Effect of Multiple-Pass Friction Stir Processing Overlapping on Microstructure and Mechanical Properties of As-Cast NiAl
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INTRODUCTION
THE NiAl bronze alloy (NAB) is extensively used for propulsion and seawater handling systems[1–6] due to the high strength, good fracture toughness, and excellent erosion-corrosion resistance.[1–4,7] The as-cast NAB is characterized by coarse Widmansta¨tten a phase, nickeliron-aluminum j phases, and island martensite b¢ phase, and the transformation products and their precipitation sequence are complex.[8–13] Furthermore, cast defects such as the shrinkage porosity and pores is a common problem for the NAB castings due to the poor casting properties. This microstructure is disadvantageous to the mechanical and corrosion properties of components. For example, the ASTM B148 cast alloy is used to make the ship propellers; however, the yield strength (YS) of slowly cooled regions in large propeller blades is typically 25 pct lower than the tensile property requirements for the keel block for this alloy (241 MPa).[14] Therefore, in order to improve the service life of the cast NAB components, it is necessary to refine the coarse as-cast structure and eliminate the porosity defects. In the past decade, a multifunctional metal working technique named friction stir processing (FSP) has been rapidly developed based on the basic principles of friction stir welding (FSW), which was invented at The Welding Institute (TWI, United Kingdom) in 1991.[15–17] This technique was first used by Mishra et al. to produce D.R. NI, Assistant Professor, P. XUE, Postgraduate, and Z.Y. MA, Professor, are with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Contact e-mail: [email protected] Manuscript submitted September 3, 2010. Article published online February 9, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
fine-grained aluminum alloys,[18,19] and then applied to modify the heterogeneous microstructure,[20] produce surface composites,[21,22] and synthesize composites and intermetallic compounds.[23,24] An important function of FSP is to provide localized modification and control of microstructure in the near-surface region of castings without changing the shape of components, with the aim to repair defects and to refine and homogenize the coarse microstructures. This provides a simple and promising way to solve the problem of the NAB, as mentioned previously. Previous investigations showed that FSP could effectively refine the coarse microstructure of the NAB, heal the porosity defect, and homogenize the microstructure, thereby increasing the hardness, tensile properties, fatigue strength, and corrosion resistance.[14,25–27] Oh-ishi and co-workers[28–32] reported that the main characteristic of the stir zone (SZ) of the FSP NAB was inhomogeneous microstructures in various subregions, including Widmansta¨tten structure, equiaxed fine grain structure, and banded or lamellar structure. Similar results were also reported by Mahoney et al.[25] and Fuller et al.[33] More recently, our study further showed that the inhomogeneous microstructure w
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