Session: 01-01: Shape Memory Alloy Characterization I

Paper Number: 98912

98912 - Miniature Self-Biasing High Temperature Sma Actuators: Production and Characterization 

In past work we have reported on the development of miniature high temperature SMA actuators fabricated from ternary NiTiHf alloys. The current contribution describes the production and characterization of actuator devices fabricated using various advanced manufacturing techniques: including the spin melting process, femto-second laser machining, electrical discharge machining and MMM laser processing.

Self-biasing SMA devices use the material’s inherent properties (pseudoeleastic and shape memory effect) to add recoverable strain without an external biasing mechanism. The result is to reduce complexity and cost of any device. In this sense, functionally graded devices can prove valuable by adding differential properties to the overall microstructure. In the current work, functional grading was added using the MMM laser processing technique. Using this method, discrete high-resolution property variations are imparted on a monolithic device via high energy intensity laser pulses. The resolidified fusion zone resulting from a laser pulse has altered composition balance resulting in a concomitant shift in the material’s thermal transformation properties.

Novel melt spun NiTiHf ribbons having various compositional balances were fabricated, cycle tested and microstructurally characterised. Tensile coupons were cut with a femto-second laser machining process and tested in a load frame in cyclic tension following a modified actuator test method. Mechanical properties were tested in the longitudinal casting direction with functional fatigue measured up to 100 actuation cycles. Metallographic examination of the melt spun ribbons was completed in the longitudinal, lateral and transverse planes using optical and scanning electron microscopy. A Eucken-Hirsch texture was observed in the direction normal to the casting direction. The morphology and area fraction of inclusions was also analyzed.

MMM laser processing was conducted on the melt spun ribbons. Effects of laser processing was observed using mechanical and metallographic methods described above. Compositional variations between the laser processed and base metal sections was completed using energy-dispersive x-ray spectroscopy.

As-cast and laser processed coupons were compared to melt spun material. Furthermore, functionally graded actuators were cut using femto-second machining in the melt spun ribbons and EDM cut sheets. Actuation performance of the two devices was characterized and compared with results showing that miniature high temperature actuators can be rapidly fabricated using melt spun ribbons. Ongoing work is underway to improve the consistency of the ribbons in thickness, and in optimizing the thermomechanical processing for actuation performance.

Presenting Author: Michael Kuntz Smarter Alloys