Thermal and Mass Balance in Reactive Thermal Processing of Nickel Aluminide Coatings on Steel Substrates
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Thermal and Mass Balance in Reactive Thermal Processing of Nickel Aluminide Coatings on Steel Substrates Tarek Alaeddine 1, Rajesh Ranganathan2, Teiichi Ando2, Charalabos C. Doumanidis1, Peter Y. Wong1. 1 – Mechanical Engineering Department, Tufts University, Medford, MA 02155. 2 – Dept. of Mechanical Industrial and Manufacturing Engineering, Northeastern University, Boston, MA 02115. ABSTRACT Nickel aluminide coatings were produced on steel substrates by reactive thermal processing of pre-plated precursor layers of nickel and aluminum using plasma arc as the heat source. Controlled rapid heating melted the outer aluminum layer, which then dissolved nickel to facilitate the nucleation and growth of a nickel aluminide. The resultant coating microstructures varied from a duplex or triplex structure, consisting of either NiAl3 and a eutectic; Ni2Al3, NiAl3 and a eutectic; to a fully monolithic Ni2Al3 structure, with the latter resulting at high heat input rates and/or low heat-source traverse rates. The temperature of the reaction layer was simulated for the experimental conditions by a numerical model based on Green’s function analysis. The nickel concentration at the liquid-solid interface just before any nickel aluminide nucleation was calculated by assuming local equilibrium interface conditions between the liquid layer and the fcc nickel-rich solution. The depth of nickel dissolution, which consequently determines the extent of nickel aluminide growth, was also predicted by the model. Numerical results of the nickel dissolution compared well with experimental observations. INTRODUCTION Intermetallic nickel-aluminide coatings enhance the lifetime of a product and impart superior properties such as resistance to oxidation and corrosion, and wear. Due to their excellent dependability at elevated temperatures, nickel-aluminide coatings are used to protect jet engine turbine blades [1,2]. Nickel aluminide coatings are also used for restoring the dimensions of worn out high temperature parts. Another use of these coatings is as tube-end liners for mild carbon steel heat-exchanger tubes, and when applied on steel, they can effectively reduce cavitation-erosion in wear rings (hydraulic turbines), water turbine buckets, diesel engine cylinder liners and pumps [3,4]. Different fabrication processes, e.g., plasma spraying, pack cementation, metalliding, PVD, CVD, etc. are employed to produce these nickel aluminide coatings. However these processes suffer from poor bonding, porosity, structural coarsening due to bulk heat treatment, or high capital cost and low productivity [5,6]. A novel method to produce nickel aluminide coatings on steel substrates is developed in this work. The desired coatings are produced on steel substrates by in-situ processing of prefabricated precursors, which were prepared by plating the substrates with nickel initially and then with aluminum. Plasma-Arc (PA) was employed for controlled surface heating, resulting in a reaction between aluminum and nickel that produces nickel aluminide coat
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