Spray forming of Cu–11.85Al–3.2Ni–3Mn (wt%) shape memory alloy

Abstract

Cu-based shape memory alloys (SMA) in the range of Cu–(11.8–13.5)Al–(3.2–4)Ni–(2–3)Mn (wt%) exhibit high thermal and electrical conductivity, combine good mechanical properties with a pronounced shape memory effect, and are low cost (Dutkiewicz et al., 1999). Their processing requires high cooling rates to reduce grain size, prevent decomposition of the ß phase into equilibrium phases, and induce martensite transformation. In this investigation, Cu–11.85Al–3.2Ni–3Mn (wt%) shape memory alloy was processed by spray forming, a rapid solidification technique that involves cooling rates of 101 to 104K/s, to determine the potential of producing deposits with adequate microstructure, homogeneity and porosity for the manufacture of SMA near net shape parts. To this end, 5.2kg of alloy with nominal composition was atomized with nitrogen gas under a pressure of 0.5MPa and a gas–metal ratio (GMR) of 1.93. The atomized material was deposited at 60rpm on a rotating steel substrate positioned 350mm below the gas nozzle. The microstructure of the deposit was characterized by optical and scanning electron microscopy, X-ray diffraction and differential scanning calorimetry. The deposit with an effective diameter of 240mm and 75mm height presented equiaxial grains with a martensite microstructure. Grain sizes varied from 25μm in the lower region (contact with the steel substrate) to 160μm in the upper region of the deposit. Measurements of the reverse martensite transformation temperature of the deposit in different regions revealed its strong influence on the grain size.