A welding method for a large-size stringer mold of a nickel-iron alloy

Abstract

This invention belongs to the field of welding technology and relates to a welding method for a large-size long girder mold made of nickel-iron alloy, comprising the following steps: 1. Preparing various plates of the mold; 2. Clamping the prepared plates on a machine tool and milling bevels at the joints of the plates; 3. Joining the plates together to form a mold blank, and welding the joints of the plates in layers and passes; 4. Annealing the welded mold blank; 5. CNC machining the annealed mold blank; 6. Inspecting the machined mold.

Description

Technical Field

[0001] This invention belongs to the field of welding technology and relates to a welding method for a large-size long stringer mold made of nickel-iron alloy. Background Technology

[0002] The large-size stringer mold described in this invention refers to a mold with a stringer moving and fixed part having an external length dimension greater than 3000mm. It is used to meet the requirements of dimensional stability and airtightness of the mold during the hot autoclave molding process of composite material components, under process conditions such as heating (180℃), pressurization (0.8MPa) and vacuuming (vacuum leakage rate not exceeding 0.017MPa / 5min).

[0003] The molding dies made of nickel-iron alloy have a similar coefficient of thermal expansion to composite material components, effectively solving the problem of out-of-tolerance dimensional accuracy caused by the difference in thermal expansion coefficients between ordinary steel and aluminum and composite materials. Furthermore, nickel-iron alloy molds possess excellent structural stability and vacuum performance. The domestically produced nickel-iron alloy material is designated 4J36, with main components of 36% nickel, 63.8% iron, and 0.2% carbon. It has poor thermal conductivity, is prone to sticking to the cutting tool during machining, and exhibits poor metal flow during welding, easily leading to deformation, porosity, cracks, and undercut, making it a difficult-to-machine material. Figure 1 The diagram shown is a moving mold of the stringer forming die. The die material is 4J36.

[0004] Because the mold body has a slender shape and several light-reducing holes, it is very easy to deform during processing. Currently, the industry generally adopts a thin-plate assembly and welding process for processing large-size stringer molds. This involves using 20mm thick 4J36 steel plates, cutting them into N layers along the height of the moving mold using water jet cutting equipment, and then assembling and welding them together. The outer shape, along with several lightening holes in the middle, is cut out together. After cutting, V-shaped bevels are machined at the joints of the N layers, and then the layers are stacked and welded one by one. This method is defined as the "layer stacking method". Because the weld is long, the internal stress is large after full welding, which easily causes the ends to warp upwards, with a deformation of about 10-15mm. Before heat treatment, reverse pressure was applied on the platform for straightening. After annealing, a 200T hydraulic press was used for straightening. After CNC precision machining of the surface, the exposed pores were repaired. Even after trial production in a hot autoclave, there is still a high probability that the formed parts will have fiber wrinkles, cavities, and gaps. The long repair cycle of large-size stringer molds seriously affects the aircraft delivery schedule. Therefore, controlling the welding deformation of large-size stringer molds is the primary problem to be solved, and it is also the main research direction of this invention. Summary of the Invention

[0005] The purpose of this invention is to provide a welding method for large-size stringer molds made of nickel-iron alloy, which reduces residual internal stress, effectively reduces welding deformation and defects, ensures good airtightness of the surface of the large-size stringer forming mold made of 4J36 material, and enables carbon fiber composite components to be formed into qualified products under the heating and pressurization of a hot autoclave.

[0006] Technical solution

[0007] A welding method for a large-size nickel-iron alloy stringer mold includes the following steps:

[0008] 1. Prepare the various plates for the mold;

[0009] 2. Clamp the prepared plates on the machine tool and mill bevels at the joints of the plates.

[0010] 3. Overlap the various plates to form a mold blank, and weld the overlaps of the plates in layers and passes.

[0011] 4. Anneal the welded mold blank;

[0012] 5. Perform CNC machining on the annealed mold blank;

[0013] 6. Inspect the processed mold.

[0014] Furthermore, in the first step, the mold is divided into multiple plates, each of which is a flat plate without bending.

[0015] Furthermore, the mold is divided into eight types of plates: top plate, bottom plate, left side plate, right side plate, right side inclined plate, front inclined plate, front side plate, rear inclined plate, rear side plate, and multiple reinforcing ribs.

[0016] Furthermore, in the second section, the bevel angle is 30°, and the overlap of two adjacent plates forms a 60° V-shaped bevel;

[0017] The blunt edge has a reserved length of 2mm and a blunt edge gap of 0.5mm.

[0018] Furthermore, in the third stage, an argon arc welding method is adopted for the root pass and a gas shielded welding method for the sealing.

[0019] The parameters for argon arc welding are as follows: rated current range 200A, argon flow rate 20L / min, and welding wire selected as INVAR M933mm.

[0020] The parameters for CO2 gas shielded welding are as follows: rated current range 200A, shielding gas flow rate 20L / min, and welding wire selected as INVAR M93 1.2mm.

[0021] Furthermore, in the third step, during welding, the V-groove is divided into multiple layers according to the depth direction; the thickness of a single layer is 2mm to 3mm; welding is carried out layer by layer from the bottom layer to the top layer, and the next layer is welded after the next layer is completely cooled. The weld after each layer is welded is then ground and polished.

[0022] Furthermore, the number of weld beads in the first layer is 1, and the number of weld beads increases by 1 with each subsequent layer; and the upper layers are welded from both sides towards the middle.

[0023] Weld continuously for 10 minutes, then rest for 5 minutes.

[0024] Furthermore, the bottom layer is argon arc welding;

[0025] The other layers are gas shielded welded, and are welded in sections every 200mm along the length.

[0026] Furthermore, in section four, the annealing process is as follows: furnace entry temperature ≥100℃, heating to 820~850℃, heating rate ≤100℃ / hour, holding time ≥2H; furnace cooling to 200℃, cooling rate ≤80℃ / hour.

[0027] Furthermore, in the sixth round of testing, the following tests were conducted:

[0028] The profile tolerance of the surface shall not exceed ±0.1mm;

[0029] The vacuum leakage rate of the molded surface shall not exceed 0.017 MPa / 5 min.

[0030] Beneficial effects: Compared with the original "layer stacking method", the "box-type overlapping method" is used for welding the moving and fixed mold bodies of large-size long stringer forming molds. This reduces the total length of the mold body welds, reduces air leakage points caused by welding defects on the working surface, reduces welding deformation caused by welding internal stress, and improves welding quality and airtightness. Because the overall welding workload is small, it not only reduces the processing cycle of the welding process, but also reduces the processing costs of special Invar steel welding wire, auxiliary materials, etc. Attached Figure Description

[0031] This invention includes 6 figures, and the figures and their descriptions are as follows:

[0032] Figure 1 This is a schematic diagram of the moving mold structure of a large-size stringer forming mold.

[0033] Figure 2 A comparative schematic diagram of welding schemes for the moving mold body.

[0034] Figure 3 This is a schematic diagram of a movable mold box-type overlapping structure.

[0035] Figure 4 This is a breakdown and cutting diagram of the moving mold body parts.

[0036] Figure 5 This is a diagram showing the layered welding sequence along the length of the moving mold weld bead.

[0037] Figure 6 This is a diagram showing the welding sequence of 4 layers and 10 passes in the width direction of the moving mold weld bead. Detailed Implementation

[0038] The specific embodiments of the present invention will be described below with reference to the accompanying drawings and technical solutions:

[0039] Figure 2 The diagram shown is a simplified comparison of the parting lines of a large-size stringer forming mold. This invention employs a "box-type overlapping method," specifically addressing... Figure 1 The typical moving mold parts shown have structural features. The welding structure overlap scheme is optimized and split into 8 different shapes and sizes of 4J36 thin plate parts. After water jet cutting, they are sequentially overlapped to form a long box-shaped mold blank. Figure 3 As shown, one piece each of the top plate D-01, bottom plate D-08, left vertical plate D-02, right vertical plate D-04, and right inclined plate D-03, two pieces each of the two inclined plates D-05 and two vertical plates D-06, and a total of N pieces of reinforcing rib plate D-07 are sequentially lapped and welded together.

[0040] In the process of lap welding of the mold structure, in order to further control the welding deformation, the following optimization measures were taken: (1) In the weld design, the 60° V-shaped bevel has a smaller welding angle deformation than the 90° bevel under the same filling strategy. Therefore, the original 90° V-shaped bevel was changed to a 60° V-shaped bevel with a blunt edge of 2mm and a blunt edge gap of 0.5mm. (2) In the process of V-shaped welding of the mold, the bottom of the V-shaped weld is the weakest link of the entire weld and is related to the airtightness of the mold. It is the most critical link of the entire welding. The original V-shaped welding method of using only carbon dioxide gas shielded welding was changed to the welding process of "argon arc welding for the root and gas shielded welding for the end" to ensure that no cracks are generated. (3) In the process of sealing the end with carbon dioxide gas shielded welding, too few filling layers are prone to porosity, and too many filling layers will reduce the welding efficiency. The welding sequence method of four layers and ten passes, welding from both sides to the middle, was adopted.

[0041] After welding, the mold must undergo full annealing to refine the grains, homogenize the microstructure, and eliminate thermal stress. For steel parts, an annealing temperature of approximately 550℃ is typically chosen, with a holding time of 2 hours. This invention has been experimentally verified to achieve the following results for the 4J36 mold: furnace entry temperature ≥100℃, heating to 820-850℃ at a rate ≤100℃ / hour, holding time ≥2 hours; furnace cooling to 200℃ at a rate ≤80℃ / hour. Using these annealing process parameters, the 4J36 mold exhibits optimal comprehensive mechanical properties.

[0042] 4J36 thin plate welding is prone to defects such as incomplete penetration, undercut, hot cracks and porosity. In the process of lap welding of the mold structure, in order to further control the welding heat and thermal stress-induced deformation, the following schemes were adopted: (1) In the weld design, the 60° V-shaped bevel is smaller than the 90° bevel under the same filling strategy. Therefore, the original 90° V-shaped bevel was changed to a 60° V-shaped bevel with a blunt edge of 2mm and a blunt edge gap of 0.5mm. (2) In the 4J36 mold V-shaped welding process, the surface weld is usually milled off in the subsequent CNC machining process. The bottom of the V-shaped weld is the weakest link of the entire weld and is related to the airtightness of the mold. It is the most critical link of the entire welding process. The heat input of the root welding process parameters is strictly controlled. Lower heat input results in faster cooling of the molten pool, which easily leads to the formation of a hardened structure and reduces weld toughness. Conversely, higher heat input increases the fusion ratio of the root pass, making the weld metal more susceptible to the formation of a low-melting-point eutectic structure due to harmful impurities, causing hot cracking. It also results in coarse weld grains and reduced weldability. During manufacturing, through ANSYS software simulation and experimental verification, we adopted a welding process of "argon arc welding for the root pass and gas shielded welding for the end pass." Therefore, the root pass uses argon arc welding with oscillating welding, a welding current of 200A, and a welding speed of 0. 0.51m / min, ensuring no hot cracks or cold cracks are generated; (3) In the process of multi-layer and multi-pass welding of 4J36 material, too small a number of filler layers means a large amount of single-pass weld deposition, which is easy to generate porosity; too many filler layers will reduce welding efficiency. Using ANSYS finite element simulation software, the bevel angle is selected as 60°. Three layers and six passes, and four layers and ten passes are selected for comparison. Through software simulation analysis, the conclusion is that four layers and ten passes are filled with a 60° bevel angle. The weld sequence from both sides to the middle can effectively reduce welding deformation. In practice, when welding multiple layers and multiple passes, it is required to weld one layer and grind one layer. There should be no interlayer. Welding should be done according to the airtightness requirements. Before welding the second layer, the first layer must be completely cooled before the second layer can be welded. Grind and polish the weld. The second to fourth layers of welds are welded along the length direction according to the following. Figure 4 As shown, welding is performed in 200mm segments to ensure that shrinkage deformation during welding occurs in the form of small waves. Segmented welding also limits the amount of shrinkage deformation to a small range; the weld width direction is as follows... Figure 5 The diagram shows 4 layers and 10 passes welded from both sides to the middle in sequence to ensure that the shrinkage of the weld is controlled to the minimum; (4) During the welding process, control the heat input, weld continuously for 10 minutes, rest for 5 minutes, and the temperature of the mold body does not exceed 50°.

[0043] Example

[0044] Taking the moving mode of the stringer of the lower panel of the outer aileron of an aircraft as an example, Figure 3The diagram shows the construction drawing of the welded structure of the moving mold blank. The moving mold is made of 4J36 material, with machining allowance. The external dimensions are 3320mm in length, 230mm in width, and 115mm in height. To ensure the structural dimensions and welding quality, the following control measures were taken:

[0045] Step 1: Figure 4 As shown, the moving mold structure is divided into 8 different sized and shaped parts, which are then cut and blanked using a CNC waterjet cutting machine. The blanking is done along the fiber direction of the sheet metal, resulting in minimal deformation.

[0046] Step 2: On a three-axis CNC machine tool, clamp each plate onto a simple V-milling tool and mill a 60° bevel at the joint of each part, with a blunt edge of 2mm and a blunt edge gap of 0.5mm.

[0047] Step 3: Weld the moving mold box-shaped blank on the platform. 4J36 stainless steel is extremely prone to rust at room temperature. To avoid adverse effects on the welded joint, before welding, use a wire brush to clean and grind the welding areas of each plate to remove rust, oil, and other impurities, followed by cleaning with acetone. The welding method adopted is "argon arc welding for the root pass and gas shielded welding for the end pass." The V-shaped bottom is welded using argon arc welding with a rated current of 200A, an argon flow rate of 20L / min, and INVAR M93 3mm welding wire. For CO2 gas shielded welding, the rated current is 200A, the shielding gas flow rate is 20L / min, and INVAR M93 1.2mm welding wire is used. Welding is performed according to airtightness requirements, and the welded parts are inspected. The V-shaped end pass is sealed using CO2 gas shielded welding. Figure 5 As shown, the second to fourth weld layers are welded in 200mm segments along the length to ensure that shrinkage deformation during welding occurs in the form of small waves. Segmented welding also limits the amount of shrinkage deformation to a small range. The weld width consists of 4 layers and 10 passes. Figure 6 As shown, weld from both sides towards the middle to minimize weld shrinkage. After welding, clean with a wire brush to remove slag, residual flux, and other debris, ensuring a smooth and uniform weld along its entire length. During welding, control heat input, welding continuously for 10 minutes followed by a 5-minute rest, ensuring the mold temperature does not exceed 50°C.

[0048] Step 4: Stress-relief annealing of the moving mold body. Perform stress-relief annealing according to the annealing parameters in the technical plan. The mold body is a thin-walled frame structure. To prevent deformation caused by uneven heating during annealing, the bottom reference surface must be placed flat during furnace annealing. Laser measurement can be used to ensure that the coplanarity of the bottom and the support block is ≤1mm. After the annealing heat treatment is completed, correction is performed.

[0049] Step 5: CNC Machining of the Mold Body. The moving mold body is narrow and long, making it prone to deformation during machining, which affects mold accuracy. Clamping the workpiece in the vise can cause mold deformation. After machining the datum surface, remove the mold from the vise and clamp it on the milling machine table. Machin the surrounding planes layer by layer from top to bottom, simply by changing the position of the pressure plates. After rough machining, remove the mold and allow it to age to relieve internal stress while machining another mold part. After rough machining and aging of the moving mold, the datum surface will deform, requiring remachining of each datum surface. Place the moving mold on a five-axis machining center to complete the remachining of the datum surfaces and subsequent machining processes. Adjust the orientation according to the datum surface, changing the conventional clamping method of four pressure plates on the datum surface to side pressure at both ends, minimizing pressure on the mold, reducing deformation in the length direction during machining, and not affecting the machining of the top and side working surfaces, nor requiring frequent changes in clamping position. Welding defects exposed on the mold surface after machining are repaired using a cold welding machine, and the fitter polishes the surface with sandpaper until the surface roughness is ≤Ra0.8.

[0050] Step 6: Inspect the profile. After assembling the mold, fixed mold and other parts, perform CNC measurement (profile accuracy less than ±0.1mm), and vacuum test the profile (vacuum leakage rate less than 0.017MPa / 5min).

[0051] Step 7: Verification. After the mold was delivered, it was put into production for trial pressing. After hot pressing and removal from the can, the T-shaped stringer was slightly deformed. After being glued to the skin and then put back into the autoclave for molding, the stringer surface was tightly attached to the skin, and the overall product quality met the design requirements.

Claims

1. A welding method for a large-size nickel-iron alloy stringer mold, characterized in that: Includes the following steps:

1. Prepare the mold components; the mold is divided into eight components, namely top plate, bottom plate, left side upright plate, right side upright plate, right side inclined plate, front inclined plate, front upright plate, rear inclined plate, rear upright plate and multiple reinforcing ribs. Each component is a flat plate without bending.

2. Clamp each of the prepared plates on the machine tool and mill a bevel at the joint of each plate; the bevel angle is 30°, and the joint of two adjacent plates forms a 60° V-shaped bevel.

3. Assemble the various plates into a mold blank, and weld the joints of the plates in layers and passes. During welding, divide the V-groove into multiple layers according to the depth direction. The thickness of a single layer is 2mm~3mm. Weld layer by layer from the bottom layer to the top layer. After the next layer is welded and completely cooled, weld the next layer. Grind and polish the weld after each layer is welded. The bottom layer is argon arc welding. The other layers are gas shielded welding. Weld in sections every 200mm along the length direction.

4. Anneal the welded mold blank; 5. Perform CNC machining on the annealed mold blank; 6. Inspect the processed mold.

2. The method according to claim 1, characterized in that: No. 2 Middle School The blunt edge has a reserved length of 2mm and a blunt edge gap of 0.5mm.

3. The method according to claim 2, characterized in that: Third Middle School The parameters for argon arc welding are as follows: rated current range 200A, argon flow rate 20L / min, and welding wire selected as INVAR M93 3mm. The parameters for CO2 gas shielded welding are as follows: rated current range 200A, shielding gas flow rate 20L / min, and welding wire selected as INVARM93 1.2mm.

4. The method according to claim 3, characterized in that: The first layer has 1 weld bead, and the number of weld beads increases by 1 with each subsequent layer; the upper layers are welded from both sides towards the middle. Weld continuously for 10 minutes, then rest for 5 minutes.

5. The method according to claim 1, characterized in that: The annealing process is as follows: furnace entry temperature ≥100℃, heating to 820~850℃, heating rate ≤100℃ / hour, holding time ≥2H; cooling to 200℃ with the furnace, cooling rate ≤80℃ / hour.

6. The method according to claim 1, characterized in that: The sixth test includes: The profile tolerance of the surface shall not exceed ±0.1mm; The vacuum leakage rate of the molded surface shall not exceed 0.017 MPa / 5 min.

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