Prototyping of Co-Cr-Mo alloy flat spiral spring by electron beam melting

Flat spiral spring (i.e. coils of metal ribbon such as clock-mainsprings) can store elastic strain energy and release the energy as kinetic energy on demand. Recently, devices to generate electricity from the elastic strain energy are attracting attention as energy saving tools and emergency power source. However, a higher energy density and downsizing are necessary for practical use. In the present study, the electron beam melting (EBM) has been applied to Co-Cr-Mo alloy, which has a potential ability to store approximately 30 % higher elastic strain energy than the currently best material, in order to maximize its Young's modulus by controlling the crystal orientation texture. A high Young's modulus of 275 GPa and a high strength of 1.2 GPa have been achieved with a rod by selecting its longitudinal axis parallel to the space diagonal of the space defined by the x- and y- scanning direction of electron beam and the z-build direction. The mechanical property of the rod is very attractive for a high energy-density clock-mainspring material. Moreover, the single-pieced fabrication of a spring-shaft-balance disk unit was tested, and the surface-smoothing and the control of local crystal texture during the EBM process have been emerged as necessary challenges.