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Research on turning process of L-shaped thin-walled ring of nickel-based superalloy

The L-shaped thin-walled ring of a gas turbine burner is a thin-walled part of high-temperature alloy material, which has the dual difficult-to-machine attributes of difficult-to-machine materials and difficult-to-machine structures. Its structural characteristics and processing difficulties are deeply analyzed. By discussing the technical problems of the thin-walled ring in turning processing, such as easy deformation and difficulty in ensuring the accuracy of parts due to the influence of cutting force, clamping force, cutting heat, and residual stress, the turning process route, tool selection, fixture scheme and cutting parameter selection scheme of L-shaped thin-walled ring of gas turbine burner is put forward. The deformation of parts is controlled and the shape and position accuracy is guaranteed. Finally, the process scheme of batch processing is given. Introduced and summarized the turning process of L-shaped thin-walled ring of a gas turbine burner, providing a reference for thin-walled parts of other difficult to machine materials.

0. Preface

The L-shaped thin-walled ring of the gas turbine burner is made of Hastelloy alloy. Hastelloy alloy is a nickel-based superalloy with very low carbon content and silicon content. It has high strength, toughness, high temperature corrosion resistance, and high mechanical properties at high temperatures. Hastelloy alloy is widely used in high temperature, high-pressure, and corrosive working environments such as high temperature parts of gas turbines. However, Hastelloy alloy has the characteristics of easy hardening and poor thermal conductivity during processing, and the internal stress is large during processing. It is easy to produce deformation, resulting in defects such as structural size out of tolerance.
The turning deformation of Hastelloy alloy thin-walled ring parts is a difficult problem in turning processing, and the turning of high-temperature alloy thin-walled ring parts is more difficult. To solve the problems of difficult processing and easy deformation of Hastelloy alloy materials and thin-walled ring parts in the cutting process, many scholars have done in-depth research on its cutting mechanism and processing technology. Shi Yusheng proposed the selection scheme of a carbide turning tool and drill material for Hastelloy alloy processing and provided the geometric angle reference of the turning tool and its cutting parameters. Zhang Yufeng takes a single thin-walled ring workpiece made of 30CrMnSiA high-strength steel as the research object. Through the optimization of process parameters and the improvement of fixtures, the turning deformation problem of thin-walled ring parts is effectively improved. Sun et al. studied the turning deformation control method of thin-walled annular parts. Taking the outer ring of an aero-engine flame tube as an example, a step-by-step layered turning method was proposed to reduce the turning deformation of the part. Yang Zhenhua designed a special clamping tool for turning annular thin-walled parts. The four-jaw chuck process groove is used to install the fixture, and the clamping method of inner and outer ring positioning is adopted to reduce the deformation of the workpiece. The L-shaped thin-walled ring of the gas turbine burner studied in this paper is made of Hastelloy X, which has the dual characteristics of difficult-to-machine materials and thin-walled ring-like difficult-to-machine structures.

1. The characteristics of parts and processing difficulties

1.1 Structural analysis

The structure of an L-shaped thin-walled ring is a typical thin-walled ring part. As shown in Figure 1, the dimensions of each part have high tolerance requirements. The inner hole Φ175+0.05mm and the boss 8mm ± 0.02mm thickness are the most difficult to control. The verticality of the left end face relative to the inner hole reference A is 0.015mm, the wall thickness of the right end is only 5mm, the diameter-thickness ratio is 37, and the roughness of the main machined surface is less than Ra1.6. Under such a large diameter-to-thickness ratio, ensuring so many dimensional tolerances, geometric tolerances, and surface roughness is very difficult.
20230726043920 67942 - Research on turning process of L-shaped thin-walled ring of nickel-based superalloy
Fig.1 L-shaped thin-walled ring

1.2 Analysis of material properties

The material of the L-shaped thin-walled ring is Hastelloy X, which is a nickel-based alloy. It has excellent high-temperature corrosion resistance and is suitable for manufacturing high-temperature components of gas turbines. Its tensile strength is 690 MPa, yield ratio is 0.4, plasticity is excellent, and it is easy to stick the knife during processing. The thermal conductivity of the material is 1/4 of that of ordinary steel. The temperature of the cutting area is high, and the tool is easy to wear. In short, the material has great processing difficulty. The chemical composition is shown in Table 1, the physical properties in Table 2, and the mechanical properties in Table 3.
Table.1 Chemical constituents of Hastelloy X (%)

Ni Cr Fe Mo W Co C Mn Si S Cu Al Ti
Base 20.5-23 17-20 8-10 0.2-1 0.5-2.5 0.05-0.15 1 1 0 1 0 1

Table.2 Physical properties of Hastelloy X

Density/(g/cm3) Melting point/℃ Thermal
conductivity λ/ (W/(m·℃))
Specific heat
capacity/(J/(
kg·℃))
Elastic
modulus/Gpa
Linear expansion
coefficient a/(10
-6/℃)
8.23 1295 1381 13.38(100℃) 372.6 199 12.1(20℃-100℃)

Table.3 Mechanical properties of Hastelloy X (minimum mechanical properties at 20 °C)

Heat
treatment method
Tensile strength
σ
b/Mpa
Yield strength σP0.2/Mpa Elongation σ5/%
Solid solution
treatment
690 275 30

1.3 Processing Difficulties
Through the above analysis, the turning process of an L-shaped thin-walled ring mainly has the following difficulties:

  • (1) The thermal conductivity of Hastelloy alloy material is small, the cutting area temperature is high, the turning tooltip is easy to burn, and the accuracy after tool change is difficult to guarantee.
  • (2) The cutting force is large in the cutting process. The unit cutting force is 2-2.5 times that of cutting 45# steel. Thin-walled parts are easy to deform, tool wear is fast, and machining accuracy is not easy to guarantee.
  • (3) Serious work hardening, greater cutting resistance, tool wear fast.
  • (4) The yield ratio of the material is 0.4, the plasticity is strong, and the bonding wear is easy to occur during the cutting process. 
  • (5) The diameter-thickness ratio of the part reaches 37, and the structural rigidity is extremely poor. It is easy to produce deformation and vibration due to the influence of clamping force, cutting force, and cutting heat, and the size and shape accuracy are not easy to meet the requirements.
  • (6) The parts have high dimensional accuracy and shape accuracy. The left end face of the part is 0.015 mm perpendicular to the reference A of the inner hole. It is necessary to process the reference inner hole and the left end face in the same clamping to ensure the perpendicularity requirement. However, due to the deformation of the inner hole and the distortion of the 8mm ± 0.02mm step, the perpendicularity is out of tolerance.
  • (7) In turn, it is necessary to optimize the processing technology, reasonably design the processing method and clamping method, optimize the cutting tool and cutting parameters, reduce the cutting heat, eliminate the turning vibration, and realize the deformation control of the parts.

2. Turning process scheme

2.1 Special fixture design

The machining process and clamping scheme are very important to prevent the deformation of parts. Due to the special structure of the parts, a special fixture must be designed. The special fixture should not only ensure the accuracy of the parts but also prevent the clamping and processing deformation of the parts and, simultaneously, make the operation simple. The specific design scheme is as follows:
(1) The special fixture adopts the axial clamping method to bear the clamping force in the direction of the large part stiffness and reduce the clamping deformation of the part. The part fixture is shown in Fig.2.
20230726045815 27971 - Research on turning process of L-shaped thin-walled ring of nickel-based superalloy
Fig.2 Parts clamping schematic diagram. 
(2) The fixture matrix installs the L-shaped thin-walled ring. The positioning end face and the positioning outer circle cooperate with the left end face and the inner hole of the L-shaped thin-walled ring to determine the correct position of the L-shaped thin-walled ring in the clamp. The positioning outer circle and the inner hole of the L-shaped thin-walled ring are clearance fit, and the fit clearance should be 0.02mm-0.04mm, which can effectively ensure the coaxiality of the parts and make the parts can be loaded and unloaded smoothly when the inner hole shrinks slightly after the force is applied.
(3) When used, the lathe chuck clamps the fixture matrix to correct the positioning end face and the positioning outer circle to ensure that the positioning end face is perpendicular to the spindle axis of the lathe and the positioning outer circle axis coincides with the spindle axis of the lathe. The design is helpful to ensure the position accuracy of the parts.
(4) The cone hole of the right end face of the pressure plate is set to a 60 ° cone hole with the top tip to ensure that the axis of the central cone hole is perpendicular to the end face of the pressure plate and coincides with the axis of the pressure plate. It is used to axially press the right end face of an L-shaped thin-walled ring, change the action point of force, force the left and right ends of an L-shaped thin-walled ring, ensure that the position of L-shaped thin-walled ring in the fixture is fixed during turning, and effectively prevent the clamping deformation of thin-walled parts.
(5) A plurality of through-holes is evenly arranged on the fixture matrix and the pressure plate to reduce the mass and rotational inertia during rotation, which is conducive to stable rotation. The through-hole is also used as a gripper during the loading and unloading process of the pressure plate to prevent falling during the loading and unloading process of the pressure plate.

2.2 Turning tool and cutting parameter optimization scheme

To reduce the cutting force in the machining process, thereby reducing the amount of deformation, while taking into account the cost and processing efficiency, through theoretical analysis and a large number of cutting tests, the selection of turning tools is determined:

  • (1) The rough turning selects the S20 type blade with a large rake angle and large back angle, which is suitable for high-temperature alloy turning. The radius of the tooltip is R0.8mm, such as WALTER brand, WNMG160608 WSM20S, which can improve the tool’s service life and reduce the cutting force and cutting heat. It can be applied to turning the end face, the outer circle, and the inner hole and reduce the tool change frequency to improve the machining efficiency.
  • (2) To further reduce the cutting force and prevent vibration during finishing, a normal WSM20S turning insert with a small tip arc radius of R0.2mm-R0.4mm is selected, and the angle between the cutting edge and the end face is ensured to be 15 ° -20 ° during tool installation, which can effectively prevent the excessive contact area between the cutting edge and the end face from affecting the quality of the part when finishing the 8mm ± 0.02mm step end face. The inner hole of the finish turning should increase the diameter of the tool rod and improve the rigidity of the tool rod. The smaller tooltip angle is beneficial to reduce the contact area between the auxiliary cutting edge and the part, effectively preventing the cutting vibration and improving the quality of the part.

In the processing of Hastelloy alloy materials, due to their material properties, the cutting parameters should be reduced and controlled, the number of cutting times should be increased, and the cutting speed should be uniform to help reduce the cutting force and cutting heat. The influence of cutting speed on cutting force is constantly changing. For thin-walled parts of Hastelloy alloy, a lower cutting speed can reduce cutting heat and prolong tool life. The specific recommended cutting parameters are as follows:

  • (1) Cutting depth: rough machining ap is 2mm, finishing is 0.15mm;
  • (2) Cutting speed: rough machining Vc choose 60m/min, finishing chooses 75m/min;
  • (3) Feed rate: rough machining f selection 0.25mm/r, finishing selection 0.1mm/r.

Adequate cooling and lubrication during turning are necessary to reduce parts’ thermal deformation. Coolant should choose cutting fluid with large specific heat capacity, low viscosity, and good fluidity, such as water-based emulsified cutting fluid, which can absorb a large amount of heat, reducing the cutting temperature and the thermal deformation of L-shaped thin-walled rings.

2.3 Cutting scheme

2.3.1 Turning process arrangement

According to the comprehensive analysis of the structure and material of the parts, the parts are divided into rough machining, semi-finishing, and finishing. The blank of the part is cut by gas. To protect the accuracy of the CNC lathe, the rough machining of the general car is used to remove the gas cutting layer, and a 2mm machining allowance is left on each surface. To ensure that the clamping surface of the part has sufficient rigidity when finishing the inner hole, the outer circle of the design process reference is used to increase the thickness of the clamping surface of the part, and the soft claw clamping is used to increase the clamping contact area and reduce the clamping deformation of the part. The arrangement of no less than 48 hours of room temperature aging after rough machining is mainly to fully release the residual stress generated by the part after the rough machining force is heated, and to prevent the stress release from continuous processing to the finished product, resulting in part deformation and dimensional tolerance. The semi-finishing of the CNC lathe ensures that the machining allowance of each surface of the parts is consistent, especially for batch processing.
In the finishing process, the outer circle of the finishing process datum is first used as the positioning and clamping datum of the turning datum inner hole. At the same time, the finishing of the small end face of the part is used as the limit of the total length and the finishing inner hole. The chamfering of the small end inner hole is pre-processed to avoid finishing the inner hole and then to turn around and process multiple clamping, resulting in the deformation of the part. The left end face and inner hole are processed by the fan-shaped jaw clamping process reference outer circle to prevent the parts from being deformed by the clamping force. When the inner hole and the left end face are used as the reference positioning to prevent the clamping and to cut force deformation of the parts, the special fixture is used to locate and clamp (Figure 2), which can effectively avoid the deformation of the inner hole and the distortion of the steps of the parts, and ensure the dimensional accuracy and coaxiality of the thin-walled ring. At the same time, there is more allowance for finishing this clamping, so it is necessary to reduce the cutting amount, the cutting force, and the cutting heat generated during processing.
According to the above processing technology analysis, aiming at the structural characteristics and processing difficulties of the L-shaped thin-walled ring, the turning process flow shown in Figure 3 is comprehensively formulated.
20230726050527 54922 - Research on turning process of L-shaped thin-walled ring of nickel-based superalloy
Fig.3 Turning process

2.3.2 Processing scheme of inner hole and left end face

In conventional processing, rough machining is usually a uniform machining allowance. Still, the clamping part of the inner hole of the L-shaped support ring is very thin and rigid, and the force is easily deformed. Therefore, in the rough machining stage, the process reference outer circle (Fig.4) is designed to increase the thickness of the clamping part to ensure sufficient rigidity. The inner hole is machined by a single process reference outer circle. After many tests and adjustments, the deformation of the inner hole is always between 0.02 mm and 0.03 mm. Given this situation, the clamping rigidity of the parts is further improved by using the limit envelope type large arc fan-shaped soft claw (Fig.5) and spring jacket (Fig.6).
At the same time, to ensure that the clamping part of each piece is compatible with the arc surface of the soft claw or the inner hole of the spring jacket during batch processing, the outer circle of the process base reference is set to Φ190+ 0.10 mm, and the inner hole of the large arc fan-shaped soft claw and the spring jacket is set to Φ1900-0.1mm. The length is set to 20mm to ensure a sufficiently large clamping contact surface when the part is clamped. The clamping force is evenly distributed on the entire cylindrical surface of the process reference outer circle so that the inner hole of the large arc fan-shaped soft claw or the spring jacket can be limited. Batch processing and effective control of the deformation of the inner hole and the left end face of the part is within 0.01 mm.
20230726050742 16922 - Research on turning process of L-shaped thin-walled ring of nickel-based superalloy
Fig.4
20230726050959 32298 - Research on turning process of L-shaped thin-walled ring of nickel-based superalloy
Fig.5
20230726051219 67910 - Research on turning process of L-shaped thin-walled ring of nickel-based superalloy
Fig.6

2.3.3 Right outer circle and thickness processing scheme

When turning the right end of the part, the step of 8mm ± 0.02mm is easily distorted by the influence of cutting force and clamping force, and the accuracy is very difficult to guarantee, which will destroy the perpendicularity of 0.015mm. After many experimental studies, a special fixture was designed and manufactured. As shown in Figure 2, the lathe chuck is used to clamp the fixture matrix, and the dial indicator is corrected within 0.005 mm to ensure that the positioning end face of the fixture matrix is perpendicular to the spindle axis of the lathe, and the positioning outer circle axis coincides with the spindle axis of the lathe. The L-shaped thin-walled ring is installed on the outer circle of the specific positioning of the clamp, and the left end face of the L-shaped thin-walled ring is fitted with the fixture matrix’s positioning end face to limit the part’s axial position.
The positioning outer circle of the pressure plate is loaded into the inner hole of the L-shaped thin-walled ring, and the 60 ° cone hole of the pressure plate is tightened by the top of the tailstock of the lathe to make the pressure plate move axially. The positioning end face of the pressure plate is closely attached to the right end face of the L-shaped thin-walled ring, and the L-shaped thin-walled ring is compressed axially. This clamping scheme changes the action point and direction of the force and effectively reduces the machining deformation of the L-shaped thin-walled ring.

3. Conclusion

Based on the investigation of many scholars’ literature, this paper takes the L-shaped thin-walled ring as the research object, which has the dual difficult-to-machine characteristics of Hastelloy alloy material and thin-walled ring. Through the design of a special clamping scheme for parts, the matching and optimization of turning tools and cutting parameters, the scientific arrangement of turning process flow and specific processing scheme, the deformation control of L-shaped thin-walled ring turning with Hastelloy X material is realized, and the processing quality of parts is ensured. Through the research work done in this paper, the following conclusions are obtained:

  • (1) Finish machining adopts the clamping scheme described in this paper. The rigid end face of the part is pressed axially, and the action point of the force is changed to force the left and right ends of the part, effectively preventing the clamping deformation of the thin-walled part. The fixture design scheme can be applied to turning similar thin-walled parts.
  • (2) In the processing of thin-walled parts that are easy to produce work-hardening materials, after rough machining and semi-finishing, a room temperature aging can be arranged to release the residual stress generated during the processing of the parts, which is conducive to preventing the continuous processing of thin-walled parts to the finished product. The stress release leads to the deformation of the parts and the dimensional tolerance. This processing flow is of great practical significance for thin-walled parts of difficult-to-machine materials.
  • (3) The cutting tool with a large rake angle and sharp cutting edge is selected to reduce the cutting force and prevent the machining deformation of the parts. Roughing To improve the service life of the tool, the tooltip arc uses an R0.8 blade; the normal blade of R0.4 is used in finish machining, the cutting parameters and cutting tools are matched, and the cutting amount of finish machining is optimized, which can prevent vibration during turning.

Author: Wen Xiaoshan

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