Direct metal laser sintering of titanium alloys for biomedical applications

Abstract

Ongoing scientific progress shifts conventional methods to the much celebrated Additive Manufacturing (AM) due to its freedom of design, flexibility in feedstock and material optimization. It has shown that Direct Metal Laser sintering (DMLS), one of the AM technologies, is an attractive manufacturing route for the biomedical applications. Ti6Al4V is the most widely used titanium alloy for the implants. However, there still remain issues of relative low ductility of DMLS Ti6Al4V and infections after implantation which have triggered the current research into producing implants of high ductility with antibacterial properties by DMLS, while establishing a body of knowledge about the relationship between the laser-matter interaction, microstructure, and mechanical properties. The type of material used in biomedical applications depends on specific implant applications and different types of implant need different mechanical properties. The current study is designed to investigate DMLS lattice structures from traditional Ti alloy such as T6Al4V ELI and the possibility of producing novel alloys by in-situ alloying for DMLS process. Learning from nature, it can be understood that cellular structures would be more preferable for biomedical implants than dense solid structures’ since the architecture of bone tissues in the human body are not completely dense and solid. Cellular structures of different nodes and strut sizes were produced and mechanically investigated to mimic the anisotropic porous nature of bones. A finite element analysis (FEA) was conducted to determine the applicability of graded/gradient implant based on each patient requirement. From the FEA it was hypothesized that implant design with cellular structures with relative low Elastic modulus would bridge the Elastic modulus gradient between dense solid metallic implants and the porous bones. An advanced lightweight mandible model was proposed whereby a damaged mandible could be replaced with a graded material based on the functional requirements of the damaged part. Mixing different elemental powders for in-situ alloying by DMLS would definitely increase the material pallet for AM. Understanding the effects of the parameters on DMLS process is paramount to gaining full control over density, microstructure and the mechanical properties of the DMLS parts. Only a careful combination of the process parameters would result in optimum process parameters for each type of powder. A wide range of process parameters were investigated to gain in-depth knowledge into the interaction between the laser beam and the powder bed by in-situ alloying powders with vastly different melting points and similar particle size distribution (45 μm). Due to difference in thermo-physical properties between the powders (Ti6Al4V, Cu, Mo, Ti), sintered materials were inhomogenous. Rescanning was employed but there was no significant change in the volume fraction of the unmelted Mo particles in the Ti15Mo alloy matrix. Due to the inherent high rate of heating and cooling simultaneously of the DMLS process, martensitic phase was found in the as-built Ti15Mo and Ti6Al4V–1at.%Cu samples. The martensitic properties reduce the ductility of the as-built samples significantly. Optimum process parameters were determined for both molybdenum-bearing titanium alloy (85% Ti and 15% Mo) and copper-bearing titanium alloy (Ti6Al4V and 1at.%Cu). Successful manufacturing of non-porous samples was done. In-situ alloying Ti6Al4V+1%Cu was successful and therefore there are promising ways to manufacture materials with embedded antibacterial properties. Incorporating copper into the bulk material by in-situ alloying would prevent the fall-off of antibacterial deposition coatings used in the past, since the material matrix (implant) would be antibacterial agent. The mechanical properties investigations with mini-samples presented ductility values below what was recommended for biomedical materials. It was concluded that finer Mo particles have to be chosen for in-situ alloying Ti15Mo for producing biomedical objects. Future work have to be done with elaboration of heat treatment procedures for higher ductility for structural bearing implants in a single step by the DMLS process. The results obtained developed new knowledge that is important for understanding the in situ alloying process during DMLS and new material production. The illustrated effects of process parameters on the properties of the synthesized material would be paramount for advanced implants with unique properties.