Study On The Influence Mechanism Of Microstructure Modification On Thermal Conductivity Of Al-Si Based Cast Aluminum Alloys

Aluminum alloy is one of the main materials for heat dissipation components due to its high specific strength,good thermal conductivity and low cost.The wireless communication system and its equipment are developing rapidly in the direction of larger-scale integration,higher power and lighter weight.In addition,the frequency of wireless communication signals is also increasing.All of these raises high requirement for the heat dissipation performance of the communication system and its equipment.The sealed housings of the Active Antenna Unit（AAU）of 5G communication base station are normally made of die-cast Al alloy.However,the commercial cast Al alloy such as ADC12 possess relatively low thermal conductivity,which is hard to meet the requirement of AAU sealed housings material.To tackle this problem,this research aimed to develop cast Al alloys with high thermal conductivity,and systematically study the influence of nine alloying elements on the thermal conductivity of hypoeutectic Al-Si alloys.The base alloy composition was optimized and the alloying elements used for modification were determined by orthogonal experiment.The mechanism was studied for the simultaneous improvement of both thermal conductivity and mechanical properties by composite modification.The relationship between microstructure and thermal conductivity/mechanical properties was revealed.Following are the main conclusions.The influence of nine alloying elements（Fe,Cu,Zn,Co,Sb,Sr,B,Sm and RE）on the electrical conductivity of hypoeutectic Al-Si alloys was systematically studied.When the Si content was high,the electrical conductivity of Al-Si alloys was always improved by the nine elements,among which the improvement of Sr,B and RE was the most significant.Based on the above result,a four-factor and three-variable orthogonal experiment was designed to analyze the influence of alloying elements on the electrical conductivity,hardness and tensile strength.The composition of the base alloy was determined to be Al-7Si-0.6Fe-0.5Zn,and the modified elements were Sr,B and RE.The electrical conductivity（thermal conductivity）of hypoeutectic Al-Si alloys was slightly improved with the addition of 0.3%Fe,which was related to the change of microstructure.In the hypoeutectic Al-Si alloys,Fe-richβ（Al₅Si Fe）phase was induced to precipitate when Fe was added.Theβphase was precipitated in two stages.When the Fe content was lower than the critical value(Fe _Crit),the addition of Fe led to the precipitation ofβ_T phase.When the Fe content was higher than Fe _Crit,there was little change of the precipitation ofβ_T phase,while the precipitation ofβ_B phase was increased with Fe content.The higher was the Si content,the higher was Fe _Crit.The solubility of Si in Al matrix was reduced due to the precipitation ofβphase,which was beneficial to increase the thermal conductivity ofα-Al matrix.On the other hand,moreα-Al/βinterface was formed by the precipitation ofβphase,resulting in the decrease of the thermal conductivity.Comparing withβ_B phase,theβ_T phase was more effective in reducing the solubility of Si inα-Al matrix,and the aspect ratio ofβ_T phase was larger than that ofβ_B phase.Therefore,the thermal conductivity was increased with the increase ofβ_T phase,and was decreased with the increase ofβ_B phase.The electrical conductivity（thermal conductivity）of the hypoeutectic Al-Si alloys（3%～12%Si）was effectively increased when a small amount of Sr（0.05%Sr）was added,which was attributed to the morphological modification of eutectic Si phases.The morphology of eutectic Si phases was transformed from coarse flake to a mixture of fiber and fine flake.When the Si content was increased,the dendrite gap and proportion of fibrous eutectic Si phases were increased with the decrease of flaky eutectic Si phase.When the Si content was higher than 9%,the Sr-modified effect on the eutectic Si phases was stable,and the proportion of flaky eutectic Si phases was negligible.The scattering effect and scattering probability of theα-Al/Si interface with a circular cross-section on electrons were less than those of theα-Al/Si interface with a rectangle cross-section.Therefore,the increment and increasing rate of thermal conductivity were gradually increased with the increase of Si content in Sr-modified alloys.When the Si content was higher than 9%,the increment and increasing rate of thermal conductivity were stable.The thermal conductivity was slightly improved by reducing the solubility of Si in Al matrix when Fe was added,while it was significantly improved by the morphological variation of eutectic Si phases when Sr was added.The mechanism of these two elements in increasing the thermal conductivity was basically different.The mechanism of simultaneous improvement in the thermal conductivity and mechanical properties of the base alloy（Al-7Si-0.6Fe-0.5Zn）by B/La/Sr composite modification was systematically investigated.The increase of both thermal conductivity and mechanical properties was the highest for 0.1%B/0.05%La/0.1%Sr composite modified alloy.Both the thermal conductivity and mechanical properties were decreased with the increase of La content.La effectively suppressed the co-poisoning effect between B and Sr when the modifiers of B,La and Sr were added in turn to the melt.Consequently,B and Sr modified the primaryα-Al phase and the eutectic Si phase,respectively,which was beneficial to the improvement of the thermal conductivity and mechanical properties.A large amount of flaky La-rich intermetallic was precipitated with the further increase of La content,which produced an adverse effect on both thermal conductivity and mechanical properties.Based on above result,the improvement in the thermal conductivity and mechanical properties of0.1%B/0.05%La/x Ce/0.1%Sr composite modified process was further studied.Compared with the 0.1%B/0.05%La/0.1%Sr composite,the 0.1%B/0.05%La/x Ce/0.1%Sr composite modification process resulted in further improvement of the thermal conductivity and mechanical properties.Both the thermal conductivity and mechanical properties were firstly increased and then decreased with the increase of Ce content.When the Ce content was 0.5%,the improvement in the thermal conductivity and mechanical properties was the highest.The morphology variation of the Fe-rich phases was the main reason of the enhancement.When the Ce content reached 0.8%,the precipitation of the Ce-rich phase with larger ratio aspect was responsible for the decrease in thermal conductivity and mechanical properties.Through the optimization of alloy system and composite modification process,the thermal conductivity and mechanical properties of the basic alloy were simultaneously improved.The optimized alloy was successfully applied in the manufacture of AAU sealed housings in a domestic communication company’s 5G communication base station.The technology developed here is also promising in the manufacture of important parts of new energy vehicles,such as the motor housings,compressor housings,inverter housings,etc.This research might provide valuable guidance for the development of Al alloys with both high thermal conductivity and high tensile strength.