Heated Mold Continuous Casting Of Cu-al-ni Shape Memory Alloys

Properties have been studied to establish the possibility of industrial application of Cu-Al-Ni SMA with columnar grain structure. Although the Cu-Al-Ni single crystal shows excellent mechanical and memory properties, almost all Cu-Al-Ni SMA are polycrystalline in practice. Researches have shown that stress concentration formed at grain boundary and rupture along the boundary when deforming, which resulted in the large elastic anisotropy factor of Cu-Al-Ni SMA. At present, most researches focus on grain refinement, which can disperse the stress concentration, in order to improve the performance of mechanism and memory. For example, adding alloying elements, rapid solidification, powder metallurgy, inoculation treatment, etc. The purpose of this study is to improve the mechanical and memory performance of Cu-Al-Ni SMA by strengthening texture, i.e. obtaining a longitudinal columnar structure by OCC (Ohno continuous casting) process, and to analyse the relationship between repeat tensile fatigue and structure.Cu-Al-Ni alloy wires 1.5mm and lmm in diameter are cast by OCC process at varies casting speeds. The tensile strength and elongation are measured with tensile experiment equipment at room temperature. The repeat tensile fatigue life and memory performance is determined by a self-designed repeat tensile fatigue testing equipment under 4%, 6% and 8% strain. The characteristic phase transformation temperatures were measured by Digital Scan Calorimeter (DSC). The crystal orientation of Cu-Al-Ni SMA wires is analyzed with X-ray Diffraction (XRD). The structure of as-cast, solid solution treated and tensile fatigue fracture were observed by metallography microscope. The fracture morphologies of tensile and fatigue were observed by Scan Electron Microscope (SEM).The result shows that: The 01.5mm wires cast in 50mm/min show a ultimate tensile strength of 633MPa, elongation of 14.95%. Under 4% strain, the number of repeat tensile fatigue rupture is 38,560 circles; Under 6% strain, the number of repeat tensile fatigue rupture is 30,378 circles, and the ultimate resumable strain of the wires is 10%. The other wires lmm in diameter cast in 75mm/min show a ultimate tensile strength of 910MPa, elongation of 18.76%. Under 4% strain, the repeat tensile fatigue life is 17,108 circles; Under 6% strain, the repeat tensile fatigue life is up to 16,215 circles, and the ultimate resumable strain of the wires is 10.5%. The recovery rates of all wires keep up 100% before fatigue rupture.As the casting speed increase, the grain diameter decreases and grain number in unittransversal area increases. The XRD results show that the crystal orientation varies between 13 in all studied casting speeds. The fatigue fracture morphologies showed that two factors affect the fatigue life: (1) the area and quantity of transverse grain boundary. (2) casting defects and surface quality. Longitudinal grain boundary has little effect on fatigue life.The Cu-Al-Ni SMA cast by OCC process have excellent mechanism, fatigue and memory performance. Through optimizing the process parameters, improving surface quality of wires, eliminating casting defects, enhancing casting speeds, increasing grain number in unit transversal area, reducing the size and quantity of transverse grain boundary, can be further improved the mechanical and fatigue performance.