Hydrogen embrittlement affected by three types of plating (low-P and high-P types electroless Ni-P plating and electrolytic Zn-platin) was investigated by means of slow-strain-rate three-point bending test on three aluminum alloys (2017-T3, 6061-T6 and 7075-T651). Hydrogen generated by the Ni-P and zinc plating was absorbed by the aluminum alloy substrates, and the trap sites in the aluminum alloy substrates for the absorbed hydrogen differed between Ni-P and zinc plating. Hydrogen embrittlement was able to be evaluated by the three-point bending tests on plated aluminum alloys. Zinc plating did not cause embrittlement for all the alloys, but Ni-P plating induced embrittlement only for the 6061-T6 aluminum alloy. The result of embrittlement of the Ni-P plated 6061-T6 aluminum alloy corresponds to the highest amount of hydrogen desorbed below 240°C than the other alloys.

A novel analytical system was developed to monitor the environmental hydrogen embrittlement of aluminum alloys dynamically and quantitatively under atmospheric air pressure. Use of this system revealed that hydrogen atoms are generated during the plastic deformation of T6-tempered Al-Zn-Mg-based alloys caused by the chemical reaction between the water vapor in air and the alloy surface without oxide films. The low-purity Al-Zn-Mg-based alloy with high amounts of iron and silicon was less ductile and released more hydrogen gas in the SSRT than another one with low impurities deformed in a humid air environment.

High strength Al-Zn-Mg-Cu alloys are susceptible to hydrogen embrittlement based on intergranular cracking. In this study, the effect of retrogression and re-aging (RRA) treatment on hydrogen embrittlement sensitivity of an Al-5.3Zn-2.4Mg-1.5Cu-0.2Cr alloy was investigated by a slow strain rate tensile test in humid air (HA) and dry nitrogen gas (DNG). Grain boundary precipitates of heat-treated specimens were characterized by observing the intergranular fracture surfaces prepared using liquid gallium embrittlement. The elongation of T6-tempered specimen was smaller in HA than in DNG due to hydrogen-induced intergranular cracking. The hydrogen embrittlement resistance in RRA-treated specimens was improved greatly, while maintaining strength as compared to the T6-tempered specimen.

An increase in the volume fraction of pores in aluminum alloys causes a decrease in the elongation and the strength of alloys. To improve the mechanical properties of aluminum alloys, it is important to understand the growth and shrinkage behavior of pores. In this study, we analyzed the relationship between hydrogen desorption behavior and the growth/shrinking behavior of pores in A6061 alloys and pure aluminum using thermal desorption analysis and synchrotron radiation X-ray tomography. In pure aluminum, the fine pores began to annihilate at temperatures above 500°C and the relatively large pores coarsened. In contrast, the pores shrank with increasing temperature in A6061 alloy. The influence of second-phase particles has been discussed as a possible explanation for the difference in the nature of pores at elevated temperatures in pure aluminum and A6061 alloys. As in the A6061 alloy, much of the hydrogen desorbed from the pores due to heating is released externally from the second-phase particles on the aluminum surface, resulting in pore shrinkage due to the internal pressure drop of pores.

Tensile properties of 7000 series aluminum alloys at different strain rates were investigated to clarify the influence of internal hydrogen which is introduced during casting process. The yield and tensile strengths slightly increased with decreasing the strain rate in 7000 series alloys containing low and high hydrogen content (7000L and 7000H). The uniform and total elongations of 7000H decreased at low strain rates whereas those of 7000L were hardly changed with the strain rate. A lot of cracks were observed at the large precipitates consisting of Al-Zn-Mg-Cu with relatively high concentrations of Zn and Cu in both 7000H and 7000L after tensile test. The cumulative counts analyzed by acoustic emission of 7000H tensile-tested at low strain rate remarkably increased at the elastic region and in an early stage of plastic deformation tested at low strain rate. The decrease of the uniform and total elongations of 7000H containing high content of internal hydrogen tensile-tested at low strain rate might be attributed to the cracking at the large precipitates at the beginning of the tensile deformation and the acceleration of the crack initiation and propagation owing to the internal hydrogen.

Strain rate dependences on tensile properties of the Al-10%Mg alloys containing high (5000H) and low (5000L) internal hydrogen were investigated. The total and uniform elongations increased with decreasing strain rate although the changes of tensile and yield strengths were small in 5000L. On the other hand, the total and uniform elongations also increased with decreasing strain rate whereas the tensile strength slightly increased and yield strength were almost constant in 5000H. The total and uniform elongations of 5000L at low strain rate were high in comparison with those of 5000H. A lot of cracks were observed at the particles consisting of Al-Mg-Mn in both 5000H and 5000L after tensile test. A lot of acoustic emission events (counts) were detected during plastic deformation in 5000H in comparison with those in 5000L. The low total and uniform elongations of 5000H at low strain rate might be attributed to the promotions of void and crack initiation, growth and propagation owing to the acceleration of cracking of particles due to the internal hydrogen which is introduced during casting process.

During brazing of aluminum heat exchangers, flux is used to remove oxide film from filler surface, and to promote molten filler to wet and flow. Composition of brazing flux is eutectic mixture of KAlF₄ and K₃AlF₆. Flux starts to melt at 562°C during brazing, and the oxide film is removed mainly based on electro-chemical reaction between molten flux and substrate of filler beneath the oxide film. Flux also reacts with oxygen and water vapor in brazing atmosphere, and effectiveness in brazing is reduced. However, reaction between flux and water vapor during brazing is not clear, and there are almost no data on quantitative limit of dew point in brazing atmosphere under various heating rates. Here, we show detailed mechanism how the effectiveness of flux is reduced by reaction with water vapor. We found that KAlF₄ reacts with H₂O and becomes K₃AlF₆ under high dew point atmosphere, and eutectic temperature increased above brazing temperature resulting low or no effectiveness in brazing. Furthermore, we found that H₂O concentration which brazing becomes impossible is approximately one-tenth compared to that of O₂. Our results can be used to estimate condition of dew point and heating rate which the effectiveness of brazing could be reduced.

Responding to the Carbon Neutral requirement, Japan Aluminium Association set a target that aluminum circulation rate in wrought products should be raised to 50% by year of 2050 from the present rate of around 10%. When discussing and verifying measures for the improvement of circulation rate, the prospective amount of aluminum scraps is necessary information. However, there has been no statistics of aluminum scraps, and no reliable estimation of aluminum scraps has been available. Previous trials of aluminum scraps estimation in Japan were not highly complete since effects of imports and exports of manufactured products to scrap amounts were not considered. Therefore, firstly, we examined effects of imports and exports by a simple index. Secondly, we set up an estimation formula for aluminum scrap recovery amounts, which enables to explicitly reflect imports and exports information of manufactured products. In addition, we elaborated parameters in the formula including retirement rate distributions of manufactured products and accumulated broad related data. A reliable estimation system for aluminum scrap recovery amounts was developed. The system would provide a significant basis for policy discussions on aluminum resource circulation.