Study Of Power Ultrasonic And Solute Synergistic Effect On The Microstructure Of Aluminum And Its Alloys

Pure aluminum is characterized by low density,low melting point,high plasticity,good electrical and thermal conductivity and high reflectivity. But its low strength and hardness make it impossible to be raw material for load-carrying mechanical parts. A series of aluminum alloy based on aluminum has its own respective characteristics and because of its outstanding properties of high strength,corrosion resistance,easy recovery etc.,the aluminum alloy has a great application in the field of machinery,transportation,aviation and military,and it occupies an increasingly important role in national defense and people’s life. Because the grain size and organization form of Al alloy have a direct influence on its performance,the key to obtain good performance is to break the original large dendritic crystal into tiny uniform equiaxed grains. Ultrasonic wave produces many special effects when spreading in liquid,such as cavitation,acoustic flow,mechanical and so on through which we can degas,deslag and reduce the composition segregation, refine grain. Therefore the study of ultrasonic-solute treatment’s influence on the microstructure of aluminum and its alloy has great theoretical significance and practical value.The effect and mechanism of ultrasonic and solutes on the microstructure of Al alloys when growth restriction factor,i.e. Q changes were investigated in this article by adding seven solutes（Cu,Mg,Si,Ti,Nb,Zr,V） into the aluminum melt under two contrary conditions-with ultrasonic vibration treatment and without ultrasonic vibration for comparison through optical microscopy, scanning electron microscopy, energy dispersive spectrometer analysis and x-ray diffraction. The following main innovative conclusions have been obtained:When we do ultrasonic vibration treatment to aluminum alloy ingot,isometric crystal area appears in the interior of the ingot and increases with the increasing of alloy composition and the grain refinement effect is significant. Compared with the eutecticforming solutes（Cu,Mg,Si）,the microstructure of adding of peritectic-forming solutes（Ti,Nb,Zr,V） is more uniform and tinier.When Ultrasonic and solute synergistic effect,ultrasonic treatment just change the morphology of the second phase but not the type of that. However,due to the cavitation and acoustic streaming effect of the ultrasonic,the liquid metal flows more rapidly and then appears great instantaneous pressure when the bubble ruptures, the secondary dendrite arm spacing and the second phase are refined,partial grains transform from dendrite to crystal axis,and in a certain range, with the increasing of solute elements,grain refinement effect becomes more obvious.The nucleation ability of ultrasonic-peritectic solutes is stronger than that of ultrasonic-eutectic solutes. The refinement mechanism of the grain size is in the same way with eutectic-forming solutes when the content of peritectic-forming solutes is less than the maximum solubility Cm,the grain size decreases with the increasing of peritectic-forming alloying elements,and it is approximately linear with 1/Q. The grain size increases with the increasing of eutectic-forming alloying elements when the the content of eutectic-forming solutes is more than the maximum solubility Cm. But when the content of peritectic-forming solutes is more than the maximum solubility Cm,the Q value stays constant and the grain size keeps decreasing i.e. the grain size has nothing to Q value and is not linear with 1/Q. Part of the test results are iisted as follows:under ultrasonic condition,the grain size reduces from about 260μm to 75μm when the content of the peritectic-forming solute Ti（Cm=0.15%Ti） increase gradually. When the content of the peritectic-forming solute Zr（Cm=0.11%Zr） is 0.1wt.%,the grain size is 240μm. The grain size decreases to 125μm when the content is 0.2wt.%. And the grain size decreases continuously to 116.5 μm when the content is 0.3wt.%.It is found that the low degree of grain refinement by the adding of eutectic-forming solutes（Cu,Mg,Si） is mainly caused by the constitutional undercooling contribution arsing from the segregating power of solutes. However, grain refinement of peritectic-forming solutes（Ti,Nb,Zr,V） are not only cause grain refinement by their solute segregation power but,more importantly,they introuduce potent nucleantion particles into the melt,such as Al₃Ti,Al₃Nb,Al₃Zr,Al₃Zr₄,Al₂₁V₂,Al₃V etc. and promote grain refinement via heterogeneous nucleation.