The effects of homogenizing treatment and iron content on the tensile properties of 6061 aluminum alloy billet were investigated mainly by tensile test and transmission electron microscopic observation. With the specimens produced by the vertical semi-continuous casting process, the iron content is 0.18, 0.7 and 1.4 mass%, the DAS is 9 and 25 μm, and the grain size is constant at about 80 μm. In the 0.18 mass% Fe alloy, elongation in as-cast is larger in DAS–9 μm than in DAS–25 μm. If they were air cooled after the homogenizing treatment, the difference in elongation between them is decreased with the increase of homogenizing time. On the other hand, if these samples were water quenched after the homogenizing treatment, even when DAS is different, no difference in elongation was found. The cause for this was presumed to be that the cooling condition after the homogenizing treatment accelerated the fracture of specimens by precipitating a coarse β' phase in the air cooling, and its elongation was reduced. It was revealed that the second phase formed between dendrite arms increased with the increase of iron content, the smaller the DAS is, the larger the elongation, and the difference of DAS did not disappear even in a long time homogenizing treatment.
