Functionally graded materials composed of Al foam and Al/spacer composites were fabricated by a sintering dissolution process, in which removal of the spacers was stopped during the dissolution process. The deformation of the material started in the low-strength Al foam layer then in the high-strength layer with the remaining spacers.
The fatigue crack initiation mechanism of cast aluminum alloy has been addressed by using the synchrotron radiation which gives a high resolution CT images to visualize the microstructure. By the 4D imaging, we have observed the crack initiation and propagation by low-cycle fatigue where the source of crack was the fracture of Si particle or debonding of Al-Si interface. The micromechanical finite element analysis can be performed with the precise microstructure based on the CT images. To investigate the effect of a small uncertainty in the applied cyclic loading on the stress and strain around the silicon particles which were fractured and initiated a crack in the fatigue test, the elastic-plastic finite element analyses was carried out with ten cycles of loading. The fractured silicon particles were identified from chronological CT observation. A method to make a reasonable waveform with precise mean and standard deviation was proposed. The small uncertainty coming from the experimental error was considered, and two types of cyclic loading waveform was made and used in the simulation. From those results, we found that the small uncertainty of applied stress had an significant effect on the maximum stress in silicon particles and increased it gradually by the cyclic loading.
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.
Porous aluminum is expected to be applied as a multifunctional material in automobiles because of very lightweight, high specific strength and high energy absorptivity. When aluminum alloy die castings are used as a starting material in the fabrication of porous aluminum, the gases intrinsically contained in the die casting can be used to generate pores. From the fact, it can be expected that porous aluminum with high porosity can be fabricated by adding a small amount of blowing agent. In this study, the amount of blowing agent from 0 to 1.4 mass% is added to ADC12 aluminum alloy die castings containing three different amounts of gases and porous aluminum is fabricated by the FSP (friction stir processing) route precursor method. The variations of porosity and pore structure with the amount of added blowing agent are investigated. Through the experimental results, it is shown that, by using a blowing agent of approximately 0.6 mass%, ADC12 porous aluminum can be fabricated with high porosity and good pore structure.
Al foam has been used in a wide range of applications owing to its light weight, high energy absorption and high sound insulation. One of the promising processes for fabricating Al foam involves the use of a foamable precursor. In this study, ADC12 Al foams with porosities of 67%-78% were fabricated from Al alloy die castings without using a blowing agent by the friction stir processing route. The pore structure and tensile properties of the ADC12 foams were investigated and compared with those of commercially available ALPORAS. From X-ray computed tomography (X-ray CT) observations of the pore structure of ADC12 foams, it was found that they have smaller pores with a narrower distribution than those in ALPORAS. Tensile tests on the ADC12 foams indicated that as their porosity increased, the tensile strength and tensile strain decreased, with strong relation between the porosity, tensile strength, and tensile strain. ADC12 foams exhibited brittle fracture, whereas ALPORAS exhibited ductile fracture, which is due to the nature of the Al alloy used as the base material of the foams. By image-based finite element (FE) analysis using X-ray CT images corresponding to the tensile tests on ADC12 foams, it was shown that the fracture path of ADC12 foams observed in tensile tests and the regions of high stress obtained from FE analysis correspond to each other. Therefore, it is considered that the fracture behavior of ADC12 foams in relation to their pore structure distribution can be investigated by image-based FE analysis.
Effect of wall-thinning dimensions on the strength of piping elbows against internal pressure and in-plane bending was examined through the finite element analysis considering material nonlinearity and geometrical nonlinearity. The pipe used for simulation was MS SMTP370 80A Sch160. Assuming the wall-thinning area was fixed to relatively narrow area, the effect of depth and position of wall-thinning was investigated. The capped elbow specimen was first subjected to a constant internal pressure, and second subjected to the close-mode in-plane bending until the prescribed displacement The maximum load was defmed as the collapse load. From the simulation results, we found that the effect of relatively narrow wall-thinning on the collapse load was small under the internal pressure and close-mode in-plane bending.
