Residual stresses and porosity in Ti-6Al-4V produced by laser powder bed fusion as a function of process atmosphere and component design

The influence of the process gas, laser scan speed, and sample thickness on the build-up of residual stresses and porosity in Ti-6Al-4V produced by laser powder bed fusion was studied. Pure argon and helium, as well as a mixture of those (30% helium), were employed to establish process atmospheres with a low residual oxygen content of 100 ppm O-2. The results highlight that the subsurface residual stresses measured by X-ray diffraction were significantly lower in the thin samples (220 MPa) than in the cuboid samples (645 MPa). This difference was attributed to the shorter laser vector length, resulting in heat accumulation and thus in-situ stress relief. The addition of helium to the process gas did not introduce additional subsurface residual stresses in the simple geometries, even for the increased scanning speed. Finally, larger deflection was found in the cantilever built under helium (after removal from the baseplate), than in those produced under argon and an argon-helium mixture. This result demonstrates that complex designs involving large scanned areas could be subjected to higher residual stress when manufactured under helium due to the gas\u27s high thermal conductivity, heat capacity, and thermal diffusivity

Micromechanical behavior of annealed Ti-6Al-4V produced by Laser Powder Bed Fusion

the micromechanical behavior of an annealed ti-6al-4V material produced by laser Powder Bed Fusion was characterized by means of in-situ synchrotron X-ray diffraction during a tensile test. the lattice strain evolution was obtained parallel and transversal to the loading direction. the elastic constants were determined and compared with the conventionally manufactured alloy. in the plastic regime, a lower plastic anisotropy exhibited by the lattice planes was observed along the load axis (parallel to the building direction) than in the transverse direction. also, the load transfer from α to β phase was observed, increasing global ductility of the material. The material seems to accumulate a significant amount of intergranular strain in the transverse direction

Separation of the Formation Mechanisms of Residual Stresses in LPBF 316L

Rapid cooling rates and steep temperature gradients are characteristic of additively manufactured parts and important factors for the residual stress formation. This study examined the influence of heat accumulation on the distribution of residual stress in two prisms produced by Laser Powder Bed Fusion (LPBF) of austenitic stainless steel 316L. The layers of the prisms were exposed using two different border fill scan strategies: one scanned from the centre to the perimeter and the other from the perimeter to the centre. The goal was to reveal the effect of different heat inputs on samples featuring the same solidification shrinkage. Residual stress was characterised in one plane perpendicular to the building direction at the mid height using Neutron and Lab X-ray diffraction. Thermography data obtained during the build process were analysed in order to correlate the cooling rates and apparent surface temperatures with the residual stress results. Optical microscopy and micro computed tomography were used to correlate defect populations with the residual stress distribution. The two scanning strategies led to residual stress distributions that were typical for additively manufactured components: compressive stresses in the bulk and tensile stresses at the surface. However, due to the different heat accumulation, the maximum residual stress levels differed. We concluded that solidification shrinkage plays a major role in determining the shape of the residual stress distribution, while the temperature gradient mechanism appears to determine the magnitude of peak residual stresses

Influence of residual stress and microstructure on mechanical performance of LPBF TI-6AL-4V

Additive manufacturing technologies provide unique possibilities in the production of topologically optimized, near-net shape components. The main limiting factors affecting the structural integrity of Laser Powder Bed Fusion (LPBF) parts are manufacturing defects and residual stress (RS) because both of them are virtually inevitable. Taking into account the complex thermal history of LPBF materials, a prediction of the material behavior is not possible without experimental data on the microstructure, defect distribution, and RS fields. Therefore, this thesis aims to understand the factors that influence the LPBF Ti-6Al-4V material performance the most, covering both the production and the post-processing steps of manufacturing. Indeed, a parametric study on the influence of manufacturing process and post-processing on RS, defects and microstructure was performed. It was found that the volumetric energy density (EV), commonly used for the LPBF process optimization, does neither consider the pore shapes and distribution, nor the influence of individual parameters on the volume fraction of pores. Therefore, it was recommended not to use EV without great care. It was shown that the position on the base plate has a great impact on the amount of RS in the part. The micromechanical behavior of LPBF Ti-6Al-4V was also studied using in-situ synchrotron X-ray diffraction during tensile and compression tests. Diffraction elastic constants (DEC), connecting macroscopic stress and (micro) strain, of the LPBF Ti-6Al-4V showed a difference from the DEC of conventionally manufactured alloy. This fact was attributed to the peculiar microstructure and crystallographic texture. It was therefore recommended to determine experimentally DECs whenever possible. Low Cycle Fatigue (LCF) tests at a chosen operating temperature were performed to evaluate the effect of post-treatment on the mechanical performance. Through the information on the microstructure, the mesostructure, and the RS, the LCF behavior was (indirectly) correlated to the process parameters. It was found that the fatigue performance of LPBF samples subjected to hot isostatic pressing is similar to that of hot-formed Ti-6Al-4V. The tensile RS found at the surface of LPBF as-built samples decreased the fatigue life compared to the heat-treated samples. The modification of the microstructure (by heat treatment) did not affect the fatigue performance in the elastic regime. This shows that in the absence of tensile RS, the manufacturing defects solely control the failure of LPBF components and densification has the strongest effect on the improvement of the mechanical performance

The role of reinforcement orientation on the damage evolution of AlSi12CuMgNi +15% Al2O3 under compression

Internal damage of an AlSi12CuMgNi alloy reinforced with planar randomshort fibres has been investigated after compression. This damage strongly influences the load partition between matrix and reinforcement. For fibres perpendicular to the applied load, breakage and interconnected cracks appear in significantly higher volume fraction than with fibres parallel to load

Influence of manufacturing parameters on microstructure and subsurface residual stress in SLM Ti-6Al-4V

Using non-optimum combination manufacturing parameters in selective laser melting (SLM) may lead to reduction of quality of component: defects generation, distortion of geometry and even cracking. Usually, the optimization of parameters is performed at first with the goal to minimize porosity values, which are easy to measure. However, not only low porosity but also a stable microstructure and low residual stresses will help to achieve the desired mechanical behavior of the material and, thus, the component. In present work, we investigated cuboid-shaped Ti-6Al-4V samples produced with different manufacturing parameters. Residual stresses in subsurface region were investigated by synchrotron X-ray diffraction, which allows to penetrate around 100 µm into the surface and therefore to overcome the problem of high roughness of SLM components without additional sample preparation and resulting artifacts. Only tensile stresses were found along the building direction that can play critical role especially during cyclic loading. The pore shape and spatial distribution obtained by computed tomography varied for samples produced with the same Ev. Importantly, by adjusting specific process parameters it was possible to decrease residual stresses while at the same time obtaining a uniform α+β Ti microstructure and relatively low porosity

Exploring the Correlation between Subsurface Residual Stresses and Manufacturing Parameters in Laser Powder Bed Fused Ti-6Al-4V

Subsurface residual stresses (RS) were investigated in Ti-6Al-4V cuboid samples by means of X-ray synchrotron diffraction. The samples were manufactured by laser powder bed fusion (LPBF) applying different processing parameters, not commonly considered in open literature, in order to assess their influence on RS state. While investigating the effect of process parameters used for the calculation of volumetric energy density (such as laser velocity, laser power and hatch distance), we observed that an increase of energy density led to a decrease of RS, although not to the same extent for every parameter variation. Additionally, the effect of support structure, sample roughness and LPBF machine effects potentially coming from Ar flow were studied. We observed no influence of support structure on subsurface RS while the orientation with respect to Ar flow showed to have an impact on RS. We conclude recommending monitoring such parameters to improve part reliability and reproducibility

Analyse von oberflächennahen Eigenspannungen in SLM Ti-6Al-4V-Komponenten

Bei der additiven Fertigung können sich während des Prozesses aufgrund von hohen Aufheiz- und Abkühlraten Eigenspannungen ausbilden, die potentiell zu einem Verzug von Bauteilen führen und sich negativ auf das mechanische Verhalten auswirken. In dieser Studie wurden Ti-6Al-4V Proben durch Selektives Laserschmelzen mit verschiedenen Lasergeschwindigkeiten additiv gefertigt, um die Auswirkung der Laserenergiedichte auf den Eigenspannungszustand zu untersuchen. Die oberflächennahen Eigenspannungsanalysen wurden mittels energiedispersiver Synchrotronbeugung durchgeführt. Insgesamt wurden hohe Zugspannungen an den Seitenflächen der Proben gefunden. Es wurde festgestellt, dass je höher die Laserenergiedichte während der Fertigung ist, desto geringer fallen die Eigenspannungen aus. Eine nachträglich durchgeführte Wärmebehandlung führt zu einem vollständigen Abbau von Eigenspannungen