The Study On The Microstructure And Mechanical Properties Of AlN/Al Composites Prepared By Mechanical Alloying

Aluminium nitride(AlN)has excellent material properties including good thermal conductivity and resistance,low coefficient of thermal expansion,low density,high Young’s modulus,compressive strength and hardness,good thermal stability,and excellent corrosion and thermal shock resistance.As a result,they are widely used in electronics,optoelectronics,aerospace,military and other fields.These properties make AlN an ideal reinforcing material for the preparation of lightweight aluminium matrix composites.The following problems still exist in the research of AlN ceramic particle-reinforced aluminium matrix composites: the small size of AlN and its large specific surface energy make it easy to agglomerate together,making it difficult to achieve uniform dispersion in the aluminium matrix;and the current research work on AlN particle-reinforced aluminium matrix composites is mostly focused on nanocomposites prepared by in-situ synthesis,with relatively few reports on the incorporation of non-in-situ reinforced nanoparticles into aluminium alloy matrix.There are relatively few reports on the preparation of composites with non-in situ reinforced nanoparticles.In this paper,aluminium powder was used as the matrix and AlN ceramic particles were used as the reinforcement.5%～15% AlN/Al composites were prepared by ball milling and dispersing the mixed powder and by vacuum hot pressing sintering process.The effects of different ball milling speeds and times on the mixed powders were investigated and characterised using scanning electron microscopy and particle size analyser.The microstructure and mechanical properties of the AlN/Al composites were also investigated under different ball milling and vacuum hot-press sintering processes using a metallographic microscope and an electronic universal testing machine.The results of the study are as follows.(1)At a constant ball milling speed,the mixed powder undergoes multiple stages of plastic deformation,cold welding,crushing and process hardening as the ball milling time increases.According to these changes,the entire ball milling process can be divided into three stages according to the amount of energy obtained: low-energy ball milling,medium-energy ball milling and high-energy ball milling.In the low-energy phase(short ball milling time),the particles are cold-welded together and crushed,resulting in a significant increase in particle size.In the medium-energy ball milling stage(longer ball milling time),the particles begin to harden and gradually break down,leading to a decrease in particle size.Finally,in the high energy ball milling stage(long ball milling),the particles will again aggregate and increase in size.A uniform fine AlN/Al mixed powder can be obtained by ball milling at 275 rpm for 9 hours.(2)At a constant ball milling speed,the mechanical properties of the composites prepared by hot-press sintering first increased and then decreased with increasing ball milling time.The AlN/Al composites prepared at 275 r/min for 9 h had the best mechanical properties.As the AlN particle content increased from 5 wt.% to 15 wt.%,the pores in the AlN/Al composites decreased and the mechanical properties increased and then decreased.10 wt.%AlN compressive strength of the composites was 756.3 MPa,which was 11.3% higher than that of the 5 wt.% AlN/Al composites and 23.4% higher than that of the 15 wt.% AlN/Al composites.(3)The effects of vacuum hot-pressing temperature and time on AlN/Al composites were investigated.The AlN/Al composites prepared by vacuum hot-pressing and sintering at 650℃ for 60 min had the best hardness and compression properties with non-porous organization and the best homogeneity.The Vickers hardness of the composites reached 122 HV,and the compressive strength also reached 756.3 MPa.In summary,by optimising the ball milling and vacuum hot press sintering processes for AlN/Al composites,the best process for preparing the mechanical properties of AlN/Al composites was determined as follows: 275r/min ball milling for 9h,10 wt.% AlN,vacuum hot press sintering at 650℃ for 60 min.