M.Tech (Nanotechnology), B.Tech (Electronics and Communication Engg)
Structure-Property Correlations of Artificially Aged AA6063 Al-Mg-Si Alloy
This investigation aims to achieve a comprehensive understanding on the structure-property correlations of a 6xxx Al-Mg-Si alloy subjected to various artificial ageing treatments. The selected alloy, namely AA6063 which is Cu-free and contains minor amounts (< 1wt.%) of inexpensive solute (Mg and Si) elements, has received renewed attention to meet the ever growing industrial demands of high-performance, light-weight and low-cost materials. Specimens of Al-Mg-Si alloy have been subjected to over ninety different ageing treatments at various combinations of temperature (373-523 K) and time (0.0083-1440 h). Measurements of mechanical properties include hardness, tensile, low-cycle fatigue and high-stress abrasive wear apart from the susceptibility to intergranular and pitting corrosion. These are supplemented by microstructural characterizations and critical examinations of fractured, worn and corroded surfaces by employing optical, FESEM, TEM, XRD, SAD, EDS and 3D profilometry techniques. The generated results are utilized to verify the models based on fundamental strengthening mechanisms to simulate the yield strength, as well as to develop simple hardness-based models capable to determine the state of ageing and to predict the yield strength.
The major findings of the present study are: (i) The selected Al-(0.5 wt.%) Mg - (0.43 wt.%) Si alloy exhibits excellent age-hardening response resulting wide range of mechanical properties like hardness (44-110 VHN), yield strength (67-260 MPa) and total elongation (35-14 %) as it follows the precipitation sequence of GP zone, β'' (Mg5Si6), β' (Mg1.8Si) and β (Mg2Si). (ii) The experimental results are utilized to relate hardness with yield strength following modified Tabor's equation, while Johnson–Mehl–Avarami and Lifshitz–Slyozov–Wagner equations are considered to predict kinetics of under- and over-ageing regimes, respectively. The developed hardness-based models satisfactorily describe the entire ageing kinetics and predict the yield strength of 6063 alloy within ±15%. (iii) It is established that when nucleation-growth-coarsening phenomena are incorporated in the simulation scheme, the classical dislocation-particle interaction model provides a closer prediction of yield strength over the entire period of ageing as compared to the modified Orowan model, since the former model takes into account of both cutting and by-pass mechanisms. (iv) Cyclic deformation behavior of the age-hardenable Al-alloys varies markedly with microstructure as determined by the state of the ageing. For under-aged alloy, cyclic deformation induces dynamic precipitation even at room temperature resulting significant monotonic hardening till failure. In contrast, over-aged alloy undergoes hardening for few initial cycles followed by saturation of stress due to the formation and annihilation of the Orowan loops around the β precipitates. Shearing of β'' precipitate causes cyclic softening for the peak-aged alloy. (v) High-stress abrasive wear characteristics of the differently aged alloys have been studied and correlated with hardness and tensile properties. The wear rate is found to follow linear relation with the strain hardening exponent, and inverse linear relationships with hardness, yield strength, tensile strength and strength coefficient. (vi) The progress of artificial ageing enhances the susceptibility to intergranular corrosion, and deteriorates the resistance to pitting corrosion in alkaline and acidic media. Mechanisms and modes of corrosion are identified and discussed with the state of ageing.
Finally, structure-property correlations for the alloy are established via construction of various 3D contour maps; each map deals with a particular property and is plotted against the artificial temperature and time of ageing. The developed maps are expected to provide appropriate guidelines for the selection of artificial ageing parameters to achieve a given set of mechanical property.