Iranian Journal of Materials Forming
Volume:1 Issue: 1, Spring and Summer 2014

In this work, dynamic mechanical properties of three high-manganese steels with TRIP/TWIP or fully TWIP characteristics are studied. High strain rate experiments in the range of true strain rates between ~500 and 1800 /s are conducted using a dynamic torsional testing setup. All the three steels show a positive strain rate sensitivity in the intermediate range of strain rates (up to 500 /s). But, they behave differently as the strain rate is further increased. The steel with the lowest carbon content shows a strain rate softening behavior, while the other two exhibit strain rate hardening. The adiabatic temperature rise of the material greatly influences its stacking fault energy during the high rate dynamic deformation. Thus, unlike the quasi-static experiments, the dominant deformation mechanism of the steel during a dynamic test changes as the deformation progresses. In this regard, the variations of the stacking fault energy during the deformation are calculated for the three steels.

Using cold working fastener holes is a simple life enhancement procedure that strengthens metallic components by retarding crack growth around a hole. In the present paper, the residual stress field that is induced by the hole expansion process is obtained based on different material models namely elastic perfect plastic, isotropic hardening and kinematic hardening using three-dimensional finite element method. Moreover, crack closure analysis was performed following introduction of residual stress. The effect of modeling parameters such as percent of cold work, remote tensile stress and crack length is considered. Crack closure analysis indicates that the material model would affects normalized crack opening loads especially at low level of remote tensile stress where small scale yielding is dominant. Cold work simulation results indicate that the residual stress variation through the thickness is considerable. At low stress level, the effect of material behavior model is considerable and at high stress levels the crack opening load is dropped. Moreover, increasing the cold work interference percent results in a higher and deeper compressive residual stress.

In the present study, the capability of accumulative roll bonding (ARB) and subsequent heat-treatment to fabricate solid solution from a multilayered Cu/Zn/Al was investigated. Two post-heat treatment (one-step and two-step) routes were proposed. In the first, the ARB processed material was heated to 800 C for 5:30 hours and allowed to cool slowly in a furnace (one-step route). In the second (two-step route), the composite was heated to 800 C for 3 hours and quenched in the cold water (0 ºC). The microstructures were characterized using a field emission scanning electron microscope (SEM) equipped with energy dispersive x-ray detector (EDX). The x-ray diffraction (XRD) pattern was used to identify the phases. Results showed that even after imposing high ARB cycles (up to 14 cycles), the solid solution could not be formed so far. However, after the subsequent heat-treatment, Cu, Zn and Al distributed uniformly in the microstructure and a copper-based solid solution was formed.

In this paper, we investigate effect of Fe–intermetallic compounds on plastic deformation of closed-cell composite Aluminum Foam as filler of thin-walled tubes. However, deformation of the Aluminum foam-filled thin-walled tubes as crushed-box will be presented in Part (II). Composite foams of AlSi7SiC3 and AlSi7SiC3-(Fe) as closed cell were synthesized by powder metallurgy foaming method. Macro and Micro structures of the produced foams revealed a non-homogenous cell dependent on Fe-rich intermetallic compounds. The cellular structure of AlSi7SiC3 foam shows a non-uniform and small size of bubbles with a few central big voids in comparison with the AlSi7SiC3-(Fe) foams. Analysis of EDAX results show, there are (αβ)-Fe- intermetallic compound within foams with Fe 2 wt. %, which presence of them, depended on the cooling rate during solidification of liquid foams. The uniaxial compressive stress–strain curves of the filled thin-walled tubs with the composite closed cell foams will be investigated in next part. The stress-strain curve of the AlSi7SiC3 foam shows a smooth plastic deformation behavior in the plastic region, throughout the compression tests. However with increasing the weight percent of Fe from 1 up to 3%, the curve shows some variety at the slope of curves within the plastic zone. Results shown all of the compression response is due to the micro-porosity in plateau region and presence of the Fe-Al-Si-intermetallic within the cells wall that both them have the role of a stress concentration point during compressive loading.

Superplastic materials show a very high ductility. This is due to both peculiar process conditions and material intrinsic characteristics. However, a number of superplastic materials are subjected to cavitation during superplastic deformation. Evidently, extensive cavitation imposes significant limitations on their commercial application. The deformation and failure of superplastic sheet metals are a result of a combination and interaction process between tensile instability and internal cavity evolution. Thus, this study carried out modeling of the uniaxial superplastic tensile test using a code based on the finite element method, that used a microstructure based constitutive model and a deformation instability criterion. These models are the criterion account for both geometrical instabilities and cavitation. It is observed that the proposed approach captures the characteristics of deformation and failure during superplastic forming. In addition, the effects of the cavitation on the superplastic forming process were investigated. The results clearly indicated the importance of accounting for these features to prevent premature failure.

Recent developments in nanostructured products draw considerable attention to ultrafine grained materials. These materials are normally manufactured by different severe plastic deformation (SPD) methods. In the present study, analytical models and finite element method (FEM) are used to calculate strain imposed to a specimen that was deformed by equal channel angular pressing (ECAP). In addition, strain inhomogeneity in term of coefficient of deviation (CV) for an aluminum alloy (AA6101) processed under ECAP was calculated. Dies with 90º, 105º and 120º intersecting angles were modeled based on FEM. Furthermore, the effect of friction on force-displacement curves was investigated using analytical and numerical approaches. Moreover, the energy loss that is due to friction was computed. Strains calculated by FEM for different die angles were identical to those evaluated by analytical models. Based on numerical and analytical models, it has been shown that strain inhomogeneity increases when the angle between two channels decreases.