Al–Mg–Si alloys for autobody sheet have a paint bake hardenability, but at present, the paint bake temperature is so low that the paint bake hardenability is insufficient although the alloys have a hardening potential. Weight reduction and dent resistance will be improved by promoting paint bake hardenability. After water quenching, natural aging, reversion treatment, natural aging as a storage and paint bake cycle were carried out. The effects of natural aging and reversion treatment condition on the paint bake hardenability of the Al–1.2Si–0.5Mg alloy were investigated. The conclusions were as follows. 1) The paint bake hardenability improved by reversion treatment. The reason may be that with GP zones being thermally decomposed by reversion treatment, some clusters formed and inhibit the formation of new GP zones. 2) The reversion efifect (decomposition of GP zones) depends on natural aging temperature. High natural aging temperature was effective to improve paint bake hardenability.

View full abstract

Electrochemical investigation and metallurgical observation have been made to compare between two aluminum alloys for beverage can containers: one is alloy 3004 in the Japanese market and the other is used aluninum alloy (compositions adjusted), in a deaerated 500 ppm NaCl solution containing 0.1 g/l citric acid. It is found that the pitting attack resistance of used aluminum alloy shows less corrosion performance in comparison with the aluminum alloy 3004: the anodic polarization shows almost same behavior, but the cathodic polarization behavior of used aluminum alloy shows detrimental tendency to the pitting attack.

View full abstract

The effect of microstructural features on the fatigue crack initiation and propagation behaviors of Al–Si–Cu system casting alloys with different microstructures by changing Si and Ca contents and mold temperature (cooling rate) was investigated. The fatigue strength is strongly dependent on the sizes of casting defects which become crack initiation site. Data of S−N curves were converted into the ΔK₀–N_f √area relationship to compare the fatigue characteristic where ΔK₀, N_f and area are the range of initial cyclic stress intensity factor, number of cycles to fracture and size of casting defects on the fracture surface of fatigue specimen, respectively, assuming that the crack propagation rate follows the Paris law. The fatigue strength increases in proportional to Si content, which was attributed to small size of casting defect and reduced crack propagation rate. Crack propagation rate of the alloys with coarse eutectic Si particles increases due to the high stress concentration effect of the particles, while these coarse particles also play a role in decreasing the crack propagation rate by crack deflection and branching. Due to the two competitive effects, the macroscopic crack propagation rate is not much dependent on the size of eutectic Si particles.

View full abstract

TiB₂ dispersed aluminum alloy was synthesized by the reaction. Starting materials used in this experiment were titanium, AlB₁₂ and AlB₂. The heat of reaction between titanium and AlB₁₂ was too high to maintain the original configuration of the powder compact. Thermodynamical calculation reveales that the adiabatic temperature of the reaction between titanium and AlB₁₂ exceeds the boiling temperature of aluminum. Titanium and aluminum were added in the [Ti, AlB₁₂] powder mixture to control the adiabatic temperature of the reaction. The addition of both titanium and aluminum can successfully control the adiabatic temperature. The average size of the TiB₂ particles was strongly depends on the amount of titanium added as the diluent of the heat of reaction. As the amount of titanium addition in powder phase increases, the average size of TiB₂ particles, synthesized by the reaction, decreases. Differential thermal analysis (DTA) data reveales that the reaction between titanium and AlB₂ initiates at around 1280 K (the decomposing temperature of AlB₂) whereas the reaction between titanium and AlB₁₂ does not occur at temperatures below 1473 K. The addition of aluminum powder in powder mixture of [Ti, AlB₁₂] system, however, contributes to decreasing the initiation temperature of the reaction.

View full abstract

Effect of precipitate state of Mg₂Si as well as Si content in Al–Mg₂Si alloys on the reduction of a die-line defect on the surface of extruded products was investigated in industrial scale. The solid product was extruded by 1500 us ton direct extrusion press at billet temperature 460°C, ram speed 9~13 mm/s and extrusion ratio 66.5. When Si content is low and a large amount of coarsened Mg₂Si is present in the alloy after homogenization at 400°C for 2 h, the extruded samples have bright smooth surface and nonbright rough surface. When a microscopic Mg₂Si is present in the alloy after homogenization at 200°C for 2 h, the extruded samples have high-quality surface with only nonbright and uniform surface roughness. When Si content is high and a large amount of coarsened Mg₂Si is present in the alloy after homogenization at 400°C for 2 h, the extruded samples have poor-quality surface with a tearing defect. However, when a microscop ic Mg₂Si is present in the alloy, which is caused by a widely used homogenizing method or after homogenization at 200°C for 2 h, the samples extruded at ram speed 9~13 mm/s have high-quality surface with only nonbright and uniform surface roughness.

View full abstract