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The modified surface properties and fatigue life of Incoloy A286 face-milled at different cutting parameters
Materials Science & Engineering A 704 (2017) 1–9
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
The modified surface properties and fatigue life of Incoloy A286 face-milled at different cutting parameters
MARK
⁎
Guoliang Liua,b, Chuanzhen Huanga,b, , Hongtao Zhua,b, Zhanqiang Liua,b, Yue Liua,b, Chengwu Lic a b c
Center for Advanced Jet Engineering Technologies (CaJET), School of Mechanical Engineering, Shandong University, PR China Key Laboratory of High-efficiency and Clean Mechanical Manufacture (Shandong University), Ministry of Education, PR China Jinan Power Co. Ltd. of China National Heavy Duty Truck Group Co., Ltd., Jinan, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Incoloy A286 Surface integrity Mechanical property Fatigue life Parameter optimization
Incoloy A286 alloy is an iron-base superalloy that widely used in gas turbine jet engines and other similar applications, and the high reliability demand of such applications drew significant attention to the machining induced surface integrity and fatigue life. In this work, the Incoloy A286 was face-milled at different cutting parameters, and the surface properties and fatigue life of the modified surface were measured and analysed. It is found that the cutting parameters can modify the surface integrity and mechanical properties significantly, and further obviously influence the fatigue life of the workpiece. The modified fatigue life under different cutting parameters was mainly determined by the roughness and mechanical properties, including the yield strength and fracture toughness, rather than the residual stresses, because of the severe stress relaxation. The surface yield strength had strong positive correlation with the work hardening, while the surface fracture toughness (KJC) was related to the work hardening formation mechanism and grain refinement. The optimal cutting parameters were recommended according to the surface integrity and fatigue life measurement results at last.
1. Introduction Incoloy A286 alloy is a typical corrosion, oxidation and heat resistant iron-base superalloy, which is widely used in gas turbine jet engines and superchargers, including the turbine wheels and blades, after-burner parts and fasteners, casings, jet engine rotors, and other similar applications [1–3]. In these applications, high reliability and safety of components are of extreme importance, so the good fatigue performance of components under cyclical mechanical and thermal loads is important. It has been proven experimentally for a long time that the fatigue cracks usually initiate from the machined surfaces, and the fatigue performance is highly dependent on the machining induced surface integrity [4,5]. However, several reasons hindered the studies on the effect of cutting operations on the fatigue performance in the last several decades: Firstly, the cost and time consuming of fatigue life test are considerable, resulting in the relatively rare direct fatigue life tests of machined workpiece; Secondly, the grinding or polishing are usually employed to finish the components after cutting operations, the effect of cutting operations on the fatigue performance of the machined components was not taken seriously. However, in recent years, the high speed machining technology has made the cutting operation play more
⁎
and more important role as the finishing process [6]. Because of that, some researchers began to compare the fatigue performance of workpiece from cutting and grinding process, for example, turning vs. grinding [7–10] and milling vs. grinding [11]. It is found in these studies that under the appropriate cutting parameters, the fatigue life of turned or milled workpiece could be longer than the ground ones. The role of cutting operations in determining the fatigue performance has been studied by several researchers in recent years [12–15]. However, these studies were limited in few types of fatigue performance tests, workpiece materials and cutting parameters. When studying the effect of cutting operations on the fatigue performance of the machined components, the internal connection between the surface integrity characteristics and the fatigue performance should be studied thoroughly. However, in the existing research results, there are still many flaws and even contradictions. There is no doubt that the poor surface roughness is detrimental to the fatigue performance because of the stress concentration [16,17]. While some researchers pointed out that the surface roughness value was usually comparatively small in the finishing process, making its effect to be non-significant, especially in the milling process where curved feed marks that may be perpendicular to the early initiation of cracks were
Corresponding author at: Center for Advanced Jet Engineering Technologies (CaJET), School of Mechanical Engineering, Shandong University, PR China. E-mail address: [email protected] (C. Huang).
http://dx.doi.org/10.1016/j.msea.2017.07.072 Received 25 May 2017; Received in revised form 22 July 2017; Accepted 22 July 2017 Available online 23 July 2017 0921-5093/ © 2017 Elsevier B.V. All rights reserved.
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where P is the load, N; m is the Meyer index of this material; D is the diameter of spherical indenter, mm; dt is the diameter of indentation under loading (Seen in Fig. 1(b)), which can be calculated by the formula (1.3), mm:
generated [18]. Residual stress was regarded as the main factor to influence the fatigue performance of the machined surface in many literatures, and it is widely accepted that the compressive residual stress could improve the fatigue performance [7,14,15,19]. Nevertheless, many researchers found that the machining induced residual stress may be released under cyclic load, and the severe residual stress relaxation may weaken the influence of residual stress on the fatigue performance [20–23]. Therefore, the residual stress relaxation must be taken into consideration when analysing the effect of residual stress on the fatigue performance, which is relatively rare in the existing fatigue life researches. The biggest vulnerability and contradiction existed on the effect of work hardening and microstructure evolution. Some researchers believed that the fatigue life could be prolonged by the work hardening and grain refinement, for that the yield strength of the machined surface could be increased along with the increased work hardening [12]. On the other hand, Mantle et al. [24,25] pointed out that the work hardening could reduce the ductility and fracture toughness of the machined surface, and thus was beneficial to the crack propagation for the rough surface. In fact, the above mentioned two influence mechanism may exist simultaneously. The actual yield strength and fracture toughness should be directly measured to reveal which one played a dominant role. However, even the continuous ball indentation technique has been invented to characterize the mechanical properties of the surface layer [26–28], relatively rare studies employed this technique to deeply reveal the true influence mechanism of work hardening and grain refinement. Therefore, it is meaningful to deepen the study of the internal connection between the surface integrity characteristics and the fatigue performance after cutting operations. In the existing studies, there is almost no direct research on the effect of cutting operations on the surface properties and fatigue life of Incoloy A286 alloy, although it is of great importance on its application. In this work, the face-milling experiments on Incoloy A286 alloy at different cutting parameters were conducted, and the surface properties and three points bending fatigue life were tested directly. The effect of cutting parameters on the surface integrity, mechanical properties and its further influence on the fatigue life were deeply investigated by considering the residual stress relaxation, actual yield strength and fracture toughness of the machined surfaces. There is no doubt that the presented results were obtained with specific insert and workpiece material, and there may be some changes with different inserts and workpiece materials. Nonetheless, the influence mechanism showed in this paper is universal. Thus, the present work can signally promote the understanding of the generation mechanism of the surface integrity, mechanical properties and fatigue performance of machined surfaces.
dt = 2 ht D − ht2
where ht is the indentation depth under loading in Fig. 1, mm. Fracture toughness (KJC) was determined on the concept of continuum damage mechanics which is based on the formation of voids beneath the indenter during indentation. Indentation with a small spherical indenter generated concentrated stress field near and beneath the indenter and it is similar to the concentrated stress field ahead of a crack tip. The deformation energy at the center of the impression is hence comparable to that at the front of a crack tip. Therefore, the KJC can be calculated based on the so-called ‘indentation energy to fracture’ (Wf ) by the formula (1.4) [33]:
KJC =
(1.4)
hf
2Wf =
4P S D ⎞ dh = ln ⎛ ∫ πd 2 π D−hf 0
t
⎜
⎟
⎝
⎠
(1.5)
where S is the slope of the load–indentation depth curve; hf is the indentation depth at the fracture initiation point, mm. The determination of hf has been discussed by Lee et al. [33] in detail. 2. Experimental procedures The workpiece used in the experiments was a block of solution and aging treated Incoloy A286 alloy with the size of 110 mm × 50 mm × 110 mm. The microstructure of the workpiece is illustrated in Fig. 2, and its chemical composition and mechanical properties are shown in Tables 1 and 2, respectively. All the face-milling tests were carried out on a vertical CNC machining center DMU-70V (Germany), and the symmetric down milling was conducted on the face of 110 mm × 50 mm, fixing the cutting width (ae) at 50 mm. A single factor experimental design, shown in Table 3, was employed to study the cutting speed (V), the feed rate (f) and the depth of cut (ap), which were selected considering the recommendation in the insert manual and the tool life we got in preexperiment. The TiAlN coated carbide inserts (DIJET, JC8015-ODHW606AEN) were employed to finish the milling experiments, and was mounted on the OTC-06100-32R cutter, with a diameter of 100 mm. Only one new insert was fixed on the cutter each time to avoid the detrimental effect of tool wear and run-out of teeth, and no cutting fluid was used in all the operations. The milling process was also simulated using the professional cutting simulation software Third Wave Systems AdvantEdge FEM (AE) to obtain the cutting forces, cutting zone temperature, cutting zone strain rate, and so on. After finishing the milling process, the surface morphology and roughness were observed using a KEYENCE VK-X200 K laser microscope (Japan). The X-ray diffraction technique was adopted to measure the residual stress on the machined surfaces with an Xstress 3000 stress instrument (Stresstech Oy, Finland). Not only the initial residual stress on the machined surfaces, but also that on several surfaces suffered 40,000 number of fatigue cycles were measured to estimate the residual stress relaxation. The work hardening and grain refinement were indicated by the Vickers hardness and the full width at half maximum (FWHM) from the X-ray diffraction (XRD) (HitachiRAX10AX), respectively. The actual yield strength and fracture toughness (KJC) of the machined surfaces were tested by the continuous ball indentation technique using the SSM-B4000 Stress-Strain Microprobe System (America). The incident software of this system integrates all the above-mentioned processes and can calculate the yield strength and
The basic principle of the continuous ball indentation technique is multiple indentations by a spherical indenter at a single test location on the test sample with intermediate partial unloading [27]. This technology has been developed in the 1980s [29–31], and has been adopted by many researchers to study the mechanical properties of materials [26–28]. The schematic diagram of the indentation load–depth curve and indentation geometry obtained in continuous ball indentation test is shown in Fig. 1. The yield strength and fracture toughness (KJC) can be calculated by following these steps: The yield strength σy can be calculated by the relation [32]: (1.1)
where βm is a coefficient related to the properties of the material; A is a parameter related to the indentation, and can be calculated by the formula (1.2):
P d m −2 = A⎛ t ⎞ 2 dt ⎝D⎠
2EWf
where E is the elastic modulus of the test material, GPa; Wf can be calculated from the indentation load–depth curve as follows:
1.1. Continuous ball indentation technique
σy = βm A
(1.3)
(1.2) 2
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Fig. 1. Schematic diagram of (a) indentation load–depth curve and (b) indentation geometry in continuous ball indentation test.
Table 2 The mechanical properties of A286 used in this investigation. Material
GH2132
Yield strength σ0.2 (MPa)
Hardness
Elongation (%)
Average Grain Size (Grade)
(HRC)
Tensile strength σb (MPa)
830
30
1050
31.0
8
Table 3 The face milling cutting parameters.
Fig. 2. Microstructure of A286 used in this investigation.
KJC automatically. A tungsten carbide spherical indenter with a 0.76 mm diameter was adopted, and the maximum indentation depth was 15% of the diameter. The size of the fatigue life test samples was 110 mm×10 mm×4 mm, and they were cut from the center of the workpieces using the wire cut electrical discharge machining (WEDM). The edges between the surfaces of face-milling and WEDM were manual rounded to eliminate the effect of electro-discharge machining. The three-point bending fatigue life tests were conducted using a servohydraulic fatigue testing machine (Instron 8801, USA) in this work, and face-milled surface was under tensional status. The schematic diagram of the test is shown in Fig. 3. During the test, a dynamic sine wave load was applied on the specimens, and the maximum tension stress on the face-milled surface was 750 MPa. The loading frequency was 7 Hz, and the stress ratio was 0.1. When the specimen fractured in the fatigue life test, the number of cycles was record as its fatigue life. At least three specimens were tested for each cutting condition to eliminate the error. Three polished specimens were tested as a reference.
Parameters
Group Ⅰ
Group Ⅱ
Group Ⅲ
V (m/min) f (mm/tooth)
30, 45, 60, 75, 90 0.15
60 0.15
ap (mm)
0.3
60 0.05, 0.1, 0.15, 0.2, 0.25 0.3
0.1, 0.2, 0.3, 0.4, 0.5
Fig. 3. Schematic diagram of the three points bending fatigue life test.
concentration factor [16,17]. Therefore, the Ra and Rz values were measured in the present work, and summarized in Fig. 4. It can be seen that the feed rate has the most significant effect on the surface roughness, while the depth of cut has a trivial effect in the selected range. When the cutting speed increased, the roughness decreased continuously, although the extent was limited. The roughness increased obviously with the increase of feed rate from 0.1 mm/tooth. There was significant difference on different parts of the machined surface at the feed rate of 0.05 mm/tooth, resulting in the large deviation of the Ra and Rz values. For the normal area, the severe friction and squeezing between the cutting edge and workpiece caused by the little feed rate led to the less obvious tooth marks, and thus the surface was smooth, just as shown in Fig. 5a). However, the excessive friction and squeezing
3. Results It is acknowledged that the surface roughness can influence the fatigue life of the machined surfaces mainly by improving the stress concentration factor, and the amplitude parameters Ra and Rz are the most important roughness indexes in calculating the stress Table 1 The chemical composition of A286 used in this investigation. Element
C
Mn
Cr
Ni
Si
Mo
Cu
Ti
Al
V
Nb
Fe
Content (wt%)
0.042
1.05
14.92
25.01
0.267
1.37
0.042
2.07
0.191
0.298
0.042
Bal
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Fig. 4. The surface roughness at different cutting parameters.
According to the common belief, the work hardening could increase the machined surface yield strength, but reduce its ductility and fracture toughness [12,24,25]. The grain refinement was believed to increase the machined surface yield strength and fracture toughness simultaneously [34,35]. However, it has been found in the above part that the work hardening and grain refinement changed simultaneously. In order to obtain the combined influence, the surface yield strength and fracture toughness (KJC) were measured directly using the continuous ball indentation tests, and the calculated results are shown in Figs. 8 and 9. It can be seen from Fig. 8 that the milling machined surface yield strength was all higher than the polished samples, and showed the same variation trend with the surface Vickers hardness when cutting parameters changed, i. e., decreased firstly and then increased with an increase of each cutting parameter. The KJC testing results in Fig. 9 showed that the trend of KJC was not completely contrary to the surface hardness. When the cutting speed increased, the KJC remained stable at first and finally showed a marginal increase. When the feed rate increased from 0.05 to 0.2 mm/tooth, the KJC decreased significantly. With further increasing of feed rate to 0.25 mm/tooth, the KJC showed an obvious increase. With the increased depth of cut, the fracture toughness decreased overall, no matter how the surface Vickers hardness changed. The milling machined surface residual stresses along two directions, parallel and vertical to the feed direction, at different cutting parameters are shown in Fig. 10. It can be seen that tensile residual stresses
also produced much more chip debris adhering on the machined surface, just as shown in Fig. 5b), seriously damaged the surface roughness. The machined surface work hardening and microstructure changes, especially the grain refinement, are another two important surface integrity characteristics that have important influence on the fatigue life of workpiece. According to the Scherrer formula, the larger FWHM from the XRD atlas stands for the smaller grain size. Although the grain size of this material may be too large to be calculated accurately, the FWHM can be used to compare the grain size as well. The machined surface hardness and FWHM at different cutting parameters are shown in Figs. 6 and 7, respectively. It is obvious that, compared with the polished specimens, the face-milled workpieces all possessed higher hardness and much smaller grain size. Serious work hardening and grain refinement occurred on all of the face-milling machined surfaces. It can also be seen from Fig. 6 that the cutting speed, feed rate and depth of cut have the similar effect on the work-hardening of the machined surface. When any of the three cutting parameters increased, the surface Vickers hardness decreased firstly and then increased, and the surface of the lowest hardness was produced at the cutting speed of 60 m/min, the feed rate of 0.15 mm/tooth or the depth of cut of 0.3 mm. The FWHM shown in Fig. 7 indicated that the grain refinement also decreased firstly and then increased with the increase of cutting speed and feed rate. When the depth of cut increased, the grain size decreased firstly, and then remained stable.
Fig. 5. a) Smooth and b) damaged parts of the machined surface when face milling the Incoloy A286 at f = 0.05 mm/tooth. (Colour should be used in print).
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Fig. 6. The machined surface hardness at different cutting parameters.
was an obviously prolonged fatigue life at the feed rate of 0.25 mm/ tooth. It can be seen from Fig. 11(c) that the fatigue life declined overall with the increased depth of cut.
were generated on all the face-milled surfaces, and the tensile residual stresses increased firstly and then decreased with the increased cutting parameters. It is well known that the residual stress is caused by both mechanical and thermal loads, and typically the mechanical effect could result in compressive stress while the thermal effect could lead to tensile stress. Therefore, the increased tensile residual stresses when the cutting parameters increased firstly were caused by the higher cutting temperature. With further increasing of cutting parameters, the improved plastic strain rate or cutting forces resulted in the lower tensile residual stresses. Many researchers have found that although the machining induced residual stresses rarely relax fully to zero in applications, significant residual stress relaxation will occur when the summation of the residual stress and applied stress reached or exceed the yield strength of the material [20–23]. The residual stresses measurement on the machined surfaces after 40,000 number of fatigue cycles proved that the residual stress relaxation occurred in the early stage of fatigue test, and the remaining stresses on the machined surfaces after fatigue cycles were all about 100 MPa. Therefore, it is reasonably to speculate that the machined residual stresses had an adverse effect on the fatigue life of face-milled workpieces, but had no important influence under different cutting parameters. The relationship between the three points bending fatigue life of the machined workpiece and the cutting parameters is shown in Fig. 11. The fracture surfaces of specimens were checked after the fatigue life tests, and made sure that all the cracks initiated from the machined surface. It is obvious in Fig. 11 that all the face-milled specimens own a shorter fatigue life than the polished ones, and the fatigue life was obviously influenced by the cutting parameters. It can be seen from Fig. 11(a) that with an increased cutting speed, the fatigue life decreased firstly and increased when the cutting speed exceeded 60 m/ min, which is similar with the trend of surface yield strength. At the feed rate of 0.05 mm/tooth in Fig. 11(b), although a relatively long average fatigue life was obtained, the deviation of the fatigue life was large. When the feed rate increased, the fatigue life decreased significantly before the feed rate of 0.25 mm/tooth was reached. There
4. Discussions 4.1. Formation of surface yield strength and KJC The above-mentioned measurement results have shown that the surface yield strength showed the same variation trend with the surface Vickers hardness when cutting parameters changed, while the trend of KJC was not completely contrary to the surface hardness. After analysing the internal formation mechanism of the work hardening, it can be speculated that the KJC was concerned with the grain refinement and formation mechanism of work hardening. It is well-known that the work hardening is mainly resulted from the plastic deformation, which is determined by both mechanical and thermal loads. The simulated cutting forces along three dimensional coordinates are shown in Fig. 12, while the cutting temperature and the plastic strain rate of the cutting zone were also extracted from the simulation models and shown in Figs. 13 and 14, respectively. It can be seen that when the cutting speed increased, the cutting forces remained stable, while the temperature and plastic strain rate of the cutting zone increased overall. It is well known that the high temperature can soften the workpiece, leading the grain easier to deform while harder to break. Therefore, the decreased surface work hardening and grain refinement when the cutting speed increased from 30 to 60 m/min should be attributed to the high temperature caused thermal softening effect. With further increasing of the cutting speed, the dramatic increased plastic strain rate (Seen in Fig. 14 GroupⅠ) strengthened the workpiece, and thus resulted in the increased surface work hardening. When the feed rate increased, the three dimensional cutting forces and cutting temperature increased on the whole, and the slightly declined cutting temperature at the feed rate of 0.25 mm/tooth was caused by the accelerated heat source and larger chips. The increased temperature with the feed rate increased from 0.05 to 0.2 mm/tooth made the surface
Fig. 7. The machined surface FWHM at different cutting parameters.
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Fig. 8. The surface yield strength at different cutting parameters.
materials easier to flow and thus decreased the plastic strain rate significantly. The slightly increased plastic strain rate at the feed rate of 0.25 mm/tooth can be explained by the lower temperature, higher cutting forces and accelerated insert movement. As a result, the serious friction and squeezing caused high plastic strain rate resulted in the serious surface work hardening at the feed rate of 0.05 mm/tooth. When the feed rate firstly increased, the increased cutting temperature and decreased plastic strain rate caused the declined surface Vickers hardness. With further increased feed rate, the increased cutting force led to the increased hardness at the feed rate of 0.2 mm/tooth, while the severe surface work hardening at the feed rate of 0.25 mm/tooth was the combined action of increased cutting force and plastic strain rate. The depth of cut had a constant effect on the cutting forces, cutting temperature and plastic strain rate in the selected range. It can be seen from Figs. 12–14 that the cutting forces and temperature increased overall with the increased depth of cut, while the plastic strain rate decreased steadily. Therefore, the decreased surface Vickers hardness with the depth of cut increased from 0.1 to 0.3 mm was also mainly because of the increased thermal softening effect and decreased strain rate strengthening effect. The increased surface Vickers hardness with further increasing of depth of cut was mainly caused by the higher cutting forces. Fig. 9 has shown that when the cutting speed increased, the KJC remained stable at first and finally showed a marginal increase. According to the above analysis, it is the dramatic increased plastic strain rate that caused the increased surface work hardening and grain refinement with an increase of cutting speed over 60 m/min. Therefore, we can speculate that the strain rate strengthening has a limited adverse effect on the fracture toughness, and the increased KJC was mainly determined by the grain refinement. When the feed rate increased from 0.05 to 0.2 mm/tooth, the cutting forces increased significantly, resulting in the severe plastic strain and decreased KJC. With further increasing of feed rate to 0.25 mm/tooth, the increased work hardening was the common result of cutting forces and plastic strain rate, thus the adverse effect on fracture toughness was limited, and the decreased grain size improved the KJC. It has been described above, with the
increased depth of cut, the grain refinement decreased on the whole and the increased surface Vickers hardness was mainly caused by the higher cutting forces. Therefore, the fracture toughness decreased overall with an increase of depth of cut.
4.2. Effect of modified surface properties on the fatigue life Fig. 11 has shown that all the face-milled specimens own a shorter fatigue life than the polished ones. According to the measuring results of the above-mentioned surface integrity characteristics, the shortened fatigue life of the face-milled workpieces could be contributed to mainly two reasons, i. e., the rough surfaces and the tensile residual stresses. When compared with the polished workpiece which had similar mirror surfaces and the arithmetic average surface roughness (Ra) values were no bigger than 0.02 µm, the surfaces generated with facemilling operations were relatively poor. Nor only the tooth marks, but also some pits may be generated on the machined surfaces, both of which may work as the stress concentration source and increased the risk of crack initiation. Secondly, although significant residual stress relaxation occurred during the fatigue life tests, the tensile residual stresses did not relax fully to zero. Therefore, the remaining tensile stresses on the machined surfaces still could promote the crack initiation and propagation, and thus shorten the fatigue life. As for the changed fatigue life under different cutting parameters, the changed surface roughness and mechanical properties, including yield strength and fracture toughness (KJC), should be the dominant factors in our selected range. It has been found in the surface integrity measurements that the remaining stresses on the machined surfaces after some fatigue cycles had no obvious difference under various cutting parameters, thus its effect could be ignored. During the range of stress fatigue, the smaller gap between the applied stress and yield stress was beneficial to the crack initiation and propagation. When the cutting speed increased from 30 to 60 m/min, the thermal softening effect led the surface yield strength obviously decreased, while the surface fracture toughness remained stable. Thus, the fatigue life decreased because of the declined surface yield strength.
Fig. 9. The surface fracture toughness (KJC) at different cutting parameters.
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Fig. 10. The surface residual stresses at different cutting parameters.
Fig. 11. Fatigue life of the specimens when milled at different (a) cutting speed, (b) feed and (c) depth of cut.
Fig. 12. The simulated three dimensional cutting forces at different cutting parameters.
Fig. 13. The simulated cutting temperature at different cutting parameters.
deviation of the fatigue life at this condition was large, indicating that the poor surface roughness played an obvious adverse influence. When the feed rate increased to 0.15 mm/tooth, the decreased surface yield strength and fracture toughness caused the shortened fatigue life. With further increasing of feed rate to 0.2 mm/tooth, although the surface
With further increasing of cutting speed, the surface roughness decreased and the surface yield strength and KJC increased, jointly prolonged the fatigue life. At the feed rate of 0.05 mm/tooth, although a relatively long average fatigue life was obtained because of the high surface yield strength and KJC, it can be seen from Fig. 11(b) that the 7
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30 m/min
45 m/min
60 m/min Group
75 m/min
90 m/min
0.05 mm/tooth
0.1 mm/tooth
0.15 mm/tooth Group
0.2 mm/tooth
0.25 mm/tooth
0.1 mm
0.2 mm
0.3 mm Group
0.4 mm
0.5 mm
Fig. 14. The plastic strain rate of cutting zone at different cutting parameters (Colour should be used in print).
5. Conclusions
yield strength started to increase, the fracture toughness decreased significantly and the surface roughness values increased obviously, resulting in the continuously declined fatigue life. The prolonged fatigue life at the feed rate of 0.25 mm/tooth should be attributed to the higher yield strength and fracture toughness as well. Fig. 9 has shown that the fracture toughness (KJC) continuously decreased when the depth of cut increased. Therefore, when the surface yield strength decreased as well with the depth of cut increasing from 0.3 to 0.5 mm, the fatigue life declined rapidly. With further increasing of depth of cut, the surface yield strength increased. Therefore, although the fatigue life continued to decrease, the decline rate reduced.
In the present work, face-milling experiments of Incoloy A286 alloy at different cutting parameters were conducted, and the machined surface properties and the three points bending fatigue life of the workpiece were measured. The following conclusions were derived from the investigations: 1. The face-milling process under different cutting parameters can modify the surface integrity and mechanical properties significantly, and further influence the fatigue life of the workpiece. 2. In our selected experiment range, with the increase of the cutting speed, the surface roughness decreased slightly, and the work hardening, grain refinement and surface yield strength all decreased firstly and then increased. The surface fracture toughness increased slightly, and the tensile residual stresses increased firstly and then declined. When the feed rate increased, the surface roughness, work hardening, grain refinement, surface yield strength and fracture toughness all decreased firstly and then increased, but the minimum values obtained at different feed rate. The depth of cut had no obvious effect on the surface roughness. The work hardening and surface yield strength decreased firstly and then increased with an increase of depth of cut, but the grain refinement and surface fracture toughness decreased overall. 3. The surface yield strength had strong positive correlation with the work hardening, while the surface fracture toughness (KJC) was concerned with the grain refinement and formation mechanism of work hardening. The increased surface hardness could be caused by the high cutting forces or severe plastic strain rate. The high cutting forces caused work hardening had an obvious adverse effect on the KJC, while the adverse effect of plastic strain rate caused work hardening was negligible. The grain refinement could increase the
4.3. Optimization of cutting parameters The cutting parameter optimization was conducted according to the surface integrity and fatigue life measurement results in our work. When just considering the fatigue life, the optimal cutting parameter combination should be minimum combination of parameters in our selected range, i.e., the cutting speed of 30 m/min, feed rate of 0.05 mm/tooth and depth of cut of 0.1 mm. However, the surface roughness was poor under the feed rate of 0.05 mm/tooth, thus the optimal feed rate was selected as 0.1 mm/tooth, and the optimal cutting parameter combination became the cutting speed of 30 m/min, feed rate of 0.1 mm/tooth and depth of cut of 0.1 mm. The verification test found that average surface roughness (Ra) value under the optimal cutting parameters was no more than 0.2 µm, the fatigue life of machined workpieces was about 3.9×105 cycles, and the tool life was longer than 30 min. Therefore, the optimal cutting parameters can meet the demand of practical application.
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ijfatigue.2008.01.005. [15] Y. Choi, Influence of feed rate on surface integrity and fatigue performance of machined surfaces, Int. J. Fatigue 78 (2015) 46–52, http://dx.doi.org/10.1016/j. ijfatigue.2015.03.028. [16] D. Arola, C. Williams, Estimating the fatigue stress concentration factor of machined surfaces, Int. J. Fatigue 24 (2002) 923–930, http://dx.doi.org/10.1016/S01421123(02)00012-9. [17] M. Suraratchai, J. Limido, C. Mabru, R. Chieragatti, Modelling the influence of machined surface roughness on the fatigue life of aluminium alloy, Int. J. Fatigue 30 (2008) 2119–2126, http://dx.doi.org/10.1016/j.ijfatigue.2008.06.003. [18] D. Taylor, O.M. Clancy, The fatigue performance of machined surfaces, Fatigue Fract. Eng. Mater. Struct. 14 (1991) 329–336, http://dx.doi.org/10.1111/j.14602695.1991.tb00662.x. [19] A.R.C. Sharman, The effects of machined workpiece surface integrity on the fatigue life of γ -titanium aluminide, Int. J. Mach. Tools Manuf. 41 (2001) 1681–1685, http://dx.doi.org/10.1016/S0890-6955(01)00034-7. [20] I. Altenberger, B. Scholtes, U. Martin, H. Oettel, Cyclic deformation and near surface microstructures of shot peened or deep rolled austenitic stainless steel AISI 304, Mater. Sci. Eng. A. 264 (1999) 1–16, http://dx.doi.org/10.1016/S09215093(98)01121-6. [21] W.Z. Zhuang, G.R. Halford, Investigation of residual stress relaxation under cyclic load, Int. J. Fatigue 23 (2001) 31–37, http://dx.doi.org/10.1016/S0142-1123(01) 00132-3. [22] R.C. Mcclung, A literature survey on the stability and significance of residual stresses during fatigue, Fatigue Fract. Eng. Mater. Struct. 30 (2007) 173–205, http://dx.doi.org/10.1111/j.1460-2695.2007.01102.x. [23] M. Benedetti, V. Fontanari, P. Scardi, C.L.A. Ricardo, M. Bandini, Reverse bending fatigue of shot peened 7075-T651 aluminium alloy: the role of residual stress relaxation, Int. J. Fatigue 31 (2009) 1225–1236, http://dx.doi.org/10.1016/j. ijfatigue.2008.11.017. [24] L. Wagner, J.K. Gregory, Thermomechanical surface treatment of titanium alloys, Mater. Sci. Forum 163–165 (1994) 159–172, http://dx.doi.org/10.4028/www. scientific.net/MSF.163-165.159. [25] A. Mantle, D. Aspinwall, Surface integrity and fatigue life of turned gamma titanium aluminide, J. Mater. Process. Technol. 72 (1997) 413–420, http://dx.doi.org/10. 1016/S0924-0136(97)00204-5. [26] L. Ferranti, R.W. Armstrong, N.N. Thadhani, Elastic / plastic deformation behavior in a continuous ball indentation test, Mater. Sci. Eng. A. 371 (2004) 251–255, http://dx.doi.org/10.1016/j.msea.2003.12.003. [27] G. Das, M. Das, S. Sinha, K. Kumar, Characterization of cast stainless steel weld pools by using ball indentation technique, Mater. Sci. Eng. A. 514 (2009) 389–393, http://dx.doi.org/10.1016/j.msea.2009.02.007. [28] K. Chung, W. Lee, J. Hoon, C. Kim, S. Ho, D. Kwon, K. Chung, Characterization of mechanical properties by indentation tests and FE analysis – validation by application to a weld zone of DP590 steel, Int. J. Solids Struct. 46 (2009) 344–363, http://dx.doi.org/10.1016/j.ijsolstr.2008.08.041. [29] F.M. Haggag Field indentation microprobe for structural integrity evaluation: U.S. Patent 4,852,397. 1989-8-1. [30] F.M. Haggag, R.K. Nanstad, D.N. Braski, Structural integrity evaluation based on an innovative field indentation microprobe, ASME PVP 170 (1989) 101–107. [31] F.Y.M. Haggag, G.E. Lucas, Determination of lüders strains and flow properties in steels from hardness/microhardness tests, Metall. Mater. Trans. A. 14 (1983) 1607–1613, http://dx.doi.org/10.1007/BF02654388. [32] F.M. Haggag, J.A. Wang, Nondestructive detection and assessment of damage in aging aircraft using a novel stress-strain microprobe system, Nondestruct. Eval. Aging Aircr., Airpt., Aerosp. Hardw. 2945 (2008) 217–228, http://dx.doi.org/10. 1117/12.259096. [33] J. Lee, J. Jang, B. Lee, Y. Choi, S. Gun, D. Kwon, An instrumented indentation technique for estimating fracture toughness of ductile materials: a critical indentation energy model based on continuum damage mechanics, Acta Mater. 54 (2006) 1101–1109, http://dx.doi.org/10.1016/j.actamat.2005.10.033. [34] J. Liao, M. Hotta, K. Kondoh, Enhanced impact toughness of magnesium alloy by grain refinement, Scr. Mater. 61 (2009) 208–211, http://dx.doi.org/10.1016/j. scriptamat.2009.03.044. [35] M. Calcagnotto, D. Ponge, D. Raabe, Effect of grain refinement to 1 μm on strength and toughness of dual-phase steels, Mater. Sci. Eng. A. 527 (2010) 7832–7840, http://dx.doi.org/10.1016/j.msea.2010.08.062.
KJC. 4. The fatigue life was influenced by the interactions of the surface properties. The shorter fatigue life of the face-milled workpieces than the polished ones was because of the rough surfaces and tensile residual stresses. The changed fatigue life under different cutting parameters was mainly determined by the roughness and mechanical properties, including the yield strength and fracture toughness (KJC). The residual stresses showed a trivial effect on the fatigue life under different cutting parameters, because of the severe stress relaxation. Acknowledgement This work is financially supported by National Natural Science Foundation of China (51675312) and Taishan Scholars Program (TS20130922). References [1] H. Dehghan, S.M. Abbasi, A. Momeni, A.K. Taheri, On the constitutive modeling and microstructural evolution of hot compressed A286 iron-base superalloy, J. Alloy. Compd. 564 (2013) 13–19, http://dx.doi.org/10.1016/j.jallcom.2013.01. 156. [2] H. De Cicco, M.I. Luppo, L.M. Gribaudo, Microstructural development and creep behavior in A286 superalloy, Mater. Charact. 52 (2004) 85–92, http://dx.doi.org/ 10.1016/j.matchar.2004.03.007. [3] M. Esfandiari, H. Dong, Improving the surface properties of A286 precipitationhardening stainless steel by low-temperature plasma nitriding, Surf. Coat. Technol. 201 (2007) 6189–6196, http://dx.doi.org/10.1016/j.surfcoat.2006.11.013. [4] V.J.A. Reed E C, The Influence of surface residual stress on fatigue limit of titanium, J. Eng. Ind. 82 (1960) 76–78, http://dx.doi.org/10.1115/1.3663004. [5] D. Novovic, R.C. Dewes, D.K. Aspinwall, W. Voice, P. Bowen, The effect of machined topography and integrity on fatigue life, Int. J. Mach. Tools Manuf. 44 (2004) 125–134, http://dx.doi.org/10.1016/j.ijmachtools.2003.10.018. [6] G. Liu, B. Zou, C. Huang, X. Wang, J. Wang, Z. Liu, Tool damage and its effect on the machined surface roughness in high-speed face milling the 17-4PH stainless steel, Int. J. Adv. Manuf. Technol. (2015) 257–264, http://dx.doi.org/10.1007/s00170015-7564-6. [7] S. Smith, S.N. Melkote, E. Lara-Curzio, T.R. Watkins, L. Allard, L. Riester, Effect of surface integrity of hard turned AISI 52100 steel on fatigue performance, Mater. Sci. Eng. A. 459 (2007) 337–346, http://dx.doi.org/10.1016/j.msea.2007.01.011. [8] F. Hashimoto, Y.B. Guo, a.W. Warren, Surface integrity difference between hard turned and ground surfaces and its impact on fatigue life, CIRP Ann. - Manuf. Technol. 55 (2006) 81–84, http://dx.doi.org/10.1016/S0007-8506(07)60371-0. [9] Y.B. Guo, A.W. Warren, The impact of surface integrity by hard turning vs. grinding on fatigue damage mechanisms in rolling contact, Surf. Coat. Technol. 203 (2008) 291–299, http://dx.doi.org/10.1016/j.surfcoat.2008.09.005. [10] A.W. Warren, Y.B. Guo, The Impact of Surface Integrity by Hard Turning vs. Grinding on Rolling Contact Fatigue, ASME 2007 Int. Manuf. Sci. Eng. Conf, (2007), pp. 473–481. [11] Y. Matsumoto, D. Magda, D.W. Hoeppner, T.Y. Kim, Effect of machining processes on the fatigue strength of hardened AISI 4340 Steel, J. Eng. Ind. 113 (1991) 154, http://dx.doi.org/10.1115/1.2899672. [12] H. Sasahara, The effect on fatigue life of residual stress and surface hardness resulting from different cutting conditions of 0.45%C steel, Int. J. Mach. Tools Manuf. 45 (2005) 131–136, http://dx.doi.org/10.1016/j.ijmachtools.2004.08.002. [13] S. Jeelani, M. Musial, Effect of cutting speed and tool rake angle on the fatigue life of 2024-T351 aluminium alloy, Int. J. Fatigue 6 (1984) 169–172, http://dx.doi.org/ 10.1016/0142-1123(84)90034-3. [14] A. Javidi, U. Rieger, W. Eichlseder, The effect of machining on the surface integrity and fatigue life, Int. J. Fatigue 30 (2008) 2050–2055, http://dx.doi.org/10.1016/j.
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The modified surface properties and fatigue life of Incoloy A286 face-milled at different cutting parameters
MARK
⁎
Guoliang Liua,b, Chuanzhen Huanga,b, , Hongtao Zhua,b, Zhanqiang Liua,b, Yue Liua,b, Chengwu Lic a b c
Center for Advanced Jet Engineering Technologies (CaJET), School of Mechanical Engineering, Shandong University, PR China Key Laboratory of High-efficiency and Clean Mechanical Manufacture (Shandong University), Ministry of Education, PR China Jinan Power Co. Ltd. of China National Heavy Duty Truck Group Co., Ltd., Jinan, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Incoloy A286 Surface integrity Mechanical property Fatigue life Parameter optimization
Incoloy A286 alloy is an iron-base superalloy that widely used in gas turbine jet engines and other similar applications, and the high reliability demand of such applications drew significant attention to the machining induced surface integrity and fatigue life. In this work, the Incoloy A286 was face-milled at different cutting parameters, and the surface properties and fatigue life of the modified surface were measured and analysed. It is found that the cutting parameters can modify the surface integrity and mechanical properties significantly, and further obviously influence the fatigue life of the workpiece. The modified fatigue life under different cutting parameters was mainly determined by the roughness and mechanical properties, including the yield strength and fracture toughness, rather than the residual stresses, because of the severe stress relaxation. The surface yield strength had strong positive correlation with the work hardening, while the surface fracture toughness (KJC) was related to the work hardening formation mechanism and grain refinement. The optimal cutting parameters were recommended according to the surface integrity and fatigue life measurement results at last.
1. Introduction Incoloy A286 alloy is a typical corrosion, oxidation and heat resistant iron-base superalloy, which is widely used in gas turbine jet engines and superchargers, including the turbine wheels and blades, after-burner parts and fasteners, casings, jet engine rotors, and other similar applications [1–3]. In these applications, high reliability and safety of components are of extreme importance, so the good fatigue performance of components under cyclical mechanical and thermal loads is important. It has been proven experimentally for a long time that the fatigue cracks usually initiate from the machined surfaces, and the fatigue performance is highly dependent on the machining induced surface integrity [4,5]. However, several reasons hindered the studies on the effect of cutting operations on the fatigue performance in the last several decades: Firstly, the cost and time consuming of fatigue life test are considerable, resulting in the relatively rare direct fatigue life tests of machined workpiece; Secondly, the grinding or polishing are usually employed to finish the components after cutting operations, the effect of cutting operations on the fatigue performance of the machined components was not taken seriously. However, in recent years, the high speed machining technology has made the cutting operation play more
⁎
and more important role as the finishing process [6]. Because of that, some researchers began to compare the fatigue performance of workpiece from cutting and grinding process, for example, turning vs. grinding [7–10] and milling vs. grinding [11]. It is found in these studies that under the appropriate cutting parameters, the fatigue life of turned or milled workpiece could be longer than the ground ones. The role of cutting operations in determining the fatigue performance has been studied by several researchers in recent years [12–15]. However, these studies were limited in few types of fatigue performance tests, workpiece materials and cutting parameters. When studying the effect of cutting operations on the fatigue performance of the machined components, the internal connection between the surface integrity characteristics and the fatigue performance should be studied thoroughly. However, in the existing research results, there are still many flaws and even contradictions. There is no doubt that the poor surface roughness is detrimental to the fatigue performance because of the stress concentration [16,17]. While some researchers pointed out that the surface roughness value was usually comparatively small in the finishing process, making its effect to be non-significant, especially in the milling process where curved feed marks that may be perpendicular to the early initiation of cracks were
Corresponding author at: Center for Advanced Jet Engineering Technologies (CaJET), School of Mechanical Engineering, Shandong University, PR China. E-mail address: [email protected] (C. Huang).
http://dx.doi.org/10.1016/j.msea.2017.07.072 Received 25 May 2017; Received in revised form 22 July 2017; Accepted 22 July 2017 Available online 23 July 2017 0921-5093/ © 2017 Elsevier B.V. All rights reserved.
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where P is the load, N; m is the Meyer index of this material; D is the diameter of spherical indenter, mm; dt is the diameter of indentation under loading (Seen in Fig. 1(b)), which can be calculated by the formula (1.3), mm:
generated [18]. Residual stress was regarded as the main factor to influence the fatigue performance of the machined surface in many literatures, and it is widely accepted that the compressive residual stress could improve the fatigue performance [7,14,15,19]. Nevertheless, many researchers found that the machining induced residual stress may be released under cyclic load, and the severe residual stress relaxation may weaken the influence of residual stress on the fatigue performance [20–23]. Therefore, the residual stress relaxation must be taken into consideration when analysing the effect of residual stress on the fatigue performance, which is relatively rare in the existing fatigue life researches. The biggest vulnerability and contradiction existed on the effect of work hardening and microstructure evolution. Some researchers believed that the fatigue life could be prolonged by the work hardening and grain refinement, for that the yield strength of the machined surface could be increased along with the increased work hardening [12]. On the other hand, Mantle et al. [24,25] pointed out that the work hardening could reduce the ductility and fracture toughness of the machined surface, and thus was beneficial to the crack propagation for the rough surface. In fact, the above mentioned two influence mechanism may exist simultaneously. The actual yield strength and fracture toughness should be directly measured to reveal which one played a dominant role. However, even the continuous ball indentation technique has been invented to characterize the mechanical properties of the surface layer [26–28], relatively rare studies employed this technique to deeply reveal the true influence mechanism of work hardening and grain refinement. Therefore, it is meaningful to deepen the study of the internal connection between the surface integrity characteristics and the fatigue performance after cutting operations. In the existing studies, there is almost no direct research on the effect of cutting operations on the surface properties and fatigue life of Incoloy A286 alloy, although it is of great importance on its application. In this work, the face-milling experiments on Incoloy A286 alloy at different cutting parameters were conducted, and the surface properties and three points bending fatigue life were tested directly. The effect of cutting parameters on the surface integrity, mechanical properties and its further influence on the fatigue life were deeply investigated by considering the residual stress relaxation, actual yield strength and fracture toughness of the machined surfaces. There is no doubt that the presented results were obtained with specific insert and workpiece material, and there may be some changes with different inserts and workpiece materials. Nonetheless, the influence mechanism showed in this paper is universal. Thus, the present work can signally promote the understanding of the generation mechanism of the surface integrity, mechanical properties and fatigue performance of machined surfaces.
dt = 2 ht D − ht2
where ht is the indentation depth under loading in Fig. 1, mm. Fracture toughness (KJC) was determined on the concept of continuum damage mechanics which is based on the formation of voids beneath the indenter during indentation. Indentation with a small spherical indenter generated concentrated stress field near and beneath the indenter and it is similar to the concentrated stress field ahead of a crack tip. The deformation energy at the center of the impression is hence comparable to that at the front of a crack tip. Therefore, the KJC can be calculated based on the so-called ‘indentation energy to fracture’ (Wf ) by the formula (1.4) [33]:
KJC =
(1.4)
hf
2Wf =
4P S D ⎞ dh = ln ⎛ ∫ πd 2 π D−hf 0
t
⎜
⎟
⎝
⎠
(1.5)
where S is the slope of the load–indentation depth curve; hf is the indentation depth at the fracture initiation point, mm. The determination of hf has been discussed by Lee et al. [33] in detail. 2. Experimental procedures The workpiece used in the experiments was a block of solution and aging treated Incoloy A286 alloy with the size of 110 mm × 50 mm × 110 mm. The microstructure of the workpiece is illustrated in Fig. 2, and its chemical composition and mechanical properties are shown in Tables 1 and 2, respectively. All the face-milling tests were carried out on a vertical CNC machining center DMU-70V (Germany), and the symmetric down milling was conducted on the face of 110 mm × 50 mm, fixing the cutting width (ae) at 50 mm. A single factor experimental design, shown in Table 3, was employed to study the cutting speed (V), the feed rate (f) and the depth of cut (ap), which were selected considering the recommendation in the insert manual and the tool life we got in preexperiment. The TiAlN coated carbide inserts (DIJET, JC8015-ODHW606AEN) were employed to finish the milling experiments, and was mounted on the OTC-06100-32R cutter, with a diameter of 100 mm. Only one new insert was fixed on the cutter each time to avoid the detrimental effect of tool wear and run-out of teeth, and no cutting fluid was used in all the operations. The milling process was also simulated using the professional cutting simulation software Third Wave Systems AdvantEdge FEM (AE) to obtain the cutting forces, cutting zone temperature, cutting zone strain rate, and so on. After finishing the milling process, the surface morphology and roughness were observed using a KEYENCE VK-X200 K laser microscope (Japan). The X-ray diffraction technique was adopted to measure the residual stress on the machined surfaces with an Xstress 3000 stress instrument (Stresstech Oy, Finland). Not only the initial residual stress on the machined surfaces, but also that on several surfaces suffered 40,000 number of fatigue cycles were measured to estimate the residual stress relaxation. The work hardening and grain refinement were indicated by the Vickers hardness and the full width at half maximum (FWHM) from the X-ray diffraction (XRD) (HitachiRAX10AX), respectively. The actual yield strength and fracture toughness (KJC) of the machined surfaces were tested by the continuous ball indentation technique using the SSM-B4000 Stress-Strain Microprobe System (America). The incident software of this system integrates all the above-mentioned processes and can calculate the yield strength and
The basic principle of the continuous ball indentation technique is multiple indentations by a spherical indenter at a single test location on the test sample with intermediate partial unloading [27]. This technology has been developed in the 1980s [29–31], and has been adopted by many researchers to study the mechanical properties of materials [26–28]. The schematic diagram of the indentation load–depth curve and indentation geometry obtained in continuous ball indentation test is shown in Fig. 1. The yield strength and fracture toughness (KJC) can be calculated by following these steps: The yield strength σy can be calculated by the relation [32]: (1.1)
where βm is a coefficient related to the properties of the material; A is a parameter related to the indentation, and can be calculated by the formula (1.2):
P d m −2 = A⎛ t ⎞ 2 dt ⎝D⎠
2EWf
where E is the elastic modulus of the test material, GPa; Wf can be calculated from the indentation load–depth curve as follows:
1.1. Continuous ball indentation technique
σy = βm A
(1.3)
(1.2) 2
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Fig. 1. Schematic diagram of (a) indentation load–depth curve and (b) indentation geometry in continuous ball indentation test.
Table 2 The mechanical properties of A286 used in this investigation. Material
GH2132
Yield strength σ0.2 (MPa)
Hardness
Elongation (%)
Average Grain Size (Grade)
(HRC)
Tensile strength σb (MPa)
830
30
1050
31.0
8
Table 3 The face milling cutting parameters.
Fig. 2. Microstructure of A286 used in this investigation.
KJC automatically. A tungsten carbide spherical indenter with a 0.76 mm diameter was adopted, and the maximum indentation depth was 15% of the diameter. The size of the fatigue life test samples was 110 mm×10 mm×4 mm, and they were cut from the center of the workpieces using the wire cut electrical discharge machining (WEDM). The edges between the surfaces of face-milling and WEDM were manual rounded to eliminate the effect of electro-discharge machining. The three-point bending fatigue life tests were conducted using a servohydraulic fatigue testing machine (Instron 8801, USA) in this work, and face-milled surface was under tensional status. The schematic diagram of the test is shown in Fig. 3. During the test, a dynamic sine wave load was applied on the specimens, and the maximum tension stress on the face-milled surface was 750 MPa. The loading frequency was 7 Hz, and the stress ratio was 0.1. When the specimen fractured in the fatigue life test, the number of cycles was record as its fatigue life. At least three specimens were tested for each cutting condition to eliminate the error. Three polished specimens were tested as a reference.
Parameters
Group Ⅰ
Group Ⅱ
Group Ⅲ
V (m/min) f (mm/tooth)
30, 45, 60, 75, 90 0.15
60 0.15
ap (mm)
0.3
60 0.05, 0.1, 0.15, 0.2, 0.25 0.3
0.1, 0.2, 0.3, 0.4, 0.5
Fig. 3. Schematic diagram of the three points bending fatigue life test.
concentration factor [16,17]. Therefore, the Ra and Rz values were measured in the present work, and summarized in Fig. 4. It can be seen that the feed rate has the most significant effect on the surface roughness, while the depth of cut has a trivial effect in the selected range. When the cutting speed increased, the roughness decreased continuously, although the extent was limited. The roughness increased obviously with the increase of feed rate from 0.1 mm/tooth. There was significant difference on different parts of the machined surface at the feed rate of 0.05 mm/tooth, resulting in the large deviation of the Ra and Rz values. For the normal area, the severe friction and squeezing between the cutting edge and workpiece caused by the little feed rate led to the less obvious tooth marks, and thus the surface was smooth, just as shown in Fig. 5a). However, the excessive friction and squeezing
3. Results It is acknowledged that the surface roughness can influence the fatigue life of the machined surfaces mainly by improving the stress concentration factor, and the amplitude parameters Ra and Rz are the most important roughness indexes in calculating the stress Table 1 The chemical composition of A286 used in this investigation. Element
C
Mn
Cr
Ni
Si
Mo
Cu
Ti
Al
V
Nb
Fe
Content (wt%)
0.042
1.05
14.92
25.01
0.267
1.37
0.042
2.07
0.191
0.298
0.042
Bal
3
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Fig. 4. The surface roughness at different cutting parameters.
According to the common belief, the work hardening could increase the machined surface yield strength, but reduce its ductility and fracture toughness [12,24,25]. The grain refinement was believed to increase the machined surface yield strength and fracture toughness simultaneously [34,35]. However, it has been found in the above part that the work hardening and grain refinement changed simultaneously. In order to obtain the combined influence, the surface yield strength and fracture toughness (KJC) were measured directly using the continuous ball indentation tests, and the calculated results are shown in Figs. 8 and 9. It can be seen from Fig. 8 that the milling machined surface yield strength was all higher than the polished samples, and showed the same variation trend with the surface Vickers hardness when cutting parameters changed, i. e., decreased firstly and then increased with an increase of each cutting parameter. The KJC testing results in Fig. 9 showed that the trend of KJC was not completely contrary to the surface hardness. When the cutting speed increased, the KJC remained stable at first and finally showed a marginal increase. When the feed rate increased from 0.05 to 0.2 mm/tooth, the KJC decreased significantly. With further increasing of feed rate to 0.25 mm/tooth, the KJC showed an obvious increase. With the increased depth of cut, the fracture toughness decreased overall, no matter how the surface Vickers hardness changed. The milling machined surface residual stresses along two directions, parallel and vertical to the feed direction, at different cutting parameters are shown in Fig. 10. It can be seen that tensile residual stresses
also produced much more chip debris adhering on the machined surface, just as shown in Fig. 5b), seriously damaged the surface roughness. The machined surface work hardening and microstructure changes, especially the grain refinement, are another two important surface integrity characteristics that have important influence on the fatigue life of workpiece. According to the Scherrer formula, the larger FWHM from the XRD atlas stands for the smaller grain size. Although the grain size of this material may be too large to be calculated accurately, the FWHM can be used to compare the grain size as well. The machined surface hardness and FWHM at different cutting parameters are shown in Figs. 6 and 7, respectively. It is obvious that, compared with the polished specimens, the face-milled workpieces all possessed higher hardness and much smaller grain size. Serious work hardening and grain refinement occurred on all of the face-milling machined surfaces. It can also be seen from Fig. 6 that the cutting speed, feed rate and depth of cut have the similar effect on the work-hardening of the machined surface. When any of the three cutting parameters increased, the surface Vickers hardness decreased firstly and then increased, and the surface of the lowest hardness was produced at the cutting speed of 60 m/min, the feed rate of 0.15 mm/tooth or the depth of cut of 0.3 mm. The FWHM shown in Fig. 7 indicated that the grain refinement also decreased firstly and then increased with the increase of cutting speed and feed rate. When the depth of cut increased, the grain size decreased firstly, and then remained stable.
Fig. 5. a) Smooth and b) damaged parts of the machined surface when face milling the Incoloy A286 at f = 0.05 mm/tooth. (Colour should be used in print).
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Fig. 6. The machined surface hardness at different cutting parameters.
was an obviously prolonged fatigue life at the feed rate of 0.25 mm/ tooth. It can be seen from Fig. 11(c) that the fatigue life declined overall with the increased depth of cut.
were generated on all the face-milled surfaces, and the tensile residual stresses increased firstly and then decreased with the increased cutting parameters. It is well known that the residual stress is caused by both mechanical and thermal loads, and typically the mechanical effect could result in compressive stress while the thermal effect could lead to tensile stress. Therefore, the increased tensile residual stresses when the cutting parameters increased firstly were caused by the higher cutting temperature. With further increasing of cutting parameters, the improved plastic strain rate or cutting forces resulted in the lower tensile residual stresses. Many researchers have found that although the machining induced residual stresses rarely relax fully to zero in applications, significant residual stress relaxation will occur when the summation of the residual stress and applied stress reached or exceed the yield strength of the material [20–23]. The residual stresses measurement on the machined surfaces after 40,000 number of fatigue cycles proved that the residual stress relaxation occurred in the early stage of fatigue test, and the remaining stresses on the machined surfaces after fatigue cycles were all about 100 MPa. Therefore, it is reasonably to speculate that the machined residual stresses had an adverse effect on the fatigue life of face-milled workpieces, but had no important influence under different cutting parameters. The relationship between the three points bending fatigue life of the machined workpiece and the cutting parameters is shown in Fig. 11. The fracture surfaces of specimens were checked after the fatigue life tests, and made sure that all the cracks initiated from the machined surface. It is obvious in Fig. 11 that all the face-milled specimens own a shorter fatigue life than the polished ones, and the fatigue life was obviously influenced by the cutting parameters. It can be seen from Fig. 11(a) that with an increased cutting speed, the fatigue life decreased firstly and increased when the cutting speed exceeded 60 m/ min, which is similar with the trend of surface yield strength. At the feed rate of 0.05 mm/tooth in Fig. 11(b), although a relatively long average fatigue life was obtained, the deviation of the fatigue life was large. When the feed rate increased, the fatigue life decreased significantly before the feed rate of 0.25 mm/tooth was reached. There
4. Discussions 4.1. Formation of surface yield strength and KJC The above-mentioned measurement results have shown that the surface yield strength showed the same variation trend with the surface Vickers hardness when cutting parameters changed, while the trend of KJC was not completely contrary to the surface hardness. After analysing the internal formation mechanism of the work hardening, it can be speculated that the KJC was concerned with the grain refinement and formation mechanism of work hardening. It is well-known that the work hardening is mainly resulted from the plastic deformation, which is determined by both mechanical and thermal loads. The simulated cutting forces along three dimensional coordinates are shown in Fig. 12, while the cutting temperature and the plastic strain rate of the cutting zone were also extracted from the simulation models and shown in Figs. 13 and 14, respectively. It can be seen that when the cutting speed increased, the cutting forces remained stable, while the temperature and plastic strain rate of the cutting zone increased overall. It is well known that the high temperature can soften the workpiece, leading the grain easier to deform while harder to break. Therefore, the decreased surface work hardening and grain refinement when the cutting speed increased from 30 to 60 m/min should be attributed to the high temperature caused thermal softening effect. With further increasing of the cutting speed, the dramatic increased plastic strain rate (Seen in Fig. 14 GroupⅠ) strengthened the workpiece, and thus resulted in the increased surface work hardening. When the feed rate increased, the three dimensional cutting forces and cutting temperature increased on the whole, and the slightly declined cutting temperature at the feed rate of 0.25 mm/tooth was caused by the accelerated heat source and larger chips. The increased temperature with the feed rate increased from 0.05 to 0.2 mm/tooth made the surface
Fig. 7. The machined surface FWHM at different cutting parameters.
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Fig. 8. The surface yield strength at different cutting parameters.
materials easier to flow and thus decreased the plastic strain rate significantly. The slightly increased plastic strain rate at the feed rate of 0.25 mm/tooth can be explained by the lower temperature, higher cutting forces and accelerated insert movement. As a result, the serious friction and squeezing caused high plastic strain rate resulted in the serious surface work hardening at the feed rate of 0.05 mm/tooth. When the feed rate firstly increased, the increased cutting temperature and decreased plastic strain rate caused the declined surface Vickers hardness. With further increased feed rate, the increased cutting force led to the increased hardness at the feed rate of 0.2 mm/tooth, while the severe surface work hardening at the feed rate of 0.25 mm/tooth was the combined action of increased cutting force and plastic strain rate. The depth of cut had a constant effect on the cutting forces, cutting temperature and plastic strain rate in the selected range. It can be seen from Figs. 12–14 that the cutting forces and temperature increased overall with the increased depth of cut, while the plastic strain rate decreased steadily. Therefore, the decreased surface Vickers hardness with the depth of cut increased from 0.1 to 0.3 mm was also mainly because of the increased thermal softening effect and decreased strain rate strengthening effect. The increased surface Vickers hardness with further increasing of depth of cut was mainly caused by the higher cutting forces. Fig. 9 has shown that when the cutting speed increased, the KJC remained stable at first and finally showed a marginal increase. According to the above analysis, it is the dramatic increased plastic strain rate that caused the increased surface work hardening and grain refinement with an increase of cutting speed over 60 m/min. Therefore, we can speculate that the strain rate strengthening has a limited adverse effect on the fracture toughness, and the increased KJC was mainly determined by the grain refinement. When the feed rate increased from 0.05 to 0.2 mm/tooth, the cutting forces increased significantly, resulting in the severe plastic strain and decreased KJC. With further increasing of feed rate to 0.25 mm/tooth, the increased work hardening was the common result of cutting forces and plastic strain rate, thus the adverse effect on fracture toughness was limited, and the decreased grain size improved the KJC. It has been described above, with the
increased depth of cut, the grain refinement decreased on the whole and the increased surface Vickers hardness was mainly caused by the higher cutting forces. Therefore, the fracture toughness decreased overall with an increase of depth of cut.
4.2. Effect of modified surface properties on the fatigue life Fig. 11 has shown that all the face-milled specimens own a shorter fatigue life than the polished ones. According to the measuring results of the above-mentioned surface integrity characteristics, the shortened fatigue life of the face-milled workpieces could be contributed to mainly two reasons, i. e., the rough surfaces and the tensile residual stresses. When compared with the polished workpiece which had similar mirror surfaces and the arithmetic average surface roughness (Ra) values were no bigger than 0.02 µm, the surfaces generated with facemilling operations were relatively poor. Nor only the tooth marks, but also some pits may be generated on the machined surfaces, both of which may work as the stress concentration source and increased the risk of crack initiation. Secondly, although significant residual stress relaxation occurred during the fatigue life tests, the tensile residual stresses did not relax fully to zero. Therefore, the remaining tensile stresses on the machined surfaces still could promote the crack initiation and propagation, and thus shorten the fatigue life. As for the changed fatigue life under different cutting parameters, the changed surface roughness and mechanical properties, including yield strength and fracture toughness (KJC), should be the dominant factors in our selected range. It has been found in the surface integrity measurements that the remaining stresses on the machined surfaces after some fatigue cycles had no obvious difference under various cutting parameters, thus its effect could be ignored. During the range of stress fatigue, the smaller gap between the applied stress and yield stress was beneficial to the crack initiation and propagation. When the cutting speed increased from 30 to 60 m/min, the thermal softening effect led the surface yield strength obviously decreased, while the surface fracture toughness remained stable. Thus, the fatigue life decreased because of the declined surface yield strength.
Fig. 9. The surface fracture toughness (KJC) at different cutting parameters.
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Fig. 10. The surface residual stresses at different cutting parameters.
Fig. 11. Fatigue life of the specimens when milled at different (a) cutting speed, (b) feed and (c) depth of cut.
Fig. 12. The simulated three dimensional cutting forces at different cutting parameters.
Fig. 13. The simulated cutting temperature at different cutting parameters.
deviation of the fatigue life at this condition was large, indicating that the poor surface roughness played an obvious adverse influence. When the feed rate increased to 0.15 mm/tooth, the decreased surface yield strength and fracture toughness caused the shortened fatigue life. With further increasing of feed rate to 0.2 mm/tooth, although the surface
With further increasing of cutting speed, the surface roughness decreased and the surface yield strength and KJC increased, jointly prolonged the fatigue life. At the feed rate of 0.05 mm/tooth, although a relatively long average fatigue life was obtained because of the high surface yield strength and KJC, it can be seen from Fig. 11(b) that the 7
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30 m/min
45 m/min
60 m/min Group
75 m/min
90 m/min
0.05 mm/tooth
0.1 mm/tooth
0.15 mm/tooth Group
0.2 mm/tooth
0.25 mm/tooth
0.1 mm
0.2 mm
0.3 mm Group
0.4 mm
0.5 mm
Fig. 14. The plastic strain rate of cutting zone at different cutting parameters (Colour should be used in print).
5. Conclusions
yield strength started to increase, the fracture toughness decreased significantly and the surface roughness values increased obviously, resulting in the continuously declined fatigue life. The prolonged fatigue life at the feed rate of 0.25 mm/tooth should be attributed to the higher yield strength and fracture toughness as well. Fig. 9 has shown that the fracture toughness (KJC) continuously decreased when the depth of cut increased. Therefore, when the surface yield strength decreased as well with the depth of cut increasing from 0.3 to 0.5 mm, the fatigue life declined rapidly. With further increasing of depth of cut, the surface yield strength increased. Therefore, although the fatigue life continued to decrease, the decline rate reduced.
In the present work, face-milling experiments of Incoloy A286 alloy at different cutting parameters were conducted, and the machined surface properties and the three points bending fatigue life of the workpiece were measured. The following conclusions were derived from the investigations: 1. The face-milling process under different cutting parameters can modify the surface integrity and mechanical properties significantly, and further influence the fatigue life of the workpiece. 2. In our selected experiment range, with the increase of the cutting speed, the surface roughness decreased slightly, and the work hardening, grain refinement and surface yield strength all decreased firstly and then increased. The surface fracture toughness increased slightly, and the tensile residual stresses increased firstly and then declined. When the feed rate increased, the surface roughness, work hardening, grain refinement, surface yield strength and fracture toughness all decreased firstly and then increased, but the minimum values obtained at different feed rate. The depth of cut had no obvious effect on the surface roughness. The work hardening and surface yield strength decreased firstly and then increased with an increase of depth of cut, but the grain refinement and surface fracture toughness decreased overall. 3. The surface yield strength had strong positive correlation with the work hardening, while the surface fracture toughness (KJC) was concerned with the grain refinement and formation mechanism of work hardening. The increased surface hardness could be caused by the high cutting forces or severe plastic strain rate. The high cutting forces caused work hardening had an obvious adverse effect on the KJC, while the adverse effect of plastic strain rate caused work hardening was negligible. The grain refinement could increase the
4.3. Optimization of cutting parameters The cutting parameter optimization was conducted according to the surface integrity and fatigue life measurement results in our work. When just considering the fatigue life, the optimal cutting parameter combination should be minimum combination of parameters in our selected range, i.e., the cutting speed of 30 m/min, feed rate of 0.05 mm/tooth and depth of cut of 0.1 mm. However, the surface roughness was poor under the feed rate of 0.05 mm/tooth, thus the optimal feed rate was selected as 0.1 mm/tooth, and the optimal cutting parameter combination became the cutting speed of 30 m/min, feed rate of 0.1 mm/tooth and depth of cut of 0.1 mm. The verification test found that average surface roughness (Ra) value under the optimal cutting parameters was no more than 0.2 µm, the fatigue life of machined workpieces was about 3.9×105 cycles, and the tool life was longer than 30 min. Therefore, the optimal cutting parameters can meet the demand of practical application.
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