Research On Primary Dendritic Spacing Under Unidirectional Solidification Of Al-Cu Alloys

In the paper many unidirectional solidification experiments of Al-4.5%Cu alloys are done with the help of 5kg crucible directional solidification stove. Firstly, primary dendritic spacing under unidirectional solidification of Al-Cu alloys is studied by changing the quantity of copper and adding different compound RE . Then , the change law of the primary dendritic spacing is researched with different temperature gradient and different growing rate of crystal.The result shows that under the same condition of other solidification parameter ,the primary dendritic spacing λ₁ increases as the quantity of Cu increases ,but the increase tendency is slow and the changing tendency agrees with the model of Hunt .The fitted regression line is λ₁ = 125.4+110 ( C₀^0.25－1.2) .When the quantity of copper is below 3.01%,the samples have many hot cracks and shrinkage porosities .When the other solidification parameter is same ,different quantity of compound RE is adding into the Al-4.5%Cu alloys. The result shows that advanced fir-tree crystal is formed with the help of a small amount of compound RE and the structure of unidirectional solidification is not apparent. As the quantity of compound RE increases the unidirectional solidification structure is gradual apparent ,the distribution of treelike crystal is gradual regular and the primary dendritic spacing becomes small .Compared with the samples of not adding compound RE ,the hot cracks and shrinkage porosities is less in the samples of adding compound RE.When the other solidification parameter is not changed the primary dendritic spacing becomes small as the temperature gradient G increases and the changing is very apparent .Besides the result of experiments is consistent with the model of Hunt, but inaccuracy exists between the measured value and the theoretical value. The approximate expression is λ₁ =26.7G^-0.5－135.9 and the result of regression analysis is y = 2843478.3x²-70217.4x+543.8。 When the temperature gradient G is invariable and the draw velocity is below 60r/min the structure of unidirectional solidification is not perfect. As the growing rate of crystal increases the primary dendritic spacing becomes small and the changing is very apparent .The badness of fit is y = 0.125x²-18.73x+802.92 and the minimal value of the primary dendritic spacing is 104.7um .Besides ,when the growing rate of crystal is above 60 r/min there is linear relation between λ₁ and V^-0.25 and the expression is λ₁ = 1000 V^0.25－229 .