A detailed investigation of residual stresses after milling Inconel 718 using typical production parameters for assessment of affected depth

Materials Today Communications 24 (2020) 100958

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A detailed investigation of residual stresses after milling Inconel 718 using typical production parameters for assessment of affected depth

T

Jonas Holmberga,b,*, Anders Wretlandc, Johan Berglunda, Tomas Benob a

RISE IVF AB, Argongatan 30, 431 53 Mölndal, Sweden University West, Production Technology, 461 86 Trollhättan, Sweden c GKN Aerospace Sweden AB, 461 81 Trollhättan, Sweden b

A R T I C LE I N FO

A B S T R A C T

Keywords: High speed milling Material removal rate Allowance determination Alloy 718 Surface integrity Residual stresses

Production of superalloy gas turbine parts involves time consuming milling operations typically performed in a sequence from rough to finish milling. Rough milling using ceramic inserts allows high removal rates but causes severe sub-surface impact. A relatively large allowance is therefore left for subsequent cemented carbide milling. With increased knowledge of the affected depth it will be possible to reduce the machining allowance and increase efficiency of the manufacturing process. Milling Inconel 718 using typical production parameters has been investigated using new and worn ceramic and cemented carbide inserts. Residual stresses in a milled slot were measured by x-ray diffraction. Stresses were measured laterally across the slot and below the surface, to study the depth affected by milling. The most important result from this work is the development of a framework concerning how to evaluate the affected depth for a milling operation. The evaluation of a single milled slot shows great potential for determining the optimum allowance for machining. Our results show that the residual stresses are greatly affected by the ceramic and cemented carbide milling; both regarding depth as well as distribution across the milled slot. It has been shown that it is important to consider that the stresses across a milled slot are the highest in the center of the slot and gradually decrease toward the edges. Different inserts, ceramic and cemented carbide, and tool wear, alter how the stresses are distributed across the slot and the affected depth.

1. Introduction The choice of alloy for gas turbine components is given by the combined requirements for low weight and high mechanical strength at high temperatures. Unfortunately, these requirements also render the alloys difficult to machine. This becomes a production challenge since many of these components typically involve removal of 60–80 % of the starting material weight. Hence, there is a significant driver for process development that aims to increase material removal rates (MRR) using more efficient processes, such as high-speed milling. This is of particular interest when it comes to components for the hot-aft-section of engines. These components are typically made from nickel-based alloys like Inconel 718, which is known to be difficult to machine even when milling with ceramic inserts. Machining of gas turbine components is typically performed in a number of different operations, from rough milling to finish milling.

Ceramic milling, commonly used for rough machining, efficiently removes material but has the disadvantage of inducing high tensile residual stresses and surface deformation. The initial rough machining is done to leave a substantial dimensional allowance with respect to the final component. This is to ensure the correct microstructure is obtained after the primary forming and shaping operations [1,2]. A small part of the original allowance is reserved for finish machining to meet geometry tolerances and surface integrity requirements. Engineering requirements are not the only reason for the time-consuming machining operations. The high strength of superalloys causes high cutting forces that act close to the cutting edge of the tool. The situation is worsened because these materials do not soften as other materials during machining due to their inherent strength at elevated temperatures. Shallow depths of cut limit the mechanical loads on the cutting edge, but still cause high wear rates and also reduce the overall MRR. Superalloys are also prone to significant strain hardening which results in

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Corresponding author at: RISE IVF AB, Argongatan 30, 431 53 Mölndal, Sweden. E-mail addresses: [email protected], [email protected] (J. Holmberg), [email protected] (A. Wretland), [email protected] (J. Berglund), [email protected] (T. Beno). https://doi.org/10.1016/j.mtcomm.2020.100958 Received 25 September 2019; Received in revised form 7 January 2020; Accepted 23 January 2020 Available online 24 January 2020 2352-4928/ © 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/BY/4.0/).

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with an inner TiAlN coating and an outer TiN coating. The milling tests were performed both dry and using cutting media applied by minimum quantity lubrication (MQL) [17]. Lubrication improved the process in terms of prolonged tool-life by reduced cutting force. Tool-life investigations were also reported by Costes et al. for turning using round inserts of solid CBN and CBN brazed on a tungsten carbide insert [18]. The results showed that a CBN content of 45–60 % suffered less wear for cutting speeds in the interval of 250–300 m/min. Evaluation of tool life during end milling has also been reported, e.g. Alauddin et al. published results from dry end milling tests using uncoated cemented carbide inserts [19,20]. It was shown that full immersion during slot milling and complete tool engagement during cutting increased the tool life compared to half-immersion. In regards to surface deformation induced by machining, Chen et al. reported highly deformed surfaces after broaching Inconel 718 [21]. It was found that the deformation resulted in recrystallization and nanocrystalline grains, which alter the material properties compared to the undeformed material. The temperatures involved in broaching are lower and the process is much slower in comparison to milling, which may influence the recrystallisation somewhat differently. As shown by Chen et al. the material properties of the deformed layer are altered after broaching, lowering the Young’s modulus by 10 %. Even though ceramic milling induces deformation and high tensile stresses in the surface, the benefits of a high MRR makes this machining method preferable for industrial applications. However, to use its full potential, increased knowledge about how stresses are induced in, and beneath, the surface is needed. With such knowledge, the machining allowance can be optimized based on a scientific approach rather than on “best practice”. Only then can the optimized and minimized machining allowance for finish machining be set. As indicated, there is limited research on high speed milling of Inconel 718, especially regarding the residual stress distribution after machining. Therefore, it is currently not possible to optimize the required allowance for milling with ceramic tools. The motivation of this work is to determine the minimum possible allowance consistent with retained product quality. By studying the residual stresses and depth of deformation it is possible to determine the optimum allowance for finish machining. This is realized by mapping the residual stresses generated by a rough milling operation using round ceramic inserts and comparing these to a semi-finish or finish milling operation using round cemented carbide inserts. The effect of tool wear on stress distribution is also studied. Our results allow us to define an optimized manufacturing chain, enabling a maximum depth of cut for rough milling with retained component integrity for the succeeding finishing operations.

high local wear of the cutting edge [3]. All in all, these factors lead to long production times for superalloy components. Major developments aimed at increasing the MRR have been performed by optimisation of the process as such as well as improving cutting tool performance. This has been undertaken by the cutting tool manufacturers, as they have developed new materials and tool grades. Several high capacity “super abrasive” derivatives of insert materials, such as various grades of ceramics, cubic boron nitride (CBN) and polycrystalline diamonds (PCD) have been developed. Two important classes of ceramic tool materials for machining Inconel 718 are (i) silicon nitride-based and (ii) whisker-reinforced ceramics [4]. The use of ceramic tools has grown due to the toughness of these materials. This allows higher cutting speeds, providing an increase in MRR [5]. Concept development aiming for cost effective material removal of superalloys is based on extensive use of ceramic inserts, while subsequent finishing is performed by using cemented carbide tools. Advances reported in the literature mainly relate to improvements in terms of cutting parameters and tool consumption. Studies investigating the physics of the cutting process itself have also been reported. Thus, some researchers have investigated the thermal conditions related to the cutting zone [6–8] where the focus has been on various ways of applying cutting media while using cemented carbide tools, while others have focused on tool geometry. For example, it has been shown that an alteration of the texture on the flank face of the insert may have a great influence on material removal rate [9,10]. Surface integrity after machining with different tools has been thoroughly investigated and reported for turning operations [11–13]. It is well known that machining generates high tensile residual stresses in the surface of the workpiece, and that low cutting speeds and small depth of cut are required to minimize these. Arunachalam et al. exemplified this in a comparative investigation of turning using a CBN insert and a mixed ceramic insert of aluminum oxide and titanium carbide [11]. It was shown that the ceramic tool generated high tensile residual stresses in the surface. It was suggested that a round CBN insert, at a rather low cutting speed (150 m/min) and a small depth of cut (0.05 mm) in wet conditions, is the best way to perform facing of aged Inconel 718. In contrast, surface integrity after milling with indexable inserts is less frequently reported. This is most likely due to the fact that it is much easier to isolate the influence from the insert during a turning operation. As opposed to turning, a milling operation generates a complex wear mechanism by continuously changing the cutting conditions. However, some researchers have studied the surface conditions after milling with ceramic inserts and recommend subsequent machining with either cemented carbide inserts or mixed cermet grades consisting of titanium, tantalum, niobium carbides or CBN [4]. Machining superalloys with ceramic tools is often performed dry to avoid thermal fatigue and cracking of the tools. The increased temperature this causes will, to some extent, soften the work material and thus lower the cutting forces. More importantly, it will cause deformation and induce high tensile stresses in the workpiece surface. Reports of milling using indexable ceramic inserts are very limited. Akthar et al. reported a detailed investigation of how cutting parameters influence the surface integrity when finish milling GH4169/ Inconel 718 [15]. Ceramic milling showed very high tensile stress levels up to 1800 MPa when machining using low cutting speed. Other comparative investigations of cemented carbide and ceramic milling only cover limited aspects of surface integrity, such as tool life or topography, but not residual stresses [16]. Related to cemented carbide milling, Cai et al. investigated the surface integrity after end milling using an AlTiN-TiCN-TiN coated solid cemented carbide tool [14]. The resulting surface topography and residual stress was greatly influenced by cutting speed and cutting feed. The milling operation generated high tensile residual stresses of almost 1000 MPa in the surface at cutting speeds of 80−110 m/min and a feed of 0.15 mm. Zhang et al. investigated wear of a cemented carbide insert

2. Material and method 2.1. Material All test specimens were taken from a forged Inconel 718 billet. The material was triple melted into a billet which was forged in an open die followed by heat treatment and electrical discharge machined (EDM) into a rectangular bar. The heat treatment was performed according to AMS 5662, solution annealing at 975 °C for 60 min followed by quenching in a polymer bath. Aging was done in two temperature steps followed by air cooling. The resulting hardness of the shaft was 261 HB after forging, and 432 HB after the final heat treatment. The alloy composition is shown in Table 1. 2.2. Experimental details Test specimens were taken from the heat-treated bar. The milling test setup is shown in Fig. 1. All tests were performed on a Quaser, MV204 CPL, 3-axis milling machine using both new and worn ceramic inserts (RPGN 090300E 6060) from Sandvik in dry condition. The 2

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Table 1 Chemical composition, in weight-%, of the billet used for test specimens. Ni [%]

Cr [%]

Fe [%]

Nb [%]

Mo [%]

Ti [%]

Al [%]

Co [%]

C [%]

Mn [%]

S [%]

P [%]

Si [%]

W [%]

Cu [%]

B [%]

Zr [%]

V [%]

53.9

17.96

Bal.

5.21

2.89

0.99

0.48

0.23

0.024

0.65

< 0.0003

0.0095

0.7

0.02

0.04

0.0036

< 0.01

0.03

2.3. Evaluation

cemented carbide milling was performed for new and worn inserts (RNGJ 10T3M0 SGDJ) from Kennametal using flood cooling. This insert has a geometry suitable for milling in superalloys. Further information on the inserts are summarized in Table 2. The machining parameters for the test are typically used for production of a complex gas turbine component in Alloy 718. Machining parameters are presented in Table 3. An overview of the set-up including the tool holder is shown in Fig. 1. Milling was performed in only one direction across the sample, seen in Fig. 1B), using only one insert in the tool holder, Fig. 1C). This was done in order to isolate the influence of individual inserts. In a normal production set-up these tool holders are equipped with 3 inserts. The resulting 16 mm wide and 74 mm long milled slot is shown in Fig. 2. Each insert was used for milling only one slot. The surface integrity, in terms of topography and residual stresses, was evaluated in the lengthwise middle position of the test specimen, see Fig. 2. This minimizes the risk of including effects of vibration when the insert enters and exits the workpiece. Fig. 2 further shows the definition of the residual stress directions and the orientations and motion of the milling tool in feed and perpendicular direction relative to feed. The inserts classified as “worn” were defined from a production perspective, characterized by a worn edge radius for both inserts. Fig. 3 shows the edge after the milling tests, for both inserts in new and worn conditions. The wear from the tests is shown for the two new inserts. The wear for the worn inserts was evaluated before and after the tests and no significant increased wear was noticed.

2.3.1. Topography The surface topography after milling was measured with coherence scanning interferometry using a Sensofar S Neox instrument. Measurements were performed over a 5.7 × 4.29 mm stitched area in the center of the sample at one position with a lateral resolution of 1.29 μm. The measured data was filtered using a spatial median noisereduction filter to reduce short wavelengths. The specific filter settings were a window size with 5 × 5 points. The topography was evaluated according to the ISO 25178-2:2012 standard. Parameters measured include: the arithmetic mean height, Sa; the root mean square height, Sq; ten point height, S10z, and the developed interfacial area ratio, Sdr [22]. Sa describes the average roughness of the surface, Sq the standard deviation of the height distribution, S10z the average height difference between the 5 highest peaks and 5 deepest pits and Sdr describes the complexity of the surface.

2.3.2. Residual stress Residual stress measurements were performed by X-ray diffraction, using a Stresstech G2R XStress 3000 diffractometer equipped with a Mn X-ray tube (λ = 0.21031 nm). X-ray diffraction measures the interplanar spacing in the atomic lattice. The modified sin2χ method was used with ± 5 tilt (psi) angles (45°…-45°) and the (311) lattice plane located at 151.88° was evaluated. Residual stress was calculated assuming elastic strain theory according to Hooke’s law using 199.9 MPa as Young’s modulus and 0.29 for Poisson’s ratio, as described by Noyan

Fig. 1. Overview of A) the spindle with tool holder, B) work piece set up in the machine and C) tool holder for the indexable insert. 3

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Table 2 Details regarding the two inserts. Property/ Insert material

Holder diameter [mm]

Slot diameter [mm]

Cutting length [mm]

Insert diameter [mm]

No. Teeth

Clearance angle [°]

Ceramic Cemented Carbide

25 25

16 16

74 74

9 10

1 1

30 30

Table 3 Machining settings used for the two milling inserts. Setting/ Insert material

vc [m/min]

vc at engaged diameter [m/min]

vf [mm/min]

s [rev/min]

fz [mm/tooth]

ap [mm]

Ceramic Cemented Carbide

800 40

500 26

713 153

10186 509

0.07 0.3

1 0.75

half-maximum height (FWHM) parameter. It has previously been shown that the diffraction peak broadening (FWHM) is efficient when evaluating for example deformation from shot peening [25,26]. The peak broadens and FWHM increases with increasing deformation. During evaluation of the diffraction peaks, great care was taken when fitting the Kα1 and Kα2 peaks. This was done using the methodology described by Prevéy [27] using Pearson VII peak fitting and a parabolic background of the Kα1 and Kα2 doublet. 3. Results The milling operation resulted in four slots, as shown in Fig. 4. Ocular inspection of the surfaces showed a clear difference, especially when comparing the ceramic and cemented carbide milling. It can be seen how the wear patterns from the inserts differ between the center and edge of the slots, where it is clear that the wear marks occur in the opposite direction. This difference is a result of the linear tool path, which was chosen so the feed direction is in the center of the slot. Closer to the outer edge of the slot, the tool is rotating behind the center of the mill holder and causes a wear pattern in the opposite direction.

Fig. 2. Overview of the milled slot for one of the test specimens with the definition of orientations: Feed and Perpendicular (Perp.) feed directions.

3.1. Topography As Fig. 5 shows, the surface topography is greatly influenced by the milling operation and tool wear. The 3D visualizations of the four different surfaces, and the corresponding ISO25178-2 parameters in Table 4 show how the surfaces differ. Note that in Fig. 5 the height scale is increased for the surface machined with a worn ceramic insert. The Sa and Sq parameters show similar trends, where the surfaces produced with a new ceramic insert gave the lowest Sa value of 0.26 μm, followed by the surfaces produced with the two cemented carbide inserts. The surface produced with the worn ceramic insert shows, unsurprisingly, the highest roughness, which is caused by adhered and smeared workpiece material. The two cemented carbide milled surfaces have similar roughness values but S10z is higher for the worn tool, which indicates a greater peak to valley height. The surface milled using a new ceramic insert shows S10z values of the same magnitude but a much lower average roughness. The Sdr parameter further distinguishes the surface milled with worn ceramic inserts, having a higher complexity caused by the different features in the surface. Additionally, it can be observed that the Sdr values for the surfaces milled with cemented carbide inserts are higher due to the more defined wear tracks as compared to the new ceramic insert milled surface.

Fig. 3. Overview of the tool edge after the milling tests of the ceramic and cemented carbide tools showing the classification of tool wear.

and Cohen [23]. Additional measurements to obtain residual stress profiles were performed using layer removal to measure the depth profiles. This was done using successive material removal by electro-polishing with a Struers Movipol and A2 electrolyte. All measurements were performed in an accredited laboratory in accordance with the SS-EN 15,305:2008 standard [24]. The error bars in the residual stress measurements represent the error from peak fitting of the individual diffraction peaks. The diffraction data contains additional information regarding deformation and dislocation density which is recorded in the full-width

3.2. Residual stresses 3.2.1. Surface residual stresses The stress distribution across the milled slots is shown in Fig. 6 4

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Fig. 4. Overview of the surfaces machined using new and worn ceramic and cemented carbide inserts.

(milled with ceramic insert) and Fig. 7 (milled with cemented carbide insert). These measurements were performed using a 1 mm circular measurement spot, at 2 mm intervals. At each position measurements were performed parallel to, and perpendicular (perp.) to the feed direction. The error bars represent the error when calculating the peak position, from peak fitting of the individual diffraction peaks. The two types of inserts, ceramic and cemented carbide, generate different stress signatures across the slots. The tool wear appears to have induced similar stress signatures, but the magnitudes differs somewhat. Differences related to the directions along/perpendicular to the feed direction can also be observed. The stress levels are extremely high, especially at the center of the slot milled with ceramic inserts. Stresses of 1600 MPa were measured which itself is questionable since the ultimate strength of this material is typically 1200 GPa at room temperature. This is further discussed in the discussion section. In addition, it is observed that for the cemented carbide insert, the maximum stress amplitude shifts from tensile to compressive as the insert passes the center position of the slot and the tool motion is towards the down milling side of the slot. The ceramic milled surface, perpendicular to the feed direction, exhibits an almost perfectly symmetrical curve with high tensile

Table 4 Topography results for selected surface texture parameters from ISO25178-2. Parameter/ Specimen

Sa [μm]

Sq [μm]

Sdr [%]

S10z [μm]

New Ceramic Worn Ceramic New Cemented carbide Worn Cemented carbide

0.26 1.33 0.88 0.93

0.31 1.83 1.12 1.11

0.02 3.07 0.51 0.44

5.55 19.49 4.80 6.85

stresses at the center of the slot which gradually decrease towards the outer edges of the slot. A slight asymmetry can be observed with higher tensile stresses for the new inserts towards the up milled side, to the left in Fig. 5. In the feed direction, the stresses are significantly lower and the difference between new and worn inserts is much greater. For the worn tool, the stresses do not show any obvious trend but for the new tool the tensile stresses increase towards the down milling side. The cemented carbide milled surfaces show another type of stress signature with a clear asymmetry of the tensile stress distribution, see Fig. 7. This asymmetry is characterized by the highest tensile stresses being toward the down milled part of the slot for direction

Fig. 5. 3D visualizations of the topography of the surfaces machined using new and worn ceramic and cemented carbide inserts. Please note that the height scale is increased for the surface machined with the worn ceramic tools. 5

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Fig. 6. Surface residual stresses across the ceramic milled slot in feed and perpendicular directions.

Fig. 7. Surface residual stresses across the cemented carbide (CC) milled slot in feed and perpendicular to feed direction.

deformation gradually increases toward the down mill side. Furthermore, the worn cemented carbide shows the highest deformation of all the inserts tested.

perpendicular to the feed and towards the up milled part of the slot in the feed direction. The magnitude of the stresses for the two directions is not as high as for the ceramic milled surface, but the difference between the highest and lowest stresses differ by more than 1000 MPa across the surface compared to 600 MPa for the ceramic milled surface. In addition, it was noted that the milling operation has resulted in compressive stresses at the very outer edge of the slot on the down milling side. Diffraction peak broadening is a useful parameter to estimate the degree of deformation. In Figs. 8 and 9 the FWHM values from the diffraction peak are presented for the ceramic and the cemented carbide milled surfaces. The center of the ceramic milled surface shows the highest degree of deformation and exhibits correspondingly broad peaks (high FWHM values). The surface milled with the worn ceramic tool shows a slightly lower FWHM value and therefore a lower degree of deformation. In comparison, the FWHM values for the cemented carbide inserts (Fig. 9) show a significant difference between the new and worn inserts. The new tool appears to produce much lower deformation where the center has higher tensile stresses which gradually decay toward the outer part of the slot. The worn tool results in a profile in which

3.2.2. Residual stress profiles The residual stress profiles are presented individually for the ceramic and cemented carbide inserts in Figs. 10 and 11. The ceramic milled surfaces result in very high tensile stresses in the surface region, and the tool wear has a great influence on the stress profile. These results will be discussed in more detail later. Stress levels in the surface are as high as 1700 MPa for the new insert, and 1370 MPa for the worn insert in the perpendicular direction. Stress levels in the feed direction are considerably lower, 1200 MPa for the new insert and 660 MPa for the worn insert. The high tensile stresses in the surface decrease rapidly below the surface, and at 20 μm depth a local minimum in the stress profile is observed. The corresponding depth for the worn ceramic insert is somewhat deeper: 30–60 μm. For both ceramic inserts a tensile stress peak is located at 0.15 – 0.25 mm below the surface. The total depth influenced also differs between the new and worn inserts. The new insert reaches the core or natural stress state at a depth 6

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J. Holmberg, et al.

Fig. 8. FWHM across the ceramic milled slot in feed and perpendicular to feed directions.

These depths are less than the corresponding affected residual stress depths. Deformation induced by the cemented carbide insert shows the same magnitude at the surface, but FHWM decreases much faster and the core state is reached for the new insert at slightly less than 0.1 mm. However, the worn insert shows a shift, with elevated FWHM at all the measured depths, indicating higher deformation. The FWHM profile decreases somewhat slower and the core state is reached at a depth of approximately 0.2 mm.

of 0.80 mm, while the worn insert shows an elevated stress level of 300 MPa at the same depth. Extrapolating these values linearly results in a total influenced depth of 1.10 mm, seen in Fig. 9. In general, the residual stress profiles for the new and worn cemented carbide inserts, shown in Fig. 10 indicate a much shallower affected depth. The stress magnitude close to the surface is lower as compared to the ceramic milled surfaces, with values up to 1000 MPa in the perpendicular feed direction. However, insert wear causes a significant stress difference in the feed direction, where the worn insert has induced a stress of 750 MPa, as compared to 530 MPa for the new insert. Generally, the new and worn inserts show similar results in the surface region but differ more at greater depths, where the worn insert has induced a tensile stress peak. The new cemented carbide milled surface shows a stress state of zero located at a depth of 70 μm while the worn insert shows a tensile stress peak which decays relatively slowly and is still measurable at a depth of 0.5 mm where the stress is approximately 140 MPa. In Figs. 12 and 13 the FWHM profiles of the diffraction peaks for various depths are illustrated. These clearly show the deformation as a function of depth. In the case of ceramic milling deformation decreases to zero at 0.2 mm for the new insert, and at 0.5 mm for the worn insert.

4. Discussion The milling inserts and machining parameters used in this investigation are typical for production of a complex and volumetrically optimized part for a gas turbine. This type of manufacturing involves different milling operations performed in different steps in order to reach the final product dimensions. Apart from making a part that is within the allowed tolerances it is of great importance to assure the integrity of the final part surface. This includes topology and residual stress. Since the part undergoes several different machining steps it is essential to study each of them separately to understand the entire

Fig. 9. FWHM across the cemented carbide (CC) milled slot in feed and perpendicular to feed directions. 7

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Fig. 10. Residual stress profiles from surfaces machined with new and worn ceramic inserts.

work piece. Regarding the penetration depth for the ceramic milled surfaces, it is shown that the core stress state is reached at a depth of 0.8 mm for a new tool, while a worn tool leaves a deeper residual stress impact. The depth of the deformation zone of the worn inserts is somewhat unexpected, but it appears to be reasonable if the following aspects are considered.

manufacturing process. It was observed that tool wear greatly influences surface roughness. Surface topography study showed that the new ceramic insert resulted in the lowest surface roughness, Sa of 0.26 μm. Tool wear greatly influenced the roughness which is shown as an increased peak to valley height of the grooves from the wear tracks, in combination with an increased amount of smeared material. The two cemented carbide milled surfaces were similar, with quite high surface roughness, Sa, close to 1 μm and relatively independent of the tool wear. This result is similar to other reports [14], where high depth of cut and feed results in higher roughness. The different milling operations clearly have a strong influence on residual stress, both at the surface and just as importantly to significant depths below the surface. Surface stresses across a milled slot (perpendicular to the feed direction) differ considerably, where the center of the slot may exhibit stresses more than three times higher than the outer part of the slot. In addition, it was observed that cemented carbide milling produces asymmetric stresses across the slot, with higher tensile stresses towards the down mill side. The reason for this asymmetry needs further study but it is closely related to the tool engagement and tool motion in the

• Initial analysis suggests that the core stress state may be higher for •

these specimens. However, the milling test was performed on the same specimens for the new and worn inserts; hence, they should have the same core stress state. The FWHM profiles for the two worn inserts indicate a clear difference with increased stress levels for these inserts. It is therefore concluded that both the worn ceramic and cemented carbide inserts cause a deeper deformation-affected zone.

The difference in deformation depth between a new and worn insert is closely related to the tool edge geometry and how that changes for a worn insert. This will result in a redistribution of the forces over a larger area when the insert is worn. In turn, this will also relocate the

Fig. 11. Residual stress profiles from surfaces machined with new and worn cemented carbide (CC) inserts. 8

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Fig. 12. FWHM profiles from surfaces machined with new and worn ceramic tools.

is typically 1200 MPa at room temperature. There are only limited investigations in the literature to refer to for comparison. However, Akthar et al. showed results from surface measurements after face milling GH4169/Inconel 718 using ceramic tools with similar magnitude, ranging from 1400−1800 MPa depending on cutting speed and feed [15]. Additionally, Arunachalam reported quite high tensile stresses, > 1000 MPa when using dry cemented carbide cutting [12]. Questions arise regarding the possibility for these stresses to be retained since it exceeds both the yield and ultimate strength of the alloy. The reason is most likely due to the severe deformation that occurs, especially in the case of dry ceramic milling. This deformation results in a transformation that induces an increase of the yield strength, which becomes much higher than that of the original material. In support of this hypothesis Chen et al. reported that the white layer transformation caused by broaching resulted in an increased dislocation density and nanocrystallization which affected the material strength [21]. Similar deformation and consequently material transformations are expected for milling operations, but the temperatures and deformation rates are higher during milling. Such a nanocrystallization might be the reason that the surface can retain these high tensile stresses. Further evaluation of the deformation zone, including the high

Hertzian pressure maximum below the surface in a similar fashion as from a shot peening operation, when different size of shot is used [28]. This offset was observed as a shift to deeper impact for both worn ceramic and cemented carbide tools. However, this is far greater than the impact from a shot peening operation. Tool geometry will greatly influence the residual stresses, but the cutting parameters also play an important role. Typical production parameters were used in this investigation. These are: a relatively high cutting speed for ceramic milling, 500 m/min at the outer diameter of the slot, without cutting fluids. The cemented carbide milling was performed at a much lower cutting speed, 26 m/min, and with cutting fluids. Thus, the results from the two operations with respect to cutting parameters are difficult to compare. However, both operations generate high residual tensile stresses in the surface, but the stresses after cemented carbide milling are significantly lower than for those after ceramic milling. Additionally, the penetration depth is much lower for the cemented carbide tools. For the new tool the core state is reached after only 70 μm while for the worn tool the impact appears to be much deeper. The high tensile stresses measured in the surface region for these specimens could be questioned since the ultimate strength of this alloy

Fig. 13. FWHM profiles from surfaces machined with new and worn cemented carbide (CC) tools. 9

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this zone the stresses in both feed and perpendicular directions coincide, indicating that the directional impact from the milling operation diminishes. If the FWHM affected depth of 0.30-0.50 mm is taken into consideration, an appropriate total allowance would be 0.50 mm in this specific case. This methodology to estimate minimum allowable rough machining tolerance is shown to be very useful to evaluate for the operation of interest. Evaluation of the results for the cemented carbide tools indicates a much shallower impact depth, which allows for a smaller tolerance. For the machining parameters used in this case only two zones could be isolated, where Zone 1 is less than 0.10 mm while Zone 2 extends much deeper for the worn tool. Using the FWHM to estimate depth impact, a total tolerance of 0.20-0.30 mm would include a sufficient margin. An optimized total tolerance for ceramic milling in this specific case is 0.5 mm which is a reduction of 40–50 % compared to the machining allowance used today. This reduction means it is possible to use ceramic tools for removal of an additional 10 % material, which allows for increased productivity. However, it is difficult to estimate the exact productivity gain since this depends on the depth of cut and cutting feed being used, as well as a difference in how fast the inserts wear out. The gain in this case would be that a cemented carbide insert would need to make an extra pass in order to remove the material that could be removed more efficiently using a ceramic insert. A significant increase in manufacturing productivity of such parts seem to be possible, with no negative effect on surface integrity and performance.

tensile stresses, using methods such as electron back scattering will be used to understand this in future research. Additionally, one can obtain an estimate of the change of material properties through hardness measurements, as this is an indication of the strength of the material. The residual stresses related to milling with cemented carbide inserts also show quite high tensile stresses. However, these results are similar in magnitude to results reported for solid end mill tools by Cai et al. [14]. In that case a much lower feed was employed, but with a higher cutting speed, which may account for the minor difference in the results. In relation to the accuracy of the measurement method, XRD, some assumptions may affect the accuracy of the results. Calculation of the stresses is based on elastic strain theory using Hooks law assuming that Young’s modulus and Poisson’s ratio, are the same for the deformed surface and the core of the material. This is not likely the case for the highly deformed surface and, as shown by Chen et al., a reduction of Young’s modulus occurs for the white layer after broaching by almost 20 GPa, i.e. 10 % [21]. A recalculation of the surface values showed that a decrease of Young’s modulus will decrease the stress values proportionally. A decrease of 10 %, from 199.9 GPa to 179.9 GPa will result in a decrease of tensile stresses from 1723 to 1550 MPa. A corresponding increase of Poisson’s ratio from 0.29 to 0.305 will instead marginally decrease the stresses from 1723 to 1703 MPa. The residual stress profiles were measured to considerable depth, 0.8 mm, in order to fully study how the stresses evolve with depth. These relatively deep measurements are necessary as the ceramic milling operation had such a deep impact. However, the depth to which it is possible to measure the stresses using x-ray diffraction and the layer removal technique, electro-polishing, is uncertain. In order to minimize the effect of stress relieving from electro-polishing, a minimal spot size of 5 mm was protected by masking during electro-polishing. However, with regard to stress relieving effects from layer removal the author has in other investigations compared the layer removal method with nondestructive methods such as synchrotron/neutron diffraction [29]. In that investigation the residual stresses of an induction hardened steel bar were measured through the complete hardening depth of 10 mm and compared by destructive layer removal and non-destructive synchrotron/neutron measurements. That experiment indicated that the two techniques measured similar stresses in the surface region and down to 1 mm below the surface, while at greater depth a stress relieving effect from the layer removal was obvious. We conclude that the results to a depth of 0.8 mm in this study are reliable. The penetration depth with respect to residual stresses and deformation (by FWHM) differs by 0.2-0.3 mm. This suggests that FWHM and, correspondingly, a hardness profile lack the ability to fully capture the true penetration depth of the milling operation. Hence, it is important to perform stress measurements in order to verify the affected depth. Future investigations will be performed to study the alterations and deformation depth that can be measured in the microstructure in detail. As suggested above, electron back scattering diffraction can capture the deformation well by measuring the grain misorientation angle. This has been shown to give valuable information concerning the turning process, exemplified by M’Saoubi et al. [30]. The residual stress profile indicates that there are different zones to consider: the outermost zone, with high tensile stresses in the surface region (Zone 1), below this the compressive/low tensile region (Zone 2) and the third and deepest which contains the tensile stress balancing peak (Zone 3). Each of these zones has characteristic features which can be described by the stress profile, or quantitatively by depth. The three zones are shown in Fig. 14 for the ceramic milled surfaces, which shows an affected region with stresses that vary widely in the interval 00.20 mm. The total affected depth is even greater. In this case, zone 1 has an impact depth of 0.05 mm while zone 2 extends to a depth of 0.20 mm. In the third zone a slowly decreasing effect from milling occurs and the core stress state is reached, which is located at 0.80 mm depth for a new tool and 1.10 mm for a worn tool. It was noticed that in

5. Conclusions A methodology has been developed and verified for an accurate measurement of the affected depth from ceramic and cemented carbide insert milling of Inconel 718. Surface topography and residual stresses are seen to be greatly affected by the insert (cemented carbide or ceramic) and by the state of the insert (new or worn). Using typical production parameters, it has been shown that ceramic milling resulted in lower surface roughness compared to cemented carbide milling. The residual stress distribution across the milled slot is closely related to the insert engagement in the working material where the outer edges exhibit much lower tensile stresses compared to the center for all investigated inserts. Up- and down- mill part of the slot for ceramic milling show similar trends. However, for cemented carbide milling, the down mill side shows significantly lower stresses compared to the up mill part of the slot. All ceramic milled surfaces exhibit high tensile surface stresses, up to 1700 MPa and consequently a steep stress gradient in the first 50 μm, followed by a tensile stress peak located at 200−300 μm below the surface. A worn insert results in a lower tensile stress in the surface region, but with a greater affected depth. The cemented carbide milled surfaces show lower and shallower tensile stresses compared to the ceramic milled surfaces. Tool wear induces higher and deeper tensile stresses. The resulting residual stress profiles were quantitatively divided into three affected depth zones showed a total affected depth of 0.5 mm for ceramic milling and 0.2-0.3 mm for cemented carbide milling. Declaration of Competing Interest None. CRediT authorship contribution statement Jonas Holmberg: Conceptualization, Methodology, Investigation, Writing - original draft, Writing - review & editing, Visualization. Anders Wretland: Conceptualization, Resources, Writing - review & 10

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Fig. 14. Residual stress profiles for ceramic milling showing the different allowance zones.

editing. Johan Berglund: Conceptualization, Supervision, Writing review & editing. Tomas Beno: Supervision, Writing - review & editing. [12]

Acknowledgements [13]

The results presented in this paper are part of the research project SWE DEMO MOTOR [grant number 2015-06047] financed by VINNOVA, the Swedish government agency for Enterprise and Innovation. Special thanks are due to GKN Aerospace Sweden AB for supplying test materials, information and expertise and to Tooltec Trestad AB for help in manufacturing the test specimens. The authors would also like to acknowledge the Knowledge Foundation (KK Stiftelsen) and the SiCoMaP research school.

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