A method for microstructure regulation and harmless treatment of oxides of Fe-based alloy material based on LMD forming

Abstract

The method for controlling the microstructure of FeCrNiCoMo steel based on LMD and harmless treatment of oxides belongs to the technical field of laser additive manufacturing. The element composition of the steel powder is as follows: C: 0.001-0.003 wt%, Cr: 10.5-12.0 wt%, Ni: 6.0-9.0 wt%, Co: 3.0-7.0 wt%, Mo: 1.5-3.5 wt%, V: 0.01-0.03 wt%, Ti: 0.1-0.2 wt%, Al: 0.02-0.03 wt%, Si: 0.1-0.2 wt%, Mn: 0.1-0.7 wt%, S: 0.001-0.002 wt%, P: 0.001-0.002 wt%, and the balance of Fe. In view of the defects such as holes and incomplete fusion in the process of LMD forming steel material and the microstructure evolution of the material, the application provides optimized LMD process parameters and targeted heat treatment system. In view of the harmful oxides generated in the forming process of the steel material, 1.00-5.00 wt% of CaZr intermediate alloy (Ca: 50 wt%, Zr: 50 wt%) is added to the bulk material to improve the sensitivity of the steel material to oxygen in the forming process and to be used for oxide slagging during forming. The prepared parts can be used in the fields of aerospace, low-temperature storage and the like.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of laser additive manufacturing, and relates to a harmless method for controlling the structure and oxidizing of Fe-based alloy materials based on LMD forming. BACKGROUND

[0002] Laser melting deposition (LMD) technology uses a laser beam as a high-temperature heat source to melt the surface of a base material to form a molten pool. Metal powder / solid wire is synchronously fed into the molten pool through a powder / solid wire feeding device. After rapid melting and cooling, the powder / solid wire solidifies and forms a metallurgical bond with the base material. The laser deposition head moves according to a pre-planned path under computer control, and the part is manufactured by layer-by-layer accumulation. Depending on the deposited material, the entire process is usually carried out in an inert gas atmosphere such as argon or nitrogen. The LMD additive manufacturing system mainly consists of a laser deposition system, a motion control system and a protection and monitoring system. The laser deposition system is the core of the entire additive manufacturing system, mainly composed of a laser, a powder / solid wire feeding device, a cooler, a deposition head and a forming platform, which is used for material delivery and melting deposition on the forming platform. The motion control system is mainly composed of a CNC machine tool / robot, a program controller and CAM programming software, which is used to realize the spatial positioning movement of the deposition head / forming platform and manufacture components of different shapes. The protection and monitoring system is mainly composed of a safety enclosure, a gas chamber, online monitoring equipment and supporting software, which is used to protect the machining safety and monitor the entire forming process to ensure the machining precision.

[0003] Fe-Cr-Ni-Co-Mo system is a new type of maraging stainless steel that can balance high strength and good toughness. Maraging stainless steel is a high-strength stainless steel that combines low-carbon martensite phase transformation strengthening and aging strengthening. Gradually replacing precipitation-hardened stainless steel with maraging stainless steel is an important trend in the development of high-strength stainless steel. Forming FeCrNiCoMo steel using LMD technology has broad application prospects and great market value. However, due to the high degree of alloying of this material, the solidification and solid state transformation process during LMD forming is complex, and the additive manufacturing process maturity is still far from reaching a high manufacturing level. Laser additive manufacturing of high-strength steel may have organizational stress caused by multiple-phase structures such as alloy carbides, residual austenite and martensite, and may also have high thermal stress caused by high temperature gradients during the manufacturing process. The existence of these two types of stress can easily lead to deformation and cracking of the built structure, so heat treatment is needed to improve the structure and optimize the performance.

[0004] Laser melting deposition (LMD) technology is distinguished from traditional casting and additive manufacturing technology due to its faster cooling speed and large size of molten pool characteristics, but it is still essentially in the category of "solid-liquid-gas" three-phase reaction, similar to traditional casting technology. In the LMD additive manufacturing process, the alloying elements in the micro molten pool are combined with the dissolved oxygen in the molten steel and the free oxygen in the surrounding gas environment to form oxides, so the oxygen content has an extremely important influence on the generation and evolution of inclusions in the LMD process. SUMMARY

[0005] The present application is directed to the needs in the fields of aerospace, cryogenic storage, etc., to prepare a new type of maraging steel powder, and to provide a forming process, and to obtain the best forming effect by adjusting the process parameters, to control the microstructure and mechanical properties, and to add 1.00-5.00wt% CaZr intermediate alloy to make oxide slag during the forming process, Ca element can modify the oxide impurities, as a deoxidizer and inoculant, causing microalloying; Zr element has strong chemical affinity with O element, can preferentially react with O to form ZrO2, generate a surface layer of slag on the matrix and fall off, can reduce the oxygen content of the steel and improve the morphology and distribution of the oxides to improve the performance; both can also refine the oxide impurity particles. Finally, further optimization is carried out through heat treatment.

[0006] The present application relates to a method for microstructure control and harmless treatment of oxides of Fe-based alloy materials based on LMD forming, the mass percentage of each component of the powder is as follows: C: 0.001-0.003wt%, Cr: 10.5-12.0wt%, Ni: 6.0-9.0wt%, Co: 3.0-7.0wt%, Mo: 1.5-3.5wt%, V: 0.01-0.03wt%, Ti: 0.1-0.2wt%, Al: 0.02-0.03wt%, Si: 0.1-0.2wt%, Mn: 0.1-0.7wt%, S: 0.001-0.002wt%, P: 0.001-0.002wt%, the balance being Fe. 1.00-5.00wt% CaZr intermediate alloy (Ca: 50wt%, Zr: 50wt%) is added to the bulk material. The preforming size of the sample block is preset, the data and instructions are input into the ABB mechanical hand demonstrator through the numerical control system, and the laser metal deposition forming is carried out by controlling the synchronous powder feeding laser cladding head. The process parameters are as follows: laser power: 1100-4000W, laser scanning speed: 3.0-12mm / s, powder feeding rate: 12.0-70.0g / min, spot diameter: 1-5mm, overlap rate: 30-60%, laser cladding head protective gas (argon) flow: 20-40L / min, argon tank protective gas (argon) flow: 10-30L / min. Single layer overlap can form a 0.8-1.6mm thick FeCrNiCoMo steel material coating.

[0007] After laser metal deposition forming is completed and machining is carried out, the sample block is heat treated as a whole, and the process route is: hot isostatic pressing treatment (1000-1300℃, 120-140MPa, 4-4.5h), high-temperature solid solution treatment (1000-1200℃, 1-3h), low-temperature solid solution treatment (600-800℃, 1-3h), cold treatment (-60--80℃, 2-4h), aging treatment (400-600℃, 3-5h). BRIEF DESCRIPTION OF DRAWINGS

[0008] Figure 1 : schematic diagram of laser additive manufacturing technology

[0009] Figure 2 : morphology diagram of the material powder of the example

[0010] Figure 3 : molten pool morphology under high-definition video monitoring

[0011] Figure 4 : physical map of the LMD formed sample block

[0012] Figure 5 : microstructure diagram of the LMD formed sample block of Comparative Example 1 in the original state

[0013] Figure 6 : microstructure phase diagram of the LMD formed sample block of Example 1 in the heat treated state

[0014] Figure 7 : oxide inclusion diagram found after forming the material of the comparative example DETAILED DESCRIPTION

[0015] Example 1

[0016] comprises the following steps:

[0017] (1) The powder of the example is FeCrNiCoMo steel bulk material added with CaZr intermediate alloy, and the specific mass percentage of each component of the bulk material powder is as follows: C: 0.001wt%, Cr: 10.5wt%, Ni: 6.0wt%, Co: 3.0wt%, Mo: 1.5wt%, V: 0.01wt%, Ti: 0.1wt%, Al: 0.02wt%, Si: 0.1wt%, Mn: 0.2wt%, S: 0.001wt%, P: 0.001wt%, and the balance is Fe.

[0018] The mass of the added CaZr intermediate alloy (Ca: 50wt%, Zr: 50wt%) is 1.0wt% of the total mass of the bulk material powder, and the powder is mixed in a ball mill for 2.5 hours. After mixing, a uniform powder is obtained.

[0019] (2) Before use, the mixed powder is placed in a vacuum drying oven for drying, the temperature of the vacuum drying oven is 110℃, and the time is 2 hours.

[0020] (3) A 30CrMnSiNi2A substrate with a thickness of 25 mm is taken, and the surface of the substrate is uniformly polished with coarse sandpaper, which can increase the absorption rate of the laser energy on the surface of the substrate and reduce the reflection of the laser energy. Then the surface of the substrate is wiped clean with alcohol and placed in a sealed argon tank.

[0021] (4) The powder feeder containing FeCrNiCoMo steel powder is connected to the laser, and by controlling the programmer in the platform, the program corresponding to the two-dimensional contour information of the pre-written model is called to automatically execute the LMD process experiment. The laser scanning mode is multi-pass reciprocating scanning. The process parameters are as follows: laser power: 1450W, laser scanning speed: 5.0mm / s, powder feeding rate: 12g / min, spot diameter: 5mm, overlap rate: 60%, laser cladding head protection gas (argon) flow: 40L / min, argon tank protection gas (argon) flow: 30L / min. A single layer of FeCrNiCoMo steel material coating with a thickness of 0.9mm can be formed, and then the program is continued to be executed;

[0022] (5) After the forming is completed, the prepared sample block is separated from the substrate by mechanical processing, and heat treatment is performed. The heat treatment process route is: hot isostatic pressing treatment (1200℃, 130MPa, 4h), high temperature solid solution treatment (1000℃, 2h), low temperature solid solution treatment (800℃, 2h), cold treatment (-80℃, 2h), aging treatment (500℃, 3h).

[0023] (6) After heat treatment, the sample block is processed into standard tensile specimens and standard U-shaped impact specimens according to the standard sample drawing, and the mechanical properties are detected to obtain the mechanical property data report with CNAS certification. Metallographic specimens for OM and SEM testing are processed, and specimens for CT and EBSD detection are processed for microstructure analysis of the material.

[0024] Example 2

[0025] (1) The powder of the example is FeCrNiCoMo steel with the addition of CaZr intermediate alloy. The specific mass percentage of each component of the bulk material powder is as follows: C: 0.002wt%, Cr: 11.0wt%, Ni: 8.0wt%, Co: 5.0wt%, Mo: 2.25wt%, V: 0.02wt%, Ti: 0.2wt%, Al: 0.03wt%, Si: 0.1wt%, Mn: 0.4wt%, S: 0.001wt%, P: 0.001wt%, and the balance is Fe.

[0026] The CaZr intermediate alloy (Ca: 50wt%, Zr: 50wt%) was added in an amount of 2.5wt% of the total mass of the bulk material powder, and the powder was mixed in a ball mill for 2.5 hours. After mixing, a uniform powder was obtained, and the mixed powder was placed in a vacuum drying oven for drying. The temperature of the vacuum drying oven was 110°C, and the time was 2 hours. Subsequently, the powder was loaded into two powder feeding barrels, respectively, for standby.

[0027] (2) The process parameters were as follows: laser power: 1500W, laser scanning speed: 5.0mm / s, powder feeding rate: 12g / min, spot diameter: 5mm, and overlap rate: 60%. The other experimental methods and process parameters were the same as in Example 1, which are not repeated here.

[0028] (3) After forming, the prepared sample block was separated from the substrate by mechanical processing, and heat treatment was performed. The heat treatment process route was as follows: hot isostatic pressing treatment (1250°C, 135MPa, 4h), high-temperature solid solution treatment (1050°C, 2h), low-temperature solid solution treatment (800°C, 2h), cold treatment (-80°C, 2h), and aging treatment (500°C, 3h).

[0029] (4) The mechanical properties and microstructure of the formed piece of this example were analyzed, and it was found that the tensile strength was 201MPa higher than that of the comparative powder formed block, the yield strength was 400MPa higher, the elongation was reduced by 5.7%, and the impact toughness was increased by 20.2J / cm 2 .

[0030] Example 3

[0031] (1) The example powder was FeCrNiCoMo steel bulk material with the addition of CaZr intermediate alloy. The specific mass percentages of the components of the bulk material powder were as follows: C: 0.003wt%, Cr: 12.0wt%, Ni: 9.0wt%, Co: 7.0wt%, Mo: 3.5wt%, V: 0.02wt%, Ti: 0.2wt%, Al: 0.03wt%, Si: 0.1wt%, Mn: 0.6wt%, S: 0.001wt%, P: 0.001wt%, and the balance was Fe.

[0032] The CaZr intermediate alloy (Ca: 50wt%, Zr: 50wt%) was added in an amount of 5wt% of the total mass of the bulk material powder, and the powder was mixed in a ball mill for 2.5 hours. After mixing, a uniform powder was obtained, and the mixed powder was placed in a vacuum drying oven for drying. The temperature of the vacuum drying oven was 110°C, and the time was 2 hours. Subsequently, the powder was loaded into two powder feeding barrels, respectively, for standby.

[0033] (2) Process parameters: laser power: 1600 W, laser scanning speed: 5.0 mm / s, powder feeding rate: 12 g / min, spot diameter 5 mm, overlap rate 60%, other experimental methods and process parameters are as in Example 1, which are not repeated here.

[0034] (3) After the forming is completed, the prepared sample block is separated from the substrate by mechanical processing, and heat treatment is performed. The heat treatment process route is: hot isostatic pressing treatment (1300℃, 140MPa, 4h), high-temperature solid solution treatment (1050℃, 2h), low-temperature solid solution treatment (800℃, 2h), cold treatment (-80℃, 2h), aging treatment (500℃, 3h).

[0035] (4) The mechanical properties and microstructure of the formed parts of this example are analyzed, and it is found that the tensile strength is higher than that of the formed block of the comparative powder by 411 MPa, the yield strength is higher by 513 MPa, the elongation is reduced by 0.3%, and the impact toughness is increased by 15.2 J / cm 2 .

[0036] (5) Compared with the material of the comparative composition (bulk material), the mechanical properties of the material of the example composition are better. After the comparative composition material is formed, oxide inclusions are found, as shown in Figure 7 . After the same heat treatment is performed on both, the changes in mechanical properties are shown in Table 1, and all data are average values obtained by testing three samples. The tensile strength of the material of the example is increased by about 101 MPa, the yield strength is increased by about 201 MPa, the elongation is increased by 3%, and the impact toughness is slightly decreased by about 5.5 J / cm 2 . Table 1

[0037]

[0038] The results show that the addition of 1-5wt% CaZr intermediate alloy (Ca: 50wt%, Zr: 50wt%) can effectively reduce the oxide inclusions in the LMD formed FeCrNiCoMo steel material, reduce its harmfulness, and cooperate with heat treatment to significantly improve its mechanical properties.

Claims

1. A method for microstructure control and oxide neutralization of Fe-based alloy materials based on LMD forming, characterized in that, The mass percentages of each component in the bulk powder material used are as follows: C: 0.001–0.003 wt%, Cr: 10.5–12.0 wt%, Ni: 6.0–9.0 wt%, Co: 3.0–7.0 wt%, Mo: 1.5–3.5 wt%, V: 0.01–0.03 wt%, Ti: 0.1–0.2 wt%, Al: 0.02–0.03 wt%, Si: 0.1–0.2 wt%, Mn: 0.1–0.7 wt%, S: 0.001–0.002 wt%, P: 0.001–0.002 wt%, with the balance being Fe; Before the material is formed, add 1.00wt%-5.00wt% of CaZr master alloy by weight of the total bulk material and mix thoroughly, wherein Ca: 50wt% and Zr: 50wt%, for oxide slag formation during forming; The process parameters for laser melting deposition (LMD) of this hybrid material are as follows: laser power 1100–4000 W, laser scanning speed 3.0–12.0 mm / s, powder feeding rate 12.0–70.0 g / min, spot diameter 1–5 mm, overlap rate 30–60%, protective gas flow rate of laser cladding head 20–40 L / min, and protective gas flow rate of argon chamber 10–30 L / min. The formed material is then subjected to subsequent heat treatment, with the following heat treatment regime: hot isostatic pressing at 1000-1300℃ and 120-140MPa for 4-4.5h, high-temperature solution treatment at 1000-1200℃ for 1-3h, low-temperature solution treatment at 600-800℃ for 1-3h, cold treatment at -60 to -80℃ for 2-4h, and aging treatment at 400-600℃ for 3-5h.

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