A paper from Californian federal research facility Lawrence Livermore National Laboratory demonstrates a method for predicting the structural integrity of 3D printed metal structures. The research focuses mainly on 3D printed lattice geometries, favored for their relative strength to weight ratios.

Mathematical simulation of these structures helps researchers better understand an object’s inner geometry, i.e. where strengths and weaknesses lie. This understanding is essential for fabricating high-grade metal parts for use in areas such as aerospace, energy and medicine.

Example 3D printed titanium lattices. Photo via: Metalysis/TWI.

Example 3D printed titanium lattices. Photo via: Metalysis/TWI.

The strength of lattice shapes

It is difficult to understand the deformation of 3D printed metal lattice as strain is not distributed evenly throughout an object when stress is applied. And so, to better understand this behaviour, and to develop the potential for scaling up lattice structures for large-scale parts, the study examines two different shapes:

  1. An ocet-truss defined as a “stretch-dominated lattice”
  2. A rhombic-dodecahedron, “a bend-dominated lattice.”
Mathematical models of 3D printed metal lattices, shown before and after yield. Image via Carltona, Linda, Messnera, Volkoff-Shoemakera, Barnardb, Bartona & Kumara.

Mathematical models of 3D printed metal lattices, shown before and after yield. Image via Carltona, Linda, Messnera, Volkoff-Shoemakera, Barnardb, Bartona & Kumara.

The lattice shapes were 3D printed on a Concept Laser M2 Cusing machine in six different types of titanium 64 alloys. Non-destructive tomographic x-rays were taken of the structures, and used to create the 3D mathematical models.

Simulated stresses were then applied to the computer generated models to the point of failure (as seen in the right in the images above) and researchers recorded the results on a micro, i.e. pore size, and macro, i.e. surface, level.

Mathematical accuracy

Conclusions showed that the 3D lattice models were “fairly accurate” at predicting the force-displacement in these shapes. In spite of defects in the 3D printed metal counterparts, the lattices also proved to be “fairly good at distributing the load in order to avoid early catastrophic failure.” 

Researchers also found that the 3D models could accurately predict the point at which the octet-truss would go from buckling to a full yield.

Buckling observed in octet-truss structures. Image via Carltona, Linda, Messnera, Volkoff-Shoemakera, Barnardb, Bartona & Kumara.

Buckling observed in octet-truss structures. Image via Carltona, Linda, Messnera, Volkoff-Shoemakera, Barnardb, Bartona & Kumara.

In a paper published in Advanced Structured Materials journal Volume 69, researchers also used mathematical models to predict the performance of 3D printed metamaterials and “fully exploit” their “exotic behaviour”. Additionally, MIT is taking a theoretical approach to understand a potential structure for 3D graphene.

In other research, Lawrence Livermore National Laboratory has collaborated on a new method of 3D printing termed ‘Direct Metal Writing’, and is the first institute to successfully print aerospace-grade carbon fiber composite materials for lightweight and strong parts.

Mapping local deformation behavior in single cell metal lattice structures is published in Acta Materialia volume 129. It is co-authored by Holly D. Carltona, Jonathan Linda, Mark C. Messnera, Nickolai A. Volkoff-Shoemakera, Harold S. Barnardb, Nathan R. Bartona and Mukul Kumara.

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Featured image shows a mathematical model of a 3D printed octet-truss before (left) and after (right) stress. Photo via Lawrence Livermore National Laboratory

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