Assessment of Residual Stresses in Laser Powder Bed Fusion Manufactured IN 625

Residual stresses pose significant challenges in the powder bed fusion of metals using a laser (PBF-LB/M), impacting both the dimensional accuracy and mechanical properties. This study quantitatively analyzes deformation and residual stresses in additively manufactured Inconel 625. Investigating both as-built and stress-relieved states with varied scanning strategies (90°, 67°, strip, and 90° chessboard) in PBF-LB/M/IN625, distortion is evaluated using the bridge curvature method. Quantitative measurements are obtained through 3D laser surface scanning on pairs of bridge specimens-one measured before and after detachment from the build plate, and the other undergoing stress-relieving heat treatment at 870 °C for 1 h. The findings reveal that, among as-built specimens, the 90° and 90° strip strategies induce the least distortion, followed by the 67° and chessboard 90° strategies. Furthermore, stress-relief treatment significantly reduces residual stress levels. After post-treatment, the deformation in X-axis samples with 90° and 90° strip strategies decreases by 39% and 42%. In contrast, the samples with the 67° and 90° checkerboard strategies exhibit more pronounced reductions of 44% and 63%, respectively. These quantitative results contribute useful insights for optimizing PBF-LB/M/IN625 processes in additive manufacturing.

Assessment of Additive Manufactured IN 625's Tensile Strength Based on Nonstandard Specimens

The study aimed to evaluate the tensile strength of additively manufactured (AMed) IN 625 using sub-sized test pieces and compare them to standard specimens. Cylindrical round coupons of varying diameters were manufactured along the Z-axis using the laser powder bed fusion technique and subjected to heat treatment. The simulation of the alloy solidification predicted the formation of several intermetallics and carbides under equilibrium conditions (slow cooling), apart from the γ phase (FCC). Sub-sized tensile specimens with different gauge diameters were machined from the coupons and tensile tested at ambient temperature. The results showed that sub-sized specimens exhibited lower tensile and yield strengths compared to standard specimens, but still higher than the minimum requirements of the relevant ASTM standard for AMed IN 625. The lower strength was attributed to the "size effect" of the test specimens. Fracture surfaces of the sub-sized test specimens exhibit a mixed character, combining cleavage and microvoid coalescence, with improved ductility compared to standard test pieces. The study highlights the importance of adapting characterization methods to the particularities of manufactured parts, including reduced thicknesses that make sampling standard-size specimens impractical. It concludes that sub-sized specimens are valuable for quality control and verifying compliance with requirements of AMed IN 625 tensile properties.

Laser Powder Bed Fusion Process Parameters' Optimization for Fabrication of Dense IN 625

This paper presents an experimental study on the influence of the main Laser Powder Bed Fusion (PBF-LB) process parameters on the density and surface quality of the IN 625 superalloy manufactured using the Lasertec 30 SLM machine. Parameters' influence was investigated within a workspace defined by the laser power (150-400 W), scanning speed (500-900 m/s), scanning strategy (90° and 67°), layer thickness (30-70 µm), and hatch distance (0.09-0.12 µm). Experimental results showed that laser power and scanning speed play a determining role in producing a relative density higher than 99.5% of the material's theoretical density. A basic set of process parameters was selected for generating high-density material: laser power 250 W, laser speed 750 mm/s, layer thickness 40 µm, and hatch distance 0.11 mm. The 67° scanning strategy ensures higher roughness surfaces than the 90° scanning strategy, roughness that increases as the laser power increases and the laser speed decreases.

Tensile Notch Sensitivity of Additively Manufactured IN 625 Superalloy

The notch sensitivity of additively manufactured IN 625 superalloy produces by laser powder bed fusion (LPBF) has been investigated by tensile testing of cylindrical test pieces. Smooth and V-notched test pieces with four different radii were tested both in as-built state and after a stress relief heat treatment for 1 h at 900 °C. Regardless of the notch root radius, the investigated alloy exhibits notch strength ratios higher than unity in both as-built and in stress-relieved states, showing that the additive manufactured IN 625 alloy is not prone to brittleness induced by the presence of V-notches. Higher values of notch strength ratios were recorded for the as-built material as a result of the higher internal stress level induced by the manufacturing process. Due to the higher triaxiality of stresses induced by notches, for both as-built and stress-relieved states, the proof strength of the notched test pieces is even higher than the tensile strength of the smooth test pieces tested in the same conditions. SEM fractographic analysis revealed a mixed mode of ductile and brittle fracture morphology of the V-notched specimens regardless the notch root radius. A more dominant ductile mode of fracture was encountered for stress-relieved test pieces than in the case of the as-built state. However, future research is needed to better understand the influence of notches on additive manufactured IN 625 alloy behaviour under more complex stresses.

Microstructural and Tensile Properties Anisotropy of Selective Laser Melting Manufactured IN 625

The present study was focused on the assessment of microstructural anisotropy of IN 625 manufactured by selective laser melting (SLM) and its influence on the material’s room temperature tensile properties. Microstructural anisotropy was assessed based on computational and experimental investigations. Tensile specimens were manufactured using four building orientations (along Z, X, Y-axis, and tilted at 45° in the XZ plane) and three different scanning strategies (90°, 67°, and 45°). The simulation of microstructure development in specimens built along the Z-axis, applying all three scanning strategies, showed that the as-built microstructure is strongly textured and is influenced by the scanning strategy. The 45° scanning strategy induced the highest microstructural texture from all scanning strategies used. The monotonic tensile test results highlighted that the material exhibits significant anisotropic properties, depending on both the specimen orientation and the scanning strategy. Regardless of the scanning strategy used, the lowest mechanical performances of IN 625, in terms of strength values, were recorded for specimens built in the vertical position, as compared with all the other orientations.

Total body irradiation prior to bone marrow transplantation--the experience of the Institute of Oncology Prof. Dr. Al. Trestioreanu Bucharest

PURPOSE

To present the technique of total body irradiation (TBI), applied for the first time in Romania, at the Institute of Oncology Bucharest, as part of stem cell transplantation for hematological malignancies.

PATIENTS AND METHODS

The total dose administered was 12 Gy at the reference point, 2 Gy/fraction, one fraction per day, 6 consecutive days, with a total dose of 8 - 11.4 Gy delivered to the lung, using Mevatron Primus linear accelerator (6 MV & 15 MV, 200-300 cGy/min in isocenter), in vivo dosimetry detectors and equipment for the reference dosimetry, personalized blocks for lung shielding sustained by polymethylmethaacrylate (PPMA) plate, Simulix HP simulator, and computer tomographic (CT) scans. Techniques used were: a) two parallel opposed anteroposterior / posteroanterior (AP/PA) fields with the patient in prone and supine position; b) two parallel opposed lateral fields with the patient placed on a lateral table, at 320 cm from the source. The percentage depth dose, tissue maximum ratio (TMR), off axis ratio (OAR) and the reference dose rate were measured for every patient's geometrical characteristics, with an uncertainty of +/- 2.2% and were used to calculate monitor units and to evaluate the dose in organs at risk (lungs, gonads, eyes etc).

RESULTS

5 patients (3 with the AP/PA technique and 2 with the lateral technique) were irradiated. All patients completed their irradiation in good clinical condition. The acute side effects were minimal (WHO grade 1: nausea/ vomiting--all patients; diarrhea--1 patient; headache--2 patients; photophobia and diplopia--1 patient; head and neck skin erythema--all patients). Because of the short follow-up period no safe evaluation of late side effects can be done. However, during this period one patient developed a non-aggressive form of chronic liver graft vs. host disease (GVHD) and one patient died due to acute GVHD.

CONCLUSION

TBI as part of stem cell transplantation for hematological malignancies was successfully realized at our Institute, with favorable clinical results. This technique is easy to carry out and reproducible.