Method for ultra-fining slm formed cualfe shape memory alloy structure

Abstract

The application belongs to the field of shape memory alloys, and particularly relates to a method for forming CuAlFe shape memory alloy organization by SLM (Selective Laser Melting) in superfine state, wherein metal element powders are mixed uniformly and then formed by SLM, and the method comprises the following components with mass percentage: 81wt%-83wt% of Cu and 12wt%-14wt% of Al as a matrix, and 4wt%-6wt% of Fe as a grain refiner; the powder particle size is: Cu 20-40um, Al 20-40um, and Fe 3-8um. The preparation steps are as follows: after the metal element powders are weighed, the powders are added into a ball mill jar, stainless steel balls with different sizes are added into the ball mill jar for ball milling, the metal element powders are mixed uniformly, and then dried; finally, SLM forming is performed. The application can generate more martensite phase, can adjust the energy density of laser and the particle size of metal powder, realize incomplete melting of iron powder, can greatly refine the alloy organization, and make the alloy have excellent mechanical properties and memory performance.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of shape memory alloys, and particularly relates to a method for superfine SLM forming of CuAlFe shape memory alloy organizations. BACKGROUND

[0002] With the development of aerospace and military industries, the application demand of high-temperature shape memory alloys is increasing. Although the traditional Ni-Ti-based shape memory alloy has good shape memory performance, it is difficult to meet the requirements of high-temperature work, and the cost is high, which limits the development of shape memory alloys in high-temperature fields.

[0003] Among the many materials of shape memory alloys, CuAlFe shape memory alloy has high-temperature shape memory performance and low manufacturing cost, only one-tenth of the cost of nickel-titanium-based alloy. In addition, copper-aluminum-based alloy has good ductility, high electrical conductivity and thermal conductivity, and corrosion resistance, and has become a very promising alloy material. Preparing high-performance CuAlFe shape memory alloy is a key research field at present.

[0004] Zhenghua Deng, Haiqing Yin, et al. prepared Cu-12Al-6Ni shape memory alloy by powder metallurgy method, see (Deng Z, Yin H, et al. Microstructure and mechanical properties of Cu-12Al-6Ni with Ti addition prepared by powder metallurgy [J]. Materials Science and Engineering A, 2020, 803: 140472), according to the composition, configure pure metal powder, and mix in the tubular mixer at a rotation speed of 60 rpm for 3 h. The mixed powder is compacted into a 20 mm diameter disc, and then sintered at 1000℃ for 1h in a hydrogen atmosphere to obtain the required alloy. However, the alloy obtained by this method has a compression strain of only 412MPa, and the mechanical properties are poor, which cannot be applied to actual environment.

[0005] SLM forming can regulate alloy organization by controlling laser energy density, and due to the characteristics of integrated forming, high degree of freedom, fast cooling and the like, the demand in production is gradually increasing. Dennis GERA, Jonadabe SANTOS and the like take pre-alloyed powder as raw material, and compare SLM forming with powder metallurgy to prepare Cu-Al-Ni-Mn-Zr alloy, see (GERA D, SANTOS J, KIMINAMI C S, et al. Comparison of Cu-Al-Ni-Mn-Zr shape memory alloy prepared by selective laser melting and conventional powder metallurgy [J]. Transactions of Nonferrous Metals Society of China, 2020, 30(12): 3322-3332.), and it is found that SLM forming can realize the refinement of Cu-based shape memory alloy organization to a certain extent through rapid cooling. However, due to the limited refinement effect of single SLM forming on CuAlFe shape memory alloy organization, the mechanical properties and memory performance are difficult to meet the application needs of the next generation of intelligent devices.

[0006] In summary, continuously optimizing the method for preparing high-performance CuAlFe shape memory alloy has become the focus of research of researchers in the field. SUMMARY

[0007] Therefore, the present application provides a method for ultra-fining SLM forming CuAlFe shape memory alloy organization to solve the problem that the refinement degree of the memory alloy organization is not ideal in the prior art, and therefore the mechanical properties and memory performance are difficult to meet the application needs of the next generation of intelligent devices.

[0008] To achieve the above-mentioned purpose, the present application provides the following technical scheme: a method for ultra-fining SLM forming CuAlFe shape memory alloy organization, metal element powder is mixed uniformly and then formed by selective laser melting (SLM), and the method comprises the following components with a mass percentage: 81wt%-83wt% of Cu and 12wt%-14wt% of Al as a substrate, and 4wt%-6wt% of Fe as a grain refiner; the powder particle size is: Cu 20-40μm, Al 20-40μm, and Fe 3-8μm.

[0009] Further, the method steps are as follows:

[0010] (1) The metal element powder is weighed according to the proportion and poured into a ball mill jar;

[0011] (2) Put the stainless steel balls of different sizes into the ball mill jar for ball milling to realize the uniform mixing of the metal element powders, and then take out the powders after ball milling for drying treatment;

[0012] (3) Take the powders of step (2) as raw materials to perform SLM forming;

[0013] Further, in step (2), the metal powders are put into a ball mill jar, stainless steel balls are added, the mass of the stainless steel balls is 1-2 times the mass of the metal powders, the diameters of the stainless steel balls are 3 mm, 5 mm and 10 mm, and the mass ratio of the two kinds of steel balls is 2:2:1; after the ball mill jar is sealed, vacuumizing is performed for 10-60 min, and then ball milling is performed on a planetary ball mill, and the ball milling time is 1-24 h.

[0014] Further, in the SLM forming of step (3), the parameters are as follows: laser power 200 W-600 W, scanning speed 100 mm / min-1000 mm / min, scanning interval 0.03-0.2 mm, powder layer thickness 0.02-0.04 mm, and laser rotation angle per layer 67°, then the model slices are output and introduced into the machine.

[0015] The prepared mixed metal powders are loaded into a powder bin, after the oxygen content is less than 800 ppm, the stainless steel substrate is preheated to 100-200 ℃, the printing is started, and the laser scanning is performed twice in the first five layers and once in each subsequent layer to obtain a copper alloy.

[0016] Compared with the prior art, the application has the following advantages:

[0017] 1. The raw material powder used in the application is mixed pure metal powder obtained by ball milling, 5wt% iron powder of a suitable size is added on the basis of 81wt%-83wt% pure copper powder and 12wt%-14wt% pure aluminum powder, since the melting point of iron is as high as 1538℃, the iron powder is only partially melted in the SLM forming process, and the un-melted iron particles exist in the molten pool, since the misfit degree of iron and copper is small, the un-melted iron particles can play the role of heterogeneous nucleating agent, greatly refining the structure of the alloy, so that the alloy has excellent mechanical properties and memory performance.

[0018] 2. By adjusting the energy density of the laser and the particle size of the metal powder, the number, size and cooling speed of the un-melted iron powder during the solidification of the molten pool can be adjusted, so that the rapid solidification of SLM and the refining effect of the refractory Fe core-shell particles on the alloy can be simultaneously exerted, and the ultra-fining of the CuAlFe shape memory alloy structure is realized.

[0019] 3. This project will introduce refractory Fe core-shell particles with high strength and good plasticity into Cu-based shape memory alloys. The refractory Fe core-shell particles can refine the alloy structure and strengthen the Fe alloy to some extent. For CuAlFe shape memory alloys, the refractory Fe core-shell particles are high-strength non-brittle particles, which can increase the strength of the alloy while reducing the toughness of the alloy. Under the combined action of the ultra-fine organization of the SLM formed CuAlFe shape memory alloy containing unburned Fe particles and the improvement of the mechanical properties of the CuAlFe shape memory alloy by the Fe core-shell particles, the mechanical properties and shape memory properties of the SLM formed CuAlFe shape memory alloy containing unburned Fe particles are greatly improved. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 For the SLM formed CuAlFe shape memory alloy in Example 1 and its microstructure after heat treatment under optical microscope (OM) and scanning electron microscope (SEM):

[0021] Figure 1 a is the OM image of the SLM formed sample, Figure 1 b is the SEM image of the SLM formed sample, Figure 1 c is the OM image of the SLM formed sample after heat treatment, Figure 1 d is the SEM image of the SLM formed sample after heat treatment;

[0022] Figure 2 For the XRD, DSC and mechanical property test results of the SLM formed CuAlFe shape memory alloy (SLM) and its heat treatment (SLM-HT) in Example 1:

[0023] Figure 2 a is the XRD of the SLM formed sample, Figure 2 b is the DSC curve of the SLM formed sample, Figure 2 c is the compressive stress-strain curve of the SLM formed sample, Figure 2 d is the hardness of the SLM formed sample;

[0024] Figure 3 For the SEM image and XRD of the SLM formed CuAlFe shape memory alloy with different particle sizes of Fe powder in Example 2:

[0025] Figure 3 a is the SEM image of the SLM formed sample of 4 μm diameter Fe powder, Figure 3 b is the SEM image of the SLM formed sample of 1 μm diameter Fe powder, Figure 3 c is its XRD image;

[0026] Figure 4The microstructure of the CuAlFe shape memory alloy cast in the comparative example:

[0027] Figure 4 a is the OM image of the cast sample, Figure 4 b is the SEM image of the cast sample;

[0028] Figure 5 The test results of the compressive stress-strain curves of the SLM formed CuAlFe shape memory alloy and the comparative example cast alloy;

[0029] Figure 6 The test results of the hardness of the SLM formed CuAlFe shape memory alloy and the comparative example cast alloy. DETAILED DESCRIPTION

[0030] In order to further understand the present application, the technical solutions provided by the present application will be described in detail below in combination with the embodiments, and the protection scope of the present application is not limited by the following embodiments.

[0031] The present application provides a preparation method for realizing ultra-fining of the microstructure of the SLM formed CuAlFe shape memory alloy by unburnt iron particles:

[0032] In one aspect, an ultra-fined SLM formed CuAlFe shape memory alloy microstructure is composed of the following mass percentage components: 81wt%-83wt% of base Cu, 12wt%-14wt% of Al, and 4wt%-6wt% of grain refiner Fe.

[0033] In another aspect, a method for ultra-fining the microstructure of the SLM formed CuAlFe shape memory alloy includes the following steps:

[0034] (1) Mix the pure metal powders according to the required proportion and put them into a ball mill jar;

[0035] (2) The mass of the stainless steel balls is 1-2 times the mass of the metal powders, the diameter of the stainless steel balls is 3mm, 5mm and 10mm, and the mass ratio of the three kinds of steel balls is 2:2:1; after sealing the ball mill jar, vacuumize it for 10-60min, then ball mill it on a planetary ball mill, the ball milling speed is 10-300rad / min, the ball milling mode is forward rotation for 10-30min, stop for 3-10min, reverse rotation for 10-30min, stop for 3-10min, cycle periodically, and the ball milling time is 1-24h; after ball milling, take out the metal powders and put them into a drying oven to heat them to 100-120℃, and dry them for 1-48h;

[0036] (3) SLM forming:

[0037] Clean the key parts such as the working chamber, laser lens, powder chamber, etc., wipe all powder-contacting positions with alcohol to prevent contamination of the powder, finally install the base plate and adjust the scraper position, and load the prepared mixed powder into the powder chamber;

[0038] Gas washing is performed on the working chamber, the gas washing and lens gas switches are opened, argon is introduced, and when the oxygen content is lower than 3%, the fan is turned on and the fan voltage is set to 3-5V to ensure that the oxygen content is lower than 800ppm;

[0039] The base plate is heated to 100-200℃;

[0040] Import the model into the machine and start printing, the process parameters for printing are: laser power 200W-600W, scanning speed 100-1000mm / min, scanning pitch 0.03-0.2mm, powder layer thickness 0.02-0.04mm, and laser rotation angle per layer 67°.

[0041] Example 1:

[0042] (1) The metal powder is composed of Cu 82wt%, Al 13wt%, and Fe 5wt%, and the powder particle sizes are 30μm, 30μm, and 5μm, respectively. After mixing, it is added to a ball milling tank.

[0043] (2) Add stainless steel balls with twice the mass of the metal powder, the stainless steel balls have diameters of 3mm, 5mm, and 10mm, and the mass ratio of the three steel balls is 2:2:1; after sealing the ball milling tank, vacuumize it for 30min, then ball mill in a planetary ball mill at a speed of 150rad / min for 100min, the ball milling mode is forward rotation for 15min, stop for 10min, reverse rotation for 15min, stop for 10min, and the cycle is repeated, the ball milling time is 20h, and the mixed metal element powder is obtained. The mixed metal element powder is placed in a drying box and dried at a temperature of 120℃ for 6h.

[0044] (3) SLM forming:

[0045] First, the part to be printed is modeled using solidworks, UGNX, and Proe software, and saved as an STL format; then it is imported into Magics2.0 software, and the SLM forming related process parameters are input in the software, the process parameters are: laser power 380W, scanning speed 700mm / min, scanning pitch 0.1mm, powder layer thickness 0.03mm, and laser rotation angle per layer 67°.

[0046] Clean the key parts such as the working chamber, laser lens, powder chamber, etc., wipe all powder-contacting positions with alcohol to prevent contamination of the powder, finally install the base plate and adjust the scraper position, and load the prepared mixed powder into the powder chamber;

[0047] The working chamber is washed with argon, the argon and lens gas switches are opened, argon is introduced, the fan is turned on when the oxygen content is less than 3%, and the fan voltage is set to 4V to ensure that the oxygen content is less than 800ppm; the substrate is heated to 150℃;

[0048] After the model is sliced and output, the machine is imported, and printing is started (laser scanning twice in the first five layers, and laser scanning once in each subsequent layer) to obtain a 10x10x10mm block-shaped CuAlFe shape memory alloy sample; finally, part of the sample is heat treated at 900℃ for 24h and then water cooled.

[0049] The microstructure of the CuAlFe shape memory alloy prepared in Example 1 (SLM formed state, without heat treatment) under an optical microscope (OM) and a scanning electron microscope (SEM) is shown in Figure 1 It can be seen from Figure 1 that the microstructure of the CuAlFe shape memory alloy prepared in Example 1 (SLM formed state) has the following structural characteristics:

[0050] The microstructure of the alloy is very fine, with a grain size of 5-8μm, and it can be observed that there are indeed un-melted iron points in the grains, as shown in Figure 1 a and Figure 1 b.

[0051] X-ray diffraction detection shows that the microstructure of the alloy is composed of β' martensite and iron points, as shown in Figure 2 a.

[0052] The sample wire is cut into a φ4x0.5mm cylinder, and the measured DSC curve is shown in Figure 2 b, and it is found that the martensite-austenite phase transition temperature of the alloy is about 300℃.

[0053] The sample wire is cut into a φ4x4mm cylinder, and its compression stress-strain curve is measured Figure 2 c, and it is found that its compression strength exceeds 1000MPa; the hardness reaches 300HV Figure 2 d.

[0054] The metallographic and scanning microstructure of the CuAlFe shape memory alloy prepared in Example 1 (SLM formed state, after heat treatment) is shown in Figure 1 c and Figure 1 d, and it can be seen that the SLM formed CuAlFe shape memory alloy prepared in this example has the following structural characteristics:

[0055] After heat treatment, the grain size of the alloy is 15-20μm, and martensite and Fe phase can be observed, and the increase in the size of the Fe points is related to the solid solubility of iron in the alloy.

[0056] The sample wire was cut into a φ4x0.5mm cylinder, and the measured DSC curve is shown in Figure 2 b, it was found that the martensite-austenite phase transition temperature of the alloy after heat treatment was close to 350℃, which was higher than that of the untreated sample.

[0057] The sample wire was cut into a φ4x4mm cylinder, and the measured compression stress-strain curve is shown in Figure 2 c, it was found that the compression strength reached 1300MPa. However, the hardness of the heat treated alloy decreased (270HV), as shown in Figure 2 d.

[0058] Example 2:

[0059] (1) The metal powders were mixed according to the following composition: Cu 82wt%, Al 13wt%, Fe 5wt%, and the powder particle sizes were 30μm, 30μm, 1μm or 4μm, respectively.

[0060] (2) Add stainless steel balls with twice the mass of the metal powder, with diameters of 3mm, 5mm and 10mm, and a mass ratio of 2:2:1; seal the ball mill jar, vacuum for 10in, and then ball mill at a speed of 140rad / min for 150min in the ball mill, with a cycle of forward rotation for 15min, stop for 10min, reverse rotation for 15min, and stop for 10min, with a cycle time of 1h, to obtain the mixed metal powder. The mixed metal element powder was placed in a drying box, and the metal powder was dried at a temperature of 120℃ for 3h.

[0061] (3) SLM forming:

[0062] First, the part to be printed was modeled using solidworks, UGNX, Proe software and saved as STL format; then it was imported into Magics2.0 software, and the SLM forming related process parameters were input in the software, the process parameters were: laser power 380W, scanning speed 700mm / min, scanning interval 0.1mm, powder layer thickness 0.03mm, laser rotation angle per layer 67°.

[0063] Clean the key parts of the work tank, laser lens, powder tank, etc., wipe all positions where the powder contacts with alcohol to prevent contamination of the powder, finally install the substrate and adjust the position of the scraper, and put the prepared mixed powder into the powder tank;

[0064] Gas washing was performed on the work tank, the gas washing and lens gas switches were opened, argon was introduced, when the oxygen content was lower than 3%, the fan was turned on, and the fan voltage was set to 5V to ensure that the oxygen content was lower than 800ppm; the substrate was heated to 150℃;

[0065] After the model slices are output, they are imported into the machine and printing begins (the first five layers are laser-scanned twice, and each subsequent layer is laser-scanned once), resulting in a 10×10×10mm block CuAlFe shape memory alloy sample.

[0066] The microstructure and XRD results of CuAlFe shape memory alloy (10×10×10mm block sample) formed by SLM of iron powder with 1μm and 4μm particle size are shown in the figure. Figure 3 .from Figure 3 It can be seen that the SLM morphology prepared in this embodiment has the following structural characteristics:

[0067] The grain size of the Fe powder SLM-formed sample with a particle size of 1 μm is 20-30 μm. Figure 3 a), while the grain size of the 4μm Fe powder SLM formed sample is 5-8μm ( Figure 3 (b) Clearly, the grain size of the 1 μm Fe powder SLM-formed sample is significantly larger than that of the 4 μm Fe powder SLM-formed sample. Strong Fe diffraction peaks can be observed in the X-ray diffraction pattern of the 4 μm Fe powder SLM-formed sample, while almost no Fe diffraction peaks are observed in the X-ray diffraction pattern of the 1 μm Fe powder SLM-formed sample. Figure 3 c.

[0068] Comparative example (traditional casting):

[0069] The microstructure and properties of CuAlFe shape memory alloys with the same composition obtained by conventional casting methods were studied in a comparative manner. The alloy composition is as follows: Cu 83 wt%, Al 13 wt%, Fe 5 wt%.

[0070] The traditional casting process for obtaining this alloy composition is as follows:

[0071] (1) Mix the metal powders in an appropriate proportion and place them in an alumina crucible.

[0072] (2) Place the corundum crucible into a non-consumable vacuum arc melting furnace to melt the alloy within a temperature range of 1000℃-1500℃, and melt the alloy six times to ensure uniform composition. After melting, cool to obtain the alloy with the specified composition.

[0073] Traditional cast CuAlFe shape memory alloys have grain sizes in the millimeter range. Figure 4 Compared to traditional cast CuAlFe shape memory alloys (grain size on the millimeter scale), Figure 4 ) and SLM formed CuAlFe shape memory alloy (grain size 20-30μm, Figure 3 b) In contrast, Example 2 of this invention utilizes unmelted Fe particles to refine SLM-formed CuAlFe shape memory alloys (grain size 5-8 μm). Figure 3a) has finer structure. In addition, Fe(Al, Cu) phase can be observed near the grain boundary of the cast CuAlFe alloy, see Figure 4 . This is because during the alloy casting process, as the solubility decreases, iron precipitates near the grain boundary. There is no such phenomenon inside the SLM formed CuAlFe alloy, and the corresponding structure is as shown in Figure 1 a- Figure 1 d, and it can be found by comparison that Fe(Al, Cu) exists inside the grain. This explains why the alloy grain is fine, because during the heating process, iron changes from body-centered cubic structure to face-centered cubic structure, resulting in that its lattice constant is close to Cu, and the misfit between them is very small, which can play a role in heterogeneous core to refine the structure. At the same time, the cooling speed of SLM forming is faster, which also helps to refine the grain size. It can be summarized that the ultra-fine grain is realized by the combined action of the two.

[0074] The significant improvement of the mechanical properties and memory properties of the alloy is caused by the refinement of the structure. The compressive strength of the 4μm particle size Fe powder SLM formed CuAlFe alloy (unmelted Fe particle refinement + rapid solidification) is 1200MPa, the compressive strength of the 4μm particle size Fe powder SLM formed CuAlFe alloy is 1450MPa; The compressive strength of the 1μm particle size Fe powder SLM formed (rapid solidification) alloy is also more than 1030MPa, and the compressive strength of the cast alloy is only 680MPa, see Figure 5 . In terms of hardness, the hardness of the 4μm particle size Fe powder SLM formed CuAlFe alloy (unmelted Fe particle refinement + rapid solidification) is 310HV, the hardness of the 1μm particle size Fe powder SLM formed CuAlFe alloy (rapid solidification) is 295HV, and the hardness of the cast sample is only about 250HV, see 6. The ε rec (9.3%) of the 4μm particle size Fe powder SLM formed CuAlFe alloy (unmelted Fe particle refinement + rapid solidification) after solid solution water quenching reaches 2.1 times of the ε rec (4.2%) of the cast CuAlFe alloy sample. The above performance difference is related to the number of unmelted iron particles and the grain size and number of the alloy.

[0075] In summary, the CuAlFe shape memory alloy of the comparative example is obtained by vacuum arc melting, and the CuAlFe shape memory alloy of the embodiment 2 of the present application is in the form of SLM forming. Through the above comparison, it is obvious that the SLM formed CuAlFe shape memory alloy prepared by the embodiment of the present application can adjust the size and number of the unmelted iron particles by adjusting the particle size of the Fe powder and the laser power, so that the structure of the alloy is ultra-fined, and the mechanical and memory properties of the alloy are obviously improved.

[0076] The above is only the preferred embodiment of the present application, and does not limit the present application in any form. Any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present application are still within the scope of the technical solution of the present application.

Claims

1. A method of ultrafining SLM formed CuAlFe shape memory alloy microstructure, characterized by: The metal element powder is mixed uniformly and formed by SLM, and includes the following mass percentage components: the matrix is 81wt%-83wt% of Cu and 12wt%-14wt% of Al, and the grain refiner is 4wt%-6wt% of Fe; the powder particle size is: Cu 20-40μm, Al 20-40μm, and Fe 3-8μm; The method comprises the following steps (1) the metal element powder is weighed according to the proportion, poured into the ball mill jar; (2) different sizes of stainless steel balls are added to the ball mill jar for ball milling to realize uniform mixing of the metal element powder, and after ball milling, the powder is taken out for drying treatment; (3) the powder of step (2) is used as raw material for SLM forming; In step (2), the metal powder is put into the ball mill jar, stainless steel balls are added, the mass of the stainless steel balls is 1-2 times the mass of the metal powder, the diameter of the stainless steel balls is 3mm, 5mm and 10mm, and the mass ratio of the two kinds of steel balls is 2:2:1; after the ball mill jar is sealed and vacuumized for 10-60min, the planetary ball mill is used for ball milling, and the ball milling time is 1-24h; In the SLM forming of step (3), the parameters are: laser power 200W-600W, scanning speed 100mm / min-1000mm / min, scanning interval 0.03-0.2mm, powder layer thickness 0.02-0.04mm, and laser rotation angle per layer 67°, then the model slice is output and introduced into the machine; The prepared mixed metal powder is loaded into the powder bin, after the oxygen content is less than 800ppm, the stainless steel substrate is preheated to 100-200℃, the printing is started, and the laser scanning is performed twice in the first five layers and once in the subsequent layers, and the CuAlFe shape memory alloy is obtained.

Images