Materials Science Forum Vols. 519-521

Abstract: The effect of heat treatment conditions such as atmosphere, temperature, annealing time and alloying elements on the hydrogen content in Al-Mg based alloys was investigated. The hydrogen content after annealing depends on the annealing atmospheres and alloying elements. When annealed in a wet atmosphere, the release of the hydrogen in the Al-Mg alloys to outside and the absorption of hydrogen from atmosphere into Al-Mg alloys are presumed to occur at the same time. The oxide layer on the surface is revealed to prevent the hydrogen from being released to outside. The spheroidal MgO particles can be seen on the surface of Al-Mg alloys after annealing at 550°C. The number of the MgO particles increases with increasing impurity elements such as Si and Fe, reducing the shielding effect against hydrogen permeration. Therefore, the condensation of hydrogen near the surface after annealing occurs more easily in an Al-Mg alloy of a high-purity than that of an ordinary purity.

Abstract: The work hardening of alloys hardened by precipitate heat treatments depends on the distribution of the precipitate sizes and the solute level left in the metal matrix. A mean field theory for precipitation is first applied for the ageing and subsequently it is coupled to a work hardening model to study the stress-strain responses of age hardened conditions of AA6xxx alloys. The predictions are compared to mechanical experiments and to TEM characterisations.

Abstract: The application of 6000 series alloys is widespread and of particular importance to the automotive sector. Their functionality depends on the detailed behaviour of the strengthening phases. In this study, transmission electron microscopy (TEM) supplemented with a variety of mechanical tests were used to examine the precipitates and their role in aspects such as the Bauschinger effect, damage and fracture events, and in recovery and recrystallization processes.

Abstract: Thixoforming offers the possibility of forming complex aluminum parts with a superior quality and a reduction of processing steps. The production of a fine, equiaxed, globular microstructure is essential for thixoforming. Strain Induced Melt Activation (SIMA) and the Cooling Slope (CS) casting processes were employed to produce AA6082 thixotropic feedstock in the present work.. SIMA process produces such a microstructure through recrystallization of heavily deformed billets and a subsequent heat treatment in the mushy zone. Molten metal with a suitable superheat is cast over a water-cooled, inclined metal plate into a permanent mould to produce the thixotropic billet in the CS casting route. The effect of cold work and heating temperature and time were investigated for the SIMA process where as the effect of CS length, casting temperature on the final microstructures were investigated for a fixed set of heating conditions in the CS casting route.

Abstract: The hot deformation behaviors and microstructure in Al-Cu-Li alloy containing small amount of Ag and Mg were investigated by transmission electron microscopy and isothermal compression tests.When the strain rate is 0.1, 0.01 and 0.001s-1(the deformation temperature within the range of 360-520􀀀 ) and 1 s-1(deformation temperature 520 􀀀 )respectively, the flow stress decreases after a peak value, showing dynamic recrystallization,while the steady-state flow characteristics exist on the other deformation conditions. The flow stress of Al-Cu-Li-Mg-Ag alloy during hot deformation can be expressed by a Zener-Hollomon parameter in the hyperbolic-sine function with the hot deformation activation energyDH of 250.45kJ/mol. The dislocations may climb with support from many vacancies generated during hot deformation, thus forming lots of helical dislocations. The dynamic precipitation and successive dynamic particles coarsening during hot compression have been assumed to be responsible for further flow softening when deformed at low strain rates.

Abstract: The effects of trace Ce, Ag on the microstructure and mechanical properties of Al-Cu-Li alloy after thermal exposure have been investigated. It’s found that the addition of Ce may lead to a slight increase in mechanical properties after thermal exposure at 107  . The stability of T1 phase is enhanced by independent Ag addition and the combined additions of Ag and Ce,which results in higher strength compared with Ag-free alloy at 150  thermal exposure. However, at 200  exposure, a great number of q¢ precipitates at the expense of T1 may be responsible for higher tensile strength in the Ag-free alloy than that of the independent addition of Ag and combined additions of Ag and Ce alloys.