The most striking thing about the first 3D printed objects was their size. They were primarily small prototypes, a far cry from the car parts and building materials routinely manufactured today. But in the last decade, the field has evolved tremendously. Now, 3D printed objects aren’t just getting bigger; they’re also getting better.
The next frontier of 3D printing will enable a generation of functionally graded materials (FGM), objects with complex structures and compositions that impart unique functionalities. For example, FGMs might have intricate lattice pores of variable diameters or be made with mixtures of starting materials. These designs give FGM an edge over materials with uniform properties as FGM object has both the durability of metal and the heat-resistance of ceramic.
Conventional methods of manufacturing FGMs can only produce very basic outputs. “Only a few of them can achieve a structural gradient and have limitations in controlling the precision and shape of the microstructure,” explained Chen-Nan Sun from A*STAR’s Singapore Institute of Manufacturing Technology (SIMTech).
Envisioning that 3D printing could open up new possibilities in FGM manufacturing, Sun teamed up with his SIMTech colleague Pan Wang to enable the precision manufacturing of FGMs through electron beam melting (EBM). As its name suggests, EBM uses an electron beam rather than a laser to melt metal powders in a vacuum environment, reducing impurity contaminations. EBM processes at elevated build temperature with a semi-sintered powder bed to support the minimized residual stress lattice without an additional support structure, thereby enabling printing in a floating and stacking way. “EBM increases productivity and design freedom and is, therefore, suitable for mass production of FGMs and other complex designs,” stated Wang.
Using specialized computer software, the team designed cubic and honeycomb-shaped FGM lattice structures in two distinct orientations — one where cells were parallel to the surface and another where cells were perpendicular. They then gradually increased the FGM’s strut diameters from one end of the structure to the other. “The densities of all structures were graded by changing the diameter of the lattice struts from 0.6 mm to 1.2 mm continuously and linearly in the z-direction,” explained Sun. "With careful control, EBM can realize the FGM design accurately, reproducibly, and quickly," commented Wang.
Next, they put their newly designed materials to the test. Assessing for compression stress, they discovered that the EBM-built FGMs structures collapsed in a highly ordered sequence, starting from the thinnest to the thickest struts.
This was strikingly different from how uniform-density materials behaved under pressure; they collapsed randomly and unpredictably. This property of the new EBM-built FGMs points toward the potential application as shock absorbers.
Notably, one of their honeycomb structures achieved an average specific energy absorption of 33.3 J/g, outperforming other designs previously described in the literature. “Superior performance can be achieved for FGM in applications in lightweight structures, thermal barriers and biomaterials compared to conventional uniform materials,” commented Sun. Given the promising findings, the team is currently looking to partner with the industry to launch these materials in the market.