Abstract:In the aerospace industry，titanium and nickel-based alloys are important metal materials for aircraft structures and engine components．Thesecritical structural components are manufactured to meet high reliability requirements，and surface integrity is one of the most relevant parame-ters for evaluating the quality of finished surfaces．The residual stresses and surface changes produced by titanium and nickel-based alloys during processing are critical to their safety and sustainability．This paper reviews the research progress on the surface integrity of titanium alloys and nickel-based alloys，and reports on many different types of surface integrity problems，including the study of surface residual stress，white layer and work hardened layer，and microstructure changes to improve the final surface quality of the product．Many parameters affect the surface qua lity of the workpiece，where cutting speed，feed rate，depth of cut，tool geometry and machining process，tool wear and workpiece performance are among the most worthwhile issues．In order to better understand the surface integrity introduced by processing，experimental and empirical studies as well as methods based on analysis and finite element modeling are required．However，at the current state of the art，there is still a lack of a comprehensive，systematic approach based on physical process that is suitable for industrial processes．The results show that while explaining the effects of various parameters on the processing of titanium alloys and nickel-based alloys，it is necessary to establish a predictive physical model consistent with reliable experiments．

Key words：titanium alloy，nickel-based alloy，surface integrity，surface defects，surface residual stress

1 Introduction

In the field of aerospace, automotive and biomedical manufacturing, a key issue that people have been concerned about is reliability. People need to understand that the performance of the processed parts has the greatest safety and predictability. The performance of a component is largely affected by the quality and reliability of its surface morphology, that is to say, the quality and performance of the product is directly related to its surface integrity. Surface integrity includes mechanical properties ( residual stress, hardness, etc. ), metallurgical state ( phase transformation, microstructure and related performance changes, etc. ) and surface parameters ( surface finish and surface morphology characteristics ) of the workpiece during processing. In the aerospace industry, surface integrity is one of the most relevant parameters to evaluate the quality of machined surface when machining key structural components for high reliability. In particular, titanium alloys and nickel-based alloys will generate residual stress during processing, which has received extensive attention due to its impact on the safety and sustainability of parts.

In the past few decades, many researchers have been trying to study the relationship between processing parameters and their effects on surface quality and product performance.The driving force behind this is the growing demand for component performance, reliability, and durability, which promotes the development of materials, enhances their resistance to harsh bearing conditions and environments, and develops high-performance manufacturing methods. For example, additive manufacturing technology, which has rapidly developed in the past decade, has the ability to quickly develop products and directly produce metal parts with complex geometric shapes, while significantly improving material utilization and reducing costs. However, there is still a certain gap between additive manufacturing parts and forgings in terms of critical performance such as fatigue. Therefore, people are actively exploring effective post-processing methods, such as using laser shot peening to strengthen additive manufacturing of TC4 titanium alloy, studying the improvement effect of additive manufacturing TC4 titanium alloy surface integrity, and comparing the strengthening effect with conventional forged titanium alloy of the same brand. The results indicate that laser shot peening has little effect on the surface roughness of the two materials, and the residual compressive stress introduced by laser shot peening on the TC4 surface of additive manufacturing is lower than that introduced by conventional forging TC4 surface.

In addition, a large number of literature has reported research on the related issues of nickel based alloy machining, such as rapid tool wear and adverse changes in the machined surface, as well as the hardening layer caused by machining. The low thermal conductivity of nickel based alloys leads to an increase in the temperature of the cutting edge of the tool, which in turn causes adhesion between the workpiece material and the cutting edge. In addition, hard particles (such as carbides) in the alloy will accelerate tool wear. The local high temperature, temperature gradient, and high pressure stress caused by machining will change the microstructure, resulting in white layer/surface damage in the deep direction of the material, which will have adverse effects on the performance of the parts. Griffith conducted a comprehensive analysis of surface integrity parameters and their control on enhancing fatigue life.

Degree gradient and high pressure stress will change the microstructure and produce white layer / surface damage in the deep direction of the material, which will adversely affect the performance of the parts. Griffith conducted a comprehensive analysis of surface integrity parameters and their control on enhanced fatigue life.

2 Surface integrity

Titanium alloy is a kind of material with high specific strength and specific stiffness, good high temperature mechanical properties, corrosion resistance and creep properties. Therefore, titanium alloy is suitable for manufacturing various fuselage and engine components ( fan blades, compressor blades and disks, stator blades, inlet and outlet guide vanes, etc. ). Titanium alloy is a typical difficult-to-machine material due to its low thermal conductivity, high tensile strength and good chemical activity at high temperature. Nickel-based alloy is another important material for aero-engines. Its main advantage is heat resistance, and it still maintains high mechanical and chemical properties at high temperatures. It has high melting point temperature, good corrosion resistance, and heat resistance. Fatigue, thermal shock, creep and erosion properties. The major challenge is also the low thermal conductivity of nickel-based thermal resistance alloys, which increases the heat impact during processing, often exhibits work hardening behavior, and the tool surface has high adhesion characteristics. Therefore, many scholars and researchers have done a lot of research on the microstructure, mechanical properties, cutting performance and application of titanium alloy and nickel-based alloy. Zhang Xiangqin et al.found that the tangential force and cutting depth resistance increased with the increase of feed rate, and obtained serrated chips which were in good agreement with the experimental results by finite element simulation. They believed that serrated chips were generated by adiabatic shear bands [ 3 ]. Yang Shucai studied the influence of tool edge on cutting TC4 through experiment and finite element method, and established the cutting force model under blunt edge [ 4 ]. Chen Ming et al.studied the influence of cutting parameters on cutting force and cutting temperature, and obtained the tool wear mechanism when drilling 3D printed titanium alloy through experiments [ 5 ]. The surface integrity, residual stress and machinability of nickel-based alloys have also been extensively studied [ 6-7 ]. For example, the surface damage of nickel-based alloy during turning, the finite element analysis of white layer formation, and the experimental observation of white layer formation. At the same time, scholars have also studied the microstructure and yield strength of the material, as well as the residual stress, hardness distribution and surface roughness of the material during the milling process [ 8-9 ], as well as the thermal response and mechanical response of surface damage to the cutting process.

The surface integrity of parts plays a vital role in the machining process. In most applications, the surface is required to be as smooth as possible, especially for the fatigue life of the machined part, and the surface integrity of the part is important. [ 10 ] The characteristics of various integrity attributes that affect the surface integrity of parts introduced or processed by the machining process can be divided into :

( a ) morphological characteristics, such as texture, waviness and surface roughness ;

( b ) Affected mechanical properties, such as residual stress and hardness ;

( c ) Metallurgical state, such as microstructure, phase transformation, grain size and shape, inclusions, etc. The surface properties of these changes can be divided into five categories : mechanical properties, thermal properties, metallurgical properties, chemical properties and electrical properties. In a word, scholars have done a lot of research on the surface integrity of machined parts, and have carried out extensive review on this kind of research [ 11-12 ]. This paper only introduces titanium and nickel-based alloys and different tool materials, and summarizes various surface integrity problems under the influence of different cutting parameters.

2.1 Surface defects

Parts of surface defects have various forms, as shown in figure 1 figure 3, its main form is surface resistance, material pull / cracking, feed trace, sticky material particles, tear surface, debris, surface pull, deformation particles, surface cavity, slip area 'wealth' seven according to reports, when the heat of material softening degree increases, compression stress also added. The defects of the processed surface also disappear, allowing the work piece close to the surface. These types of defects were observed by the researchers in many different shackles and titanium alloys, such as NiCr20TiA groaning, IN 718, titanium based Ti-6242S ", and Ti44 ®

Fig.1 Surface damages in machining of nickel-and titanium-based alloys: (a)metallographical microstructure of Ni Cr20TiAl after turning at V=60 m/ min，f=0.15 mm/rev and Do C=1 mm［15 ］，(b)lay pattern after drymilling Ti6242S at V=125 m/min，f=0.2 mm/tooth，Do C=2.5 mm［14 ］，(c)me tal debris after turning IN718 at V=125 m/min，f=0.05 mm/rev，and Do C= 1 mm［14 ］，and(d)smeared material and feed marks after turning IN718 at V=125 m/min，f=0.1 mm/rev，and Do C=0.75 mm［14 ］

 

Fig.2 Surface defects in turning IN718 with V=40—120 m/min，f=0.15— 0.25 mm/rev，Do C=0.25 mm:microstructural deformations in(a)carbide cracking in the deformed layer，and(b)surface tearing and cavities.

Fig.3 Microstructural deformations of turned IN718 at V=40—120 m/min， f=0.15—0.25 mm/rev，Do C=0.25 mm with(a)new tool and(b)worn tool

The cutting parameters have a certain influence on these surface defects. The feed trace is obvious in the processing, but its severity can be adjusted by changing and optimizing the feed rate. Cutting speed affects the number of surface micro-debris, while cutting depth and other parameters affect the tearing, resistance and coating of the material. [ 16 ] Many surface defects will be found in the processing of nickel and titanium alloys, especially in the processing of micron precision, so it is very important to optimize the cutting conditions. The largest defects in the surface defects are the cutting marks, the re-deposition of the chips to the surface and the grain deformation. In addition, pulling particles from the surface and redepositing them on the surface will produce two different defects, and these particles will also cause drag and tear defects when passing through the surface next time. It is very difficult to adjust the cutting parameters according to these defects, and it is difficult to completely eliminate surface defects.

In addition, the structure of some parts materials contains carbide particles, and many coating materials also contain some carbides. When the tool wears and the part is machined, these carbide particles are sometimes removed from the machined surface or the tool and stuck to the surface of the part. [ 15 ] This phenomenon is called carbide crack ( Fig.2 ), which leads to a sudden increase in shear stress during the cutting process, resulting in voids and cracks on the surface due to drawing, and even more serious surface problems. Both titanium alloy and nickel-based alloy are prone to carbide cracks, and the existence of cracks greatly reduces the fatigue life of the material. Therefore, the surface integrity of carbide cracks at the microscopic scale may be a serious problem. Especially when the cutting depth and feed rate are very small, the carbide particles are close to a certain scale, and the carbide cracks may be extremely important on the surface of the final product.

2.2 Surface structure changes

During the processing of parts, the material is exposed to thermal energy, mechanical energy and chemical energy, which leads to strain aging and recrystallization of the material, so that the material may become harder but less ductile. These thermal ( high temperature, rapid quenching ) and mechanical ( high stress, strain ) effects are the main reasons for the change of microstructure, phase transformation and plastic deformation of materials.

The study of Ti-64 and Ti-6246S shows that a very thin layer of plastic deformation is formed on the subsurface of the workpiece. With the wear of the tool, due to the change of the microstructure, the plastic deformation and the thickness of the subsequent deformation layer increase [ 20 ]. When the cutting speed and feed rate increase, the depth of microstructural changes under these surfaces is observed to increase [ 16 ]. In addition, long-term use of worn tool processing will also increase the microstructure changes of the material, manifested as severe plastic deformation and a thicker ' interference ' layer on the machined surface. [ 21 ] Fig.3 shows that the grain boundary deformation in the machining direction is related to the increase of feed rate or tool wear. Fig.4 shows the microstructural changes of Ti-6242S alloy before and after milling ( V = 100 m / min, f = 0.15 mm / tooth, Do C = 2 mm ) [ 16 ]. When these microstructures are dominant in strain aging, white layers are formed and recrystallization is also observed.Under an optical microscope, another darker image can be observed, with a hardness value between the white layer and the matrix material, which has been widely studied in steel materials.

Fig.4 Microstructural alterations in Ti-6242S after milling at V=100 m/ min，f=0.15 mm/tooth，and Do C is 2 mm:(a)at the beginning of machining;(b)after the tool is worm.

2.3 Surface plastic deformation

The main problem of surface integrity comes from the plastic deformation of the workpiece during the machining process. It is necessary to study the influencing factors of these deformations. These deformations are caused by many parameters such as cutting parameters ( cutting speed, feed rate, cutting depth ), tool parameters ( rake angle, blade radius, shape, coating, wear ) and workpiece parameters ( material, structure ). In order to find out the main reason for the plastic deformation of the workpiece, a lot of research has been carried out. Studies have shown that during the processing of titanium alloys Ti-64 and Ti-6246, as the tool wears, the plastic deformation of the workpiece increases, forming a white layer [ 17,21 ]. In addition, the nickel-based alloy IN-718 was processed with processing parameters V = 32 ~ 56 m / min, f = 0.13 ~ 0.25 mm / rev, and Do C = 1 ~ 2 mm. After 1 min, it was observed that although there was no obvious plastic deformation, the subsurface of the material showed severe plastic deformation when the processing time was extended by 15 min [ 23 ] ( Fig.5 ). This is due to the influence of high stress ( mechanical effect ) caused by local heating ( thermal effect ) and the increase of force, which occurs at the initial and later stages of processing. The grain boundary deformation, slip and grain elongation indicate the seriousness of the plastic deformation. The tool wear is the main cause of the deformation.

Fig.5 Plastic deformation in the form of intense slip bands in milling(a) Ti64 and(b)Ti834 at V=200 m/min，f=0.05 mm/tooth，Do C= 1 mm

The occurrence of plastic deformation is usually not considered as a surface integrity problem. It mainly occurs on the subsurface of the material, and the time is very short, the area is very narrow, so it is difficult to be measured or observed. The main problem caused by plastic deformation is that the surface of the workpiece is hardened due to excessive plastic strain and excessive residual stress. It is found that both thermal and mechanical effects have significant effects on plastic deformation. For IN-718, when the processing parameters are V = 225 m / min and f = 0.15 mm / rev, the mechanical effect is more dominant, and the plastic deformation is closer to the material surface through hardening, while the thermal effect plays a leading role in deepening the material softening [ 24 ]. When milling γ-Ti Al material, the plastic deformation area is on the surface of 0 ~ 20 μm with V = 70 ~ 120 m / min, f = 0.06 ~ 0.12 mm / tooth and Do C = 0.1 ~ 0.5 mm [ 25 ]. However, when milling Ti-64 and Ti-834 alloys, the plastic deformation area is on the surface of 30 ~ 50 μm with V = 200 m / min, f = 0.05 mm / tooth and Do C = 1 mm. As shown in Fig.5, the plastic deformation in these cases is interpreted to be caused by phase dislocation slip [ 26 ]. Similarly, with the increase of tool wear, plastic deformation will also increase, especially at lower cutting speeds, such as 40m / min.This can be explained as that with the increase of tool wear, the contact area between the tool and the workpiece will increase with the decrease of the tool clearance angle, resulting in more friction on the surface of the workpiece [ 27 ]. In most cases, a very narrow plastic deformation region can be observed, indicating that plastic deformation is not the only factor leading to poor surface quality. Studies have shown that high pressure generated by high temperature processing is the main reason for plastic deformation.After machining Ti-6242 at parameters V=100-125 m/min, f=0.150.2 mm/tooth, Do C=2-2.5 mm, cutting conditions and tool wear are important factors affecting the plastic deformation of the machined subsurface. The cutting speed increased from 100 m/min to 125 m/min, and the feed rate increased from 0.15 mm/tooth to 0.2 mm/tooth, resulting in an increase in plastic deformation. When the tool wear increases from Initial condition to VB (vertical distance between the machined surface and the surface to be machined)=0.3 mm, it will affect the change of deeper microstructure [16]. On the contrary, Sadat et al.'s research shows that increasing the cutting speed reduces the depth of IN-718 plastic deformation at V=0.2~1.61 m/s and Do C=0.028 mm.

2.4 Surface hardening layer formation and microhardness

The surface of parts is usually processed by continuous machining processes ( such as rough machining, semi-finishing and finishing ). The characteristics of the machined surface layer produced by the previous process have an important influence on the processing performance of the subsequent process. This effect becomes more important for materials that exhibit high work hardening properties, such as nickel alloys. The part is easy to form a work hardening layer, which is a response to the processing-induced subsurface deformation. This is mainly due to the work hardening tendency of nickel alloy under excessive strain loading, forming a highly hardened surface layer, making subsequent cutting extremely difficult. The cutting depth of sequential cutting should be kept greater than the work hardening layer, which is a difficult problem for industrial applications. In order to overcome this problem, it has been reported that continuously changing cutting depth helps to improve the processing level of nickel-based alloys.

It has been found that after machining, the hardness of the material surface is greater than in the depth of the material, where most of the thermal and strain effects of the material are offset. These types of microhardness changes are also related to surface integrity, and have been studied by many scholars. Figure 6 shows that I1V718 at the processing parameter V=500 m / min% / * =0. 1 mm / rev> DoC = 0.35 mm1291When, from a higher surface hardness decreased to a lower hardness of the matrix material, the compression layer is caused by the material processing process hardening. Figure 7 shows the microhardness distribution'cutting of INJ 18 at different processing parameters (" = 40 ~ 120 min / min and / =0. 15-0. 25 mm / rev).

Fig.6 Microhardness profiles through the depth of IN718 after turning at V=500 m/min，f=0.1 mm/rev，and Do C=0.35 mm(C:commercial chamfer，M:modified chamfer)

Fig.7 Microhardness profiles through the depth of the material after turning IN718 at V=40—120 m/min，f=0.15—0.25 mm/rev，and Do C=0.25 mm.

2.5 Surface roughness

There are many methods to quantify the surface integrity of parts, among which the most widely used is the surface roughness. It is considered the main indicator of surface optical, and has been reported by many scholars. In titanium alloy and shackles alloy, the traditional processing process can not produce low enough surface roughness to meet the requirements of the final product, therefore, usually using laser impact shot or ball grinding and other post-treatment technology to optimize the material surface oIN-718 and Ti-6AI-6V-2 Sn in high speed processing (cutting and turning) temperature plays an important role in tool wear, is an important factor affecting the surface roughness of the material. The stacking layer formed on the side of the tool can make the tool out of the original motion trajectory, thus affecting the roughness of the tool and the choice of cutting parameters. Cutting parameters also have a great influence on the surface roughness. Some studies have reported that r cutting speed " 7 penalty, feed speed and cutting depth have a certain effect on improving the surface roughness of the material, but mainly with the increase of cutting speed. The surface roughness of Ti-6246 and Ti64 is increased. In addition, the tool parameters. Such as tool insertion shape, cutting edge parameters, insertion front blade type, front knife radius, coolant selection can also affect the surface roughness 8. According to this study, since the contact length of circular blades is longer than that of square blades, they can produce better surface finish, and honed edges can also produce better surface finish because of their roundness.

According to the research, when f = 0.35 mm / rev and Do C = 2 mm, the surface roughness of Ti-64 increases gradually with the increase of cutting speed from 45 m / min to 100 m / min [ 17,21 ] ( Fig.8 ). The same material showed similar behavior at V = 40 ~ 160 m / min, f = 0.1 mm / tooth and Do C = 1 mm [ 31 ]. Their results are for two kinds of tools without carbide coating and PCD, indicating that the results of surface roughness of these two kinds of tools are similar. For carbide coated tools, increasing the cutting speed, the surface roughness of PCD tools increases faster and more obviously ( Figure 9 ). In addition, the feed rate is also one of the main parameters affecting the surface roughness. For example, when V = 32 ~ 125 m / min, f = 0.075 ~ 0.6 mm / rev and Do C = 0.5 ~ 2 mm, the surface quality of IN-718 decreases with the increase of feed rate.

Fig.8 Surface roughness values when turning Ti64 with V=45—100 m/ min，f=0.35 mm/rev，and Do C=2 mm.

Fig.9 Average surface roughness values for changing cutting speed for Ti64 at V=40—160 m/min，f=0.1 mm/tooth，Do C=1 mm.

In addition, for IN-718, when the cutting parameters V = 32 ~ 56 m / min and Do C = 1 ~ 2 mm, increasing the feed rate from 0.13 mm / rev to 0.25 mm / rev leads to higher surface roughness. Some researchers have also found that for some materials and conditions, when the cutting depth increases, the surface quality decreases again. [ 32 ] Some studies have also shown that the cutting depth has no significant effect on the surface roughness [ 12 ]. It has been reported that these effects are caused by thermal and mechanical cycles, microstructural transformations, and mechanical and thermal deformation during processing. [ 33 ] The parameters affecting f vary with the material. Yang et al. [ 18 ] believed that surface integrity can be studied under several interrelated independent topics, such as residual stress, metallurgical changes, and changes in the mechanical properties of workpiece materials. In any case, these will affect the surface roughness of the final product, so they need to be included in the analysis process. In general, researchers have found that cutting speed and feed parameters in different fields, different materials and different cutting conditions have adverse effects on surface roughness. However, when the process material removal rate decreases with the decrease of feed rate or cutting speed, or both, the surface quality of the part is better and the surface roughness value is reduced.

2.6 Surface residual stress

Usually, after the end of the machining process, the workpiece material releases the top thermal mechanical load due to machining, but not all energy can be recovered, some of which is used for plastic deformation, which causes the material to exhibit some stress, especially on its free end surface. These stresses remain in the material after loading and are called residual stresses. They are believed to be mainly caused by the tensile plastic deformation of the sub surface of the workpiece material and the thermal effect of cutting tool conditions on the workpiece surface [34]. These residual stresses pose potential risks to the initiation, propagation, and fatigue failure of cracks in the final product, and the generation of tensile surface residual stresses must be eliminated or prevented during the machining process. The prediction and prevention methods of residual stress are currently one of the hotspots in research.

Residual stress is difficult to measure, and it is difficult for scholars to model this phenomenon, which results in the diversity of literature results. Many researchers claim that the surface residual stress is tensile [ 19, 24 ] ; some claim that they are compressed [ 35-36 ]. The existence and degree of compressive residual stress peak and the depth of residual stress elimination quantification have not reached an agreement. At the same time, the influence of cutting parameters and tool parameters on surface residual stress, different literatures also show different results. These different results may be due to different workpiece materials and different cutting conditions and tool parameters. Studies have shown that residual stress is a very important problem in the processing of steel, titanium and nickel workpieces. As the depth of the workpiece increases to about 50μm, the residual stress has a stronger tensile property on the surface of the workpiece and gradually becomes a compressive stress. However, when the depth of the workpiece increases to about 300μm, the residual stress decreases and disappears. The tensile residual stress layer may be related to the formation of the white layer, and the compressive residual stress layer after 50μm may be related to the formation of the black layer, but this possibility needs further study. In general, the increase of feed rate will produce more tensile residual stress and larger peak compression depth on the surface [ 19,30 ], as shown in Fig.10, especially at higher cutting speed. At a lower cutting speed, the peak residual stress depth may decrease with the increase of feed rate [ 37 ]. Outeiro et al. [ 38 ] found that if the coating tool is used, the surface residual stress becomes smaller, and the peak compressive residual stress increases in the compression direction and occurs deeper ( Fig.11 ). In addition, the geometry of the cutting edge affects the compressive / tensile behavior and residual stress.The research results show that by changing the orientation of the cutting tool or the inclination angle of the workpiece, without changing the working parameters, residual stress may change from tensile stress to compressive stress, or the magnitude of residual stress may change. It is understood that as tool wear increases, for example, IN-718 exhibits tensile residual stress on the surface at V=40-120 m/min, f=0.15-0.25 mm/rev, Do C=0.25 mm [19], and V=40-80 m/min, f=0.35 mm/rev, Do C=0.25 mm [21]. This is mainly due to the increase in plastic deformation and the temperature rise caused by friction caused by frictional effects.

Fig.10Ｒesidual profiles of(a)new and(b)worn tools while turning IN718 at V=40 m/min，f=0.15—0.25 mm/rev，Do C=0.25 mm.

Fig.11Ｒesidual stress profile of IN718 after turning at V=70 m/min，f= 0.2 mm，and Do C=0.5 mm.

3 Characterization method of surface integrity

The development of advanced surface integrity measurement methods has received more and more attention, and many studies are constantly reassessing surface integrity. Many scholars have carried out experimental studies on different surface integrity characterization methods such as surface roughness, microhardness and residual stress. The results are also very useful for analysis or finite element-based analysis methods. These experiments themselves may not provide the best answers, nor can they summarize the effects of all possible cases through experiments. However, the results of these experiments can be verified and compared by analysis or based on predictions. They can also be used as a basis for building models and empirical research, as these studies are based on accurate data, which makes it easy to find the relationship coefficient between input and output parameters.

The development of advanced surface integrity measurement methods has received more and more attention, and many studies are constantly reassessing surface integrity. Many scholars have carried out experimental studies on different surface integrity characterization methods such as surface roughness, microhardness and residual stress. The results are also very useful for analysis or finite element-based analysis methods. These experiments themselves may not provide the best answers, nor can they summarize the effects of all possible cases through experiments. However, the results of these experiments can be verified and compared by analysis or based on predictions. They can also be used as a basis for building models and empirical research, as these studies are based on accurate data, which makes it easy to find the relationship coefficient between input and output parameters.

With respect to residual stress, in the last 10 years, different experimental techniques have been developed, optimized and adapted to determine the residual stress generated by different types of processing. In order to quantify the residual stress of low-pressure turbine blades, Glaser et al research studies compared different residual stress measurement techniques such as synchrotron radiation X-ray diffraction (SXRD), laboratory X-ray diffraction (XRD) and sequential electropolishing, neutron diffraction (ND), progressive hole drill (IHD) and contour method (CM). Using complementary techniques facilitates facilitate understanding of residual stress within materials. For example, XRD is a good means for surface stress measurement, and the full stress field can be changed by using the fine resolution and contour measurement provided by SXRD.

In addition, different indentation techniques have also been used to determine the residual stress of machined parts in recent years. In response to these technical problems, Warren et al.studied nanoindentation [ 41 ], focusing on understanding the basic relationship between mechanical properties, microstructure, and residual stress introduced in the processed parts. Later, Wyatt et al. [ 42 ] developed a micro-indentation method to study the residual stress of high-speed milling parts. Diaz et al. [ 20 ] used the optimized indentation method to study the effects of thermal and mechanical effects on the residual stress in high-speed milling of different iron-containing aluminum alloy samples.

4 Conclusion

This paper summarizes the current work on the surface integrity of aviation parts processing, aiming to emphasize the necessity of carrying out work on this integrity, so as to establish a prediction model based on scientific research and its application in industry. Although the characteristics of surface defects caused by machining are described in this paper, the numerical simulation method and analysis prediction model of surface integrity are not described and introduced, and there are still many problems that have not been solved. For example : how to control the processing conditions, reduce or even eliminate such defects ? What is the importance of surface defects to fatigue life ? And considering the multiple interactions between different surface defects, how to predict the fatigue life of parts ? In particular, the fatigue life prediction of parts is very important for the industry, and has high academic value and application. Therefore, further detailed research is needed to establish an accurate and reliable prediction model to ensure the final function and performance of the processed parts.

FAQ 1: What is machining surface integrity, and why is it crucial for aviation parts?

Answer: Machining surface integrity refers to the quality and condition of the surface of aviation parts after the machining process. It includes factors such as surface roughness, residual stresses, microstructure, and metallurgical changes. Achieving excellent surface integrity is critical for aviation parts as it directly impacts the performance, durability, and reliability of aircraft components. High-quality surface integrity ensures reduced fatigue, improved wear resistance, and enhanced structural integrity, all of which are vital for the safety and efficiency of aircraft operations.

FAQ 2: How can manufacturers ensure superior surface integrity during the machining of aviation parts?

Answer: Manufacturers employ several techniques to ensure superior surface integrity in aviation parts:

• Cutting Parameters: Optimizing cutting parameters, such as cutting speed, feed rate, and depth of cut, helps control heat generation and minimize the risk of thermal damage.
• Tool Selection: Using high-quality cutting tools with appropriate coatings and geometries helps achieve smoother surface finishes and reduces the likelihood of tool-induced defects.
• Coolant and Lubrication: Proper coolant and lubrication management dissipate heat during machining, preventing overheating and minimizing the formation of surface defects.
• Post-Machining Treatments: Applying post-machining treatments like shot peening or stress relieving can further enhance surface integrity by mitigating residual stresses and improving material properties.

FAQ 3: What are the benefits of superior surface integrity in aviation parts?

Answer: Superior surface integrity in aviation parts offers numerous benefits:

• Extended Service Life: Enhanced surface integrity increases the fatigue life of aircraft components, leading to longer service intervals and reduced maintenance costs.
• Improved Performance: Parts with superior surface quality contribute to smoother operation, reduced friction, and improved fuel efficiency of aircraft systems.
• Enhanced Safety: High-quality surface integrity ensures the structural integrity and reliability of critical aircraft components, enhancing overall flight safety.
• Regulatory Compliance: Meeting stringent aviation industry standards for surface integrity is crucial for obtaining certifications and approvals for aviation parts.

 

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