A gradient forging method for a high-strength steel rotor shaft

Abstract

The present invention discloses a gradient forging method for a high-strength steel helicopter rotor shaft. This method addresses the problem that thick-walled tubes cannot be formed to near-net shape by spinning. By utilizing additive manufacturing technology, the fully dense forged rods of the raw material are transformed into incompletely dense additive tube blanks. By utilizing hot extrusion technology, the dual goals of tube blank forming and property control are achieved. The forming is the same as hot extrusion of conventional forged rods, with plastic deformation occurring in the wall thickness and length directions. The property control 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 of thick-walled tube blanks and reduces costs.

Description

Technical Field

[0001] The invention relates to 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 from solid forgings. This resulted in poor microstructure and performance consistency, and defects such as transitional mixed crystal zones were unavoidable, which weakened fatigue performance. Furthermore, the forging process required multiple firings, a long process flow, poor dimensional stability, and excessive machining allowances, leading to lengthy processing cycles. With advances in hollow drive shaft manufacturing technology abroad, by the 1960s, leading aerospace manufacturers such as General Motors and Pratt & Whitney in the United States, Rolls-Royce in the United Kingdom, and Snecma and Auboudova in France had transitioned from traditional solid forging machining to hollow extrusion and machining. This resulted in forgings with more uniform microstructure and performance while saving significant amounts of raw material. Furthermore, the reduced machining allowances reduced machining cycle time and costs. Currently, hollow extrusion technology for producing high-performance hollow drive shafts shows promising prospects. Researchers in aviation powerhouses such as the United Kingdom, the United States, and Germany have conducted extensive research, and the precision forming technology for large aspect ratio hollow drive shafts has achieved mature application.

[0003] For hollow drive shaft components with variable diameters and cross-sections, the sole use of hollow extrusion technology restricts the precision forming of hollow shaft components. However, precision power spinning technology can further reduce machining allowances, improve material utilization, and optimize mechanical properties. Power spinning is an advanced manufacturing process that applies external forces to a rotating blank to induce continuous localized plastic deformation, ultimately achieving thinned-wall hollow rotating parts with minimal chipping.

[0004] However, in recent years, domestic manufacturers that have used the extrusion + spinning process to near-net-shape rotor shaft parts have 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 object of the present invention is achieved through the following technical solutions:

[0007] In the gradient forging method of a high-strength steel helicopter rotor shaft of the present invention, the helicopter rotor shaft is a tubular member, and the forging method is characterized in that the steps are as follows:

[0008] Step 1: Raw material preparation

[0009] Prepare 4340 steel material powder with a powder particle size of -200 to -300 mesh and a powder purity of 99.99%;

[0010] Step 2: Preform preparation

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

[0012] Step 3: Preformed blank thermoforming

[0013] The preformed blank is heated to 900-1140° C. in a heating furnace, kept warm for 1 hour, and then subjected to forward extrusion at an extrusion speed of 30-50 mm / s and an extrusion ratio of 4:1. The inner and outer diameter surfaces of the circular tubular part are then polished to obtain an intermediate blank for a helicopter rotor shaft. 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. 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, and 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. The outer diameter of the intermediate blank is processed to the outer diameter of the helicopter rotor shaft part by spinning. The spinning reduction 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 worked to the inner diameter 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. After cooling, the 4340 high-strength steel helicopter rotor shaft parts are finally obtained.

[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 step 2, the wall thickness of the preform 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 type of high-strength steel helicopter rotor shaft parts in China is forging bars + machining. Although this method can ensure the parts are processed, the plastic flow line of the bars is cut during the processing, which reduces the fatigue life of the parts. In addition, the part 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 microstructure and properties. This process route has a single-side machining allowance of 20mm, high cost, long cycle time and poor performance.

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

[0023] On this basis, in response to the technical problems of thick-walled tube spinning, a process route of additive manufacturing tube billet + hot extrusion + spinning was further proposed. The forged bar was replaced by the additive manufacturing tube billet 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 shape of the thick-walled tube was achieved without mechanical processing, while 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 the helicopter transmission system. Thanks to the efforts of the technical solution of the present invention, the tensile properties of this product and various similar shaft components used in engines and reduction boxes can be improved by 20%, the fatigue life can be increased by 100%, and the single-side processing 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 with reference to the embodiments:

[0026] In this embodiment, the helicopter rotor shaft is a tubular member made of 4340 steel. The steps for 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 with a powder particle size of -200 to -300 mesh and a powder purity of 99.99%;

[0029] Step 2: Preform preparation

[0030] Based on the digital model of the helicopter rotor shaft, the blank is processed using additive manufacturing technology. The blank is in the shape of a circular tube. The outline of the blank completely covers the outer dimensions of the helicopter rotor shaft, and the machining allowance is 15-40mm.

[0031] The inner and outer diameter surfaces of the blank are then polished by machining to obtain a preform of a 4340 high-strength steel helicopter rotor shaft. The outer diameter of the preform is at least 10 mm larger than the outer diameter of the helicopter rotor shaft, and the inner diameter of the preform is at least 10 mm smaller than the inner diameter of the helicopter rotor shaft. That is, the machining allowance of the outer and inner diameters of the preform is at least 10 mm. The density of the tube wall of the preform along the thickness direction is 40-50%, and the wall thickness of the preform is 60 mm.

[0032] Step 3: Preformed blank thermoforming

[0033] The preformed blank is heated to 900-1140° C. in a heating furnace, kept warm for 1 hour, and then subjected to forward extrusion at an extrusion speed of 30-50 mm / s and an extrusion ratio of 4:1. The inner and outer diameter surfaces of the circular tubular part are then polished to obtain an intermediate blank of the helicopter rotor shaft. 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. 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 30 mm.

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

[0035] The intermediate blank is placed on a spinning machine, and 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. The outer diameter of the intermediate blank is processed to the outer diameter of the helicopter rotor shaft part by spinning. The spinning reduction 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 worked to the inner diameter 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. After cooling, the 4340 high-strength steel helicopter rotor shaft parts are finally obtained.

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

[0039] 1. 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 produced by this process achieve a fully dense state and, compared to other parts, have almost no machining allowance, achieving a near-net shape.

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

[0041] Third, the present invention overcomes the technical difficulty of the traditional spinning process in achieving near-net-shape thick-walled tubes, thereby improving material utilization.

[0042] 4. The solution of the present invention addresses the problem that thick-walled tubes cannot achieve near-net shape through spinning. By utilizing additive manufacturing technology, the fully dense forged rods of the raw material are transformed into incompletely dense additive tube blanks. By utilizing 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, with plastic deformation occurring in the wall thickness and length directions. The controllability is that the center of the wall thickness is not fully dense, which reserves operating space for subsequent spinning and reduces the difficulty of spinning. This process route truly achieves near-net shape of thick-walled tube blanks and reduces costs.

Claims

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

1. The inner and outer diameter surfaces of the circular tubular part are then polished to obtain an intermediate blank for a helicopter rotor shaft. 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. 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, and 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. The outer diameter of the intermediate blank is processed to the outer diameter of the helicopter rotor shaft part by spinning. The spinning reduction 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 worked to the inner diameter 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. After cooling, the 4340 high-strength steel helicopter rotor shaft parts are finally obtained.

2. The gradient forging method for 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 for 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 for 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.