Study On Microstructure And Mechanical Properties Of Laser Selective Melting CLF-1 Steel

Nuclear fusion energy is regarded as the first choice of clean energy in the future by many countries in the world due to its advantages of large energy release,abundant raw material reserves,low mining cost and high safety performance.Low activation ferritic/martensitic（RAFM）steel has a lower thermal expansion coefficient,high thermal conductivity,good resistance to swelling and irradiation embrittlement resistance and is considered to be the future fusion structure material of choice for cladding,in response to the ITER TBM program requirements and the development strategy of China’s fusion DEMO,independent research and development by the southwest of China national nuclear physics institute at low activation ferritic/martensitic steel（CLF-1）.At present,due to the large volume of TBM,the large number of parts required in the manufacturing process,and the need for a variety of technologies to process the overall assembly,there are many problems to be solved,such as long manufacturing cycle,unstable structure and high cost.As a new metal material processing technology,laser selective melting（SLM）technology has many advantages,such as high processing accuracy,high production efficiency,high material utilization rate,and manufactability of complex parts,etc.It is hopeful to solve the existing problems in TBM manufacturing.In order to deal with the problems existing in TBM manufacturing,laser selective melting（SLM）technology was used to prepare low activated ferritic/martensitic CLF-1 steel and the microstructure of CLF-1 steel after solution aging and thermal isostatic pressure subsequent heat treatment.OM,SEM,TEM and other methods were used to characterize the structure of the material under deposition,solid solution aging and thermal isostatic pressure.The defect was characterized by X-CT technology.After that,the mechanical properties of CLF-1 steel were tested by tensile method,and the fracture was analyzed.The results showed that the deposited CLF-1 steel had higher tensile strength and yield strength but lower toughness,and the fracture was small and shallow dimmable,while the toughness increased after solution aging and thermal isostatic pressure treatment,but the tensile strength and yield strength decreased,and the fracture was large and deep dimmable.By comparing the microstructure of the material,we can see that the sedimentary state is a mixture of a large amount of martensite and part of ferrite,with defects such as pores and unmelted particles.After heat treatment,carbon atoms in martensite are precipitated,martensite is transformed into ferrite,and carbon atoms combine with trace elements in grain boundaries and grains to grow to form M₂₃C₆ carbides and MX compounds.X-CT data showed that there were many pores in the sedimentary state,and the improvement was not obvious after the solution aging,while the porosity of the sample decreased significantly after the thermal isostatic pressure.The conclusion is that the precipitation of M₂₃C₆ carbide and the grain size growth after heat treatment are the main factors for the reduction of tensile strength and yield strength,while the transformation of some microstructure into ferrite improves the toughness of the material,and the decrease of porosity after thermal isostatic pressure is considered to be the main reason for the improvement of tensile performance.