Microstructure Control And Properties Of In Situ Fabricating Particulate Reinforced Aluminum Matrix Composites

Much attention has been paid to the particulate reinforced aluminum matrix composites.Prior studies focused on exploring the new in situ fabrication processes and systems.But it is essential to control the feature and distribution of particulates.It has important theoretical and current meanings to optimize the in situ system,fabrication and solidifying processes so as to control the microstructure and properties of composites.The paper is based on two academic theories.One is to control the microstructure of in situ composites,such as kinds,feature,and distribution of particulates and grain size of matrix et al.It may offer the theoretical basement to produce composites on an industrial scale.The other is to explore the appliances of electromagnetic technologies in composite materials process.The in situ(Al₂O₃+Al₃Zr)_p/Al composites is fabricated composites using Al-Zr(CO₃)₂ components by direct melt reaction method.The in situ(Al₂O₃+Al₃Zr+ZrB₂)_p/Al composites are synthesized using Al-Zr(CO₃)₂-KBF₄ and Al-Zr(CO₃)₂-H₃BO₃ components.The sizes of particulates are in the range of 2～3μm.In order to refine and promote the uniform distribution of particulates the foreign electromagnetic fields are introduced to control the microstructure of composites.The (Al₂O₃+Al₃Zr)_p/Al composites are synthesized by electromagnetic stirring method with 0.025T magnetic induced intensity.The size of particulates is refined to 1～2μm.They are well distributed in matrix.The vortex flow occurs due to tangential electromagnetic force,which makes it easy for reactant particles to enter into the molten aluminum.The contact opportunity between reactants is increased.Meanwhile the electromagnetic forces accelerate the mix convection moving and add up the velocity of heat,mass transfer and diffusion.When the magnetic induced intensity is 0.04T the electromagnetic force is raised largely.The gradient(Al₂O₃+Al₃Zr)_p/Al composites are produced due to the centrifugal force.The high frequency sinusoid modified electromagnetic field is combined when the low frequency sinusoid modification signal imposed on the high frequency carrier wave. The magnetic induced intensity is 0.029T when the frequency and voltage of sinusoid signal is 10Hz and 7V separately.Meanwhile the current of high frequency magnetic field is 30A.The distribution rules in coils are studied systematically.Turbulence, stirring and oscillation phenomena take place due to the modified electromagnetic field, which improve the kinetic conditions of in situ fabrication.The volume fraction of particulates in composites is increased largely.The size of particulates are refined to 0.5～1μm and the particulates distribute uniformly in matrix.Magnet chemistry analysis illustrates that the melt receive the energy of electromagnetic field.The entropy and energy status of in situ reaction system are improved,which promote the in situ reaction.Conventional pinhole and shrinkage would appear in the cast composites with permanent mold.When the squeeze casting technique is utilized to cast composites,the tightness of microstructure is increased and the grain size of matrix is refined.The feature of particulates changes slightly.So it is suitable to produce composite parts. Semi continuous casting and electromagnetic stirring semi continuous casting are practiced to cast the composite.The casting temperature is in the range of 710～720℃,the casting speed is 9cm/min and the cooling water amount is 0.8t/h.The features of particulates almost do not change whilst the distribution state is more uniform than ever.When the magnetic induced intensity is 0.025T the grain size is refined and large areas with equiaxed grain appeare.With the increase of stirring intensity the pattern of silicon phases change from plate to needle and slender rod.Later high frequency electromagnetic continuous casting processes are used to increase the surface quality of slab.The special closed water cooling slit mold is introduced.The soft contact between slab and mold is reached.The surface of slab is smooth when the current is 80A.There are apparent oscillation marks on the surface of slab when using square wave modified the high frequency electromagnetic field.Higher frequency of modified wave,smaller space of oscillation marks.Basing on the above conclusions an "integrate electromagnetic fabrication of composites"is brought forward,which combines the low frequency melting with high frequency continuous casting.Microstructure observation shows that the size of particulates is 1～2μm and they are well distributed in matrix.The technology is suitable to produce composite slab with high properties and excellent surface quality.The tensile property and wear performance of composites are further studied.For A356,15vol%(Al₂O₃+Al₃Zr)_p/A356 cast by permanent mold,squeeze casting,semi continuous casting and integrate electromagnetic fabrication,the tensile strengthes are 184.6,263.2,345.7,355.8 and 340.6MPa,the elongationa are 7.1%,4.7%, 15.1%,14.2%and 10.1%individually.The fracture mechanisms are the mixed forms of brittle and plastic ones.The abradability study shows that it is not the case that high adding amount of reactants has high wear resistance property.Because the particulates would sustain the wear before falling and aggregate the wear after falling.For (Al₂O₃+Al₃Zr)_p/A356 composites the best value of theoretical volume fraction is 10%.The paper study extends the system of in situ composites and expands the appliance domain.Electromagnetic stirring and modified high frequency electromagnetic field are successfully introduced into the fabrication of particulate reinforced aluminum matrix composites.The electromagnetic forces enhance the thermodynamic and kinetic conditions.Hence the sizes of particulates are refined and they are well distributed in matrix.Meanwhile the parameters of electromagnetic field are optimized.The process mechanisms about squeeze casting,semi continuous casting with electromagnetic stirring and integrate electromagnetic fabrication are put forward.The aim of control the microstructure and surface quality of composite slab are realized.Furthermore the gradient particulate reinforced composites are produced under electromagnetic field.