Measurements of cycles to fatigue crack onset and to failure and of fatigue crack propagating rates of 7075 alloy heat treated under various conditions were made to determine the effect of mean stress of them. The number of cycles to crack onset from sharp notch in the alloys overaged under a high mean stress is unexpectedly greater than that under low mean stress. But, the fatigue life is independent of such a tendency because of acceleration by mean stress for crack propagation. The influence of mean stress is uneven through the range of ΔK values and for different types of microstructures. Both C and m of the propagation law are affected by mean stress. Proposed theories for the effect of mean stress are unsatisfied this experimental result, but a concept of accumlated damage appears to offer the most promising solution. A specimen slightly overaged shows better fatigue property than the standard treated one.

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The Al-4%Cu binary and Al-4%Cu-0.065%Sn ternary alloys were solution-treated, step-quenched into bath kept at -30°C100°C for 1sec2min and then quenched into liquid nitrogen. The specific electric resistivity of both alloys showed a maximum at the step quenching temperature fo about 50°C when the step quenching time is longer than 10sec. The results of high resolution electron micrographs showed, in the quenched state, that in binary alloy, densely distributed precipitates of about 15 Å at maximum were frequently observed; in ternary alloy, they were by no means detected and a small number of precipitates much larger than about 15 Å accompanied by tin rich phases were always observed. Those precipitates were suggested to form during step-quenching. The difference in low temperature aging behavior between both alloys would relate to the difference in their quenched structure or their nature of quenched clusters.

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The alternate bending tests were conducted on the butt-welded 5083-0 plates 6mm in thickness in which the angular distorted joint having angles 5, 10 and 15 degrees had been aligned by simple bending. The fatigue strength is reduced by about 15% due to the influence of residual tensile stress introduced into weld toe by pre-bending. Moreover, non-propagating cracks are found at the weld toe on the specimens not yet fractured at 10⁷ cycles. It is recommended from the fatigue tests that the reinforcement on the compression side of bending are machined off before pre-bending.

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Aging behaviors of the heat-affected zones in Al-24.5%Zn-1.24%Mg alloys welds were examined by means of hardness measurements and X-ray small angle scattering methods. The heat-affected zones in all the Al-Zn-Mg alloys welds are regions in the alloys which have been heated at 100°C or above by weld thermal cycles, where dissolution, reversion or partial reversion occurs. A re-solution treated or reverted area in the heat-affected zone is extended in the alloys containing more of Zn. Recovery rate of hardness in the heat-affected zone also increases with increase in Zn content except for the alloys containing small amounts of Mn, Cr, Zr, Cu and Fe. Softened areas are formed in the heat-affected zones of all the Al-Zn-Mg alloys welds during post-weld aging at room temperature. It is wider and remains longer in the Al-Zn-Mg alloys containing more of Mg.

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Effects of such cutting fluids as spindle and machine oils were investigated in orthogonal cutting and turning of AC3A. In orthogonal cutting, when the cutting fluid is applied to the work surface, the cutting resistances and the coefficient of friction on tool face slightly decrease, and the cutting ratio increases. When the work piece is immersed in the cutting fluid, the cutting resistances and the coefficient of friction greatly decrease, the cutting ratio increases and the roughness of cut surface is improved. In turning, these facts almost the same are found. The cut surface in wet cutting is good at slow cutting speeds. It is further improved in wet and dry cutting at faster cutting speeds. The cutting fluid up to a certain amount (5cm³/sec in this work) leads to decrease in the cutting resistance, the coefficient of friction and the cutting temperature, increase in the cutting ratio, and improvement of the cut surface. Spindle oil is superior to machine oil.

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