Study On Microstructure Regulation And Rheological Properties Of Hypoeutectic Al-NI Alloys With High Thermal Conductivity

With the rapid development of new-generation electronic communications,new energy vehicles and other fields,the problem of effective heat dissipation of power-intensive electronic devices used in them needs to be solved urgently,and thermal management materials with excellent thermal conductivity are urgently needed.Aluminum alloy has the advantages of light weight and high thermal conductivity,and has beenwidely used in the field of heat dissipation components of electronic devices and systems.The current traditional high thermal conductivity aluminum alloys such as Al-Si cast alloys have good castability,but the thermal conductivity in the as-cast state is difficult to exceed 200 W/(m·K).In this project,Ni element with low solid solubility is selected,and the near-eutectic Al-5Ni alloy is used as the base alloy to control the microstructure of its trace addition elements.The alloy is expected to have both excellent thermal conductivity and castability.The trace elements added to the near-eutectic Al-5Ni alloy in this subject are 0?0.5wt.%semi-metal Si element,rare earth element Ce,transition group Fe,Co,Cu elements.The Al-5Ni-X alloy was first prepared by gravity casting.Then the microstructure and phase composition were studied by OM,SEM and Xrd.Finally,the modification effect of micro-added elements was evaluated based on the statistical results of?-Al grain area.The results show that with the increase of alloying element content,the?-Al grains all show the characteristics of first refinement and then coarsening,while the eutectic Al₃Ni phase will first spheroidize and then coarsen.When the alloying element content is too high(for example,0.5wt.%),it will further undergo metallurgical reaction with the Al matrix,and precipitate a coarse second phase(for example,Al-5Ni-0.5Fe has long rod-shaped Al₃Fe phase,Al-5Ni-0.5Co branch Al₉Co₂ phase),while weakening the modification effect of alloying elements.Due to the extremely low solid solubility of Ni in Al,the size and distribution of the second phase of Al-5Ni-X alloy are the main factors affecting its thermal conductivity.This subject quantifies the sphericity of the second phase by the Maxwell model.The results show that the sphericity of the second phase of the alloy is proportional to the thermal conductivity.The spherical second relative to the free electron motion has a low degree of interference,which reduces the probability of free electrons being scattered during heat transfer motion.Therefore,the higher the sphericity of the second phase of Al-5Ni-X alloy,the better its thermal conductivity.When a small amount of 0.1wt.%Ce is added,the sphericity of the second phase of the Al-5Ni-X alloy increases,and the thermal conductivity in the as-cast state increases from 211.41 W/(m·K)to 219.35 W/(m·K).The rheological behavior test shows that the addition of 0.5wt.%of the above elements can slightly reduce the apparent viscosity of Al-5Ni alloy.When the test temperature was550°C to 575°C,the apparent viscosity increase rate was different,the increase rate was the lowest when 0.5wt.%Ce was added,and the highest when 0.5wt.%Fe was added.The rheological factor n-1 obtained by power-law model fitting can reflect the decreasing trend of the apparent viscosity of the alloy melt when the shear rate increases,that is,the severity of the shear-thinning effect.The smaller the n-1value obtained from the linear fitting,the more obvious the decreasing trend of apparent viscosity.The addition of Fe,Co and Si elements can significantly reduce the value of n-1.Among them,the Al-5Ni-0.5Si alloy has the lowest n-1value at 640°C,which is-1.636 Pa·s.Compared with the Al-5Ni alloy,the reduction rate is23.0%.The lower the apparent viscosity value,the more obvious the decreasing trend of viscosity,the more favorable for the die-casting of the alloy melt,and the better the castability of the alloy.In conclusion,the thermal conductivity of the Al-5Ni-X series alloys developed in this project can exceed 200 W/(m·K)in the as-cast state.At the same time,the influence of alloying elements on the microstructure,thermal conductivity and rheological behavior of the alloy is clarified,which can provide theoretical reference and practical reserve for the industrial production of the alloy.