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Laser machining of WC-Co green compacts for cutting tools manufacturing
International Journal of Refractory Metals & Hard Materials 81 (2019) 316–324
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International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Laser machining of WC-Co green compacts for cutting tools manufacturing a,⁎
b
b
a
a
a
B. Guimarães , D. Figueiredo , C.M. Fernandes , F.S. Silva , G. Miranda , O. Carvalho a b
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Center for Micro-Electro Mechanical Systems (CMEMS-UMinho), University of Minho, Campus de Azurém, 4800-058 Guimarães, Portugal Palbit S.A., P.O. Box 4, 3854-908 Branca, Portugal
ARTICLE INFO
ABSTRACT
Keywords: Cutting tools WC-Co Green compacts Laser machining
Nowadays, conventional machining of WC-Co green compacts is used in industries in order to achieve desired dimensions, complex geometries and good surface quality. However, conventional machining presents some problems, namely tool wear, tool breakage, chatter, vibration and deflection besides mechanically induced damage to the compact. Laser machining is a promising approach to machine WC-Co green compacts, since it is performed without contact and allows great flexibility for producing several geometries, high material removal rate, good surface quality and precision, also for complex shapes. It also allows the production of details smaller than 0.2 mm, hardly manufactured by conventional machining, due to the brittle nature of cutting tools of very small dimensions. Due to the abovementioned reasons, laser machining presents a great potential for lowering the production costs of cemented carbide tools. This work addresses the laser machining of WC-Co green compacts, using a Nd:YAG laser and performing different strategies and combinations of laser parameters to obtain different types of profiles (grooves, areas and specific geometries). Results showed that an effective laser machining of WC-Co green compacts is attained when using laser power of 3 W, scan speed of 128 mm/s, 8 passages and line spacing of 0.08 mm. These parameters were effective for obtaining around 800 μm depth geometries, where the addition of a finishing step (1.5 W, 256 mm/s and 8 passages) improved the quality of the edge of the machined geometry. The laser machined compacts were sintered using a SinterHIP process and no undesirable phases were detected, as eta-phase or graphite.
1. Introduction Cemented carbides are commonly used in industrial applications, such as cutting tools, mechanical seal rings, and press-stamping dies [1]. The most commonly used constituents of cemented carbides are fine tungsten carbide (WC) particles, which are hard and brittle, and cobalt (Co) as metal binder, which is relatively soft and ductile [2,3]. These constituents are responsible for the high toughness and hardness, wear and deformation resistance typical of these materials [2,4]. Cemented carbides are widely used as cutting tool materials to machine demanding materials, including gray cast iron, ductile nodular iron, austenitic stainless steel, nickel-base alloys, titanium alloys, among others [5]. According to some data from Dedalus Consulting (Cutting Tools 2014), the global market for cutting tools reached 16.33 billion dollars in 2013 with a foreseen annual growth of 4–5%. Also, in 2013, cemented carbide tools represented about 53% of worldwide consumption [6]. With the advance of technology, new materials with higher melting
⁎
point, higher hardness and lower thermal conductivity are being used in modern industries. These materials create enormous challenges during the machining processes. The combination of high hardness and low thermal conductivity lead to a rapid temperature increase in the cutting zone, which increases wear, thus reducing the lifetime of the cutting tool [7,8]. Furthermore, it also affects the dimensional precision, surface quality and integrity of the machined parts [9]. This scenario creates the need for improved cutting tools. Recently, promising surface modification approaches are being proposed for obtaining cutting tools with higher tribological performance [10,11]. Surface texturing, which involves flat and smooth lands interrupted by depressions is one of the approaches that has been developed for enhancing wear resistance and lowering the coefficient of friction [12–14]. These depressions can trap wear debris, store lubricant and also contribute to increase the load carrying capacity of the sliding surface under fluid lubrification [10,15]. Powder metallurgy routes, i.e. milling, pressing and sintering, are used to process WC-Co once by other routes their fabrication is difficult or at least not allowing to obtain the required specific properties
Corresponding author. E-mail address: [email protected] (B. Guimarães).
https://doi.org/10.1016/j.ijrmhm.2019.03.018 Received 21 December 2018; Received in revised form 28 February 2019; Accepted 17 March 2019 Available online 18 March 2019 0263-4368/ © 2019 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 81 (2019) 316–324
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Fig. 1. SEM images of WC-Co granules. Table 1 Summary of the laser processing parameters used on the machining of grooves.
Fig. 2. Schematic representation of the grooves machining by means Nd:YAG laser.
[16,17]. WC-Co presents high hardness after sintering which makes the machining of this material into complex shapes and with precise dimensions very expensive and time consuming, with a high tendency to initiate micro cracks [18–20]. Therefore, the creation of surface details that cannot be moulded during compaction and tight dimensional tolerances are usually made before sintering, by machining green compacts [21,22]. Conventional machining still faces some challenges, like tool wear, tool breakage, chatter, vibration and deflection, mechanically induced tool damage, tool costs, inability to perform small details, as grooves having dimensions smaller than 0.2 mm [23–25]. When using laser for machining hard and brittle materials, like sintered WC-Co, micro cracks are usually introduced, due to locally induced thermal mismatches, besides typically presenting a low machining efficiency and high cost [25,26]. To avoid most of the previously mentioned problems, laser machining of green compacts arises as a promising approach. The adoption of laser for green compacts machining allows great flexibility, since lasers are optically manipulated and the machining is made without contact, allowing a high material removal rate, good surface quality, precise dimensions and the production of complex shapes [23]. Several studies have been made on the production of ceramics by green state machining [25–31]. Yang et al. [25] successfully used a CO2 laser to machine Al2O3 green bodies to the desired shape. The influence of laser output power and scanning speed on the machining of green Al2O3 ceramic complex shape bodies were studied and better surface quality and deeper machining depth were obtained when using a power of 30 W. Dadhich el al. [26] reported that laser machining of green alumina compacts allows the formation of microfeatures with better edge retention and precision in comparison to conventional machining. In order to withstand the machining process, a pressing binder is used to increase these green compacts strength and machinability [21,30]. When using laser technologies, besides imparting the necessary strength, this binder will allow the machining of these green compacts by being sublimated. To these authors knowledge, there are no available studies in literature regarding laser green machining of
Experiment
Laser Power (W)
Scan Speed (mm/s)
Number of Passages
A1 A2 A3 A4 A5 B1 B2 B3 B4 B5 C1 C2 C3 C4 C5 D1 D2 D3 D4 D5 E1 ⋮ E5 F1 ⋮ F5 G1 ⋮ G5 H1 ⋮ H5 I1 ⋮ I5 J1 ⋮ J5 K1 ⋮ K5 L1 ⋮ L5
0.75
64
1.5
64
3
64
6
64
0.75
128
1.5
128
3
128
6
128
0.75
256
1.5
256
3
256
6
256
1 2 4 8 16 1 2 4 8 16 1 2 4 8 16 1 2 4 8 16 1 ⋮ 16 1 ⋮ 16 1 ⋮ 16 1 ⋮ 16 1 ⋮ 16 1 ⋮ 16 1 ⋮ 16 1 ⋮ 16
WC-Co. In this work, a comprehensive study regarding the laser machining of grooves, areas and a selected geometry is presented, assessing the optimum laser parameters combinations.
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granulation process was performed, with the proportionated powders being homogenized during 48 h in a ball mill. During this process the granules are formed by adding 2 wt% of paraffin wax (binding agent). After ball milling, the powders were dried and sieved, resulting in granules in the range 100–150 μm, as depicted in Fig. 1. The compaction was performed using a uniaxial pressure of 200 MPa for 30 s to produce WC-10wt.%Co cylindrical green compacts with 12 mm diameter, 4.5 mm thickness and 4.2 g. Fig. 3. Schematic representation of the areas machining by means Nd:YAG laser.
2.2. Green compacts laser machining The green compacts were then laser machined using a Nd:YAG laser. The Nd:YAG laser used in this work has a maximum working power of 6 W, a laser spot size of 3 μm and a wavelength of 1064 nm. The laser beam was focused on the surface of the sample with a focal length of 160 mm. The experimental work was divided in three different laser machining strategies (with an increase in complexity) to investigate the effect of different laser parameters on the WC-Co green compacts machinability:
Table 2 Summary of the laser processing parameters used on the machining of areas. Experiment
Laser Power (W)
Scan Speed (mm/s)
Number of passages
Line Spacing (mm)
A4.1 A4.2 A4.3 A4.4 A4.5 A4.6 A4.7 A4.8 A4.9 A4.10 B4.1 ⋮ B4.10 G4.1 ⋮ G4.10 H4.1 ⋮ H4.10
0.75
64
8
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.01 ⋮ 0.1 0.01 ⋮ 0.1 0.01 ⋮ 0.1
1.5
64
8
3
128
8
6
128
8
(i) Laser machining of grooves; (ii) Laser machining of wide areas; (iii) Laser machining of selected geometry; In these strategies, different laser parameters (laser power, scan speed, number of passages and line spacing) were tested to produce the abovementioned machined parts. (i) Laser machining of grooves Fig. 2 shows a schematic representation of the process used for laser machining of grooves. For this approach, grooves were produced using different combinations of laser parameters, i.e. laser power between 0.75 up to 6 W, scan speed in the range 64–256 mm/s and number of passages from 1 to 16, as presented in Table 1.
Table 3 Summary of the laser processing parameters used on the machining of the selected geometry. Experiment
A4.7 B4.6 G4.8 H4.7
Laser Power (W)
Scan Speed (mm/s)
Number of Passages
Line Spacing (mm)
Energy Density (J/ mm2)
0.75 1.5 3 6
64 64 128 128
8 8 8 8
0.07 0.06 0.08 0.07
1.34 3.13 2.34 5.36
(ii) Laser machining of wide areas Fig. 3 illustrates the process used for laser machining wide areas. To perform this second strategy, four different combinations of laser parameters were selected from the previous study, corresponding to those that achieved the best results. These parameters were combined with different line spacing values (from 0.01 to 0.1 mm, interspaced 0.01 mm), as shown in Table 2. The energy density was calculated using the following equation, being depicted in Table 3 for some of the parameter combinations selected:
Energy density (J/mm2) =
P ×N V×d
where: P = Laser power (W); V = Scan speed (mm/s); d = Line spacing (mm); N = Number of passages.
Fig. 4. Schematic representation of the geometry machining by means Nd:YAG laser.
2. Materials and methods
(iii) Laser machining of selected geometry;
2.1. Materials and green compacts production
The analysis of Scanning Electron Microscopy (SEM) images of the machined areas made using the parameters found in Table 2, allowed to elect four conditions for the machining of a selected geometry (see Fig. 4). The selected laser parameters combinations are presented in Table 3. To achieve a higher depth, for each experiment, the laser parameters combination was repeated four times.
WC and Co powder from Palbit S.A. with an average particle size of 0.8 μm and 1.5 μm, respectively were used to prepare WC-10wt.%Co powder mixture. In order to achieve a granular material with a spherical shape a 318
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Fig. 5. SEM images of the grooves produced using the parameters from Table 1.
(XRD) analysis was performed for crystalline phase identification using a Bruker AXS D8 Discover diffractometer with a Cu-Kα radiation (λ = 1.5418 Å) in θ/2θ mode, range 20° to 90°, step of 0.05 and an integration time of 1 s. 3D Optical Profilometry was performed to evaluate the topography/roughness after laser machining the cavity. A Sensofar S-neox, using surface ISO 25178 (2012), coupled to a SensoSCAN software was used for this analysis. 3. Results and discussion As indicated in Table 1, sixty different parameters combinations were tested for WC-Co grooves machining. Fig. 5 shows SEM images of the grooves produced using these different combinations of laser parameters. It is possible to observe that when combining a scan speed of 64 mm/s (the lowest among the tested), with higher laser powers (3 and 6 W) and a high number of passages (8 and 16), the formation of cracks occurs, due to an excessive energy used, which compromises the green compact integrity. Furthermore, it is possible to conclude that the surface quality of grooves produced using high laser power combined with low speeds is very low. Cracks formation are related with high thermal stress induced by the very localized energy delivered by the laser. To avoid the formation of these cracks, the delivered energy must be lowered, for example by increasing the scan speed to 128 mm/s (Fig. 5). These results show that there is a critical energy value for which the formation of cracks occurs, being this value dependent on the laser parameters combination. By comparing these results with the ones for a scan speed of 64 mm/s, a higher surface quality is found, although with lower material removal (indicated by an apparent lower deepness of the grooves), especially for lower laser power values. This effect is more pronounced the smaller the number of passages. As expected, for a scan speed of 256 mm/s the material removal was the lowest, not having practical effect for laser power of 0.75 and 1.5 W. In a general way, the increase in the number of passages and/or laser power results in grooves apparently deeper. Relatively to the thermally affected zone, in general, this outcome is found for combinations leading to higher energies. As indicated in Table 2, forty different parameters combinations were tested for WC-Co areas machining. The effect of these parameters combinations was assessed by analysing corresponding SEM images and
Fig. 6. Effect of the laser parameters combinations for experiments in Table 2.
Aiming to improve the quality of the machined parts, for experiment G4.8 (that revealed the best results concerning laser machining), a finishing step was performed on the edge of the geometry. This step was made using a laser power of 1.5 W, scan speed of 256 mm/s and 8 passages. 2.3. Dewaxing and sintering This stage was performed at Palbit S.A. according to their specifications. The samples were dewaxed at 600 °C in hydrogen atmosphere and then sintered in a sinter HIP furnace (20 bar, argon atmosphere) at 1480 °C. 2.4. Morphological and crystallographic characterization SEM was used to verify the influence of laser parameters on the green compacts morphology/topography, on the amount of material removal and on the quality of the machined parts/surface. A NanoSEMFEI Nova 200 (FEG/SEM) was used for this analysis. X-Ray Diffraction 319
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Fig. 7. SEM images of the areas produced using the parameters from Table 2.
qualifying the obtained outcome. It was possible to observe three different effects as a result from the laser parameters combinations: melting, removal or no removal. These findings are presented in Fig. 6, linked to the corresponding laser power, line spacing and scan speed. From Fig. 6 it is possible to conclude that for each power/speed combination, there is an optimal line spacing range where an effective material removal is obtained. This range is variable, both regarding its
position and its extent, depending on the power/speed combination. When analysing the line spacing, it is possible to verify that for all the tested power/speed combinations, spacings from 0.01 to 0.03 mm generally lead to ineffective machining due to Co melting. Inversely, line spacings from 0.06 to 0.08 mm allow an effective machining, for all the power/speed combinations. Fig. 7 shows SEM images of the areas produced using these different 320
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Fig. 8. SEM images for experiments a) G4 and b) H4, with parameters P = 3 W, V = 128 mm/s, d = 0.08 mm, N = 8 and P = 6 W, V = 128 mm/s, d = 0.07 mm, N = 8, respectively. Table 4 SEM images of the machined geometries (green vs sintered).
Parameters
G ree n
Sintered 2
A4.7 (1.34 J/mm ) P = 0.75 W V = 64 mm/s d = 0.07 mm N=8
B4.6 (3.13 J/mm2) P = 1.5 W V = 64 mm/s d = 0.06 mm N= 8
G4.8 (2.34 J/mm2) P=3W V = 128 mm/s d = 0.08 mm N= 8
H4.7 (5.36 J/mm2) P=6W V = 128 mm/s d = 0.07 mm N= 8
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Fig. 9. SEM images of specimen from experiment G4.8 (a) before and (b) after the sintering stage.
Fig. 10. Roughness values and 3D optical profilometry images from experiment G4.8, a) 2D view and b) 3D view.
effective machining. In fact, the same energy density value, obtained by combining different laser parameters often results in different outcomes or properties [32,33]. As an example, for the same energy density of 4.69 J/mm2 cobalt melting occurs for experiment G4 but not for experiment B4, where material removal is obtained (see Fig. 8). Table 4 shows SEM images of a geometry machined with the four best laser parameters combinations from the previous study (marked yellow on Fig. 7). Although, the parameters used to machine this geometry were the same from the previous study, the machining strategy was different once machining using parallel lines is more suitable for reproducing complex shapes. By analysing the SEM images from Table 4 it is possible to conclude that for experiment A4.7 the energy density used was too low (1.34 J/ mm2), especially due to the high line spacing (0.07 mm) that caused the removal of material not to be complete for the intended area. For experiments B4.6 and H4.7 the observed effects are similar, with some cobalt melting occurring, resulting in high roughness on the bottom of the cavity, however this effect is more pronounced for experiment H4.7 due to the higher energy density used. As previously mentioned, for lower scan speed and higher laser power (i.e. higher energy density), the material removal is higher, but in this particular case, even though experiment B4.6 has higher energy density in comparison with experiment G4.8, this led to an increase of the roughness in the bottom of the cavity, which is not desired. Therefore, experiment G4.8 is the one that presents the best results, achieving a deep and fully defined geometry with surface quality on the bottom, allowing to conclude that these laser parameters combination (resulting in an energy density value of 2.34 J/mm2) are the most suitable for machining areas on WC-Co green compacts. The contraction of the material after the sintering stage was very significant, resulting in approximately 18% (linear contraction) and about 43% (volume contraction), being these values in accordance with the ones presented in literature for this material [3,34]. Fig. 9 shows the difference in depth of this geometry in a specimen from experiment
Fig. 11. SEM image after finishing step of specimen from experiment G4.8.
combinations of laser parameters, showing the abovementioned three different effects. In a general way, it was found that energy density can be directly correlated with the machining efficiency. While for lower energy densities no material removal occurs, the highest energy density values led to the cobalt melting. Intermediate energy densities values were found most efficient for the machining of these green compacts, however the bounds for this optimum range of energy is dependent on the selected combination of parameters, as seen in Fig. 7. In fact, while for experiment G4, laser power of 3 W, scan speed of 128 mm/s, 8 passages and line spacing of 0.08 mm (Table 2), the energy density values where material removal occurred are in a very narrow zone, for experiment H4, laser power of 6 W, scan speed of 128 mm/s, 8 passages and line spacing of 0.07 mm, a bigger range of values allow to achieve the same effect. This observation allows concluding that the analysis per se of the energy density value used in laser technologies and in this case for laser machining, although valid, is not enough to guarantee the most 322
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comparison purpose, an XRD of the (c) initial WC-Co granules is also included. Fig. 12 shows XRD peaks locations consistent with the characteristic peaks for HC-WC and FCC-Co with no other phases detected. Cobalt characteristic peak is not detected in XRD pattern for WC-Co granules due to the presence of paraffin wax. These analyses led to conclude that laser machining WC-Co green compacts do not leads to the formations of carbon deficient phases like W2C, Co3W3C and Co6W6C or free carbon, which would affect significantly the mechanical properties of WC-Co [35]. 4. Conclusions
• The laser parameters used on this work, namely laser power, scan • • • • •
Fig. 12. X-ray diffraction patterns of WC-Co a) laser machined surface (bottom); b) top surface and c) WC-Co granules.
•
G4.8 (energy density 2.34 J/mm2), before and after sintering. A depth of about 949 μm was achieved in the green compact, while the sintered specimen exhibited 778 μm depth, corresponding to a linear contraction of 18%. To evaluate the influence of laser machining in the roughness of the cavity, 3D optical profilometry was performed to experiment G4.8, as depicted in Fig. 10. 3D optical profilometry results show big irregularities, i.e. deep valleys, presenting a maximum valley depth (Sv) of 47.18 μm probably due to WC-Co granules removal during laser machining process, once this process is performed in a green compact. Results also show that laser machining of WC-Co green compacts induces a roughness on the surface of the machined cavity, achieving a roughness value (Sa) of 10.66 μm for the laser parameters used for experiment G4.8. This induced roughness depends on the laser parameters combination used, being possible to control it by adding a post processing step (see Fig. 11). Fig. 11 shows SEM image of the machined geometry with a finishing step for experiment G4.8, which was found to present the best results. When analysing Fig. 11, it is clear that the addition of a finishing step improved the quality of the edges, thus enhancing the geometrical and dimensional precision of the geometry. These finding indicate that the surface roughness is dependent on two aspects, firstly the laser path together with the laser parameters combination used, that will dictate the surface topography, secondly the powder granules size (between 100 and 150 μm) and the compaction that was performed when applying pressure to the mixture. Aiming to verify the influence of the laser on the WC-Co green compacts, namely regarding the formation of any new constituents, XRD analyses were made, for experiment G4.8 specimen. Fig. 12 shows the XRD patterns of the (a) machined surface (bottom) and (b) top (nonmachined) surface of WC-Co sintered specimen (see Fig. 11). For
speed, number of passages and line spacing influences the surface finishing and material removal efficiency of WC-Co green compacts. Generally, the increase in the number of passages and/or laser power results in higher material removal, leading to grooves apparently deeper. The isolated analysis of the energy density value used for the laser machining was shown insufficient to guarantee the most effective machining. In this sense, it was necessary to assess the influence of each processing parameter individually. The best parameters for laser machining areas/geometries on WC-Co green compacts were laser power of 3 W, scan speed of 128 mm/s, 8 passages and line spacing of 0.08 mm, followed by a finishing step to improve the quality on the edge of the geometry. 3D optical profilometry showed that these parameters induce a roughness value (Sa) of 10.66 μm. The laser machining of WC-Co green compacts was proven adequate for retaining the original phases, non‑leading to the formations of carbon deficient phases that can affect these tools properties. This study showed that laser machining WC-Co green compacts is a viable alternative to conventional machining, performed today in the cutting tools industries.
Acknowledgements This work was supported by FCT (Fundação para a Ciência e Tecnologia) through the project POCI-01-0145-FEDER-030353 (SMARTCUT) and also by the project NORTE 01-0145_FEDER-000018 (HAMaBICo). This work was also financed by FCT with the reference project UID/EEA/04436/2019. References [1] M. Eriksson, M. Radwan, Z. Shen, Spark plasma sintering of WC, cemented carbide and functional graded materials, Int. J. Refract. Met. Hard Mater. 36 (2013) 31–37, https://doi.org/10.1016/j.ijrmhm.2012.03.007. [2] E.M. Trent, P.K. Wright, Cutting tool materials II: cemented carbides, Met. Cut., Fouth Edi, Butterworth-Heinemann, 2000, pp. 175–226. [3] G.S. Upadhyaya, Materials science of cemented carbides — an overview, Mater. Des. 22 (2001) 483–489, https://doi.org/10.1016/S0261-3069(01)00007-33. [4] S.H. Chang, S.L. Chen, Characterization and properties of sintered WC-Co and WCNi-Fe hard metal alloys, J. Alloys Compd. 585 (2014) 407–413, https://doi.org/10. 1016/j.jallcom.2013.09.188. [5] C.M. Fernandes, A.M.R. Senos, Cemented carbide phase diagrams: a review, Int. J. Refract. Met. Hard Mater. 29 (2011) 405–418, https://doi.org/10.1016/j.ijrmhm. 2011.02.004. [6] Dedalus Consulting, Cutting Tools (New York),, 2014. www.dedalusconsulting.com. [7] S. Sulaiman, A. Roshan, S. Borazjani, Effect of cutting parameters on tool-chip interface temperature in an orthogonal turning process, Adv. Mater. Res. 903 (2014) 21–26, https://doi.org/10.4028/www.scientific.net/AMR.903.21. [8] N.A. Abukhshim, P.T. Mativenga, M.A. Sheikh, Heat generation and temperature prediction in metal cutting : a review and implications for high speed machining, Int. J. Mach. Tools Manuf. 46 (2006) 782–800, https://doi.org/10.1016/j. ijmachtools.2005.07.024. [9] S. Pervaiz, I. Deiab, E. Moustafa, A. Rashid, M. Nicolescu, A coupled FE and CFD approach to predict the cutting tool temperature profile in machining, Procedia CIRP 17 (2014) 750–754, https://doi.org/10.1016/j.procir.2014.01.104. [10] R. Meng, J. Deng, Y. Liu, R. Duan, G. Zhang, Improving tribological performance of
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Laser machining of WC-Co green compacts for cutting tools manufacturing a,⁎
b
b
a
a
a
B. Guimarães , D. Figueiredo , C.M. Fernandes , F.S. Silva , G. Miranda , O. Carvalho a b
T
Center for Micro-Electro Mechanical Systems (CMEMS-UMinho), University of Minho, Campus de Azurém, 4800-058 Guimarães, Portugal Palbit S.A., P.O. Box 4, 3854-908 Branca, Portugal
ARTICLE INFO
ABSTRACT
Keywords: Cutting tools WC-Co Green compacts Laser machining
Nowadays, conventional machining of WC-Co green compacts is used in industries in order to achieve desired dimensions, complex geometries and good surface quality. However, conventional machining presents some problems, namely tool wear, tool breakage, chatter, vibration and deflection besides mechanically induced damage to the compact. Laser machining is a promising approach to machine WC-Co green compacts, since it is performed without contact and allows great flexibility for producing several geometries, high material removal rate, good surface quality and precision, also for complex shapes. It also allows the production of details smaller than 0.2 mm, hardly manufactured by conventional machining, due to the brittle nature of cutting tools of very small dimensions. Due to the abovementioned reasons, laser machining presents a great potential for lowering the production costs of cemented carbide tools. This work addresses the laser machining of WC-Co green compacts, using a Nd:YAG laser and performing different strategies and combinations of laser parameters to obtain different types of profiles (grooves, areas and specific geometries). Results showed that an effective laser machining of WC-Co green compacts is attained when using laser power of 3 W, scan speed of 128 mm/s, 8 passages and line spacing of 0.08 mm. These parameters were effective for obtaining around 800 μm depth geometries, where the addition of a finishing step (1.5 W, 256 mm/s and 8 passages) improved the quality of the edge of the machined geometry. The laser machined compacts were sintered using a SinterHIP process and no undesirable phases were detected, as eta-phase or graphite.
1. Introduction Cemented carbides are commonly used in industrial applications, such as cutting tools, mechanical seal rings, and press-stamping dies [1]. The most commonly used constituents of cemented carbides are fine tungsten carbide (WC) particles, which are hard and brittle, and cobalt (Co) as metal binder, which is relatively soft and ductile [2,3]. These constituents are responsible for the high toughness and hardness, wear and deformation resistance typical of these materials [2,4]. Cemented carbides are widely used as cutting tool materials to machine demanding materials, including gray cast iron, ductile nodular iron, austenitic stainless steel, nickel-base alloys, titanium alloys, among others [5]. According to some data from Dedalus Consulting (Cutting Tools 2014), the global market for cutting tools reached 16.33 billion dollars in 2013 with a foreseen annual growth of 4–5%. Also, in 2013, cemented carbide tools represented about 53% of worldwide consumption [6]. With the advance of technology, new materials with higher melting
⁎
point, higher hardness and lower thermal conductivity are being used in modern industries. These materials create enormous challenges during the machining processes. The combination of high hardness and low thermal conductivity lead to a rapid temperature increase in the cutting zone, which increases wear, thus reducing the lifetime of the cutting tool [7,8]. Furthermore, it also affects the dimensional precision, surface quality and integrity of the machined parts [9]. This scenario creates the need for improved cutting tools. Recently, promising surface modification approaches are being proposed for obtaining cutting tools with higher tribological performance [10,11]. Surface texturing, which involves flat and smooth lands interrupted by depressions is one of the approaches that has been developed for enhancing wear resistance and lowering the coefficient of friction [12–14]. These depressions can trap wear debris, store lubricant and also contribute to increase the load carrying capacity of the sliding surface under fluid lubrification [10,15]. Powder metallurgy routes, i.e. milling, pressing and sintering, are used to process WC-Co once by other routes their fabrication is difficult or at least not allowing to obtain the required specific properties
Corresponding author. E-mail address: [email protected] (B. Guimarães).
https://doi.org/10.1016/j.ijrmhm.2019.03.018 Received 21 December 2018; Received in revised form 28 February 2019; Accepted 17 March 2019 Available online 18 March 2019 0263-4368/ © 2019 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 81 (2019) 316–324
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Fig. 1. SEM images of WC-Co granules. Table 1 Summary of the laser processing parameters used on the machining of grooves.
Fig. 2. Schematic representation of the grooves machining by means Nd:YAG laser.
[16,17]. WC-Co presents high hardness after sintering which makes the machining of this material into complex shapes and with precise dimensions very expensive and time consuming, with a high tendency to initiate micro cracks [18–20]. Therefore, the creation of surface details that cannot be moulded during compaction and tight dimensional tolerances are usually made before sintering, by machining green compacts [21,22]. Conventional machining still faces some challenges, like tool wear, tool breakage, chatter, vibration and deflection, mechanically induced tool damage, tool costs, inability to perform small details, as grooves having dimensions smaller than 0.2 mm [23–25]. When using laser for machining hard and brittle materials, like sintered WC-Co, micro cracks are usually introduced, due to locally induced thermal mismatches, besides typically presenting a low machining efficiency and high cost [25,26]. To avoid most of the previously mentioned problems, laser machining of green compacts arises as a promising approach. The adoption of laser for green compacts machining allows great flexibility, since lasers are optically manipulated and the machining is made without contact, allowing a high material removal rate, good surface quality, precise dimensions and the production of complex shapes [23]. Several studies have been made on the production of ceramics by green state machining [25–31]. Yang et al. [25] successfully used a CO2 laser to machine Al2O3 green bodies to the desired shape. The influence of laser output power and scanning speed on the machining of green Al2O3 ceramic complex shape bodies were studied and better surface quality and deeper machining depth were obtained when using a power of 30 W. Dadhich el al. [26] reported that laser machining of green alumina compacts allows the formation of microfeatures with better edge retention and precision in comparison to conventional machining. In order to withstand the machining process, a pressing binder is used to increase these green compacts strength and machinability [21,30]. When using laser technologies, besides imparting the necessary strength, this binder will allow the machining of these green compacts by being sublimated. To these authors knowledge, there are no available studies in literature regarding laser green machining of
Experiment
Laser Power (W)
Scan Speed (mm/s)
Number of Passages
A1 A2 A3 A4 A5 B1 B2 B3 B4 B5 C1 C2 C3 C4 C5 D1 D2 D3 D4 D5 E1 ⋮ E5 F1 ⋮ F5 G1 ⋮ G5 H1 ⋮ H5 I1 ⋮ I5 J1 ⋮ J5 K1 ⋮ K5 L1 ⋮ L5
0.75
64
1.5
64
3
64
6
64
0.75
128
1.5
128
3
128
6
128
0.75
256
1.5
256
3
256
6
256
1 2 4 8 16 1 2 4 8 16 1 2 4 8 16 1 2 4 8 16 1 ⋮ 16 1 ⋮ 16 1 ⋮ 16 1 ⋮ 16 1 ⋮ 16 1 ⋮ 16 1 ⋮ 16 1 ⋮ 16
WC-Co. In this work, a comprehensive study regarding the laser machining of grooves, areas and a selected geometry is presented, assessing the optimum laser parameters combinations.
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granulation process was performed, with the proportionated powders being homogenized during 48 h in a ball mill. During this process the granules are formed by adding 2 wt% of paraffin wax (binding agent). After ball milling, the powders were dried and sieved, resulting in granules in the range 100–150 μm, as depicted in Fig. 1. The compaction was performed using a uniaxial pressure of 200 MPa for 30 s to produce WC-10wt.%Co cylindrical green compacts with 12 mm diameter, 4.5 mm thickness and 4.2 g. Fig. 3. Schematic representation of the areas machining by means Nd:YAG laser.
2.2. Green compacts laser machining The green compacts were then laser machined using a Nd:YAG laser. The Nd:YAG laser used in this work has a maximum working power of 6 W, a laser spot size of 3 μm and a wavelength of 1064 nm. The laser beam was focused on the surface of the sample with a focal length of 160 mm. The experimental work was divided in three different laser machining strategies (with an increase in complexity) to investigate the effect of different laser parameters on the WC-Co green compacts machinability:
Table 2 Summary of the laser processing parameters used on the machining of areas. Experiment
Laser Power (W)
Scan Speed (mm/s)
Number of passages
Line Spacing (mm)
A4.1 A4.2 A4.3 A4.4 A4.5 A4.6 A4.7 A4.8 A4.9 A4.10 B4.1 ⋮ B4.10 G4.1 ⋮ G4.10 H4.1 ⋮ H4.10
0.75
64
8
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.01 ⋮ 0.1 0.01 ⋮ 0.1 0.01 ⋮ 0.1
1.5
64
8
3
128
8
6
128
8
(i) Laser machining of grooves; (ii) Laser machining of wide areas; (iii) Laser machining of selected geometry; In these strategies, different laser parameters (laser power, scan speed, number of passages and line spacing) were tested to produce the abovementioned machined parts. (i) Laser machining of grooves Fig. 2 shows a schematic representation of the process used for laser machining of grooves. For this approach, grooves were produced using different combinations of laser parameters, i.e. laser power between 0.75 up to 6 W, scan speed in the range 64–256 mm/s and number of passages from 1 to 16, as presented in Table 1.
Table 3 Summary of the laser processing parameters used on the machining of the selected geometry. Experiment
A4.7 B4.6 G4.8 H4.7
Laser Power (W)
Scan Speed (mm/s)
Number of Passages
Line Spacing (mm)
Energy Density (J/ mm2)
0.75 1.5 3 6
64 64 128 128
8 8 8 8
0.07 0.06 0.08 0.07
1.34 3.13 2.34 5.36
(ii) Laser machining of wide areas Fig. 3 illustrates the process used for laser machining wide areas. To perform this second strategy, four different combinations of laser parameters were selected from the previous study, corresponding to those that achieved the best results. These parameters were combined with different line spacing values (from 0.01 to 0.1 mm, interspaced 0.01 mm), as shown in Table 2. The energy density was calculated using the following equation, being depicted in Table 3 for some of the parameter combinations selected:
Energy density (J/mm2) =
P ×N V×d
where: P = Laser power (W); V = Scan speed (mm/s); d = Line spacing (mm); N = Number of passages.
Fig. 4. Schematic representation of the geometry machining by means Nd:YAG laser.
2. Materials and methods
(iii) Laser machining of selected geometry;
2.1. Materials and green compacts production
The analysis of Scanning Electron Microscopy (SEM) images of the machined areas made using the parameters found in Table 2, allowed to elect four conditions for the machining of a selected geometry (see Fig. 4). The selected laser parameters combinations are presented in Table 3. To achieve a higher depth, for each experiment, the laser parameters combination was repeated four times.
WC and Co powder from Palbit S.A. with an average particle size of 0.8 μm and 1.5 μm, respectively were used to prepare WC-10wt.%Co powder mixture. In order to achieve a granular material with a spherical shape a 318
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Fig. 5. SEM images of the grooves produced using the parameters from Table 1.
(XRD) analysis was performed for crystalline phase identification using a Bruker AXS D8 Discover diffractometer with a Cu-Kα radiation (λ = 1.5418 Å) in θ/2θ mode, range 20° to 90°, step of 0.05 and an integration time of 1 s. 3D Optical Profilometry was performed to evaluate the topography/roughness after laser machining the cavity. A Sensofar S-neox, using surface ISO 25178 (2012), coupled to a SensoSCAN software was used for this analysis. 3. Results and discussion As indicated in Table 1, sixty different parameters combinations were tested for WC-Co grooves machining. Fig. 5 shows SEM images of the grooves produced using these different combinations of laser parameters. It is possible to observe that when combining a scan speed of 64 mm/s (the lowest among the tested), with higher laser powers (3 and 6 W) and a high number of passages (8 and 16), the formation of cracks occurs, due to an excessive energy used, which compromises the green compact integrity. Furthermore, it is possible to conclude that the surface quality of grooves produced using high laser power combined with low speeds is very low. Cracks formation are related with high thermal stress induced by the very localized energy delivered by the laser. To avoid the formation of these cracks, the delivered energy must be lowered, for example by increasing the scan speed to 128 mm/s (Fig. 5). These results show that there is a critical energy value for which the formation of cracks occurs, being this value dependent on the laser parameters combination. By comparing these results with the ones for a scan speed of 64 mm/s, a higher surface quality is found, although with lower material removal (indicated by an apparent lower deepness of the grooves), especially for lower laser power values. This effect is more pronounced the smaller the number of passages. As expected, for a scan speed of 256 mm/s the material removal was the lowest, not having practical effect for laser power of 0.75 and 1.5 W. In a general way, the increase in the number of passages and/or laser power results in grooves apparently deeper. Relatively to the thermally affected zone, in general, this outcome is found for combinations leading to higher energies. As indicated in Table 2, forty different parameters combinations were tested for WC-Co areas machining. The effect of these parameters combinations was assessed by analysing corresponding SEM images and
Fig. 6. Effect of the laser parameters combinations for experiments in Table 2.
Aiming to improve the quality of the machined parts, for experiment G4.8 (that revealed the best results concerning laser machining), a finishing step was performed on the edge of the geometry. This step was made using a laser power of 1.5 W, scan speed of 256 mm/s and 8 passages. 2.3. Dewaxing and sintering This stage was performed at Palbit S.A. according to their specifications. The samples were dewaxed at 600 °C in hydrogen atmosphere and then sintered in a sinter HIP furnace (20 bar, argon atmosphere) at 1480 °C. 2.4. Morphological and crystallographic characterization SEM was used to verify the influence of laser parameters on the green compacts morphology/topography, on the amount of material removal and on the quality of the machined parts/surface. A NanoSEMFEI Nova 200 (FEG/SEM) was used for this analysis. X-Ray Diffraction 319
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Fig. 7. SEM images of the areas produced using the parameters from Table 2.
qualifying the obtained outcome. It was possible to observe three different effects as a result from the laser parameters combinations: melting, removal or no removal. These findings are presented in Fig. 6, linked to the corresponding laser power, line spacing and scan speed. From Fig. 6 it is possible to conclude that for each power/speed combination, there is an optimal line spacing range where an effective material removal is obtained. This range is variable, both regarding its
position and its extent, depending on the power/speed combination. When analysing the line spacing, it is possible to verify that for all the tested power/speed combinations, spacings from 0.01 to 0.03 mm generally lead to ineffective machining due to Co melting. Inversely, line spacings from 0.06 to 0.08 mm allow an effective machining, for all the power/speed combinations. Fig. 7 shows SEM images of the areas produced using these different 320
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Fig. 8. SEM images for experiments a) G4 and b) H4, with parameters P = 3 W, V = 128 mm/s, d = 0.08 mm, N = 8 and P = 6 W, V = 128 mm/s, d = 0.07 mm, N = 8, respectively. Table 4 SEM images of the machined geometries (green vs sintered).
Parameters
G ree n
Sintered 2
A4.7 (1.34 J/mm ) P = 0.75 W V = 64 mm/s d = 0.07 mm N=8
B4.6 (3.13 J/mm2) P = 1.5 W V = 64 mm/s d = 0.06 mm N= 8
G4.8 (2.34 J/mm2) P=3W V = 128 mm/s d = 0.08 mm N= 8
H4.7 (5.36 J/mm2) P=6W V = 128 mm/s d = 0.07 mm N= 8
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Fig. 9. SEM images of specimen from experiment G4.8 (a) before and (b) after the sintering stage.
Fig. 10. Roughness values and 3D optical profilometry images from experiment G4.8, a) 2D view and b) 3D view.
effective machining. In fact, the same energy density value, obtained by combining different laser parameters often results in different outcomes or properties [32,33]. As an example, for the same energy density of 4.69 J/mm2 cobalt melting occurs for experiment G4 but not for experiment B4, where material removal is obtained (see Fig. 8). Table 4 shows SEM images of a geometry machined with the four best laser parameters combinations from the previous study (marked yellow on Fig. 7). Although, the parameters used to machine this geometry were the same from the previous study, the machining strategy was different once machining using parallel lines is more suitable for reproducing complex shapes. By analysing the SEM images from Table 4 it is possible to conclude that for experiment A4.7 the energy density used was too low (1.34 J/ mm2), especially due to the high line spacing (0.07 mm) that caused the removal of material not to be complete for the intended area. For experiments B4.6 and H4.7 the observed effects are similar, with some cobalt melting occurring, resulting in high roughness on the bottom of the cavity, however this effect is more pronounced for experiment H4.7 due to the higher energy density used. As previously mentioned, for lower scan speed and higher laser power (i.e. higher energy density), the material removal is higher, but in this particular case, even though experiment B4.6 has higher energy density in comparison with experiment G4.8, this led to an increase of the roughness in the bottom of the cavity, which is not desired. Therefore, experiment G4.8 is the one that presents the best results, achieving a deep and fully defined geometry with surface quality on the bottom, allowing to conclude that these laser parameters combination (resulting in an energy density value of 2.34 J/mm2) are the most suitable for machining areas on WC-Co green compacts. The contraction of the material after the sintering stage was very significant, resulting in approximately 18% (linear contraction) and about 43% (volume contraction), being these values in accordance with the ones presented in literature for this material [3,34]. Fig. 9 shows the difference in depth of this geometry in a specimen from experiment
Fig. 11. SEM image after finishing step of specimen from experiment G4.8.
combinations of laser parameters, showing the abovementioned three different effects. In a general way, it was found that energy density can be directly correlated with the machining efficiency. While for lower energy densities no material removal occurs, the highest energy density values led to the cobalt melting. Intermediate energy densities values were found most efficient for the machining of these green compacts, however the bounds for this optimum range of energy is dependent on the selected combination of parameters, as seen in Fig. 7. In fact, while for experiment G4, laser power of 3 W, scan speed of 128 mm/s, 8 passages and line spacing of 0.08 mm (Table 2), the energy density values where material removal occurred are in a very narrow zone, for experiment H4, laser power of 6 W, scan speed of 128 mm/s, 8 passages and line spacing of 0.07 mm, a bigger range of values allow to achieve the same effect. This observation allows concluding that the analysis per se of the energy density value used in laser technologies and in this case for laser machining, although valid, is not enough to guarantee the most 322
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comparison purpose, an XRD of the (c) initial WC-Co granules is also included. Fig. 12 shows XRD peaks locations consistent with the characteristic peaks for HC-WC and FCC-Co with no other phases detected. Cobalt characteristic peak is not detected in XRD pattern for WC-Co granules due to the presence of paraffin wax. These analyses led to conclude that laser machining WC-Co green compacts do not leads to the formations of carbon deficient phases like W2C, Co3W3C and Co6W6C or free carbon, which would affect significantly the mechanical properties of WC-Co [35]. 4. Conclusions
• The laser parameters used on this work, namely laser power, scan • • • • •
Fig. 12. X-ray diffraction patterns of WC-Co a) laser machined surface (bottom); b) top surface and c) WC-Co granules.
•
G4.8 (energy density 2.34 J/mm2), before and after sintering. A depth of about 949 μm was achieved in the green compact, while the sintered specimen exhibited 778 μm depth, corresponding to a linear contraction of 18%. To evaluate the influence of laser machining in the roughness of the cavity, 3D optical profilometry was performed to experiment G4.8, as depicted in Fig. 10. 3D optical profilometry results show big irregularities, i.e. deep valleys, presenting a maximum valley depth (Sv) of 47.18 μm probably due to WC-Co granules removal during laser machining process, once this process is performed in a green compact. Results also show that laser machining of WC-Co green compacts induces a roughness on the surface of the machined cavity, achieving a roughness value (Sa) of 10.66 μm for the laser parameters used for experiment G4.8. This induced roughness depends on the laser parameters combination used, being possible to control it by adding a post processing step (see Fig. 11). Fig. 11 shows SEM image of the machined geometry with a finishing step for experiment G4.8, which was found to present the best results. When analysing Fig. 11, it is clear that the addition of a finishing step improved the quality of the edges, thus enhancing the geometrical and dimensional precision of the geometry. These finding indicate that the surface roughness is dependent on two aspects, firstly the laser path together with the laser parameters combination used, that will dictate the surface topography, secondly the powder granules size (between 100 and 150 μm) and the compaction that was performed when applying pressure to the mixture. Aiming to verify the influence of the laser on the WC-Co green compacts, namely regarding the formation of any new constituents, XRD analyses were made, for experiment G4.8 specimen. Fig. 12 shows the XRD patterns of the (a) machined surface (bottom) and (b) top (nonmachined) surface of WC-Co sintered specimen (see Fig. 11). For
speed, number of passages and line spacing influences the surface finishing and material removal efficiency of WC-Co green compacts. Generally, the increase in the number of passages and/or laser power results in higher material removal, leading to grooves apparently deeper. The isolated analysis of the energy density value used for the laser machining was shown insufficient to guarantee the most effective machining. In this sense, it was necessary to assess the influence of each processing parameter individually. The best parameters for laser machining areas/geometries on WC-Co green compacts were laser power of 3 W, scan speed of 128 mm/s, 8 passages and line spacing of 0.08 mm, followed by a finishing step to improve the quality on the edge of the geometry. 3D optical profilometry showed that these parameters induce a roughness value (Sa) of 10.66 μm. The laser machining of WC-Co green compacts was proven adequate for retaining the original phases, non‑leading to the formations of carbon deficient phases that can affect these tools properties. This study showed that laser machining WC-Co green compacts is a viable alternative to conventional machining, performed today in the cutting tools industries.
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