The Effect of Pre-deformation on Precipitation and Mechanical Properties during Aging of Al-Mg-Si Alloys: An Experimental Study
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The present work reports on the effect of small deformations on the natural and artificial aging behaviour of two industrial aluminium alloys. The main focus has been on the mechanical properties response of these alloys, supplemented with microstructural and thermoanalytical investigations. The investigated alloys were AA6060 and AA6082 which both represent high volume production in the extrusion of alloys in Al-Mg-Si system. The objective of this work has been to get a better understanding and quantitative description of the aging behaviour in the investigated alloys as a function of processing conditions. The amount of introduced deformation and storage at room temperature prior to artificial aging have a significant importance in industrial practice and they are often unavoidable in the manufacturing processing line. Therefore, the effect of predeformation followed by natural aging on the mechanical properties as strength, ductility and work hardening of an AA6060 and AA6082 alloy has been thoroughly investigated by tensile testing. It was found that in samples with no pre-deformation the hardening after natural aging is achieved mostly due to a higher density of Mg-Si coclusters while in samples with introduced pre-deformation the hardening is due to the increased dislocation density. In the latter case, the precipitation hardening is suppressed, most probably due to loss of quenched-in vacancies to dislocations sinks. In order to investigate the effect of pre-deformation on the artificial aging response of the AA6060 alloy, a series of hardness measurements and tensile tests were designed and carried out. In addition to the mechanical properties tests, transmission electron microscopy and differential scanning calorimetry were used to characterize dislocation evolution, microstructure and precipitation state for various combinations of predeformation and aging time. The artificial aging part of the investigations individually considered two different routes of applying the pre-deformation, i.e., sequential mode and simultaneous mode. It was found that for the AA6060 alloy processed in sequential mode of predeformation and artificial aging, increasing pre-deformation accelerates the kinetics of the precipitation sequence, increases both yield strength and ultimate tensile strength, and changes the nucleation mechanism for precipitation from homogeneous nucleation, as observed in undeformed material, to heterogeneous nucleation. Both nucleation mechanisms, homogeneous and heterogeneous, were supposedly taking part during simultaneous mode of pre-deformation and artificial aging of AA6060, which resulted in the best performing alloys with respect to yield strength and ultimate tensile strength among all the investigated alloy conditions. It was shown, that these superior final mechanical properties depend on a careful configuration of aging parameters (aging temperature, aging time, and strain rate) and chosen amount of predeformation. Further improvement in mechanical properties has been achieved with use of split aging procedure, when an additional aging after simultaneous mode of aging was performed in the way that after reaching the desired pre-deformation, the specimen was kept at the temperature with no prior change in temperature and aged for an additional time Based on the work of this thesis, it may be concluded that small deformations prior to or together with artificial aging generally leads to improved strength of the investigated Al-Mg-Si alloys due to the combined effects of strain and precipitation hardening, and that this fact should be taken into account when designing the processing routes of extruded products of these materials.