Research On 18Ni300 Fabricated By Laser Additive Manufacturing And Vision Sensing

Maraging steel is a new type of ultra-high strength steel lacked of carbon and strengthed by precipitate.it is widely used in aviation,aerospace,mechanical manufacturing,atomic energy and other important fields,Because of its good toughness,weldability and simple heat treatment.Because of its short production cycle,good economy and limited material structure design,additive manufacturing(AM)is more characteristic than traditional technology.The manufacture of maraging steel parts by means of AM has a good prospect.The evolution and characteristics of the molten pool during the deposition using the coaxial visual sensing technology is studied by three plans-single-trace-single-layer(STSL)test,multi-trace-multi-layers(MTML)test and single-trace-multi-layers(STML)test.The evolution of microstructure and mechanical properties of the sample-AM was analyzed.Since the good strength and toughness of 18Ni300 come from heat treatment,two types of solid solution,aging treatment and aging treatment have been studied to investigate the microstructure evolution behavior of the sample-AM.In the STSL test,the orthogonal experimental was designed.The result showed that the laser power,scanning speed and powder feeding rate was influented on the forming significantly.Through STML test,it is found that the bottom of the part is mainly small columnar crystal,the middle part is columnar crystal with upward growth,and the columnar to equiaxed transformation occurs at the top.the misorientation between martensite and martensite is 45°[001] In parts martensite was majority,but austenite was also found at the bottom due to element diffusion.Austenite was at the top of parts due to incomplete martensite transformation.martensite with good mechanical stemed from a large number of small angle grain boundary on intragranular.Porosity and cracks are easy to occur in STML test.The porosity defects are mainly caused by defects in the powder,and the cracks are closely related to the oxidation of elements.Although the non-dense defects can be avoided and the density is increased,the oxide inclusions of Ti and Al can be easily produced in the MTML test because of the oxidation of the elements.In order to suppress the inclusion in MTML test,the test with low oxygen content was designed.It has been found that the surface roughness and the accuracy of the parts-AM are greatly improved by decreasing the oxygen content.In low oxygen content,the area of equiaxed crystal at the top is obviously decreased due to the laser scattering by plasma.In addition,the primary spacing in the lower oxygen content is larger than that in the atmospheric.In mechanical properties,because of the obvious reduction of the defects in the lower oxygen,the tensile properties have been greatly increased to 907.8Mpa without heat treatment.Intermetallic compounds with Ni,Ti,Mo and other alloy elements were found in the matrix of the specimen under low oxygen content on heat treatment.These intermetallic compounds was dispersed fine and coherent matrix,these intermetallic compounds can greatly improve the strength of the sample,the maximum strength of the sample after heat treatment can reach 1586.7Mpa.In order to study the feature of molten pool,the real-time the molten pool image geometric characteristics by using coaxial vision sensing technology were carried out.In STSL test,with laser power increases,the area of molten pool almost increases linearly.Scanning speed increasing caused the area of molten pool decreasing,and the increase of feed rate decreases the area of molten pool.In STML test,the evolution of the molten pool area can be divided into three stages.Time-frequency analysis shows that the main components of the optimized process parameters are concentrated in two frequency bands of 0 Hz~1.2 Hz and 1.3 Hz~1.8 Hz.In the MTML test,the two frequency bands with 0 Hz~1.2 Hz and 1.3 Hz~1.8 Hz also have higher frequency than the long-side parallel repeated scanning(LPRS)method in either atmosphere or argon.In argon environment,the molten pool was relatively stable,the brightness of molten pool was higher,the shape was regular,and it had higher stability of molten pool.Therefore,higher forming accuracy can be obtained in argon environment.