Session: 01-03 Shape Memory Alloys 2

Paper Number: 167950

167950 - Solid-State Joining of Shape Memory Alloys Using Cold Spray Deposition 

The implementation of adaptive and intelligent mechanisms that take advantage of smart materials is an innovative approach to achieving the complex application requirements expected of the next generation of spacecraft. Strong and lightweight smart material-driven deployables and actuators are one solution to the need for systems that respond to their environment. Consideration into the design of how these systems connect to and interact with their supporting structure and other spacecraft systems demand that they be joined together in a lightweight yet high strength manner.

Traditionally, joining shape memory alloy (SMA) and other structural aerospace alloy components require the use of either mechanical fasteners or a welding process. Welding processes are often preferred as a more reliable and lighter alternative to mechanical fasteners, however, fusion welding of SMAs comes with a number of challenges. The formation of brittle intermetallics upon solidification as well as the production of heat affected zones negatively affect the strength of the joint. The melting process can also volatilize alloy components, drastically shifting the material’s crucial transformation temperature.

To avoid such defects, a solid-state process known as cold spray deposition was explored as a method for joining SMAs and common structural aerospace alloys. In the cold spray process, metal feedstock powder is accelerated to ballistic velocities via high pressure gas stream through a de Leval nozzle. Upon impact with a substrate, the powder consolidates, forming strong, dense metallic bonds due to the high strain rate mechanical deformation. This process takes place at temperatures substantially lower than the powder’s melting point, eliminating the defects typical of SMA fusion welding. With an intentional interface design, components can be joined with cold sprayed powder in the solid state.

Leveraging extensive characterization of nitinol (NiTi) feedstock powder and experimentation with cold spray conditions and parameters, we sought to produce NiTi-to-NiTi and NiTi-to-non-SMA joints in various combinations. Here we will provide account of that effort as well as an early look at some examples of cold spray joints in several geometries. This includes an analysis of the consolidated material and interface as well as comparison with non-SMA (traditional structural alloy) baseline cases. We then evaluate mechanical properties of these proof-of-concept joints between NiTi, Al 6061, and Ti-6Al-4V.

It was found that cold spray coatings using NiTi powder can produce high quality, dense consolidations. When applied to cold spray joints, those sprayed using NiTi feedstock measured higher strength than those with Al 6061; however, the adhesion strength of the NiTi consolidated material to its substrate requires further improvement. With continued optimization of the cold spray process for this material system, and further validation of mechanical properties, the use of cold spray deposition is promising as an effective technique for the solid-state joining of SMAs.

Presenting Author: Matthew Gleason Worcester Polytechnic Institute

Presenting Author Biography: Matthew Gleason and Ashton Lyon are PhD students at Worcester Polytechnic Institute working in the Cote Research Lab. Both are studying cold spray deposition for a variety of material systems.