Understanding nanoscale material behaviour for improved precision machining of shape memory alloys; testbed study on elliptical vibration assisted cutting of CuZr SMA.

Abstract

The field of ultra-precision machining has gained significant importance in the manufacture of components for the electronic, optical and medical industry. Two crucial factors that play a key role in the machinability of materials are the machining parameters and the material’s physical properties. Certain materials such as hardened steel or nickel-based superalloys are difficult-to-machine but innovations in the field of precision machining have developed a technique known as elliptical vibration assisted machining, which enables to improve the machinability of these materials. CuZr high-temperature shape memory alloy is categorized as a difficult-to-cut material and although EVAM has been applied to a wide range of metals it hasn’t yet been studied in CuZr HTSMA. In this context, the purpose of this thesis is twofold: On the one hand, to characterise the mechanical properties of CuZr SMA using Molecular Dynamics and, on the other hand, to explore the nanoscale mechanism of material removal of CuZr shape memory alloy (SMA) during elliptical vibration assisted machining (EVAM). The conclusions of this thesis can be summarized as follows. To characterise the mechanical properties of Cu₅₀Zr₅₀, Cu₂Zr and Cu₅Zr, a tensile and shear test were carried out using MD. Tensile test was done with crystal orientation and direction of tensile pulling as <010>. The results showed that Cu₅₀Zr₅₀ and Cu₂Zr exhibited a phase transformation (pseudoelasticity) during loading. However, Cu₅Zr showed dislocation nucleation as the main plastic deformation mechanism followed by fracture. Shear tests were done in the same phases with crystal orientation and direction of shear pulling as <100>. Interestingly, the shear test results showed no phase transformation for Cu₅₀Zr₅₀ and Cu₂Zr but the Cu₅Zr composition did show phase transformation during loading. It is important to highlight that all three phases of CuZr binary alloy that we have tested showed a different plastic response during the tensile test and the shear test. As far as machining is concerned, we observed indications that EVAM shows improved machinability compared with conventional machining. Although cutting forces were lower in EVAM, the stresses on the workpiece were slightly higher and both techniques showed the same mechanism of plasticity during machining. Neither dislocation nucleation or martensitic transformation was exhibited in either of the two machining techniques and instead, amorphisation was observed as the main plastic deformation mechanism in both cases. Interestingly, amorphisation has been previously observed by Saitoh and Kubota (2010) during loading NiTi SMA [1]; however, it didn’t show up in every crystal orientation confirming that NiTi shows significant changes in response to loading in different lattice directions. One of the main outcomes from this thesis is that CuZr SMA exhibits different modes of plastic deformation; namely amorphisation, dislocation nucleation and martensitic transformation during loading. The governing mechanism that arises during loading highly depends in the lattice direction in which the load is being applied. These findings can potentially enable reliable predictions and provide guidelines of the microstructural design of CuZr SMA systems