Effect Of Phosphorus Modification And Heat Treatment On Microstructure,Hardness And Wear Behavior Of Mg₂Si/Al Composites

In recent years, in the field of metal matrix composites, the development of particle reinforced aluminum matrix composites is rapid especially. This is not only because of its light weight, strength, stiffness than high, high shear strength, low thermal expansion coefficient, good thermal stability and thermal conductivity, conductive properties, as well as good anti-wear resistance, anti-corrosive organic fluids and solvents a series of advantages, but also because the world has abundant resources of aluminum.In addition, conventional equipment and techniques available for processing aluminum matrix composite molding, to enable the preparation and production of particle reinforced aluminum matrix composites more economical and practical. With the car running and other vehicles to high-speed, lightweight and low power technology development, particle reinforced aluminum matrix composites in the automotive and other devices on the application of carriage is of special significance, such as to increase speed ,reduce fuel consumption and reduce environmental pollution. Therefore, the particle reinforced aluminum matrix composites have become the world today the field of metal matrix composites of one of the most important hot spots, and increasingly to the industrial scale production and application of the direction.The Mg₂Si/Al composites is a new bright spot in composite materials development and practical applications.The intermetallic compound Mg₂Si exhibits a high melting temperature of 1085℃, low density of 1.99×103kgm^-3, high hardness of 4.5×10⁹Nm^-2, a low thermal expansion coefficient (TEC)7.5×10^-6K^-1 and a reasonably high elastic modulus of 120GPa. It is very suitable for aluminum matrix composites reinforced. Mg₂Si/Al composite materials in particular, the preparation method is simple, low cost, and has excellent machining performance, these features make this material as a possible large-scale production applications. Therefore, as a motor cylinder, disc, such as selling important components of lightweight material, Mg₂Si/Al composite materials have enormous market potential and wide application prospect. Over the past decade, researchers have been at home and abroad intermetallic compound Mg₂Si conducted a preliminary study.However, the primary Mg₂Si phases in normal in situ Mg₂Si/Al composites are usually very coarse, and thus, lead to poor properties to these composites. Accordingly, the poor properties restrict the material application in much field. So it is a principal topic to modify the morphology of reinforced phase, size and distribution as well as enhancing the poor properties. Therefore, this thesis mainly uses appropriate modification and heat treatment to improve the organization,size and distribution, improving the mechanical and wear resistance of composites and promoting its applied development in Engineering and practical process.In the present thesis, the crystalline and growth characters as well as the effect of cooling rate on growth manner of primary Mg₂Si crystal in Al melt are studied. The microstructure of Mg₂Si/Al composites are modified effectively via the addition of P-Cu alloy elements and heat treatment.Discussed the questions related to the mechanism of the process; Metamorphic Mg₂Si/Al studied before and after the mechanical properties of composite materials and dry sliding wear behavior.The major research efforts of the present study are as follows:(1) According to phase diagram and probative experiment, the solidification process of Mg₂Si/Al of the series of Al-20%Mg₂Si-9%Si was obtained: L→L1+Mg₂Sip→L2+(Al+Mg₂Si)e+Mg₂Sip→(Al+Si+Mg₂Si)e+ (Al+Mg₂Si)e+ Mg₂Sip; he solidification process of Mg₂Si/Al of the series of Al-20%Mg₂Si-13%Si was obtained: L→L1+Mg₂Sip→L2+(Si+Mg₂Si)e +Mg₂Sip +Sip→(Al+Si+Mg₂Si)e+(Si+Mg₂Si)e+Mg₂Sip +Sip ; According to thermodynamic theory analysis, Reaction of Mg and Si can be spontaneously generated Mg₂Si;(2) It was found that the coarse primary Mg₂Si and Si could be refined as fined polygonal morphology after modification, the size also could be reduced. The large dendritic primary phase Mg₂Si is refined into small polygon size crystal block. The large irregular lump Primary phase Si has changed into smaller quadrilateral. In addition, it also was found that the eutectic Mg₂Si and Si phase were refined by phosphorus in the Al-20Si system.But in the Al-20Si system, the eutectic Mg₂Si and Si phase have no significant changes.The modification is the best when the amount of P is 0.6 % in Al-20% Mg₂Si-9%Si system; The modification is the best when the amount of P is 0.8 % in Al-20% Mg₂Si-13%Si system;(3) In the solution heat treatment process, the hardness of composite materials is gradually increasing with the increase in solution temperature, and the time of reaching the peak hardness is shorted. But the hardness of composite materials is decreased when the temperature is higher than 510℃; The curve of hardness of composite materials has a "double peak" phenomenon when the temperature is above the statute of limitations. The first and second peak hardness of Al-20%Mg₂Si-9%Si composites have appeared in 6h and 12h, respectively;the first and second peak hardness of Al-20%Mg₂Si-13%Si composites have appeared in 8h and 14h, respectively, when the aging temperature is 175℃The hardening speed of the alloys is faster, but hardening ability is decreased with the aging temperature increasing. The hardness is increased with the increasing of Si content. The hardness of composite materials has improved markedly after P modification. The hardness of all samples processed after T6 will increase about 15 18% than untreated;(4) It can be observed that Mg₂Si and Si primary phase has not changed, but eutectic phase of needle-like or fibrous globular pre-heat treatment changed into a worm-like and granular after aging treatment. This allows the mechanical properties of the material have been improved significantly. The precipitation speed is up with the continuous improvement of the aging temperature;(5) Wear tests revealed that at a fixed sliding speed under variable load conditions, the composite material has higher wear resistance after T6 + modification in Al-20Si or Al-25Si system. With no deterioration compared to the composite material, T6 + deterioration of the composite materials has a strong resistance to abrasion; And Al-Si hypereutectic alloy has better wear resistance than non-perishable composite materials. Load change at a fixed sliding velocity conditions, the conclusions with the same test I.The wear resistance of T6 + modification of Al-20%Mg₂Si-9%Si increased 5 times than the non-perishable,2 times than Al-20Si; The wear resistance of T6 + modification of Al-20% Mg₂Si-9% Si increased 0.5 times than the Al-25Si.(6) The experimental results show that the reasons for the enhanced wear resistance is in the process of wear and tear, Mg₂Si particles are changed smaller,dense and more uniform distribution after T6 + metamorphic,eutectic structure is changed more uniform small also. It can play a stronger role.Because the structures are not easily broken, inhibited to a large extent the appearance of plastic deformation. The ability of anti-plastic flow has been noticeably enhanced, resulting in the wear-resistance of the composite materials after modification is stronger than the composite material unmodified. The microstructure of Al-Si hypereutectic alloy is smaller than the microstructure of the composite material unmodified, so the wear resistance of Al-Si hypereutectic alloy is better than the composite materials unmodified.