| Magnesium alloys have been increasingly used in electronic products and other fields with its advantages of light weight,high specific strength,high thermal conductivity and excellent electromagnetic shielding performance.In recent years,with the rapid development of electronic technology,heat dissipation has become more and more critical in high-density,high-power electronic products.In order to dissipate heat effectively and coordinate assembly,electronic packaging materials should have high thermal conductivity,low thermal expansion coefficient and certain mechanical properties.However,the traditional magnesium alloy materials can no longer meet the development requirements,so the development of magnesium alloys or magnesium matrix composites with excellent comprehensive thermophysical properties is of great significance.Hexagonal boron nitride(h-BN)is a ceramic material with a crystal structure similar to graphite.It has the advantages of low thermal expansion coefficient and density,high thermal conductivity and so on,which can significantly improve the thermophysical properties of magnesium matrix composites,however,its wettability with magnesium melt is very poor and preparation is difficult.Long Period Stacking Ordered Structure(LPSO)in magnesium alloy can significantly strengthen the mechanical properties of the alloy,and has low thermal expansion coefficient.Combining it with h-BN can not only improve the mechanics of the composite,but also further improve the thermal expansion performance.Therefore,in this paper,h-BN particles are selected as the reinforcement,and LPSO structure is formed in the composite,the h-BN+LPSO hybrid reinforced magnesium matrix composite is obtained.This paper also studied the influence of ultrasonic treatment(UT),squeeze casting(SC)and heat treatments on the microstructure and properties of composites.The electroless nickel plating treatment of h-BN with a bath system using hydrazine hydrate as the reducing agent and ammonia as the complexing agent was studied.Through appropriate process parameters,a coating that meets the requirements of this research is obtained,that is,the coating is complete,uniform,compact,and as thin as possible.The specific process parameters are:after pretreatment,the electroless plating reaction temperature is 78°C,the reaction time is 30min,the h-BN loading is 5g/L,and the p H is9?10.The plating layer obtained by this process is pure nickel,and the bonding strength with h-BN is high,with an average thickness of about 0.17?m.Electroless nickel plating significantly improves the wettability of h-BN and magnesium,and h-BN can be successfully added to magnesium melt through liquid stirring method.The preparation process of h-BN+LPSO hybrid reinforced magnesium matrix composites and the effects of UT and SC on the microstructure and properties of composites were studied.Combining the previous research results on LPSO structure in the laboratory,this study uses the Ni element melted from the nickel-plating layer,and adds Y element into the matrix at the Ni/Y atomic ratio of 0.5/0.9,the LPSO structure composed of Mg-Ni-Y was successfully formed.However,the addition order of the Y element will seriously affect the microstructure of the composites.If the order is not correct,the Y element will oxidize and produce a large amount of YOF impurity phase.The h-BN+LPSO hybrid reinforced magnesium matrix composite prepared by adding h-BN particles first and then adding Y element,the YOF impurities were obviously reduced and the microstructure was relatively ideal.The volume fractions of h-BN and LPSO in the composite are 3%and16%,respectively.UT and SC have significant impacts on the microstructure and properties of the composites.UT can eliminate h-BN agglomeration,improve the morphology of LPSO structure,and make them distributed uniformly.SC can refine the grain size and the LPSO structure at the grain boundary,and significantly reduce the porosity of the composites.The composite formed by SC after UT has the best comprehensive properties,with the thermal conductivity at room temperature is 107.64W/(m·?),the average coefficient of thermal expansion at 20-100?is 18.90×10-6/?,which is significantly lower than that of LPSO single-phase reinforced magnesium alloy,the as-cast tensile strength is 173MPa,and the elongation is 4.2%.The effect of T6 and T1 heat treatment on the microstructure and properties of composite formed by SC after UT was studied.The results show that after T6 treatment,the microstructure of the composite has no obvious change.The LPSO structure has good thermal stability and the morphology has not changed,only a small amount of Mg2Ni phase dissolved.With the increase of the aging temperature,the thermal conductivity and thermal expansion coefficient of the composite increase first and then decrease,and the maximum thermal conductivity is 118.41W/(m·?).After T1 treatment,the microstructure of the composite is basically unchanged,the thermal conductivity increases with the increase of temperature to a maximum of 117.86W/(m·?),which is 9.5%higher than that of as-cast,while the thermal expansion coefficient is basically unchanged.Compared with T6treatment,T1 treatment has a better effect on improving the thermophysical properties of the composite.The thermal conductivity is improved to a certain extent on the basis that the thermal expansion coefficient and mechanical properties of the composite are basically unchanged.Under T1 heat treatment at 220?/24h,the room temperature thermal conductivity of the composite reaches 118.21W/(m·?),the average coefficient of thermal expansion at 20-100?is 18.91×10-6/?,the tensile strength and elongation are 170MPa and 4.5%,respectively.A magnesium matrix composite with good thermophysical properties and certain mechanical properties is obtained. |
PDF Full Text Request