PSI - Issue 19

Lloyd Hackel et al. / Procedia Structural Integrity 19 (2019) 452–462 Valentin LOURY--MALHERBE/ Structural Integrity Procedia 00 (2019) 000–000

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1. Abstract This work on Additively Manufactured (AM) Inconel 718 (In718) centers on a laser peening surface treatment intermixed with thermal processing and evaluated on the basis of fatigue enhancement. AM parts are not currently used for safety critical applications because of concern for material integrity, impact fatigue, and thermal/stress induced creep. However, it is commonly admitted that additive manufacturing (AM) will in the future attain performance to allow use for safety critical parts and further, by enabling advanced component designs, AM parts will bring about a revolution in performance and fuel efficiency to systems. With a specific interest in jet engines to enhance fatigue performance for high temperature components such as turbine blades, we evolved a new type of process employing repeated applications of a laser peening (LP) and thermal exposure. Our data shows that this approach enhances fatigue performance after prolonged thermal exposures. The term laser peening plus thermal microstructure engineering (LP+TME) is used to identify the process. This process coupled with AM designs using intricate cooling passages not attainable in standard design and when applied to engine hot section blades and discs could dramatically increase thermal performance and fuel efficiencies. Initial work looks at retention of residual stress after thermal exposures of 760°C and evaluates the important fatigue performance after thermal exposure of 600°C for up to 350 hours. 2. Motivations In 2018, commercial aircraft used $100B in fuel meaning that a 1% improvement in fuel efficiency would result in a $1B cost savings [1]. High efficiency gas turbines, including jet engines, electric power generation turbines and ship and tank engines all would benefit from improved efficiencies, efficiencies that are highly dependent on operating temperature capability of the turbine hot section blades and discs. First level motivation and evaluation for our work was interest in high temperature (600 o C) performance of commercial engine turbine blades which run hot for approximately 2 minutes during typical takeoffs and climb outs. Thermal loading for 2 minutes times 10,500 operational takeoffs equates to about 350 hours. Work started by evaluating approaches that allowed retaining residual stress in wrought In718 through thermal exposures. The LP + TME approach was developed to improve retention of residual stress and improve fatigue performance after long duration exposures at high temperature. AM In718 test samples were designed, peened and tested in 4-point bending fatigue performance under longer and longer thermal exposures of 600°C. Shot peening [2] and laser peening [3] are technologies that are used to increase strength and/or lifetimes in fatigue prone areas of components. The processes generate pressures that plastically yield material within the near surface layer with the resulting component response generating protective compressive stress. In this work, with an interest in materials used for high temperature applications, a new process combining laser peening and thermal treatment of materials was introduced to improve retention of residual stress and thereby increase high-temperature fatigue performance.