Fabrication And Properties Of GNPs/Al2024 Composites By Powder Semi-solid Processing

In order to meet the increasing performance requirements of aluminum matrix composites in the fields of aerospace,power electronics,automotive and optical instruments,a"comprehensive"material with high mechanical properties and good thermal properties was expected to prepare.Graphene material with two-dimensional was planned to add to the aluminum matrix as the reinforcing phase.It is expected to improve the dispersion of graphene materials and the interface problems encountered in the improvement of thermal properties during the preparation of graphene/Al2024 composites,developing its high strength and high thermal properties,and exploiting a new process for the preparation of graphene/Al2024 matrix composites.In this paper,graphene nanoplatelets（GNPs）reinforced Al2024 matrix composites were prepared by powder semi-solid processing（PSSP）.Combined with friction stir processing（FSP）,the agglomeration problem of high proportion GNPs was improved,and the surface of GNPs was modified to improve the thermal conductivity of the composites.The PSSP forming of GNPs/Al2024 composites was studied.The dispersion of GNPs during ball milling and the microstructure evolution of composites during PSSP preparation were systematically studied.The results show that the ideal milling time is the initial stage of plastic deformation and micro-broken.GNPs are evenly dispersed in the matrix alloy powder with structural integrity,and agglomeration within a certain degree does not significantly affected the performance of the composite.The matrix of the composites consists of primaryα-Al particles,secondaryα-Al particles and precipitated Al₂Cu+Al₂Cu Mg.The optimal liquid phase content of PSSP is about 30%-50%,and the primary particles are nearly spherical immersed in the liquid phase,which is a typical semi-solid slurry.There are interfacial reactions in the composites,and the weak interfacial reaction product Al₄C₃is small in size and quantity when formed at 600℃,which has no obvious effect on the properties.The effects of the main parameters of PSSP on the microstructure and mechanical properties of composites were studied.The results show that the heating temperature directly affects the liquid fraction.PSSP composites have better quality at 600℃-620℃.The typical semi-solid structure is not formed below the temperature range,and liquid phase segregation occurs and the performance deteriorates when forming temperature is higher than the range.The size ofα-Al phase is uniform when the holding time is 40 min.The pressure affects the properties of composites by affecting the density when the liquid phase content is less than 30%,whereas the composites can be formed at low pressure when liquid volume fraction≥30%,pressure increasement has little effect on the performance.GNPs distributed in both grain boundaries and grains,which can significantly refine the secondary solidification microstructure and increase the dislocation density.With the parameters of 600℃,40 min and 300 MPa,the yield strength,ultimate tensile strength and the elongation of 1%GNPs/Al2024 composite prepared by PSSP reach 338 MPa,562 MPa and 12.2%,respectively.The yield strength and tensile strength increased by 53.6%and 34.4%,and the elongation decreased slightly.The modified SiC nanorods（SiCn）treatment on GNPs surface was carried out,and SiCn-GNPs/Al2024 composites were prepared by PSSP method.The results showed that the in-situ SiCn SiCn bridges GNPs and Al2024 matrix and establishes the heat conduction path between GNPs and Al2024 matrix in GNPs/Al2024composite.SiCn acts as the heat transfer path between GNPs and Al2024 matrix,and connects the heat transfer network in the composite,improving the thermal conductivity of the composite.When the weight content of GNPs is 1%and the volume ratio of SiCn to GNPs is 30%,the yield strength and tensile strength of SiCn-GNPs/Al2024 composites are 380 MPa and 586 MPa,respectively,which are72.7%and 40.2%higher than that of the matrix material.The maximum thermal conductivityof the composites is 178 Wm^-1k^-1,respectively,which is 50.8%than that of the matrix material.The FSP behavior of GNPs/Al2024 composites was studied.The results show that the agglomerations of GNPs can be broken and distributed evenly by friction agitation.The composites containing 1.5%and 2%GNPs are distributed evenly by FSP.The heat input is a direct factor affecting the quality of composites.When the heat input is enough,recrystallization will occur to refine the grains.After 1-6passes of FSP stirring,the grain size is refined from 3.13μm to 0.7μm.The effect of FSP on the properties of composites was studied.The results show that the average microhardness of the composites with 1.5%GNPs and 2%GNPs is 184 HV and 191 HV,and the microhardness is 16.5%and 46.9%higher than that of the matrix,respectively.The maximum tensile strength reaches 569MPa and 578 MPa at 4 passes.The maximum elongation was 13.6%and11.5%.The maximum strength of FSPed SiCn-GNPs/Al2024 composites with GNPs content of 1.5%and 2%are 583 MPa（elongation of 12.9%）and 562 MPa（elongation of 9.6%）,respectively.The thermal conductivity is up to 195 Wm^-1k^-1and 203 Wm^-1k^-1,which are 65.3%and 72%higher than that of the matrix.The distribution,structural integrity of GNPs,mechanical properties and thermal conductivity can be optimized by coordinating the process parameters.