Microstructure And Mechanical Properties Of Mo And Mo Alloys Prepared Using Laser Additive Manufacturing Processes

Molybdenum and molybdenum alloys have been extensively employed in aerospace,national defense and electronic industry owing to its excellent properties of high melting point,high temperature resistance,slow creep,low expansion,high thermal conductivity and high corrosion resistance.Molybdenum and molybdenum alloys are conventionally prepared by powder metallurgy process,while post welding and forging are usually used form parts with complex structures.There processes are usually time-cost and are difficult to prepare complex parts.Recently,the development of laser additive manufacturing（LAM）technology provides the a possibility for rapid near net shape forming of molybdenum and molybdenum alloys.Two main LAM processes,including laser melting deposition（LMD）and selective laser melting（SLM）,are first applied to the fabrications of pure molybdenum samples,with the aim to obtain the optimized process parameters.The optimized SLM processes are then selected to prepare molybdenum alloys to investigate the composition effects on the microstructure and mechanical properties.Finally,the hot isostatic pressing（HIP）process is adapted to modify the microstructure and mechanical properties.The OM,SEM,EBSD,XRD,etc are applied to investigate the microstructures,and the Vickers hardness tests and tensile tests are applied to test the mechanical properties.The main conclusions are as follows:（1）It is hard to fabricate thin walled plate molybdenum samples continuously using LMD process,since the top of the plate samples are prone to have rather wavy shapes.It is also hard to avoid the cracking of the plate samples.The LMD of tantalum is then carried out for comparison.It is found that tantalum has a wider process window and can be successfully prepared with a high density of above 98%and a good comprehensive strength and ductility.The difficulties of the LMD of molybdenum are supposed to due to the high cracking susceptibility to the impurity interstitials,and the balling phenomenon caused by the high conductivity of molybdenum and the evaporations of molybdenum oxides,which lead to the wavy shapes of the plate samples.（2）A series of pure molybdenum cubic samples are prepared by adjusting process parameters of laser power,the scanning rate,the overlap ratio,and layer thickness.The continuity of solidified molten pool tracks,the porosity,and cracks are investigated and compared to select optimized process parameters.Pure molybdenum samples with low porosities of lower than 0.5%,good surface finishing,and continuous molten pool tracks can be obtained.When the surface energy densities are higher than 17 J/mm²,and a 6 mm×6 mm area partition scanning strategy is applied,the corresponding samples can have continuous molten pool tracks and good surface finishing.The effects of energy density on the porosity can be fitted into a U-shaped curve.When the energy density is too high,the evaporations of the titanium substrate and the molybdenum oxides will form gas bubbles and be trapped in the molten pool,resulting in the residual round shaped pores.When the energy density is too low,the lack of fusion defects will from,resulting in the residual irregular shaped pores.（3）Using the optimized SLM process parameters,molybdenum alloys samples are then prepared by adding 0.45 wt.%carbon or 0.9 wt.%La₂O₃ into the molybdenum powders.The results shows that both alloyed samples can have high densities of above 99.5%.With the addition of La₂O₃,the formability of molybdenum alloy is improved significantly,showing better surface finishing and fewer cracks.With the addition of carbon,the formability of molybdenum alloy is improved slightly,show reductions of both cracks and pores.What is more,the grain sizes are reduced,columnar to equiaxed transitions of grains are prompted,and the microhardness is increased significantly.（4）The SLM molybdenum and molybdenum alloys are finally treated using HIP technique.The density of the sample with 0.9 wt.%La₂O₃ addition is improved obviously to 99.7%.Although the number densities of cracks are not reduced,however,the wideness of cracks are narrowed.The microhardness of all samples is reduced after HIP treatment,probably due to the release of thermal stresses.