Residual Stresses in Machining Operations

Definition

By definition, the residual stresses are multiaxial static stresses that exist in an isolated component without any applied external force or moment, and they are in mechanical equilibrium. Residual stresses are the response to the mechanical and structural history of the component during its manufacturing (metal casting, metal forming, machining, heat treatment, etc.) and in service when submitted to external loadings (thermal, mechanical, and chemical). They are caused by the elastic response of the material to the heterogeneous plastic deformation at any scale of the component or structure.

Theory and Applications

Introduction

The quality of mechanical components depends on large extend on the surface integrity, which is characterized by the mechanical, metallurgical, and chemical states of the machined affected layers (Jawahir et al. 2011). The residual stresses, together with the hardness, yield stress, tensile strength, etc., characterize the mechanical state of the machined affected layers. The study of machining operations inducing residual stresses is particularly important when critical structural components are machined, especially, if the objective is to reach high reliability levels and long service life.

Both magnitude and distribution of the residual stresses in the machined components can be critical for the functional performance and life of components. Residual stresses can cause a decrease in the static and dynamic strength, a decrease in the corrosion resistance, a dimensional instability (part distortion), changes in the magnetically properties, etc. (Brinksmeier et al. 1982) (see Fig. 1). Therefore, they must be taken into consideration during the design and manufacturing of components. In general, tensile residual stresses at the components superficial layers are unwanted, since they can induce premature fatigue and corrosion failures. On the contrary, the compressive residual stresses at the components superficial layers are beneficial, since they increase the fatigue and corrosion resistances.

Residual Stresses in Machining Operations, Fig. 1

Effects of the residual stresses (Outeiro 2002)

Residual stress distribution in the machined components results from the machining history but also from the previous materials processing. The machining history consists of a sequence of machining operations (turning, milling, drilling, etc.) and corresponding machining parameters. In this sequence, the effect of successive machining passes should be also considered (Guo and Liu 2002). Defining a logical machining sequence for a given component, the resulting residual stress distribution in the machined surface layers will depend on the machining parameters used in each operation, being the strongest contribution given by the last one. The final goal is the selection of the optimal machining parameters in order to obtain an acceptable/desirable residual stress distribution in the component.

Mechanisms of Residual Stress Formation in Metal Cutting Operations

Residual stresses in metal cutting are essentially generated by heterogeneous plastic deformation existing in a component. This plastic deformation results from the combined action of the thermal and mechanical phenomena generated by metal cutting operations. These two phenomena are usually designed as the origins of the residual stresses. Usually, phase transformation is also considered as other origin of the residual stresses. However, phase transformation is a consequence of the thermal and mechanical phenomena (see Fig. 2). Depending on the volume variation, it can generate both tensile and compressive residual stresses (Scholtes 1987).

Residual Stresses in Machining Operations, Fig. 2

Schematic representation of the machining residual stress formation (Outeiro 2002)

The heat generated in machining, which is produced by plastic deformation and friction, represents the thermal phenomena. However, only the portion of the heat conducted to the workpiece can generate residual stresses. Usually, this heat will contribute to the formation of tensile residual stresses due to the thermal expansion and contraction of the surface machined affected layers. Since the core of the workpiece is not deformed plastically, heterogeneous plastic deformation in the cross section of the component is created and consequently the residual stresses.

The mechanical phenomenon also induces heterogeneous plastic deformation due to the mechanical action of the tool over the workpiece surface layer. Liu and Barash (1982) proposed an approach to explain this action, which is based in the loading cycle that a material element is submitted through his movement along the cutting direction (see Fig. 3). According to this approach, the material element experiences deformation in compression when it is located ahead of the tool, followed by yielding. Then, it experiences deformation in tension when it passed through the tool tip, followed by an eventual second yielding. The residual stress in the machined layer will depend on the relative magnitude of the tensile and compressive loads. As shown (see Fig. 3), a predominantly compressive load will induce tensile residual stress, while a predominantly tensile load results in compressive residual stress. This cyclic loading is influenced by the work material properties and chip formation process.

Residual Stresses in Machining Operations, Fig. 3

Schematic representation of the residual stress formation in machining proposed by Liu and Barash (1982) which considers the mechanical action of the tool over the surface of the workpiece

Experimental Techniques for Determining the Residual Stresses

Several techniques can be applied to evaluate the residual stresses in engineering components/applications. These techniques can be classified into mechanical (hole-drilling, contour, curvature, and layer removal), diffraction (X-ray diffraction, neutron diffraction, synchrotron radiation), and others (magnetic, ultrasonic, Raman spectroscopy). The selection of the proper technique is a critical issue, and the decision will depend on practical (size of the component, availability of the equipment, level of expertise required, cost, etc.), material (type of material to analyze, surface condition, etc.), and measurement issues (spatial resolution, penetration, type of stress and gradient that can be analyzed/evaluated, accuracy of the measure) (Kandil et al. 2001). Most of the residual stress measurements have been carried out in metals, including cast iron, steels, light alloys (such as aluminum, titanium, and magnesium alloys), and nickel-based superalloys. However, there is also an increasing interest to measure the residual stresses in composite, polymer, ceramic materials and other nonmetallic materials.

The mechanical techniques rely on the monitoring of changes in component distortion, either during the generation of the residual stresses or afterward, by deliberately removing material to allow the stresses to relax (Withers and Bhadeshia 2001). Measuring these distortions using contact or noncontact techniques, the residual stresses can be calculated using the elasticity theory. The major advantages of these techniques are their relatively simplicity, quickness, and low cost, and they can be applied to wide range of materials. The major disadvantages are their low resolution, and they are destructive.

The diffraction techniques relay to the use of the radiation, such as X-rays and neutrons, to access to changes in the interplanar atomic spacing of a specific family of lattice planes and, therefore, to calculate the elastic (residual) strains and stresses (Noyan and Cohen 1987). Indeed, the presence of residual stress within a polycrystalline material causes elastic strain and thus changes in the spacing of the lattice planes from their stress-free value to a new value, which corresponds to the magnitude of the applied residual stress. Using X-ray or neutron diffraction, it is possible to measure the shift in the angular position of the diffraction peak in relation to its position when the material is without residual stresses. The interplanar atomic spacing can be calculated knowing the angular position of the diffraction peak and applying the Bragg law. Knowing the interplanar spacing, the elastic strain can be calculated, and applying elasticity theory, the residual stress can be determined. The major advantages of these techniques are their good resolution (in particular the case of X-ray and synchrotron), they operate without contact and, consequently, they are nondestructive (except when applying X-ray diffraction and electrochemical removal process to evaluate the residual stresses below surface). The major disadvantages are the high cost of the equipment and measurement and high level of expertise required, and they are limited to crystalline materials.

The other methods are not used so frequently for residual stress evaluation. In general such techniques like magnetic and ultrasonic are nondestructive, cheap, simple to use, very fast, and portable, which are well suited to routine inspections. However, the low resolution is their major limitation, in particular when compared with the diffraction techniques. The Raman spectroscopy is an exception. Its high resolution (less than 1 μm, thus higher than the diffraction techniques) makes it suitable to evaluate the residual stresses in extremely narrow regions of a few micrometers as is the case of fiber composites, providing basic information about the residual stress distribution from fiber ends to centers (Withers and Bhadeshia 2001). Unfortunately, the major disadvantages of this technique relay to its calibration and limited range of material that can be analyzed. Many other techniques are being developed for measuring residual stresses, most of which are still in the research and development stage.

From the range of techniques above described, the hole-drilling and the X-ray diffraction techniques are the most used in practice. A survey carried out in the United Kingdom (Kandil et al. 2001) covering a representative cross section of UK industry and academia shows that more than 55% use the hole-drilling and the X-ray diffraction techniques for residual stress analysis, because they fit most of the practical, material, and measurement issues. This survey also shows that almost 50% consider the residual stresses of high importance to their business, while 30% ranked them as of medium importance.

A relatively good agreement between the results of both hole-drilling and X-ray diffraction techniques is obtained, in particular in the interior of the samples (Nobre et al. 2000). However, the hole-drilling was found to be unsuitable to evaluate the residual stresses in very near-surface and also strong residual stress gradients, such as those generated by machining and some metal forming processes. The observed discrepancies between the residual stresses measured by both techniques are often attributed to the basic shortcoming of the hole-drilling technique, which is its limitation to residual stresses up to 60% of the material’s yield strength (Beaney 1976). Because the drilling operation induces plastic deformations, the so-called plasticity effect can strongly affect the residual stress evaluation, which assumes linear elastic material behavior.

In conclusion, among all available techniques for determining the residual stresses in the machined affected layers, the X-ray diffraction, the synchrotron radiation, and the hole-drilling techniques are probably the most used. In the case of the two radiation-based techniques, they can provide very localized measurements due to their high spatial resolution and low penetration of the radiation in almost engineering materials. So, they are suitable to detect strong in-depth residual stress profiles, characteristic of the metal cutting processes. In the case of amorphous materials (like composites), the hole-drilling technique is a good alternative to the radiation-based techniques, although the abovementioned limitation of this technique.

Residual Stresses in Metal Cutting Operations

Residual stresses induced from metal cutting operations have been studied for several decades, resulting in a significant number of scientific publications covering a wide range of metal cutting operations (turning, milling, drilling, boring, etc.), work materials (plain carbon steels (Brinksmeier et al. 1982; Capello 2005; Henriksen and Ithaca 1951; Outeiro et al. 2006; Scholtes 1987; Torbaty et al. 1982), hardened steels (Matsumoto et al. 1986; Thiele et al. 2000; Umbrello et al. 2010), stainless steels (Jang et al. 1996; Outeiro et al. 2002, 2006), and superalloys (Mantle and Aspinwall 2001; Outeiro et al. 2008; Sharman et al. 2001, 2015; Sridhar et al. 2003)), tool geometries, tool materials, and cutting parameters. This section presents the residual stress distribution induced by several machining operations of different work materials, using several tool materials/geometry and cutting parameters.

Residual Stresses in Difficult-to-Cut Materials

The difficult-to-cut materials are a group of alloys that requires higher cutting energy when compared with low strength alloys (e.g., plain carbon steel). This group includes several alloys used for aerospace and nuclear applications, which can be classified into three major categories: nickel-based alloys (e.g., Inconel), iron-based alloys (e.g., austenitic stainless steels), and titanium-based alloys. As metal cutting is the purposeful fracture of the layer to be removed, not only the strength of the work material but also the strain at fracture should be considered. The product of these two mechanical characteristics indicates the energy that has to be spent in fracturing a unit volume of the work material, allowing chip formation. Due to high strength and fracture strain of such alloys, high cutting forces and heat are generated during their machining. Moreover, their low thermal conductivity and high mechanical and microstructural sensitivity to strain and stress-rate induce mechanical modifications and behavior heterogeneity on the machined surface, and this leads to unstable chip formation and vibrations. Their low thermal conductivity also leads to heat concentration in the cutting zone resulting in high localized temperatures. As a result, machining of such difficult-to-cut alloys when compared with machining of plain carbon steels (see Fig. 4) may induce (i) higher residual stress levels (sometimes reaching more than 1000 MPa at the component’s surface), (ii) larger thickness of the layer affected by tensile residual stress, (iii) high work-hardening rate, and (iv) larger thickness of the work-hardened layer.

Residual Stresses in Machining Operations, Fig. 4

In-depth profiles of the residual stresses generated in turning of AISI 316L steel, AISI 1045 steel, and IN718, using coated and uncoated cemented carbide cutting tools (Outeiro et al. 2006, 2008)

Such high tensile residual stress levels allow cracks to nucleate and grow, and thus decreasing the component’s fatigue life. Moreover, the residual stresses are also responsible for the dimensional instability phenomenon leading to part distortion, which can pose major difficulties during assembly.

Residual Stresses in the Process Chain

Figure 5 shows the residual stress distribution induced by different machining operations used to produce a mold in H13 tool steel. As shown in this figure, the residual stress components of the tensor (parallel to the direction of the feed motion, σ₁₁, and perpendicular to feed motion, σ₂₂) generated during face milling of AISI H13 tool steel are predominantly compressive (see Fig. 5a), where their magnitude at the surface depends on the machining parameters employed. However, they can become tensile after EDM (see Fig. 5b). These residual stresses decrease and may become compressive after manual polishing or grinding (see Fig. 5c and d, respectively).

Residual Stresses in Machining Operations, Fig. 5

In-depth profiles of normal stress components (σ11 and σ22) in AISI H13 tool steel (Outeiro et al. 2007). (a) After annealing and milling. (b) After annealing, milling, quenching, tempering (50 HRC), roughing EDM, and finishing EDM. (c) After annealing, milling, quenching, tempering (50 HRC), and grinding. (d) After annealing, milling, quenching, tempering (50 HRC), roughing EDM, finishing EDM, and manual polishing

Besides, today a progressive replacement of the expensive and time-consuming EDM and manual polish/finish operations by high-speed machining technology is observed. Due to the low efficiency of cutting fluids at high cutting speeds, this technology can be associated with dry or near-dry machining conditions. This imposes new challenges for the proper characterization of the residual in the components’ machined surface layer.

Influence of Cutting Parameters

The influence of the cutting speed (V_c), feed (f), and depth of cut (a_p) on the residual stresses has been investigated by several researchers. The influence of V_c on the residual stresses is not evident. The results show that residual stresses can increase or decrease with V_c depending on the other cutting parameters, tool geometry/material, and work material (Outeiro 2002). Identical behavior is observed with a_p. In this case, surface residual stress remains constant or decreases with a_p (Outeiro 2002). In both cases (V_c and a_p), the thickness of the layer affected by tensile residual stresses slightly decreases with the increases of V_c and a_p (Outeiro et al. 2002). As far as the feed is concerned, its influence on the surface residual stresses seems to be more evident. In general, both the surface residual stresses and the thickness of the layer affected by tensile residual stresses increase with the feed (f) (Capello 2005; Outeiro 2002; Outeiro et al. 2002). Figure 6 shows an example of the in-depth residual stress profiles obtained by turning AISI 316L stainless steel, varying V_c (Fig. 6a), f (Fig. 6b), and a_p (Fig. 6c).

Residual Stresses in Machining Operations, Fig. 6

In-depth circumferential (cutting direction) residual stress profiles for different Vc (a), f (b), and a_p (c) (Outeiro et al. 2002)

Influence of Tool Geometry and Tool Material (Coating)

Tool geometry as well tool material has an important role in the residual stresses generated in the machined surface. Several studies have been conducted to show the influence several tool geometry parameters on the residual stress distribution. For example, an experimental study on turning of three work materials performed by Capello (2005) shows that axial residual stress increases with the tool nose radius (r_ε) and decreases with the tool cutting edge angle (K_r). Another experimental study on turning AISI 316L stainless steel performed by Outeiro et al. (2010) shows that both axial and circumferential residual stresses increase when the cutting edge radius increases up to the value of the uncut chip thickness (40–50 μm), which corresponds to a ratio between the uncut chip thickness and the cutting edge radius equal to 1 (Fig. 7a). For larger cutting edge radius (when this ratio is less than 1), the residual stresses do not change significantly, being almost constant. These residual stresses are largely due to the plowing process. Moreover, Fig. 7b shows the variation of the circumferential residual stresses as a function of the distance from the machined surface. As seen in this figure, the residual stresses are maximal at the machined surface, and then they decrease as the distance from the machined surface increases, stabilizing around the residual stress value found in the work material before machining (in the range of 150–300 MPa in compression). This figure also shows that an increase in the edge radius from 15 to 55 μm causes an increase in the thickness of the tensile layer from 26 to 55 μm.

Residual Stresses in Machining Operations, Fig. 7

Influence of cutting edge radius on residual stresses at the machined surface (orthogonal cutting; AISI 1045; uncoated tungsten carbide; V_c, 175 m/min; h₁, 0.050 mm; dry cutting) (Outeiro et al. 2010)

Concerning to the tool material, Fig. 4b shows that turning IN718 superalloy using coated (TiAlN coating) cutting tool when compared with uncoated tool generates higher thickness of the layer affected by tensile residual stresses, but lower surface residual stress value. As demonstrated by Outeiro et al. (2006), although the coated tool generated slightly less total thermal energy when compared to the uncoated tool, more heat is conducted into the workpiece when the coated tool is used. As a result, high temperatures and thermally affected layers are produced on the machined surface.

Influence of Tool Wear

Figures 8 and 9 show the influence of tool wear on the in-depth residual stress profiles. With the increase of tool wear, the thermal and mechanical phenomena acting in the workpiece become more intense. The result of the mechanical action of tool on the machined surface is similar to the Hertz stress (Brinksmeier et al. 1994). So, as the tool becomes worn, the residual stress maximum below surface increases, and its location shifts further below the surface. Depending on the amount of heat generated and of the thermophysical properties of the tool and workpiece, thermal dilatations can be produced and may enough to plastically deform the superficial layer, which may result in an increase of tensile residual stress (Fig. 8).

Residual Stresses in Machining Operations, Fig. 8

Evolution of the residual stresses in function of the cutting time (Goldstein 1991). (a) New tool, cutting time = 0 min. (b) Worn tool, cutting time = 40 min

Residual Stresses in Machining Operations, Fig. 9

Effect of tool wear on residual stress profile in milling of H13 steel-coated carbide (V_c = 80 m/min, f = 0.15 mm/rev, and a_p = 0.8 mm) (M’Saoubi et al. 2008)

Residual Stress Control in Practical Machining Operations

There is a need to develop a number of feasible means to control the residual stresses during practical machining operations. An example of such control resulted from an extensive study of the residual stresses obtained varying different machining parameters and tool geometry performed by Outeiro (2002) in turning operation of AISI 316L. In this study, the feed, tool nose radius, and tool cutting edge angle were the most influencing parameters on the residual stresses. The feed seems to be the parameter that has the strongest influence on residual stresses. In order to reduce the magnitude of the residual stress, the feed must be kept as low as possible. However, decreasing the feed decreases the material removal rate (keeping the other cutting regime parameters unchanged). Therefore, in order to increase the material removal rate without compromise, the residual stresses (sometimes even improving it) the depth of cut can be increased.

This finding allowed us to introduce the parameter f/p, defined as the ratio between the feed and the depth of cut. This new parameter can be used in the control process of the residual stress. As shown in Fig. 10a, the residual stress increases with the parameter f/p. Therefore, if the objective is to reduce the residual stresses, this parameter must be kept as low as possible.

Residual Stresses in Machining Operations, Fig. 10

Influence of the f/p parameter in surface residual stresses when turning AISI 316L using uncoated tungsten carbide cutting tool (Outeiro 2002)

Another interesting result is related with the equivalent tool cutting edge angle, K_r^eq, defined as the tool cutting edge angle of the equivalent cutting edge. This cutting edge replaces the major and minor cutting edges and the tool nose in the manner shown in Fig. 10b. As formulated by Colwell (1954), this equivalent cutting edge is defined as a straight line that connects the end of the major and minor cutting edges as shown in Fig. 10b. Once the equivalent cutting edge is constructed, the direction of chip flow is assumed to be perpendicular to this edge. Because the f/p parameter and K_r^eq have opposite evolutions, the residual stresses decrease with the K_r^eq angle. Similar results are also obtained by Capello (2005) who showed that residual stresses decrease with the major tool cutting edge angle, κ_r. This suggests that the effect produced by the same cutting tool and different values of feed and depth of cut, therefore, different K_r^eq angles, is equivalent to that produced by identical cutting tools having different κ_r angles.

It is interesting to note that there is a relationship between the residual stresses and the chip side-flow angle (η_s). This angle is defined between the chip flow direction and the direction of the feed motion, and it’s measured on the tool rake face Fig. 11. As seen, the chip side-flow angle depends on the applied cutting conditions, in particular on the feed, depth of cut, tool cutting edge angle, and tool nose radius. These parameters are among those which have strong influence on residual stresses. Therefore, it can be deduced that there is a relationship between the chip side-flow angle and the residual stresses. Indeed, as shown in Fig. 11, the residual stresses increase as the chip side-flow angle increases. As a consequence, the chip side-flow direction can be used to control of residual stresses.

Residual Stresses in Machining Operations, Fig. 11

Relation between chip side-flow angle and residual stresses, when turning AISI 316L using uncoated tungsten carbide cutting tool (Outeiro 2002)

Influence of Residual Stresses in the Functional Performance and Life of Components

As known, compressive residual stresses are beneficial for fatigue endurance. For example, Ghanem et al. (2002) have investigated the role of residual stress state in two operations, namely, hard milling and EDM of a tool steel (SAEJ438b at 30 HRc). Three-point bending fatigue tests of the notched specimens revealed a loss of 35% in fatigue endurance in the case of EDM (Fig. 12).

Residual Stresses in Machining Operations, Fig. 12

Influence of machining on fatigue life (after Ghanem et al. 2002)

In spite of the rather low cutting regime (4 mm diameter HSS cutter at V_c = 30 m/min and f = 0.05 mm/rev) in milling in this test, the effects of residual stress (−300 MPa in milling and +750 MPa in EDM) are obvious.

Cross-References

• Residual Stress (Abrasive Processes)

• Residual Stress (Forming)

• Surface Integrity