The strengthening of aluminium 2024 sheet by accelerated aging / Syafruddin Noer

This study is based on daily experience in the Aerospace Industries. Aircraft structures are mostly made from Aluminium Alloys, especially the 2024 alloy. Strengthening of this material to achieve the T42 condition is by heat treating and quenching (solution treatment) after which it is left at room temperature (natural aging) for 96 hours approximately. This is time consuming and will adversely affect productivity. Precipitation hardening of aluminium and aluminium alloys occurs either at room temperature (natural aging) or at elevated temperature (artificial aging or precipitation treatment). An attempt was made to accelerate this aging process and examine the properties of the material when introduced at above room temperature (40°C, 50°C, 60°C, 70°C, 80°C and 90°C) but below the normal precipitation treatment temperature (120°C) after being solution heat treated. The material selected for this study was aluminium 2024 sheet with 1.6 mm thickness, manufactured by ALCOA. Microstructures of aged materials were difficult to differentiate between natural and artificial aged, the grains were almost equiaxed which consist of rounded Al2CuMg as an undissolved excess phase , irregularly shaped particles of unreacted (Mn,Fe)3SiAI12 and reaction product Al7Cu2Fe, along with fine dispersoid of Cu2Mn3Al20. Aging at elevated temperatures (artificial aging), the ultimate strength tended to decrease by increasing the aging temperature. Although the strength was low at temperature 40°C but the minimum acceptance criteria for T42 condition has been achieved within 24 hours of aging time. The highest ultimate strength was achieved by aging at temperature 50°C and the lowest strength was by aging at temperature 90°C. One of the factor caused this phenomenon was by exposure to temperature whereby a number of microstructural changes were introduced including coarsening of the matrix precipitate s and grain boundary precipitation or development of a precipitate free - zone. Conditioning this material above room temperature showed that strengthening process is successfully done and aging time to achieve T42 condition was decreased by increasing the ambient temperature and the optimum ranges of accelerating aging temperatures are 40, 50 and 60°C. The ability to accelerate this aging process will be of great benefit to processors (manufactures) particularly where time is concerned.