Research On The Microstructure And Properties Of Endogenous Mg₂Si Particles Reinforced Aluminum Matrix Composites

Compared with commonly used steel materials,aluminium alloys are widely used in transportation,aerospace,ordnance industry,electronic and electrical fields,because they have the advantages of light weight,high specific strength,good corrosion resistance and formability,low cost and reusability.In-situ Mg₂Si particle reinforced aluminium matrix composites have become an important development direction of aluminium matrix composites and have a wide range of applications because of the advantages of clean interface,good compatibility with matrix,high specific strength and specific rigidity,good thermal stability and low preparation cost,etc.But the morphology and size of primary Mg₂Si phase precipitated by traditional process are usually coarse dendrites and uneven distribution,and that of eutectic Mg₂Si phase is usually flake-like which makes aluminium matrix become discontinuous.So it is easy to cause stress concentration and cracks which will weaken the mechanical properties of Al-Mg-Si alloy and hinders the application of this kind of composites material.In order to refine the morphology,size and distribution of Mg₂Si phase,modification and heat treatment are commonly used by researchers.Previous studies have shown that the primary Mg₂Si phase can be refined by adding single element modifier and compound modifiers.Further solution and aging treatment can passivate the sharp edges and corners of polygon primary Mg₂Si phase and refine the morphology of Chinese-character eutectic Mg₂Si phase.Since the modification effects of compound modifiers are usually superior to single modifier as long as choosing appropriate compound modifiers,the research will focus on compound modification in the future to further refine the size and shape of Mg₂Si phase in the in-situ particle reinforced aluminium matrix composites.Therefore,the effects of Bi-Sb,Cu-P modification and heat treatment on the microstructures and mechanical properties of Al-20wt.%Mg₂Si composites were studied in this paper,and the mechanisms were explored.The experimental purpose was to find a simple and effective processing technology to manipulate the Mg₂Si phase in Al-Mg₂Si composites.The major study results are as follows:?1?Adding 0.2wt.%Bi or 2wt.%Sb alone in Al-20wt.%Mg₂Si composites could transform the morphology of primary Mg₂Si phase from dendrite to polygon,and the average size of Mg₂Si phases was decreased from 130.85?m to 21.30?m and 20.46?m,respectively,and both of them reduced the lamellar spacing of eutectic Mg₂Si phase;the Brinell hardness of experimental materials was increased from HB 94.53 to HB 123.11 and HB 128.10,respectively;?2?With compound modification of 0.2wt.%Bi and 2wt.%Sb,the mean size of primary Mg₂Si phase was decreased to 10.21?m,which increased the Brinell hardness to HB 130.25,and the ultimate tensile strength and elongation were increased from 216.41MPa and 1.51%to 257.24MPa and 2.11%,respectively.The modification mechanism can be described as follows:the preformed Mg₃Sb₂ and AlSb phase acted as the heterogeneous nucleation for the primary Mg₂Si,and the Bi atoms absorbed on the growing surface of the primary Mg₂Si to suppress the dendritic preferential growth;?3?With the addition of 6.9wt.%Cu-P,the primary Mg₂Si phase in Al-20wt.%Mg₂Si composites was decreased from 82.80?m to 8.95?m,resulting in the increases of the Brinell hardness,ultimate tensile strength and elongation?HB 155.53,227.36MPa and 1.74%?.The modification mechanism could be that the preferential formation of AlP phase increased the primary Mg₂Si heterogeneous core.In addition,the three-dimensional morphology of primary Mg₂Si was changed from dendrite to truncated octahedron.However,with the increase of Cu-P addition,more brittle Al₂Cu phases were formed,which led to the insufficient improvement of mechanical properties of the alloy;?4?The microstructures of Al-20wt.%Mg₂Si composites modified by Bi-Sb and Cu-P were further refined by solution treatment.The edges and corners of primary Mg₂Si phase were rounded,and the eutectic Mg₂Si phases were changed from Chinese character to short rod and dot,which optimized the microstructures and mechanical properties of the experimental aluminum alloy;?5?T6?500?/24h+215?/6h?treatment not only manipulated the primary and eutectic Mg₂Si of Al-20wt.%Mg₂Si composites modified by Cu-P,but also dispersed the network Al₂Cu phase which distributed along the grain boundary,and the tensile properties of the experimental alloys were increased to 241.35MPa and 2.38%respectively.The micro-Vickers hardness of Al matrix was increased from HV 64.17 after solution treatment to HV 89.75after T6 treatment,which increased by 39.85%.