3D printing honeycomb structures

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In a recent study published in the open access journal Materialsthe researchers compared the mechanical properties between computer and experimental models of honeycomb lattice structures of polymers and metals.

Study: Additive Manufacturing of a Honeycomb Lattice Structure – From Theoretical Models to Polymer and Metallic Products. Image Credit: BokehStore/Shutterstock.com

Stratasys Vero PureWhite polymer honeycomb structure was manufactured using PolyJet technology and that of 316L stainless steel and titanium alloy Ti6Al4V was obtained by laser powder bed fusion. The finite element model (FEM) accurately predicted the stiffness of the metal structure but overestimated the stiffness of the polymer structure.

context

Additive manufacturing (AM) can produce structures with complex design and tunable mechanical, thermal, electrical and acoustic properties. However, unlike subtractive manufacturing (SM), the properties of the finished structure are difficult to predict.

In SM the properties of the final product and the initial part are almost the same, but in AM a subtle variation in operating temperature, deposition rate and environmental conditions can significantly deviate the properties of the finished structure from its objective.

Dimensions of hexagonal honeycombs [14].  A material unit cell is highlighted.

Dimensions of hexagonal honeycombs. A material unit cell is highlighted. Image Credit: Goldmann, T et al., Materials

Additionally, prediction of optimal processing parameters and material condition during AM is crucial to minimize inherent manufacturing imperfections, defects and reduce the number of prototypes required. Computer-aided design (CAD) has been a versatile tool for predicting the mechanical properties of structures fabricated by SM. The availability of initial material properties, process parameters and environmental factors is sufficient to anticipate the properties of the finished structure in SM. However, it faces significant AM challenges.

Each type of material, viz. ceramics, polymers and metals, and their properties have a different correlation with the processing parameters in AM. Thus, AM requires a detailed study of the difference between the properties of CAD and experimental models depending on the type of material.

About the study

In this study, the researchers fabricated hexagonal honeycomb lattice structures of a polymer and two metals and compared the mechanical properties of their FEM and experimental models. Stratasys Vero PureWhite RGD837 acrylic-based photopolymer was fabricated by the PolyJet method with a layer thickness of 27 µm and a resolution of 600 dpi.

316L stainless steel and titanium alloy Ti6Al4V were fabricated by the laser powder bed fusion method using an “M2 cusing” 3D printer. The particle size ranged from 20 to 50 µm. The laser beam had a power of 200 W with a scanning speed of 0.5 m/s, a layer thickness of 20 µm and an operating temperature of 840 °C.

(a) Von Mises stress distribution for the Stratasys Vero PureWhite sample loaded by the axial tensile force of 200 N. The stress value is expressed in MPa.  Stress values ​​are expressed in MPa.  The maximum stress is identified at the connection nodes of the individual honeycomb cells.  Some nodes with stress concentration are marked with arrows.  Experimental samples of Stratasys Vero White polymer (b), Ti6Al4V titanium alloy (c) and 316L stainless steel (d) show fractures at the connection modes.

(a) Von Mises stress distribution for the Stratasys Vero PureWhite specimen loaded by the axial tensile force of 200 N. The stress value is expressed in MPa. Stress values ​​are expressed in MPa. The maximum stress is identified at the connection nodes of the individual honeycomb cells. Some nodes with stress concentration are marked with arrows. Experimental samples of Stratasys Vero White polymer (b), titanium alloy Ti6Al4V (vs) and 316L stainless steel (D) show fractures at connection modes. Image Credit: Goldmann, T et al., Materials

The FEMs of the honeycomb structures were constructed and meshed using ABAQUS/CAE software and analyzed with an equivalent elastic material model. This gave the effective Young’s modulus (E*) and Poisson’s ratio (v). A 3D scanner was used to measure the dimension of the honeycomb structures. Finally, for the experimental data, an electromechanical test system was used for uniaxial tests at a constant strain rate of 0.04 s at room temperature.

Comments

Stratasys Vero PureWhite polymer exhibited the lowest stiffness among all samples. You6Al4Alloy V exhibited the lowest extensibility with an average elongation at break of 3.5 ± 0.3%, while the stainless steel specimens exhibited the highest plastic deformation with an average elongation at break of 25 .2 ± 0.2%.

Each sample showed maximum stress concentration at the honeycomb cell nodes; therefore, most of the cracks were generated at these points after the load test. Scanning electron microscope (SEM) images revealed that both metal samples exhibited a ductile transcrystalline fracture.

Experimentally obtained E* of the polymer, Ti6Al4Alloy V and stainless steel had z-test values ​​of (z=-144.2, pp = 0.018), respectively. E* values ​​obtained from FEM analysis for the same samples had z-test values ​​of (z=−122.16, p

Micrographs of fracture surfaces of specimens after tensile testing in (a) Ti6Al4V titanium alloy and (b) 316L stainless steel.

Micrographs of specimen fracture surfaces after tensile testing in (a) titanium alloy Ti6Al4V and (b) in 316L stainless steel. Image Credit: Goldmann, T et al., Materials

conclusion

To conclude, the researchers of this study analyzed the difference in stiffness, i.e. E* of the Stratasys Vero PureWhite polymer, Ti6Al4Alloy V and 316L stainless steel honeycomb structures obtained from the experimental model and FEM. FEM overestimated the E* value for the polymer, while it was a close approximation for the two metal alloys.

This shows that FEM is a suitable tool to predict the mechanical properties of 3D printed metals and alloy structures. However, it can only be used for the first-order approximation of the mechanical properties of polymeric structures.

Source

Goldmann, T., Huang, W., Rzepa, S., Džugan, J., Sedláček, R., Daniel, M., Additive Manufacturing of a Honeycomb Lattice Structure – From Theoretical Models to Polymer Products and metallic. Materials 2022, 151838. https://www.mdpi.com/1996-1944/15/5/1838

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