Recrystallization of Ti 6Al-4V to Improve Hardness for Hip Implants Noah Baughman, Kendal Hiatt, Ana Michelena, Nikolas Vega
Department of Materials Science and Engineering, University of Florida, Gainesville
The experiment was designed to create a more viable hip implant by increasing the hardness of Ti 6Al-4V by annealing, and consequently, decrease its grain size.
Eight samples were used to test the hardness of the alloy after heat treating. Four were treated and four were not. Then, hardness testing, tensile testing, and SEM experiments were carried out to study the differences in hardness between treated and not treated Ti 6Al-4V samples.
After comparing the results obtained, it can be said that heat treating Titanium alloys does increase its hardness.
For future experiments, it is better to choose your application before buying the material to be tested. The thickness of the sample needed depends on the application chosen.
For an experiment involving a heat treatment, EDS should be required to insure you are testing the correct material.
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This research was made possible by the support and guidance of Dr. Nancy Ruzycki, as well as donors and staff of the UF MSE department.
Overall, the annealing did increase the hardness of the sample. However, due to the heat treatment used the thickness of the sheets was too thin to get proper tensile data. The heat treatment made our sample more brittle, which combined with the small thickness of the sample made it fail under the pressure of the grips. Consequently, if we had had a thicker sample, we could have had a better tensile data.
Also, for this metal, decreasing grain size required a different heat treatment.
The result was a more brittle and harder sample.
Samples
Treated
2 Hardness Tested
3 Tensile Tested
1 Grain Size
Abstract
Experimental Design
Results
Conclusion
Future Experiments
References
Acknowledgements
Treated Samples
Hardness Testing
Tensile Testing
Grain Size
Total hip arthroplasty (THA) is one of the most successful
surgical interventions performed in medicine. Commercially,
α + β titanium alloys, such as titanium-6Al-4 V have been the
most common metallic implant biomaterials used in THA [1],[2].
This, because of its comparatively low density, high mechanical
strength, excellent corrosion resistance, and biocompatibility with
bone [2].
As the purpose of this experiment was to increase the hardness of
Ti 6Al-4V for a more resistant hip implant, some alterations to the
grain size had to be made. Larger grains will reduce strength and
toughness of the material [4]. This is why the goal was to recrystallize
and obtain smaller grains in order to increase hardness. A harder
surface for THA would be a better fit for highly active or heavy patients
because it prevents it from breaking or deforming.
Introduction
Samples
Utilizing MiniTab 1-Sample T-Test it is determined 8 samples will be required, however, to get accurate values for Tensile Testing we used a sample size of 3 dog bones. 12 samples total
Rockwell Hardness Testing
Tensile Testing
Grain Size
Heat Treatment
Figure 2: Experimental Design Explanation
Control
2 Hardness Tested
3 Tensile Tested
1 Grain Size
Figure 3: Samples Used on Experiment
Figure 1: Hip Implant Demonstration [6]
Figure 4: Stress vs Strain Tensile Testing for Control Samples [5]
Figure 5: Stress vs Strain Tensile Testing for Treated Samples [5]
Control Samples
Treated Samples
Figure 7: Microscopic view of Control Sample grains
Figure 8: Microscopic view of Treated Sample grains
Tensile testing of titanium alloy control samples illustrates a highly ductile material capable of withstanding immense tensile force. Treated samples after the annealing process result in a lower Young’s modulus and decreased ductility. The treated sample is significantly more brittle than the control.
Control Samples
Rockwell hardness testing (type-B) indicates the annealing process was successful in raising the hardness of the titanium sample. Samples experienced an average increase in hardness of approximately 50%. This hardness increase is in the optimal range to allow for hip transplants of sufficient integrity and wear resistance, without creating a problematically brittle device. Additionally, the tighter standard deviation indicates a superior quality control process to ensure more uniform properties across the device.
Titanium samples subjected to heat treatment exhibit larger, columnar grain patterns. This is due to an increase in the alpha grains, which increases hardness. However, the columnar grain sizes would weaken the tensile strength, which is shown in our results. This is unexpected as the sample was heat treated to decrease grain size.
Figure 9: Rockwell Hardness Testing of Control and Treated Samples
Young’s Modulus | 296 MPa | 180 MPa |
Failure Stress | Did not Fail | 74.6 MPa |
Yield Stress | 227 MPa | 73.7 MPa |
Increase Tensile Strength
Decrease Grain Size
Increase Hardness
Annealing increased Hardness by 50%
Annealing increased Grain Size
Annealing decreased Tensile Strength
Figure 6: Tensile Testing data Comparison between control and Treated Samples
Figure 10: Conclusion of Results Table