Effect Of Oxides On Microstructure And Properties Of Aluminum And Aluminum Alloys

The extensive application of oxides in aluminium matrix composites enhances the mechanical properties of the composites and enlarges the application scope of aluminium alloys,which also reflects the great influence of oxides on the structure and properties of aluminium and aluminium alloys.The application of oxides as modifiers in the production of aluminium and aluminium alloys has always been a blind spot for scholars at home and abroad.In order to explore the effect of oxides on the structure and properties of aluminium and aluminium alloys and to find out the mechanism of the role of oxides in aluminium and aluminium alloys,two kinds of modifiers of oxide systems are used to modify pure aluminium and 8079 aluminium alloys.And optical microscopy（OM）,X-ray diffraction（XRD）,scanning electron microscopy（SEM）,energy dispersion spectrum（EDS）and hardness tester are used to systematically explore the mechanism of modification of oxides as modifiers.MgO-MgAl₂O₄ modifier is prepared by calcining Al-MgO powders mixed by ball milling in a muffle furnace at 1000℃for 3 hours.The effect of the modifier on the structure and properties of 8079 aluminium alloy is studied.The results show that:（1）MgO-MgAl₂O₄ powder modifier can refine the structure and grain of 8079aluminum alloy.When the addition content is 0.05wt%,the dendrite disappears and transforms into equiaxed structure.The average grain size is about 30um.（2）MgO-MgAl₂O₄ modifier can improve the morphology and size of needle-like second phase in 8079 aluminium alloy.When the content of MgO-MgAl₂O₄ is 0.1wt%,the needle-like second phase of Al₃Fe is transformed into granular phase with a size of about 3 um.（3）After modification,the hardness of 8079 aluminium alloy increased by21%.The analysis shows that the crystal structure of MgAl₂O₄ is the same as that ofα-Al,and the lattice mismatch is small,so it can be used as the nucleation core of 8079aluminium alloy heterogeneous nucleation,which refines the matrix structure of 8079aluminium alloy;and the addition of Mg oxide can increase the viscosity of aluminium alloy melt,making it more difficult for Fe atoms which are difficult to diffuse in 8079aluminium alloy melt to diffuse and reducing the segregation of the second phase at grain boundaries.Al-Ti-MgAl₂O₄ master alloys are prepared by direct melt reaction method with TiO₂ powder and Al-Mg alloy as raw materials（holding for 3 hours at 950℃）.And Al-Ti-MgAl₂O₄ master alloys are added to pure aluminium and 8079 aluminium alloys to study their refinement effects.Analysis from EDS and XRD show that Al-Ti-MgAl₂O₄ master alloys mainly contain Al₃Ti phase and MgAl₂O₄ phase.which can be observed by optical microscope to cluster and distribute in the master alloys.With adding 3wt%master alloy into pure aluminium,microstructure of 8079Aluminum Alloy of pure aluminium changes from large-size columnar grain to fine equiaxed grain.The average grain size decreases from 970 um to 37 um,and the hardness of the alloy increases by 23.7%.When 3wt%master alloy is added to 8079aluminum alloy,the grain size decreases from 186 um to 29 um,and the hardness of the alloy increases by 35%.Further analysis shows that the fine refining effect of Al-Ti-MgAl₂O₄ master alloys is due to the potential of both Al₃Ti and MgAl₂O₄ phases as nucleation cores ofα-Al.And MgAl₂O₄ phase（111）[110]is parallel to Al₃Ti phase（112）[20-1]and the lattice mismatch is very small.In aluminium melt,The nucleation of Al₃Ti phase is based on MgAl₂O₄ phase,while the nucleation ofα-Al phase is based on Al3Ti.This nucleation method improves the nucleation rate of aluminium and aluminium alloys.Therefore,Al-Ti-MgAl₂O₄ master alloy prepared in this paper is a very effective grain refiner forα-Al.