Porous aluminum can be a good shock absorber, and its collapse behavior and shock absorbing property is desired to be optimized by the pore structure. To unravel the collapse mechanism, we have utilized the image-based finite element analysis. The pore structure was captured by the micro-focus X-ray computed tomography, and the finite element model was automatically and easily generated by using the uniform cubic element, that is, voxel element. The elastic property of two samples of porous aluminum was evaluated by the finite element analysis, and compared with the experimental result of static compression test. The effect of element size on the elastic numerical results was also investigated by changing the size of voxel element. However, the elastic property obtained by the finite element analysis completely differed from the initial gradient of the stress–strain curve obtained by the experiment. On the other hand, the elastic-plastic analysis gives a good estimation of the initial gradient of the stress–strain curve. We concluded that the compressive collapse behavior was affected by the elastic-plastic property of material along with the high stress concentration from the beginning of contact with the compression plate and the plateau region was realized by the finite deformation of the cell walls.
The presence of heterogeneous deformation is a one of key issue in a prediction model of deformation texture development. Recently, not only local strain mapping but also crystallographic orientation mapping is possible to obtain within aluminum alloy by using synchrotron radiation. In this study, local strain mapping and orientation mapping were obtained by microtomography and three-dimensional X-ray diffraction using synchrotron radiation in Al–4Pb alloy. Simulation of deformation texture evolution has been performed based on crystal plasticity model. The simulation started from actual microstructure and the obtained local strain was applied. The simulation produced similar microstructure to the actual one. Therefore, heterogeneous strain distribution is very important for the development of deformation texture.
It is difficult to evaluate the corrosion pit using normal observation methods because of the complex shape of the pit and its passive film. In this study, measurements of pit and crack formation in corrosion fatigue tests of high-strength aluminum alloy, 7075-T651 were conducted by synchrotron radiation CT imaging. 3D/4D analysis was applied to the observation of corrosion pit growth and crack initiation in corrosion fatigue tests. Pits and inclusions were recognized individually in 3D/4D analysis. The corrosion pits showed complex shapes, and parts of pits grew under the surface of specimens along inclusions. It was found that the behavior of the corrosion pit under the surface was affected by the inclusions distribution. The stress intensity factor range ΔK was calculated based on the measured shape and dimensions of the corrosion pit. The value of ΔK increased with increasing loading cycles, and the value of ΔK at the location of crack initiation indicated larger than 0.9 MPa m1/2, which was similar to the threshold stress intensity factor range ΔKth for corrosion fatigue of A7075-T651.
The crack initiation mechanism of cast aluminum alloy was addressed to quantitatively evaluate the fatigue strength. We employed the synchrotron radiation microtomography to visualize the three-dimensional damages around pores and silicon particles. Two types of specimens underwent a low-cycle fatigue test. The temperature of solution treatment was different, and it yielded a difference in the shape, size and distribution of silicon particles. After a certain cycles of fatigue, a catastrophic damage around many silicon particles occurred, and they connected to each other so as to form a crack especially within the high stressed region around a relatively large pore. The scanning electron microscopy after the test showed that the type of damage was the breakage of long-shaped silicon particles or the aluminum–silicon interface debonding of round silicon particles. The high temperature solution treatment facilitated the interface debonding by the enlargement of silicon particles.
The solute atom distribution of the L12 ordered phase (Al3X, X=Sc, Zr) was investigated for an Al–0.15 mol%Sc alloy and an Al–0.10 mol%Zr–0.05 mol%Si alloy using three-dimensional atom probe (3DAP) analysis and Monte Carlo simulation. Precipitates of the L12 ordered phase (Al3X, X=Sc, Zr) in the Al–Sc and Al–Zr–Si alloy were detected using 3DAP analysis. The L12 ordered phase (Al3X, X=Sc, Zr) in the Al–Sc and Al–Zr–Si alloy was evaluated quantitatively. Formation of the L12 ordered phase (Al3X, X=Sc, Zr) was observed in the simulation with the interaction energy calculated using the first principle calculation. Si atoms were concentrated in the Al3Zr phase and were positioned at Al sites in the Al3Zr phase in the Al–Zr–Si alloy. The interaction energy between Si atom and Zr atom is negative: mutually attractive force exists between Si and Zr atoms. These results suggest that Si atoms are positioned at Al sites in the Al3Zr phase and that they increase the frequency of the Al3Zr phase nucleation.
Porous aluminum was fabricated by tool-traversing friction powder sintering process with the sintering and dissolution process (SDP). In this process, the starting material was a mixture of aluminum powder and sodium chloride (NaCl) as spacer particles. After the powder mixture was placed in a mold, compaction and sintering was conducted only by the traversing of a rotating tool as in friction stir welding. Namely, no external heat source was necessary for the fabrication of porous aluminum, except for the friction heat generated by the traversing of the tool. In this study, porous aluminum with porosities of 60%, 70% and 80%, and a length equal to the tool traversing length was successfully fabricated. By X-ray computed tomography (CT) and scanning electron microscope (SEM) observations of the pore distribution and shapes, it was found that fabricated porous aluminum had a uniform pore distribution with pore shapes similar to the NaCl morphology, regardless of the porosity and the position along the tool traversing direction. In a compression test, the fabricated porous aluminum was observed to exhibit ductile fracture behavior, indicating that the aluminum powder was sufficiently sintered. The fabricated porous aluminum had almost the same plateau stress regardless of the position along the tool traversing direction for each porosity.
Functionally graded aluminum foam (FG Al foam), in which the properties vary with the position, is expected to improve the performance of Al foam. In this study, A1050/A6061/ADC12 three-layered FG Al foam was successfully fabricated by a friction stir welding route precursor foaming method. Stepwise compression tests and continuous X-ray computed tomography (CT) nondestructive observation of the pore structures of the FG Al foam was conducted. It was revealed that the deformation of the fabricated FG Al foam started from the low strength A1050 Al foam layer, and thereafter the middle strength A6061 and high strength ADC12 Al foam layers sequentially deformed. The stress–strain curves during compression tests revealed that three plateau regions appeared independently in the FG Al foams, which corresponded to the plateau regions appearing in the uniform Al foams. The three plateau regions of the FG Al foam had almost the same plateau stresses as that for each corresponding uniform Al foam. These results suggest that the FG Al foam has the potential to be deformed at a controlled and desired location and with a desired plateau stress.
Recently, a friction powder compaction process by the sintering and dissolution process route for fabricating porous aluminum (Al) has been developed. The sintering of the mixture of Al and NaCl powders was conducted only by a rotating tool plunged into the mixture. In this process, elongation of pores, which was caused by shear deformation generated by rotating tool, was observed. In this study, shear deformation was observed using X-ray computed tomography (CT) by fabricating Al–Fe compact. It was shown that the shear deformation was marked at the vicinity of the rotating tool, but largely decreased as the distance from the rotating tool became large. By comparing with the pore structures of porous aluminum with porosity of 60%, elongation of pores were only observed at the vicinity of the rotating tool. From these results, it was indicated that although the slight shear deformation was generated at the remote part from the rotating tool, the NaCl remained its shape and therefore no deformation of pore structures was observed.
Interpenetrating phase composites (IPCs), in which each phase forms a 3-dimentionally interconnected network, show a new class of reinforcement morphology. In this study, syntactic foams, which are made of hollow glass microspheres embedded in a polymer matrix, are reinforced with open-cellular porous aluminum foams. Both deformation behavior and a strengthening mechanism of the syntactic foam/aluminum foam IPCs (SF/AF IPCs) are investigated with the help of an X-ray Computed Tomography (X-ray CT). Deformation behavior of the aluminum foam in the SF/AF IPC during compressive deformation is observed by the X-ray CT. The von-Mises stress distribution in the aluminum foam under compression was simulated by a finite element method for both the aluminum foam alone and the SF/AF IPC. High von-Mises stresses are detected in a layer with a relatively higher porosity of aluminum foam alone. Cell edges in the aluminum foam of the SF/AF IPC are fractured in the perpendicular direction to the compressive direction.
Dissimilar friction stir welding of aluminum alloy and steel is attractive method to fabricate light structure for transportation vehicle. The welding conditions, such as heat input (tool rotation speed and welding speed) and welding tool shape influence on the weld material and the material flow during welding. Control of material flow is important for fabrication of sound weld without defect. Thus, the material flow in the weld was investigated by using X-ray CT technique after welding. Chip of steel from weld interface was used as marker material for 3D visualization of material flow in aluminum alloy. From the observation, the heat input and probe shape affected on the material flow and formation of weld defect. Screw probe effectively stirred aluminum, induced downward and circumferential material flow around the probe and fabricated defect free weld with high weld strength.