Research On The Structure And Properties Of Semi-solid Extruded Diamond/2A12 Composites

Diamond has the characteristics of high hardness,high modulus,and low expansion coefficient,as a reinforcing phase of aluminum matrix composite which can significantly improve the elastic modulus and mechanical properties of the material.However,the diamond with large surface tension which lead to agglomeration easily,and does not wet with the aluminum liquid,which result that the interface between diamond and Al matrix is not tightly integrated.Therefore,the key points in preparing diamond particle reinforced aluminum matrix composite is improving the wettability between diamond and aluminum alloy melt and the dispersibility of the particles in the matrix.To solve these difficulties discussed above.In this paper,the diamond/2A12composite was prepared by semi-solid repeated extrusion and the diamond was plated a thickness of Nickel by electroless nickel plating.Investigated the stability of Nickel plating and the interfacial reaction between the diamond nickel plating layer and in matrix during the procession of preparation.Discussed the dispersion of particles in Al matrix and the organization of the composites.Explored the effect of repeated extrusion passes and T6 heat treatment on the microstructure and properties of the composite material.The main conclusions are as follow:?1?During the procession of preparation,the nickel plating on the diamond surface remained stable without shedding and loosening.The unplated diamond reacted with the matrix and generated Al₄C₃,which formed the interface relationship of Diamond-Al₄C₃-Al;nickel-plated diamond particles in the aluminum matrix is diamond-Ni-NiAl-Al;the T6 heat treatment process promotes the diffusion between Al and Ni,and the interface relationship is Diamind-Ni-Al₃Ni-Al.?2?The dispersibility of diamond particles in the composites which prepared by semi-solid extrusion was significantly improved.The particle uniformity increased from 54.3%to 73.6%.With the increasing of the number of semi-solid extrusion,the dispersion of diamond particles in the matrix got better and better,the particle uniformity increased from 73.6%to 87.9%.The microstructure of the diamond particle/2A12 composite was mainly composed of primary phase?-Al,D phase?diamond particles?,S phase?Al₂CuMg?and?phase?Al₂Cu?.Semi-solid extrusion refines the grain.After the extrusion is performed once,the grain size is 30⁵0?m.After extrusion 5^th times,the grains size is range from10 to 30?m.?3?With the increase of the mass fraction of diamond particles,the mechanical properties of the material increase first and then decrease.At the mass fraction of 2.5%,the composite material achieves the best performance,the tensile strength is 329.6 MPa,yield strength is 223.5 MPa,the elongation is 10.9%,and the hardness value is 130.3HB.With the increasing times of extrusions,the tensile strength and the yield strength of the 2.5%.wt diamond/composite both increase at first and then decrease.The hardness and elastic modulus increase gradually,the hardness increases from 130.0HB to 136.1 HB,and the range of modulus of elasticity is from 96.1 GPa to 102.4 GPa;when the number of extrusions is 3 times,the tensile strength reaches 356.6 MPa,the yield strength reaches 253 MPa,but when the extrusion passes is performed 5 times,the tensile strength decreases to 321 MPa,the yield strength decreases to 185 MPa.Elongation has been increasing from 10.8%to 12.4%.?4?After the T6 heat treatment to composites,the second phase particles S??Al₂CuMg?and?phase?Al₂Cu?precipitated from matrix.The hardness of the composite material was greatly improved to reach 158.7 HB,increased by 18.2%.The tensile strength is 342 MPa,increased by 9.8%,the yield strength is 228.5 MPa,increased by 4.1%,and the elongation has decreased by 4.5%.The elastic modulus increases from 96.1 GPa to 96.5 GPa.The fracture mechanism of the material after heat treatment is a dilute-based mixed fracture mechanism,but ductile fracture is more obvious.In this paper,the low-content diamond/2A12 composites were prepared by semi-solid extrusion,which solved the problem of serious agglomeration of diamond particles in the matrix.When a small amount of diamond particles?2.5%.wt?was added to matrix,not only improved the the elastic modulus?96.5 GPa?of composite,but also the composites with well tensile strength?356 MPa?and elongation?11.4%?.It lays a theoretical foundation for the research and development ofhigh-strength and high-elastic-modulus of Al-based composite materials that applied in precision instrument.