A method of strengthening and toughening a metal material by additive manufacturing and a strengthened and toughened stainless steel workpiece

Abstract

The application discloses a kind of toughening treatment methods of additive manufacturing metal material, comprising obtaining the metal workpiece to be handled prepared using additive manufacturing method;Forming encapsulation layer on the surface of metal workpiece to be handled, to obtain encapsulation workpiece;After annealing the encapsulation workpiece at 1-10GPa, 550-1800 ℃, the encapsulation layer is stripped to obtain toughened metal workpiece.The method can eliminate the defects of metal workpiece produced by additive manufacturing, such as porosity, crack and microstructure inhomogeneity.The material prepared by the method has high yield strength and good plasticity.The application also discloses a toughened stainless steel workpiece prepared by the method.

Description

Technical Field

[0001] This invention belongs to the field of additive manufacturing technology, specifically relating to a method for strengthening and toughening additive manufacturing metal materials and a strengthened and toughened stainless steel workpiece. Background Technology

[0002] High-strength alloys, with their superior mechanical properties, have become essential materials in industrial fields, particularly in applications with stringent material requirements such as surgical instruments, ship components, and bearing manufacturing. Despite their extreme durability, high-strength alloys face a number of challenges in traditional production processes (such as cutting and forging). These challenges include long lead times, high material costs, and limitations imposed by design complexity. Furthermore, the complex microstructures of high-strength alloys often result in both rigidity and brittleness during forming and subsequent processing, further increasing manufacturing difficulties.

[0003] Additive manufacturing, as a modern manufacturing technology, offers an effective way to solve these challenges. It constructs objects by adding materials layer by layer, greatly increasing design flexibility and allowing the production of complex shapes that are difficult to manufacture using traditional methods. In terms of resource efficiency, additive manufacturing minimizes material waste and accelerates the overall production process from design to finished product. This technology not only produces near-final shapes to reduce or eliminate subsequent machining needs but also improves surface quality, ensuring that manufactured parts possess the mechanical and physical properties to meet stringent application requirements. With the increasing maturity and optimization of additive manufacturing technology, its applications are rapidly expanding into aerospace, automotive manufacturing, and medical devices, opening up new possibilities for the application of high-strength alloys. Through continued research and improvement of additive manufacturing technology, we can expect even more significant progress in cost reduction, efficiency improvement, and innovative design.

[0004] While additive manufacturing offers a precise process for producing high-strength alloy parts with complex shapes, challenges remain in improving the overall quality of the parts. Currently, even after fine-tuning parameters, additive manufacturing sometimes struggles to produce perfectly dense components. Particularly under cyclic loading, these unfused voids can become stress concentration areas, accelerating crack formation and weakening the fatigue strength of the part. Furthermore, the microstructure generated during additive manufacturing often exhibits significant anisotropy and inhomogeneity, which can affect the final properties of the material and thus typically requires specialized post-processing steps to improve it. Additionally, for precipitation-strengthened or solution-strengthened alloys, such as martensitic stainless steel, achieving the ideal balance of strength, hardness, and toughness in additive manufacturing requires heat treatment processes such as tempering.

[0005] Among numerous post-processing techniques, hot isostatic pressing (HIP) is a widely used method that improves the density and homogeneity of materials by annealing under a certain load pressure. However, while traditional HIP can effectively reduce internal porosity, it cannot completely eliminate pores communicating with the outside environment due to the applied pressure and the limited medium. Furthermore, it can sometimes cause parts to soften, thus limiting its effectiveness in improving the strength of high-strength alloy parts manufactured through additive manufacturing.

[0006] To overcome these limitations, new post-processing methods need to be developed to optimize the performance of additively manufactured high-strength alloy parts and ensure their reliability and durability in harsh environments. Summary of the Invention

[0007] This invention provides a method for strengthening and toughening additively manufactured metallic materials, the resulting materials exhibiting high yield strength and good toughness.

[0008] This invention provides a method for strengthening and toughening additively manufactured metallic materials, comprising:

[0009] (1) Obtain the metal workpiece to be processed using additive manufacturing method;

[0010] (2) An encapsulation layer is formed on the surface of the metal workpiece to be processed, thereby obtaining an encapsulated workpiece;

[0011] (3) After annealing the packaged workpiece at 1-10 GPa and 550-1800℃, the packaged layer is peeled off to obtain a toughened metal workpiece.

[0012] The higher pressure provided by this invention can counteract the expansion of pores due to annealing softening, resulting in fewer pores in the final metal workpiece, reducing stress concentration inside the metal, reducing crack formation, and obtaining additive manufacturing metal materials with higher yield strength.

[0013] Preferably, the material of the metal workpiece to be processed is a high-temperature alloy, titanium alloy, stainless steel, aluminum alloy, or magnesium alloy.

[0014] Preferably, the material of the metal workpiece to be processed is 420 stainless steel, and the annealing process in step (3) has a pressure of 4-8 GPa, a temperature of 850-1050℃, and a time of 10-60 min.

[0015] This invention enables 420 stainless steel to form fine-grained martensite and equiaxed grains through controlled isostatic pressing (IP) annealing pressure, temperature, and pressing time. This results in higher yield strength with less reduction in toughness. The introduction of high pressure not only closes the pores generated during additive manufacturing but also increases the nucleation rate of phase transformation structures and precipitates, thereby refining the microstructure and precipitates. Furthermore, the high pressure allows for higher annealing temperatures without grain coarsening, enabling greater control over the material's microstructure and properties.

[0016] Preferably, forming an encapsulation layer on the surface of the metal workpiece to be treated includes: sequentially forming a first heat insulation layer, a heating layer, a second heat insulation layer, an outer encapsulation layer, and a pressure conduction layer on the surface of the metal workpiece to be treated.

[0017] More preferably, the heating layer is made of graphite or lanthanum chromate, the first and second insulation layers are made of zirconium oxide, magnesium oxide, boron nitride or aluminum oxide, the encapsulation layer is made of pyrophyllite, magnesium oxide or molybdenum, and the pressure conduction layer is made of tungsten carbide and / or pyrophyllite.

[0018] More preferably, the heating layer is made of graphite; the first insulation layer and the second insulation layer are both made of zirconium oxide, magnesium oxide or aluminum oxide.

[0019] On the other hand, the present invention provides a toughened stainless steel workpiece, which is prepared by the toughening treatment method of the additive manufacturing metal material.

[0020] Preferably, the microstructure of the toughened stainless steel workpiece comprises martensite and equiaxed grains, with the martensite accounting for 60%-90% of the area and the equiaxed grains accounting for 10%-40% of the area. The equiaxed grains are distributed at the grain boundaries of the original austenite, thereby achieving good strength and toughness.

[0021] Preferably, the martensite grain diameter of the toughened stainless steel workpiece is 0.1-1 μm, and the equiaxed grain diameter is 0.05-0.1 μm. The relatively coarse martensite laths and ultrafine equiaxed grains can generate a synergistic deformation mechanism similar to a gradient heterostructure. The ultrafine equiaxed grains distributed at the original austenite grain boundaries provide most of the strength at low strain levels, while the relatively coarse martensite laths, aided by the fine grains, can accommodate more dislocations, thereby improving the overall plasticity of the sample.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] This invention addresses the common problems of cracks, porosity, and anisotropy caused by inhomogeneous microstructure in additive manufacturing materials by using high-pressure annealing. This significantly strengthens and toughens the additively manufactured metal materials. Compared to traditional annealing and hot isostatic pressing, this invention offers significant advantages in improving workpiece strength and eliminating near-surface defects. This invention also expands the application scope of additive manufacturing technology in producing high-strength alloys with enhanced mechanical properties. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the laser selective melting technology used in a specific embodiment of the present invention;

[0025] Figure 2 This is a schematic diagram of the high-pressure annealing process used in Embodiment 1 of the present invention;

[0026] Figure 3 This is a microstructure diagram of the metal workpiece obtained in Example 1 of the present invention;

[0027] Figure 4 This is a microstructure diagram of the metal workpiece prepared in Comparative Example 2 of the present invention;

[0028] Figure 5 The stress-strain curves obtained after compressive mechanical property testing are provided in Embodiment 1, Comparative Example 1, and Comparative Example 2 of the present invention.

[0029] Figure 6 This is a comparison diagram of defects between the metal workpiece obtained in Embodiment 1 of the present invention and the original metal workpiece. Detailed Implementation

[0030] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the specific implementation schemes and embodiments of the present invention will be briefly introduced below.

[0031] To address the current lack of methods for strengthening and toughening additive manufacturing of metallic materials, this invention proposes a specific technical solution. The technical solution of this invention will be clearly and completely described below, along with a specific embodiment. It should be noted that the described embodiment is only a part of, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention. Operating methods not specifically specified in the following embodiments are generally performed under conventional conditions or as recommended by the manufacturer.

[0032] like Figure 1As shown, selective laser melting (SLM) technology uses a laser to melt metal powder layer by layer, fabricating metal workpieces in an additive manufacturing process. The metal workpiece is pre-modeled and sliced, and finally printed on a substrate, then separated from the substrate by cutting.

[0033] Example 1

[0034] This embodiment provides a method for strengthening and toughening additively manufactured metallic materials, such as... Figure 2 As shown, it includes:

[0035] Step 1: Model and slice the required 3D model of the workpiece. After selecting a suitable printing process, generate the corresponding processing file. Then, import the required processing file into the control computer of the 3D printer and load the file.

[0036] Step 2: Add 420 stainless steel powder into the powder feeding hopper of the 3D printer.

[0037] Step 3: Seal the printing chamber and purge it with high-purity argon gas to reduce the oxygen concentration in the printing chamber to below 200 ppm. At the same time, preheat the printing substrate to 50°C.

[0038] Step 4: After the gas washing and preheating are completed, prepare the metal workpiece to be processed. After the metal workpiece is prepared, use wire cutting technology to remove the metal workpiece.

[0039] Step 5: After cleaning the surface of the metal workpiece to be treated, sequentially wrap the surface of the metal workpiece with a magnesium oxide insulation layer, a graphite heating layer, a zirconium oxide insulation layer, a molybdenum sheet encapsulation layer, and a pressure-conducting layer composed of tungsten carbide and pyrophyllite (in... Figure 2 (as indicated in the diagram) forms an encapsulation layer to complete the encapsulation process.

[0040] Step Six: Place the regular cubic assembly block into a high-temperature, high-pressure equipment for high-pressure annealing. The high-pressure annealing temperature is 950℃, the pressure is 7GPa, and the time is 15 minutes. After peeling off the encapsulation layer, a toughened 420 stainless steel workpiece is obtained.

[0041] like Figure 3 As shown, the main microstructure of the additively manufactured 420 steel sample after high-pressure annealing is martensite, with very fine equiaxed grains at the original austenite grain boundaries (marked by yellow lines).

[0042] Example 2

[0043] Unlike Example 1, Example 2 had an annealing pressure of 7 GPA, an annealing temperature of 850°C, and a annealing time of 15 minutes.

[0044] Comparative Example 1

[0045] Unlike Example 1, Comparative Example 1 used a box annealing furnace to anneal at 950 degrees Celsius under normal pressure for 15 minutes.

[0046] Comparative Example 2

[0047] Unlike Example 1, the annealing temperature was 500°C, such as Figure 4 As shown, the grain size of Comparative Example 2 is coarser and contains more austenite, which leads to softening of the workpiece.

[0048] Performance Analysis:

[0049] The metal workpieces obtained in Examples 1 and 2 and Comparative Examples 1 and 2 were subjected to compressive mechanical property tests, and the results are as follows: Figure 3 As shown, the 420 stainless steel treated with high-pressure annealing exhibits an extremely high yield strength of 1828 MPa, which is 43% higher than that of the original sample, while the plastic deformation capacity only decreases slightly, from 36% to 28%. The specific results are shown in Table 1.

[0050] Table 1

[0051]

[0052]

[0053] like Figure 5 As shown, the yield strength of the sample annealed at 950℃ under high pressure in Example 1 increased by 43%, while the yield strength of the sample annealed at 850℃ under high pressure in Example 2 increased, and the plasticity also increased slightly. However, the yield strength of the samples annealed at 500℃ under high pressure in Comparative Example 2 and at 950℃ under normal pressure under Comparative Example 1 showed a significant decrease. This indicates that high pressure annealing using a specific process is significantly effective as a toughening post-treatment for additive manufacturing metals.

[0054] like Figure 6 As shown, the three-dimensional defect characterization was performed using an X-ray microscope, and the magnified two-dimensional view was captured using a scanning electron microscope. It can be seen that the microstructure inhomogeneity of the sample after high-pressure annealing is eliminated, and the pores generated by additive manufacturing are closed.

Claims

1. A method for strengthening and toughening additively manufactured metallic materials, characterized in that, include: (1) Obtain the metal workpiece to be processed using additive manufacturing method; (2) An encapsulation layer is formed on the surface of the metal workpiece to be processed, thereby obtaining an encapsulated workpiece; (3) After annealing the packaged workpiece at 7-10 GPa and 850-1050℃, the packaged layer is peeled off to obtain a toughened metal workpiece. The material of the metal workpiece to be processed is 420 stainless steel, and the annealing time is 10-60 min; The process of forming an encapsulation layer on the surface of the metal workpiece to be processed includes: sequentially forming a first heat insulation layer, a heating layer, a second heat insulation layer, an outer encapsulation layer, and a pressure conduction layer on the surface of the metal workpiece to be processed; The heating layer is made of graphite or lanthanum chromate. The materials for both the first and second insulation layers are zirconium oxide, magnesium oxide, boron nitride, or aluminum oxide; The material of the outer packaging layer is pyrophyllite, magnesium oxide, or molybdenum; The pressure-conducting layer is made of tungsten carbide and / or pyrophyllite.

2. The method for strengthening and toughening additively manufactured metallic materials according to claim 1, characterized in that, The heating layer is made of graphite; The materials of the first insulation layer and the second insulation layer are both zirconium oxide, magnesium oxide or aluminum oxide.

3. A toughened stainless steel workpiece, characterized in that, It is prepared by the toughening treatment method of additive manufacturing metal material as described in claim 1 or 2.

4. The toughened stainless steel workpiece according to claim 3, characterized in that, The microstructure of the toughened stainless steel workpiece includes martensite and equiaxed grains, with the area of ​​martensite accounting for 60%-90% and the area of ​​equiaxed grains accounting for 10-40%.

5. The toughened stainless steel workpiece according to claim 4, characterized in that, The martensite grain diameter of the toughened stainless steel workpiece is 0.1-1 μm, and the equiaxed grain diameter is 0.05-0.1 μm.

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