Study On The Fabrication And Properties Of In Situ Cu-Ti(Zr)-B Particulate Reinforced Copper Matrix Composites

Because of the excellent electrical conductivity,thermal conductivity and ductility,as well as good wear resistance,high strength and thermal stability,particulate reinforced copper matrix composites are widely used in high-voltage switch electric contact,electromagnetic rail gun,rail transit contact line and high-speed rail brake pad.Conventionally,particulate reinforced copper matrix composites are prepared by powder metallurgy or mechanical alloying,of which the processes are always complex and costly.In contrast,in-situ reaction casting method based on the casting technology,features in simple technology and low cost.During the in-situ reaction casting process,particulate strengthening phases are generated through the reaction of the raw material in the melt,leading to the good interface between the particles and copper matrix.Further,complex-shaped copper matrix composite parts can be singly formed using in-situ reaction casting method.Besides,the internal density of the composite parts prepared by in-situ reaction casting method can also be improved obviously.However,there are two important problems to be solved for the particulate reinforced copper matrix composites prepared by in-situ reaction casting.The first one is the aggregation of strengthening particles,these particles tend to aggregate during the solidification process of composite melt due to the demand of lower surface free energy.The aggregation place will serve as crack initiation site in tensile process,and accelerate the crack growth until rupture.As a result,the aggregation of strengthening particles will deteriorate the mechanical properties of composite materials seriously.The other one is the inevitable harmful impurities introduced in the casting process and the incomplete reaction,even small amounts of solute impurities can decrease the conductivity of material significantly.How to prepare high-performance copper matrix composites with homogeneous microstructures and dispersed particles becomes a hot topic.The in-situ reaction casting method was used to prepare Cu-Ti(Zr)-B composites in this paper.In-situ particulate reinforced Cu-TiB2 and Cu-Zr-ZrB2 composites were prepared.The distribution of TiB2 reinforcing particles was improved by rotating stirring magnetic field,and the structure and properties of Cu-TiB2 composites were improved by rare earth La.Cu-Zr-B system composites were designed and prepared,which effectively avoided the residue of solute elements caused by incomplete reaction.By the synergistic strengthening of micron-sized ZrB2 and nano-scale CusZr,the composites have high strength,high electrical conductivity and good wear resistance.Additionally,the as-cast samples were rolled to further improve the mechanical properties of the composites and verify their deformation ability.The microstructures,mechanical property,conductivity property,friction and wear behavior of the rolled composites were investigated systematically.The contents of this research are mainly as follows:(1)Cu-Ti-B system was selected for the preparation of TiB2 particulate reinforced copper matrix composites,and Cu-TiB2 composites were successfully prepared through the in-situ reaction of Ti and B in Cu melt by casting method.The average diameter of TiB2 particles generated by in-situ reaction was about 1 ?m,and the fully grown TiB2 particles were mostly hexagonal.TEM results showed that the layer fault at the interface between in-situ generated TiB2 particles and Cu matrix effectively compensated the lattice mismatch,leading to the formation of a good interface.It was also found that the tensile strength of rolled Cu-TiB2 composite increased with the content of TiB2(from 397 MPa for pure Cu to 442 MPa for Cu-1.5wt.%TiB2).The conductivity of composites remain at about 80%IACS,a little decreased with pure copper.The friction and wear tests of Cu-TiB2 composite were carried out under different loads,sliding speeds and sliding distances.The results showed that the volume loss of the composites decreased significantly with the increasing TiB2 content.At higher load and sliding speed,the improvement in abrasion resistance of the material was more noticeable.Due to the load transfer enhancement of large TiB2 particles,the wear mechanism of composites was changed from the adhesive and fatigue wear(pure copper)to the adhesive and abrasive wear.(2)Rotating stirring magnetic fields with different intensity were applied at the solidification stage of Cu-TiB2 composite.The results showed that with the rotating stirring magnetic field,the bulk agglomeration disappeared and the TiB2 particles dispersed well in Cu matrix,namely the distribution of TiB2 particles was improved effectively.The electromagnetic volume force generated from the stirring magnetic field will cause the forced convection of composite melt.In addition to rotational motion,the radial pressure gradient will also generate secondary convection in the meridian plane.It is this forced convection that disperses the aggregated clusters of TiB2,resulting in uniformly dispersed Cu-TiB2 composite.However,when the intensity of magnetic field is too high,severe forced convection increases the probability of collision of TiB2 particles,the increased melt temperature prolongs the solidification time,and thus resulting in reaggregation of dispersed TiB2 particles.Therefore,the intensity of stirring magnetic field has an optimal value.In this study,the most homogenous distribution of TiB2 particles is obtained when the excitation current is 60 A.According to the reinforcement theory for composite,the more diffused TiB2 particles under stirring magnetic field,alleviated the negative influence of the particle aggregation on the strengthening effect,thus effectively improved the tensile strength and hardness of composite.The tensile strength and hardness of Cu-1 wt.%TiB2 were increased from 406 MPa and 125 HV without magnetic field applied to 470 MPa and 171 HV.(3)Trace rare earth La was added into the composite.It was found that La could significantly refine the original self-generated TiB2 particles in the Cu melt,and the average size of the particles decreased from 1,132 nm(without La addition)to 422 nm at 0.04 wt.%La.This is mainly due to the fact that the surface active element La can reduce the interface energy of the melt and particles,thereby reducing the critical nucleation free energy of TiB2 and increasing the nucleation rate of TiB2.In condition of equal TiB2 content,higher nucleation rate of TiB2 means smaller particle size.With the addition of rare earth La,the transition of the interface between TiB2 particles and Cu matrix from angular facetted growth to non-facetted growth was observed.The theoretical analysis showed that the reduction in the interface energy decreased the wetting angle between TiB2 particles and Cu melt.The good wettability weakens the facetted features of TiB2,and the interface morphology becomes rounder.Due to the dual effect of rare earth La on the improvement of both TiB2 particle size and morphology,the strength,hardness and elongation of Cu-TiB2 composite were improved with the La addition.In addition,the rare earth La can purify the melt and remove impurities,which significantly enhances the electrical conductivity of the Cu-TiB2 composites.The electrical conductivity of the composites with 0.04 wt.%La reached to 88.7%IACS.(4)Based on the results above,dual-scale particulate reinforced copper matrix composites Cu-Zr-B were design and prepared,i.e.,micron-sized ZrB2 particles formed through the in-situ reaction of Zr and B in Cu melt,and nanoscale Cu5Zr phase precipitated from the supersaturated solid solution Cu matrix,which avoids the negative effects of the solute atoms on the electrical conductivity.These composites with the dual-scale particles behaves high strength,high electrical conductivity and good wear resistance.The tensile strength of Cu-0.3wt.%Zr-2 wt.%ZrB2 after rolling and aging is 575 MPa with a high conductivity of 78.4%IACS.Compared with that of Cu-0.3 wt.%Zr,the volume wear amount of Cu-0.3 wt.%Zr-2 wt.%ZrB2 significantly decreased under different loads and sliding speeds.The friction coefficient decreased from 0.8 to 0.6,and the wear mechanism also changed from adhesive wear and fatigue wear to adhesive wear and abrasive wear.