| 7075 aluminum alloy（7075 Al）has the characteristics of light weight,high strength,high corrosion resistance and good processing performance,etc.It is widely used in aerospace,ship,automobile and other transportation fields.However,with the rapid development of science and technology,7075Al has been unable to meet the growing demand,and needs to be improved in terms of strength,wear resistance and other properties.Aluminum matrix composites are favored by various engineering fields because of their excellent physical,mechanical and friction properties.At present,most of the preparation processes applied to aluminum matrix composites have the problems of long sintering time and slow heating rate,which easily cause the grain growth and grain coarsening of the composites,and they are not conducive to the improvement of the performance of the composites.Spark plasma sintering（SPS）has the advantage of low temperature and rapid sintering,which can significantly inhibit the coarsening of crystal grains during the sintering process,improving the density of and the overall performance of the composites.In this paper,7075Al as the matrix,silicon nitride（Si3N4）,graphene nanoplates（GNPs）as reinforcement,Si3N4/7075 Al composites and GNPs/7075Al composites were prepared using the SPS process,respectively.The influences of the process parameters and the content of Si3N4 on the mechanical properties,friction and wear,and coefficient of thermal expansion（CTE）of the Si3N4/7075Al composites were analyzed systematically,and the effects of GNPs on the hardness and friction and wear properties of GNPs/7075Al composites were also investigated.The strengthening mechanism of the two composites were analyzed with the microscopic morphology.The molecular dynamics method was used to simulate the sintering process and mechanical properties of the Si3N4/7075Al composites and the GNPs/7075Al composites.Meanwhile,the changes of microstructure of the composites and strengthening mechanism of composites were study during the sintering process,which can provide a theoretical basis for the design of composites and the preparation of high-performance composites.The main conclusions are as follows:（1）The orthogonal experiment method was used to prepare Si3N4/7075Al composites with excellent mechanical properties using SPS for the first time.The structure and composition of the Si3N4/7075Al composites were studied,and the best preparation process parameters were obtained:sintering temperature was500℃,sintering pressure was 50 MPa,heating rate was 50℃/min,and holding time was 6 min.Under this process,the hardness and the tensile strength of the Si3N4/7075Al composite were 184.0 HV and 481.7 MPa,respectively,which are90.5%and 60.4%higher than that of the matrix..（2）The single factor experiment was used to study the influence of various process parameters on the properties of composites.The results showed that with the increase of the sintering temperature,the increase of the pressure,the acceleration of the heating rate,and the extension of the holding time,the hardness and tensile strength of the Si3N4/7075Al composites all showed a trend of first increasing and then decreasing.And when the temperature was 500℃,the sintering pressure was50 MPa,the heating rate was 50℃/min,and the holding time was 6 min,respectively,the hardness and tensile strength of the composites are optimal.（3）The optimal process parameters were used to prepare Si3N4/7075Al composites,and the influence of the particle size of the 7075Al powder and Si3N4content on the mechanical properties of Si3N4/7075Al composites was studied.It is found that the mechanical properties of Si3N4/7075Al composites prepared by7075Al powder with particle size of 10μm are better than those prepared by 7075Al powder with particle size of 35μm.With the increase of Si3N4 content,the hardness of the Si3N4/7075Al composites prepared by 7075Al powder with particle size of 10μm gradually increase,and when Si3N4 content is 20wt%,the tensile strength of the composite is the highest,481.7 MPa,which is 60.57%higher than that of 7075Al.This is mainly due to the grain refinement and low porosity of the composite,which improves the comprehensive strength.In addition,the strengthening mechanism of composites is also explained,including load transfer strengthening,dislocation strengthening and grain refinement strengthening.（4）The effect of process parameters on the friction and wear properties of Si3N4/7075Al composites was studied systematically.The results showed that with the increase of sintering temperature,sintering pressure,heating rate and holding time,the wear volume of Si3N4/7075Al composites all showed a trend of first decreasing and then increasing.When the sintering temperature was 500℃,the sintering pressure was 50 MPa,the heating rate was 50℃/min,and the holding time was 8 min,the Si3N4/7075Al composite had the smallest wear volume and the best wear resistance.It provides the basis foundation for the preparation of the composites with high wear resistance.（5）The effect of Si3N4 content on the friction and wear properties of Si3N4/7075Al composites was studied.The results showed that the addition of Si3N4significantly improved the wear resistance of the composites.With the increase of Si3N4 content,both the friction coefficient and wear volume of the composites decreased first and then increased.When the Si3N4 content was 25wt%,the wear scar was relatively shallow,almost no cracks were generated,and the wear resistance was the best.And the friction coefficient and wear volume were 0.33 and0.10 mm3,respectively,which was off by 15.38%and 87.01%compared to 7075 Al matrix,indicating that the addition of Si3N4 can significantly improve the wear resistance of composites.However,when the content of Si3N4 was too high,the agglomeration of Si3N4 will occur,the internal pores increase,which can reduce the bonding strength of the interface between the matrix and Si3N4 particles,leading to the reduced wear resistance.（6）The applied load and rotation speed during the friction process significantly affect the friction and wear properties of 7075Al and Si3N4/7075Al composites.It was found that with the increase of applied load,the wear rate of7075Al matrix gradually decreased.When the applied load was 10 N,the wear rate was the lowest,which was 1.54×10-3mm3/（N·m）.However,the wear rate of the composites showed a trend of first decreasing and then increasing with the increase of the applied load.When the applied load was 5 N,the thickness of the mechanical mixed layer was stable,and there was no crack on the subsurface,and the protective effect on the friction surface was better.At this time,the wear rate was the lowest,1.84×10-4mm3/（N·m）,which was only 11.95%of that of the 7075 Al.With the increase of rotation speed,the wear rate of 7075Al matrix gradually decreased.When the rotation speed was 400 r/min,the wear rate was lowest,which was1.44×10-3mm3/（N·m）.However,with the increase of rotation speed,the wear rate of the composites increased first and then decreased.Studies have shown that the wear rate of the composites were far lower than that of the 7075Al under two conditions,indicating that the addition of Si3N4 can positively promote the wear resistance of the composites.（7）The influence of Si3N4 content,sintering pressure and temperature on the thermal stability of Si3N4/7075Al composites was studied.The studies showed that the content of Si3N4 had a significant effect on the CTE of the composites.With the increase of Si3N4 content,the CTE of the composites gradually decreased.When the Si3N4 content was 30wt%,the CTE of the Si3N4/7075Al composite was 13.7×10-6/℃,which was 43%lower than that of the matrix.It showed that the addition of Si3N4 played a positive role in improving the thermal stability of Si3N4/7075Al composites.（8）Molecular dynamics was used to simulate the sintering process of Si3N4/7075Al composites and the influence of the ratio of the particle size of the matrix to the reinforcing particle on the tensile strength of the composites.Meanwhile,the strengthening mechanism of the composites was studied.The studies showed that as the sintering time increases,sintering necks were gradually formed between the matrix particles,and the pores in the composite gradually decreased.In addition,the particle size ratio had an important effect on the internal pore volume of the composite.When the particle size ratio was less than 7/2,the tensile performance of the composite was mainly affected by the maximum pore size in the vertical stretching direction.When the particle size ratio was 1,the mechanical properties of the composite were the best.（9）The SPS was used to prepare GNPs/7075Al composites.The influence of GNPs on the hardness and wear resistance of GNPs/7075Al composites was investigated.The results showed that the addition of GNPs could significantly improve the hardness and wear resistance of the composites.When the GNPs content was 0.3wt%,GNPs were evenly dispersed in the matrix,and the hardness of GNPs/7075Al composite was 123.2 HV and the wear rate was 0.00273 mm3/（N·m）.Compared with the 7075Al matrix,the hardness was increased by 21.60%and the wear rate was reduced by 20.18%.（10）Molecular dynamics was used to simulate the sintering process and the mechanical tensile behavior of the graphene/Al composites.The changes of aluminum particles and graphene during sintering process were described from the microscopic side.The results showed that when the graphene content was 14.07%,the carrying capacity was better.At this time,the composite had the smallest grain size,the highest tensile strength（23.75 GPa）and the highest elastic modulus（483.14 GPa）,which are 94.35%and 64.24%higher than pure aluminum,respectively,which indicated that the addition of graphene could significantly improve the mechanical properties of the composites. |
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