Microstructure And Mechanical Properties Of The 18Ni(350) Ultra-high Strength Steel Fabricated By Gas Metal Arc Additive Manufacturing

Wire and Arc Additive Manufacturing（WAAM）is a kind of rapid prototyping technology for metal parts,which has the characteristics of high forming efficiency and low equipment cost.Ultra-high-strength steel has received widespread attention due to its excellent mechanical properties.At this stage,the research on the use of AM technology to manufacture ultra-high-strength steel is mostly focused on selective laser melting additive manufacturing（SLM-AM）,while the use of arc additive manufacturing technology is less.SLM-AM technology has the problems of low deposition efficiency and difficulty in ensuring the density of additive parts.It is not suitable for medium and large-scale building additives.Based on this,this article takes the self-made 18Ni（350）ultra-high-strength steel welding wire as the object,and uses the gas metal arc additive manufacturing（GMA-AM）process to manufacture 18Ni（350）ultra-high-strength steel.Mainly studied the forming process of parts,the microstructure and properties of the as deposited parts,the heat treatment process and the microstructure and properties of the heat-treated parts.At the same time,a test of adding ultrasonic impact treatment to the additive process was carried out to study the structure and properties of the formed parts under the action of ultrasonic impact treatment.First,a single-pass single-layer forming process test of 18Ni（350）martensitic steel was carried out,and the range of process parameters for good forming was determined.Research shows that the forming quality of single pass and single layer in pulse mode is better than that in CMT mode.In the pulse mode,the surface of the single layer is smooth,without holes and other defects that affect the forming quality,and the good forming parameters have a wide range.Secondly,a single-pass multi-layer forming experiment was carried out using the optimized additive process parameters to study the structure and performance of the single wall under different process parameters.The study found that the microstructure of each single wall is relatively small,and they are all composed of dendrites.Among them,the dendritic region is martensite and the interdendritic region is austenite.The average hardness of the top of the single wall is 325 HV_1.0,and the hardness of the middle and bottom is 510 HV_1.0.The average tensile strength in the transverse direction is 1212.3±173.2 MPa,and the elongation is0.9±0.6%;the average tensile strength in the longitudinal direction is 1481.1±5.9 MPa,and the elongation is 4.8±3.5%.On this basis,combined with the strengthening characteristics of 18Ni（350）martensitic steel and the properties characteristics of the as deposited state,the heat treatment process test was carried out.Studies have shown that the sample is in good condition within the solid solution temperature range of 1050℃～1150℃,the element segregation of the sample disappears after solid solution,and the microstructure changes from dendrites to equiaxed crystal grains;the microhardness values differ between the sample regions small,the anisotropy of tensile strength disappears,and the elongation increases significantly.After the solid solution sample is aged at 480℃×6 h,dispersed Ni₃Ti and Ni₃Mo strengthening phases are formed in the matrix.The average microhardness of the sample is 624±24.8 HV_1.0,and the transverse and longitudinal tensile strengths are equivalent,2123 MPa respectively,2163 MPa,the elongation is not 6.8%and 5.4%,respectively,and the property of the sample under the best state of the traditional process（tensile strength 2450 MPa,elongation 7%）is basically equivalent.Finally,an ultrasonic impact test was carried out in the process of GMAAM 18Ni（350）,and the structure and properties of the formed parts under ultrasonic impact were studied.The results showed that the structure of the ultrasonic impact parts changed from columnar dendrites to short rods or granular crystals.The average grain size decreased from 10.4μm to 6.2μm.The transverse and longitudinal tensile properties are equivalent,the tensile strength is 1104±28MPa,the elongation is 2.6±0.5%,and the hardness is 482±40.3 HV_1.0.After heat treatment,the tensile strength of ultrasonic impact parts is 2079±46 MPa,the elongation is 6.8±0.8%,and the hardness is 561±19.7 HV_1.0.The property of ultrasonic impacted parts after heat treatment is significantly improved compared with that of unimpacted parts under the same heat treatment process.