Advanced Solutions for Dimensional Stability in Thin-Walled Component Turning

This paper examines deformation control strategies using two case studies involving thin-walled ring parts made from the nickel-based superalloy GH4169. It concludes that employing a fixture to support the inner cavity of the ring and filling that cavity with auxiliary materials can enhance the rigidity of the machining area. This improvement effectively reduces turning deformation in thin-walled parts. Additionally, this approach has broad applicability and demonstrates significant benefits.

 

01 Introduction

Nickel-based superalloys, such as GH4169, typically have a hardness ranging from 346 to 450 HBW. This material is ideal for manufacturing aero-engine components due to its high thermal strength, excellent mechanical properties, and resistance to corrosion. However, it is also a challenging material to machine, as it results in high cutting forces and temperatures, significant tool wear, work hardening, and tool sticking. The characteristics of the material lead to considerable stress and thermal deformation during machining.

Thin-walled ring parts offer advantages like lightweight design, a high thrust-to-weight ratio, material savings, and a compact structure, making them widely used in the aerospace industry. In manufacturing, these thin-walled ring parts are primarily produced through turning, with a material removal rate typically exceeding 90%. However, during machining, thin-walled annular parts are susceptible to deformation, which can lead to dimensional inaccuracies.

Nickel-based superalloy thin-walled rings present unique challenges due to their composition and structure. To mitigate deformation during machining, it is essential to employ appropriate methods. One of the most common and effective approaches is to provide adequate support or fill the thin-walled sections. For thin-walled rings made of superalloy, the chosen support or filling methods must vary based on the specific shape, size, and manufacturing process of each part.

This paper examines two thin-walled rings of different sizes and structures as case studies and highlights three methods applied in practice to enhance the turning process and reduce deformation.

 

02 Fixture Auxiliary Support

The part illustrated in Figure 1 is a conical ring made from the GH4169 nickel-based high-temperature alloy, utilized for sealing purposes in aero engines. In the cross-sectional view, the part features a maximum diameter of 356.1 mm, a minimum diameter of 197.11 mm, and a total length of 80.9 mm. Its design includes two annular cavities with depths of 51 mm and 62.8 mm, respectively. Additionally, the thickness of the peripheral wall ranges from 1.5 mm to 2.5 mm, which categorizes it as a typical thin-walled ring component.

 

The basic turning process consists of several key steps. First, there are two roughing operations to remove the majority of excess material. This is followed by a heat treatment operation, which helps relieve the stresses from the roughing process, and a datum repair operation to correct any deformation caused by the heat treatment.

Next, two semi-finishing operations are performed to further reduce the material, leaving a 1mm allowance. Finally, two finishing operations complete the final shape of the workpiece.

Regardless of the specific process route used, the final finish turning will always involve turning one end, then flipping the piece and turning the other end. To minimize deformation, it is advisable to finish turn the end with the two annular cavities first, followed by the other end, as illustrated in Figure 2.

 

 

When using conventional clamping methods, deformation can occur due to the pressure exerted by the cutting tool, leading to machining defects. Additionally, the inherent properties of the material mean that repeated cutting can result in surface hardening and even cause overcutting.

To address these issues during precision machining of one end of a deep cavity, a floating auxiliary support can be employed to enhance the rigidity of the machined area, thereby reducing deformation. The basic structure of this auxiliary support consists of a spring coil and a locking nut, as depicted in Figure 3.

To use this system, keep the locking nut loose while mounting the part on the fixture. Then, adjust the locking nut using a dial indicator to properly support the part. This method has advantages over rigid supports, as it minimizes over-positioning interference caused by machining errors and maintains support without gaps.

However, there are some drawbacks to this approach. The part’s structure is relatively complex and requires high dimensional accuracy, which can make it challenging to manufacture. Additionally, each use demands readjustment of the tightness with a dial indicator to ensure even force distribution, which can be inconvenient.

 

03 Shaped Rubber Filler Support

Case 1: When the part is precision machined at the other end, it features two annular cavities that enhance its rigidity. Conventional machining methods can be negatively impacted by tool extrusion, which not only causes deformation but also leads to significant vibrations during the machining process. This vibration can make it nearly impossible to complete the machining.

Since the machining work at this end occurs around the outer edge of the annular cavity, research has shown that filling the cavity to improve rigidity is more effective than adding auxiliary supports to address stability and vibration issues. To achieve this, a ring-shaped support filler made of rubber is utilized. The manufacturing method is as follows:

1) Choose rubber materials that possess the following characteristics:

- Applicable temperature range: -30 to 120°C
- Shore hardness: 30 to 60 HS
- Elastic modulus (G): 525 to 1260 N/m²
- Compression deformation under static load: less than 15%
- Compression deformation under dynamic load: less than 5%

2) To determine the size of the vibration damping rubber ring (refer to Figure 4), consider the size of the cavity on the reverse side of the machined part. Due to the significant elasticity of rubber, the rubber ring and the part should create an interference fit. Therefore, the size of the rubber ring should be slightly larger than the cavity, specifically by 1 to 2 mm. Once the dimensions of the rubber ring are obtained, the ring can be machined. This machining can be done either by turning the rubber or by heating it to a molten state and then molding it.

 

3）When machining parts, simply place the rubber ring into the cavity of the part. If the rubber ring is not securely in place, you can use bolts or adhesive to attach it to the fixture before clamping the part for machining. The filling support annular cavity is illustrated in Figure 5.

 

Verified results demonstrate that this method significantly enhances the rigidity of hollow CNC milling components, effectively minimizes the elastic deformation of the workpiece caused by cutting forces, and offers excellent vibration absorption through the use of rubber, which helps to reduce machining vibrations. Additionally, this method is relatively easier to fabricate compared to traditional auxiliary supports.

 

04 Low-Melting-Point Alloy Filled Support

Using the part depicted in Figure 6 (Case 2) as an example, this component is constructed from GH4169, a nickel-based high-temperature alloy. It serves as a ring for connections in aero-engines. The part features a maximum diameter of 1331 mm, a minimum diameter of 1284 mm, and a total height of 31 mm. Its design includes an inclined U-shaped structure, with an axial depth of 29.3 mm in the U-shaped groove and a peripheral wall thickness ranging from 1.04 mm to 1.27 mm, with a tolerance of ±0.15 mm.

 

The machining sequence must be carefully planned. If machining is carried out directly without a specific order, the part may suffer from insufficient rigidity and be adversely affected by tool pressure when the wall thickness is machined last. This can lead to difficulties in controlling the wall thickness.

Given that the part has a simple structure, with the raw material being a rectangular ring, the basic process involves machining a mounting edge on the raw material, completing all finishing operations in one go, and then cutting the piece into its final shape. The key challenge is enhancing the rigidity of the machined area.

The most effective way to improve rigidity is to first machine the ring groove. By using supports to hold the U-shaped groove, the part’s rigidity can be ensured. Then, the inner and outer circles can be machined separately to maintain proper wall thickness. If the inner and outer circles are machined first, followed by the U-shaped groove, designing a support scheme for the inner and outer circles becomes more complicated. The machining approach is illustrated in Figure 7.

 

When selecting a filler material, using auxiliary fixtures for support complicates the design of a perfectly fitting structure. Even if such a design is feasible, the high cost associated with the required precision poses challenges. While rubber fillers are beneficial for absorbing vibrations and providing auxiliary support, their high elasticity limits their effectiveness in supporting thin-walled, large ring components. Improper use of rubber can lead to deformation before machining due to its high coefficient of expansion. Therefore, it is necessary to choose a material with lower elasticity and greater hardness than rubber for filling purposes.

 

The first attempt involved infusing common paraffin wax. After it solidified, a gap of 0.1–0.2 mm remained between the paraffin wax and the part wall, resulting in poor adhesion. Consequently, the rigidity of the part system did not meet expectations, and the gap caused tool deflection during metal CNC machining, making it impossible to maintain a tolerance of ±0.15 mm.

In the second attempt, rosin was used as a reinforcing material. This time, there was nearly no gap between the rosin and the part, which met the machining requirements. However, it was discovered that the rosin needed to be heated to over 160°C to melt and remove, a temperature difficult to achieve on-site.

 

The third attempt employed plaster as a reinforcing material. Although the surface layer of plaster solidified within 10 minutes of pouring, the bottom layer remained unset, resulting in poor uniformity. Additionally, the complete solidification process was time-consuming, which reduced work efficiency.

As a result, a material was needed that could solidify quickly and uniformly, have a low coefficient of thermal expansion, high hardness, and be easily removed under normal conditions after processing. After repeated comparisons, a low-melting-point alloy composed primarily of lead, tin, and bismuth was selected. This alloy features a low coefficient of thermal expansion, a melting point of only 70°C, and minimal thermal impact during hot filling, machining, and melting-separation processes, reducing the risk of deformation in the parts.

 

The entire operation process is as follows:

1) Mount the workpiece on the machine tool and heat the low-melting-point alloy to a molten state using an electric furnace or induction cooker.

2) Pour the molten low-melting-point alloy into the groove of the part, filling it evenly to the predetermined depth. The pouring process should take only 5 to 7 minutes.

3) Allow the low-melting-point alloy to cool completely for 3 to 5 minutes.

4) Inspect the surface for any defects from the pouring process, such as porosity, dents, and gaps between the low-melting-point alloy and the part.

5) Check the internal dimensions. If there are no abnormal changes, proceed with machining.

6) After machining, place the part in a hot water bath at a temperature of 70 to 80°C to melt away the low-melting-point alloy. Following this, check its dimensions and geometric accuracy.

7) The low-melting-point alloy can be recycled.

This method effectively ensures part rigidity and allows for a wall thickness accuracy of ±0.15 mm. Although low-melting-point alloys require casting and recycling, they are the best overall option compared to paraffin, rosin, and gypsum.

 

05 Conclusion

This article provides practical examples of controlling machining deformation in two typical high-temperature alloy thin-walled ring parts: using fixtures for auxiliary support and filling with auxiliary material. Practical verification demonstrates that choosing either the auxiliary support method or the auxiliary material filling method can enhance the rigidity of the machined part. The appropriate selection, based on specific circumstances, not only ensures the required dimensional accuracy but also improves machining efficiency. This approach is valuable and applicable to the machining of other similar thin-walled ring parts.

 

 

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