What are the types of lattice structures and AM created lattice geometries?

 

Cellular lattice structures are becoming a more common feature of parts designed for additive manufacturing. Cellular lattice structures can exhibit a combination of high strength, relatively low mass, good energy absorption characteristics and good thermal and acoustic insulation properties. However, lattice structures are difficult to manufacture via conventional methods such as CNC machining, injection molding, investment casting, brazing or metal wire approaches. Conventionally manufactured lattices are limited to simple geometries, with relatively large dimensions compared to the critical dimensions required for use in many advanced metal components.

 

                            Body centred cubic(BCC)                          Face centred cubic(FCC)                                 Hexagonal centred cubic(HCC)

 

Lattice structures in AM

 

Honeycomb structure

A hexagonal honeycomb is a pattern with hexagonally shaped walls, all orthogonal to the same plane. This is a common lattice structure for the infill of FFF/FDM parts, and for some SLA parts such as sacrificial patterns used in investment casting. The in-plane stiffness and strength of honeycombs are lowest because stresses in this plane make the cell walls bend. The out-of-plane stiffnesses and strengths are much larger because they require axial extension or compression of the cell wall.

When a honeycomb structure is being deformed in a plane, it broadly exhibits a linear-elastic regime followed by a plateau of roughly constant stress, leading into a final regime of steeply rising stress. The linear-elastic regime is due to bending of the cell walls, but when critical stress is reached the cells begin to collapse resulting in the plateau region and eventually at high strains, the cells collapse sufficiently that opposing cell walls touch resulting in densification and steep increase in the stress. A widespread application of honeycomb lattices made using traditional manufacturing techniques is the reinforcement of load-bearing structural panels, and the flexibility of AM to tailor the dimensions of the honeycomb cells according to local loading conditions offers new opportunities in these applications.

 

Bending of cell walls due to linear-elastic property

 

                       3D Printed honeycomb structure & honeycomb sandwich structure

 

Open-and Closed-Cell foams

Three-dimensional foam structures can be open-cell or closed-cell structures and be regular or random in their arrangement. Synthetic foams, manufactured on a large scale, are used for absorbing mechanical impact energy, and managing weight-efficient strength, thermal conductivity, and buoyancy. The important features of foam are relative density, the degree to which the cells are open or closed, and the extent of anisotropy.

The cell shapes in real foams are more complex than regular open-cell structures. However, the modes of deformation are similar such that the scaling of mechanical properties can be understood by using dimensional arguments that are independent of the geometry. Open-cell lattices can have many topologies, defined by the arrangement of the beams in the unit cell, and how the unit cells are arranged and attached to one another. Two important categories of open-cell lattices are those with mechanical behaviour dominated by bending, and by stretching.