Now, it is hard to find a published academic paper on forging design. You can find a textbook or handbook, where there are some directions for flash design. Fox example.:
George E. Dieter,Howard A. Kuhn, S. Lee Semiatin, Handbook of Workability and Process Design, Materials Park, OH : ASM International, c2003.
Chapter 14, Processing design of impression die forging
You can search it by Google, and preview some papes of the Chapter 14, Processing design of impression die forging. It is enough.