Effect of Thermal Annealing on Machining-Induced Residual Stresses in Inconel 718
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JMEPEG DOI: 10.1007/s11665-017-2824-2
Effect of Thermal Annealing on Machining-Induced Residual Stresses in Inconel 718 A. Madariaga
, J. Aperribay, P.J. Arrazola, J.A. Esnaola, E. Hormaetxe, A. Garay, and K. Ostolaza (Submitted August 19, 2016; in revised form May 8, 2017)
Nickel-based alloys are widely employed in the manufacturing of aero-engines. These alloys are difficult to machine, and tensile residual stresses are generated during machining. These tensile residual stresses can negatively affect the performance of aero-engine components. Nevertheless, residual stresses can vary due to thermal or mechanical loading. These variations must be considered to evaluate the real influence of residual stresses on component behavior. This paper studies the effect of thermal loads on machininginduced residual stresses in the alloy Inconel 718. A ring-shaped Inconel 718 part was face-turned, and specimens were extracted from it. Specimens were exposed at 550 and 650 °C for 10 min, 1 and 10 h. Residual stresses were measured, and microstructure was observed before and after thermal exposure. Residual stress variations found after thermal exposure were the consequence of two factors: relaxation of strain bands during the early stage of exposure and diffusion-controlled creep. In addition, a modified Zener-Wert-Avrami model is proposed to predict residual stress relaxation caused by the diffusion-controlled creep. Once having fitted the modified Zener-Wert-Avrami model, the study was extended for a wider range of temperatures (400-650 °C). This analysis showed that surface residual stresses do not relax significantly at temperatures below 500 °C. Keywords
aerospace, hole drilling technique, machining, residual stress, superalloys, thermal relaxation
1. Introduction Nickel-based alloys were developed in the 1940s and have been widely employed in the aero-engine industry (Ref 1). These alloys have excellent mechanical and chemical properties at high temperatures. Therefore, they are appropriate materials for components that have to withstand high mechanical loads operating at high temperatures. As a result of that, however, these alloys are more difficult to process and manufacturing becomes more challenging (Ref 2). Beyond this scope, aeroengine manufacturers are continuously striving to improve component performance and reliability while seeking to reduce manufacturing costs (Ref 3). The last stage of the manufacturing chain is usually where the machining operations of critical aero-engine components, such as turbine disks, are carried out. Importantly, nickel-based alloys exhibit low machinability due to their characteristics during the cutting process (Ref 2): prone to work-hardening, retention of high strength levels at high temperatures, reactivity with cutting tool materials, tendency to form built-up edge, the presence of abrasive carbides in their microstructure and low thermal conductivity. Consequently, if machining process conditions are not selected appropriately, the surface integrity A. Madariaga, J. Aperribay,
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