An Investigation on Chemical Machining of NiTi SMA Prepared by Powder Metallurgy

Abstract

Chemical machining is very important in solving problems constantly arising due to the requirement of high surface finish along with high productivity through machining of metals and alloys. Such requirement appears in machining of NiTi shape memory alloy (55wt%Ni and 45wt% Ti) which has widespread engineering applications. The present work aimed at utilizing of Taguchi design of experiments with the method of desirability function to optimize the chemical machining parameters (machining temperature and machining time) of NiTi alloy prepared by powder metallurgy technique. Taguchi experimental design concept, L25 (2×5) orthogonal array was used to determine the S/N ratio. Based on the analyses of multiple regression method, mathematical predictive models for the studied parameters had designed and validated. To achieve the objectives of the present work Datafit ver9, and Mini Tab14 softwares had employed. The powders with the required percentages were mixed for 5hours, compacted at pressure of 900MPa, and sintered in two stages, at 500°C for 2 hours and then at 950°C for 6hours under vacuum conditions (10-4 torr ). The X-ray diffraction test showed that the prepared sample are consisting of three phases (NiTi monoclinic phase, NiTi cubic phase, and Ni3Ti hexagonal phase). Machining temperature appears as the most significant factor, which controls both surface roughness and metal removal rate. Two mathematical models were developed to predict the metal removal rate and the surface roughness for any collection of machining temperature, and machining time. A minimum surface roughness of 0.666μm with a maximum metal removal rate of 0.0041g/min can be associated with machining temperature of 650C and machining time of 2min in solution consisting of 6%HCl, 19%HNO3, and 75%H2O.