Ingenieurwissenschaften und zugeordnete Tätigkeiten
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Microstructural changes in porous cordierite for diesel particulate filter applications caused by machining were characterized using microtensile testing and X-ray computed tomography (XCT). Young’s modulus was determined on ~215-380 m thick machined samples by digital image correlation. Results show a decrease of Young’s modulus due to machining of the thin samples. Explanation of this phenomenon was provided by XCT: the presence of debris due to machining and the variation of porosity due to removal of the outer layers were quantified and correlated with the introduction of further microcracking.
While additive manufacturing (AM) is blossoming in nearly every industrial field, and the most different process are being used to produce components and materials, little attention is paid on the safety concerns around AM materials and processes.
Leveraging on our leading expertise in non-destructive testing (NDT) and materials characterization, we approach AM at BAM under two important viewpoints: first the on-line monitoring of the process and of the product, second the evolution of the (unstable) microstructure of AM materials under external loads.
These two subjects are the core of the two new-born internal projects ProMoAM and AGIL, respectively.
A detailed view of the goals and the organization of these two projects will be given, together with the expected output, and some preliminary results.
The combination of microstructural data with other experimental techniques and with modeling is paramount, if we want to extract the maximum amount of information on porous material properties. In particular, quantitative image analysis, statistical approaches, direct discretization of tomographic reconstructions represent concrete possibilities to extend the power of the tomographic 3D representation to insights into the material and component performance. I will show a few examples of possible use of X-ray tomographic data for quantitative assessment of porosity in ceramics.
Moreover, I will show how not-so-novel 2D characterization techniques, based X-ray refraction, can allow a great deal of insights in the damage evolution in microcracked (and porous) ceramics. I will show how X-ray refraction can detect objects (e.g. microcracks) below its own spatial resolution.
Finally, I will discuss the link between the microstructural findings and the mechanical properties of porous microcracked ceramics.
Having been introduced almost two decades ago, Additive Manufacturing (AM) of metals has become industrially viable for a large variety of applications, including aerospace, automotive and medicine. Powder bed techniques such as Selective Laser Melting (SLM) based on layer-by-layer deposition and laser melt enable numerous degrees of freedom for the geometrical design. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The residual stress distribution in IN718 elongated prisms produced by SLM was studied non-destructively by means of neutron (bulk) and laboratory X-ray (surface) diffraction. The samples with different scanning strategies, i.e. hatching length, were measured in as-built condition (on a build plate) and after removal from the build plate.
While surface stress fields seem constant for AB condition, X-ray diffraction shows stress gradients along the hatch direction in the RE condition. The stress profiles correlate with the distortion maps obtained by tactile probe measurements.
Neutron diffraction shows bulk stress gradients for all principal components along the main sample directions. We correlate the observed stress patterns with the hatch length, i.e. with its effect on temperature gradients and heat flow. The bulk stress gradients partially disappear after removal from the baseplate.
Together with surface scan utilizing a coordinate-measuring machine (CMM), it is possible to link the stress release to the sample distortion.
We finally propose an explanation of those stress profiles based on the deposition strategy.
We investigated lattice structure manufactured by laser beam melting with computed tomography on difference scales, such as powder scale, strut scale and lattice scale.
The raw powder has been evaluated by means of synchrotron computed tomography (CT) at the BAM-Line (HZB Bessy II, Berlin). Therefore, the particle size distribution and even the pore size distribution was investigated and compared with results received by the producer by means of sieving. Studies with laboratory X-ray CT of porosity and roughness of manufactured struts in dependence of the build angle exhibited the tendency that elongated pores appear solely in a certain range near the edge. The integrity and load-bearing capacity of a lattice structure was investigated by means of in-situ CT during compression. The lattice structure was compressed by 10 % in height with an applied maximum force of 5 kN. We applied digital volume correlation algorithm on volumes of different load steps to quantifies the displacement within the structure.