Investigation On The Process And Oxidation Properties Of Mo-Co Alloys By Selective Laser Melting

With the development of ship technology,the service environment of the workpiece changes,and higher requirements are placed on the material and its preparation process.Some traditional structural materials are difficult to meet the actual working conditions of ships,and the research of new materials and their preparation processes has become a key issue to be solved urgently.Molybdenum-based alloy is expected to replace nickel-based alloy and aluminum-based alloy due to its high strength and stable mechanical properties in high temperature environments,making them a forward-looking structural material for ship development.The preparation of the impurity element by the conventional method is complicated,and the shape of the formed part is limited.The Mo-6Co alloy prepared by the selective laser melting technique studied in this thesis has compact structure and good oxidation resistance.In this experiment,spherical pure metal powder was used as raw material.The effects of laser power,scanning strategy,powder thickness,scanning speed and scanning pitch on the microstructure of the formed parts were studied to determine the optimal process parameters.The addition of cobalt to the molybdenum material can improve the oxidation resistance of the alloy.After studying the mixing process of the powder,the shaped parts with different cobalt content were prepared by changing the addition amount of cobalt in the molybdenum,and the influence of cobalt on the microstructure and mechanical properties of the molybdenum was studied.The effect of cobalt on the high temperature oxidation performance of molybdenum was studied by using related characterization instruments.The heat treatment process of molybdenum-cobalt alloy with better comprehensive performance was selectedLaser power and scanning speed can seriously affect the forming quality.In this paper,by comparing the density and internal pores of the formed parts under different process parameters,the optimal preparation process of the formed parts is obtained:the laser power is 325W,the scanning speed is 400mm/s,the powder thickness is 30μm,and the scanning pitch is 40μm.At this time,the formed workpiece has the largest density,which can reach 94.92%.By comparing the different molybdenum-cobalt alloys designed in this experiment,it can be found that the cobalt element is solid-dissolved in the molybdenum matrix,which has the effect of refining the internal grains of the molybdenum material,improving the hardness and compressive strength of the material,but promoting the internal The generation of cracks.The microstructure of the sample melted by the selected area laser has anisotropy.The hardness in the vertical direction is higher than the hardness in the horizontal direction,and the compressive strength is also larger.At this time,the compression curve is more in line with the characteristics of brittle fracture.By observing the fracture of the material,it is judged that the fracture modes of the four molybdenum-based alloys designed in this experiment are quasi-cleavage fractures.The weight loss of pure molybdenum sample,Mo-3 Co alloy sample,Mo-6Co alloy sample and Mo-9Co alloy sample at 400℃,600℃,800℃ and 1000℃ after oxidation for 1 h was systematically analyzed.The surface of the layer and the shape of the section were observed.It can be found that the Mo-6Co alloy sample is violently oxidized at 800℃ to 1000℃ to produce volatile MoO3.The results of the heat treatment experiment show that the grains inside the sample tend to become larger as the heat treatment temperature increases.