Study On As-Cast Microstructure Mechanism Of Two-Dimensional Casting Refined Al-Si Alloy

Metal materials often produce defects such as porosity,shrinkage cavity,crack and coarse internal structure during casting,which seriously affect the mechanical properties of materials.In order to refine the as-cast structure and reduce defects,this thesis proposes a new casting process which called two-dimensional casting.Al-20%Si alloy was selected as the experimental material.Two-dimensional casting and conventional casting were used to prepare Al-Si alloy ingots.The microstructure morphology was observed by optical microscope.The macroscopic morphology and microstructure of the ingots obtained by the two processes were compared.The effects of casting flow rate,cooling method and casting method on the solidification structure of ingots were analyzed.The results show that the two-dimensional casting process can improve the macro morphology,reduce the core defects and refine the as-cast microstructure of the ingot.When the mold is wrapped with insulation material,the gate diameter is 10 mm,and the casting temperature is about 710 °C,the conventional casting method with the casting drop point at the center of the mold produces a large shrinkage cavity defect at the center of the ingot.Under the same conditions,the shrinkage cavity defects in the obtained Al-Si alloy ingots disappear when the rotary casting method with the casting point 25 mm away from the edge of the mold is adopted.When the ingot of the same size is cast,the primary silicon and eutectic silicon structures in the core,1/4 thickness and edge of the ingot obtained by rotary casting are refined to a certain extent,especially the average grain size of primary silicon is reduced from 192μm to 141 μm.The analysis shows that the two-dimensional casting changes the fixed falling point of conventional casting and makes the temperature field more uniform,thus reducing the core defects and achieving grain refinement.When the gate diameter decreases from 10 mm to 8 mm for rotary casting,the grain size of primary silicon and eutectic silicon decreases to a certain extent.The primary silicon is passivated,and the average grain size decreases from 141 μm to113 μm.The number of needle-like eutectic silicon decreases,but the shrinkage cavity length at the center of the ingot increases to 55 mm.When the cooling mode of the ingot is changed,the cooling rate and supercooling degree of the ingot are changed.Compared with the mold wrapped insulation material,when the mold is not wrapped with insulation material,the primary silicon gradually becomes smooth from the core position to the edge position of the ingot,the eutectic silicon becomes fibrous,and the grain size is greatly reduced.The average grain size of primary silicon is reduced from 141 μm to 87 μm,but the shrinkage defect with a length of about 12 mm is generated in the core of the ingot.When the mold rotates and traverses,a shrinkage defect with a length of about 25 mm is generated in the middle and lower part of the ingot.The grain size of the primary silicon from the core to the edge of the ingot is not much different,and the average grain size is about 91 μm.In addition,under this process condition,when the cooling of the ingot is strengthened,the shrinkage cavity in the ingot is reduced,and the average grain size of primary silicon is reduced to 58 μm.It shows that when the casting flow rate decreases,the liquid aluminum dissipates heat quickly,the viscosity increases,the fluidity becomes worse,and the shrinkage cavity is easy to form in the core of the ingot.With the increase of the cooling intensity of the mold,the as-cast structure is refined,but the core is also prone to shrinkage.At the same time,the transverse movement of the mold increases the heat in the core of the ingot,and the core will also produce shrinkage cavity.The two-dimensional casting process can be used for near-net-shape casting to prepare thin-walled Al-Si alloy tube blanks.The obtained tube blanks are dense and regular in shape.The internal structure of the tube blanks is uniform and the grain size is small.The average grain size of primary silicon is about 43 μm.