Ingenieurwissenschaften und zugeordnete Tätigkeiten
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Organisationseinheit der BAM
Influence of deposition hatch length on residual stress in selective laser melted Inconel 718
(2018)
The present study aims to evaluate the bulk residual stresses in SLM parts by using neutron diffraction measurements performed at E3 line -BER II neutron reactor- of Helmholtz-Zentrum für Materialien und Energie (HZB) Berlin. Together with microstructure characterization and distortion measurements, it is possible to describe the stress state throughout the whole sample. The sample was measured in as-build condition (on a build plate) and after releasing from the build plate. The used material is the nickel based superalloy 718. This alloy is widely used in aerospace and chemical industries due to its superior corrosion and heat resistant properties.
Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component. The normal and transversal component exhibits a rather compressive behavior while the longitudinal was tensile in the center part of the sample and became compressive towards the tip. As expected, the absolute values of all stress components decreased after releasing the sample from the building plate. A surface scan utilizing a coordinate-measuring machine (CMM) allowed us to present top surface distortion before and after releasing. The top surface showed a distortion around ±80µm after releasing. Microstructure evolution in the scanning-building cross-section is largely dominated by columnar grains. In addition, many small random orientated grains are prominent in the regions of a laser overlap during SLM.
In summary, for the sample of superalloy 718 manufactured by SLM, a small distortion occurred when removing the sample from the build plate whereby the residual stress state decreases. Moreover, the observed columnar grains in the building direction could give a reason for the lowest stress values in that normal direction. However, the most important parameter controlling the residual stresses is the temperature gradient. Hence, future investigations are planned for a different scan strategy to distribute the laser impact in a more homogenous manner.
Most of the Al alloys used in additive manufacturing (AM), in particular Laser Powder Bed Fusion (LPBF), do not exceed a strength of 200 MPa, whereas conventionally high-performance alloys exhibit strengths exceeding 400 MPa. The availability of such Al alloys in AM is limited due to difficulties in printability, requiring synergetic material and AM process development to satisfy harsh processing conditions during LPBF [1]. One approach is the addition of reinforcement to the based powder, allowing tailoring composition and properties of a Metal Matrix Composite (MMC) by AM. Still, the effect of the reinforcement on the resulting mechanical properties must be studied to understand the performance and limits of the newly developed material. The goal of this work was to investigate the failure mechanism of LPBF Al-based MMC material using in-situ Synchrotron X-ray Computed Tomography (SXCT) during mechanical testing.
In this work, we present the recent in-situ imaging developments at the BAMline (of synchrotron BESSY II, HZB), focused on the in-situ characterization and understanding of microstructural evolution of additively manufactured materials subjected to different environments. Two show cases are presented. In the first, X-ray refraction radiography (SXRR) was combined with in-situ heat treatment to monitor the microstructural evolution as a function of temperature in a laser powder bed fusion (LPBF) manufactured AlSi10Mg alloy. We show that SXRR allows detecting the changes in the Si-phase morphology upon heating using statistically relevant volumes. SXRR also allows observing the growth of pores (i.e., thermally induced porosity), usually studied via X-ray computed tomography (XCT), but using much smaller fields-of-view. In the second case study, XCT was combined with in-situ tensile test to investigate the damage mechanism in a LPBF Aluminum Metal Matrix Composite (MMC). In-situ SXCT test disclosed the critical role of the defects in the failure mechanism along with pre-cracks in the reinforcement phase of MMC. We found that cracks were initiated from lack-of-fusion defects and propagated through coalescence with other defects.
In the present study, internal damage to an AlSi12CuMgNi alloy reinforced with planar random Al2O3 short fibres was investigated after compression testing. Due to the alloy composition, this composite contains a second reinforcement phase in the form of eutectic Si, which builds interpenetrated networks in the volume and increases the creep resistance and load-bearing capacity of the material. Materials with their fibre plane parallel and transversal to the load direction were characterized in order to investigate the dependence of load partition and damage on fibre plane orientation. In-situ compression testing during neutron diffraction measurements showed that internal damage is strongly influenced by the load partition between matrix and reinforcement. Moreover, micro-computed tomography was performed in the same material after ex-situ compression for damage analysis. In the case of a fibre plane perpendicular to the applied load, breakage and interconnected cracks appeared in a significantly higher volume fraction than with a fibre plane parallel to load.
Selective Laser Melting (SLM) technique allows to produce parts with complex geometry. Due to the rapid solidification and heat gradient during production, SLM results in presence of residual stress. The present study shows the influence of manufacturing parameters on surface residual stress of Ti-6Al-4V SLM parts. High tensile stresses in the front surface are found. Heat Treatment conditions relax residual stresses almost to zero. High scanning speed during manufacturing results in higher tensile stresses in the surface.
Using non-optimum combination of 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 by changing volumetric energy density (Ev) and selecting parameters giving low porosity values. However, not only low porosity but also stable microstructure and low residual stresses will help to achieve advanced mechanical behavior of the component.
In present work, we investigated cuboid-shaped Ti-6Al-4V samples produced with different manufacturing parameters. The parameters leading to the same Ev were considered as well as parameters which are not included in Ev. Residual stresses in subsurface region were investigated by synchrotron X-ray diffraction, which allows to penetrate around 100 µm from the surface therefore overcome the problem of high roughness of SLM components without additional sample preparation. 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. However, by using some process parameters it was possible to decrease residual stresses and obtain uniform α+β Ti microstructure and relatively low porosity. Additionally, it was found that not included in Ev (e.g., base plate position, focus distance) should be considered as additional manufacturing parameters during SLM process.
Al-Si alloys produced by Laser Powder Bed Fusion (PBFLB) allow the fabrication of lightweight free-shape components. Due to the extremely heterogeneous cooling and heating, PBF-LB induces high magnitude residual stress (RS) and a fine Si microstructure. As the RS can be deleterious to the fatigue resistance of engineering components, great efforts are focused on understanding their evolution in as-built state (AB) and after post-process heat treatments (HT). RS in single edge notch bending (SENB) subjected to different HT are investigated (HT1: 1h at 265°C and HT2: 2h at 300°C).
The availability of high-performance Al alloys in AM is limited due to difficulties in printability, requiring both the development of synergetic material and AM process to mitigate problems such as solidification cracking during laser powder bed fusion (LPBF). The goal of this work was to investigate the failure mechanism in a LPBF 7017 Aluminium alloy + 3 wt% Zr + 0.5 wt% TiC. The processing leads to different categories of Zr-rich inclusions, precipitates and defects.