A uniform, fine -phase microstructure enhances the mechanical properties of Al-Si alloys, however, it is an open question how the phase affects the crack-growth behavior. This paper addresses the effects of the morphology and distribution of phase on the fracture behavior in a model dual-phase Al-7%Si alloy with different microstructures. The influences of microstructural factors on crack-growth behavior are examined using in-situ experiments. The results show that a globular -phase microstructure produces a straight crack-growth path, whereas a dendritic, orientational -phase microstructure leads to a deflected crack profile. Finite-element modeling is performed to simulate the fracture behavior, and to rationalize the observed phenomena. The near-tip J-integral based fracture criterion is used to predict the fracture path. Numerical results indicate that a variation in the morphology and distribution of phase changes the symmetry and intensity of the near-tip stress, strain and displacement fields due to the strong mismatch in elastic-plastic properties of the phase and eutectic phase, which have major influences on both crack-growth direction and crack-tip driving force.
HIROYUKI TODA, SEISHI NISHIDO, TOSHIKAZU AKAHORI, MITSUO NIINOMI, and TOSHIRO KOBAYASHI Hypoeutectic Al-Si alloys consist of primary ␣-Al and Al-Si eutectic phases and show typical elasticplastic fracture. To understand their fracture behavior, fracture processes were simulated using an elastic-plastic finite-element method. The validity of the J-integral-based criterion was verified and applied to the simulations. A complicated model was used to simulate the fracture in an idealized dendritic microstructure, and four simplified models were intended to more clearly understand the interaction between a crack and individual ␣ phases. Results show that the crack is attracted to the soft ␣ phase when passing by the ␣ phase, whereas it is repelled when the ␣ phase is close in front of or behind the crack tip. The presence of ␣ phase close in front of or behind the crack tip leads to an amplification of the driving force. However, the ␣ phase beside the tip reduces the driving force. Furthermore, the fracture behavior is mainly affected by the adjacent ␣ phase on one side around the crack tip, while the remote ␣ phase on the opposite side has an offsetting effect. The local stress-strain fields were examined to analyze the simulated behavior. The simulated crack-growth path in the dendritic model was compared and verified with the experimentally observed path.
In the present study, initiation and evolution of damage at eutectic and primary Si particles during monotonic and cyclic loadings were investigated utilising in situ studies. Al-7%Si and Al-20%Si binary alloys were produced as model materials. Their damage behaviour was analysed in terms of composite theory. In the tensile tests, particles were found to crack perpendicular to a loading axis, being triggered by a far-field uniaxial stress. The damage is accumulated gradually with an increase in applied strain. The in situ strengths of the Si particles are 500-900 MPa for the small eutectic Si particles, whereas it is as low as 200 MPa for the coarse primary Si particles. During the cyclic loading, gradual accumulation of damage at the Si particles was observed with the increase in number of cycles. Propagating through such a weakened zone, a fatigue crack seems to receive some acceleration by the existence of the cracked particles ahead of it. The degree of acceleration was quantitatively evaluated by experiments.
Die casting in mass production has the advantage of producing complex shapes precisely, but the disadvantage of air entrapment in high-speed injection molding. Plunger velocity control very effectively avoids air entrapment. Using computational fluid dynamics (CFD), we analyzed fluid behavior, the amount of air trapped, and air shutting caused by die casting plunger movement. We calculated optimum die casting plunger velocity control input to reduce or prevent air entrapment and air shutting in die casting products. We also conducted optimization using a genetic algorithm incorporating a CFD simulator.
In-situ SEM observation has been used to characterize crack propagation behaviors and microscopic damage evolution at Si particles in three model hypoeutectic Al-Si cast alloys with different eutectic microstructures. It was clarified that the extents of damage and crack propagation path are significantly affected by the microstructures. A crack propagates in an eutectic region in a gravity cast alloy with dendritic a phase. Meanwhile, in the case of a rheo-cast alloy, a crack often propagates into a phases. Generally, the observed tendencies appear to be dominated by the fundamental crack-tip shielding/anti-shielding behaviors. However, the actual crack paths can not be fully assessed only from the experimental efforts. The crack propagation simulations are therefore performed for this purpose. Similar complicated crack propagation paths are well reproduced in the simulations, thereby providing mechanistic insights for the crack deflection behaviors.
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