Processing-surface integrity-fatigue relationship in electrical discharge machining of nitinol shape memory alloy
Nitinol is a nearly equiatomic nickel-titanium shape memory alloy (SMA) with two unique properties, i.e., thermal shape memory and superelasticity. Nitinol has broad applications in medical device, aerospace, actuator, machine tool, and civil industries due to its excellent mechanical properties, high fatigue strength, superior corrosion resistance, and good biocompatibility. However, the grand challenges for manufacturing Nitinol components are manifested in two aspects: (i) Nitinol is exceedingly difficult to machine by mechanical cutting due to the very high material strength, high ductility, low temperature of phase transformation, rapid tool wear, and burr formation; and (ii) The requirements of complex geometry, delicate micro features, and superior surface integrity are very stringent for Nitinol components. Electrical discharge machining (EDM) is a competitive technique to machine difficult-to-cut materials. The contact free nature between the workpiece and the electrode avoids severe tool wear and other issues inherited in mechanical cutting processes. In addition, EDM has the advantages in machining of high aspect ratio and complex structures. However, very few studies have been done on EDM of Nitinol. The influence of EDM induced thermal damage on surface integrity and fatigue performance of Nitinol components has not been understood yet. The fundamental relationship between process-surface integrity-fatigue is still unknown. To reveal the underlying EDM process mechanism and its impact on surface integrity and fatigue of Nitinol components, the research focuses are to: (1) Perform a critical assessment on the challenges and outlooks on EDM of Nitinol shape memory alloys. (2) Investigate the surface integrity characteristics of Nitinol components machined by wire-EDM at main cutting and finish trim cutting. (3) Study the crystallography, compositions, and mechanical properties of white layer and reveal the relationship between microstructure and properties of white layers of EDMed Nitinol. (4) Determine the effect of surface integrity on fatigue performance of EDMed Nitinol components. (5) Create a 3-dimensional finite element model accounting for massive random discharges during EDM to simulate the random discharge phenomenon and process effects.