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Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy
RMHM-04378; No of Pages 10 Int. Journal of Refractory Metals and Hard Materials xxx (2016) xxx–xxx
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Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy Erdem Sireli a,⁎, Nilufer Orhon a, Guldem Kartal Sireli b a b
Böhler Sert Maden, GOSB Ihsan Dede Cad. 1600. Sk. No: 1602, Gebze, Kocaeli, Turkey Istanbul Technical University, Department of Metallurgical & Materials Engineering Maslak, Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 12 August 2016 Received in revised form 23 November 2016 Accepted 2 December 2016 Available online xxxx Keywords: w-EDM Surface integrity Hard metal TiN morphology
a b s t r a c t In this study, the effects of heat affected zone generated by wire electro discharge machining (w-EDM) process on the surface integrity of a K15 grade hard metal before and after TiN coating application via chemical vapour deposition (CVD) was investigated. It was observed that applying dry blasting (DB) on w-EDM'ed surfaces is a very efficient way to remove heat affected zone, namely recast layer. TiN layers deposited on w-EDM'ed surfaces showed a great diversity in their morphologies depending on the w-EDM process parameters and exhibited irregularities whereas the ones coated on dry blasted (DB'ed) surfaces constituted much finer mixture of nodular, star shaped and lenticular crystals. Adhesion of TiN layers on the DB'ed hard metal samples was better compared to the ones deposited directly on w-EDM'ed samples with a thick heat affected zone on their surfaces. Friction and wear properties of the TiN layer formed on DB'ed surface against alumina ball were also superior which were also in good agreement with its higher adhesion strength and distinctive morphology. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Hard metals (WC-Co cemented carbides) with a wide variety of compositions and grain sizes are one of the most demanded functional materials in such industries as cutting tool, die and wear parts owing to their superior properties namely, perfect hardness-toughness combination and wear resistance [1–3]. Corresponding to the developments in material science, hard metals have gained higher hardness and better wear resistance along with improved thermal and chemical properties thanks to the ability to produce ultrafine/nano sized grades through using different carbide additives [4–6], varying binder compositions [7–10] as well as applying novel sintering techniques [11–14]. However, all these good properties brought in hard metals also make them more brittle and difficult to cut through conventional methods; for instance, grinding which requires expensive metal/resinoid bonded diamond discs and cutting fluids [16–19]. Particularly in forming of complex shaped hard metal parts with a precise dimensional accuracy, the drawbacks of the traditional forming methods become more prominent. Among several alternative routes to shape hard metals, wire electro discharge machining (w-EDM) is the one commonly used technique in fabrication of hard metal parts [20,21]. W-EDM is a process which erodes material via controlled sparks generated between the moving tool (wire) and the workpiece in a dielectric fluid. Since there is no physical contact in this process, it can be applied to any kind of ⁎ Corresponding author. E-mail address: [email protected] (E. Sireli).
electrically conductive materials regardless of their mechanical properties and complexity in their geometries. On the other hand, w-EDM causes a heat affected zone, namely recast layer underneath the shaped surface. It contains high amount of tensile stress, cracks and voids in a condensed WC-Co alloy through cooling after spark depending on the process parameters. Decarburisation, corrosion and material transfer from wire onto the machined surface are the other issues that can be faced in w-EDM process [16,17,22–24]. There are some ways to get rid of or alleviate these effects such as executing multi step sequential cuts (numerous passes between wire and workpiece to reach final dimension) [22], conducting blasting process afterwards [25] or introducing compressive stress via PVD coatings [15,26]. Among them, applying a blasting process, as a post treatment is the most cost effective approach compared to the others which require higher operating costs. Since cutting tools and dies which are composed of hard metals are mostly coated, different kinds of coatings might also be applied on the w-EDM'ed surfaces. TiN is still one of the most demanded coatings, especially as an adhesion layer between hard metal substrates and following layers in multilayer coatings deposited through CVD [27–29]. As an adhesion layer, TiN's morphological structure has a crucial importance in terms of interlocking with following layers and its mechanical properties. Numerous studies have been carried out to evaluate TiN coatings applied via CVD on ground or as-sintered hard metal surfaces. However, there is no a published study in the literature about the investigation on the morphological changes in TiN coatings deposited on w-EDM'ed surfaces.
http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001
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Table 1 Designation of the samples (N/A: not applied; A: Applied). Treatment condition
Number of passes
Dry blasting
TiN coating
Sample code
w-EDM'ed
1 4 5 7 1 4 5 7 1 4 5 7 1 4 5 7 N/A
N/A N/A N/A N/A N/A N/A N/A N/A A A A A A A A A N/A
N/A N/A N/A N/A A A A A N/A N/A N/A N/A A A A A A
1EDM 4EDM 5EDM 7EDM 1EDM + 4EDM + 5EDM + 7EDM + 1EDM + 4EDM + 5EDM + 7EDM + 1EDM + 4EDM + 5EDM + 7EDM + G + TiN
w-EDM'ed and TiN coated
w-EDM'ed and DB'ed
w-EDM'ed and subsequently DB'ed then TiN coated
Ground and TiN coated
In this study, different hard metal surface conditions were prepared by applying one step or multi-step sequential w-EDM process. After wEDM application(s), selective DB was executed on the w-EDM'ed surfaces as the post treatment procedure. It was aimed to inspect the effects of w-EDM caused damaged zones on the surface integrity of a hard metal alloy and the morphology of TiN layer deposited afterwards. In addition, the adhesion, friction and wear properties of TiN layers grown on different surface states were evaluated to correlate them with morphological variations induced by the w-EDM process and the following DB process.
TiN TiN TiN TiN DB DB DB DB DB + TiN DB + TiN DB + TiN DB + TiN
Adhesion of the TiN layers was evaluated through The Daimler-Benz Rockwell-C adhesion test. The principle of this method was given in Ref. [31]. Wear tests were carried out in a CSM ball on disc test apparatus. During the tests Al2O3 ball with a 5 mm diameter, 10 N of load, 150 m of distance, 10 cm/s of linear speed and 2.5 mm of wear track radius were used. The wear tracks on the hard metal discs were inspected via SEM and energy dispersive X-ray spectrometry (EDS)/back scattering (BS) detectors. Wear tracks and wear debris were analyzed through elemental mapping in SEM. The wear scars on the balls were visualized by Wyko Nt1100 optical profiling system.
2. Material and methods 2.1. Sample designation A plain WC-Co hard metal which corresponds to Boehlerit's HB30F grade with 10% wt Co content and 0.8 μm WC grain size was chosen as the base material to be shaped by w-EDM. Disc shaped samples with 10 mm diameter were cut with a w-EDM equipment (AC Classic V2) in one pass or multi pass of the wire sequentially. It is known that w-EDM'ed surface conditions vary in a wide range from rough to microfinished depending on the process parameters such as discharge current, power, etc. [30]. For eliminating the high roughness and recast layer which was created during the first pass, following passes were conducted by lowering discharge current gradually. 4, 5 and 7 passes were chosen in multi pass w-EDM tests, since these procedures ensure considerable differences in surface quality of hard metals. The wire chemical composition was Cu-37 wt% Zn and its diameter was 0.25 mm. One of the samples was ground with a metal bonded diamond wheel with a grain size of 46 μm using a rotational speed of 2800 rpm. This sample was used as the reference to make comparisons with w-EDM'ed hard metal surfaces in terms of surface roughness and the morphology of following TiN layer. A group of samples were DB'ed following the w-EDM with a conventional blasting equipment (Graf Reinigungs systeme). 320 mesh sized Al2O3 powder was used for 10 s with 5 bar of pressure. Surface roughness measurements before and after DB process were carried out in a Wyko Nt1100 optical profiling system. TiN coating was applied through Bernex, hot wall vertical moderate temperature CVD (MT-CVD) equipment. Liquid TiCl4 and N2 gas were used as the precursors. The applied coating temperature, the pressure and the time were 900 °C, 150 mbar and 1 h, respectively without any additional post treatments. The phase analysis of the TiN layers was conducted in Philips PW 3710 XRD thin film diffraction equipment using CuKα radiation (10 kV–10 mA) with the glancing angle of 2°. The inspection of the surface integrity of the w-EDM'ed samples and the morphology of TiN layers deposited on different surface states were conducted by a Jeol 5410 Scanning Electron Microscope (SEM).
In this study, there were four main test sets as follow: w-EDM subsequently TiN coating, w-EDM plus DB then TiN coating, DB and TiN coating, grinding then TiN coating. Entire samples were designated referring to the number of passes in w-EDM process and their treatment situations such as DB'ed or ground (G) samples. For example, “4EDM + DB + TiN” denotes to the sample cut in 4 passes of w-EDM and additionally DB applied then TiN coating conducted. All the samples along with their codes and processed conditions were illustrated in Table 1. 3. Results and discussion 3.1. Surface inspection SEM images taken from the top surfaces and the cross sections of wEDM'ed samples were presented in Fig. 1. As clearly seen in Fig. 1(a-2), a single pass of wire caused severe recast layer in which considerable amount of voids and droplets existed. When the number of passes was increased, the defective structure and the resultant roughness diminished gradually. The cross section micrographs also showed that the thickness of the recast layer decreased and the rough surface obtained after a single pass of wire became smoother. Cross sectional SEM micrographs of TiN coatings deposited on the surfaces of sole w-EDM'ed and w-EDM'ed plus subsequently DB'ed as well as only G samples were given in Fig. 2. As seen in Fig. 2-a, TiN layer grew very irregularly on just w-EDM'ed sample which comprised a thick heat affected zone on its surface. However, TiN layer deposited on the w-EDM'ed and then DB'ed specimen was much more uniform with an apparent columnar structure which was comparable to the one deposited on G sample. Superior mechanical properties cannot be expected from 1EDM + TiN, considering the irregular, rugged morphology of coating resulting from the very thick recast layer at the interface, as indicated in Ref. [32].
Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001
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Fig. 1. SEM images of w-EDM'ed samples along with (1) surface and (2) cross sectional views: (a) 1ED, (b) 4EDM, (c) 5EDM, (d) 7EDM.
3.2. Surface roughness Average roughness values measured before and after DB along with the ones after the application of TiN coating were given in Table 2. According to the SEM observations (Fig. 1), the surface roughness
decreased gradually as the number of passes in w-EDM process got increased. Also, it is clear that the Ra values were diminished to an extent when DB process was carried out after w-EDM owing to its mechanical action to reduce the thickness of recast layer. The smoothening effect of DB was more prominent in 1EDM which was prepared by single pass w-
Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001
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diffractograms. It is obvious that the DB process eliminated the precipitated etaphase resulting from the w-EDM process and decrease d the thickness of the recast layer which was composed of condensed WC and Co. There was also another peak at 50.1° which corresponds to Cu-Zn alloy in the pattern of 1EDM + TiN sample. Since 1EDM + TiN had the roughest surface resulting from the single pass w-EDM without applying DB process, the presence of the transferred wire material (Cu37%Zn) at the coating-substrate interface was highly possible. The backscatter SEM image (Fig. 6) proved that there was a precipitation of wire material between the recast layer and the coating. The light gray zone, between the bright recast layer and the dark gray TiN coating referred to the transferred wire material corresponding to Cu and Zn. 3.4. Coating morphology
Fig. 2. Cross sectional morphologies of TiN coatings deposited on differently treated surfaces: (a) 1EDM + TiN, (b) 1EDM + DB + TiN, (c) G + TiN.
EDM process. However, Ra values for DB'ed specimen started to increase again beginning from 5EDM + DB compared to sole w-EDM'ed samples (i.e., 5EDM and 7EDM), as a result of the fact that the impingement effect of the DB process roughened the surface. It seemed that the same trend in Ra values was conserved even after TiN deposition with some little variance which probably stayed in measurement uncertainty of the technique. In Fig. 3, 2-D images of the surfaces which belong to the w-EDM'ed and coated samples with/without DB were given. The impingement effect of the DB which caused hill/valley structure was apparent (see Fig. 3). As a comparison with the w-EDM process, G sample was also TiN coated and its surface roughness, 159 nm was given in Fig. 4. As seen, the minimum Ra value was obtained in G + TiN. However, parallel grinding traces were still obvious even after TiN coating deposition.
3.5. Adhesion test
3.3. X-ray diffraction analysis Thin film XRD patterns of TiN coatings deposited on differently treated samples were given in Fig. 5. The patterns belonging to 4EDM + TiN (Fig. 5-d) and 1EDM + TiN (Fig. 5-e) differed from the others since they contained W6Co6C (etaphase) at the diffraction angles of 32.8, 43.1 and 47.1. High temperatures and rapid cooling rate at the end of sparks during w-EDM might have caused the formation and retainment of this brittle phase which normally forms at around 1100 °C and transforms into WC and Co via slow cooling in the sintering of hard metal alloys, as mentioned in Ref. [16]. Also, WC peaks which located at 31.3, 35.6 and 48.3 were more prominent in 4EDM + TiN (Fig. 5-d) and 1EDM + TiN (Fig. 5-e) patterns with respect to the ones present in 4EDM + DB + TiN (Fig. 5-b), 1EDM + DB + TiN (Fig. 5-c)
Table 2 Average surface roughness values of the samples with different surface conditions. Sample code
Ra (nm)
Sample code
Ra (nm)
1EDM 4EDM 5EDM 7EDM 1EDM 4EDM 5EDM 7EDM
2060 518 211 142 858 303 287 282
1EDM 4EDM 5EDM 7EDM 1EDM 4EDM 5EDM 7EDM
1710 493 225 177 936 342 273 221
+ + + +
DB DB DB DB
Morphological diversity of the TiN coatings with respect to the conditions of the substrate surfaces were shown in Fig. 7. The TiN coating deposited on 1EDM had an irregular, rugged morphology imitating the roughly (single pass) EDM'ed surface of the base material, however, a nodular and star shaped morphology (bright spots on the image) began to prevail as the number of passes in w-EDM was increased. It is clear that the morphology of TiN layer became finer and more regular, as the surface roughness of base materials decreased as a result of the varying w-EDM parameters which favored the finished surface condition. However, a w-EDM process with multiple passes takes longer time depending on product geometry and higher costs due to increased amount of energy, labor and consumables. Finer and denser TiN morphology obtained on 5EDM + TiN with respect to the one obtained on 4EDM + TiN was compatible with the 2D images and the surface roughness values of base materials given in Fig. 3-(c, e) and Table 2. There was no considerable difference in the morphologies of TiN coatings deposited on the DB'ed samples (see Fig. 7b, d, f). The general appearance was a mixed morphology of nodular, star shaped and lenticular crystals which is very similar to the one stated by Cheng et al. [33]. The morphology of TiN layer deposited on the G sample revealed the resemblance with the DB'ed samples (Fig. 8).
+ + + + + + + +
TiN TiN TiN TiN DB + TiN DB + TiN DB + TiN DB + TiN
Rockwell-C adhesion test was applied on 1EDM + TiN, 4EDM + TiN, 1EDM + DB + TiN, 1EDM + DB + TiN specimens and the SEM images of the indentations taken in backscatter mode were shown in Fig. 9. 1EDM + TiN exhibited an unacceptable adhesion performance with considerable amount of chips and flakes in/around the indentation (seen as bright zones in the image). There was a significant difference between the adhesion strength of 1EDM + TiN and 4EDM + TiN. The coating deposited on 4EDM'ed sample displayed much better adhesion thanks to the thinner heat affected zone at the interface (Fig. 1-b) despite some small and isolated chipped areas, especially inside the trace. On the other hand, it was clear that the DB process applied following the w-EDM improved the adhesion of TiN layer to a great extent due to the removal of the heat effected zone (Fig. 9-b, d). Apparently, this recast layer formed at the interface between the TiN coating and the substrate had a negative impact on the adhesion. Moreover, DB eliminated the brittle etaphase precipitated region (Fig. 5) which generally deteriorates the adhesion by leading to formation of cracks at the interface and hence flaking of the coating. 3.6. Wear test The friction curves and the wear track images taken in backscatter mode for 1EDM + TiN and 1EDM + DB + TiN samples were given in Fig. 10. Higher amount of wear debris was detected at the contact area of 1EDM + TiN compared to 1EDM + DB + TiN. The comparison of the friction curves revealed longer running-in period, greater
Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001
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Fig. 3. 2D surface images of TiN coating: (a) 1EDM + TiN, (b) 1EDM + DB + TiN, (c) 4EDM + TiN, (d) 4EDM + DB + TiN, (e) 5EDM + TiN, (f) 5EDM + DB + TiN.
fluctuations and higher mean friction coefficient for 1EDM + TiN with respect to those observed for 1EDM + DB + TiN. X ray maps taken from the wear track on 1EDM + TiN pointed to the formation of complex oxide layer ((W,Cu,Ti)-oxide) inside the track (Fig. 11). The SEM image of the wear track (Fig. 12) indicated that a tribolayer was produced. The light gray zone was the represented the tribolayer whereas the dark gray and bright areas referred to TiN coating and exposed recast layer, respectively.
Fig. 4. 2D image of ground and TiN coated sample (G + TiN).
Fig. 5. XRD patterns of TiN coatings deposited on differently treated samples: (a) G + TiN, (b) 4EDM + DB + TiN, (c) 1EDM + DB + TiN, (d) 4EDM + TiN, (e)1EDM + TiN.
Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001
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Fig. 8. Surface morphology of TiN coating deposited on ground sample (G + TiN). Fig. 6. Backscattered cross sectional SEM image of 1EDM + TiN coated hard metal.
As the mechanism of the tribolayer formation in the wear track on 1EDM + TiN, following depiction could be offered. The higher surface roughness of 1EDM + TiN sample caused considerable amount of wear debris since the compressive and tangential forces leaded to the fracture of the protruding grains on the initial sliding surface and this
brought about longer running-in which was about 80 m. As the sliding went on, the fractured fragments broke into smaller ones triggering the accumulation due to the higher surface area of the wear debris as well as frictional heating and this leaded to the formation of the tribolayer consequently. When a critical thickness of the tribolayer
Fig. 7. Variation of TiN surface morphology depending on pervious treatments: (a) 1EDM + TiN, (b) 1EDM + DB + TiN, (c) 4EDM + TiN, (d) 4EDM + DB + TiN, (e) 5EDM + TiN, (f) 5EDM + DB + TiN.
Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001
E. Sireli et al. / Int. Journal of Refractory Metals and Hard Materials xxx (2016) xxx–xxx
Fig. 9. SEM images of Rockwell-C indentations: (a) 1EDM + TiN, (b) 1EDM + DB + TiN, (c) 4EDM + TiN, (d) 1EDM + DB + TiN.
Fig. 10. Friction curve and wear track micrograph of TiN coatings: (a) 1EDM + TiN, (b) 1EDM + DB + TiN.
Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001
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Fig. 11. Elemental maps taken from the wear track of 1EDM + TiN.
was reached, it started to crack and delaminate into particles again and these particles were dragged away to the periphery of the wear track, as illustrated in Fig. 10-a. In addition to long running-in, higher friction coefficient with the big fluctuations was given rise by this continuous tribolayer which seized the counterface. On the other hand, X ray maps taken from the wear track on 1EDM + DB + TiN (Fig. 13) denoted that considerable amount of Al transferred from the ball onto the wear track due to the wear of the counterpart. The wear scars on the alumina ball were also inspected through 2-D profilometry and correlated with the 3-D images of the wear tracks (Fig. 14). The smoothening of the ball used against 1EDM + DB + TiN was quite clear exhibiting significant amount of wear, while there was no noticeable wear on the one used against 1EDM + TiN. This observation coincided with the fact that there was a significant amount of Al on the wear track on 1EDM + DB + TiN. The 3-D image of the wear track on 1EDM + DB + TiN pointed that the contact area got broadened due to the severe deformation of the ball material, while the wear track on 1EDM + TiN was narrower and deeper since the geometry of the ball was mostly preserved. TiN coatings with mixed crystal structure comprising star shaped and lenticular ones might have better mechanical properties such as hardness and wear resistance since the presence of these structures concurrently retards dislocation movements as mentioned in earlier studies [33,34]. The mixed type of crystal structure of TiN deposited on 1EDM + DB, including star shaped and nodular grains in which lenticular subgrains grew was revealed in Fig. 15. For the materials used in cutting tool industry, deforming workpiece while conserving their surface integrity is mandatory regardless of the fact that they are coated or not. The inspections conducted either on the wear track on 1EDM + DB + TiN, or on the ball material indicated that 1EDM + DB + TiN exhibited better friction behavior and wear resistance in this respect. Additionally, much better adhesion of TiN coatings deposited on DB'ed samples (Fig. 9-d) contributed to the superior wear
Fig. 12. Backscatter SEM image taken from the inside of the wear track of 1EDM + TiN.
resistance of 1EDM + DB + TiN with respect to the one belonging to 1EDM + TiN. 4. Summary and conclusions In this study, the effects of heat affected zones formed as a result of w-EDM process on the surface integrity of a hard metal alloy and the morphology of TiN layer deposited subsequently were investigated. Adhesion, friction and wear properties of TiN layers grown on the surfaces with different treatment conditions were examined. The results of this research could be summarized as follows: (1) Even 10 s of DB could be helpful to remove recast layer, etaphase precipitation and transferred wire material caused by a rough wEDM process. (2) The surface conditions generated by w-EDM had a considerable influence on the morphology of TiN coating deposited subsequently. TiN coatings applied directly on w-EDM'ed surfaces had a rough, irregular morphology; whereas, the ones applied on DB'ed surfaces subsequent to w-EDM exhibited much finer and mixed crystal structures consisting of nodular, star shaped and lenticular grains regardless of w-EDM parameters. (3) Applying multi pass w-EDM up to 5 passes could create fine TiN crystals which were similar to the ones obtained the on DB'ed surfaces, however, this long procedure would not be cost effective taking the operation time, labor, energy and consumable costs into account.
Fig. 13. Al Accumulation rich wear debris at the periphery of the wear track on 1EDM + DB + TiN.
Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001
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Fig. 14. 2-D profiles of the wear scars on the balls along with the 3-D images of the wear tracks on (a) 1EDM + TiN, and (b)1EDM + DB + TiN.
(4) TiN layers deposited on the DB'ed samples had a better adhesion than the ones deposited on w-EDM'ed surfaces directly. (5) The friction and wear properties of TiN coatings deposited on the DB'ed sample (1EDM + DB + TiN) subsequent to rough w-EDM (single pass) were superior with respect to the one deposited on the rough w-EDM'ed surface (1EDM + TiN). (6) While TiN layer on the DM'ed specimen (1EDM + DB + TiN) protected its integrity during the wear test, causing material transfer from the alumina ball onto the wear track, the one applied on the roughly w-EDM'ed surface (1EDM + TiN) could not resist sliding forces exposing the recast layer underneath and causing high amount of wear debris which turned into a complex tribolayer. (7) w-EDM could be a very efficient alternative route for forming
Fig. 15. Mixed crystal structure of TiN coating deposited on 1EDM + DB.
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Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001
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Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy Erdem Sireli a,⁎, Nilufer Orhon a, Guldem Kartal Sireli b a b
Böhler Sert Maden, GOSB Ihsan Dede Cad. 1600. Sk. No: 1602, Gebze, Kocaeli, Turkey Istanbul Technical University, Department of Metallurgical & Materials Engineering Maslak, Istanbul, Turkey
a r t i c l e
i n f o
Article history: Received 12 August 2016 Received in revised form 23 November 2016 Accepted 2 December 2016 Available online xxxx Keywords: w-EDM Surface integrity Hard metal TiN morphology
a b s t r a c t In this study, the effects of heat affected zone generated by wire electro discharge machining (w-EDM) process on the surface integrity of a K15 grade hard metal before and after TiN coating application via chemical vapour deposition (CVD) was investigated. It was observed that applying dry blasting (DB) on w-EDM'ed surfaces is a very efficient way to remove heat affected zone, namely recast layer. TiN layers deposited on w-EDM'ed surfaces showed a great diversity in their morphologies depending on the w-EDM process parameters and exhibited irregularities whereas the ones coated on dry blasted (DB'ed) surfaces constituted much finer mixture of nodular, star shaped and lenticular crystals. Adhesion of TiN layers on the DB'ed hard metal samples was better compared to the ones deposited directly on w-EDM'ed samples with a thick heat affected zone on their surfaces. Friction and wear properties of the TiN layer formed on DB'ed surface against alumina ball were also superior which were also in good agreement with its higher adhesion strength and distinctive morphology. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Hard metals (WC-Co cemented carbides) with a wide variety of compositions and grain sizes are one of the most demanded functional materials in such industries as cutting tool, die and wear parts owing to their superior properties namely, perfect hardness-toughness combination and wear resistance [1–3]. Corresponding to the developments in material science, hard metals have gained higher hardness and better wear resistance along with improved thermal and chemical properties thanks to the ability to produce ultrafine/nano sized grades through using different carbide additives [4–6], varying binder compositions [7–10] as well as applying novel sintering techniques [11–14]. However, all these good properties brought in hard metals also make them more brittle and difficult to cut through conventional methods; for instance, grinding which requires expensive metal/resinoid bonded diamond discs and cutting fluids [16–19]. Particularly in forming of complex shaped hard metal parts with a precise dimensional accuracy, the drawbacks of the traditional forming methods become more prominent. Among several alternative routes to shape hard metals, wire electro discharge machining (w-EDM) is the one commonly used technique in fabrication of hard metal parts [20,21]. W-EDM is a process which erodes material via controlled sparks generated between the moving tool (wire) and the workpiece in a dielectric fluid. Since there is no physical contact in this process, it can be applied to any kind of ⁎ Corresponding author. E-mail address: [email protected] (E. Sireli).
electrically conductive materials regardless of their mechanical properties and complexity in their geometries. On the other hand, w-EDM causes a heat affected zone, namely recast layer underneath the shaped surface. It contains high amount of tensile stress, cracks and voids in a condensed WC-Co alloy through cooling after spark depending on the process parameters. Decarburisation, corrosion and material transfer from wire onto the machined surface are the other issues that can be faced in w-EDM process [16,17,22–24]. There are some ways to get rid of or alleviate these effects such as executing multi step sequential cuts (numerous passes between wire and workpiece to reach final dimension) [22], conducting blasting process afterwards [25] or introducing compressive stress via PVD coatings [15,26]. Among them, applying a blasting process, as a post treatment is the most cost effective approach compared to the others which require higher operating costs. Since cutting tools and dies which are composed of hard metals are mostly coated, different kinds of coatings might also be applied on the w-EDM'ed surfaces. TiN is still one of the most demanded coatings, especially as an adhesion layer between hard metal substrates and following layers in multilayer coatings deposited through CVD [27–29]. As an adhesion layer, TiN's morphological structure has a crucial importance in terms of interlocking with following layers and its mechanical properties. Numerous studies have been carried out to evaluate TiN coatings applied via CVD on ground or as-sintered hard metal surfaces. However, there is no a published study in the literature about the investigation on the morphological changes in TiN coatings deposited on w-EDM'ed surfaces.
http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001 0263-4368/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001
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Table 1 Designation of the samples (N/A: not applied; A: Applied). Treatment condition
Number of passes
Dry blasting
TiN coating
Sample code
w-EDM'ed
1 4 5 7 1 4 5 7 1 4 5 7 1 4 5 7 N/A
N/A N/A N/A N/A N/A N/A N/A N/A A A A A A A A A N/A
N/A N/A N/A N/A A A A A N/A N/A N/A N/A A A A A A
1EDM 4EDM 5EDM 7EDM 1EDM + 4EDM + 5EDM + 7EDM + 1EDM + 4EDM + 5EDM + 7EDM + 1EDM + 4EDM + 5EDM + 7EDM + G + TiN
w-EDM'ed and TiN coated
w-EDM'ed and DB'ed
w-EDM'ed and subsequently DB'ed then TiN coated
Ground and TiN coated
In this study, different hard metal surface conditions were prepared by applying one step or multi-step sequential w-EDM process. After wEDM application(s), selective DB was executed on the w-EDM'ed surfaces as the post treatment procedure. It was aimed to inspect the effects of w-EDM caused damaged zones on the surface integrity of a hard metal alloy and the morphology of TiN layer deposited afterwards. In addition, the adhesion, friction and wear properties of TiN layers grown on different surface states were evaluated to correlate them with morphological variations induced by the w-EDM process and the following DB process.
TiN TiN TiN TiN DB DB DB DB DB + TiN DB + TiN DB + TiN DB + TiN
Adhesion of the TiN layers was evaluated through The Daimler-Benz Rockwell-C adhesion test. The principle of this method was given in Ref. [31]. Wear tests were carried out in a CSM ball on disc test apparatus. During the tests Al2O3 ball with a 5 mm diameter, 10 N of load, 150 m of distance, 10 cm/s of linear speed and 2.5 mm of wear track radius were used. The wear tracks on the hard metal discs were inspected via SEM and energy dispersive X-ray spectrometry (EDS)/back scattering (BS) detectors. Wear tracks and wear debris were analyzed through elemental mapping in SEM. The wear scars on the balls were visualized by Wyko Nt1100 optical profiling system.
2. Material and methods 2.1. Sample designation A plain WC-Co hard metal which corresponds to Boehlerit's HB30F grade with 10% wt Co content and 0.8 μm WC grain size was chosen as the base material to be shaped by w-EDM. Disc shaped samples with 10 mm diameter were cut with a w-EDM equipment (AC Classic V2) in one pass or multi pass of the wire sequentially. It is known that w-EDM'ed surface conditions vary in a wide range from rough to microfinished depending on the process parameters such as discharge current, power, etc. [30]. For eliminating the high roughness and recast layer which was created during the first pass, following passes were conducted by lowering discharge current gradually. 4, 5 and 7 passes were chosen in multi pass w-EDM tests, since these procedures ensure considerable differences in surface quality of hard metals. The wire chemical composition was Cu-37 wt% Zn and its diameter was 0.25 mm. One of the samples was ground with a metal bonded diamond wheel with a grain size of 46 μm using a rotational speed of 2800 rpm. This sample was used as the reference to make comparisons with w-EDM'ed hard metal surfaces in terms of surface roughness and the morphology of following TiN layer. A group of samples were DB'ed following the w-EDM with a conventional blasting equipment (Graf Reinigungs systeme). 320 mesh sized Al2O3 powder was used for 10 s with 5 bar of pressure. Surface roughness measurements before and after DB process were carried out in a Wyko Nt1100 optical profiling system. TiN coating was applied through Bernex, hot wall vertical moderate temperature CVD (MT-CVD) equipment. Liquid TiCl4 and N2 gas were used as the precursors. The applied coating temperature, the pressure and the time were 900 °C, 150 mbar and 1 h, respectively without any additional post treatments. The phase analysis of the TiN layers was conducted in Philips PW 3710 XRD thin film diffraction equipment using CuKα radiation (10 kV–10 mA) with the glancing angle of 2°. The inspection of the surface integrity of the w-EDM'ed samples and the morphology of TiN layers deposited on different surface states were conducted by a Jeol 5410 Scanning Electron Microscope (SEM).
In this study, there were four main test sets as follow: w-EDM subsequently TiN coating, w-EDM plus DB then TiN coating, DB and TiN coating, grinding then TiN coating. Entire samples were designated referring to the number of passes in w-EDM process and their treatment situations such as DB'ed or ground (G) samples. For example, “4EDM + DB + TiN” denotes to the sample cut in 4 passes of w-EDM and additionally DB applied then TiN coating conducted. All the samples along with their codes and processed conditions were illustrated in Table 1. 3. Results and discussion 3.1. Surface inspection SEM images taken from the top surfaces and the cross sections of wEDM'ed samples were presented in Fig. 1. As clearly seen in Fig. 1(a-2), a single pass of wire caused severe recast layer in which considerable amount of voids and droplets existed. When the number of passes was increased, the defective structure and the resultant roughness diminished gradually. The cross section micrographs also showed that the thickness of the recast layer decreased and the rough surface obtained after a single pass of wire became smoother. Cross sectional SEM micrographs of TiN coatings deposited on the surfaces of sole w-EDM'ed and w-EDM'ed plus subsequently DB'ed as well as only G samples were given in Fig. 2. As seen in Fig. 2-a, TiN layer grew very irregularly on just w-EDM'ed sample which comprised a thick heat affected zone on its surface. However, TiN layer deposited on the w-EDM'ed and then DB'ed specimen was much more uniform with an apparent columnar structure which was comparable to the one deposited on G sample. Superior mechanical properties cannot be expected from 1EDM + TiN, considering the irregular, rugged morphology of coating resulting from the very thick recast layer at the interface, as indicated in Ref. [32].
Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001
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Fig. 1. SEM images of w-EDM'ed samples along with (1) surface and (2) cross sectional views: (a) 1ED, (b) 4EDM, (c) 5EDM, (d) 7EDM.
3.2. Surface roughness Average roughness values measured before and after DB along with the ones after the application of TiN coating were given in Table 2. According to the SEM observations (Fig. 1), the surface roughness
decreased gradually as the number of passes in w-EDM process got increased. Also, it is clear that the Ra values were diminished to an extent when DB process was carried out after w-EDM owing to its mechanical action to reduce the thickness of recast layer. The smoothening effect of DB was more prominent in 1EDM which was prepared by single pass w-
Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001
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diffractograms. It is obvious that the DB process eliminated the precipitated etaphase resulting from the w-EDM process and decrease d the thickness of the recast layer which was composed of condensed WC and Co. There was also another peak at 50.1° which corresponds to Cu-Zn alloy in the pattern of 1EDM + TiN sample. Since 1EDM + TiN had the roughest surface resulting from the single pass w-EDM without applying DB process, the presence of the transferred wire material (Cu37%Zn) at the coating-substrate interface was highly possible. The backscatter SEM image (Fig. 6) proved that there was a precipitation of wire material between the recast layer and the coating. The light gray zone, between the bright recast layer and the dark gray TiN coating referred to the transferred wire material corresponding to Cu and Zn. 3.4. Coating morphology
Fig. 2. Cross sectional morphologies of TiN coatings deposited on differently treated surfaces: (a) 1EDM + TiN, (b) 1EDM + DB + TiN, (c) G + TiN.
EDM process. However, Ra values for DB'ed specimen started to increase again beginning from 5EDM + DB compared to sole w-EDM'ed samples (i.e., 5EDM and 7EDM), as a result of the fact that the impingement effect of the DB process roughened the surface. It seemed that the same trend in Ra values was conserved even after TiN deposition with some little variance which probably stayed in measurement uncertainty of the technique. In Fig. 3, 2-D images of the surfaces which belong to the w-EDM'ed and coated samples with/without DB were given. The impingement effect of the DB which caused hill/valley structure was apparent (see Fig. 3). As a comparison with the w-EDM process, G sample was also TiN coated and its surface roughness, 159 nm was given in Fig. 4. As seen, the minimum Ra value was obtained in G + TiN. However, parallel grinding traces were still obvious even after TiN coating deposition.
3.5. Adhesion test
3.3. X-ray diffraction analysis Thin film XRD patterns of TiN coatings deposited on differently treated samples were given in Fig. 5. The patterns belonging to 4EDM + TiN (Fig. 5-d) and 1EDM + TiN (Fig. 5-e) differed from the others since they contained W6Co6C (etaphase) at the diffraction angles of 32.8, 43.1 and 47.1. High temperatures and rapid cooling rate at the end of sparks during w-EDM might have caused the formation and retainment of this brittle phase which normally forms at around 1100 °C and transforms into WC and Co via slow cooling in the sintering of hard metal alloys, as mentioned in Ref. [16]. Also, WC peaks which located at 31.3, 35.6 and 48.3 were more prominent in 4EDM + TiN (Fig. 5-d) and 1EDM + TiN (Fig. 5-e) patterns with respect to the ones present in 4EDM + DB + TiN (Fig. 5-b), 1EDM + DB + TiN (Fig. 5-c)
Table 2 Average surface roughness values of the samples with different surface conditions. Sample code
Ra (nm)
Sample code
Ra (nm)
1EDM 4EDM 5EDM 7EDM 1EDM 4EDM 5EDM 7EDM
2060 518 211 142 858 303 287 282
1EDM 4EDM 5EDM 7EDM 1EDM 4EDM 5EDM 7EDM
1710 493 225 177 936 342 273 221
+ + + +
DB DB DB DB
Morphological diversity of the TiN coatings with respect to the conditions of the substrate surfaces were shown in Fig. 7. The TiN coating deposited on 1EDM had an irregular, rugged morphology imitating the roughly (single pass) EDM'ed surface of the base material, however, a nodular and star shaped morphology (bright spots on the image) began to prevail as the number of passes in w-EDM was increased. It is clear that the morphology of TiN layer became finer and more regular, as the surface roughness of base materials decreased as a result of the varying w-EDM parameters which favored the finished surface condition. However, a w-EDM process with multiple passes takes longer time depending on product geometry and higher costs due to increased amount of energy, labor and consumables. Finer and denser TiN morphology obtained on 5EDM + TiN with respect to the one obtained on 4EDM + TiN was compatible with the 2D images and the surface roughness values of base materials given in Fig. 3-(c, e) and Table 2. There was no considerable difference in the morphologies of TiN coatings deposited on the DB'ed samples (see Fig. 7b, d, f). The general appearance was a mixed morphology of nodular, star shaped and lenticular crystals which is very similar to the one stated by Cheng et al. [33]. The morphology of TiN layer deposited on the G sample revealed the resemblance with the DB'ed samples (Fig. 8).
+ + + + + + + +
TiN TiN TiN TiN DB + TiN DB + TiN DB + TiN DB + TiN
Rockwell-C adhesion test was applied on 1EDM + TiN, 4EDM + TiN, 1EDM + DB + TiN, 1EDM + DB + TiN specimens and the SEM images of the indentations taken in backscatter mode were shown in Fig. 9. 1EDM + TiN exhibited an unacceptable adhesion performance with considerable amount of chips and flakes in/around the indentation (seen as bright zones in the image). There was a significant difference between the adhesion strength of 1EDM + TiN and 4EDM + TiN. The coating deposited on 4EDM'ed sample displayed much better adhesion thanks to the thinner heat affected zone at the interface (Fig. 1-b) despite some small and isolated chipped areas, especially inside the trace. On the other hand, it was clear that the DB process applied following the w-EDM improved the adhesion of TiN layer to a great extent due to the removal of the heat effected zone (Fig. 9-b, d). Apparently, this recast layer formed at the interface between the TiN coating and the substrate had a negative impact on the adhesion. Moreover, DB eliminated the brittle etaphase precipitated region (Fig. 5) which generally deteriorates the adhesion by leading to formation of cracks at the interface and hence flaking of the coating. 3.6. Wear test The friction curves and the wear track images taken in backscatter mode for 1EDM + TiN and 1EDM + DB + TiN samples were given in Fig. 10. Higher amount of wear debris was detected at the contact area of 1EDM + TiN compared to 1EDM + DB + TiN. The comparison of the friction curves revealed longer running-in period, greater
Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001
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Fig. 3. 2D surface images of TiN coating: (a) 1EDM + TiN, (b) 1EDM + DB + TiN, (c) 4EDM + TiN, (d) 4EDM + DB + TiN, (e) 5EDM + TiN, (f) 5EDM + DB + TiN.
fluctuations and higher mean friction coefficient for 1EDM + TiN with respect to those observed for 1EDM + DB + TiN. X ray maps taken from the wear track on 1EDM + TiN pointed to the formation of complex oxide layer ((W,Cu,Ti)-oxide) inside the track (Fig. 11). The SEM image of the wear track (Fig. 12) indicated that a tribolayer was produced. The light gray zone was the represented the tribolayer whereas the dark gray and bright areas referred to TiN coating and exposed recast layer, respectively.
Fig. 4. 2D image of ground and TiN coated sample (G + TiN).
Fig. 5. XRD patterns of TiN coatings deposited on differently treated samples: (a) G + TiN, (b) 4EDM + DB + TiN, (c) 1EDM + DB + TiN, (d) 4EDM + TiN, (e)1EDM + TiN.
Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001
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Fig. 8. Surface morphology of TiN coating deposited on ground sample (G + TiN). Fig. 6. Backscattered cross sectional SEM image of 1EDM + TiN coated hard metal.
As the mechanism of the tribolayer formation in the wear track on 1EDM + TiN, following depiction could be offered. The higher surface roughness of 1EDM + TiN sample caused considerable amount of wear debris since the compressive and tangential forces leaded to the fracture of the protruding grains on the initial sliding surface and this
brought about longer running-in which was about 80 m. As the sliding went on, the fractured fragments broke into smaller ones triggering the accumulation due to the higher surface area of the wear debris as well as frictional heating and this leaded to the formation of the tribolayer consequently. When a critical thickness of the tribolayer
Fig. 7. Variation of TiN surface morphology depending on pervious treatments: (a) 1EDM + TiN, (b) 1EDM + DB + TiN, (c) 4EDM + TiN, (d) 4EDM + DB + TiN, (e) 5EDM + TiN, (f) 5EDM + DB + TiN.
Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001
E. Sireli et al. / Int. Journal of Refractory Metals and Hard Materials xxx (2016) xxx–xxx
Fig. 9. SEM images of Rockwell-C indentations: (a) 1EDM + TiN, (b) 1EDM + DB + TiN, (c) 4EDM + TiN, (d) 1EDM + DB + TiN.
Fig. 10. Friction curve and wear track micrograph of TiN coatings: (a) 1EDM + TiN, (b) 1EDM + DB + TiN.
Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001
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Fig. 11. Elemental maps taken from the wear track of 1EDM + TiN.
was reached, it started to crack and delaminate into particles again and these particles were dragged away to the periphery of the wear track, as illustrated in Fig. 10-a. In addition to long running-in, higher friction coefficient with the big fluctuations was given rise by this continuous tribolayer which seized the counterface. On the other hand, X ray maps taken from the wear track on 1EDM + DB + TiN (Fig. 13) denoted that considerable amount of Al transferred from the ball onto the wear track due to the wear of the counterpart. The wear scars on the alumina ball were also inspected through 2-D profilometry and correlated with the 3-D images of the wear tracks (Fig. 14). The smoothening of the ball used against 1EDM + DB + TiN was quite clear exhibiting significant amount of wear, while there was no noticeable wear on the one used against 1EDM + TiN. This observation coincided with the fact that there was a significant amount of Al on the wear track on 1EDM + DB + TiN. The 3-D image of the wear track on 1EDM + DB + TiN pointed that the contact area got broadened due to the severe deformation of the ball material, while the wear track on 1EDM + TiN was narrower and deeper since the geometry of the ball was mostly preserved. TiN coatings with mixed crystal structure comprising star shaped and lenticular ones might have better mechanical properties such as hardness and wear resistance since the presence of these structures concurrently retards dislocation movements as mentioned in earlier studies [33,34]. The mixed type of crystal structure of TiN deposited on 1EDM + DB, including star shaped and nodular grains in which lenticular subgrains grew was revealed in Fig. 15. For the materials used in cutting tool industry, deforming workpiece while conserving their surface integrity is mandatory regardless of the fact that they are coated or not. The inspections conducted either on the wear track on 1EDM + DB + TiN, or on the ball material indicated that 1EDM + DB + TiN exhibited better friction behavior and wear resistance in this respect. Additionally, much better adhesion of TiN coatings deposited on DB'ed samples (Fig. 9-d) contributed to the superior wear
Fig. 12. Backscatter SEM image taken from the inside of the wear track of 1EDM + TiN.
resistance of 1EDM + DB + TiN with respect to the one belonging to 1EDM + TiN. 4. Summary and conclusions In this study, the effects of heat affected zones formed as a result of w-EDM process on the surface integrity of a hard metal alloy and the morphology of TiN layer deposited subsequently were investigated. Adhesion, friction and wear properties of TiN layers grown on the surfaces with different treatment conditions were examined. The results of this research could be summarized as follows: (1) Even 10 s of DB could be helpful to remove recast layer, etaphase precipitation and transferred wire material caused by a rough wEDM process. (2) The surface conditions generated by w-EDM had a considerable influence on the morphology of TiN coating deposited subsequently. TiN coatings applied directly on w-EDM'ed surfaces had a rough, irregular morphology; whereas, the ones applied on DB'ed surfaces subsequent to w-EDM exhibited much finer and mixed crystal structures consisting of nodular, star shaped and lenticular grains regardless of w-EDM parameters. (3) Applying multi pass w-EDM up to 5 passes could create fine TiN crystals which were similar to the ones obtained the on DB'ed surfaces, however, this long procedure would not be cost effective taking the operation time, labor, energy and consumable costs into account.
Fig. 13. Al Accumulation rich wear debris at the periphery of the wear track on 1EDM + DB + TiN.
Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001
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Fig. 14. 2-D profiles of the wear scars on the balls along with the 3-D images of the wear tracks on (a) 1EDM + TiN, and (b)1EDM + DB + TiN.
(4) TiN layers deposited on the DB'ed samples had a better adhesion than the ones deposited on w-EDM'ed surfaces directly. (5) The friction and wear properties of TiN coatings deposited on the DB'ed sample (1EDM + DB + TiN) subsequent to rough w-EDM (single pass) were superior with respect to the one deposited on the rough w-EDM'ed surface (1EDM + TiN). (6) While TiN layer on the DM'ed specimen (1EDM + DB + TiN) protected its integrity during the wear test, causing material transfer from the alumina ball onto the wear track, the one applied on the roughly w-EDM'ed surface (1EDM + TiN) could not resist sliding forces exposing the recast layer underneath and causing high amount of wear debris which turned into a complex tribolayer. (7) w-EDM could be a very efficient alternative route for forming
Fig. 15. Mixed crystal structure of TiN coating deposited on 1EDM + DB.
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Please cite this article as: E. Sireli, et al., Effects of wire-electro discharge machining process on surface integrity of WC-10Co alloy, Int J Refract Met Hard Mater (2016), http://dx.doi.org/10.1016/j.ijrmhm.2016.12.001