Preparation method of invar alloy thick plate with δ≥70mm for aviation mold

Abstract

The present invention discloses a preparation method of an Invar alloy thick plate with a thickness ≥δ70mm for aviation molds. The melting is carried out by combining non-vacuum induction furnace melting and vacuum consumable melting. Forging with a large deformation ratio is adopted, and upsetting and drawing forming are used. Through subsequent heat treatment, the grains grow again, reducing the grain size difference between the edge and the core. The Invar alloy thick plate produced by the present invention has the characteristics that the grain size is finer than grade 5, and the absolute value difference of the grain size grades between the edge and the core (along the thickness direction) ≤ 0.5 grade. After butt welding the edge and the core respectively, the absolute value difference between the welding mechanics and the plate mechanics ≤ 20MPa. After welding at different positions, the weld properties are relatively consistent.

Description

Technical Field

[0001] The present invention belongs to the technical field of aviation special alloys, and relates to a preparation method of an invar alloy thick plate with δ≥70mm for aviation molds. Background Art

[0002] Invar alloy (Invar36, 4J36) in low-expansion alloys, according to the provisions of YB / T 5241 "Low-Expansion Iron-Nickel and Iron-Nickel-Cobalt Alloys", when the temperature range is 20 - 100°C, its expansion coefficient should be ≤1.5×10 -6 / °C. It has an extremely low expansion coefficient and is widely used in fields such as LNG tanks, thermostatic bimetals, aviation industry molds, and OLEDs.

[0003] Currently, the development of large aircraft has become an important part of China's medium- and long-term science and technology development plan. Due to characteristics such as high strength and low specific gravity, when composite materials are applied to the aerospace field, they can bring advantages such as reduced airframe structure weight, extended maintenance intervals, and decreased operating costs. Therefore, composite materials are one of the basic materials for manufacturing large aircraft.

[0004] Composite materials are compounded and formed in a designed mold, and generally no further processing is carried out after forming. Therefore, the mold largely determines the quality of the composite materials; the forming method of composite materials is generally hot pressing, which has high requirements for the expansion coefficient, fatigue resistance, etc. of the mold. Currently, commonly used molds are made of materials such as carbon steel and aluminum alloy. The advantages are low cost and mature manufacturing processes, but the expansion coefficients are significantly different from those of composite materials. Therefore, there are serious problems in aspects such as dimensional accuracy and residual stress of the formed parts.

[0005] In view of the large size and complexity of the composite materials used in large aircraft, it is difficult to form an invar mold in one step, and welding is required for forming. The currently commonly used thickness specifications of invar molds range from δ6mm to δ25.4mm. Because the specifications and sizes of various profiles are different, the invar mold plates need to go through processes such as hot press bending to form various profiles and then be welded.

[0006] In the past two years, the domestic demand for thick plates with a thickness specification of more than δ70mm has increased sharply. This is mainly because thick plates can directly machine some profiles, thereby reducing the number of welds and minimizing the risk of air leakage at the welds. If the thick plates for aviation molds need to be formed by welding, then both the ingot weight and the purity of the alloy ingot need to be ensured; moreover, there is a problem of whether the overall performance of the invar alloy thick plate is uniform; finally, there may be a situation where different parts of the plate are welded, which also puts forward requirements for the uniformity of the welding performance of the thick plate. Summary of the Invention

[0007] The problem to be solved by the present invention is to provide a preparation method for invar alloy thick plates with δ≥70mm for aviation molds, which can obtain thick plates with uniform properties and fine grain size, and the weld properties of different parts of the thick plates are close after welding.

[0008] The present invention is realized by adopting the following technical solutions:

[0009] A preparation method for invar alloy thick plates with δ≥70mm for aviation molds includes the following operations:

[0010] 1) Alloy melting: According to the invar alloy composition ratio, select Fe-Ni return materials, pure iron, electrolytic nickel plates, metallic manganese and crystalline silicon as melting materials, and NiMg alloy, Al powder and SiCa powder as additive materials;

[0011] Add the melting materials into the melting crucible of a non-vacuum induction furnace, heat with a current of 500 - 900A, and add 1 - 1.5% of the mass of the melting materials of lime for slag making when the molten steel is seen; after the materials are completely melted, raise the temperature to 1510 - 1530°C and then pour the crucible to remove the slag;

[0012] After slag removal, add 1 - 1.5% of the mass of the melting materials of lime and 0.5 - 0.75% of fluorite to re-make the slag, and at the same time raise the temperature to 1550 - 1570°C with a current of 800 - 1200A. During the temperature rise process, add a mixture of SiCa powder and Al powder for deoxidation. After the slag turns white, take a sample for composition analysis;

[0013] According to the composition analysis results, make supplementary additions according to the Mn and Si contents;

[0014] After changing the slag, continue to use a mixture of SiCa powder and Al powder for deoxidation. Add NiMg alloy before tapping. After the slag turns white, take a sample for analysis; after the steel liquid composition meets the invar alloy composition, raise the temperature to 1610 - 1630°C and tap the steel;

[0015] 2) Alloy pouring: After tapping, let it stand for 10 - 15 minutes and then pour it under vacuum. After pouring, add a heating agent into the riser; cool the mold for ≥12h, and after demolding, obtain an alloy ingot;

[0016] 3) Alloy forging: Heat the alloy ingot with a gas furnace, the charging temperature ≤450°C, raise the temperature to 850±10°C at a rate of 60 - 80°C / h and hold for 2 - 3h, then raise the temperature to 1100±10°C along with the furnace and hold for 3 - 5h before forging; use a free forging hammer for forging to forge the alloy ingot into the size required for a consumable electrode; the starting forging temperature ≥1050°C, and the final forging temperature ≥850°C;

[0017] 4) Consumable melting: Bake the consumable electrode at 600 - 800°C for 8 - 12h, weld the consumable electrode and the auxiliary electrode with a welding wire of the same steel type and then carry out consumable melting. The vacuum degree during consumable melting is 0.2 - 0.4Pa;

[0018] The starting arc current for consumable melting is 3000 - 5000 A, the current during the stable melting stage is 7500 - 9000 A, and the melting rate during the melting process is between 2 - 5 kg / min;

[0019] When the remaining electrode weight is 10 - 20%, feeding is carried out. During feeding, the current is reduced, and the feeding time is 1 - 3 h;

[0020] After the consumable melting is completed, it is cooled in the mold for 10 - 15 h, and a consumable ingot is obtained after demolding;

[0021] 5) Finished product forging: The consumable ingot is charged into the furnace at room temperature, heated to 800 - 850 °C at a rate of 50 - 60 °C / h and held for 3 - 3.5 h, then heated to 1050 - 1100 °C with the furnace and held for 3.5 - 4 h before starting upsetting and drawing forging;

[0022] The starting forging temperature ≥1050 °C, the final forging temperature ≥850 °C, and upsetting and drawing forging are carried out to a finished product plate with a thickness δ≥70 mm; If the temperature during the forging process is lower than the final forging temperature, it is reheated in the furnace. The reheating temperature shall not be higher than 1040 °C, and the reheating holding time shall not exceed 3 / 5 of the holding time of the consumable ingot;

[0023] 6) Heat treatment: The finished product plate is charged into the furnace at ≤300 °C, and each plate is supported by pads with a thickness ≥ the thickness of the finished product plate; heated to 800 - 850 °C at a rate of 60 - 80 °C / h, held for 6 - 8 h, and cooled in the furnace to below 200 °C and then taken out for air cooling.

[0024] Among the melting materials and additives, the Fe-Ni return material and pure iron are baked at 600 - 850 °C for at least 24 h, the electrolytic nickel plate is baked at 800 - 900 °C for 4 - 5 h, and the metallic manganese, crystalline Si, NiMg alloy, Al powder, and SiCa powder are baked at 100 - 200 °C for at least 24 h;

[0025] In terms of mass percentage, the composition of the Invar alloy is: C≤0.02%, Mn≤0.30%, Si≤0.20%, P≤0.015%, S≤0.015%, Al≤0.10%, Ti≤0.10%, Ni is 35.5 - 37.0%, O≤10 ppm, N≤10 ppm, H≤0.5 ppm, and the balance is Fe and unavoidable impurities.

[0026] Compared with the prior art, the present invention has the following beneficial technical effects:

[0027] The preparation method of Invar alloy thick plates with a thickness ≥ δ70mm for aviation molds provided by the present invention uses a combination of non-vacuum induction furnace melting and vacuum consumable melting for melting, which can reduce the gas content in the alloy; to ensure the uniformity of thick plate forging, forging with a large deformation ratio and upsetting and drawing forming are adopted to refine grains, homogenize the structure, and improve the uniformity of the alloy plate; moreover, when using upsetting and drawing forming, a 12T hammer is used for upsetting treatment, which can break the grain size, especially the grains in the core, and then through subsequent heat treatment, the grains grow again, reducing the grain size difference between the edge and the core.

[0028] The preparation method of Invar alloy thick plates with a thickness ≥ δ70mm for aviation molds provided by the present invention uses spacers (thickness ≥ plate thickness) during heat treatment to ensure that the plates are thoroughly burned; if the spacing between the plates is small, it is possible that the grains on the side where the two plates face each other are coarser or uneven.

[0029] The preparation method of Invar alloy thick plates with a thickness ≥ δ70mm for aviation molds provided by the present invention. Since the Invar alloy ingot has a low gas content, with O, N ≤ 10.0ppm and H ≤ 0.5ppm in the alloy ingot; the Invar alloy thick plates produced by forging have the characteristics that the grain size is finer than grade 5, and the absolute value difference of the grain size grades between the edge and the core (along the thickness direction) ≤ 0.5 grade. After butt welding the edge and the core respectively, the absolute value difference between the welding mechanics and the plate mechanics ≤ 20MPa; after welding different parts, the weld properties are relatively consistent. Description of the Drawings

[0030] Figure 1 Schematic diagram of the grain size of the edge of the finished plate with a thickness δ ≥ 70mm;

[0031] Figure 2 Schematic diagram of the grain size of the core of the finished plate with a thickness δ ≥ 70mm;

[0032] Figure 3 Schematic diagram of the finished plate with a thickness δ ≥ 70mm made. Detailed Embodiments

[0033] The following further describes the present invention in detail with reference to embodiments, which is an explanation rather than a limitation of the present invention.

[0034] A preparation method of Invar alloy thick plates with δ ≥ 70mm for aviation molds includes the following operations:

[0035] 1) Alloy melting: According to the Invar alloy composition ratio, raw materials are prepared, where Fe-Ni return materials, pure iron, electrolytic nickel plates, metallic manganese, and crystalline silicon are melting materials, and NiMg alloy, industrial Al powder, and SiCa powder are used as raw materials;

[0036] Among them, the Fe-Ni return material and pure iron are baked at 600-850 °C for a time ≥ 24 h. The electrolytic nickel plate needs to be baked at 800-900 °C for 4 h. Metallic manganese, crystalline Si, NiMg alloy, industrial Al powder, SiCa powder, etc. are baked at 100-200 °C for ≥ 24 h to keep them dry;

[0037] Add the melting materials into the melting crucible of a non-vacuum induction furnace, heat the materials with a current of 500-900 A. When the molten steel is seen, add 1-1.5% of the lime by the mass of the molten steel to make slag; after the materials are completely melted, raise the temperature to 1510-1530 °C and then pour the crucible to skim the slag;

[0038] After skimming the slag, add 1-1.5% of the lime by the mass of the molten steel and 0.5-0.75% of fluorite to re-make slag. At the same time, raise the temperature to 1550-1570 °C with a current of 800-1200 A. During the temperature-raising process, use a mixture of SiCa powder and Al powder for deoxidation. After the slag turns white, take a sample for composition analysis;

[0039] According to the results of the composition detection, make supplementary additions according to the Mn and Si contents;

[0040] After changing the slag, continue to use a mixture of SiCa powder and Al powder for deoxidation. The dosages of industrial Al powder and SiCa powder are respectively (0.15-0.25%) * the weight of the molten steel. After the slag turns white, take a sample for analysis;

[0041] The dosage of NiMg alloy is 0.1% * the weight of the molten steel (Kg). The addition timing of NiMg alloy: add it into the ladle before tapping;

[0042] After the composition of the molten steel reaches the target value of invar alloy, raise the temperature to 1610-1630 °C and tap the steel;

[0043] In terms of mass percentage, the composition of the invar alloy molten steel is: C ≤ 0.02%, Mn ≤ 0.30%, Si ≤ 0.20%, P ≤ 0.015%, S ≤ 0.015%, Al ≤ 0.10%, Ti ≤ 0.10%, Ni is 35.5-37.0%, O ≤ 10 ppm, N ≤ 10 ppm, H ≤ 0.5 ppm, and the balance is Fe and unavoidable impurities.

[0044] 2) Alloy casting: After tapping, hold for 10 min for calming, then start casting. After casting is completed, add a heat-generating agent into the riser; cool in the mold for ≥ 12 h, and after demolding, obtain an alloy ingot;

[0045] Specifically, the temperature of the gating system is kept above 80 °C; after casting is completed, break the vacuum and add a heat-generating agent into the riser; in terms of mass ratio, the composition of the heat-generating agent is: industrial aluminum powder 50-70%, sodium nitrate 30-50%; the dosage of the heat-generating agent is 0.2-0.6% of the weight of the alloy ingot;

[0046] 3) Alloy forging: Alloy ingots are forged into the size of consumable electrodes;

[0047] Specifically, the consumable electrode size is φ500mm*1400mm;

[0048] Generally, the size of the consumable electrode is 0.6 to 0.8 times the size of the consumable furnace crystallizer;

[0049] The forging process is as follows: heating by gas furnace, charging temperature ≤ 450℃, heating to 850±10℃ at 60~80℃ / h and keeping at that temperature for 2~3h, then heating to 1100±10℃ with the furnace, keeping at that temperature for 3~5h before starting forging, initial forging temperature ≥1050℃, final forging temperature ≥850℃, forging is required to be carried out with a free forging hammer of 6T or above; a forging hammer of 6T or above is selected to break the structure so that the loose part of the cast alloy ingot can be healed;

[0050] The round rod is forged into an electrode by a die (thrower) and used as the electrode for consumable smelting. Since the electrode is required to be round, the use of a hammer to form it avoids the consumption of electrode polishing, saves material costs, and improves the processing yield rate.

[0051] 4) Consumable smelting: The consumable electrode is sawn at the head and tail, and the end surface flatness is required to be ≤6°; at the same time, it is polished to remove the surface oxide scale and defects;

[0052] The polished consumable electrode is baked at 600-800℃ for 8-12h, and the consumable electrode and the auxiliary electrode are welded with the same steel wire and then the consumable electrode is smelted. The vacuum degree during the consumable smelting is between 0.2 and 0.4Pa.

[0053] The vacuum self-consumable arc starting current is 3000-5000A, the current in the stable melting stage is 7500-9000A, and the melting rate in the melting process is between 2-5kg / min; the voltage fluctuation in the stable period is controlled between ±2V, and the current fluctuation is controlled between ±30A;

[0054] When the electrode weight is between 10% and 20% remaining, heat shrinkage is performed. The current is gradually reduced during shrinkage. In order to improve the shrinkage hole depth, the heat shrinkage time must be ensured to be between 1 and 3 hours.

[0055] After vacuum consumable smelting is completed, it is cooled in the crystallizer for 10 to 15 hours, and a consumable ingot is obtained after demoulding.

[0056] Specifically, the weight of the consumable ingot is at least 2 tons, and it is usually cast into a cylinder;

[0057] 5) Finished product forging: After the shrinkage hole is sawed out of the consumable alloy ingot, a large-tonnage free forging electro-hydraulic hammer is used to increase the first insulation time and perform multiple upsetting and drawing processes to fully break the columnar grains and obtain uniform and refined grains.

[0058] Considering that preventing the billet temperature from being too high will cause surface cracks, which is not conducive to subsequent processing, and at the same time to save materials, the heating temperature is reduced and the holding time is extended to ensure the billet surface; the forging heating system is the same as the forging process of the alloy ingot in step 3).

[0059] Finished forging: The consumable ingot after sawing and reducing the shrinkage hole is charged into the furnace at room temperature, heated to 850°C at a rate of 60°C / h and held for 3 h, then heated to 1100°C with the furnace and forged after holding for 4 h. The initial forging temperature is 1050°C.

[0060] After the consumable ingot is taken out of the furnace, it is quickly rolled into a round shape, upset to H = 650 mm, clamped at the length and drawn out to 430×430 mm, then upset to H = 800 mm, clamped at the length and forged to a width of 1000 mm by spreading, and drawn out to 220×1000 mm, and then returned to the furnace (at this time, the forging temperature has dropped. If the temperature during the forging process is lower than the final forging temperature, it needs to be returned to the furnace for heating).

[0061] The purpose of upsetting and drawing is to refine the grains. To ensure the grain size, the return furnace temperature should be appropriately reduced, not higher than 1040°C, and the holding time should not exceed 3 / 5 of the holding time of the alloy ingot; when the temperature is relatively high, the grains grow faster, which is not conducive to obtaining fine grains. In addition, after returning to the furnace, only drawing is required to reach the finished product size. Therefore, a high return furnace temperature not only wastes resources but also causes serious oxidation of the alloy; after being taken out of the furnace, it is drawn out to the finished product size (the finished product thickness δ≥70 mm);

[0062] According to the arc size at both ends of the length, add a flat head once to ensure that the billet is straight, and pay attention to preventing the billet from being clamped and damaged by the jaws of the manipulator.

[0063] 6) The finished product plate after flaw detection and sawing is heat-treated:

[0064] Charge into the furnace at ≤300°C, heat to 850°C at a rate of 60 - 80°C / h, hold for 6 - 8 h, cut off the power and cool in the furnace to below 200°C and then take out and air-cool; it should be noted that each plate needs to be supported (thickness ≥ plate thickness) during heat treatment to ensure the uniformity of heat treatment.

[0065] Specifically, charge into the furnace at 80°C, separate two plates of δ80*1000*3000 mm with 80 mm refractory bricks; heat to 850°C at a rate of 80°C / h, hold for 8 h, cut off the power and cool in the furnace to 150°C and then take out and air-cool.

[0066] The purpose of heat treatment is to eliminate processing stress, obtain a stable expansion coefficient and uniform grains.

[0067] The following gives specific examples and comparative examples.

[0068] Example 1

[0069] Ingot preparation: The Invar alloy ingots were produced by a melting method that included non-vacuum induction furnace melting and vacuum consumable melting. Self-consumable ingots with a single weight of 2T and a size of φ500mm*1400mm were obtained.

[0070] The composition of the Invar alloy is as follows: C 0.009%, Mn 0.25%, Si 0.11%, P 0.0048%, S 0.0012%, Al 0.040%, Ti 0.048%, Ni 35.95%, O 3.5ppm, N 8.2ppm, H 0.03ppm, and the balance is Fe and unavoidable impurities.

[0071] Finished product forging: The self-consumable ingot after sawing off the shrinkage cavity was charged into the furnace at room temperature, heated to 850°C at a rate of 60°C / h and held for 3h, then heated to 1100°C with the furnace and held for 4h before starting forging, and the initial forging temperature was 1050°C.

[0072] After the ingot was taken out of the furnace, it was quickly rolled into a round shape, upset to H = 650mm, the length held by the clamp was drawn out to 430×430mm, then upset to H = 800mm, and the length held by the clamp was subjected to width forging to a size of 1000mm wide and drawn out to 220×1000mm. If necessary, it was returned to the furnace. To ensure the grain size, the return temperature should be appropriately reduced, and after being taken out of the furnace, it was drawn out to the finished product size. According to the arc size at both ends of the length, a flat head was added once to ensure that the billet was straight, and attention should be paid to preventing the billet from being scratched by the jaws of the manipulator.

[0073] Heat treatment: Two plates of δ80*1000*3000mm were separated by 80mm refractory bricks, charged into the furnace at 80°C, heated to 850°C at a rate of 80°C / h and held for 8h, then the power was cut off and the furnace was cooled to 150°C and taken out of the furnace for air cooling.

[0074] Comparative example 1:

[0075] Ingot preparation: Non-vacuum induction furnace melting, casting a 2.5T square ingot by bottom pouring, with a size of (riser 550*550mm)*(tuyere 420*420mm)*1490mm;

[0076] The composition of the Invar alloy is as follows: C 0.011%, Mn 0.22%, Si 0.12%, P 0.0068%, S 0.0015%, Al 0.0440%, Ti 0.038%, Ni 35.85%, O 39.5ppm, N 48.2ppm, H 0.05ppm, and the balance is Fe and unavoidable impurities.

[0077] Finished product forging: The square ingot after sawing off the shrinkage cavity and grinding was charged into the furnace at room temperature, heated to 850°C at a rate of 60°C / h and held for 2.5h, then heated to 1100°C with the furnace and held for 5h before starting forging, and the initial forging temperature was 1050°C.

[0078] First, use a 12T electro-hydraulic hammer to upset the alloy ingot to 900 mm, and then draw it out to δ250*700*L, mm; then put it back into the furnace and hold it at 1040 °C for 2.0 h. After taking it out of the furnace, directly draw it out to δ85*700*L, mm. After flaw detection and sawing, the size is δ85*700*4000 mm.

[0079] Heat treatment: Two plates of δ85*700*4000 mm are separated by 85 mm refractory bricks; charge the furnace at 80 °C, heat it up to 850 °C at a rate of 70 °C / h, hold for 8 h, cut off the power and cool the furnace to 180 °C, then take it out of the furnace and air-cool.

[0080] Comparative Example 2:

[0081] Alloy ingot preparation: Medium-frequency induction furnace melting → LF refining → VD refining, bottom-pouring to cast a 2.5T square ingot with dimensions of (riser 550*550 mm)*(runner 420*420 mm)*1490 mm;

[0082] The composition of the invar alloy is: C 0.020%, Mn 0.25%, Si 0.08%, P 0.0078%, S 0.0010%, Al 0.054%, Ti 0.038%, Ni 35.95%, O 15.0 ppm, N 32.0 ppm, H 0.08 ppm, and the balance is Fe and unavoidable impurities.

[0083] Finished product forging: The forging process used in this comparative example is the same as that in Comparative Example 1.

[0084] Heat treatment: Two plates of δ85*700*4000 mm are separated by 85 mm refractory bricks; charge the furnace at 80 °C, heat it up to 850 °C at a rate of 70 °C / h, hold for 8 h, cut off the power and cool the furnace to 180 °C, then take it out of the furnace and air-cool.

[0085] The chemical compositions and oxygen contents of Example 1 and Comparative Examples 1 and 2 are shown in Table 1.

[0086] Table 1 Chemical Composition Table

[0087]

[0088] The expansion coefficients of Example 1 and Comparative Examples 1 and 2 are shown in Table 2.

[0089] Table 2 Expansion Coefficient

[0090]

[0091] As can be seen from Table 1 and Table 2, the components and oxygen content of the examples all meet the requirements described in the present invention; however, the O contents of Comparative Examples 1 and 2 are 39.5 ppm and 48.2 ppm respectively, both higher than the required 10.0 ppm; the N contents of Comparative Examples 1 and 2 are 48.2 ppm and 32.0 ppm respectively, both higher than the required 10.0 ppm; the H contents of Comparative Examples 1 and 2 are 0.05 ppm and 0.08 ppm respectively, meeting the requirement of ≤1.0 ppm.

[0092] The expansion coefficients of Example 1 and Comparative Examples 1 and 2 all meet the standard requirements.

[0093] The properties of Example 1 and Comparative Examples 1 and 2 are shown in Table 3.

[0094] Table 3 Plate Properties

[0095]

[0096] The positions shown in Table 3 are along the thickness direction of the plate. The mechanical properties, grain size, and HRB uniformity of Example 1 are good; the grain size at the core of the plate in Comparative Example 1 is Grade 4, finer than the required Grade 5, and the grain size of Example 1 meets the requirements and the grain distribution is uniform (as Figure 1 、 Figure 2 shown); as shown in Table 3, the elongation at the core is 36.5%, much lower than 39.5% at the edge; the elongation at the core of the plate in Comparative Example 2 is 37.5%, lower than 39.5% at the edge; this shows that the uniformity of the plates in Comparative Examples 1 and 2 is not as good as that of Example 1.

[0097] Next, thick plate specimens prepared from Example 1 and Comparative Examples 1 and 2 are welded, and the welding method is as follows:

[0098] S1) Select welding alloy plates, and take edge and core samples along the thickness direction of the alloy plates, with thicknesses of 20 mm respectively;

[0099] S2) Before welding, open a 30° "V" groove on the alloy plate, with a root face height of 2 mm. Use a angle grinder and sandpaper to polish the groove to remove the oxide film, wipe the groove with alcohol or acetone, and wait for it to dry before welding;

[0100] S3) Welding method: TIG welding, the wire specifications and compositions are shown in Table 4; weld the root pass, with a welding current of 210 A, a welding speed of 0.1 m / min, a shielding gas of 99.99% Ar, a shielding gas flow rate of 20 L / min, and a dwell time of 0.5 s at the start and end of the arc;

[0101] S4) Weld the filling layer with a welding current of 190 A, a welding speed of 0.1 m / min, a shielding gas of 99.99% Ar, a shielding gas flow rate of 20 L / min, dwell at the arc starting and ending points for 0.3 s, and select whether to use oscillating welding according to the wire filling amount;

[0102] S5) Weld the cover layer with a welding current of 190 A, a welding speed of 0.08 m / min, a shielding gas of 99.99% Ar, a shielding gas flow rate of 20 L / min, dwell at the arc starting and ending points for 0.5 s, and adopt the oscillating welding process.

[0103] Table 4 Wire properties

[0104]

[0105] By using the above welding processes to weld the thick plates prepared in the examples and Comparative Examples 1 and 2 respectively, the welding properties are shown in Table 5.

[0106] Table 5 Welding properties of the plates prepared in Example 1 and Comparative Examples 1 and 2

[0107]

[0108] The absolute value differences between the edge and the core of each plate after welding with each wire are shown in Table 6.

[0109] Table 6 Mechanical property differences between the edge and the core

[0110]

[0111] As can be seen from Table 5, the welding effects of the 2# wire are better than those of the 1# wire; for the weldments obtained by welding with the 2# wire, the fracture positions are all at the material, with obvious necking phenomenon, while the fracture positions of the 1# wire are at the weld, and the necking phenomenon is not obvious; the strength and elongation of the weldments obtained in Example 1 can meet the requirements; in Comparative Example 1, the tensile strengths of the weldments during welding at the core of the plate are 81.25% and 89.30% respectively, not meeting the requirements (i.e., ≥98%), and the elongation of the weldments welded with the 1# wire is only 29% and 25%, not meeting the requirements (i.e., ≥30%); in Comparative Example 2, when welding with the 1# wire, the proportion of the tensile strength at the core is only 91.86%, and the elongation is only 28%, both not reaching the target values.

[0112] As can be seen from Table 6, for the thick plates produced in Example 1, regardless of which wire is used for welding, the mechanical differences between the edge and the core are <20 MPa; the mechanical property differences between the edge and the core in Comparative Example 1 are 52.70 MPa and 26.88 MPa, not meeting the requirements of the present invention; the mechanical property differences between the edge and the core in Comparative Example 2 are 21.04 MPa and 21.16 MPa, not meeting the requirements of the present invention.

[0113] In summary, an invar alloy thick plate with a thickness ≥ δ70mm for aviation molds produced by the present invention (as Figure 3 shown) has the characteristics of low gas content and stable and uniform performance; and the mechanical properties of different parts after welding are all excellent and stable, and the domestic production of invar alloy thick plates for aviation molds can be realized.

[0114] The above-given embodiments are the better examples for implementing the present invention, and the present invention is not limited to the above embodiments. Any non-essential addition or replacement made by those skilled in the art according to the technical features of the technical solution of the present invention falls within the protection scope of the present invention.

Claims

1. A method for preparing an invar alloy thick plate with a thickness δ≥70mm for aviation molds, characterized in that, It includes the following operations: 1) Alloy melting: According to the Invar alloy composition ratio, select Fe-Ni return materials, pure iron, electrolytic nickel plates, metallic manganese, and crystalline silicon as melting materials, and NiMg alloy, Al powder, and SiCa powder as additive materials; Add the melting materials into the melting crucible of a non-vacuum induction furnace, heat with a current of 500 - 900 A, and add 1 - 1.5% of the mass of the melting materials of lime for slagging when the molten steel is seen; after the materials are completely melted, raise the temperature to 1510 - 1530 °C and then pour the crucible to remove the slag; After slag removal, add 1 - 1.5% of the mass of the melting materials of lime and 0.5 - 0.75% of fluorite to re-slag, and at the same time raise the temperature to 1550 - 1570 °C with a current of 800 - 1200 A. During the temperature increase process, add a mixture of SiCa powder and Al powder for deoxidation. After the slag turns white, take a sample for composition analysis; According to the composition analysis results, make supplementary additions according to the Mn and Si contents; After changing the slag, continue to use a mixture of SiCa powder and Al powder for deoxidation. Add NiMg alloy before tapping. After the slag turns white, take a sample for analysis; when the composition of the molten steel meets the Invar alloy composition, raise the temperature to 1610 - 1630 °C and tap; Among the melting materials and additive materials, the Fe-Ni return materials and pure iron are baked at 600 - 850 °C for at least 24 h, the electrolytic nickel plates are baked at 800 - 900 °C for 4 - 5 h, and the metallic manganese, crystalline silicon, NiMg alloy, Al powder, and SiCa powder are baked at 100 - 200 °C for at least 24 h; By mass percentage, the Invar alloy composition is: C ≤ 0.02%, Mn ≤ 0.30%, Si ≤ 0.20%, P ≤ 0.015%, S ≤ 0.015%, Al ≤ 0.10%, Ti ≤ 0.10%, Ni is 35.5 - 37.0%, O ≤ 10 ppm, N ≤ 10 ppm, H ≤ 0.5 ppm, and the balance is Fe and unavoidable impurities; 2) Alloy casting: After tapping, hold for 10 - 15 min and then conduct vacuum casting. After casting is completed, add a heat-generating agent to the riser; the mold cooling is ≥ 12 h, and an alloy ingot is obtained after demolding; 3) Alloy forging: Heat the alloy ingot with a gas furnace, the charging temperature ≤ 450 °C, raise the temperature to 850 ± 10 °C at a rate of 60 - 80 °C / h and hold for 2 - 3 h, then raise the temperature with the furnace to 1100 ± 10 °C, hold for 3 - 5 h and then forge; use a free forging hammer for forging, and forge the alloy ingot into the size required for a consumable electrode; the starting forging temperature ≥ 1050 °C, and the final forging temperature ≥ 850 °C; 4) Consumable melting: Bake the consumable electrode at 600 - 800 °C for 8 - 12 h, weld the consumable electrode and the auxiliary electrode with a wire of the same steel type and then conduct consumable melting. The vacuum degree during consumable melting is 0.2 - 0.4 Pa; The starting arc current for consumable melting is 3000 - 5000 A, the current in the stable melting stage is 7500 - 9000 A, and the melting rate during the melting process is between 2 - 5 kg / min; When the weight of the consumable electrode remains 10 - 20%, perform feeding, and reduce the current during feeding. The feeding time is 1 - 3 h; After consumable melting is completed, cool in the mold for 10 - 15 h, and a consumable ingot is obtained after demolding; 5) Finish forging: Charge the consumable ingot into the furnace at room temperature, heat it up to 800 - 850°C at a rate of 50 - 60°C / h and hold for 3 - 3.5 h, then heat it up to 1050 - 1100°C along with the furnace and start upsetting and drawing forging after holding for 3.5 - 4 h; The starting forging temperature ≥ 1050°C, the final forging temperature ≥ 850°C, upset and draw forge to a finished sheet with a thickness δ ≥ 70 mm; If the temperature during the forging process is lower than the final forging temperature, reheat in the furnace, and the reheat temperature shall not be higher than 1040°C, and the reheat holding time shall not exceed 3 / 5 of the consumable ingot holding time; 6) Heat treatment: Charge the finished sheet into the furnace at ≤ 300°C, support each sheet with a spacer block, and the thickness of the spacer block ≥ the thickness of the finished sheet; Heat it up to 800 - 850°C at a rate of 60 - 80°C / h, hold for 6 - 8 h, cool in the furnace to below 200°C and then take it out for air cooling.

2. The preparation method of an invar alloy thick plate with a thickness δ≥70mm for an aviation mold as described in claim 1, characterized in that, During alloy pouring, the temperature of the gating system is maintained above 80°C; After pouring, break the vacuum and add a heating agent into the riser; By mass percentage, the heating agent composition is 50 - 70% industrial aluminum powder and 30 - 50% sodium nitrate; The dosage of the heating agent is 0.2 - 0.6% of the weight of the alloy ingot.

3. The preparation method of an invar alloy thick plate with a thickness δ≥70mm for an aviation mold as described in claim 1, characterized in that, The consumable electrode is forged to 0.6 - 0.8 times the size of the consumable furnace crystallizer; After forging the consumable electrode, saw off the head and tail, and the requirement for the end face flatness is ≤ 6°; After turning and finishing to remove surface defects, carry out consumable melting.

4. The preparation method of an Invar alloy thick plate with a thickness δ≥70mm for an aviation mold as described in claim 1, characterized in that, During the vacuum consumable stable melting stage, the voltage fluctuation is controlled between ±2 V, and the current fluctuation is controlled between ±30 A.

5. The preparation method of an invar alloy thick plate with a thickness δ≥70mm for an aviation mold as described in claim 1, characterized in that, The single weight of the consumable ingot is at least 2 t, and the shape is cylindrical.

6. The preparation method of an invar alloy thick plate with a thickness δ≥70mm for an aviation mold as described in claim 1, characterized in that, The spacer block is a refractory brick, and multiple refractory bricks are used to separate the sheets from each other.

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