Thermal Conductivity and Mechanical Properties of Casting Al-4.8Cu Alloy with Addition of La
In the era of rapid development of electronic information technology,the power per unit area of integrated circuit increases,resulting in heat concentration,prone to thermal damage of components caused by excessively high regional temperature.The thermal conductivity of pure aluminum is excellent,but the addition of other alloying elements will decrease the thermal conductivity of pure aluminum in most cases.Therefore,how to balance the thermal conductivity and mechanical properties of aluminum alloy is an urgent problem to be solved.Aluminum-copper alloy is called duraluminium alloy,which has simple casting process,good cutting perfor-mance and mechanical properties.Rare earth element La was added to Al-4.8Cu alloy to adjust its structure and phase content in this paper,so as to achieve a balance between thermal conductivity and mechanical properties,and further expand its application field.To prepare Al-4.8Cu-0.15Ti-XLa alloy,pure Al and Al-Cu alloy were first melted in a resistance furnace at 730 ℃,and then Al-Ti alloy and Al-La alloy were added and the melt was kept warm.After refining at 720 ℃,the melt temperature was adjusted to 700 ℃ and poured into a metal mold at 250 ℃.The microstructure and properties of the alloy were characterized by optical microscope(OM),scanning electron microscope(SEM),X-ray diffraction(XRD),universal tensile testing machine and thermal conductivity measuring instrument.The properties of some phases in the alloy were calculated based on first principles.When 0.38％La element was added to the alloy,the grain size of the alloy was equiaxed,most of the dendrites had been fused and disintegrated,and the grain size reached the minimum.Compared with the 149 μm without La element,the grain size decreased by 53.7％.After adding La element to Al-4.8Cu alloy,Al20Ti2La phase was formed in the alloy,which precipitated before α-Al in the solidification process.The lattice mismatch be-tween Al20Ti2La phase and α-Al matrix could be calculated as 4.11％.The results showed that Al20Ti2La phase could be used as hetero-geneous nucleation point to refine α-Al matrix.When the La content was 0.38％,the tensile strength and yield strength of the alloy reach the maximum,which were 175.69 MPa and 115.20 MPa,respectively.When La content was 0.29％,the elongation reaches the maximum of 10.04％.The thermal conductivity of the alloy with 0.78％La element reached the maximum of 210.70 W·m-1·K-1,which increased by 43.9％,43.5％,60.9％and 10.3％,respectively,compared with that without La element.In the absence of La element,there was almost no low melting point phase in the intergranular cracks of alloy.After adding La element,a certain amount of intergran-ular liquid film appeared,but the strength was low,and it was easy to be torn by the intergranular separation stress.When the number of liquid film was enough,the liquid film would only be stretched and would not be torn,which significantly improved the mechanical properties and thermal conductivity.However,with the further increase of La content,Al4La phase would converge at the grain bound-ary during solidification,causing harm to the mechanical properties and thermal conductivity of the alloy.The porosity in the alloy had great harm to the mechanical properties and thermal conductivity.By calculating the ratio of the actual density to the ideal density,it was found that the ratio increased with the increase of La content,indicating that the volume fraction of the porosity in the alloy de-creased.First,La element could react with O,H and other elements entering the melt during smelting to form compounds with high melting point,such as LaH2,La2O3,etc.,and the melt was discharged in the slag raking process.In addition,the addition of La ele-ment could shorten the solidification temperature range and make the solidification mode transition from paste solidification to sequen-tial solidification.Moreover,the addition of La element could increase the low melting point phase in the alloy,which played a good role in filling the intergranular pores caused by the intergranular separation stress.The decrease of the pores in the alloy would greatly improve the mechanical properties and thermal conductivity.The equilibrium distribution coefficient of Cu at the solidification inter-face decreased with the increase of La element,so the decrease of Cu atoms in the α-Al matrix also increased the thermal conductivity of the alloy.It was generally believed that the higher the Debye temperature of the material,the higher the strength of the chemical bond and the better the thermal conductivity.By calculating the Debye temperatures of Al2Cu,Al11La3,Al4La and Cu5La phases,it could be seen that the Debye temperature of Al11La3 phase was higher than that of the other three phases.Therefore,the Al11La3 phase formed by adding La element to the Al-4.8Cu alloy was beneficial to the improvement of the strength and thermal conductivity,while the Al4La and Cu5La phases formed by adding too much La would reduce the mechanical properties and thermal conductivity of the alloy.
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