Effects of hot isostatic pressing on the elastic modulus and tensile properties of 316L parts made by powder bed laser fusion (original) (raw)
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Materials & Design, 2021
Laser powder bed fusion additive manufacturing is used for demanding applications in industries such as aerospace. However, machine-specific, optimized process conditions and parameters are required to assure consistent part quality. In addition, differences in supplied powder can cause variation in the mechanical properties of the final parts. In this paper, the variability in mechanical properties of 316L stainless steel produced with two different laser powder bed fusion machines from two different powder batches was studied by producing an identical set of tensile and impact toughness test specimens. The samples were subjected to stress-relieving, solution annealing and hot isostatic pressing to assess the effectiveness of standardized heat-treatments in reducing variation in the mechanical properties of the built parts. Porosity, microstructure, tensile properties, and impact toughness of the specimens were measured to study the effect of changing the material, machine, and heat treatment. The maximum differences observed between the studied machine-powder combinations were approximately 7% for tensile properties and approximately 20% for impact toughness. HIP reduced the variability in all other studied properties except elongation. All the specimens fulfil the minimum requirements set in ASTM F3184-16 for AM 316L.
ESAFORM 2021
Despite the attractive capabilities of additive manufacturing (AM) technology, the industrialization of these processes remains very low. This is attributed to the complexes physical phenomena involved in the AM process and the layered structure of the produced parts. Intense research work is still needed for the prediction and optimization of AM parts mechanical properties. In this study, the influence of particle size distribution (PSD) of stainless steel 316L (SS 316L) powders on AM parts properties was investigated. Four PSD were used to produce test parts and compare the resulting porosity, surface roughness and macro-hardness. The SS 316L specimens were fabricated by Laser Powder Bed Fusion process (LPBF) on a SLM 125HL machine using variations in laser power and scan velocity. Computed scan tomography (CT) was used to characterize the defects. Lack of fusion and keyhole defects were detected. Defects were detected even in nearly dense parts. The powder size distribution was f...
Materials
The AlSi7Mg0.6 alloy, with its good tolerance against strain, is used in laser powder bed fusion (LPBF) to produce parts with complex geometries for aerospace engineering. Production of parts with good mechanical strength requires, however, the optimization of laser parameters. This study thus evaluated the influence of scanning speed, laser power, and strategy on several mechanical properties (tensile/resilience/hardness) to identify an optimal processing region. Results have shown the profound influence of laser power and scanning speed on mechanical properties, with a limited influence from the laser strategy. Tensile strength values ranging from 122 to 394 MPa were obtained, while Young’s Modulus varied from 17 to 29 GPa, and the elongation at break ranged from 2.1 to 9.8%. Surface plots of each property against laser power and speed revealed a region of higher mechanical properties. This region is found when using 50 µm thick layers for energy densities between 25 and 35 J/mm3....
Productivity optimization of laser powder bed fusion by hot isostatic pressing
Additive Manufacturing, 2020
Laser Powder Bed Fusion is today used for the serial production of parts, e.g. in the medical and aerospace markets. One of the major limitations is the comparatively low build rate of the process, which leads to low productivity and high costs when compared to conventional processes. Current approaches such as the use of multi-laser systems help increasing the build rate but come at higher investment costs. Overall, the low productivity limits the number of business cases for Laser Powder Bed Fusion and hinders the market uptake in more cost-sensitive industries. This paper suggests a combined approach of Laser Powder Bed Fusion and subsequent Hot Isostatic Pressing as a method to improve productivity. Hot Isostatic Pressing is often used as a post-process to eliminate any remnant porosity. It is shown that the process, however, is able to densify specimens that come out of Laser Powder Bed Fusion with an as-build density as low as 95 %. This opens up a larger process window for the initial Laser Powder Bed Fusion step. Experimental investigations are presented using two commercial Laser Powder Bed Fusion systems with the widely used titanium alloy Ti-6Al-4V. Instead of optimizing the process for the highest possible density, the parameters were optimized to yield the highest possible speed while maintaining a density above 95 %. A scan speed increase of 67 % was achieved and the specimens were then successfully compacted to above 99.8 % density in the Hot Isostatic Processing step. The high-speed parameter set was then applied to a demonstrator build job, where it leads to an overall saving of 26 % of build time. This research did not receive any specific grant from funding agencies in the public, commercial, or not-forprofit sectors.
Thermo-mechanical modelling of stress relief heat treatments after laser-based powder bed fusion
Laser-based powder bed fusion, due to its layer-by-layer nature, results in a unique stress profile in a part after the primary production process. The residual stresses are typically tensile near the top, while they are compressive near the bottom of the part. When it is removed without proper precautions, the part will bend excessively. In order to alleviate this deformation, a stress relief heat treatment can be applied. In this paper, such a stress relaxation heat treatment is modelled to investigate the effect of the post-processing parameters. The model uses an Arrhenius-type creep equation to simulate the influence of the heat treatment temperature and dwell time on the stress field in a relatively simple cantilever beam produced in Ti-6Al-4V. Via validation of the simulations, the effect of the heat treatment is shown to be represented accurately. The validated model is used to predict the deformation that results from the residual stresses after heat treating the part under various conditions. The results from the simulations ultimately allow choosing the optimal heat treatment conditions to obtain a given reduction in the residual stress level, while reducing the need for extensive experimental investigations.
Metals, 2017
A357 samples were realized by laser powder bed fusion (LPBF) on building platforms heated up to different temperatures. The effect of the preheating temperature and of the post processing heat treatment on the microstructure and the mechanical properties of the samples was studied. It was demonstrated that building platform heating can act as an in situ ageing heat treatment following the fast cooling that arises during laser scanning. A 17% higher ultimate tensile strength was achieved by the selection of the optimum building platform temperature. Moreover, the possibility to further increase the mechanical properties by means of a direct ageing heat treatment was investigated.
Part deflection and residual stresses in laser powder bed fusion of H13 tool steel
Materials & Design, 2021
• Martensitic transformation left behind compressive residual stresses which turned into tensile upon adding next layer. • Top surface residual stresses grew more compressive by increasing the volumetric energy density in the non-preheated case. • A great agreement between experimental and simulation results was achieved upon selecting a proper relaxation temperature. • 200°C preheating was found to eliminate the need for costly and timeconsuming stress-relief post-processing treatments.