The mechanism of decomposition and its products were investigated by resistivity measurements and optical and electron microscopy. The existence of a new metastable phase (G') was established, besides the known Al₆Mn and G phases. However, no sequence of precipitation was observed, in spite of the occurrence of metastable phases.
The precipitation process seems to be mainly affected by the degree of difficulty in nucleation of these three phases. Accordingly, grain boundaries and dislocations can act as preferential nucleation sites and thus control the precipitation behavior of the alloys.

View full abstract

Al-Fe, Al-Sn, Al-Cu, Al-Zn and Al-Zr binary alloys of various concentrations were rapidly solidified from melt into foil form. The microstructures of these alloys were studied with respect to the origin and arrangement of dislocations.
Cell boundaries could be classified into three types with respect to the dislocation arrangement as reported in our previous work⁴⁾; and a new type was found in Al-Sn alloys.
The origin of dislocations could be explained by the theory that it would be resulted from the constitutional stress on the cell boundaries due to solute segregation. Consequently, the highest dislocation density was found in the alloys, where the difference of atomic radii between solvent and solute atoms was great and the difference of solute concentrations between on the boundary and in the interior of cells was also great. The second mechanism for the origin of dislocations would be prismatic punching, which was observed at large secondary phase particles.

View full abstract

In order to investigate the effect of grain size on Portevin-Le Chatelier yielding, tensile tests were performed on Al-0.55 and 1 at%Mg alloys having various grain sizes.
In the temperature range of -20+40°C and strain rate range of 3.3×10^-51.7×10^-3/sec., the value of activation energy for P-L effect was found to be 0.60±0.02eV, which was independent of grain size.
The relation between strain rate (ε) and strain (ε₀) required for the onset of P-L effect at 0°C was expressed as follows:
ε∝ε₀^2.01±0.1μ^-0.53±0.02
where μ is grain size.
It was found that the value of ε₀ was decreased and the stress at the onset of P-L effect was increased with the decrease of grain size. The above fact is explained by the effect of grain size on the average velocity of moving dislocations.

View full abstract

Turning tests were conducted on aluminum-13%silicon alloy casting (silumin) and aluminum-23% silicon alloy casting (hypersilumin), which are known as "difficult machining aluminum alloy castings" and giving fairly large tool wear. With the view of improving their machinability with no effects on mechanical properties, 0.20.5% of tin had been added to the alloys for the tests. The results obtained were as follows:
(1) In aluminum-13%silicon alloy containing 0.20.4%tin, cutting efficiency was improved. For exampie; cutting resistance was decreased, cutting temperature was lowered and the progress of tool wear was slower. The improvement of machined surface was not distinct, but the treatment of chips was easier, because their shape was changed to sheared form. In the range of experiments, the effects of the amount of tin addition were not clear.
(2) In aluminum-23%silicon alloy containing 0.20.5%tin, the improvement in machinability was not very distinct. The decrease in cutting resistance and drop of cutting temperature were not clear. The effects of tin addition were not very decisive on tool wear and machined surface. The chip shape of the original alloy was likely to be sheared form, and the alloy containing tin gave finer chips of the same shape.

View full abstract

This study was undertaken to assess the weld crack sensitivity of "R74S", which is a weldable Al-Zn-Mg alloy of medium strength.
The experiments were conducted using an automatic MIG welding machine. 12mm thick sheet specimens of R74S in the T4 temper were welded with A5356WY, A5556WY and an Al-4%/Mg-1.5%Zn alloy as filler metals. 5083 sheet specimens in the O temper were used as control material.
The assessment of weld crack sensitivity was made by Gapped-T-joint weld cracking test, in which the sensitivity was numerically expressed by the width of gap at crack tip. The data were statistically analyzed with repetitions, because scatter of the data was considerable.
The results obtained are summarized as follows:
(1) The data of the sensitivity followed the normal distribution when the tests were conducted with repetitions under the same conditions. The assessment, therefore, must be made by statistical analysis with repetitions because of the wide scatter of the data.
(2) The sensitivity of R74S was approximately the same as that of 5083, though the former was very slightly higher than the latter. When 5083 and R74S as base metals were welded together with A5556WY as filler metal, the sensitivity was intermediate between the sensitivity of 5083-5083 welds and that of R74S-R74S welds.
(3) The weld crack sensitivity was affected by the composition of the filler alloys. An Al-Mg-Zn alloy when used as filler metal to weld R74S resulted in higher sensitivity than A5556WY or A5356WY.
It was concluded that Al-Mg alloys such as A5556WY and A5356WY were more desirable as filler metals to weld R74S than Al-Mg-Zn alloys due to lower sensitivity and easier welding operation.

View full abstract