The Effects Of Heat Treatment On The Microstructures And Properties Of Al-Si-(Mg) Alloys

Al-Si-(Mg) alloys are extensively in many industries, for instance, automotiveand aerospace, for their excellent castability, high strength-to-density ratio andcorrosion resistance. These Castings are generally heat treated to obtain a desiredcombination of strength and ductility. In this work, the effect of heat treatment on themicrostructure developments and mechanical properties of Al-Si-(Mg) alloys isinvesgigated. The first part (Chapter3) is about the microstructure evolution duringheat treatment of an Al-7Si Alloy. The second part (Chapter4) is to study the effect ofsolution treatment on the age hardening of Al-7Si-0.3Mg (A356) alloy. The third part(Chapter5) is to observe the evolution concerning solidified eutectic microstructureduring solution treatment of an Al-12Si-0.8Mg alloy semi-continuous casting. Thelast part (Chapter6) is to investigate the natural aging (NA) and its effect on thesubsequent artificial aging (AA) in Al-12Si-0.8Mg alloy. The conlusions aresummarized as follows:(1) Eutectic silicon particles of branch-like morphology become spheroidizedand coarsened to some extent with the extending of solution treatment time in anAl-7Si alloy. A certain amount of Si atoms dissolved in the Al matrix would aggregate,nucleate and grow into silicon precipitates in the matrix, and at interfaces as wellduring artificial aging. Silicon precipitates in the matrix lies on the {111} planes ofthe Al matrix. Silicon precipitates on the eutectic silicon particles grow larger withaging time. Interestingly, a large number of twins and other crystal defects arepresented in the nano-sized silicon precipitates on the eutectic silicon particles. Inaddition to ordinary Si Σ3(111) twins, some multiple twins such as five-fold twinsalso exist.(2) Prolonged solution treatment has a detrimental influence on the agehardening due to the ubiquity of porosity in the cast A356alloy materials. Besidesspheroidizing of the eutectic Si particles, the enrichment of Mg at the interfacebetween porosity and Al matrix was observed using elemental line-scanning analysiscarried out by energy dispersive X-ray spectrometer. Magnesium oxide nanoscaleparticles were found in the sample solution treated with an extended duration. Ourresults indicate that Mg would diffuse toward the porosity pre-existed in the A356alloy during solution. This phenomenon was consistent with the degraded aging kinetics and hardening potential of the A356alloys treated with a long solution time,as there was a concomitant reduction of Mg solutes used for formingMg-Si-containing strengthening precipitates, mainly monoclinic β" phase, in thematrix during the following artificial aging.(3) In Al-12Si-0.8Mg semi-continuous castings, crystallographic orientationsof Si fibers and surrounding aluminum grains in eutectic are identical, while nospecific orientations with the primary aluminum dendrites exist. These results indicatethat the eutectics nucleate heterogeneously in the interdendritic liquid and areindependent of the primary dendrites. After solution treatment, eutectic Al grains withsmall misorientations align to the same orientation, thus the number of eutectic Algrains decrease and the sizes of them increase. In addition, by the misorientationanalysis, twinning relations have been frequently found in the same silicon particle, aswell as between the silicon particles and the eutectic aluminum grains that departthem.(4) Precipitates formed in Al-12Si-0.8Mg alloy peak-aged at180C are pre-β"phases. But the precipitates in the over-aged samples are mainly coarsened β" phases,and a small portion of U2, B′and Si-rich phases. The NA causes a sluggishage-hardening response during subsequent AA within30min, while afterwards theage-hardening curves resemble the directly aged one. Transmission electronmicroscopy observations reveal that β" precipitates in AA samples with prior NA arelarger but with similar narrow particle size distributions, compared with sampleswithout NA. The abundant loose Al/Si-particle interfaces may act as sinks forquenched-in vacancies during NA. For lack of vacancies, NA clusters can not act asnuclei of β". As a result, the preferential growth of precipitates and the broadening ofthe particle size distribution in ensuing AA are prevented.