Characterisation of Ti6Al4V lattice structures produced by direct metal laser sintering for possible biomedical applications
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Date
2022
Authors
Raselabe, Kabelo Matome
Journal Title
Journal ISSN
Volume Title
Publisher
Vaal University of Technology
Abstract
The use of Ti6Al4V lattice structures as a new detergent to the stress shielding phenomenon caused during implantation has been growing. The stress shielding phenomenon is caused by the inhomogeneous transfer of load between the implant and the surrounding bone. This is mainly due to the implants containing modulus (110 GPa) being higher than that of the surrounding bone (2-20 GPa). Due to high cost and tedious process involved in manufacturing lattice structures using conventional technique, as a result of the complexity in lattice structure’s geometry, a process known as additive Manufacturing is highly recommended. Additive manufacturing processes like the use of Direct metal laser sintering (DMLS) process to create three-dimensional (3D) parts, layer by layer using computer-aided design (CAD) allows to freely build parts with geometric complexities
In this dissertation, the DMLS process was used to manufacture Ti6Al4V lattice structures with Young’s modulus like that of cortical bone or within the range of 2-20 GPa. These structures were manufactured at different build orientations (Horizontal and Vertical) and strut sizes (1 and 1.5 mm).
The Ti6Al4V lattice structures post-manufacturing were subjected to stress-relieving (SR) post-treatment of 650 ºC under argon atmosphere for 2 hours and 950 ºC under vacuum (VHT) atmosphere for 2 hours. The purpose of stress-relieving is to remove the residual stresses caused by the thermal gradients during manufacturing. While the vacuum heat treatment is for promoting ductility. Under these conditions (stress relieving and vacuum heat-treatment) the lattice structures were then analysed for microstructure, hardness, compression, and corrosion. For stress relieved lattice structures, DMLS's Ti6Al4V SR lattice structures possessed α' martensite and prior β grains along the build orientation. When comparing the mechanical properties of vertical and horizontal lattices, it was discovered that vertical lattice has higher hardness, compressive strength (324.1 MPa), and ductility (12%), as well as a lower Young's modulus (1.5 GPa). Vertically built lattices corroded at a faster pace than horizontally built. To change the microstructure and characteristics of Ti6Al4V lattices, vacuum heat treatment at 950 °C was performed on the SR lattices. The microstructure study demonstrated a change of α' to α+β lamellar type structure. Furthermore, the VHT enhanced ductility (18-29%) and demonstrated a Young modulus value of 2.8-3.3 GPa which is within the range for the biomedical area. The corrosion rate of horizontally built lattices was lower than that of vertically built lattices.
It was then concluded that, the DMLS Ti6Al4V lattice structures yielded a Young's Modulus within the range necessary for cortical bone replacement. Which in turn mean that the Ti6Al4V lattice is more likely to be accepted in the biomedical area than fully solid/dense DMLS made Ti6Al4V, which ranges from 110 – 120 GPa. Other properties of Ti6Al4V lattice structures demonstrate that VHT lattices have higher hardness than SR lattices and that VHT lattices corrode less than SR lattices. This ultimately led to both 1 and 1,5 mm strut sizes, the Horizontally formed lattice was more corrosion resistant than the Vertically created lattice.
Description
M. Tech. (Department of Metallurgical Engineering, Faculty of Engineering and Technology), Vaal University of Technology.
Keywords
Lattice structures, Strut size, DMLS, Young’s modulus, Stress-relieved, Vacuum heat-treated