Managing Residual Stress in Thin-Walled Ring Component Machining
This paper analyzes various factors that affect the surface residual stress of a typical thin, narrow, ring-shaped part model. It reveals the stress state during the machining process and proposes methods for controlling residual stress, ultimately aiming to improve the machining quality of these parts.
1. Introduction
Machining thin and narrow parts presents significant challenges, particularly for high-performance products. These components often feature thin walls and narrow, irregular edges due to their specific design requirements. The quality of these parts is largely determined by both their surface geometry and the mechanical properties of the surface layer, which can be difficult to control.
This paper examines the impact of residual stress on part quality, focusing specifically on surface layer stress during the machining process. It analyzes the causes and characteristics of this residual stress to enhance understanding of the part’s stress state both during and after machining. Additionally, methods for eliminating and controlling residual stress are proposed, offering valuable insights into the machining processes for these types of parts.
2. Part Characteristics
For illustration, let’s consider a typical metal part that is thin, narrow, and irregular in shape. This part is ring-shaped, characterized by the structure shown in Figure 1. Its thickness (δ) is less than 10 mm, and the width (b) is more than twice the thickness (δ). Additionally, the ratio of the inner hole diameter (d) to the thickness (δ) is greater than 20. The outer ring features a tapered angle of α degrees. The material used for this part is austenitic stainless steel.
This component lacks structural rigidity and shows poor resistance to machining deformation. After machining, the end face exhibits significant warping, unevenness, and out-of-roundness, resulting in immediate failure post-machining.
3. Machining Stress and Deformation
When analyzing the causes of deformation in this part, the first factor to consider is clamping. Optimizing the clamping process is crucial to ensure that the part is subjected only to planar clamping forces, avoiding any radial clamping forces that could lead to deformation. Once external forces are excluded, only the internal forces within the part remain.
It is well-known that the stress generated during machining can lead to plastic deformation afterwards. According to the concept of internal stress proposed by German scholar Machlauch, internal stress can be categorized into three types. In the context of metal part machining, it is primarily the first category that we need to analyze: this internal stress is nearly uniform within the material’s grain size range and maintains equilibrium across all cross-sections. This type of internal stress is referred to as residual stress in engineering.
In simpler terms, residual stress is the macroscopic stress that exists within a material when external forces have been removed, temperature has stabilized, and phase transformations have stopped. Based on this understanding, we can analyze the residual stress generated in this part from three perspectives: surface plastic deformation, temperature effects, and metallographic changes that occur during the machining process.
3.1 Surface Plastic Deformation
The part is made from austenitic stainless steel, a material known for its high plasticity, which makes it challenging to machine. This difficulty often leads to significant plastic deformation during the cutting process. To create the part, a CNC lathe is utilized for turning, along with specially designed fixtures to prevent clamping deformation.
The machining process begins with the flat surfaces and internal holes of the part, followed by the inclined surfaces. This sequence ensures that the part experiences continuous cutting forces throughout the operation. The cutting deformation zones are illustrated in Figure 2.
As the tool enters the material, the metal in front of the cutting edge is compressed by the rake face. This results in compressive plastic deformation along the cutting direction, known as the first deformation zone. Due to the constraints posed by the inner undeformed metal, this compressive deformation generates residual tensile stress within the material.
Meanwhile, the flank face of the tool encounters significant friction and compression against the machined surface, leading to substantial deformation, referred to as the third deformation zone. As the surface’s metallographic structure is further stretched and compressed, the surface metal undergoes tensile plastic deformation. Once the tool is removed, this deformation remains permanently.
As a result of the inner layer of metal that hasn’t experienced plastic deformation, compressive residual stress forms on the surface of the part, while tensile residual stress develops in the inner layer. It is important to note that the plastic deformation varies significantly with depth in the surface layer of the part.
The explanation above relates to the internal stress that arises from the significant deformation of both the tool and the surface of the part during cutting. Through experiments and practical observations, it has become evident that the actual residual stress is also closely linked to the cutting elements involved.
(1) Influence of Cutting Speed
Austenitic stainless steel is known for its high plasticity, making it difficult to cut. To minimize severe tool wear during the cutting process and to meet the surface quality requirements of the part, low-speed cutting is generally preferred, typically with a linear speed of ≤40 m/min. Different cutting speeds generate varying amounts of cutting heat on the surface of the part, with heat increasing as the cutting speed rises. Despite these changes in cutting heat at different speeds, the austenitic structure does not undergo phase transformation during heating. As a result, cutting heat is the dominant factor in the entire cutting process, leading to the formation of tensile residual stress on the surface of the part.
Figure 3 illustrates the relationship between cutting speed (vc) and residual stress. It shows that higher cutting speeds result in lower residual tensile stress at the surface of the part, and this effect decreases with greater depth. Even at relatively low cutting speeds, some level of residual tensile stress will ultimately develop on the surface.
(2) Influence of Feed Rate
Increasing the feed rate typically results in greater plastic deformation, leading to an increase in residual stress and the depth of the stress layer on the surface of the part. Given the material characteristics of the part, a smaller feed rate is often chosen, which results in less plastic deformation during the cutting process. Consequently, there is less heat generated in the cutting zone, which minimally affects the value and depth of residual stress.
Figure 4 illustrates the impact of feed rate (f) on the residual stress of the part when using a positive rake angle turning tool and a cutting speed (vc) of less than 80 m/min. Selecting a feed rate of 0.15 mm/r shows a minimal effect on the surface residual stress, which is nearly zero. Additionally, there is a small amount of residual compressive stress found in the inner layer.
(3) Influence of the rake angle
During the turning process, the rake angle (γ) of the cutting tool significantly affects the residual stress of the surface metal of the workpiece. Figure 5 illustrates how different rake angles influence the residual stress when the cutting speed (vc) is less than 100 m/min. As the rake angle changes from a positive to a negative value, the residual tensile stress on the surface of the part gradually decreases, while the depth of the residual stress layer increases.
With specific cutting parameters, using a negative rake angle with a larger absolute value can generate residual compressive stress in the machined surface layer. In simple terms, cutting the part with a tool that has a positive rake angle results in residual tensile stress, whereas a negative rake angle yields residual compressive stress. The greater the absolute value of the rake angle, the more significant the residual stress produced.
(4) Selection of Cutting Elements
Residual stress is an unavoidable phenomenon that occurs on the surface of a workpiece during metal cutting. The magnitude and nature of this residual stress are influenced by various factors. By designing appropriate elements based on the specific working conditions of the part, it is possible to achieve the desired residual stress properties.
During operation, the ring-shaped part will partially slide against its mating components, which can cause wear damage on the local surface. The distribution of sliding friction stress during operation is depicted in Figure 6. The frictional force generates compressive stress on the metal surface in the direction of movement. If the part retains residual compressive stress after machining, this stress will combine with the new stress, leading to increased wear and, ultimately, surface degradation.
From a fatigue resistance perspective, the selection of elements during the final machining process should ideally focus on generating residual tensile stress.
3.2 Influence of Surface Temperature
The metal cutting process generates a significant amount of heat. The high temperatures produced during cutting cause the part to expand, which in turn affects the residual stress state of the machined surface. Additionally, the elevated temperatures can relax or redistribute existing residual stresses, resulting in changes to the surface metal’s residual stress (see Figure 7).
When considering factors such as the use of cutting fluid during actual machining, the process of how the residual stress state of thin, narrow annular parts changes is illustrated in Table 1. This series of changes demonstrates that cutting temperatures ultimately lead to the formation of residual tensile stress in the part.
3.3 Influence of Metallographic Structure Changes
The significant amount of heat generated during machining raises the surface temperature of the part. If this temperature exceeds the phase transformation threshold, a phase change may occur in the surface metal structure. The austenitic stainless steel used for the part remains austenitic at both room temperature and high temperatures, which means it typically cannot undergo martensitic transformation through rapid cooling or phase changes. Although austenitic stainless steel can experience martensitic transformation during actual heating processes (ranging from 500 to 850°C), we will not discuss that here. For the purpose of this discussion, we will assume that no phase transformation occurs during the machining process, meaning that it is not affected by any factors leading to phase transformation and, as a result, no residual stress is generated.
3.4 Part Deformation
The residual stress present in the machined surface layer of a part results from a combination of several factors. The magnitude and sign of this stress are determined by which factor is dominant. While machining cutting elements can be controlled and adjusted, factors such as plasticity, cutting heat, and metallographic structure are influenced by the material itself.
During CNC turning, the part experiences significant plastic deformation, which primarily generates residual stress. This often results in residual tensile stress. The distribution of surface stress in the component can be simplified as shown in Figure 8, indicating that the residual stress is unevenly distributed across its cross-section. Because the component is thin and narrow, it has relatively poor stiffness. When its stiffness is inadequate to withstand the residual stress, the component will deform to achieve equilibrium.
Given that the factors contributing to residual stress are complex and multifaceted, it is not possible to provide a quantitative calculation model or formula in this paper. Instead, this work offers a qualitative analysis to reveal the nature and approximate distribution of surface residual stress. Designers should be mindful of this and implement appropriate control measures.
4. Residual Stress Relief Methods
Residual stress can be reduced or eliminated after machining by using appropriate treatment methods, such as heat treatment, vibration aging, and chemical treatment. Thin, narrow ring-shaped parts require specialized tooling for subsequent processing.
4.1 Heat Treatment
Heat treatment is a process that involves heating a part to a specific temperature and maintaining that temperature for a set period to relax and release any internal residual stress. It is crucial to strictly control the temperature, holding time, and cooling rate during this process to ensure that residual stress is eliminated without introducing new deformations or defects. However, heat treatment has its limitations; it can be time-consuming, energy-intensive, and improper process control may result in secondary deformation. For thin, narrow, ring-shaped parts that are both flexible and require high precision, maintaining uniform temperature and minimizing deformation can be particularly challenging.
4.2 Vibration Aging
Vibration aging is a process that eliminates residual stress by applying vibrations at a specific frequency to a part. The energy generated by these vibrations causes the microscopic lattice structure to shift and rearrange, effectively releasing the residual stress. This method has a short processing time, does not require heating, and is environmentally friendly. Vibration aging can rapidly and uniformly eliminate residual stress within parts, with particularly notable effects at stress concentration points, especially in thin, narrow annular components.
4.3 Chemical Treatment
Chemical treatment involves creating a specialized coating on the metal surface of a part through a chemical reaction, which helps eliminate residual stress. For thin, narrow annular parts, suitable chemical treatment methods include carburizing and nitriding. While chemical treatments effectively eliminate residual stress and improve surface properties with controlled coating depths, the process is complex and requires strict control of the reaction conditions and parameters. This complexity can present challenges during implementation.
5 Residual Stress Control Strategies
5.1 Detection Methods
Calculating the magnitude and distribution of stress on the surface of a part can be challenging, but these factors can be measured using existing technologies. Non-destructive methods are preferred for detecting surface stress, with X-ray diffraction, neutron diffraction, and ultrasonic testing being the most common techniques. Of these, X-ray diffraction is regarded as the most accurate and reliable method. By using X-ray diffraction to gather stress data from the part’s surface, one can analyze the distribution of residual stress and gain insights into its magnitude and direction. When combined with numerical simulation, this approach offers a more comprehensive understanding of the residual stress state.
5.2 Control Methods
Methods for controlling residual stress in parts involve optimizing material selection, structural design, and utilizing advanced processing technologies. By carefully selecting materials and limiting their composition, the microstructure and properties of the part can become more uniform and stable, thereby reducing the formation of residual stress.
Improving the structural design of parts can enhance their stiffness, which helps to counteract residual stress. A well-planned processing route is essential; this includes choosing the appropriate processing steps and tool paths, minimizing the number of tool passes on the same part, and avoiding stress accumulation due to repeated processing. For instance, roughing operations are typically conducted first to eliminate the majority of excess material, followed by finishing operations to minimize residual stress on the surface of the part.
Symmetrical and zoned machining methods can be employed to balance the stress distribution on the part’s surface. Additionally, optimizing cutting parameters—such as adjusting the depth of cut and feed rate—can help to reduce cutting forces, resulting in lower residual stress.
Auxiliary machining techniques, such as incorporating grinding or other finishing processes into the cutting operations, can improve surface quality and reduce stress concentration. Specialized machining methods like ultrasonic vibration and cryogenic cutting can also help suppress the generation of residual stress to some extent.
Pre-treatment of the part material before machining can effectively mitigate both the generation and distribution of residual stress afterward. To monitor the magnitude and state of stress during machining, real-time monitoring and feedback systems can be implemented. This could involve the installation of stress sensors or other monitoring equipment to track stress changes in real time, allowing for adjustments to cutting parameters based on the feedback received, thereby controlling residual stress more effectively.
6 Conclusion
Metal cutting is a complex process influenced by various factors. This paper focuses specifically on the factors that affect the machining quality of thin, narrow, ring-shaped parts, particularly regarding surface residual stress. Additionally, it presents several effective methods and control strategies for eliminating residual stress. These insights can help improve the quality control of machining for such parts.
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