Selective laser melting of porosity graded lattice structures for bone implants

Porosity-graded lattice structures are used in bone implants to mimic natural bone properties. Rather than having uniform pore size distribution, the size distribution is gradually changed in a certain direction to achieve specific mechanical and biological properties. Selective laser melting (SLM) has been used to print uniform metallic lattice structures with high accuracy. However, the accuracy of SLM in printing lattice structures with a wide range of pore sizes and volume fractions needs to be defined. The effect of SLM process scanning strategies on morphological properties of graded porosity metallic lattice structures is investigated in this study. Three different scanning strategies are proposed, and their effect on volume fraction, strut size, and surface integrity is investigated. Characterization of the printed parts reveals that the effect of different scanning strategies on the morphological quality is highly dependent on the design volume fraction for the chosen unit cell design. It was noted that using hatching strategies results in better dimensional accuracy and surface integrity in high-volume fraction lattice structures. While the use of total fill scanning strategy resulted in significantly distorted geometries in high-volume fractions. However, in lower-volume fractions, the dimensional accuracy as well as the surface integrity are comparable to that of hatching strategies. This work highlights the importance of understanding the limitations and capabilities of the SLM process in this application, and to enhance the printing quality of porosity-graded metallic lattice structures.