Akio Niikura

Refinement of Recrystallized Grains in Al-Mg Alloys Induced by Super-High-Reduction Rolling and Consequent Annealing

Refinement of recrystallized grains and its effects on mechanical properties have been studied in Al-Mg alloys (5xxxx aluminum alloys). By employing cold-rolled method with super-high-reduction ratio beyond 99% and consequent annealing at 573K, the grain sizes of the Al-Mg alloys have been suppressed under 4μm. The yield stress and n-value of commercial 5083 alloy are 140MPa and 0.32, respectively, have been improved to reach 185MPa and 0.33, respectively, by the previous processes.

Aging Process during Creep Deformation in an Al-Mg-Si Alloy Affected by Pre-Aging Condition

Creep behavior of an Al-0.3%Mg-0.5%Si alloy affected by pre-aging condition was investigated to obtain fundamental knowledge on the thermal stability in the service temperature range for the precipitation-hardened aluminum heat-exchanger. The alloy was aged at 175 °C for 3 h (under-aged) and 24 h (peak-aged), after solution treatment, and then subjected to creep testing at temperatures of 150 °C and 200 °C. When the creep temperature was 150 °C, the under-aged (UA) specimen showed a lower creep rate than that of the peak-aged (PA) specimen. On the other hand, when the creep temperature was 200°C, both specimens showed almost the same creep rate. In other words, the UA specimen had higher creep resistance than the PA specimen at 150 °C, whereas this advantage disappeared at 200 °C. The difference was thought to originate from the precipitation during creep testing.

Recrystallization Behaviour in Twin-Roll Continuous Cast Al-Fe-Ni Alloy

Factors controlling the recrystallization of a continuous cast Al-Fe-Ni alloy were investigated by analyzing microstructure, grain structure, texture, and electron backscattering patterns. The texture of an as-rolled sample heated at 400°C was similar to that of a sample at the reduction ratio of 50% after intermediate annealing at 550°C. The as-rolled samples had a strong (213)[36-4] (S-type) orientation that was changed to a strong (130)[3-12] (QQ-type) orientation by heating at 400°C. This recrystallization looks like continuous recrystallization. Discontinuous recrystallization due to particle-stimulated nucleation is suppressed by the fine dispersion of Al-Fe-Ni-Si particles that form during casing and intermediate annealing, and stabilize the grain and subgrain structure. We think that a course initial grain structure and moderate level of strain level are the important factors promoting continuous recrystallization in this alloy.