EOS has developed a process to additive manufacture (AM) Ti64 alloy, which, combined with hot isostatic pressing (HIP) heat treatment, can create improved fatigue properties.
The company decided to set a up a project to highlight and test these fatigue properties on a real-life hip implant design and compare the results to traditionally forged Ti64 hip stems.
To begin, fatigue tests were performed by, Orthopedic Innovation Center (OIC), based in Winnipeg, Canada. In this study, two additively manufactured hip stems were tested, and both passed to a runout of 10 million cycles. The hip implant design used in this study is the property of Monogram Orthopedics, Austin, Texas. The hip stems were tested on for neck fatigue properties according to ISO 7206-6 and withstood a load of 5340 N for 10 million cycles. This demonstrates that the fatigue properties of these AM manufactured implants were at least as good as the traditional forged Ti64 hip stem. This is the first time that such high fatigue properties have been measured on an implant manufactured with additive manufacturing.
To avoid a need for support removal and to facilitate the implant removal from the platform, the hip stems were manufactured in a “holder”, which can provide protection against the recoater forces. (To obtain the highest mechanical properties, a High-speed steel (HSS) blade was used to ensure optimal and consistent recoating behavior.) However, there is no connection between the holder and the hip stem, but rather a gap of about 0.2mm – 0.3mm. This gap size is a trade-off between easy removability and firmly keeping the hip stems from vibrating due to recoater forces. The hip stem is held in place due to the friction forces between the holder, the powder, and the hip stem.
The next step was to ensure this idea would work in practice. Additive Works’ 3D printing software Amphyon was used to confirm that the setup strategy did not lead to any issues, eliminating the need for a ‘trial and error’ building strategy. A common complaint from new AM users, and even experienced AM users trying new applications, is the need to use this strategy for first time builds.
Simulations were also run to check for recoater collision, to verify that the deformations were within tolerance, and to check for thermal stresses. This analysis provides a high-quality build before building a single job, reducing cost and lead time.
The Amphyon software also allows for pre-deformation of the hip stem to create a highly accurate part directly out of the printer, the first time. In my experience this works well, especially for small and controlled deformations which are expected for the hip stem during printing.
The hip stems were manufactured using EOS’s Titanium Ti64 Grade 23 process applying layers of 40µm thickness using an EOS M 290 3D printer. The EOS M 290 was chosen for this project because of its reliability and repeatability – exactly what is needed when superior fatigue properties are required, because a single defect can lower properties significantly. It would make sense in the future to optimize the process further for serial manufacturing, potentially improving the build rate and stability, given the unique requirements of a hip stem application.
HIP heat treatment
These excellent fatigue properties were obtained due to a combination of the AM process and an optimized hot isostatic pressure (HIP) heat treatment. In this case, conventional HIP heat treatments were optimized to improve the mechanical properties of the casted or cast-like quality and microstructure of the hip part. EOS has developed a HIP heat treatment which takes into account the unique microstructure of AM.
While conventional hip treatment, used in a range of industries, is carried out at 920°C at 100 MPa for two hours, the process developed by EOS takes place at 820°C at 140 MPa for two hours. This HIP cycle combined with EOS’ direct metal laser sintering (DMLS) process results in fatigue strength of 795 Mpa for 10^7 cycles (N=9).
In this case, post processing was rather straight forward and was performed by engineering company Precision ADM, based in Winnipeg, Canada. The AM process was set up in such a way that the same post processing steps as those used with a conventionally manufactured hip stem could be used. Therefore, the no supports approach was chosen. The taper was machined, and the neck of the hip stem was polished for optimal fatigue properties, in a similar way to implants already on the market.
The fact that it is possible to achieve forged mechanical properties on an actual application today is a huge leap in AM for orthopedics. This exciting development allows us to unlock the potential of AM for, yet again, another group of implants. It is now up to the device designers in orthopedic companies, and others, to push the boundaries of AM and realize ideas which were thought impossible.