Materials Science Forum Vols. 551-552

Abstract: Dynamic grain growth in superplastic AA5083 deformed at constant strain rates and varying strain rates has been studied. It has been found that the strain rate history has a significant influence on the grain growth behaviour. It is proposed that differences in mechanical behaviour of AA5083 for the strain rate conditions examined can be attributed to differences in the grain growth behaviour. The influence of dynamic grain growth on the superplastic behaviour of AA5083 is discussed.

Abstract: This paper deals with the cavity formation and growth behavior of fine-grained 1420 Al-Li alloy during superplastic forming. The results indicated that there were many sub-micron cavities pre-existing at the particle-matrix interface and these sub-micron cavities grew initially under deformation. Different from uniaxial tension, the cavities nucleation under biaxial tension was combinable effect of stress concentration and matrix/particle de-cohesion. With the strain and temperature increasing, the total number and the average size of cavities increased. By the calculations, it was seen that diffusional growth process dominates the initial stage of void growth, and for void radii>~1.7μm, void growth was mainly controlled by plasticity.

Abstract: 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.

Abstract: According to the characteristic of appearing cavitation in the metals during superplastic deformation, the influence of strain rate on cavity evolvement, the influence of cavity on superplastic deformation capability, and the formation, development process of cavity were investigated for Al-Cu-Mg alloy (i.e. coarse–grained LY12). The results show that: ①The pore nucleation occurs not only at triangle grain boundaries, but also along nearby the second phase particles, and even within grains. The cavities at the triangle grain boundaries are present in V-shape, others near the second phase particles and within grains are present in O-shape. These cavities may result from disharmony slippage of grain boundaries. ②The tendency of cavity development decreases with increasing of strain-rate. In lower strain-rate condition, though Al-Cu-Mg alloy has better superplasticity, many big cavities in the specimen may reduce the room temperature properties of the alloy. In higher strain-rate condition, Al-Cu-Mg alloy has certain superplasticity and room temperature properties as well as few cavities forming. By analyzing, viscous layer on grain boundaries is very thin and grain sizes can be refined during their extruding and rotating each other in higher strain-rate superplastic deformation condition. ③Growth and coalescence of cavity are the main reason of the superplastic fracture of Al-Cu-Mg alloy. And small and a certain amount of cavities with dispersion and independence state are very useful to crystal boundary slippage.

Abstract: The microstructure evolution and mechanical properties of magnesium alloy AZ31D processed by equal channel angular extrusion(ECAE) are studied. The processing temperature and the passes of extrusion are important factors to affect the microstructure and mechanical properties of the magnesium alloy during ECAE. In this paper, ECAE was performed at the temperature from 523 to 673K. The ductility increased through the grain refinement after the ECAE because the recrystallization took place and high angle grain boundary formed. The elongation of magnesium alloys AZ31D was improved after the ECAE. The maximum elongation-to-failure of magnesium alloys AZ31D processed by ECAE for 4 passes at 573K and strain rate of 0.5 10−4 s−1 × was 350%.