Optimization of the forging process window in respect of AGG, IGG and direct age effect in alloy 718 engine disks

Abstract

Alloy 718 is commonly used for turbine disks in aircraft engines. The very demanding loading conditions require top notch mechanical properties of the material. In order to maximize the mechanical properties a perfect use of the direct age (DA) effect is necessary. An optimal DA effect contributes approximately 150 MPa to the total yield strength. However, the effect is only present in a certain, narrow process window. It was observed that small deformations at specific local temperatures during the last forging step lead to an ideal DA-effect. As the ideal process window in respect to the DA-effect can hardly be maintained in the whole forging, regions with a decreased DA-effect occur. A connection between the regions with decreased DA-effect and abnormal respectively irregular grain growth (AGG and IGG) was analyzed based on forging simulations. Therefore, the meta-dynamic recrystallization velocity parameter vmrx was developed for indication of AGG and IGG. A correlation of the introduced parameter and the DA-effect was found. In addition, vmrx was used for the design of solution annealed forgings containing coarse grain regions. Those forgings were manufactured and analyzed using laser ultrasonic testing (LUS) and metallography on cut-ups. The occurrence of ALA grains was identified but no white spots (light etch indications) were found