Abstract
Copper and its alloys with excellent electrical and thermal conductivity are widely applied to electronics, electricity, metallurgy, aerospace industrial fields, etc.However, the low strength of copper are unable to meet all the requirements of the applications in industry, especially the parts used in extreme conditions,such as the conducting rails of electromagnetic railgun,contact wires for high speed electric railway,etc. The enhancement of strength is always primary investigation in copper alloys. Copper alloys prepared by traditional methods have the strength enhanced at the expense of its conductive properties, which lowers with increasing the strength of alloys. Surface modification technique (SMT) attracts wide attentions due to enhanced surface strength and remaining high conductive properties of the integral copper alloy. In all SMTs,laser technique possesses stronger competition and is widely used in the industrial manufacturing field. Whereas, so far, the investigation of employing laser technique to enhance the strength of copper alloys is relatively less because of high reflection and rapid thermal conductivity of copper. Consequently,finding or improving technology to enhance effectively surface strength of copper is the emphases of laser surface modification on copper.
The intention of the dissertation is to strengthen the surface of copper employing laser surface modification to confer the surface with high wear resistance and high electrical conductivity. For this purpose, the technology for laser surface modification of copper is designed. Laser surface alloying is selected because it has less influence on electrical conductivity of copper than laser cladding. The light absorption of copper is increased with pre-placed powders. Ni-based alloy powders with better wettability are used as strengthening material depending on selecting principle of alloying materials. The optimal parameters have been obtained through a large amount of prior experiments, which have small difference for different sizes of samples. The effects of scanning speed on microstructure and properties are investigated especially. Based on these works, firstly, the surface of copper is strengthened by employing Ni-based alloy, and then two ceramic particles (TaC and NbC) are added into Ni-based alloy layer by in-situ laser synthesis, respectively. The microstructure and formation mechanism of three modified layers are discussed. The micro-hardness, wear resistance and electrical conductivity are investigated also. The primary conclusions are as follows:
1. The absorption to CO2 laser of copper is improved through pre-placed powders.Therefore, laser surface modification of copper is accomplished with low laser power.The optimal parameters are as follows: copper substrate sizes, 50 mm x 30 mm x 5 mm; laser power, 2.2 kW; scanning speed, 4 mm/s; defocusing amount,50 mm; depth of pre-placed powders, 0.9 mm; overlapping ratio, 40%. For copper substrate with sizes of 50 mm x 25 mm x 5 mm, laser power of 2 kW is enough.
2. Scanning speed is key parameter of laser surface modification in the dissertation. With increasing the scanning speed within a certain scope, grain refinement, increased hardness, high alloying degree and high dilution rate are obtained. Consequently, Cu-based materials are prepared with high surface wear resistance and high integral electrical conductivity.
3. All of three modified layers are free from cracks with the characteristic of rapid solidification, which have homogeneous fine microstructures dispersed uniformly in the matrix. The presence of (Cu,Ni,Fe) solid solution exhibits good wettability between Ni-based alloy and copper, and the formation of metallurgical bonding between the modified layer and the substrate.
4. The average hardness of Ni-based alloy modified layer is enhanced to HVo.i650, which is 7 times of that of copper substrate. The wear resistance is improved by 4 times. It is attributed to particulate reinforcement, solid solution strengthening and grain refinement. The hardness and wear resistance of two ceramic particles reinforced Ni-based alloy modified layer are improved remarkably. TaC reinforcements has better effects on hardness and wear resistance of the modified layer, which has average hardness of 1.56 times and wear mass loss of 2/5 of Ni-based alloy modified layer. This result is attributed to the presence and the uniform distribution of in-situ synthesized ceramic particles.
5. The electrical conductivity of copper substrate is slightly affected by laser surface modification. The level of decrease of electrical conductivity depends on alloying degree (dilution rate). High dilution rate leads to high electrical conductivity.The electrical conductivity can remain above 90%IACS with suitable dilution rate.Therefore, optimizing parameters to control suitable dilution rate is critical to prepare Cu-based composite material with high surface wear resistance and high integral electrical conductivity.
6. The electrical conductivity is affected primarily by Ni-based alloy. Adding ceramic particles have little influence on it The influence of different ceramic particles on electrical conductivity is not same. For the two ceramic particles in this dissertation, the addition of NbC particles has less effect. Suitable doping of ceramic particles can improve greatly the surface property and has little effect on the electrical conductivity, which provides primary experimental foundation to prepare particulate reinforced surface modified Cu-based composite materials.
7. Comparing to other surface techniques, laser surface modification leads to metallurgical bonding between the modified layer and the substrate,and can enhance the characteristics of the metal surface, especially gives the surface special characteristics. It breaks the using limits of copper alloys, which possesses better economic benefits and promising applications.
Keywords: laser surface modification; copper; Ni-based alloy; TaC; NbC; wear resistance; electrical conductivity
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