Enhanced Fatigue Performance of AM Superalloy at Elevated Temperatures Using Laser Peening Plus Thermal Microstrructure Engineering

Additive manufacturing (AM) will in the future be used for safety critical parts and bring a revolution in performance and fuel efficiency to systems such as jet engines. AM is not currently used for critical applications because of concern for material integrity impacting fatigue and thermal/stress-induced creep. However, we show a new type of processing involving laser peening plus thermal microstructure engineering and suggest it be coupled with AM designs using intricate cooling passages not attainable in standard design. This revolution applied to engine hot section blades and discs could dramatically increase thermal performance and fuel efficiencies. We evolved this approach after tests of shot and laser peening showed a lack of retained compressive residual stress and poor fatigue performance in respectively wrought and AM In718. In our work a laser peening and annealing treatment (LP+TME) of wrought In718 retains residual stress of 420 MPa (60 ksi) and depths greater than 1 mm after 50 hours prior exposure at 760^oC. The LP+TME process enhances fatigue performance of AM In718 by over 500% after thermal exposures of 600^oC for up to 350 hours. Motivation was to support high temperature (600^oC) performance of commercial engines turbine blades which run hot for 2 minutes during typical takeoffs and climb outs. Thermal loading for 2 minutes times 10,500 operational takeoffs equates to 350 hours. In our fatigue tests of AM In718 airfoil-like test coupons in non-peened vs. shot and laser peened condition and with no thermal exposure, the peening improved fatigue performance by as much as 10 times. However this performance dramatically decreased to no value as the samples were exposed to 600^oC for longer times. Impressively, further tests showed that processing with LP+TME enabled superior fatigue performance with as much as 500% fatigue life improvement after a 600^oC and 350 hour exposure.