Microstructure And Properties Of Laser In-situ Synthesis Reinforced Cu Matrix Composites

Copper had excellent electrical and thermal conductivity,corrosion resistance and ductility,and it was widely used in electronics,transportation,aviation,military and other fields.However,with the development of science and technology and the upgrading of industrial products,increasing requirements had been placed on the comprehensive properties of copper materials,the single copper material had gradually been unable to meet the actual demand.Adding multiple reinforcement to the copper matrix by in-situ synthesis was an effective method to further improve the material properties.The in-situ synthesized hybrid reinforcements could not only form a strong interfacial bonding with the matrix,but also exhibited different strengthening mechanisms due to their different types,morphologies and sizes,which could improve the mechanical properties of the composites.Therefore,this paper selectted the combination of laser direct energy deposition technology and in situ synthesis to prepare ZrB2-(ZrC or carbon onion)reinforced copper matrix composites.According to the thermodynamic principle,the feasibility of the in-situ synthesis reaction was confirmed.According to the analysis of the phase diagram,the molar ratios of Zr and B4C were 3:1 and 2:1 respectively,and the synthesized products were ZrB2-ZrC and ZrB2-C,and two groups of raw material powders were designed.In the Cu matrix,ZrB2 whisker and ZrC particle were in-situ synthesized and formed a strong,defect-free interfacial bonding with the matrix.Increaseing of the cooling rate of the melt,the size of ZrB2 decreased but the aspect ratio increased,and the morphology of ZrC changed from square to polygon.The necessary synthesis conditions of carbon onion were analyzed and summarized.The in-situ synthesis of carbon onion was successfully achieved in liquid metal by using the heat source of a laser beam,the substrate of copper,and the carbon source of B4C.The nucleation process of carbon onion was mainly a uniform nucleation mechanism,and the growth process was dependent on the two-dimensional nucleation mechanism of graphite and the lateral growth process of steps.With the increase of undercooling,the critical nucleation of carbon onion decreased,and the growth rate of the graphite layer increased,resulting in a decrease in the size and an increase in the number of carbon onions.Under the condition of high undercooling,a Cu interlayer with a thickness ranging of 2-30 nm was formed inside the carbon onion,the nano-Cu interlayer adapted to the curvature of carbon onion by twinning,curving and bending.Cu matrix composites were mainly composed of ZrB2 whiskers,ZrC/carbon onion particles,Ni dendrites and Cu matrix.Cu composites were prepared under different laser process parameters,the changes of macroscopic morphology were analyzed.With the laser energy density increased,the height of the single-track sample decreased,the width and the dilution rate increased.From the surface of the single-layer samples to the interface with the substrate,the number of needle-like phases gradually decreased,the equiaxed dendrites gradually transformed into columnar dendrites,therefore,the change of microhardness also showed a decreasing trend.With the increase of laser energy density,the size and life time of the molten pool increased,the diameter of the needle-like phase decreased and its length increased,and the size and spacing of Ni dendrites increased.At the track interface of the multilayer samples,a continuous transition of dendrite morphology from cellular to largesized dendrites to small-sized dendrites could be observed.With the reinforcement content increased from 10 wt%to 30 wt%,the microhardness of ZrB2-ZrC/Cu matrix composites increased from 134 HV0.2 to 504 HV0.2;the tensile strength first increased and then decreased,and the 15 wt%ZrB2-ZrC/Cu matrix composite had the highest tensile strength at 535.2 MPa.With the increase of laser energy input density,the average tensile strength of ZrB2-ZrC/Cu matrix composites decreased,the elongation increased,and the fracture mechanism changed from a ductile-brittle hybrid mechanism to a ductile fracture.The tensile strength of 20 wt%ZrB2-C hybrid reinforced copper matrix composites was 548.8 MPa.The strength contribution for hybrid strengthening of in-situ synthesized reinforcements was derived from the load transfer,thermal mismatch and dispersion strengthening.Ni dendrites could transfer loads and directly reinforced the Cu matrix.The strength contribution of each reinforcement was calculated according to the reinforcement mechanism.The strength contribution of ZrB2 whisker was the highest,and its strengthening efficiency per unit volume increased with the increase of aspect ratio.The effects of different process parameters,types and contents of reinforcements on the wear resistance of Cu matrix composites were compared and tested.With the reinforcement content was increased from 10 wt%to 30 wt%,the friction coefficient and wear rate of ZrB2-ZrC/Cu matrix composites decreased by 48.5%and 67.9%,respectively.The ceramic reinforcement reduced the current-carrying wear resistance of the composite by reducing the surface roughness,electrical ablation and oxidation,so the Cu composites could maintain a relatively low wear rate under the high current and sliding speed.ZrB2-C/Cu matrix composite had both the strengthening effect of ceramic reinforcement and the lubrication and friction reducing effect of carbon onion,so it showed a lower friction coefficient and lower wear rate than ZrB2-ZrC/Cu matrix composite.