In-situ laser metal deposition of nitinol and the effect of Ni4Ti3 phase concentration on its pseudoelasticity

Abstract

Pseudoelasticity is one of the major properties of nitinol. It is highly altered by the presence of nickel-rich Ni4Ti3 phase in the nitinol matrix. Hence it is significant to make an attempt to study the connection between Ni4Ti3-concentration and the matrix-pseudoelasticity. In this work, the connection of pseudoelasticity of nitinol with the concentration of Ni4Ti3 in the matrix was investigated. A 0.5 mm thick preplaced layer of a powder mixture of nickel and titanium (55 % and 45 % by weight, respectively) on a 4 mm thick nitinol substrate was laser scanned with a laser power of 1000 W, scan speeds of 200, 400, 600, 800 and 1000 mm/min and a laser spot diameter of 3 mm under a shielding of argon gas stream. The various phases formed during the process at different scan speeds were quantified by Rietveld quantitative phase analysis. Further, load-displacement curves were obtained with nano-indentation to estimate the pseudoelastic recovery of the matrix and to show the pseudoelastic connection of the matrix with its constituent Ni4Ti3 concentration. The laser scanned tracks (with the scan speeds of 600, 800 and 1000 mm/min) having a higher concentration of Ni4Ti3 showed a higher amount of pseudoelastic recovery. On the other hand, the tracks (with the scan speeds of 200 and 400 mm/min) having a lower concentration of Ni4Ti3 showed a lower amount of pseudoelastic recovery.