The effects of aging and annealing conditions on the 0.2% proof stress of A3004-H112 aluminum alloy were investigated, in order to develop a method for evaluation of design strength taking account of the strength change caused by long-term service periods at elevated temperatures, in particular at approximately 200°C. It was confirmed that a decrease of 0.2% proof stress of the alloy during the aging and annealing was attributed largely to recovery of strain hardening initially existed in the as-extrusion state. It was also found that while content of supersaturated Mn varied depending on heat treatment conditions, its effect on strength was not dominant. It is suggested that in the case of aging treatment, holding at 250°C or 300°C for over 10³ h is necessary to obtain strength low enough at each holding temperature. Annealing treatments at 520°C for 10 h followed by a cooling rate of 0.5°C/h or smaller is also effective to reach sufficiently decreased 0.2% proof stress, by removing non-equilibrium strengthening factors such as strain hardening and supersaturated Mn solid solution. The aged and annealed A3004-H112 alloy specimens exhibited a similar 0.2% proof stress value when the conditions are selected as above.

In hot extrusion, tearing plays an important role on surface quality of profiles. In this study, extrudability of 7075 aluminum alloy without tearing was quantified by extrusion limit diagram, which is a relationship between temperatures and speeds. Tearing appearance and distribution of elements were investigated to assess tearing mechanism. According to the diagram, tearing easily occurrs at high temperature and high speed conditions. Micro-solid bridge was observed at tearing surface to indicate the localization of melting of soluble Al₂CuMg and MgZn₂ compounds because temperature increased by friction coupled with tension stress at a die region. During hot extrusion, recrystallization affects the grain growth, intermetallic compounds were formed at grain boundaries reducing bonding strength between grains promoting tearing sensitivity. Zinc and magnesium concentration is high at the vicinity of grain boundaries because of diffusion into aluminum-matrix whereas insoluble Al₇Cu₂Fe particles were large and remained at grain boundaries resulting in tearing. Grain refinement can decrease a size of insoluble compounds and promote scattering of soluble compounds. Therefore, to reduce tearing sensitivity, grain refinement has to be controlled during hot extrusion process by controlling chemical composition of billet and friction at bearing region. This causes temperature to increases and thus, induce recrystallization.

The Al–Mg–Si alloys with excess Si without copper addition have recently been found to suffer from intergranular cracking. In the present study, we have investigated the effects of microstructure adjacent to grain boundaries on the intergranular cracking. It was confirmed that the alloy with excess Si without Cu addition had lower elongation to failure caused by higher fraction of intergranular fracture surface consisting of many dimples than balanced alloy and alloy with excess Si and Cu. Fractographic observations on the pair of fractured test pieces revealed that a grain boundary precipitate was present at the bottom of a dimple in either pieces. In the alloy with excess Si without Cu, difference in the number of grain boundary precipitates was found within the pair of the pieces, while equal number of precipitates were found in the pair in the other two alloys. Both the size of the precipitates and the amount of precipitation were larger in the alloy with excess Si without Cu than the other two alloys. The larger amount of precipitation meant the larger width of PFZ in the alloy. Thus, the larger fraction of intergranular fracture was closely correlated with the larger size of grain boundary precipitates and wider PFZ.

Since β-AlFeSi intermetallic compound particles formed during solidification of Al–Si–Fe alloys are brittle, large β-AlFeSi particles will decrease the mechanical properties of the as-cast alloys. In this study, fragmentations of β-AlFeSi particles in Al–7 mass%Si–1 mass%Fe alloy are carried out by ECAP (Equal-Channel Angular Pressing), MDF (Multi-Directional Forging), symmetric rolling and asymmetric rolling. It is found that the β-AlFeSi particles in the Al–7 mass%Si–1 mass%Fe alloy can be fragmented by the above methods.