Study On The Formability Of Maraging Steel Alloy And Its Composite Materials Fabricated By Laser Metal Deposition

As an emerging laser additive manufacturing technology,laser metal deposition(LMD)has many advantages such as rapid manufacturing,personalized customization and high resource utilization.In this paper,18Ni-300 maraging steel(abbreviation as MS)was selected as the research object,and MS alloy material and its composite material fabricated by LMD were systematically studied from the improvement and optimization of laser and heat treatment process and composition design,in order to improve its microstructure and mechanical properties.It is found that the optimal laser energy density is about 90 J/mm2,and the relative density of the sample can reach 99.98%.According to the analysis of phase transformation temperature,reasonable heat treatment temperature and time of concrete aging treatment(AT)and solution-aging treatment(ST+AT)were determined.Then,the microstructure and properties of the samples were studied and analyzed in detail by density test,microstructure and composition analysis,phase structure and phase transformation analysis,microhardness test,friction and wear test and electrochemical test.The microstructure of MS is mainly equiaxed and cellular.MS fabricated by LMD mainly contains a large amount of martensite and a small amount of retained austenite.MS+Al metal matrix alloys were prepared by LMD.When the content is less than 6 at.%,the microstructure of the sample is fine equiaxed dendrites.When the content of Al reaches 12 at.%,a lot of acicular and massive martensite structures appear in the matrix,leading to the degradation of MS+12 at.%Al properties.With the addition of Al,the retained austenite in the sample gradually decreases,and the precipitation of Ni3Al intermetallic compounds increases,which promotes grain refinement.Strength,hardness and wear resistance were enhanced by fine grain strengthening,grain boundary strengthening,dislocation strengthening and Orowan strengthening.Thus the hardness increases from 312 HV of MS to 514 HV of MS+12 at.%Al.The wear resistance of MS+6 at.%Al sample is the best,which increases by 137%.The corrosion current of MS+6 at.%Al is 4.082 μA/cm2,which is 115%lower than that of MS.After MS+6 at.%Al aging treatment,a large number of Ni3(Ti,Mo,Al)precipitates appear in the microstructure,and the second phase enhanced precipitation appears in the martensitic matrix,and the grains are refined.Compared with as-fabricated(AF)sample,the hardness and wear resistance of the samples after heat treatment are significantly improved.The sample with the highest hardness and wear resistance is 460℃ AT,hardness is 638 HV,wear rate is 1.75 ×10-6 mm3/Nm,hardness and wear resistance are 68%and 118%higher than AF sample,respectively.Compared with AF sample,the corrosion resistance of AT sample significantly reduces,which is due to the formation of galvanic cells between the precipitated phase and martensitic matrix,resulting in greater corrosion tendency and faster corrosion rate of matrix in NaCl solution.The microstructure of MS+H13 composites prepared by LMD consists of martensite,retained austenite,carbides and intermetallic compounds.With the increasing of H13 mass fraction,the microstructure changes from equiaxed grain to columnar grain.The hardness of the sample increased sharply from 312 HV of MS to 558 HV of H13.Meanwhile,the hardness,wear resistance and corrosion resistance of the composite were improved obviously.The 50%MS+50%H13 samples showed good wear resistance,and the wear mechanism was adhesive wear and abrasive wear.Furthermore,the increase of Cr content contributes to the formation of a dense oxide layer on the surface of the sample during the electrochemical corrosion process,thus improving the corrosion resistance of the sample.75%MS+25%H13 and 50%MS+50%H13 have lower corrosion current,which are 3.479μA/cm2 and 3.576μA/cm2,respectively.Compared with MS,they are 1.5 times lower.Compared with the as-deposited 50%MS+50%H13 sample,the hardness and wear resistance of the sample after heat treatment are significantly improved.In addition,the hardness and wear resistance of AT samples are better than those of ST+AT samples.This may be associated with stress relief and slight grain coarsening.In addition,it should be considered that intrinsic heat treatment occurs during the LMD process,which may have resulted in coherent precipitation or concentration of atoms in rich solute zones,which may have been dissolved during the solution treatment.460℃ AT has the highest hardness of 639 HV,520℃ AT has the highest wear resistance,and the wear rate of 1.42×10-6 mm3/Nm.After heat treatment,the corrosion resistance of the sample significantly reduces.This is because the passive film is formed on the surface of AF sample at the initial stage of corrosion,which effectively obstructs the corrosion and improves the corrosion resistance of the sample.After heat treatment,the corrosion performance of the sample deteriorates because the grain is refined and there are a lot of fine and compact lath martensite in the matrix,which provide good nucleation points for pitting corrosion.