Precipitation Heat Treatment of Laser Directed Energy Deposition Additive Manufactured Ti-Rich NiTi Shape Memory Alloys
Open Access
- Author:
- Spoll, Jessica Macauley
- Area of Honors:
- Engineering Science
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- Dr. Reginald Felix Hamilton, Thesis Supervisor
Bruce Gluckman, Thesis Honors Advisor - Keywords:
- Shape memory alloys
NiTi
Precipitation heat treatment - Abstract:
- This research investigates additive manufacturing as a fabrication method for shape memory alloys. Shape memory alloys (SMAs) undergo a reversible solid-state martensitic phase transformation that results in high recoverable strains, making them useful in industrial applications. NiTi alloys are widely used SMAs. However, due to their sensitive material properties, micromachining fine features in NiTi materials is difficult. Additive manufacturing (AM) allows for complex 3D geometries to be fabricated directly to the final dimensions, albeit surface finishing may be required. Moreover, AM has the capability to design material microstructures “on demand” by controlling input feedstock composition and deposition parameters during fabrication. This opens up avenues for manufacturing gradient microstructures for functionally graded materials. For this research, Ti-rich builds were fabricated using the additive manufacturing technique of laser directed energy deposition. The feedstock was elementally blended Ni and Ti powders and resulted in a Ti53.1Ni46.9 at.% build. Previous results from the Hamilton group showed that laser directed energy deposition additive manufactured (LDEDAM) Ni-rich NiTi alloys had an anisotropic microstructure in the as-deposited condition. The degree of anisotropy was reduced using precipitation heat treatment. For LDEDAM Ti-rich NiTi alloys in the as-deposited condition, previous work showed the thermal-induced martensitic phase transformation (TIMT) occurred at temperatures higher than those for the Ni-rich compositions. Thus, the Ti-rich material is attractive for applications requiring the shape memory effect above room temperature. This work builds upon the Hamilton group’s previous work on as-deposited LDEDAM Ti-rich NiTi alloys. The goal of this research is to investigate the influence of precipitation heat treatment on the transformation temperatures of the as-deposited material condition. Two different durations are used at one precipitation heat treatment temperature. Differential scanning calorimetry (DSC) experiments are used to measure characteristic phase transformation temperatures and enthalpies. Samples for calorimetry analysis were micromachined from different height locations within the LDEDAM build in order to spatially resolve the behavior, as the layer-by-layer fabrication is expected to cause microstructure anisotropy for the Ti-rich NiTi alloys. The results from DSC analysis show that the transformation temperatures for the precipitation heat treated material are comparable to the as-deposited material. The enthalpy measurements for the TIMT on cooling and heating become equivalent for the longer duration precipitation heat treatment. Ultimately, the results show that precipitation treatments may improve the reversibility of the TIMT without altering the operating temperature ranges.