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Metals 2018, 8(7), 543; doi:10.3390/met8070543

Article
Hole Making by Electrical Discharge Machining (EDM) of γ-TiAl Intermetallic Alloys
1
Department of Mechanical Engineering, University of the Basque Country (UPV/EHU), Plaza Europa 1, 20018 San Sebastián, Spain
2
Department of Mechanical Engineering, University of the Basque Country (UPV/EHU), Nieves Cano 12, 01006 Vitoria, Spain
3
CFAA, University of the Basque Country (UPV/EHU), Parque Tecnológico de Zamudio 202, 48170 Bilbao, Spain
*
Author to whom correspondence should be addressed.
Received: 18 June 2018 / Accepted: 11 July 2018 / Published: 14 July 2018

Abstract

:
Due to their excellent strength-to-weight ratio and corrosion and wear resistance, γ-TiAl alloys are selected for aerospace and automotive applications. Since these materials are difficult to cut and machine by conventional methods, this study performed drilling tests using Electro Discharge Machining (EDM) to compare the machinability between two different types of γ-TiAl: extruded MoCusi and ingot MoCuSi. Different electrode materials and machining parameters were tested and wear, surface hardness, roughness and integrity were analyzed. The results indicate that extruded MoCuSi is preferable over MoCuSi ingots.
Keywords:
γ-TiAl intermetallic alloys; EDM; grinding

1. Introduction

Electrical discharge machining (EDM) is a widely used nontraditional machining process in industrial applications requiring high-profile accuracy and fine dimensional tolerances, especially when difficult-to-cut materials are involved. In particular, EDM drilling [1] is a commonly used process for orifice and hole machining in the aerospace, medical [2], molds and automotive sectors.
In an EDM process, the material is machined by electrical discharges. An electric arc is produced between the workpiece and the electrode until the desired shape is obtained. The workpiece and the electrode are submerged in a dielectric environment [3], both are also electrical conductors [4]. EDM is especially efficient for machining materials with high-hardness and complex geometries [5].
However, surface defects associated with EDM, such as voids, dense white layers, serious micro cracks and heat affected areas [6], require surface analysis [7] in order to guarantee machined component quality [8]. Moreover, electrode wear prediction and process parameter selection for process productivity, quality and accuracy requirements remain challenging with EDM.
In this sense, some authors study real-time monitoring techniques [9,10] in order to control electrode wear and to obtain an efficient EDM drilling process for nickel superalloys [11,12], titanium alloys [13,14,15,16], and stainless steel [17,18] workpieces.
Therefore, as it is often difficult to ensure the surface quality and the correct machining process, in this study, the results from EDM drilling tests in γ-TiAl are presented as a guide to ensure good surface quality and machining process performance. Within this context, different electrode materials and machining parameters were tested in order to determine optimal machining conditions. On this matter, electrode wear, surface hardness, roughness and integrity were analyzed and discussed. TiAl alloys are widely used in areas such as aerospace, jet engines, airframe components and automotives due to their excellent corrosion resistance, lightweight and mechanical properties. Titanium and aluminum [19,20] are the most important components together with molybdenum (1–2%), copper and silicon (0.2%). They have very good thermal and mechanical properties such as: (1) low density (4 g/cm3); (2) high temperature resistance; (3) excellent corrosion resistance and (4) good electric and thermal conductivity.
The literature regarding these materials is relatively scarce. Jabbaripour et al. [21] compared the powder mixed electrical discharge machining (PMEDM) and EDM process of γ-TiAl by changing the current, pulse on time, powder size and powder concentration. Shabgard et al. [22] used the EDM process for the machining of γ-TiAl and discussed the influence of the input parameters (discharge current and duration) on the removal rate, tool wear, or compositions and phases of machined surfaces. Holsten et al. [23] investigated the anomalous behavior of γ-TiAl, which is better machined under the conventional anodic polarity. This leads to TiC formation on the work surface.
In this work, EDM technology and process parameters are compared within two γ-TiAl types. Hole making operations using electrical discharges are made on extruded and ingot versions of this material for different machining regimes.
The MoCuSi TiAl alloy, depicted in Figure 1, is available in two versions: extruded or ingot. They are made from the same materials but differ in the way they are obtained. The former is obtained by casting, while the latter is obtained by extrusion performed in a stainless-steel sleeve. Due to this difference, the latter is covered in stainless steel.
Recently, TiAl alloys are a vast group with important differences between products. Table 1 shows the main differences between MoCuSi types and common Ti6Al4V. In the past, TiAl alloys were much easier to machine that the current TiAl alloys. As such, newer alloys have much lower density as well as higher maximum operating temperature. As both these properties are essential for current applications, this is a step forward for the industry.
Vacuum arc remelting (VAR), depicted in Figure 2a, is used in the manufacturing of ingots of γ-TiAl. The basic design of the VAR furnace has been continuously improved continuously, particularly in regard to computer control and regulation, with the objective of achieving a fully-automated remelting process. This has resulted in improved reproducibility of the metallurgical properties of γ-TiAl ingots.
The process involves melting the material within a vacuum-controlled atmosphere (0.13–13 Pa) in order to develop material homogeneity. VAR is the continuous remelting of a consumable electrode by means of an arc under vacuum. Direct current (DC) power is applied to strike an arc between the consumable electrode (cathode −) and the baseplate of a copper mold contained in a water jacket (anode +). The intense heat generated by the electric arc melts the tip of the electrode and a new ingot is progressively formed in the water-cooled mold.
MoCuSi ingots are obtained by VAR while extruded MoCuSi is obtained by extruding MoCuSi ingots. This difference in processing results in substantial differences in properties, shown in Figure 2b. The extruded material [24,25,26] has more desirable properties than the ingots. Yield strength, creep strain and endurance limit are much lower in the ingots than in the extruded material. The difference in the fracture toughness is less pronounced.
TiAl alloys are used for reliable components where surface integrity must be maintained. However, the machinability of titanium and its alloys is poor due to their low thermal conductivities, high chemical reactivity, and low elastic moduli, making it difficult to obtain good machining quality. To overcome these challenges, EDM could be an effective manufacturing process, especially for the manufacturing of complex geometries. In this process, a high temperature gradient can jeopardize process accuracy and the operating life of the part.

2. Experimental Procedure

γ-TiAl can be applied to a number of applications in the aerospace and automotive industries. This study was designed to determine the machinability and the best machining conditions for EDM of the extruded and ingot types of these titanium aluminides. The experimental procedure included the following steps: (1) experimental machining tests with different regimes, roughing and finishing, using different strategies and parameters; (2) measurements of process quality: electrode wear, hole diameter, surface finish and hardness.

2.1. Machining Tests

For the EDM tests, a conventional EDM machine (ONA© Compact 2) was used, depicted in Table 2.
The next step was the design of the roughing and finishing operations for the ingot/extruded MoCuSi workpieces. Eight holes of 0.3 mm depth, four for roughing conditions and four for finishing operations, were made in both materials, shown in Figure 3. The number of attempts for roughing (four) and finishing (four) operations were chosen in order to adequately represent a number of cutting parameters within the machine’s capabilities. Additionally, a 0.3 mm depth of cut was chosen to keep machining times reasonable as increasing this value leads to very long machining times.
In the EDM process, the electrode is one of the most important factors, accounting for around 60% of total process cost. Desirable electrode material properties are high fusion temperature, good conductivity, low expansion coefficient and low density. Regarding electrode material, in this case, graphite electrodes were used. The properties of these electrodes are shown in Table 3. Copper-tungsten electrodes were also tested, but these were discarded due to problems with erosion. Copper-tungsten electrodes required 1 h and 20 min for a 0.15 mm deep hole and it was impossible to obtain a deeper hole. Extra attempts were performed with higher machining values but this problem persisted. The main reason for such a failure is that copper-tungsten electrodes are not appropriate for titanium machining due to excessive electrode wear. Graphite’s good thermal and electric properties and its machinability make it ideal for γ-TiAl machining. The most important property of graphite is related to state transformation: it does not pass from solid to liquid but passes directly from solid to gas, a process called sublimation.
Test conditions (intensity, voltage, stages, impulse time (ti) and pause time (to)) were established for both types of operations. Initially, the workpiece and electrode were placed in the right position, and the cleaning system was run. First, the roughing test was carried out and afterwards, the finishing test was performed. For the roughing test, the hole diameter was measured with a coordinate measuring machine (CMM) (Mitutoyo, Kawasaki, Kanagawa, Japan). Additionally, the electrode wear was studied by analyzing the height and weight losses. The lost height was measured by the wear electro discharge machine, it was measured before and after holes drilling. First of all, roughing tests were performed (more than 100 mm3/min), testing different intensities, impulse and pause times. Roughing test conditions were selected depending on the gap, machining speed, and electrode wear.
The workpiece hardness changes during the EDM process due to the heat suffered by the electrode and the workpiece. Depending on the pulse time and the time between pulses, the workpiece suffers cooling and heating cycles. If the workpiece holds a temperature higher than the annealing temperature, hardening occurs. Therefore, after the EDM process, the hardness was measured in order to verify the influence of the machining parameters on surface hardness. This measurement was carried out at three different points in order to obtain reliable average results. For that purpose, a Mitutoyo© HR-300 (Mitutoyo, Kawasaki, Kanagawa, Japan) was used. The results of these experiments are shown in Table 3.
After machining the surface finish was measured with a Mitutoyo SJ-301 (Mitutoyo, Kawasaki, Kanagawa, Japan) with a measuring range of 12.5 mm. The results of these tests are shown in Table 4.

2.1.1. Roughing Tests

For roughing tests, shown in Table 4 and Table 5, the power value was set to 100 V. The established polarity was negative for the electrode and positive for the piece. EDM parameters were programmed according to roughing conditions. However, pause times were increased due to the fluid lack of time for deionization. The pulse and pause times per period are represented by ti and to respectively. The sum of ti and to times gives the time period, which is the period between two consecutive sparks. The workpiece material being removed is represented by Vw. Black rectangles indicate critical regimes that could not be successfully tested during the experiments.
In all the tests, the pause time, to, was increased for a more stable erosion. For the MoCuSi ingot workpiece, shown in Table 4, for the first and the second attempts, the pause time was increased from 6 µs to 20 µs, in the third attempt it was increased from 12 µs to 30 µs and in the fourth attempt it was increased from 6 µs to 30 µs. For the extruded MoCuSi, shown in Table 5, in the first, the third and the fourth attempts the pause time was increased from 12 µs to 30 µs and in the second attempt it was increased from 12 µs to 20 µs.

2.1.2. Finishing Tests

For finishing tests, the voltage used was 100 V, with positive polarity for the graphite electrode and negative polarity for the workpiece. For cases one and two for MoCuSi ingot component, shown in Table 6, were carried out with direct polarity, while for cases three and four, the polarity was inverted in order to decrease machining times.
For extruded MoCuSi finishing tests, shown in Table 7, in the first, second and fourth attempts, the pause time (to) was increased from 6 µs to 12 µs for a more stable erosion. Moreover, two cleaning tubes were added for the fourth attempt for a better cleaning.

3. Results and Discussion

For ingot and extruded MoCuSi, the electrode wear, surface hardness, roughness and integrity were analyzed.

3.1. Electrode Wear

For MoCuSi ingots, shown in Table 8, the lateral gap of the first and fourth attempts was the smallest one. Moreover, the machining in the fourth attempt was 20 min faster. The diameter of the hole was measured three times using the CMM machine and the three measurements were averaged. The values were approximated to the nearest one in 0.01 mm even though the machine resolution was 0.001 mm. Table 7 shows that the values obtained in the first and fourth attempts were the closest to the nominal diameter of 18.5 mm. After further surface analysis, shown below, it was concluded that the holes in the first and fourth attempts had the best surface finish so the parameters of the fourth attempt were the most efficient of tested conditions.
For the extruded MoCuSi, shown in Table 9, the lateral gap in test #4 was the smallest one. The more intensity applied, the better the hole surface finish and precision obtained. In the following section, it was concluded that the electrode wear results obtained with the extruded MoCuSi workpiece are better as less time was required for hole machining for the same machining parameters.

3.2. Hardness Results

The hardness measurements were made using a hardness testing machine HR-320MS (Mitutoyo, Kawasaki, Kanagawa, Japan). Three different hardness points were measurements in each hole surface. One was in the center of the hole and the other two were in the hole’s vertical diameter 3 mm apart. Table 10 shows the obtained hardness values for ingot and extruded MoCuSi.
Figure 4 and Figure 5 show that extruded MoCuSi presents always higher hardness values than the MoCuSi ingot in all the performed tests. For both materials, the hardness is relatively constant for low to moderate intensities. However, at high intensity values, the hardness is increased specially for the ingot type (x 1.3, from 30 to approx. 40 HB). The tendency is more accentuated for roughing operations where a hardness maximum is found for both materials at moderate intensities.

3.3. Roughness Results

Table 11 and Table 12 show the roughness results for ingot and extruded MoCuSi respectively. As these tables show, the surface finish obtained was worse than in theoretical steel. In this case, in the first and second tests worse results in comparison to the finish of theoretical steel were obtained. On the contrary, in the third and fourth tests, similar results to the finish of theoretical steel were found.
Figure 6 shows the surface roughness evolution when increasing the removed material Vw. It demonstrates that extruded MoCuSi type always presents a better surface finish than ingot MoCuSi.
The following figures show the best and worst surface finish results for the finishing test for the ingot and extruded MoCuSi. The drilled surface and geometry edges were analyzed. Figure 7 shows MoCuSi ingot’s best and worst surface finish. In this case, the best results correspond to the lowest machining volume. For higher machining volumes, the machined surfaced was damaged. Moreover, the best surface finish was obtained for the lowest intensity and impulse-time values.
Figure 8 shows the best and worst edge and surface finish for extruded MoCuSi. Similar to the ingot MoCuSi, the best surface finish results were obtained with the lowest machining volumes tested. Tested high machining volumes led to damaged surfaces. Moreover, the best surface finish was obtained for low intensity and pulse-time values.
Regarding surface integrity, it can be concluded that ingot and extruded MoCuSi present similar behavior for the same machining conditions, but extruded MoCuSi presents a better surface finish.
Due to the erosion and the corrosion (chemical and thermal attack), the electrodes suffer wear. Figure 9 shows the difference between electrode surface at the beginning of the machining process and once the machining process was finished. As shown in the figure, the graphite surface suffers wear during the machining process.
Figure 10 shows the wear suffered by the electrode profile. The angle of the electrode was 90° at the beginning and is subsequently reduced due to the suffered damage.

4. Conclusions

γ-TiAl alloys possess better mechanical properties than classical TiAl alloys and so, there is a chance to substitute the latter with the former. However, the lower machinability of γ-TiAl alloys is an obstacle that needs to be addressed. In this study, the machinability of two types of γ-TiAl alloys, ingot MoCuSi and extruded MoCuSi, was compared.
After material analysis, it can be concluded that, in general, MoCuSi alloys present a similar behavior.
Considering the surface finish, both MoCuSi alloys present a similar behavior. The best results are obtained with the extruded MoCuSi workpiece but, in general, the difference between both is subtle.
Moreover, in roughing tests, the best results were also obtained with extruded MoCuSi: the lowest times and the best surface finish results. In the roughing test, tested parameters on the first and the fourth attempts present the best surface finish results. For the fourth attempt, the machining time was shorter than the first one. In relation to this, intensity and stage number influence was observed: higher intensity values determine shorter machining times.
In relation to hardness analysis, the extruded MoCuSi workpiece presents harder values than the MoCuSi ingot. In general, after roughing tests, higher hardness values were measured than after finishing tests. Thus, the extruded type is more preferable than the MoCusi ingot due to its superior mechanical properties.
On the other hand, regarding electrode materials, although both graphite and copper-tungsten electrodes were tested, copper-tungsten electrodes were found to be inappropriate for titanium component machining due to unstable erosion and excessive electrode wear. For that reason, graphite electrodes were used for this study.

Author Contributions

A.B. designed and performed the experiments. G.U. and A.C. analyzed roughness, hardness, electrode wear and surface integrity. Finally, L.N.L.d.L. contributed with the resources (machine, tools, material, etc.) and general supervision of the work.

Funding

Acknowledgments: Thanks are given to the UFI in Mechanical Engineering of the UPV/EHU for its support to this project, and to Spanish project DPI2016-74845-R, ESTRATEGIAS AVANZADAS DE DEFINICION DE FRESADO EN PIEZAS ROTATIVAS INTEGRALES, CON ASEGURAMIENTO DE REQUISITO DE FIABILIDAD Y PRODUCTIVIDAD and project RTC-2014-1861-4, INNPACTO DESAFIO II.

Acknowledgments

Thanks are given to the assistance from the technical specialist Eng. Garikoitz Goikoetxea at UPV/EHU.

Conflicts of Interest

The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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Figure 1. MoCuSi (a) ingot and (b) extruded. (courtesy of GfE® Metalle und Materialien GmbH).
Figure 1. MoCuSi (a) ingot and (b) extruded. (courtesy of GfE® Metalle und Materialien GmbH).
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Figure 2. (a) Vacuum arc remelting (VAR) furnace (adapted from ATI Allvac©); (b) properties of extruded and ingot TiAl.
Figure 2. (a) Vacuum arc remelting (VAR) furnace (adapted from ATI Allvac©); (b) properties of extruded and ingot TiAl.
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Figure 3. Scheme of electrical discharge machining (EDM) operations for both materials ordered by hole number.
Figure 3. Scheme of electrical discharge machining (EDM) operations for both materials ordered by hole number.
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Figure 4. Comparison of hardness/intensity in finishing for both materials.
Figure 4. Comparison of hardness/intensity in finishing for both materials.
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Figure 5. Comparison of hardness/intensity in roughing for both materials.
Figure 5. Comparison of hardness/intensity in roughing for both materials.
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Figure 6. Surface finish and machining volume for the ingot and extruded MoCuSi.
Figure 6. Surface finish and machining volume for the ingot and extruded MoCuSi.
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Figure 7. Details of the best (left) and worst (right) edge and surface finish with the MoCuSi ingot.
Figure 7. Details of the best (left) and worst (right) edge and surface finish with the MoCuSi ingot.
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Figure 8. Details of the best (left) and worst (right) edge and surface finish with the MoCuSi ingot.
Figure 8. Details of the best (left) and worst (right) edge and surface finish with the MoCuSi ingot.
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Figure 9. The difference between the graphite electrode before and after being used.
Figure 9. The difference between the graphite electrode before and after being used.
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Figure 10. The profile of the graphite electrode after being used.
Figure 10. The profile of the graphite electrode after being used.
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Table 1. Properties of different TiAl alloys (courtesy of GfE® Metalle und Materialien GmbH).
Table 1. Properties of different TiAl alloys (courtesy of GfE® Metalle und Materialien GmbH).
PropertiesMoCuSi ExtrudedMoCuSi IngotTi–6Al–4V (Annealed)
Density (g/cm3)3.743.884.49
Specific modulus (GPa/kg/m3)433724
Tensile strength (MPa)6076891087
Specific strength (MPa/(g/cm3))162178242
Yield strength (MPa)589570942
Ductility (% elongation)1.72.47.8
Fracture toughness (MPa·m1/2)232052
Thermal conductivity (W/m/K)24198.6
Maximum operating temperature (°C)900865615
Table 2. ONA© Compact 2 characteristics.
Table 2. ONA© Compact 2 characteristics.
Metals 08 00543 i001X-Y-Z axes travel350 × 250 × 380 mm
Machine dimensions1000 × 750 × 2000 mm
Worktable dimensions550 × 360 mm
Allowable weight on table350 kg
Generator intensity30 A
Voltage levels9
Electrode diameter0.2–3 mm
Water tank capacity25 L
Table 3. Properties of selected graphite electrodes (courtesy of matweb©).
Table 3. Properties of selected graphite electrodes (courtesy of matweb©).
PropertiesGraphite
Density (kg/m3)1800
Particle size1–5 µm
Bending resistance (kg/m2)5.3 × 106
Compression resistance (kg/m2)10.50 × 106
Hardness (HB)70
Electrical resistivity (µΩ)16
Table 4. Roughing parameters for the MoCuSi ingot.
Table 4. Roughing parameters for the MoCuSi ingot.
TestIntensity (A)Stages Numberti (µs)to (µs)Lateral Gap (µm)Removed Material Vol. Vw (mm3/min)
124-x 32002080101
1248x 220020120170
1248x 320030140270
1248x 420030150330
Table 5. Roughing parameters for the extruded MoCuSi.
Table 5. Roughing parameters for the extruded MoCuSi.
TestIntensity (A)Stages Numberti (µs)to (µs)Lateral Gap (µm)Removed Material Vol. Vw (mm3/min)
124-x 32003080101
1248x 227020120170
1248x 320030140270
1248x 420030150330
Table 6. Finishing parameters for MoCuSi ingot.
Table 6. Finishing parameters for MoCuSi ingot.
TestIntensity (A)Stages Numberti (µs)to (µs)Ra (µm)Type of FinishRemoved Material Vol. Vw (mm3/min)
12--x 11661.6N71.2
12--x 25063.2N810
12--x 4200126.3N942
1---x 2150612.6N1084
Table 7. Finishing parameters for the extruded MoCuSi.
Table 7. Finishing parameters for the extruded MoCuSi.
TestIntensity (A)Stages Numberti (µs)to (µs)Ra (µm)Type of FinishRemoved Material Vol. Vw (mm3/min)
12--x 116121.6N71.2
12--x 250123.2N810
12--x 4200126.3N942
1---x 21501212.6N1084
Table 8. Roughing results for the ingot MoCuSi.
Table 8. Roughing results for the ingot MoCuSi.
Roughing MoCuSi Ingot
HoleTestTimeWeight (g)Height Wear (mm)Hole Diameter (mm)Hole Diameter Average (mm)Electrode Diameter (mm)Lateral Gap (mm)
Roughing 111 h 29 min9.860.9318.6818.6318.500.13
218.67
318.68
Roughing 211 h 3 min9.380.7618.6818.650.15
218.67
318.67
Roughing 3140 min9.860.8518.6918.690.19
218.68
318.69
Roughing 411 h 8 min8.640.9118.6718.630.13
218.66
318.65
Weight of the electrode at the beginning (g)8.19
Weight of the electrode at the end (g)6.08
Table 9. Roughing results for the extruded MoCuSi.
Table 9. Roughing results for the extruded MoCuSi.
Roughing MoCuSi Extruded
HoleAttemptTimeWeight (g)Height Wear (mm)Hole Diameter (mm)Hole Diameter Average (mm)Electrode Diameter (mm)Lateral Gap (mm)
Roughing 1131 min7.640.9518.6818.6818.500.18
218.67
318.68
Roughing 211 h7.080.9018.6818.670.17
218.67
318.67
Roughing 3145 min6.561.05186918.690.19
218.68
318.69
Roughing 4136 min6.081.0018.6718.660.16
218.66
318.66
Weight of the electrode at the beginning (g)8.19
Weight of the electrode at the end (g)6.08
Table 10. Hardness with MoCuSi ingot/extruded roughing.
Table 10. Hardness with MoCuSi ingot/extruded roughing.
MoCuSi IngotMoCuSi Extruded
Measurement areaTestsTests
123Average123Average
Without erosion31.728.331.130.43234.634.533.7
Finishing 130.429.832.630.934.732.334.934.0
Finishing 230.230.627.929.635.834.635.435.3
Finishing 327.930.831.730.136.435.43535.6
Finishing 437.539.537.738.235.738.943.739.4
Roughing 126.224.429.826.845.341.526.537.8
Roughing 232.948.242.441.238.542.151.844.1
Roughing 354.646.749.250.239.75259.850.5
Roughing 43225.934.730.946.83633.438.7
Table 11. Results for surface finish with the ingot MoCuSi.
Table 11. Results for surface finish with the ingot MoCuSi.
Graphite Electrode and MoCuSi Ingot Piece
TestRa (µm)Rz (µm)Rt (µm)Finish TypeTheoretical Steel Finish
2.65416.68223.676N8N7
4.58324.99934.282N9N8
6.44336.94845.181N10N9
13.16579.028116.375N11N10
Length4.8 mm
Table 12. Results for surface finish with the extruded MoCuSi.
Table 12. Results for surface finish with the extruded MoCuSi.
Graphite and MoCuSi Extruded Piece
TestRa (µm)Rz (µm)Rt (µm)Finish TypeTheoretical Steel Finish
2.78618.55420.605N8N7
6.98940.69249.935N10N8
5.03032.02045.207N9N9
10.53355.51871.361N10N10
Length4.8 mm

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