Gradient forging forming method for high-strength steel rotor shaft

Abstract

According to the gradient forging method for the high-strength steel helicopter rotor shaft, the problem that near-net forming of a thick-wall pipe cannot be achieved through spinning is solved, an additive manufacturing technology is utilized, a fully-compact forged rod of a raw material is converted into an incompletely-compact additive pipe blank, a hot extrusion technology is utilized, and the dual purposes of pipe blank forming and controllability are achieved; the forming is the same as the conventional forging rod hot extrusion, the plastic deformation occurs in the wall thickness and length directions, the controllability is that the center position of the wall thickness is not completely compact, the operation space is reserved for the subsequent spinning, the spinning difficulty is reduced, the near-net forming of the thick-wall pipe blank is really realized by the process route, and the cost is reduced.

Description

Technical Field

[0001] The invention discloses a gradient forging method for a high-strength steel rotor shaft, belonging to the technical field of hot processing. Background Art

[0002] At the beginning of the last century, hollow drive shafts were mostly machined using solid forgings, which resulted in poor consistency in microstructure and performance, and defects such as transitional mixed crystal zones were difficult to avoid, which weakened fatigue performance. In addition, the forging process had many fires, a long process, poor dimensional stability, and excessive machining allowance, resulting in a long processing cycle. With the advancement of foreign hollow drive shaft manufacturing technology, by the 1960s, foreign manufacturers represented by advanced aviation manufacturing companies such as General Motors and Pratt & Whitney in the United States, Rolls-Royce in the United Kingdom, Snecma and Aubudwa in France had transformed their hollow drive shaft manufacturing from traditional solid forging machining to hollow extrusion + machining process manufacturing, which not only produced forgings with more uniform microstructure and performance, but also saved a lot of raw materials. At the same time, due to the reduction in machining allowance, the machining cycle and cost decreased simultaneously. At present, the hollow extrusion billet making technology of high-performance hollow drive shafts shows good application prospects. Researchers in aviation industry powers such as Britain, the United States, and Germany have carried out a lot of research, and the precision forming technology of large aspect ratio hollow drive shafts has been maturely applied.

[0003] For hollow transmission shaft components with variable diameter and cross-section structures, the single use of hollow extrusion technology restricts the precision forming of hollow shaft components, while precision high-power spinning technology can further reduce processing allowances, improve material utilization, and optimize mechanical properties. Metal high-power spinning is an advanced manufacturing process that applies external force to the rotating blank to produce continuous local plastic deformation, and finally obtains hollow rotating parts with reduced wall thickness and low chip processing.

[0004] However, in recent years, the domestic use of extrusion + spinning process to near-net-shape rotor shaft parts has found that cold spinning is difficult to achieve due to the excessive wall thickness of the extruded blank and the high deformation resistance of the fully dense 4340 steel material. Summary of the invention

[0005] The present invention is designed to address the deficiencies in the above-mentioned prior art and provides a gradient forging method for a high-strength steel rotor shaft. Its purpose is to achieve blank forming before machining by spinning for high-strength steel rotor shaft parts for helicopters developed and manufactured domestically, and to retain plastic streamlines, thereby significantly improving the plastic deformation capacity of the product.

[0006] The objective of the present invention is achieved through the following technical solutions:

[0007] In the gradient forging method of the high-strength steel helicopter rotor shaft of the present invention, the helicopter rotor shaft is a tubular part, characterized in that the forging method comprises the following steps:

[0008] Step 1: Raw material preparation

[0009] Prepare 4340 steel material powder, the powder particle size is -200 to -300 mesh, and the powder purity is 99.99%;

[0010] Step 2: Pre-preparation

[0011] Based on the digital model of the helicopter rotor shaft, the blank is processed by additive manufacturing technology. The blank is in the shape of a circular tube. The inner and outer diameter surfaces of the blank are polished by mechanical processing to obtain a pre-made blank of a 4340 high-strength steel helicopter rotor shaft. The outer diameter of the pre-made blank is more than 10mm larger than the outer diameter of the helicopter rotor shaft, and the inner diameter of the pre-made blank is more than 10mm smaller than the inner diameter of the helicopter rotor shaft, that is, the machining allowance of the outer and inner diameters of the pre-made blank is at least 10mm, and the density of the tube wall of the pre-made blank along the thickness direction is 40-50%;

[0012] Step 3: Preformed blank hot forming

[0013] The pre-set blank is heated to 900-1140°C in a heating furnace, and then positively extruded after being kept warm for 1 hour, with an extrusion speed of 30-50mm / s and an extrusion ratio of 4:1. Then, the inner and outer diameter surfaces of the round tube are polished to obtain an intermediate blank of the helicopter rotor shaft, wherein the outer diameter of the intermediate blank is the same as the maximum outer diameter of the helicopter rotor shaft part, and the inner diameter of the intermediate blank is the same as the minimum inner diameter of the helicopter rotor shaft part, that is, the machining allowances at the maximum outer diameter and the minimum inner diameter of the intermediate blank are 0, and the density of the intermediate blank along the wall thickness direction gradually decreases from the inner and outer diameter surfaces to the core, the density at the inner and outer diameter surfaces of the intermediate blank is 100%, and the density of the tube wall core of the intermediate blank is 80-90%;

[0014] Step 4: Cold forming of intermediate blank and local cold working

[0015] The intermediate blank is placed on a spinning machine, the size of the intermediate blank is matched with the three-dimensional digital model of the helicopter rotor shaft part, a mandrel is inserted into the center hole of the intermediate blank, and the outer diameter size of the intermediate blank is processed to the outer diameter size of the helicopter rotor shaft part by spinning. The pressing amount of spinning is 50-60%. At this time, the density of the tube wall of the helicopter rotor shaft part along the thickness direction is 100%, the mandrel is pulled out, and the inner diameter of the helicopter rotor shaft part is cold-processed to the inner diameter size of the helicopter rotor shaft part;

[0016] Step 5: Heat treatment

[0017] The intermediate billet after spinning in step 4 is placed in a vacuum heat treatment furnace for stress relief annealing, and finally a 4340 high-strength steel helicopter rotor shaft part is obtained after cooling.

[0018] In addition, in the step 2, the outer contour of the blank completely covers the outer dimensions of the helicopter rotor shaft, and the machining allowance is 15-40 mm.

[0019] In addition, in the step 2, the wall thickness of the pre-formed blank is 60 mm.

[0020] In addition, in step three, the wall thickness of the intermediate blank is 30 mm.

[0021] At present, the mainstream process route for this kind of high-strength steel helicopter rotor shaft parts in China is forging bars + machining. Although this can ensure that the parts are processed, the plastic flow line of the bars is cut off during the processing process, which reduces the fatigue life of the parts. In addition, the size of the parts is too large, with a length of 2.6m and a maximum outer diameter of 450mm, which increases the difficulty of manufacturing forged bars with uniform organizational properties. This process route has a single-side machining allowance of 20mm, high cost, long cycle, and poor effect.

[0022] To solve this problem, the research and development team of the present invention proposed a new process route for this type of shaft component, namely, forging bar + hot extrusion + spinning + machining. Through hot extrusion + spinning, the blank before machining is transformed from bar to tube, a large number of plastic streamlines are retained, and the plastic deformation is significantly improved, the tensile performance is improved by 20%, the fatigue life is improved by 100%, and the single-side machining allowance is reduced from 20mm to 5mm. However, it was found in the production process that the spinning deformation did not meet the design expectations due to the excessive wall thickness and machining allowance.

[0023] On this basis, in response to the technical problems of thick-walled tube spinning, a process route of additive manufacturing tube blank + hot extrusion + spinning was further proposed. The forged bar was replaced by the additive manufacturing tube blank in order to reduce the deformation resistance caused by excessive wall thickness and processing allowance. Hot extrusion achieved full density in most positions, while greatly reducing the difficulty of spinning. Ultimately, the processing allowance was eliminated through spinning, and the near-net forming of the thick-walled tube was achieved without mechanical processing. At the same time, the density of the tube wall core also reached 100%.

[0024] The 4340 high-strength steel helicopter rotor shaft targeted by the technical solution of the present invention is a key component of a helicopter transmission system. Due to the efforts of the technical solution of the present invention, the tensile properties of this product and similar shaft components used in engines and reduction boxes can be improved by 20%, the fatigue life can be improved by 100%, and the single-sided machining allowance can be reduced from 5mm to 0. DETAILED DESCRIPTION

[0025] The technical solution of the present invention will be further described in detail below in conjunction with embodiments:

[0026] In this embodiment, the helicopter rotor shaft is a tubular member made of 4340 steel. The steps of preparing the high-strength steel helicopter rotor shaft using the gradient forging method of the present invention are as follows:

[0027] Step 1: Raw material preparation

[0028] Prepare 4340 steel material powder, the powder particle size is -200 to -300 mesh, and the powder purity is 99.99%;

[0029] Step 2: Pre-preparation

[0030] Based on the digital model of the helicopter rotor shaft, the additive manufacturing technology is used to process the blank. The blank is in the shape of a round tube. The outline of the blank completely wraps the outer dimensions of the helicopter rotor shaft, and the machining allowance is 15-40mm.

[0031] Then, the inner and outer diameter surfaces of the blank are polished by mechanical processing to obtain a pre-formed blank of a 4340 high-strength steel helicopter rotor shaft, wherein the outer diameter of the pre-formed blank is greater than the outer diameter of the helicopter rotor shaft by more than 10 mm, and the inner diameter of the pre-formed blank is less than the inner diameter of the helicopter rotor shaft by more than 10 mm, that is, the machining allowance of the outer diameter and the inner diameter of the pre-formed blank is at least 10 mm, the density of the tube wall of the pre-formed blank along the thickness direction is 40-50%, and the wall thickness of the pre-formed blank is 60 mm;

[0032] Step 3: Preformed blank hot forming

[0033] The pre-set blank is heated to 900-1140°C in a heating furnace, and then positively extruded after being kept warm for 1 hour, with an extrusion speed of 30-50mm / s and an extrusion ratio of 4:1. Then, the inner and outer diameter surfaces of the round tube are polished to obtain an intermediate blank of the helicopter rotor shaft, wherein the outer diameter of the intermediate blank is the same as the maximum outer diameter of the helicopter rotor shaft part, and the inner diameter of the intermediate blank is the same as the minimum inner diameter of the helicopter rotor shaft part, that is, the machining allowances at the maximum outer diameter and the minimum inner diameter of the intermediate blank are 0, and the density of the intermediate blank along the wall thickness direction gradually decreases from the inner and outer diameter surfaces to the core, the density at the inner and outer diameter surfaces of the intermediate blank is 100%, the density of the tube wall core of the intermediate blank is 80-90%, and the wall thickness of the intermediate blank is 30mm;

[0034] Step 4: Cold forming of intermediate blank and local cold working

[0035] The intermediate blank is placed on a spinning machine, the size of the intermediate blank is matched with the three-dimensional digital model of the helicopter rotor shaft part, a mandrel is inserted into the center hole of the intermediate blank, and the outer diameter size of the intermediate blank is processed to the outer diameter size of the helicopter rotor shaft part by spinning. The pressing amount of spinning is 50-60%. At this time, the density of the tube wall of the helicopter rotor shaft part along the thickness direction is 100%, the mandrel is pulled out, and the inner diameter of the helicopter rotor shaft part is cold-processed to the inner diameter size of the helicopter rotor shaft part;

[0036] Step 5: Heat treatment

[0037] The intermediate billet after spinning in step 4 is placed in a vacuum heat treatment furnace for stress relief annealing, and finally a 4340 high-strength steel helicopter rotor shaft part is obtained after cooling.

[0038] It can be seen from the specific implementation of the present invention that the beneficial effects of the technical solution of the present invention are:

[0039] 1. The scheme of the present invention combines plastic deformation with metal additive manufacturing to develop a new process for manufacturing high-strength steel rotor shaft parts for helicopters. The 4340 high-strength steel parts prepared by this process reach a fully dense state, and compared with the parts, there is almost no processing allowance, achieving near-net shape;

[0040] Second, through the scheme of the present invention, the 4340 high-strength steel rotor shaft starts with additive manufacturing, undergoes plastic deformation and cold working, and the density is increased from 50% to 100%, realizing gradient manufacturing. The lower density does not damage the final part performance, but promotes the part forming;

[0041] Third, the solution of the present invention overcomes the technical difficulty that the traditional spinning process cannot achieve near-net-shape forming of thick-walled tubes, and improves material utilization;

[0042] 4. The scheme of the present invention aims at the problem that thick-walled tubes cannot achieve near-net-shape forming through spinning. By using additive manufacturing technology, the fully dense forged rods of the raw materials are transformed into incompletely dense additive tube blanks. By using hot extrusion technology, the dual goals of tube blank forming and controllability are achieved. The forming is the same as the hot extrusion of conventional forged rods, and plastic deformation occurs in the wall thickness and length directions. The controllability is that the center of the wall thickness is not completely dense, which reserves operating space for subsequent spinning and reduces the difficulty of spinning. This process route truly achieves near-net-shape forming of thick-walled tube blanks and reduces costs.

Claims

1. A gradient forging method for a high-strength steel helicopter rotor shaft, the helicopter rotor shaft being a tubular member, characterized in that: The steps of this method are as follows: Step 1: Raw material preparation Prepare 4340 steel material powder, the powder particle size is -200 to -300 mesh, and the powder purity is 99.99%; Step 2: Pre-preparation Based on the digital model of the helicopter rotor shaft, the blank is processed by additive manufacturing technology. The blank is in the shape of a circular tube. The inner and outer diameter surfaces of the blank are polished by mechanical processing to obtain a pre-made blank of a 4340 high-strength steel helicopter rotor shaft. The outer diameter of the pre-made blank is more than 10mm larger than the outer diameter of the helicopter rotor shaft, and the inner diameter of the pre-made blank is more than 10mm smaller than the inner diameter of the helicopter rotor shaft, that is, the machining allowance of the outer and inner diameters of the pre-made blank is at least 10mm, and the density of the tube wall of the pre-made blank along the thickness direction is 40-50%; Step 3: Preform hot forming The pre-set blank is heated to 900-1140°C in a heating furnace, and then positively extruded after being kept warm for 1 hour, with an extrusion speed of 30-50mm / s and an extrusion ratio of 4:

1. Then, the inner and outer diameter surfaces of the round tube are polished to obtain an intermediate blank of the helicopter rotor shaft, wherein the outer diameter of the intermediate blank is the same as the maximum outer diameter of the helicopter rotor shaft part, and the inner diameter of the intermediate blank is the same as the minimum inner diameter of the helicopter rotor shaft part, that is, the machining allowances at the maximum outer diameter and the minimum inner diameter of the intermediate blank are 0, and the density of the intermediate blank along the wall thickness direction gradually decreases from the inner and outer diameter surfaces to the core, the density at the inner and outer diameter surfaces of the intermediate blank is 100%, and the density of the tube wall core of the intermediate blank is 80-90%; Step 4: Cold forming of intermediate blank and local cold working The intermediate blank is placed on a spinning machine, the size of the intermediate blank is matched with the three-dimensional digital model of the helicopter rotor shaft part, a mandrel is inserted into the center hole of the intermediate blank, and the outer diameter size of the intermediate blank is processed to the outer diameter size of the helicopter rotor shaft part by spinning. The pressing amount of spinning is 50-60%. At this time, the density of the tube wall of the helicopter rotor shaft part along the thickness direction is 100%, the mandrel is pulled out, and the inner diameter of the helicopter rotor shaft part is cold-processed to the inner diameter size of the helicopter rotor shaft part; Step 5: Heat treatment The intermediate billet after spinning in step 4 is placed in a vacuum heat treatment furnace for stress relief annealing, and finally a 4340 high-strength steel helicopter rotor shaft part is obtained after cooling.

2. The gradient forging method of a high-strength steel helicopter rotor shaft according to claim 1, characterized in that: In the step 2, the outer contour of the blank completely covers the outer dimensions of the helicopter rotor shaft, and the machining allowance is 15-40 mm.

3. The gradient forging method of a high-strength steel helicopter rotor shaft according to claim 1, characterized in that: In the step 2, the wall thickness of the preform is 60 mm.

4. The gradient forging method of a high-strength steel helicopter rotor shaft according to claim 1, characterized in that: In the step 3, the wall thickness of the intermediate blank is 30 mm.