were performed at room temperature according to ISO 6892-1:2009 using an electromechanical universal testing machine, Instron 3369 (Instron, Norwood, MA, USA), equipped with a 50 kN load cell. During the tensile test, the strain rate over the parallel length was set to
= 0.00025 s
−1 until the detection of 0.2% yield strength, then the extensometer was removed and the strain rate over the parallel length was set to
= 0.0067 s
−1. The tensile properties’ anisotropy was expressed as a function of the values recorded on the
Z-axis specimens, using Equation (1) [44 (
link)].
where
σi1 is the average ultimate tensile strength (UTS)/0.2% yield strength (YS)/elongation/reduction of area value obtained on test pieces manufactured along
X-axis or tilted at 45° in the
XZ plane.
σi2 is average UTS/0.2% YS/elongation/reduction of area value obtained on test pieces manufactured in vertical position, along
Z-axis, that showed the lowest strength values.
Microstructural simulations were performed using the ANSYS Additive Suite (ANSYS, Inc., Canonsburg, PA, USA), Additive Science module, R1/2020 edition. Due to software limitations, the simulations were realized only on specimens manufactured on the
Z-axis by applying all three scanning strategies, and the same manufacturing process parameters as in the case of the experimental procedure, except the laser focus, which for the simulation was 80 µm (the lowest value that can be applied).
For microstructural analysis by finite element cellular automaton method, the IN 718 was selected from the material database as currently, it is the only Ni-based superalloy available in the ANSYS Additive Suite database (ANSYS, Inc., Canonsburg, PA, USA) validated for microstructural prediction. The software was developed for IN718 alloy which belongs to the same Ni-Cr superalloys class with the investigated IN 625 alloy. As the software allows the customization of input data, the simulation was done using the actual experimental process parameters used for IN 625 in the current study.
The simulation analysis showed the microstructure evolution as a 1 mm
2 surface of the
XY,
XZ, and
YZ planes. The experimental microstructural analysis was performed by scanning electron microscopy (SEM) using an FEI F50 Inspect (FEI Company, Brno, Czech Republic) and optical microscopy using the microscope, Axio Vert.A1 MAT (Carl Zeiss Microscopy GmbH, Jena Germany) with camera (Nikon Digital Microscope Camera DS-Fi3, (Nikon Instruments Inc., Melville, NY, USA), and NIS-Elements software (version 5.02.03, Nikon Instruments Inc., Melville, NY, USA).
Microstructural analysis was performed on metallographically prepared prismatic specimens by grinding, polishing, and etching with Aqua Regia for 20 s. For the grain size measurement the intercept method was used. Optical micrographs captured at 100× magnification were processed using the Scandium software (version 5.2, Olympus Soft Imaging Solutions GmbH, Münster, Germany) by highlighting the grain boundary (red color separation, edge enhance filter, unsharp mask filter) and applying a grid consisting in 5 vertical and 5 horizontal equally spaced lines. The average grain size was determined based on measurements realized on 4 different light optical microscopy images.
Density measurements were made using the Archimedes method, according to ISO 3369 [45 ], and using an analytical balance, Pioneer PX224 (Ohaus Europe GmbH, Nänikon, Switzerland), with a density kit for solids. The relative density was expressed as the percentage of the ratio between the average of 5 measurements made on a prismatic specimen for each scanning strategy used, and the material’s theoretical density calculated based on its chemical composition. The auxiliary liquid used for measurements was ethanol (99.3% purity), and all specimens were degreased before testing using the same alcohol. The measurements were made at 20 °C room temperature, and 19.9 °C ethanol temperature.
Condruz M.R., Matache G., Paraschiv A., Frigioescu T.F, & Badea T. (2020). Microstructural and Tensile Properties Anisotropy of Selective Laser Melting Manufactured IN 625. Materials, 13(21), 4829.