ABSTRACT:
Conventional closed die forging, is widely used for manufacture of superalloy parts. Despite recent technological advancements in the area of forging design, empirical and trial-and-error methods are still widely used in forging super alloy materials. This paper investigates the effect of flash thickness and various geometrical forging design parameters including internal and external drafts and internal and external corner radii, on the die cavity filling, forging load and raw material cost when forging Nimonic80-A a superalloy material. A new design methodology based on DOE-designated fractional factorial and FE simulations is proposed to establish optimal closed die finish forging and to find the optimum values of the design parameters. Also a response-surface-methodology (RSM) based method for forging load prediction is presented.
Key words: Superalloy forging, Nimonic 80-A, D.O.E, Optimal design, F.E.M
1 INTRODUCTION
In modern gas turbines the continuous demand for higher efficiency combined with safety at minimum cost has led to an increase of the operating temperature. This target has stressed the need for new alloy with higher strength at elevated temperature. Among them, superalloys are outstanding and although gas turbines remain the major applications for such materials, where they can account for over 50% of the total weight, the range of application now includes several components for use at high temperatures such as reciprocating engines, e.g. for turbo-chargers and exhaust valves, for tool and dies in metal working, in aircraft structures, etc. Among superalloys, the nickel-base superalloys are widely used. The earliest superalloys were not overly difficult to forge, but it was quickly found that, in general, superalloys, because of their greater strength at elevated temperatures, were more difficult to forge than most other metals, the complex system of phases in the microstructure of Ni-base superalloys make them very difficult to deform so that they can be hot worked into very narrow conditions, in terms of temperature and strain rate. The Nimonic80-A superalloy is a large group of high temperature Nibase alloys intend for sophisticated application in gas turbine. [1, 2] Die design in industries often depends on experience and the trial-and-error method, which results in a great waste of time and cost. Furthermore, the design quality of the forging dies clearly affects the production efficiency directly. The main goals of optimal die design are to reduce forging load, minimize stress, control the material microstructure, and obtain a sound product with no defects. Therefore, it is very important to increase the die life and improve the product quality by means of optimizing die geometry design, proper selection of the die material, and optimum control of the process parameters. In the current study, in order to overcome the critical issues of dies with unstable and short service life in three-dimensional forging, the finite element simulation and Fractional factorial design are utilized to optimize the die geometry design for the fixed die material H 26, hot working tool alloy, used for forging the Nimonic80-A superalloy


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