Heat-treatment-free eutectic high-toughness aluminum alloy material as well as preparation method and application thereof

Abstract

The invention discloses a heat-treatment-free eutectic high-strength-and-toughness aluminum alloy material and a preparation method and application thereof, and designs a novel heat-treatment-free high-strength-and-toughness Al alloy and a matched forming process method thereof in order to solve the problems that an existing high-strength aluminum alloy is high in heat cracking performance and poor in printing performance, and a subsequent heat treatment procedure is needed. The high-strength and high-toughness aluminum alloy is prepared by taking Al-Ni as a matrix and properly adding Ti and Mn through an additive manufacturing process. The heat-treatment-free high-toughness Al-Ni-Ti-Mn alloy disclosed by the invention not only has high toughness, but also has good additive formability. An alloy sample formed by melting of a laser powder bed is free of cracks and does not need a follow-up heat treatment process, the tensile strength of a directly formed form can reach 500 MPa, and the ductility reaches 8%. According to the method, Ti and Mn elements are introduced into the Al-Ni eutectic alloy, the Ti element is introduced to refine grains, Al-Al3Ni eutectic is formed, the hot crack sensitivity is reduced, and meanwhile, the added Mn element plays a solid solution strengthening role to improve the mechanical property.

Description

Technical Field

[0001] The present invention belongs to the field of additively manufactured high-performance aluminum alloys, and particularly relates to a eutectic high-strength and tough aluminum alloy material without heat treatment, a preparation method thereof, and an application thereof. Background Art

[0002] Aluminum alloys have the advantages of low density, high specific strength, good corrosion resistance, etc., and play an important role in the manufacturing field. The increasing requirements for the service environment in the aerospace field have made the requirements for the performance of aluminum alloys increasingly stringent, and higher requirements have been put forward for the strength, toughness, and design and manufacturing flexibility of parts. Traditional manufacturing processes have technical difficulties such as low forming freedom and many processing steps, and are limited in the field of integrally forming complex aerospace components. Compared with traditional manufacturing processes, laser additive manufacturing has higher forming freedom, which provides the possibility for the integral forming of complex aluminum alloy components in the aerospace and other fields. Al-Si-based alloys have been widely used in the field of laser powder bed fusion (LPBF) due to their excellent formability. Commonly used alloys include AlSi 10 Mg, AlSi 12 and other alloys, but their strength, plasticity, and tensile strength are relatively low, which limits their application scope in the aerospace field. Other traditional high-strength and tough aluminum alloy materials such as the 2-series (Al-Cu series) and 7-series (Al-Zn series) have poor additive manufacturing processability. There are a large number of hot cracks and many pores in the microstructure of the 2-series and 7-series aluminum alloys formed by the LPBF technology, resulting in their mechanical properties being much lower than those of forgings and unable to be used in key components.

[0003] Adding rare earth elements such as Sc and Zr can solve the cracking problem to a certain extent. Currently, Al-Mg-Sc-Zr commonly used in LPBF usually requires an additional heat treatment process to optimize the performance after forming, which increases the production cost and manufacturing cycle. In summary, additive manufacturing can achieve the integral forming of parts with complex structures. However, at present, traditional high-strength and tough aluminum alloys have problems such as a high tendency for hot cracks and easy generation of pore defects in the LPBF process, making it difficult for the mechanical properties of the formed specimens to reach the performance level of forged specimens; at the same time, aluminum alloy systems modified based on trace rare earth elements have high costs and require additional post-treatment processes, etc., which increases the subsequent manufacturing process and cycle. Therefore, it is urgent to develop an aluminum alloy system with good process formability for LPBF, which can meet the requirements of high strength and toughness and does not require heat treatment, and its forming process method. Summary of the Invention

[0004] Objective of the Invention: Aiming at the problems of poor formability, high hot cracking tendency and the need for additional heat treatment processes in the existing laser additive manufacturing of high-strength and high-toughness aluminum alloys, the present invention provides a heat-treatment-free high-strength and high-toughness aluminum alloy for additive manufacturing and its forming method, realizing low hot cracking sensitivity and porosity, a wide processing window without additional heat treatment processes, and good LPBF formability and mechanical properties.

[0005] In order to achieve the above-mentioned objective of the invention, the technical solutions adopted by the present invention are as follows:

[0006] A heat-treatment-free eutectic high-strength and high-toughness aluminum alloy material, comprising components in the following mass percentages:

[0007] Ni: 2.0% - 5.6%, Ti: 0.5% - 3.5%, Mn: 0.3% - 0.5%, and the balance is aluminum;

[0008] Among them, the mass ratio of the sum of the masses of Ti element and Ni element to Mn element is 7.5 ≤ (Ni + Ti) / Mn ≤ 12.5.

[0009] Furthermore, the present invention also protects the preparation method of the above-mentioned heat-treatment-free eutectic high-strength and high-toughness aluminum alloy material, comprising the following steps:

[0010] S1. Raw material preparation: Use pure Al as the Al raw material, use Al-Ni master alloy as the Ni raw material, use pure Ti as the Ti raw material, and use Al-Mn as the Mn raw material; respectively weigh Al ingots, Ti ingots, Al-Ni master alloy ingots, and Al-Mn master alloy ingots as raw materials.

[0011] S2. Powder atomization: Using Al ingots, Ti ingots, Al-Ni master alloy ingots, and Al-Mn master alloy ingots as raw materials, melt them in a vacuum atomization furnace under the protection of inert gas to obtain alloy liquid, and use inert gas for atomization to make powder.

[0012] S3. Particle size screening: Screen the powder obtained in step S2 to obtain alloy powder with a particle size of 15 - 50 μm; then put the alloy powder into a vacuum oven and dry it to obtain the product.

[0013] Preferably, in step S2, the melting temperature is 700 - 800 °C, and the holding time is more than 0.5 h to completely melt the ingot.

[0014] Preferably, in step S2, the inert gas is argon; argon with a purity ≥ 99.99%, the atomization temperature for making powder is 750 - 850 °C, and the gas pressure is 2.4 - 3.8 Mpa.

[0015] Preferably, in step S2, the atomization powder making is carried out under an inert protective atmosphere, and the pressure of the inert gas protective atmosphere is 0.1~0.12 Mpa.

[0016] Preferably, in step S2, the collected alloy powder is vacuum-packed to prevent the powder from being oxidized.

[0017] Further, in step S3, the drying temperature is 80~120 °C; the drying time is 6~8 h; when drying, first load the alloy powder onto a tray and spread it flat, the thickness of the spread powder ≤20 mm, then load the alloy powder and the tray into a vacuum oven together, and finally dry at a temperature of 100~120 °C for 8~10 h. After drying, the alloy powder is cooled to room temperature in the furnace.

[0018] Further, the eutectic high-strength and tough aluminum alloy material prepared by the above preparation method is also within the protection scope of the present invention.

[0019] Furthermore, the present invention also claims to protect the application of the above eutectic high-strength and tough aluminum alloy material in the preparation of aluminum alloy specimens by laser powder bed melting.

[0020] Specifically, a laser powder bed melting device is used to form the eutectic high-strength and tough aluminum alloy material powder prepared. The laser powder bed melting process parameters are: the powder spreading thickness is 30~50 μm, the laser power is 200~400 W, the scanning speed is 800~1600 mm / s, and the scanning spacing is 0.1~0.2 mm; the forming is carried out under an argon atmosphere, and the oxygen content of the argon atmosphere ≤50 ppm.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] (1) The present invention realizes a high-strength and tough aluminum alloy with good formability and heat treatment-free through the synergistic effect of three elements of Ni, Ti, and Mn.

[0023] The high-strength and tough aluminum alloy of the present invention is based on the Al-Ni eutectic system. This alloy system has a relatively wide processing window and forms Al-Al3Ni eutectic in the later stage of solidification. The formation of eutectic means that the thermal stress during the solidification process is small, reducing the hot cracking tendency of the aluminum alloy. Introducing Ti element into the Al-Ni eutectic system, the Ti element forms metastable L12 structure tri-aluminide (Al3Ti) precipitate phases in Al. These precipitate phases serve as the substrate for heterogeneous nucleation, promoting the formation of fine equiaxed grains. The fine equiaxed grains can effectively inhibit the growth of columnar crystals, thereby reducing the hot cracking sensitivity.

[0024] In the L-PBF specimens prepared from the high-strength and tough aluminum alloy raw materials of the present invention, Mn elements enter the Al matrix to form a solid solution, which can improve the mechanical properties of the alloy; and a large number of Al 31 Mn6Ni 12 intermetallic compounds exist in the form of nano-quasicrystals, which hinder dislocation movement and play a strengthening role. Al 31 Mn6Ni 12 The coexistence of Al3Ni and Al3Ni two precipitation phases makes the hindrance of dislocation movement in the material more complex, thereby further improving the strength and work hardening of the alloy.

[0025] The high-strength and tough aluminum alloy of the present invention is based on the Al-Ni eutectic system, and Ti elements are additionally introduced. The formability of the aluminum alloy is improved by forming Al-Al3Ni eutectic and heterogeneous nucleation of Al3Ti to refine grains, and the hot cracking tendency of the aluminum alloy during rapid cooling is significantly reduced; at the same time, Mn elements are introduced to play a solid solution strengthening role and significantly improve the alloy strength.

[0026] (2) The present invention controls the content of the added elements to achieve synergy between the alloy strength and plasticity. The mass ratio of the sum of the masses of Ti elements and Ni elements to Mn element is 7.5 ≤ (Ni + Ti) / Mn ≤ 12.5. The solubility of Ni elements and Ti elements in the Al matrix is extremely low, and the excessive part forms Al3Ni eutectic and Al3Ti precipitation phases; Mn elements enhance the strength through solid solution strengthening, but excessive amounts are prone to generate brittle Mn-rich phases. 7.5 is the lower limit of the minimum content of the precipitation phase and the suppression of the formation of brittle phases, and 12.5 is the upper limit for restricting the content of the precipitation phase and ensuring the effectiveness of solid solution strengthening of Mn elements. Beyond this range, it will have a negative impact on the formability, mechanical properties, and plasticity of the alloy. When the sum of the masses of Ti elements and Ni elements is too high, the continuous network formed by the eutectic will weaken the strengthening effect, and too many precipitates will cause uneven microstructure and reduce the plasticity of the alloy. When the sum of the masses of Ti elements and Ni elements is too low, the improvement of the formability of the alloy is limited. When the content of Mn element is too high, brittle Mn-rich phases are formed when it exceeds its solubility limit in Al, resulting in a significant reduction in the plasticity of the alloy; when the content of Mn element is too low, the content of the solid solution decreases and the strengthening effect is reduced.

[0027] (3) The specimens formed by the alloy L-PBF of the present invention have outstanding mechanical properties, with a tensile strength of up to 500 MPa and an elongation greater than 8%, achieving synergy between strength and plasticity. Compared with the existing AlSi10Mg alloy used for additive manufacturing, the aluminum alloy of the present invention has higher mechanical properties. Compared with traditional high-strength and tough aluminum alloy materials, the aluminum alloy of the present invention has good formability and strong resistance to hot cracking. Compared with the existing AlMgScZr alloy used for additive manufacturing, the aluminum alloy of the present invention does not require heat treatment. Description of the Drawings

[0028] The following further specifically describes the present invention in conjunction with the accompanying drawings and specific embodiments, and the above and / or other advantages of the present invention will become clearer.

[0029] Figure 1 This is the process flow chart of the present invention.

[0030] Figure 2 This is the typical particle morphology of Al-Ni-Ti-Mn powder.

[0031] Figure 3 This is the microstructural diagram of the aluminum alloy in Example 1.

[0032] Figure 4 This is the microstructural diagram of the aluminum alloy in Example 2. Specific Embodiments

[0033] The present invention can be better understood according to the following embodiments.

[0034] Example 1

[0035] Combined with Figure 1 , the present invention provides a high-strength and tough aluminum alloy for additive manufacturing containing Ni, Ti, and Mn and its forming method, which includes the following components in mass percentage: Ni: 3.0%, Ti: 2.0%, Mn: 0.5%, and the balance is aluminum. Among them, (Ni + Ti) / Mn is 10.0. Specifically, it includes the following steps:

[0036] (1) After weighing Al ingots, Ti ingots, Al-Ni master alloy ingots, and Al-Mn master alloy ingots according to the mass percentage of the above aluminum alloy, place them in a crucible and melt to obtain alloy liquid. Use argon gas to break the alloy liquid to prepare alloy powder, and cool the alloy powder in an argon atmosphere. Vacuum package the collected alloy powder to prevent the powder from being oxidized.

[0037] (2) Screen the alloy powder with 325-mesh and 1000-mesh sieves to obtain alloy powder with a particle size of 15 - 50 μm, and prepare the powder as shown in Figure 2 shown.

[0038] (3) Place the alloy powder on a tray and spread it flat, with a spreading thickness of 18 mm. Then put the alloy powder and the tray into a vacuum oven together, and finally dry it at a temperature of 100 °C for 8 h. After drying, cool the alloy powder in the furnace to room temperature.

[0039] (4) Forming is carried out using a laser powder bed fusion equipment, with a powder spreading thickness of 30 μm, a laser power of 200 W, a scanning speed of 800 mm / s, a scanning spacing of 0.11 mm. During the forming process, argon is used for protection, and the oxygen content in the argon atmosphere in the cavity is controlled to be ≤50 ppm. After cooling, it is taken out.

[0040] Using the Archimedes drainage method to test, the relative density of the Al-3Ni-2Ti-0.5Mn alloy block obtained in this example reaches more than 99%, showing excellent printing performance; testing its mechanical properties, finally a high-strength and tough aluminum alloy with an ultimate tensile strength of 503 Mpa and an elongation of 8.0% is obtained.

[0041] Figure 3 The figure shows the backscattered SEM image of the cross-sectional microstructure of the specimen formed in Example 1. It can be seen that it presents a typical columnar-equiaxed phase distribution structure, and dispersed white nano-precipitated particles can be seen, and these precipitated particles are Al3Ti particle phases.

[0042] Example 2

[0043] A high-strength and tough aluminum alloy for additive manufacturing of Ni, Ti, Mn, which comprises the following components by mass percentage: Ni: 3.0%, Ti: 0.5%, Mn: 0.5%, and the balance is aluminum.

[0044] (Ni + Ti) / Mn is 7.0.

[0045] Its preparation process is the same as that of Example 1.

[0046] The difference between this example and Example 1 is that the mass percentage of Ti element is reduced.

[0047] By using the same test method as in Example 1, it is measured that the relative density of the Al-3Ni-0.5Ti-0.5Mn obtained in this example is more than 98%, the ultimate tensile strength is 410 Mpa, and the elongation is 6.0%.

[0048] Figure 4 The figure shows the backscattered SEM image of the cross-sectional microstructure of the specimen formed in Example 2. It can be seen that it presents a typical columnar-equiaxed phase distribution structure. Compared with Example 1, when the proportion of Ti is reduced, the crystal shows a coarsening feature and there is no significant white precipitation phase, indicating that a good strengthening effect is not obtained for the formed specimen at this time.

[0049] Example 3

[0050] A high-strength and tough aluminum alloy for additive manufacturing of Ni, Ti, Mn, which comprises the following components by mass percentage: Ni: 3.0%, Ti: 3.5%, Mn: 0.5%, and the balance is aluminum.

[0051] (Ni + Ti) / Mn is 13.0.

[0052] Its preparation process is the same as that of Example 1.

[0053] The difference between this example and Example 1 is that the mass percentage of Ti element is increased.

[0054] By the same testing method as in Example 1, it is measured that the density of Al-3Ni-3.5Ti-0.5Mn obtained in this example is above 99%, the ultimate tensile strength is 442 Mpa, and the elongation is 5.2%.

[0055] Example 4

[0056] A high-strength and tough aluminum alloy for additive manufacturing containing Ni, Ti, and Mn, which comprises the following components by mass percentage: Ni: 2.0%, Ti: 2.0%, Mn: 0.5%, and the balance is aluminum.

[0057] (Ni + Ti) / Mn is 8.0.

[0058] Its preparation process is the same as that of Example 1.

[0059] The difference between this example and Example 1 is that the mass percentage of Ni element is decreased.

[0060] By the same testing method as in Example 1, it is measured that the density of Al-2Ni-2Ti-0.5Mn obtained in this example is above 97%, the ultimate tensile strength is 423 Mpa, and the elongation is 8.2%.

[0061] Example 5

[0062] A high-strength and tough aluminum alloy for additive manufacturing containing Ni, Ti, and Mn, which comprises the following components by mass percentage: Ni: 5.6%, Ti: 2.0%, Mn: 0.5%, and the balance is aluminum.

[0063] (Ni + Ti) / Mn is 15.2.

[0064] Its preparation process is the same as that of Example 1.

[0065] The difference between this example and Example 1 is that the mass percentage of Ni element is increased.

[0066] By the same testing method as in Example 1, it is measured that the density of Al-5.6Ni-2Ti-0.5Mn obtained in this example is above 98%, the ultimate tensile strength is 397 Mpa, and the elongation is 4.6%.

[0067] Comparative Example 1

[0068] A high-strength and tough aluminum alloy for additive manufacturing containing Ni and Ti, which comprises the following components in mass percentage: Ni: 3.0%, Ti: 2.0%, and the balance is aluminum.

[0069] Its preparation process is the same as that of Example 1.

[0070] The difference between this example and Example 1 is that it does not contain Mn element.

[0071] By the same test method as in Example 1, it is measured that the density of Al-3Ni-2Ti obtained in this example is more than 99%, the ultimate tensile strength is 331 Mpa, and the elongation is 15.0%.

[0072] Comparative Example 2

[0073] A high-strength and tough aluminum alloy for additive manufacturing containing Ni, Ti, and Mn, which comprises the following components in mass percentage: Ni: 3.0%, Ti: 2.0%, Mn: 1.2%, and the balance is aluminum.

[0074] (Ni + Ti) / Mn is 4.17.

[0075] Its preparation process is the same as that of Example 1.

[0076] The difference between this example and Example 1 is that the mass percentage of Mn element is increased.

[0077] By the same test method as in Example 1, it is measured that the density of Al-3Ni-2Ti-1.2Mn obtained in this example is more than 99%, the ultimate tensile strength is 535 Mpa, and the elongation is 2.2%.

[0078] Comparative Example 3

[0079] An AlMgScZr alloy for additive manufacturing, its composition by mass percentage is: Mg: 4.5%, Sc: 0.7%, Zr: 0.4%, and the balance is Al. Its preparation method includes the following steps:

[0080] (1) After weighing Al ingots, Al-Mg master alloy ingots, Al-Sc master alloy ingots, and Al-Zr master alloy ingots according to the mass percentage of the above aluminum alloy, put them into a crucible to melt to obtain alloy liquid. Use argon gas to break the alloy liquid to prepare alloy powder, and cool the alloy powder in an argon atmosphere. Vacuum package the collected alloy powder to prevent the powder from being oxidized.

[0081] (2) and (3) The steps are the same as those in Example 1.

[0082] (4) Forming is carried out using a laser powder bed fusion equipment. The powder spreading thickness is 30 μm, the laser power is 400 W, the scanning speed is 800 mm / s, the scanning spacing is 0.11 mm. During the forming process, argon is used for protection to control the oxygen content in the argon atmosphere in the cavity ≤ 50 ppm, and then it is taken out after cooling.

[0083] The density of the AlMgScZr alloy block obtained in this comparative example is more than 99% as measured by the same testing method as in Example 1, the ultimate tensile strength is 410.2 Mpa, and the elongation is 1%.

[0084] Table 1 Mechanical property test results of high-strength and tough aluminum alloys in different examples

[0085]

[0086] It can be seen from Table 1 that the aluminum alloy prepared by using the alloy composition and forming process of the present invention exhibits excellent mechanical properties, as shown in Example 1.

[0087] Comparing Example 1 with Examples 2 and 3, when the content of Ti is reduced, the content of Al3Ti precipitated in the aluminum alloy obtained by additive manufacturing decreases, and the number of fine equiaxed crystals decreases, resulting in a decrease in strength and plasticity; when the content of Ti element increases, both plasticity and strength decrease, because the increase in the amount of Al3Ti precipitates leads to non-uniform microstructure.

[0088] Comparing Example 1 with Examples 4 and 5, the decrease in the content of Ni element leads to a decrease in strength and density, indicating that there are more defects in the formed specimens; when the content of Ni element increases, the eutectic content increases to form a continuous network, which has a negative impact on both strength and plasticity.

[0089] Comparing Example 1 with Comparative Example 1 and Comparative Example 2, it can be seen that adding a small amount of Mn element will greatly improve the strength of the alloy, but when the amount of Mn element is excessive, the plasticity will also decrease significantly, and at this time, the synergy of strength and plasticity cannot be achieved.

[0090] Comparing Example 1 with Comparative Example 3, Example 1 does not require heat treatment and can achieve a higher strength than the AlMgScZr alloy without heat treatment.

[0091] The present invention provides an idea and method for a eutectic high-strength and tough aluminum alloy material without heat treatment, its preparation method and application. There are many methods and ways to specifically implement this technical solution. The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and retouches can be made, and these improvements and retouches should also be regarded as the protection scope of the present invention. Each component not clearly defined in this embodiment can be realized by the existing technology.

Claims

1. A heat-treatment-free eutectic high-strength and tough aluminum alloy material, characterized in that, It comprises components with the following mass percentages: Ni: 2.0% - 5.6%, Ti: 0.5% - 3.5%, Mn: 0.3% - 0.5%, and the balance is aluminum; Among them, the mass ratio of the sum of the masses of Ti element and Ni element to Mn element is 7.0 ≤ (Ni + Ti) / Mn ≤ 15.

2.

2. The preparation method of the eutectic high-strength and tough aluminum alloy material without heat treatment according to claim 1, characterized in that, It comprises the following steps: S1. Raw material preparation: Use pure Al as the Al raw material, use Al-Ni master alloy as the Ni raw material, use pure Ti as the Ti raw material, and use Al-Mn as the Mn raw material; Weigh Al ingots, Ti ingots, Al-Ni master alloy ingots, and Al-Mn master alloy ingots respectively as raw materials; S2. Powder atomization: Use Al ingots, Ti ingots, Al-Ni master alloy ingots, and Al-Mn master alloy ingots as raw materials, and carry out melting in a vacuum atomization furnace under the protection of inert gas to obtain alloy liquid, and use inert gas for atomization to make powder; S3. Particle size screening: Screen the powder obtained in step S2 to obtain alloy powder with a particle size of 15 - 50 μm; Then put the alloy powder into a vacuum oven for drying to obtain the product.

3. The preparation method of the eutectic high-strength and tough aluminum alloy material without heat treatment according to claim 1, characterized in that, In step S2, the melting temperature is 700 - 800 °C, and keep the temperature for more than 0.5 h to completely melt the ingot.

4. The preparation method of the eutectic high-strength and tough aluminum alloy material without heat treatment according to claim 1, wherein, In step S2, the inert gas is argon; Argon with a purity ≥ 99.99%, the atomization temperature for making powder is 750 - 850 °C, and the gas pressure is 2.4 - 3.8 Mpa.

5. The preparation method of the eutectic high-strength and tough aluminum alloy material without heat treatment according to claim 1, wherein, In step S2, the atomization for making powder is carried out under an inert protective atmosphere, and the pressure of the inert gas protective atmosphere is 0.1 - 0.12 Mpa.

6. The preparation method of the eutectic high-strength and tough aluminum alloy material without heat treatment according to claim 1, characterized in that, In step S2, the collected alloy powder is vacuum-packed to prevent the powder from being oxidized.

7. The preparation method of the eutectic high-strength and tough aluminum alloy material without heat treatment according to claim 1, characterized in that, In step S3, the drying temperature is 80 - 120 °C; The drying time is 6 - 8 h; When drying, first put the alloy powder on a tray and spread it flat, the thickness of the spread powder ≤ 20 mm, then put the alloy powder and the tray into a vacuum oven together, and finally dry at a temperature of 100 - 120 °C for 8 - 10 h, and cool the alloy powder in the furnace to room temperature after drying.

8. The eutectic high-strength and tough aluminum alloy material prepared by the preparation method according to any one of claims 1 - 7.

9. The application of the eutectic high-strength and tough aluminum alloy material according to claim 8 in the preparation of aluminum alloy specimens by laser powder bed fusion forming.

10. The application according to claim 9, wherein Use a laser powder bed fusion device to form the eutectic high-strength and tough aluminum alloy material powder prepared, and the laser powder bed fusion process parameters are: the powder spreading thickness is 30 - 50 μm, the laser power is 200 - 400 W, the scanning speed is 800 - 1600 mm / s, and the scanning spacing is 0.1 - 0.2 mm; The forming is carried out in an argon atmosphere, and the oxygen content in the argon atmosphere ≤ 50 ppm.

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