Surface integrity of laser cutting nitinol shape memory alloy

Abstract

Every year, millions of people experience new or recurrent stroke or heart attack. Cholesterol plaque buildup is the culprit for these fatal diseases. One standard procedure to treat these diseases is inserting a stent into the arteries to retain the blood flow. Stent materials are critical for product performance and functionality. Nitinol, a nearly equiatomic nickel-titanium shape memory alloy, has been widely used to fabricate vascular stents due to the excellent mechanical properties, fatigue and corrosion resistance, and biocompatibility. The challenges for manufacturing Nitinol stents are manifested in two aspects: (i) stents are miniature devices that demand a very precise and complex meshed geometry; and (ii) Nitinol is very difficult to be shaped by mechanical cutting due to strong work hardening, excessive tool wear, and bur formation. These technical hurdles can be overcome by laser cutting. A fine focused laser beam can produce a micrometer size kerf on Nitinol, and the non-contact thermal process can also eliminate pressing issues inherent in mechanical cutting. Conventional dry laser cutting is a high-temperature process which may generate thermal damage such as heat affected zone (HAZ), recast layer, micro-cracks, and tensile residual stresses. The thermal damage and ablation-induced contamination are critical technical barriers for fatigue performance and biocompatibility of Nitinol stents. To reveal the underlying process mechanism of laser cutting nitinol, this research focuses on the following aspects: (1) A critical assessment on the state-of-the-art of laser cutting Nitinol was conducted. (2) Low plasticity burnishing (LPB) was used to understand the deformation behavior and material properties of superelastic Nitinol. A method of modeling superelasticity and thermal shape memory of Nitinol was developed. (3) A 3-dimensional finite element model of pulsed laser cutting was developed to better understand the process mechanism in laser cutting of Nitinol. A novel thermal loading model with high spatial accuracy was developed to simulate a moving volumetric pulsed heat flux. The predicted kerf geometry and dimensions agreed well with experimental data. Also, the effects of cutting speed, pulse power, and pulse width on kerf profile, temperature, and heat affected zone (HAZ) were investigated. (4) A fiber laser cutting system was instrumented to experimentally investigate surface integrity of Nitinol. In addition, the process window for optimal surface integrity was identified. (5) Tensile and fatigue testing was conducted to evaluate the impact of surface integrity including thermal damage of laser cut Nitinol on static strength and fatigue performance. (6) Recommendations for possible future research directions were also outlined.