Effect Of Cold Materials On Microstructure And Properties Of 2a12 Aluminum Alloy Ingots In Mold Casting

2A12 aluminum alloy is widely used in aerospace,aviation,shipbuilding,military and other fields.It is especially important to achieve uniform and fine equiaxed crystal structure during its casting and solidification process.Due to the high content of alloy elements in 2A12 aluminum alloy,especially in the production of large-scale ingots,macrosegregation can't be avoided,and coarse columnar and equiaxed crystals are easy to appear,and the internal and external distribution is uneven.The fundamental cause for this phenomenon is the large temperature difference between the internal and external during the solidification of ingots.Therefore,in this paper,the temperature field of 2A12 aluminum alloy melt is changed by inserting 2A12 aluminum alloy extrusion bar cold materials into the core of melt,and the ingots are obtained by keeping cold materials at 2A12 melt for different time.The effect of 2A12 cold materials on the solidification structure and properties of ingots is studied,and the mechanism of refining the grain by cold materials is discussed.The experiment includes two parts:(1)The effect of the melting state of the cold materials on the structure of the ingot was studied by inserting the cold materials with the diameter of 20mm into the core of 2A12 aluminum alloy melt for different immersion time,and the grain refinement mechanism of cold materials with the diameter of 20mm was discussed.(2)The effect of the melting state of the cold materials on the structure and properties of the ingot was studied by inserting the cold materials with the diameter of 10mm into the core of 2A12 aluminum alloy melt for different immersion time,and the grain refinement mechanism of cold materials with the diameter of 10mm was discussed.The experimental results show that:(1)When the immersion time is short,the cold material with the diameter of 20mm which is inserted into the core of the melt doesn't melt completely,and a coarse columnar crystal transition area is formed around the cold material.With the extension of immersion time,the cold material gradually melts,the grains in the columnar crystal transition zone and the ingot edge gradually become finer.Until the cold material and columnar crystal melt completely,the grain is fine and uniform equiaxed crystal.(2)With the extension of immersion time,the cold material with the diameter of 10mm which is inserted into the core of the melt melts gradually,and the grains become finer and more uniform.However,when the immersion time is longer,the grains of ingot grow up.(3)When the cold material with diameter of 20mm is inserted into the core of the melt,the liquid-solid ratio is low,and the melt temperature is greatly reduced,so that the melt is initially crystallized.Then,as the melt temperature rises,the columnar crystal and the cold material melt together to provide nucleation particles.However,when the cold material with diameter of 10mm is inserted into the core of the melt,the liquid-solid ratio is high,and the cold material is not enough to lower the temperature of the melt below the liquidus,and the preliminary crystallization of the melt doesn't occur.Therefore,the increased nucleation particles is all derived from the cold material.(4)The refinement effect of cold materials with diameter of 10mm on the ingot grain improves the mechanical properties of the ingot.When the immersion time is 5s,the mechanical properties of the ingot are best,and the tensile strength,yield strength and elongation are increased by 17.5%,7.3%and 40.7%respectively than that of the conventional ingot.With the extension of the immersion time,the mechanical properties are decreased.(5)In conven tional ingots and ingots with cold materials,a duplex structure of grains with coarse core and fine periphery is observed from all samples,and the content of Cu and Mg elements in fine dendrite structure is significantly higher than that in coarse dendrite structure.(6)2A12 aluminum alloy cold materials refines solidified structures by three key mechanisms:decreased degree of superheat,increased cooling rate,and a large number of small particles that acted as the nucleus of heterogeneous nucleation.