The underlying cause of mechanical anisotropy in additively manufactured (AM) parts is not yet fully understood and has been attributed to several different factors like microstructural defects, residual stresses, melt pool boundaries, crystallographic and morphological textures. To better understand the main contributing factor to the mechanical anisotropy of AM stainless steel 316L, bulk specimens were fabricated via laser powder bed fusion (LPBF). Tensile specimens were machined from these AM bulk materials for three different inclinations: 0 ° , 45 ° , and 90 ° relative to the build plate. Dynamic Young’s modulus measurements and tensile tests were used to determine the mechanical anisotropy. Some tensile specimens were also subjected to residual stress measurement via neutron diffraction, porosity determination with X-ray micro-computed tomography ( μ CT), and texture analysis with electron backscatter diffraction (EBSD). These investigations revealed that the specimens exhibited near full density and the detected defects were spherical. Furthermore, the residual stresses in the loading direction were between − 74 ± 24 MPa and 137 ± 20 MPa , and the EBSD measurements showed a preferential 〈 110 〉 orientation parallel to the build direction. A crystal plasticity model was used to analyze the elastic anisotropy and the anisotropic yield behavior of the AM specimens, and it was able to capture and predict the experimental behavior accurately. Overall, it was shown that the mechanical anisotropy of the tested specimens was mainly influenced by the crystallographic texture.

/pdf/mechanical-anisotropy-of-additively-manufactured-stainless-27hgll21a3.pdf

Mechanical anisotropy of additively manufactured stainless steel 316L: An experimental and numerical study

Modern materials science diffractometers are generally equipped with area detectors that allow a high time efficiency to be achieved by simultaneously collecting the scattering pattern over large angular regions. These area-detector-based instruments, however, produce a huge amount of data, especially if they are located at large-scale neutron or synchrotron sources. The software StressTextureCalculator (STeCa) was designed to facilitate fast, easy and automated access to such area-detector data. Its outstanding features are direct calculation of diffraction patterns from different types of area-detector measurements, automatic data treatment and peak fitting using several implemented fit options. The resulting information on intensity, peak shift and broadening can then be exported into several data formats. These in turn can be used as input for a wide range of texture, stress and microstructure analysis software packages without additional prior treatment.

StressTextureCalculator: a software tool to extract texture, strain and microstructure information from area-detector measurements

The recent performance boost of E3, the residual strain and texture diffractometer at the HMI, Berlin, has enhanced significantly the experimental turnover as well as the quality of the measurement results. The instrument was equipped with a new vertically focusing and horizontally bent perfect Si single crystal monochromator (Figure 1), which optimizes the figure of merit (Figure 2) for residual stress experiments. The increase in the figure of merit compared to the previous monochromator is more than a factor of 10.

Efficiency Boost of the Materials Science Diffractometer E3 at BENSC: One Order of Magnitude Due to a Horizontally and Vertically Focusing Monochromator

Recently, we have shown that a polycrystalline Ni–Mn–Ga magnetic shape-memory alloy, when containing two populations of pore sizes, shows very high magnetic-field-induced strain of up to 87% Here, this double-porosity sample is imaged by x-ray microtomography, showing a homogenous distribution of both pore populations The orientation of six large grains—four with 10M and two with 14M structure—is identified with neutron diffraction In situ magnetomechanical experiments with a rotating magnetic field demonstrate that strain incompatibilities between misoriented grains are effectively screened by the pores which also stop the propagation of microcracks During uniaxial compression performed with an orthogonal magnetic bias field, a strain as high as 1% is recovered on unloading by twinning, which is much larger than the elastic value of <01% measured without field At the same time, repeated loading and unloading results in a reduction in the yield stress, which is a training effect similar to that in 

Magnetic-field-induced recovery strain in polycrystalline Ni–Mn–Ga foam

Residual stress/strain and microstructure used in additively manufactured material are strongly dependent on process parameter combination. With the aim to better understand and correlate process parameters used in electron beam melting (EBM) of Ti-6Al-4V with resulting phase distributions and residual stress/strains, extensive experimental work has been performed. A large number of polycrystalline Ti-6Al-4V specimens were produced with different optimized EBM process parameter combinations. These specimens were post-sequentially studied by using high-energy X-ray and neutron diffraction. In addition, visible light microscopy, scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) studies were performed and linked to the other findings. Results show that the influence of scan speed and offset focus on resulting residual strain in a fully dense sample was not significant. In contrast to some previous literature, a uniform α- and β-Ti phase distribution was found in all investigated specimens. Furthermore, no strong strain variations along the build direction with respect to the deposition were found. The magnitude of strain in α and β phase show some variations both in the build plane and along the build direction, which seemed to correlate with the size of the primary β grains. However, no relation was found between measured residual strains in α and β phase. Large primary β grains and texture appear to have a strong effect on X-ray based stress results with relatively small beam size, therefore it is suggested to use a large beam for representative bulk measurements and also to consider the prior β grain size in experimental planning, as well as for mathematical modelling.

/pdf/residual-lattice-strain-and-phase-distribution-in-ti-6al-4v-5b1i2xfxd3.pdf

Residual Lattice Strain and Phase Distribution in Ti-6Al-4V Produced by Electron Beam Melting.

Recent progress in engineering includes the development of new materials and innovations in their processing and treatments. Material technologies, like the study of metals, alloys, ceramics and composites, especially non-destructive analyses of residual stresses profiles and textures, have gained an increasing importance. The dedicated residual stress diffractometers E3 and E7 at BENSC, HMI, Berlin are already equipped with new two-dimensional position sensitive detectors. An upgrade of the monochromator system is planned for 2006 which includes perfectly bent silicon crystals in order to optimise both intensity and angular resolution yielding a large gain of the diffractometer efficiency for strain measurements. A range of equipment for sample positioning is available, such as a closed Eulerian cradle for samples with weights of up to 5 kg, a second cradle for heavy samples (up to 50 kg) with the ability to tilt the samples up to 90° and a translation table carrying samples of up to 300 kg and 1000 mm in diameter. Gauge volumes can be adjusted by a new computer controlled variable slitsystem in a range from 1x1x1 mm³ up to several mm³. In-situ residual stress analysis can be performed within industrial components during mechanical or thermal loading (up to 2000 K). Rapid data visualization as well as evaluation is performed by the specially designed software. The powder diffraction pattern is calculated by summation over the scattering angle dependent Debye- Scherrer lines on the two-dimensional 400*400 mm² planar area detector. A large amount of beam time is exclusively used for industrial research. Among the components that were investigated are crankshafts, impellers, pistons, cylinder heads, turbine blades and welds. Both instruments are similarly designed, where E3 is set up for higher flux and therefore penetration depths and E7 is designed for higher angular resolution.

The New and Upgraded Neutron Instruments for Materials Science at HMI - Current Activities in Cooperation with Industry

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Efficiency Boost of the Materials Science Diffractometer E3 at BENSC: One Order of Magnitude Due to a Horizontally and Vertically Focusing Monochromator

The New and Upgraded Neutron Instruments for Materials Science at HMI - Current Activities in Cooperation with Industry