The Mechanical Properties And Precipitate Microstructures Of Of Al-Mg-Si Alloys Processed By Combined Deformation And Ageing

Compared with conventional metal materials such as iron and steel, aluminumalloy has the following advantages: high strength to weight ratio, light weight, goodelectrical and heat conductivity, high resistance of corrosion and easiness to recycle,therefore aluminum alloys are widely used across the world. With the flourishing ofmodern industries like machinery manufacturing (aerospace and automotive industry),construction, packageing and electronic technology, the consumption of aluminumand its alloys keep increasing, especially in manufacturing of transportation vehicles.Aluminum alloys play a crucial role in the development of the national economy, thusit has always been a primary concern from both academia and industry that producingAl alloys with high strength and good ductility by tuning the internal microstructures.The objective of present work is to achieve Al-Mg-Si alloys with excellentmechanical properties by defomation and ageing (controlling the deformationmicrostructures and solutes precipitation). By tuning the aggregation of alloyingelements before deformation using pre-ageing such as natural ageing or artificialageing as well as post-annealing tempers, a modified thermo-mechanical processingprocedure by combining deformation and ageing, referred to as M-TMP, was obtainedto improve the mechanical properties of Al-Mg-Si alloys. The mechanical propertiesand microstructure evolution during low temperature ageing plus the recovery andre-crystallization behavior during high-temperature post-annealing were investigatedby hardness test, tensile test, differential scanning calorimetry (DSC), scanningelectron microscopy (SEM), electron backscatter diffraction (EBSD), andtransmission electron microscopy (TEM).Three kinds of Al-Mg-Si alloys (Mg/Si=0.5,1,2, but with similar contents ofalloying elements) were chosen for this study. They were pre-treated by naturalageing and artificial ageing before subjecting to the deformation with the sameconditions. Then the following experiments were conducted: observation of theevolution of mechanical properties and microstructures in specimens post-aged atdifferent temperatures (70oC,120oC,150oC and180oC, respectively). In this paper,the conclusions are as follows:1. After appropriate pre-treatment, a significant enhancement of strength withoutcompromising in ductility compared with conventional T6temper was acquired onall three kinds of alloys and they both fractured in ductile manner. 2. The strength and elongation of the M-TMP alloy reaches a peak at the same timethen decline simultaneously when subjected to ageing at150oC. Degradation instrength is due to the decrease in the dislocation density and the coarsening ofprecipitates, and the reason for the decreasing of uniform elongation is earlieroccurrence of non-uniform deformation caused by coarsened precipitates, aroundwhich pre-mature cracks can easily form.3. The precipitation in the alloy processed by combining deformation and ageing islattice-defects mediated. In contrast to the homogeneous dispersion of needle-likeβ″precipitates in T6aged sample wall-like precipitates, which are merely1-2nmin width with the cross-section elongated along <001>Aldirection or <011>Aldirection, form. In addition, a novel type of precipitate possessing a monocliniclattice with parameters a=0.81nm c=0.89nm β=130°and a habit plane (110)Alisidentified.4. The mutual interaction of various types of defects and solutes during the postageing results in a concurrent process involving recovery, partial recrystallizationand precipitation. Therefore, sub-grains, lamellar structures, nano-grains, cellularnetworks, precipitates and dislocations finally constitute a hierarchicalnanostructure in the alloy processed by combining deformation and ageing. Thepost ageing temper is critical in tuning the hierarchical nanostructures which playan essential role to achieve a simultaneous improvement in strength and ductility.5. The precipitation sequence in the Al-Mg-Si alloy processed by combiningdeformation and ageing was determined as shown below:High energy deformed state�'dislocation induced wall-like precipitates,needle-like precipitates and pre-B′�'U2�'β′, B′.6. The DSC curve of M-TMP alloy is totally different with the as-quenched one inconventional T6treatment. The formation of dislocation induced precipitates(wall-like series、needle-like and Pre-B′) which is related to Peak1locating at150oC contributes to the highest strength.