To improve the adhesion and corrosion resistance of aluminum, the dual-layer anodic oxide film was formed by sequential anodizing in phosphoric acid (PA) and oxalic acid (OA) electrolytes. The upper-layer anodic oxide film, with large pore diameter and thick barrier layer, was obtained by applying a high voltage in PA electrolyte. While the large pore diameter could improve adhesion, the thick barrier layer hindered the uniform growth of the lower-layer anodic oxide film formed by anodizing in OA electrolyte. To achieve a uniform growth of the lower-layer film, the barrier layer of the upper-layer film was thinned using the current recovery method. Consequently, pores grew continuously at the interface between the upper-layer and lower-layer films, resulting in a uniform film thick-ness of the dual-layer film. When the dual-layer film was subjected to hot water sealing, few flaky hydrates formed in the upper-layer film and the pores remained open at the surface, whereas many granular hydrates formed in the lower-layer film. Thus, the dual-layer film could improve both adhesion and corrosion resistance of aluminum.

Although cast materials based on Al-7%Si-0.3%Mg alloy are well-known commercial materials that have been widely used, the details of the precipitation that occur during aging treatment, which is used to improve strength, are not well understood. As reported in the previous paper, when Al-7%Si-0.3%Mg alloy is directly aged after casting, a distribution of fine precipitates is observed near the eutectic Si in the sample at the peak of aging, and a structure in which coarse Type-A precipitates and granular Si phases are mixed in the α phase away from the eutectic is formed. In order to clarify the mechanism, the cast Al-0.5Mg₂Si alloy was processed and subjected to solution treatment to eliminate the effect of segregation during casting as much as possible and to eliminate the effect of the eutectic interface on the precipitation. In both the wrought and cast materials, the hardness increased with aging time, reached a peak, and then decreased, showing typical age-hardening behavior. In both the wrought and cast materials, the peak-aged samples at 473 K showed a mixture of areas containing coarse β' metastable phases and areas containing relatively fine β' metastable phases.

To clarify the color tone change and blackening due to oxidation at high temperatures on Mg-containing aluminum, several investigations were conducted on AC4B and Al-3mass%Mg alloys. The AC4B and Al-3Mg alloys were polished into mirror-like surfaces and used as specimens. The dripping of Ca ions and the local melting of AC4B were investigated using a digital microscope, SEM, EPMA, XRD, XPS, DTA, and X-ray CT. To examine the effect of Ca ion drips, CaCl₂ solution was dripped onto the specimens and heat-treated at 500°C for 4 hours. Regarding local melting, AC4B was heat-treated at 550°C and analyzed using X-ray CT to survey internal voids. Additionally, TG-DTA measurements were conducted to determine the melting start temperature of AC4B. As a result, the surface oxide scale is composed of MgO and CaCO₃, and voids are formed by local melting; thus, the surface becomes rough with Ca ions dripping. However, these factors are not essential to blackening. Additionally, the color tone change was investigated under the heat treatment at 500°C. The result showed that the surface color settles at a constant value over a long period, and the blackened Mg oxide formed on the Al-3Mg alloy contains metallic aluminum. Therefore, metallic aluminum-containing MgO affects the blackening of Mg-containing aluminum alloys.

To fabricate multi-materials joining between aluminum alloy and engineering plastics for automotive weight reduction, we have developed a two-step anodizing process to improve adhesion and corrosion resistance for 5052 aluminum alloy. The two-step anodizing process consists of phosphoric acid anodizing (PA) followed by sulfuric acid anodizing (SA), with the PA film on the upper layer and the SA film on the lower layer. In this study, the effect of SA anodizing time on adhesion and corrosion resistance was evaluated for the two-step anodizing process on 5052 aluminum alloy. Adhesion was around 17 MPa even when the SA anodizing time was varied. On the other hand, anodic polarization measurements showed that increasing the SA anodizing time resulted in a decrease in corrosion current density and a positive shift in corrosion potential, indicating improved corrosion resistance. In the two-step anodizing process, the morphology and chemical state of the PA film was important for adhesion, and the thickness of the SA film was important for corrosion resistance.

In this study, the effect of heat treatment at 300°C (for residual stress relief) on the mechanical properties of Al-2.5%Fe-2%Mn alloy manufactured by laser powder bed fusion (L-PBF) was investigated. Hardness measurements showed that the heat-treated sample (HT sample) exhibited comparable hardness to the as-built one (AS sample). Exposure tests at 300°C for up to 1008 hours revealed no significant change in hardness within the first 100 hours, but a decline was observed beyond this period, consistent with the AS material's behavior. Tensile tests conducted at room temperature (RT), 200°C, and 300°C demonstrated that the HT sample maintained strength and ductility comparable to literature values. Fracture surface observations revealed dimple patterns and flat regions corresponding to melt pool boundaries, indicating fracture along these boundaries. The HT sample exhibited similar fracture modes and mechanical properties to the AS sample, suggesting that the residual stress relief heat treatment does not compromise the alloy's performance. These findings indicate that the heat-treated Al-2.5%Fe-2%Mn alloy retains sufficient mechanical properties for industrial applications, making residual stress relief heat treatment a viable option for AM-fabricated components.