Study on Superplastic Grain Growth Model and Dynamic Simulation

The ability of metal plastic forming and the mechanism performance of the part are correlated with grain granularity of the metal. Grain growth is a prominent character of the microstructure evolution. It is very helpful for the design of process and die structure to study the mechanism of superpalstic deformation and microstructure evolution of superplastic forming. The microstructure evolution of material is impact directly the mechanic performance of the component in superplastic forming. It is necessary to optimize the complex process and to predict the microstructure evolution. A new simulation method that integration the finite element simulation and the microstructure simulation of superplastic forming is introduced in this paper. Monte Catlo method is an odds simulating technique and can simulate time course of microstructure evolution. Based on the studies of superplastic grain growth mechanism, the superplastic grain growth rate equation are derived in this paper by coupling static state anneal grain growth mechanism and deformation stimulated grain growth mechanism. The grain growth drive force of superplastic deformation includes mostly boundary energy and distortion energy. A new drive force model is derived based on energy model. The microstructure evolution is correlated with the stress and strain of the part, and the integration of superplastic forming FE simulation and microstructure evolution MC simulation is realized. Using the integrating simulation technique can predict not only the forming process but also presenting grain growth image of the part. Experimental studies of the part in forming process and microstructure evolution were performed. Introduction Grain growth is a prominent character of the microstructure evolution during superplastic forming. The growth grain will fall the distortion capability of materials and brings cracks and ruptures easily. Researches indicate that the grain will growth up with the distortion grade [1]. It is an important stage for planning techincs to understanding and controlling the process of the grain growth. The grain growth drive force model according with superplastic forming characters is established in the paper. Based on the microstructure evolution simulation arithmetic-Monte Catlo method (MC), an advancing MC model considered the characteristic of superplastic forming is established. The integration of superplastic forming FE simulation and microstructure evolution MC simulation is realized by the data mapping successfully between the FEM and MC in this paper. Grain Growth Model Grain growth is one of the significant character of microstructure evolution in superplastic forming. Static state anneal grain growth mechanism and dynamic grain growth mechanism or forming stimulate grain growth mechanism are mainly ways in grain growth of superplastic forming, and the forming stimulate grain growth is the mostly form in superplastic deformation [2]. The classical static state grain growth model can be described as q q s t d d / 1 0 ) ( α + = 2-1 where q is exponent, α is rate gene, 0 d is original grain dimension and t is time. Materials Science Forum Vols. 551-552 (2007) pp 639-644 online at http://www.scientific.net © (2007) Trans Tech Publications, Switzerland Online available since 2007/Jul/15 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 130.203.133.33-17/04/08,10:05:59) The brilliant boundary slippage is the main deformation mechanism of the metal superplastic behavior, so the theory “crystal lattice damnification” can describe strictly microstructure evolution in superplastic forming[3]. The forming stimulate grain growth can be expressed as ) exp( 0 βε d d p = 2-2 where β is a coefficient andε is strain. Based on superplastic deformation mechanism, the grain growth increment model can be expressed as the differential sum form of forming stimulate grain growth and static state reversion grain growth[4]. ε ε d d dt t d dd p s ∂ ∂ + ∂ ∂ = 2-3 Combining Eq. 2-1 , Eq. 2-2 and Eq. 2-3 we can obtain d qd d q ε β α & & + = −1 2-4 Differential coefficient and integral the whole course of superplastic deformation, we can obtain ( ) [ ]        + − = q q q d q q d 1 0 ) exp( 1 exp βε βε ε β α