Int J Adv Manuf Technol (2014) 72:1503–1512 DOI 10.1007/s00170-014-5765-z
ORIGINAL ARTICLE
Development and application of new composite materials as EDM electrodes manufactured via selective laser sintering Tiago Czelusniak & Fred L. Amorim & Camila F. Higa & Armin Lohrengel
Received: 1 July 2013 / Accepted: 6 March 2014 / Published online: 22 March 2014 # Springer-Verlag London 2014
Abstract The cost of a part manufactured by electrical dischargeEDM machining (EDM) is mainly determined by electrode cost. The production of electrodes by conventional machining processes is complex, time consuming, and can account for over 50 % of the total EDM process costs. The emerging additive manufacturing (AM) technologies provide the possibility of direct fabrication of EDM electrodes. Selective laser sintering (SLS) is an alternative AM technique because it has the possibility to directly produce functional components, reducing the tool-room lead time and total EDM costs. The main difficulty of manufacturing an EDM electrode using SLS is the selection of an appropriate material, once both processes require different material properties. The current work focused on the investigation of appropriate materials that fulfill EDM and SLS process demands. Three new metal-matrix materials composed of Mo–CuNi, TiB2–CuNi, and ZrB2–CuNi were developed and characterized. Electrodes under adequate SLS conditions were manufactured through a systematic methodology. EDM experiments using different discharge energies were carried out, and the performance evaluated in terms of material removal rate and volumetric relative wear. The results showed that the powder systems composed of Mo–CuNi, TiB2–CuNi, and ZrB2–CuNi revealed to be successfully processed by SLS, and the EDM experiments demonstrated that the new composite electrodes are promising materials. The work also suggests important topics for future research work on this field. T. Czelusniak : F. L. Amorim (*) : C. F. Higa Mechanical Engineering Graduate Program, Pontifical Catholic University of Parana-PUCPR, Av. Imaculada Conceição, 1155-Prado Velho, 80215-901 Curitiba, Brazil e-mail:
[email protected] A. Lohrengel IMW-Fritz-Süchting-Institut für Maschinenwesen, Technische Universität Clausthal-TUClausthal, Robert Koch Strasse, 32, 38678 Clausthal-Zellerfeld, Germany
Keywords Additive manufacturing . Selective laser sintering . EDM electrodes . Metal-matrix composite materials Nomenclature ie Discharge current [A] ui Open circuit voltage [V] ue Discharge voltage [V] te Discharge duration [μs] ti Pulse duration (μs) t0 Pulse interval time (μs) tp Pulse cycle time (ti +t0) (μs) Ve Electrode wear rate [mm3/min] Vw Material removal rate (mm3/min) ϑ Volumetric relative wear (Ve/Vw) (%) τ Duty factor (ti/tp) (%)
1 Introduction Electrical discharge machining (EDM) is a nonconventional material removal process with unique capabilities. It is widely applied in precision mechanic industry to manufacture complex and accurate 3D components in macro-, micro-, and nanoscale from any electrical conductive material despite its hardness [1]. As in EDM, the electrode shape is mirrored in the workpiece, the tool electrode is an essential element for the process, and the EDM performance is strongly dependent on the electrode material, design, and manufacturing [2]. It is reported that appropriate materials for EDM electrodes are those with properties of high electrical conductivity and high melting point, where the most commonly used are copper (Cu), graphite, and Cu–tungsten, which are generally processed by conventional methods such as turning and milling. However, each material has its advantages and disadvantages in respect to machining performance and costs [3]. It has already been
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reported that the electrode production is specifically a major cost and time spent in the EDM process cycle, which can account for over 50 % of the machining costs [4, 5]. Likewise, the increasing demand in parts complexity, size and accuracy pushed the manufacturing of EDM electrodes by conventional methods more expensive and time consuming [6]. More recently, it was discussed that an accurate method to manufacture complex EDM electrodes quickly and with minimum manual intervention would offer great potential to reduce lead times and tooling costs [7]. In this sense, an alternative approach for the EDM electrodes manufacturing is the use of additive manufacturing (AM) technologies, with the selective laser sintering (SLS) emerging as a promising technique, where the SLS is a powder-based technique that creates parts layer-by-layer using a laser beam to sinter successive layers of loose powder [8]. Additionally, the material selection for an EDM electrode to be manufactured by means of SLS is not trivial, as the demands of both processes in regards to material properties are distinct [9]. Altan et al. [10], when investigating advanced approaches for die and mold manufacturing, highlighted that the production of EDM electrodes by AM has been carried out concurrently to the AM development itself, presenting that it can be divided in two main categories: indirect methods and direct methods, where the main difference between indirect and direct methods is that the first one needs intermediate processes to achieve the desired final part, whereas the direct method claims no use of intermediate processes. Dürr et al. [11] emphasized that in regards to the application of direct methods for producing EDM electrodes the research works are mainly focused on using SLS Technique. They evaluated the effect of process parameters on the direct SLS of an electrode made of bronze, nickel (Ni), and Cu phosphate. A direct relation was found between laser scan speed and porosity and an unclear influence of the scan line spacing. They also observed that the EDM experiments showed that a decrease in porosity of the electrodes leads to a decrease of volumetric relative wear and increased material removal rate. The work conducted by Tay and Haider [12] used direct SLS as a base process for all electrodes and further Cu plated the electrodes with electroless and electroplating methods. EDM experiments were performed for roughing, semiroughing and finishing conditions, comparing electroplated, electroless and untreated electrodes with solid Cu electrodes. Results showed that for roughing and semiroughing regimes untreated electrodes are suitable, but their performances were inferior to the electroplated and electroless electrodes. For finishing regimes, only the Cu electroplated electrodes achieved the desired performance. A similar research was carried out by Dimla et al. [13] where a bronze alloy was directly produced by SLS and subsequently electroplated with Cu. The electrodes were
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found unsuitable for EDM applications, as there was not enough Cu deposition in the electrodes. Later on Meena and Nagahanumaiah [14] used the same bronze alloy to directly produce EDM electrodes by SLS. They optimized the EDM machining parameters by means of gray relational analysis together with Taguchi Method. The authors found out that the discharge current is the most influencing parameter on the EDM process. They also reported an excessive electrode wear owing to its porosity. As shown by Das [15], there are many variations of producing EDM electrodes by AM and each has its features and limitations. Specifically, the direct SLS technique has as its key point the ability to produce complex shape parts without the need of tooling and the elimination of time consuming preprocessing and postprocessing when compared with other methods. Conversely, both SLS and EDM processes are strongly dependent on the materials used, in such way that the EDM electrode material processed by SLS must be selected considering both processes demands. They also remark that one of the main goals in this research field is to obtain an electrode with low porosity aiming to achieve an adequate EDM performance. Lately, Kumar and Kruth [16] have presented that even though the direct fabrication of composites in SLS has been carried out their application in EDM as electrodes is very scarce. In this context, metal matrix composite materials are a potential alternative to unify the desired properties of different materials in an EDM electrode processed by SLS. As for that, in the present work, three different material systems were developed, characterized and evaluated: a powder system composed of ZrB2 and CuNi; another one consisting of TiB2 and CuNi and third one made of Mo and CuNi. EDM electrodes were directly manufactured via SLS and experiments were designed and conducted under different EDM regimes in order to observe the electrodes behavior and performance.
2 Experimental approach To properly investigate the direct production of EDM electrodes using the SLS Technique a systematic experimental methodology composed of two phases was designed as shown in Fig. 1. Phase 1 It refers to the SLS experimental approach. It involved the selection of materials that fulfilled both EDM and SLS processes demands, as detailed in “Materials.” To reduce the number of experiments and at the same time make possible the analysis of the influence of the main SLS parameters, the most important variables that affect the SLS processing were varied (“SLS processing”). Simultaneously, the samples were characterized after each step to
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Fig. 1 Experimental flowchart developed for the SLS and EDM experiments
analyze aspects such as densification behavior, porosity and surface morphology of the samples, as described in “Characterization.” As presented in Fig. 1, it goes through a sequence that starts with the variation of the layer thickness in order to find its best value, and then it is followed by the optimization of laser scan speed and ends up seeking out an appropriate scan line spacing. Phase 2 This stage is related to the EDM experimental approach (“EDM experiments”). To complete the experimental cycle, EDM experiments are performed with the electrodes manufactured under the optimized SLS parameters, so that the electrodes behavior and performance can be quantified when machining a tool steel workpiece. The main objective is to observe the electrodes performance in terms of material removal rate Vw and volumetric relative wear ϑ, as well as to assess the surface finish of the workpiece. Finishing, semifinishing, and roughing EDM regimes were investigated. The results are compared with solid Cu electrodes electrical discharge machined under the same conditions and also with Cu powder electrodes manufactured by SLS.
3 Materials The materials used in this study are composed of a metallic matrix (Cu–Ni) and a ceramic (ZrB2, TiB2) or metallic (molybdenum (Mo)) reinforcement. Zirconium diboride (ZrB2) and
titanium diboride (TiB2) are members of a family of materials known as ultrahigh-temperature ceramics (UHTCs). These materials present a unique combination of high melting point, high thermal and electrical conductivity, chemical inertness against molten metals, and thermal shock resistance. Such properties make these ceramics important for applications where thermal wear resistance is desirable. Mo is a refractory metal very attractive for high-temperature applications, due to its high melting point, low thermal expansion coefficient, and a thermal conductivity that exceeds most of the other metals. Cu is a well-known material in EDM because of its high thermal and electrical conductivity, being applied as EDM electrode to machine a wide range of materials. The combination of the above mentioned matrix and reinforcements may lead to interesting properties for electrical and thermal applications, being potential materials in EDM as electrodes. Ni present in the matrix acts as a sintering aid for the matrix-reinforcement systems, improving the SLS process behavior. The metallic matrix was common to all powder systems and is composed of pre-alloyed Cu–Ni powder with 90 wt.% of Cu and 10 wt.% of Ni, spherical particle shape, and a mean particle size of 33 μm. Three reinforcements materials were used: ZrB2 with irregular particle shape and a mean particle size of 8.73 μm, TiB2 with irregular structure and a mean particle size of 27.4 μm and Mo with a mean particle size of 3–5 μm. Scanning electron microscope (SEM) micrographs of the described powder systems are depicted in Fig. 2. The metallic matrix was mixed with each of the reinforcements to obtain the following compositions, which are shown in Table 1: ZrB2, 30 wt.% and Cu(90)Ni(10), 70 wt.%; TiB2, 30 wt.% and Cu(90)Ni(10), 70 wt.%; Mo, 63 wt.% and Cu(90)Ni(10), 37 wt.%.
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Fig. 2 Powder images from SEM depicting a ZrB2–CuNi powder system, b TiB2–CuNi powder system, and c Mo–CuNi powder system
3.1 SLS processing The electrodes manufacture using the direct SLS technique was performed in an EOSINT M 250 Xtended machine, with a 200-W CO2 laser with laser spot size of 0.4 mm, nitrogen atmosphere, and pre-heating temperature of 80 °C. Three SLS process parameters were chosen because of their importance in the laser energy delivery to the powder bed: layer thickness, laser scan speed and scan line spacing. The optimization of the SLS parameters was performed in three phases by means of a series of experiments. The optimized parameters for the three powder systems evaluated are shown in Table 2.
and porosity of the electrodes were measured using a Micromeritics GeoPyc 1360 (Archimedes’ Principle). Surface roughness was measured with a Mahr Perthen Perthometer S8P. The morphology of the powders was characterized using a CamScan SEM. The bulk density of the powder system was measured with a POROTEC helium pycnometer. Particle size distribution analysis was made using a SYMPA TEC HELOS. 3.3 EDM experiments
3.2 Characterization
The EDM experiments were conducted aiming to observe the general behavior of the composite electrodes manufactured by SLS. The following materials, equipment and methods were applied for all series of experiments:
Samples characterization was performed after each processing phase. For the electrodes manufactured with the optimal SLS parameters, SEM and EDX analysis of the specimens was applied in order to observe the microstructure of the electrodes and evaluate their chemical composition. Electrical conductivity was calculated from electrical resistivity measurements made with a Conrad Mo 199 Milliohmeter. Apparent density
(i) EDM machine: a Charmilles ROBOFORM 30 CNC machine equipped with an isoenergetic generator, which means that it is possible to set the discharge energy We supplied to the working gap during a spark (We =ue.ie.te (mJ)), was employed. (ii) Tool electrodes: Cubic electrodes (10×10×10 mm) using the ZrB2–CuNi, TiB 2–CuNi, and Mo–CuNi
Table 1 Powder systems composition
Table 2 SLS parameters optimized for the three powder systems
Powder system
Reinforcement fraction (wt.%)
Matrix fraction (wt.%)
Material
Layer thickness (mm)
Laser scan speed (mm/s)
Scan line spacing (mm)
ZrB2–CuNi TiB2–CuNi Mo–CuNi
30 30 63
70 70 37
ZrB2–CuNi TiB2–CuNi Mo–CuNi
0.04 0.05 0.02
50 50 50
0.3 0.1 0.2
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(iii)
(iv) (v)
(vi)
(vii)
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materials were manufactured by SLS. For purposes of performance comparison, Cu powder electrodes produced by SLS under optimized parameters were tested with the same EDM conditions applied to electrodes. Solid Cu electrodes (10×10×10 mm) produced by conventional machining were also used as a benchmark. Workpiece: AISI H13 tool steel cubic samples (12×12× 12 mm) with Ra=0.42 μm on the surface to be electrical discharge machined. The chemical composition of AISI H13 tool steel is: 0.40 % C, 1.0 % Si, 1.0 % Mn, 5.2 % Cr, 1.5 % Mo, 0.9 % V, and 0.00765 g/mm3 density at 200 °C. The AISI H13 workpieces were quenched and tempered to an average 45 HRC. Dielectric fluid: a hydrocarbon fluid with 3 cSt at 40 °C, with flash point of 134 °C and 0.01 wt.% of aromatic contents were used for the experiments. Flushing method: a jet of fluid directly to the gap plus immersion of the pair electrode/workpiece. It was sufficient to evacuate the excess of eroded particles away from the working gap as well as to promote adequate cooling. Electrical variables: the EDM experiments were carried out under four different standard machining conditions, from roughing (ie =32 A), semifinishing (ie =12 A) to finishing (ie =2 and 4 A). The main electrical parameters for each condition are shown in Table 3, which are established by the EDM machine-tool manufacturer as being the optimal conditions for the pair of solid Cu electrodes and steel workpieces. The machining time was 40 min for 4 A, 25 min for 12 A, and 8 min for 32 A. EDM performance measurement: the precise quantification of the material removal rate Vw and the volumetric relative wear ϑ was possible using a precise scale (resolution of 0.0001 g) to weigh the electrodes and workpiece before and after each EDM experiment. It is important to mention that during the EDM process the electrodes can absorb some quantity of the dielectric fluid because they are porous. To avoid any error when measuring the mass of the electrodes it was necessary to carry out a drying period. Therefore, the electrodes were kept in a furnace at 150 °C for 24 h before and after each EDM experiment.
4 Results and discussion This section presents the results for each selected material using the experimental designed methodology, starting with the SLS results found for the optimized EDM electrodes followed by the experiments using the produced EDM electrodes samples. 4.1 Selective laser sintering results After the optimization of the SLS parameters for the ZrB2– CuNi, TiB2–CuNi, and Mo–CuNi, the most adequate values (Table 2) were used to manufacture 10 mm cubic EDM electrodes for further characterization and EDM experimenting, as presented in Fig. 3. All materials could be successfully manufactured by SLS, presenting a good behavior and stability during the process. Mo–CuNi electrodes had the smoother surface roughness, followed by ZrB2–CuNi and TiB2–CuNi. This is mainly due to the particle size used for the materials, where smaller particle sizes, like Mo and ZrB2, resulted in better surfaces. The manufactured EDM electrodes were also used to perform a characterization of some properties such as density, porosity, electrical conductivity and surface roughness. Table 4 summarizes the results from the characterization measurements performed. The measured porosity of the electrodes reached a minimum of 24.80 % for the ZrB2–CuNi and the highest value for Mo–CuNi. The porosity values are in accordance when considering the presureless nature of the sintering process involved in the SLS. Electrical conductivity also showed an interesting result. Although not as conductive as solid metals such as Cu, it falls in the conductive range of materials and in the range of application for EDM. TiB2–CuNi achieved the highest electrical conductivity whereas Mo–CuNi reached the lowest. In regards to their surface roughness, all electrodes showed a relatively rough surface, with the electrodes processed with the ceramic reinforcement reaching the highest levels of Rz, due to the larger particle size and consequently layer thickness used to process these materials. Figure 4 shows SEM images taken from the vertical cross section of electrodes (parallel to the laser beam direction) made with ZrB2–CuNi (Fig. 4a, b), TiB2–CuNi (Fig. 4c, d),
Table 3 Standard EDM parameters settings (finishing, semifinishing, and roughing regimes) for the experiments with the SLS processed electrodes EDM regime
Open voltage ui (V)
Discharge current ie (A)
Discharge duration te (μs)
Interval time to (μs)
Tool polarity
Duty factor τ (τ ≈ te/te +to; %)
Finishing Finishing Semifinishing Roughing
200 160 120 120
2 4 12 32
25 25 100 200
12.8 6.4 12.8 25
(+) (+) (+) (+)
71.4 83.5 90.4 90.6
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Fig. 3 EDM electrodes (10×10× 10 mm) manufactured with the most adequate SLS parameters: a ZrB2–CuNi powder system, b TiB2–CuNi powder system, and c Mo–CuNi powder system
and Mo–CuNi (Fig. 4e–f). The pictures reveal that the sintering and densification level was relatively high for all materials processed, considering the pressureless nature of the laser sintering process. For the ZrB2–CuNi and TiB2–CuNi, the structure is formed by the metallic matrix CuNi (white areas) surrounding the uniformly dispersed ceramic reinforcement (gray areas). The chemical compositions of the dispersed ceramic reinforcements and matrix taken from EDX point scan analysis is shown in Table 5. The metallic matrix (crisscross A and C in Fig. 4) contains essentially Cu and Ni elements, whereas the reinforcement particulates (crisscross B and D in Fig. 4) revealed the presence of the ceramic dissolved with Cu and Ni atoms, indicating a good degree of bonding between the metallic and ceramic phases. The interfaces between the reinforcement particles and the matrix were continuous (Fig. 4b–d), showing a homogenous metallurgical bonding. For the Mo–CuNi part, the structure reveals the more prominent presence of the Mo phase (white areas) bond together by the solidified CuNi matrix (gray areas). EDX analysis shows the presence of Ni in the reinforcement, whereas no Mo was found dissolved in Table 4 Properties of the EDM composite electrodes manufactured by SLS Material
ZrB2–CuNi
TiB2–CuNi
Mo–CuNi
Density (g/cm3) Porosity (%)
5.89 24.80
5.18 25.10
6.87 29.2
1.01
1.14
0.93
Electrical conductivity (×106 S/m) Surface roughness Rz (μm)
74.1
89.6
65.8
the metallic matrix. This material yielded a more heterogeneous microstructure with Mo solid particles forming clusters and irregular CuNi regions observed (Fig. 4f). Mo did not suffered any sintering during laser processing as Fig. 4f reveals the presence of spherical particles very close to each other. During laser sintering, the time to which a powder region is exposed to the laser is generally very brief, in the order of milliseconds. In such extremely short heating cycles, consolidation must occur rather speedily. Therefore, the most likely sintering mechanism occurring is liquid phase sintering which takes place mainly by particle rearrangement where the localized laser induced melting and rapid solidification of the binder material and is responsible for the consolidation of the powders. It is corroborated by the work of Bourell et al. [17]. For the studied materials, laser sintering starts with the selective melting of the CuNi metal matrix (∼1,140 °C), while the reinforcement phases ZrB2, TiB2, and Mo remain in the solid state, due to their high melting point. The result is the formation of a “molten pool,” with the matrix acting as a binder and the reinforcement the so-called structural material. The molten liquid will then infiltrate through the reinforcement particles by capillary forces, which is the driving mechanism for the densification of the material during the rearrangement phase, as studied by Gu and Shen [18]. With the laser beam moving away, the molten pool undergoes rapid solidification and the reinforcement particles are bonded together by the solidified liquid. This bonding is dependent on the ability of the liquid to wet the structural material. A good wetting behavior was found for the considered materials, possibly due to the addition of Ni in the metal matrix, which
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Fig. 4 SEM pictures of EDM electrodes manufactured with the most adequate SLS parameters: a, b ZrB2–CuNi, c, d TiB2–CuNi, and e, f Mo–CuNi
a
b +B +A
Building direction
c
d +D
+C
e
f
+E +F
improved the wetting behavior. For the Mo–CuNi, the higher amount of reinforcement present when compared with the ZrB2–CuNi and TiB2–CuNi reduced the spreading kinetics of the liquid, reducing the densification and increasing the overall porosity. In addition, as Mo has a higher thermal conductivity than the ceramic materials, it helped to diminish the molten pool temperature, diminishing the wettability of the system.
4.2 EDM experiments results In this section, the results obtained for the EDM experiments using the composite electrodes manufactured with the most adequate SLS parameters are discussed in comparison to Cu powder material electrodes also processed by SLS and solid Cu electrode under its optimal EDM conditions. The results for solid Cu electrodes are not presented in Fig. 5, because of
Table 5 EDX chemical compositions for different regions of the materials microstructure ZrB2–CuNi
Label
Position
A
Matrix
TiB2–CuNi
B Label
Reinforcement Position
Mo–CuNi
A B Label
Matrix Reinforcement Position
A B
Matrix Reinforcement
Chemical composition (wt.%) Zr Cu – 86.91 94.45 3.94 Chemical composition (wt.%) Ti Cu – 87.76 96.47 1.52 Chemical composition (wt.%) Mo Cu – 85.78 94.24 –
Ni 12.01
Residual 1.08
1.61
–
Ni 12.24 2.00
Residual – –
Ni 13.53 5.22
Residual 0.7 0.54
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much higher values of material removal rate Vw achieved by this material. Figure 5 shows the results of material removal rate Vw and volumetric relative wear ϑ for the three EDM regimes. The superior performance of the new composite electrodes in comparison to SLS Cu electrodes is evident. For the finishing regime (We =2.5 mJ) the material removal rate of the Cu powder electrode was Vw =0.01 mm3/min. The ZrB2–CuNi electrodes achieved a better performance than solid Cu electrodes in terms of material removal rate, removing 2.52 mm3/min against 1.78 mm3/min for the solid Cu. This is followed by the TiB2–CuNi and Mo–CuNi electrodes, which achieved lower values of material removal rate, Vw =1.39 mm3/min and Vw =0.19 mm3/min respectively.
Fig. 5 Results for EDM experiments of SLS composite and Cu powder electrodes under three different levels of discharge energy (We =2.5, 30.6 and 182.4 mJ) depicting at a material removal rate Vw (mm3/min) and at b volumetric relative wear ϑ (%)
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In regard to volumetric relative wear ϑ, the performance of the SLS composite electrodes was also much superior to those of Cu powder electrodes. ZrB2–CuNi had the lowest wear of 21.5 %, followed by TiB2–CuNi presenting ϑ of 22.9 % and Mo–CuNi with ϑ=32.7 %, against the Cu volumetric relative wear ϑ of 450 %. Nevertheless, these values are high when compared with solid Cu electrodes which achieved a volumetric relative wear of ϑ=2 %. Here, it is also import to emphasize that for the SLS electrodes made with Cu powder the EDM performance disparity of Cu powder versus solid Cu can be credited to the Cu powder behavior during the SLS processing. Cu powder has a high thermal conductivity and a low absorption coefficient for CO2 laser. These characteristics led to a poor SLS performance, as the heat provided by the laser is not well absorbed and is dissipated very quickly resulting in a poor sintering/melting process, a porous part and consequently a poor EDM performance. For semifinishing conditions (We =30.6 mJ) the TiB2–CuNi electrodes reached Vw of 10.3 mm3/min, removing about 50 times more material than SLS Cu electrodes (Vw =0.2 mm3/ min). For solid Cu the value of Vw=18.7 mm3/min was reached. ZrB2–CuNi removed lesser workpiece material (9.15 mm3/min) than the TiB2–CuNi, whereas Mo–CuNi had again the lowest material removal. For semifinishing conditions, the TiB2–CuNi also had the best behavior with a volumetric relative wear (ϑ=8.9 %) about 21 times smaller than Cu (ϑ=186 %). ZrB2–CuNi had a higher wear of ϑ= 11.1 %, while Mo–CuNi achieved the maximum wear among the composite electrodes (ϑ=15.1 %). The solid Cu electrode volumetric relative wear was ϑ=0.9 %, much lower than the SLS processed materials. When compared with finishing regime, all SLS composite electrodes had a better performance, removing more material Vw from the workpiece and achieving a lower volumetric relative wear ϑ. This may be due to the more stable conditions during semifinishing regimes when EDM during finish conditions, so that it is possible that particles separated from the composite electrodes tended to clog the working gap, causing short circuits and arc discharges and compromising the EDM process efficiency. Figure 5 also presents results of material removal Vw rate and volumetric relative wear ϑ for roughing EDM regime (We =182.4 mJ). SLS composite electrode had the best material removal rate Vw of 14.0 mm3/min and a volumetric relative wear ϑ of 5.9 % for TiB2–CuNi, trailed by ZrB2–CuNi (Vw =13.29 mm3/min; ϑ=7.6 %) and ending up with Mo– CuNi, with the poorest performance among them (Vw = 12.21 mm3/min; ϑ=10.1 %). Once again the results are much better than those results provided by SLS manufactured Cu electrodes. When compared with the solid Cu electrodes, which obtained Vw =37.6 mm3/min and ϑ=1.6 %, this difference is less significant when compared with the other EDM conditions.
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Figure 6 presents the surface roughness Ra of the AISI H13 workpiece electrical discharge machined with the SLS composite electrodes in two different finishing regimes (We = 1.2 mJ and We =2.5 mJ). Results of surface roughness with solid Cu electrodes under the same conditions are presented for comparison. Workpieces machined with solid Cu electrodes achieved lower levels of surface roughness than SLS composite electrodes, regardless of the EDM finishing regime used. For finishing at lower discharge energy We, the SLS composite electrodes achieved a maximum of Ra =8.0 μm with TiB2– CuNi, whereas solid Cu reached Ra =1.0 μm. At the finishing with higher energy input, solid Cu electrodes more than doubled the workpiece surface roughness, while this increase was lower for the SLS electrodes, which attained a value of 6.1 μm when machined with ZrB2–CuNi. The surface roughness of workpieces is intrinsically related to the average discharge energy We =ue.ie.te (mJ). The increase in the energy We yields a higher level of roughness, since after the electrical discharge interruption deeper and wider craters are formed. Usually the energy increase is associated to the elevation of the discharge current ie or the discharge duration te. Here the parameter of influence was the discharge current, which doubled from 2 at We =1.2 mJ to 4 A at We =2.5 mJ, whereas the discharge duration used was the same at both energy levels. Interestingly, workpieces machined with TiB2– CuNi electrodes had the opposite behavior, achieving a smoother surface when higher discharge currents were set. Comparing the SLS composite electrodes workpiece surface finish, TiB2–CuNi resulted in the roughest surfaces, followed by ZrB2–CuNi and Mo–CuNi. The larger particle size and higher layer thickness used for the ceramic reinforcement TiB2 and ZrB2 may be associated to this. Mo, on the other hand, with its very small particles and consequent
Fig. 6 Workpiece surface roughness Ra machined with solid Cu electrodes and SLS composite electrodes under different finishing regimes
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smaller layer thickness used for SLS processing, yielded a smother electrode surface and as a result a better surface finish. The rougher surface achieved by the workpieces machined by the composite electrodes can be explained based on the porous nature of the SLS processed electrode. Given the pressureless laser sintering of the composite powder, the remaining porosity after the process is significant, as demonstrated in the previous sections. This porosity affects the electrical properties of the electrode, diminishing its conductivity and also promoting the formation of spark concentrations during the EDM process, which could lead to the energy discharge at localized regions in the workpiece or arcing. Such intense arc-discharges could lead to increase thermal shock within the electrode and induce the ceramic material detachment by spalling. Spalling would then increase the contamination of the working gap with ceramic particles, contributing in amplifying arcing and short-circuit, resulting in a rougher surface.
5 Conclusions In this work, the direct production of EDM electrodes by means of the SLS technique using three newly developed nonconventional metal-matrix composite materials was presented and discussed along with the electrodes performance under different EDM regimes. From the investigation, the following conclusions can be drawn: 1. The powder systems composed of Mo–CuNi, TiB2– CuNi, and ZrB2–CuNi revealed to be successfully processed by SLS, indicating a good stability for the range of parameters settings evaluated; 2. The chosen composite materials yielded a relatively high densification level, considering the SLS process nature. ZrB2–CuNi and TiB2–CuNi resulted in a more homogeneous reinforcement distribution and metallurgical bonding, whereas the Mo–CuNi microstructure revealed the formation of Mo particle clusters and irregular metallic matrix regions; 3. The SLS electrodes achieved interesting values of electrical conductivity in regards to what is expected for EDM applications. Surface roughness is directly related to the material particle size and layer thickness applied in the SLS process; 4. The composite electrodes presented a better performance than the SLS Cu powder electrodes, independently of the applied EDM regime. This performance increased as the machining conditions varied from finishing to roughing regime; 5. Among the investigated materials, TiB2–CuNi achieved the best overall EDM performance, removing more
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material from the workpiece and presenting lowest electrode wear. This is followed by ZrB2–CuNi, whose EDM performance at finishing conditions was superior, and Mo–CuNi with the poorest one; 6. The overall performance of the composite electrodes was inferior to the solid Cu electrodes. It must be remarked that the EDM conditions applied were the optimal for solid Cu electrode. To obtain higher material removal rate and lower electrode wear, further research must involve reaching denser electrodes and optimization of EDM conditions for each material developed. Acknowledgments The authors thank the following funding agencies: from the Brazilian side, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and from the German side, Deutsche Forschungsgemeinschaft (DFG). This work is part of the BrazilianGerman Collaborative Research Initiative on Manufacturing Technology Program (BRAGECRIM).
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