Abstract
Additive manufacturing (AM) techniques have gained attraction in orthopedic implant design with their ability to create unique shapes and structures. Depending on the application, there are different mechanical properties required. This study evaluated the mechanical properties of direct metal laser sintered (DMLS) Titanium alloy (Ti6Al4V) with and without hot isostatic pressure (HIP) treatment.
Three dimensional computer modeling and the DMLS manufacturing assisted in building net or near-net samples for testing. The material testing consisted of uniaxial tension, Charpy impact, rotating beam fatigue (RBF), density, and hardness. Two sets of Ti6Al4V samples were created for testing using a DMLS process and stress relieved in a vacuum furnace prior to removal from the build platform. One set of samples were HIP treated. The two sets of samples were tested and the material properties of the non-HIP treated samples were compared to those with HIP treatment.
Tension testing was conducted on fifteen (15) samples per treatment according to ASTM E8/E8M on as-built samples designed to a round specimen 3 per the standard. Fifteen (15) Charpy impact samples per treatment were built to near-net shapes. A low stress grind was performed on all surfaces and a notch was placed in the sample to comply with ASTM E23 and testing was performed in accordance with the standard. Fifteen (15) samples were built per treatment and machined for RBF per ISO 1143. RBF was performed on all samples at a frequency of 100 Hz with run out conditions of 10M cycles or failure. Density and hardness was measured on three (3) samples from each set using Archimedes' Principle and Rockwell hardness techniques respectively.
The average (standard deviation) tensile strengths between the two groups were statistically different (p < 0.05). The non-HIP treated samples had an average ultimate strength of 956(10) MPa, yield strength of 896(13) MPa, and modulus of 118(2) GPa (Table 1). The HIP treated samples had an average ultimate strength of 909(4) MPa, yield strength of 832(9) MPa, and modulus of 112(3) GPa (Table 1). There was also statistical differences in the impact strength with the HIP treatment samples having a higher required force of 23.4(1.6) J compared to the non-HIP treated group of 19.8(1.8) J (Table 1). The fatigue strength of the samples HIP treated compared to the non-HIP treated group was 650 MPa and 396 MPa respectively (Table 1).
This study shows that the HIP treatment of DMLS Ti6Al4V diminishes some mechanical strengths while greatly improving the fatigue life of the material. As we continue to evaluate these “new” materials for orthopedic devices, these mechanical and physical properties will help us understand the capabilities of this process and material.
