Additive manufacturing (AM) of soft materials has a wide variety of applications, such as customized or wearable devices. Silicone is one popular material for these applications given its favorable material properties. However, AM of silicone parts with overhang structures remains challenging due to the soft nature of the material. Overhang structures are the areas where there is no underlying structure. Typically, a support material is used and built in the underlying space so that the overhang structures can be built upon it. Currently, there is no support structure that has been used for AM of silicone. The goal of this study is to develop an AM process to fabricate silicone parts with overhang structures. We first identified and confirmed poly-vinyl alcohol (PVA), a water-soluble material, as a suitable support material for silicone by evaluating the adhesion strength between silicone and PVA. Process parameters for the support material, including critical overhang angle and minimum infill density for the support material, are identified. However, overhang angle alone is not the only determining factor for support material. As silicone is a soft material, it deflects due to its own weight when the height of the overhang structure increases. A finite element model is developed to estimate the critical overhang height paired with different overhang angles to determine whether the use of support material is needed. Finally, parts with overhang structures are printed to demonstrate the capability of the developed process.
The shape memory mechanism associated with R-phase transformation was investigated using electromechanical tests on Ni-Ti shape memory alloy thin wires. To provide a more precise insight into the effects of R-phase on phase transformation of shape memory alloys, wires were prepared in three different initial states prior to conducting the experiments: pure martensite, mixture of R-phase and martensite, and pure R-phase. The electromechanical tests were done in two different loading states (under constant and variable stresses) using specially designed and manufactured apparatuses. Tests under different constant stresses were carried out for three cases of pure martensite, a mixture of martensite and rhombohedral, and pure rhombohedral phase, while those under variable stresses were done on pure martensite as well as mixed martensite and rhombohedral. For all the expressed cases, strain–time responses were investigated during electric heating–cooling cycles to interpret possible microstructural changes in the samples. According to the results, a phase diagram containing R-phase transition strip was proposed for Ni-Ti shape memory alloys.
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