Research On The Mechanism Of High Power CO₂ Laser Sintering Of Metallic Powder With Diamond Grits

In this thesis, CO₂ laser sintering experiments of bronze-Ti and bronze-Co-Ni powder with diamond grits have been done, aiming for exploring the new technique of manufacturing peculiar-shaped diamond tools and new synthetic material. Firstly, the technique of laser sintering has been studied, while keeping focus diameter 8.5mm, thickness of compact 0.7¹.0mm and constant flow of Ar gas, the results showed as follows: (1) During laser sintering, the bronze-Ti powder is oxidized most easily and plenty of residua and lump is observed. (2)The bronze-Co-Ni powder has outstanding performance of laser sintering: â‘ Under the high laser energy input, the microstructure of casting state and fishbone shape similar to branch crystal is observed. Under the low laser energy input, the microstructure is very loose and the grains gather and felt together. But when the laser energy is excessively high, laser sintering turns into liquid-phase sintering. â‘¡Lots of sockets exist in the fracture of sintered samples, so it is typical fracture with toughness. â‘¢In a certain range, while keeping constant scanning speed, the density and abrasive resistance of sintered samples are raised with the increase of laser power. While laser power is constant, the density are decreased and the abrasive resistance raised with the increase of scanning speed. But the relation of laser power, scanning speed and linear energy with the stretch intensity shows the characteristic of Gauss function. And when the linear energy is about 42J/mm, the sample with the optimal stretch intensity will be obtained. Secondly, the densification of metallic powder with diamond grits during laser sintering and the mechanism of combination between metallic powder and diamond grits have been analysed and studied in theory: (1) The densification of metallic powder compact during laser sintering includes three steps: the low melting point metal melted, high melting point metal melted slightly and atoms diffused. (2) The metal which reacts to C easily such as Cr, Fe, Co, gathers around the diamond grits, which makes much liquid solidify around the diamond nucleus and forms metallurgical combination on the surface of diamond. On the surface of Ti-coated diamond grits, Cu exists. Finally, the thesis emphasizes on study of the defect of high power laser sintering, such as the damnification of diamond grits and pore: (1) The damnification of diamond grits mainly includes oxidation and carbonization. Under high laser energy input, shape of diamond grits are destroyed and grits are oxidized slightly. But under excessively high energy input, the diamond grits are not only oxidized seriously, also carbonized to loose graphite slice. (2) Coating Ti on the surface of diamond grits will increase capacity of combination between diamond and metallic powder, and resistance of damnification. (3) The pore is induced mostly by air remained in the compact, and gas which is formed during oxidization of diamond. The thesis has researched systematically the technique and mechanism on high power laser sintering of metallic powder with diamond grits for the first time and pioneered the new field of laser sintering application. It is important and instructive for the manufacture of big area peculiar-shaped diamond tools and new synthetic material.