X-ray Computed Tomography Procedures to Quantitatively Characterize the Morphological Features of Triply Periodic Minimal Surface Structures

Abstract

Additively manufactured (AM) metallic sheet-based Triply Periodic Minimal Surface Structures (TPMSS) meet several requirements in both bio-medical and engineering fields: Tunable mechanical properties, low sensitivity to manufacturing defects, mechanical stability, and high energy absorption. However, they also present some challenges related to quality control, which can prevent their successful application. In fact, the optimization of the AM process is impossible without considering structural characteristics as manufacturing accuracy, internal defects, as well as surface topography and roughness. In this study, the quantitative non-destructive analysis of TPMSS manufactured from Ti-6Al-4V alloy by electron beam melting was performed by means of X-ray computed tomography (XCT). Several advanced image analysis workflows are presented to evaluate the effect of build orientation on wall thicknesses distribution, wall degradation, and surface roughness reduction due to the chemical etching of TPMSS. It is shown that the manufacturing accuracy differs for the structural elements printed parallel and orthogonal to the manufactured layers. Different strategies for chemical etching show different powder removal capabilities and both lead to the loss of material and hence the gradient of the wall thickness. This affects the mechanical performance under compression by reduction of the yield stress. The positive effect of the chemical etching is the reduction of the surface roughness, which can potentially improve the fatigue properties of the components. Finally, XCT was used to correlate the amount of retained powder with the pore size of the functionally graded TPMSS, which can further improve the manufacturing process.

The effects of varying wall thickness on the surface roughness of Ti-6Al-4V by electron beam powder bed fusion

“…According to the literature [47,82], the surface morphology of downskin surfaces is dominated by partially melted attached particles and irregular surface structures. These irregular structures are created by unsupported molten material in the melt pool pulled by gravity, which penetrates the powder bed underneath [84], leading to the formation of typical downskin protrusions. The vertical displacement of molten material is aggravated by deeper melt pools generated by higher levels of accumulated thermal energy [85].…”

Micro Non-Destructive Testing and Evaluation

“…This Special Issue shows that X-ray computed tomography is becoming a major tool for μ-NDT&E, being used for small and large components [ 3 , 5 , 6 ], and for sensitive materials [ 7 ]. Indeed, new methods are also being developed [ 6 , 7 , 8 ].…”

“…This Special Issue shows that X-ray computed tomography is becoming a major tool for μ-NDT&E, being used for small and large components [ 3 , 5 , 6 ], and for sensitive materials [ 7 ]. Indeed, new methods are also being developed [ 6 , 7 , 8 ]. At the same time, optical methods are being perfected to tackle challenging problems at the micro and macro scales [ 9 , 10 ].…”

Effect of Crosshead Speed and Volume Ratio on Compressive Mechanical Properties of Mono- and Double-Gyroid Structures Made of Inconel 718

“…Structures with complex geometry within so-called Triply Periodic Minimal Surfaces (TPMSs) have become a subject of research for their extraordinary mechanical and damping properties in terms of component weight and material consumption for their production [ 26 , 27 , 28 ].…”