As the formation and breakdown of the built-up edge occurs repeatedly in drilling aluminum alloys, the behavior of the built-up edge and the appearance of the drilled surface in aluminum wrought alloys, 1100-F, 2024-T4, 5052-F and 6061-T6, have been studied by quick-stopping of cutting operation of a twist drill for the purpose to examine the aluminum drilling mechanism. The results obtained are as follows.:
In dry drilling, the built-up edge is generally formed and the adhering condition of the built-up edge changes continuously as the drill rakc angle changes from -30° to +30° from the center to the margin. But, the effective rake angle of cutting is nearly constant (about 35°), and the nose radius of the built-up edge is also nearly constant (about 60μ). In the case of 5052-F, the built-up edge is formed on the whole drill edge. For 2024-T4 and 6061-T6, the built-up edge is formed near the center and only a little near the margin of the drill edge. For 1100-F, the built-up edge is formed very little. It seems that the formation of the built-up edge depends on the material. The drilled surface (the conical surface and especially the cylindrical surface) is strongly affected by the built-up edge, and a good drilled surface is obtained when the formation of the built-up edge is suppressed in the vicinity of the margin.

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In the previous report, the effects of specimen width, plate thickness and welding residual stress on the fracture characteristics of 5083-O plate and 5183 welds, in which crack-like flaws are present, have been
investigated. In this report, the effects of notch acuity, through-thickness notch length, surface notch size and welding misfabrication on the fracture strength are investigated.
From the experimental results, the following conclusions are obtained:
1) The saw cut with the notch root radius 0.1mm is equivalent to a natural crack from the viewpoint of ductile fracture strength.
2) In order to obtain the fracture strength of these materials, the standard test specimen for the deep notch test, with 400mm width and 60mm edge cut, is proposed.
3) The ductile fracture strength of a specimen with a comparatively long and deep surface notch depends on the 0.2% proof stress of the plate. On the other hand, that with a comparatively short and shallow surface notch depends rather on the tensile strength.
4) The ductile fracture strength is considerably greater than the 0.2% proof stress, even in the case that a surface crack-like flaw and large welding mis-fabrication are present. However, the misalignment should be restrained as small as possible.

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This investigation was perfomed in order to understand the influence of heat treating conditions and minor additional elements on the exfoliation characteristics of commercial Al-Zn-Mg alloys. The results obtained are as follows:
(1) In a commercial Al-4.2%Zn-1.36%Mg alloy, exfoliation sensitivity increased with decrease of solution treatment temperature and aging temperature. As precipitation proceeds, exfoliation sensitivity decreased. In an experimental Al-4.5%Zn-1.2%Mg alloy, addition of a small amount of titanium, chromium or zirconium increased it's exfoliation sensitivity.
(2) From microscopic observation of the exfoliated specimens, it was considered necessary for exfoliation of Al-Zn-Mg alloys that compounds of the minor additional elements existed in a lamellar state.

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Al-Zn-Mg alloys containing high zinc contents (Zn≥8%) have not been used commercially because of their high stress-corrosion susceptibility. However, if proper methods of improving the resistance to stress-corrosion cracking can be found, they will become very usefull alloys because of their excellent hot-workability and high strength.
In the course of study on the effect of small amounts of additional elements on stress-corrosion, we found that the stress-corrosion susceptibility of these alloys remarkably improved by the addition of small amounts of Cu and Zr. On that account further studies on Al-8-15%Zn-0.5-10%Mg alloys with addition of Cu and Zr were carried out. It is shown that these alloys have high strength and excellent hot workability combined with good resistance to stress-corrosion.

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