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Influences of the microstructure, micro defects and the stress concentration factor on fatigue characteristics were investigated for a JIS-AC4B alloy containing 6.79%Si, 2.93%Cu, 0.17%Mg and 0.59%Fe, and for an iron free Al-Si-Cu-Mg alloy. Solidification microstructures consist of dendritic phase, eutectic Si, Al 2 Cu and Mg 2 Si phases in both alloy specimens and a few gas and shrinkage porosities appear in every specimen; while Fe compound modified by Mn appears among the dendrites in the AC4B alloy. Rotating bending fatigue tests were carried out on specimens with notches of 2, 1, 0.3 and 0.1 mm radius. Both AC4B and Al-Si-Cu-Mg alloys show the same fatigue sequence when the notch size is larger than 1 mm, indicating that the gas and shrinkage porosities act as the origins of cracking and thus govern the fatigue characteristics. Contrarily, when the notch radius becomes smaller than 0.3 mm, so that the stress concentration factor becomes larger than 2.4, the AC4B alloy has a higher fatigue strength than the Al-Si-Cu-Mg alloy, indicating that Fe-compounds may retard crack propagation.

Additive metal manufacturing-has been attracting attention for various applications such as aircraft parts. However, the parts manufactured exhibit anisotropy characteristics due to the columnar crystal structure induced by rapid melting and solidification. In this work, an equiaxed solidification structure is obtained by adding Zr to promote nucleation. Samples were fabricated by additive manufacturing using Zr-added SUS304L powder. Their microstructures were observed by scanning electron microscopy and transmission electron microscopy. The mechanical and corrosion resistance properties were evaluated by comparison with samples fabricated by additive manufacturing using SUS316L. The microstructure observation revealed that the formation of columnar crystals in the SUS304L-Zr-3DP samples was suppressed compared with the SUS316L-3DP samples and that the average grain size was reduced to 1/3 or less. The transmission electron microscopy revealed a ZrN crystallized substance, which may have contributed to the suppression of columnar crystal formation. Tensile test results showed that the SUS304L-Zr-3DP exhibited isotropic mechanical properties. Corrosion resistance tests using electrochemical methods showed that they exhibited higher corrosion resistance than the SUS316L-3DP. These results demonstrate that the solidification structure formed during additive manufacturing can be controlled by adding Zr into the alloy matrix as nucleation sites.

The influences of SiC particle distribution, surface notch size and solidification microstructure of the matrix on the fatigue characteristics of SiC reinforced JIS-AC4B alloy composites were investigated. Al-6.79 mass% Si-2.93 mass% Cu-0.17 mass% Mg-0.59 mass% Fe matrix composites with relatively homogeneously dispersed 11 mm SiC particles were fabricated through a combination of pressure infiltration and a melt stirring casting method. The matrix microstructure consisted of a dendritic alpha phase and eutectic Si with a few volume fractions of Fe intermetallic compound among the dendrites. All specimens contained some gas and shrinkage porosities, and all composite specimens contained SiC particle clusters. Vickers hardness of composites clearly increases due to the dispersion of SiC particles and age hardening. The hardening ability increases with an increasing volume fraction of SiC. Rotating bending fatigue tests were carried out on notch-free and notched specimens that had peak aging. In the notch-free matrix alloy specimen, cracks generated from porosities, whereas cracks generated from the SiC particles/the matrix interfaces in the composite specimens. Thus, the fatigue strength decreased with an increase in the SiC volume fraction. In the notch-introduced matrix alloy specimen, where the stress concentration factor is high, the notch becomes the crack generation site and dominated the fatigue strength. The cracks, however, generate near SiC particles instead of the notch bottom in the composite specimen. Moreover, it was found that the fatigue limit stress is unchanged in composite specimen even when the notch is introduced, although the critical stress for crack generation declines. Microstructural observation revealed that the cracks were spread and diverted in and around the cluster of SiC particles, suggesting that crack propagation resistance was improved in the composite specimen.