Preparation And Properties Of High Performance Heat-Conducting Cast Aluminum-Silicon Alloy

The advancement and rapid development of modern electronic information and manufacturing technology have put forward higher requirements for the heat dissipation performance of materials.Therefore,it is urgent to develop a material with high thermal conductivity to meet the high heat dissipation requirements of modern equipment.Al-Si alloys are widely used in transportation,construction,power communication,aerospace and other fields because of their good casting properties and welding performance.At present,Al-Si alloys are used as materials for key components of telecommunication products manufactured by Huawei,ZTE and other technology companies due to their good heat dissipation performance.In this dissertation,the cast Al-Si alloy is used as the research object.The permanent mould casting and controlled diffusion solidification techniques are used to prepare the Al-Si alloy.The optical microscopy（OM）and scanning electron microscope（SEM）are used to observe the structure morphology.The alloy phase composition and its thermal conductivity were tested.By adjusting the parameters of alloying elements in Al-Si alloy,forming method and heat treatment,the effects of alloying elements,forming method and heat treatment on the microstructure,thermal conductivity and mechanical properties of cast Al-Si alloy were studied.The results are as follows:（1）After the Cu element is added to the Al-Si alloy,the dendritic structure is refined to a certain extent with the increase of Cu element content,and the eutectic structure is increased.Due to the large difference in the radius of Cu atoms and Al atoms,the lattice constant of the aluminum matrix decreases,causing a large degree of lattice distortion,which makes the thermal conductivity of the alloy significantly decrease.162.37W/（m·K）decreased to 127.97W/（m·K）when the Cu content was 3%.The tensile strength of the alloy increases,Cu element is dissolved into the aluminum matrix,lattice distortion occurs,and the stress field generated by the lattice distortion interacts with the stress field around the dislocations,forming Coriolis gas groups,increasing the dislocation motion.The resistance increases the strength of the alloy,making it difficult to deform the alloy,and the strengthening effect is obvious.When Cu is not added,the strength of the alloy is at least 174.43MPa and the elongation is at the highest 8.32%.When the content of Cu is increased to 3%,the strength of the alloy is at the highest 227.27MPa and the elongation is reduced to 5.64%.It is because as the Cu content increases,the more Al₂Cu phases distributed at the grain boundaries,the more obvious the plasticity decline.（2）With the increase of Mg content,Mg and Si generate Mg₂Si phase,which consumes the Si content in the alloy.Therefore,the eutectic Si content in the structure gradually decreases,and it shows a discrete distribution,and the thermal conductivity tends to decrease.The difference in radii is not large,so the effect on the thermal conductivity of the alloy is smaller than that of Cu,from 130.9W/（m·K）without Mg to 120.82W/（m·K）when the Mg content is 2%.When Mg element is not added,the strength of the alloy is at least 217.79MPa,and the elongation is at least 1.56%.When the content of Mg element is increased to 2%,the strength of the alloy is up to242.73MPa,and the elongation is increased to 2.96%.When Mg element is added to the alloy,Mg₂Si,Al₃Mg₂,Al₇Cu₃Mg₆ phases are generated,and some Si atoms are consumed to reduce the amount of eutectic silicon in the alloy.Silicon is a brittle phase and is a source of crack propagation.As the amount of eutectic silicon decreases,stress is generated.The possibility of concentration also decreases;the size of the eutectic silicon in the alloy is reduced,the edges and corners are passivated,the deformation of the alloy is changed from brittle to plastic,and the elongation of the alloy is gradually increased.（3）The hypo-eutectic Al-Si alloy of permanent mould casting,the primaryα-Al phase is coarse dendritic,and it is easy to produce defects such as hot cracking and shrinkage;and the controlled diffusion solidification casting of the hypo-eutectic Al-Si alloy the primaryα-Al phase is refined and has a regular shape and a uniform distribution.The thermal conductivity increased significantly,from 120.96W/（m·K）during permanent mould casting to 150.6W/（m·K）during controlled diffusion solidification,an increase of approximately 24.5%,due to the primaryα-Al after controlled diffusion solidification.Theα-Al phase is refined and dense,the distribution becomes uniform,the regularity of the ion electric field is improved,the obstacle to moving electrons is reduced,the average free path of moving electrons through the material is increased,and the thermal conductivity is increased.The tensile strength of the alloy increased from 235MPa during traditional casting to245MPa during controlled diffusion solidification,an increase of 4.25%.（4）By designing orthogonal experiments and range analysis,the optimal heat treatment process parameters of Al-Si-Cu-Mg were selected as solid solution heat preservation at 520℃for 6h,and aging heat preservation at 180℃for 8h.After T6heat treatment,the alloy microstructure becomes uniform,the eutectic Si phase begins to fracture and spheroidize,and the particle size decreases from 65.89μm in the as-cast state to 32.21μm after T6 heat treatment.The thermal conductivity is 174.8W/（m·K）.On the one hand,the morphology and distribution of the eutectic Si phase changes.As the Si phase gradually begins to fracture and spheroidize from an irregular layer,the distribution also becomes uniform,the scattering of moving electrons is reduced,and the ion field is improved.Regularity.On the other hand,when the alloy is in the as-cast state,the structure is an unbalanced supersaturated solid solution.After the solution treatment,the second phase and some alloying elements will be dissolved inα-Al to form a supersaturated solid solution.After the aging treatment,the second phase precipitates,which makes the purity of the Al matrix higher,the degree of lattice distortion reduced,and the thermal conductivity significantly increased.After T6 heat treatment,dendrite segregation is eliminated,small and deep dimples and tearing edges appear,with smaller and spherical Si particles,which better coordinate with the deformation of the matrix,thereby forming a better fracture shape Appearance,showing better comprehensive mechanical properties,tensile strength reaches 345MPa,elongation is 2.8%,and the fracture mode is ductile fracture.