18.4 The potential of reducing the energy consumption for machining TiAl6V4 by using innovative metal cutting processes

E. Uhlmann 1, P. Fürstmann 1, B. Rosenau 1, S. Gebhard 1, R. Gerstenberger 1, G. Müller 2

1 Institute for Machine Tools and Factory Management, Technische Universität Berlin, Germany 2 Institute for Production Systems and Design Technology, Fraunhofer IPK, Germany

Abstract

Small and medium-sized production companies are alarmed at the increasing costs for energy. There are two possibilities presented to decrease the energy consumption per produced part. The first approach of energy saving refers to turning TiAl6V4. For this, the energy demand of machine tool, cooling system and tool wear of an internally cooled turning tool with closed cooling circuit at dry and wet machining and at combined cooling were compared. It becomes obvious that the turning tool allows for an enormous energy saving potential as well as for lifetime advantages or productivity increases respectively. The second energy saving approach in-vestigates the milling of TiAl6V4 workpieces. In this case, a machine tool’s energy consumption during con-

ventional milling was compared to the energy consumption during a trochoidal milling process. It is described that a trochoidal milling strategy offers considerable potential for improvement as regards energy consumption and process time. Keywords: TiAl6V4; Energy Consumption; Energy Efficiency; Internally Cooled Tool; Trochoidal Milling

1 INTRODUCTION

Currently, the demand for shorter and shorter product life cycles and customisation leads to a more flexible use of machine tools. That means that machine tools have to cover more and more requirements of the production process. At the same time, the production shall be as energy efficient as possible in order to minimize the variable costs. In order to meet these requirements, the machine tool, the tool and the production strategy would ideally have to be adjusted to every single task. As such an adaptation is hard to realise in an operational environment in many cases and as it is often uneconomical, new tool systems or cutting strategies might help to reduce the energy consumption at increased produc-tivity of the machine tool. The energetic efficiency of the metal cutting process de-pends on the following factors: machine tool, cutting process and tool. The two last-named factors can be amended fast and inexpensively without a high capital commitment. The material investigated in this paper is a Ti alloy grade 5, TiAl6V4. This Ti-alloy has a considerable economic influence with its market share of 50 % and is, at the same time, the most widely used Ti-alloy. This light metal is especially used by the aerospace industry as its characteristics of a high temperature resistance and a low density of 4,43 g/cm3 at high tensile strength of Rm = 1060 N/mm² complement each other positively. The last-mentioned value decreases with increasing temperature so that TiAl6V4 can usually be used until 315 °C without any risk. Due to its high corrosion re-sistance as regards hot fluids, this material is frequently used for turbine manufacturing. Ti alloys embrittle at temper-atures above 700 °C due to titanium’s high affinity for oxygen

and nitrogen. At high material removing rates or a high speed of cutting, this effect leads to unwanted subsurface damages. Together with the low head conductivity and with the reactivity with oxygen, the light metal reacts exothermic at high cutting temperatures. Therefore, the process should be cooled well whenever Ti alloys are cut. Due to their in-creasing importance, the efficient and productive machining of titanium looms larger and larger in the area of production technology as quite often more than 80 % of the unmachined workpiece are cut [1, 2, 3, 4]. 2 POTENTIAL FOR ENERGY SAVINGS

During the manufacture of rotationally symmetric parts made of TiAl6V4 there are not many possibilities to increase the energetic productivity. In the case of heavy machining it is possible that, for example, several cutting edges / tools are used simultaneously, whereas the surface quality is critical for the quality in the case of fine machining and, thus, de-fines the process parameters. One possibility to lower the energy consumption per machined workpiece in the case of fine machining, is to substitute one main consumer of the machine tool, e. g. the cutting liquids supply [5, 6]. In the industry, the cutting zone is cooled by cutting liquids (CL) when Ti alloys are cut. Additionally, minimum quantity lubri-cation systems (MQL) are used together with tools with wear protection coating geared to these. Another cooling strategy is the dry machining [7] that, however, should be avoided for tools that are not geared to the cutting process as they show a high tool wear. Nevertheless, the downstream, energy-intensive cleaning process of the workpiece can be spared in the case of dry machining and the chips can be disposed of

G. Seliger (Ed.), Proceedings of the 11th Global Conference on Sustainable Manufacturing - Innovative Solutions ISBN 978-3-7983-2609-5 © Universitätsverlag der TU Berlin 2013

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immediately without any follow-up treatment. Internally cooled tools (ICT) with closed internally cooling circuits are a further possibility of tool cooling [8]. They have a coolant circulating within the tool to reduce the temperature of the cutting edge as much as possible. Milling operations offers higher energy saving potentials compared to turning operations. This is as in most cases a reduction of process time leads to energy savings. If a com-plex surface roughness is produced, energy can e. g. be saved by the optimisation of the travel length [9]. Less com-plex 2.5-D geometries often have milling algorithms prede-termined by the machine that leave little margin for time savings. One approach can be found in machine tools with a high axis dynamic. With them, trochoidal process can be executed. A trochoidal process differs from a conventional milling process as the tool does not only make a feed movement towards the processed material but additionally a circular movement, see figure 1.

Figure 1: Kinematics of the trochoidal milling process

This leads to the fact that the radial depth of cut varies and that the wrap-around angle can be adjusted variably at the tool. In the case of conventional slot milling, the wrap-around angle is 180° and the material is removed continuously. In the case of trochoidal milling, the material is removed se-quentially with higher feed movements and lower depths of cut. During this process, the temperatures in the tool and in the workpiece are lowered; moreover, the cutting forces grow weaker. In combination with suitable cutting parame-ters and a tool geared to the process, a depth of cut up to two times of the tool diameter can be reached by the tro-choidal milling [10, 11, 12]. 3 ASSESSMENT OF THE PROCESS EFFICIENCY

REGARDING ENERGY CONSUMPTION

In engineering, the efficiency of a process of a system is demonstrated by the quotient of effort and benefit. Mori [13] and Dietmair [14] set up an equation to express the energet-ic efficiency of a cutting process. In this equation the work brought into the cutting process W is related to the material removal rate MRR. The result is expressed in the variable Y. The smaller Y, the more energetically efficient is the cutting process.

Y =𝑊

MRR (1)

Besides the specific consumed energy Y, the energy produc-tivity E of a process is of a higher interest for an economic

consideration [15]. The economic potential in relation to the used energy can be assessed by the quotient of part costs CPart, the benefit, and the amount of energy costs per part CEnergy, the effort.

E = CPart

CEnergy (2)

In this case, the used amount of energy of the machine tool or the machine components relevant for the cutting process can be taken as a basis. 4 INTERNALLY COOLED TURNING TOOL

4.1 Experimental Setup

Machine Tool, Cutting Tool and Cooling System

All cutting trials were undertaken on the CNC turning and milling centre TRAUB TNX 65. The machine tool is equipped with an opening for measurement lines and coolant hoses. Moreover, holders that prevent the contact between the rotating main spindle and the coolant hoses were installed in the workspace. The internally cooled tool consists of four major parts: shaft, selective laser sintered tool head, micro-cooling device and temperature sensors (figure 2). The geometry and shape of the tool was designed in accordance to DIN 4984, CSBPL 2525M. The tool holder is optimised for indexable inserts type SPUN 120108. Theses inserts are made of cemented carbide type K10. A pump and cooling system is connected with the tool holder to provide coolant at the right temperature.

Figure 2: Side view of the internally cooled turning tool with integrated temperature sensors

Measuring Equipment

The measurement of the tool wear, especially of the crater wear, on the indexable inserts was done by a MikroCADpico by GFMESSTECHNIK. Cutting forces were measured with a KISTLER Type 9121 three-component dynamometer. The dynamometer was connected to a charge amplifiers type KISTLER 5011. In addition to this, temperatures of the cool-ant at the inlet and outlet of the micro-cooling device were measured by thermocouples K-type. These values were used to calculate the heat that was removed by the micro-cooling device. The thermocouples and the force measure-ment setup were connected to a NATIONAL INSTRUMENTS type 6251 data acquisition system. The signals were ana-lysed and documented with LabView a NATIONAL INSTRUMENTS software.

ae: radial depth of cutap: axial depth of cutn : rotational speedvf : feed

WorkpieceTiAl6V4

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The potential of reducing the energy consumption for machining TiAl6V4 by using innovative metal cutting processes

Table 1 : Comparison of different cooling approaches for turning TiAl6V4

Variable Unit DRY DRY + ICT WET WET +

ICT

Average Mechanical Spindle Power PMECH W 281 290 332 343

Heat Removed by the ICT in Average Q̅ W – 18 – 3

Average Cutting Force F̅c N 177 179 181 186

Relative Crater Wear in Correlation to Crater Wear under Wet Cutting Conditions VCR_REL

mm³ mm³ 10 4.17 1 0.25

Work of the Tool Machine for Cutting 25 cm³ TiAl6V4 W Wh 434 466 578 610

Costs per Cutting Process in Correlation to the Energy Consumption and Tool Wear CZER € 15.04 6.29 1.55 0.43

The measurement of the power consumption was undertak-en by using the multifunctional power meter SENTRON PAC 4200 and current transformers by SIEMENS. Six current transformers were used to measure two 3-phase power lines at once. Cutting Process

The internally cooled tool was used for all cutting trials to avoid unwanted variance by e. g. alternating cutting tool stiffness. For the turning trials the process of fine machining was chosen and TiAl6V4 was cut according to the following cutting parameters: Speed of cut vc = 72.00 m/min Depth of cut ap = 0.60 mm Feed f = 0.15 mm Pre-trials showed that the lifetime criterion, crater depth of 25 µm, with these cutting parameters in dry machining is reached at a volume of removed material of 25 cm³.

4.2 Results and Discussion

The performance measurement at the TRAUB TNX 65 showed that the effective power is 5.5 ± 0.2 kW in a ready-to-operate condition. These requirements are mainly caused by the lathe’s chuck hydraulics supply, by the centralized lubrication system, by the servo amplifier of the axis drives,

by the cooling of hydraulic fluid, by the sensor- and path measuring systems, by the industry computer as well as by transformer, relays and switches. The last-mentioned parts are inductivities that can clearly be seen in the machine’s

reactive power demand as this is 2.5 ± 0.1 kW in a ready-to-operate condition. Whenever cutting trials are undertaken and the machine is switched from a ready-to-operate to an active condition, the energy consumption of the TRAUB TNX 65 increases. The increased demand is caused by the feed axis, by the main spindle, by the chip conveyor and by the cooling systems. Depending on the different cooling strategies, the machine‘s effective power lies between 5.8

and 7.7 kW, see Figure 3. In case of dry machining, the average effective power of the machine tool is 5.85 ± 0.05 kW. The average effective power in the case of wet machining is 7.6 ± 0.1 kW. If the internally cooled tool is used in dry or wet machining, the average effective power for cooling and provision of the coolant, tempered at 15.6 °C, increases by 0.4 kW to 6.2 or 8.1 kW respectively. The effective power of the main spindle correlates with the measured data of the force measurement. In the case of dry machining the cutting forces are the lowest with an average of F̅c = 177 N, whereas they increase to F̅c = 186 N in the case of wet machining in combination with the internally cooled tool, see Table 1. By cooling the cutting zone with CL the cutting temperatures are reduced.

Figure 3: Effective power of the CNC turning and milling centre TRAUB TNX 65 under different cooling approaches

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Internally Cooled Tool for SPUN 12XX08, Cemented Carbide Grade K10, uncoated,h tool = 1.00 mm

Cylindrical round bar:

DIN TiAl6V4, ISO 5832-3

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vc = 72.00 m/minap = 0.60 mmf = 0.15 mm

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Figure 4: Comparison of the effective power under different cooling approaches and the tool wear for turning TiAl6V4

This temperature reduction is paralleled by a strengthening of the TiAl6V4 [16]. In direct comparison to dry machining the measured cutting forces of wet machining are even higher, but the friction and, thus, the cutting force can be reduced by the lubricating effect of the CL. The fact that the influence of the internally cooled tool on the cutting forces is of a different dimension in the case of dry machining and wet machining has to be emphasized. If the internal cooling system is switched on in dry machining with the internally cooled tool, an increase of the average cutting force by 2 N can be monitored. This implies a reduction of cutting temperature and the influence of the internal cooling on the cutting process. The same effect can be seen in the case of wet machining: the average cutting force increases by approximately 5 N with a switched-on internal cooling. The reason for this is that the released heat energy flows into the workpiece, into the chip and into the tool in the case of dry machining. Due to the missing cooling by cooling liquid, the internally cooled tool mainly works as a heat sink. The heat absorption capacity is limited to a coolant tempera-ture of 15.6° C at the coolant inlet and at the fixed flow of the fluid. This is why the cutting temperatures cannot be further reduced by the internally cooled tool in the case of dry ma-chining and a set average mechanical spindle power of approximately 290 W, although a thermal power of approxi-mately 18 W was removed by the internally cooled tool. In contrast to this, an enormous part of the released heat ener-gy is absorbed by the CL in the case of wet machining, in which the average mechanical spindle power is 332 W due to the lower cutting temperatures. If the internally cooled tool is used additionally, the absorbed heat is approximately 3 W. In the case of wet machining, the chip, the workpiece and the tool are directly cooled by the CL. The cooling of the area of heat generation, the interface between material and tool, however, is not done directly by the CL. This is as due to the high surface pressure between workpiece material and tool and the resulting tight fit no CL can get into the heat generation zone. The cutting zone is, therefore, indirectly cooled. The CL cools the going off chip, reduces its tempera-

ture and, thus, lowers the temperature of the cutting zone. The combination of wet machining and internally cooled tool leads to a two-way cooling of the cutting zone: the cutting zone is cooled by the cooled chip, on the one hand, and by the internally cooled tool, on the other hand. The advantages of the internally cooled tool in the case of dry machining become apparent in the crater wear, a typical type of wear in the case of longitudinal turning of titanium. After a volume of removed material of 25 cm³, the average crater wear in dry machining was 1.5·10-3 mm³; a value that reached the tool life criterion. By switching on the internal cooling, the crater wear could be reduced to 6.1·10-4 mm³. In case of a conven-tional wet machining an average crater volume of 1.5·10 4 mm³ was measured; whenever the internal cooling system was switched on in addition to the wet machining, the crater volume could be reduced by 400 % to 3.78·10-5 mm³. This improvement in wear behaviour can be achieved by the use of energy intensive cooling and pump-ing systems; in the case of wet machining the CL pump and valves are switched on. These components lead to a perma-nent effective power demand of 2.3 ± 0.1 kW, no matter how much CL is needed, see Figure 4. If the wear behaviour is related to the power consumption of the machine tool, it can be seen whether the use of lubricating systems or internally cooled tools with closed cooling circuits is energetically reasonable. In order to clarify this, the equitation of energy productivity and the specific consumed energy [13, 14, 15] were combined. Thus, the variable CZER is defined. This variable contains the costs of the used electrical energy cEnergy and the costs of tool wear cTool for a specific cutting process. In this formula the work W of the process is multi-plied with the cost for energy. For the costs of the tool the crater wear V was set in relation to the crater wear of the reference process VREF. As a result of this VCR_REF is the relative crater volume, see Table 1.

CZER = cEnergyW + cToolVCR_REL (3)

As assumption: the price for electricity is cEnergy = 0.09 €/kWh

and the price for the indexable insert that was used in the internally cooled tool is cTool = 1.50 €/edge.

Machine Tool:

TRAUB TNX 65

Cutting Tool:

Internally Cooled Tool for SPUN 12XX08, Cemented Carbide Grade K10, uncoated,h tool = 1.00 mm

Cylindrical round bar:

DIN TiAl6V4, ISO 5832-3

Longitudinal turning:

vc = 72.00 m/minap = 0.60 mmf = 0.15 mm

Cooling Approach

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The potential of reducing the energy consumption for machining TiAl6V4 by using innovative metal cutting processes

The results of this calculation can be found in Table 1. The wet machining combined with the internal cooling is produc-ing the lowest costs with 0.43 € per 25 cm³. Whereas, the conventional wet machining process costs 1.55€ per 25 cm³ removed TiAl6V4. 5 TROCHOIDAL MILLING

5.1 Experimental Setup

Machine Tool, Cutting Tool and Cooling System

A 5-axis-machining centre, type RXP600DSH, of the com-pany RÖDERS GMBH was used for the cutting trials. This machine tool is equipped with three linear and two rotatory axes. Like this, a 5-axes multiprocessing is possible so that it can be used for the production of single components, com-plex geometries and free-form components, e. g. Blisks. The spindle with a maximum speed n = 160.000 U/min is a spe-ciality of this machine. The control system RMS6, developed for high speed cutting applications, is an in-house develop-ment of RÖDERS GMBH. An advantage of this machine is the high dynamic of its axes, the high stiffness and the position-ing accuracy of 3 µm linked to it. For the milling of TiAl6V4 a cemented carbide milling tool with a diameter of DT = 10 mm was used. The milling tool with six cutting edges shall increase the processing of the TiAlN coating, applied by the physical vapour deposition method, significantly. The tool’s maximum depth of cut is

ap = 22 mm. The cutting edges show a rounding of approxi-mately 4 to 6 µm. A sharp edge is one requirement in the processing of high temperature-resistant material [17, 18, 19]. To cool the tool, CL is is sprayed into the cutting zone from the outside. This is done through cooling channels in the interior of the tool holder that end at the front side and spray the CL directly into the flute of the milling tool. During this process, the pressure of the CL is p = 60 bar. Cutting Process

The reasons for the different cutting parameters of the tro-choidal and the conventional milling process are found in the developing forces and temperatures. The cutting forces are

higher in the case of conventional milling due to the wrap angle of 180°. Therefore, the depth of cut of 20 mm of the reference slot could only be reached by repeated cutting. The power measurement was done by the SENTRON PAC4200 of SIEMENS, as in the case of the turning trials. 5.2 Results and Discussion

The effective power measurement at the machining centre RXP600DSH of the company RÖDERS GmbH revealed an average basic power consumption of P = 5400 W in the non-operating state which is defined by aggregate that are per-manently in an operating condition, see Figure 5. During the cutting trials, an enormous increase of the effective power consumption can be monitored; a fact that especially results from the moving feed axis, the rotating milling spindle, and the supply of CL. An approximate loss of power of 5 % can be observed during the conventional milling with Pkonv. = 8500 W compared to the trochoidal milling with Ptrocho. = 8900 W, see Table 2. Due to the milling tool’s max-

imal possible feed of apkonv. = 2 mm, ten cuts are necessary to reach a depth ap = 20 mm in a conventional slot milling process. That leads to an effective cutting time tc = 150 sec. Compared to that a cutting time tc = 126 sec is needed to manufacture the slot by trochoidal milling, i. e. an increase of rate of material removal of 17 %. The reduction of machining time is referable to the near-net shape feed of the milling tool, aptrocho. = 20 mm, that is possible due to the kinematical-ly determined reduction of the wrap angle during the pro-cessing and the reduced cutting forces linked to it. Consider-ing the total energy consumption, it can be determined that approximately 12 % more energy is needed for the conven-tional milling strategy with W = 354 Wh. This fact can be explained by the increased cutting forces compared to the trochoidal milling strategy. Moreover, in the trochoidal milling process, the cutting temperature is lowered by a reduction of the wrap angle, accompanied by higher process parameters. In addition to that and regarding the relation of work to the volume of removed material Y, Table 2 shows that the coef-ficient decreases by 23 % could be identified for the tro-choidal milling process, compared to the conventional milling strategy.

Figure 5: Effective power consumption of the machine tool when milling TiAl6V4 at different cutting strategies

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TrochoidalCutting Parameters

vc = 120 m/minvf = 2292 mm/minfz = 0.1 mmap = 20 mmae = 0.25 mm

Cutting Parameters

vc = 100 m/minvf = 500 mm/minfz = 0.03 mmap = 2 mmae = 10 mm

Tool

Tungsten Carbide, Coating: TiAlN

DT = 10 mmz = 6 -

Work Piece

DIN TiAl6V4,ISO 5832-3

Cooling StrategyLubricant: 100% OilLubricant pressure:(p = 60 bar)

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Table 2: Comparison of the energy demand for manufactur-ing one 20 mm slot

Conventional Cutting Process

Trochoidal Cutting Process

Work of the Tool Machine for Cutting

W in Wh 354 315

Total power Ptot in W 8500 8900

Material removal rate Q in mm3/sec 142.9 173.5

Y in Wh/cm3 19.2 14.8

Finally, it has to be concluded that the trochoidal milling process enables a lower energy consumption in the pro-cessing of high temperature-resistant material due to the increased volume of removed material, although the limits of possible cutting parameters for trochoidal milling have not been reached yet. 6 SUMMARY

Two approaches for energy saving are presented in this paper for turning and milling TiAl6V4 workpieces. The major outcome of the cutting trials with the internally cooled turning tool is that the tool wear under dry cutting conditions is in between dry and wet machining, whereas the combination of the internally cooled turning tool with wet machining leads to strongly increased tool lifetimes. In con-trast to this, the energy demand under dry cutting conditions is much lower than under wet cutting conditions. With re-spect to the environmental protection, the best tool cooling is dry cutting combined with the internally cooled tool. To achieve higher productivity rates or increased tool life-times, the combination of wet machining and the internally cooled turning tool is the best solution. In comparison to conventional milling, the average effective power consump-tion of the machine tool under trochoidal cutting conditions is increased by 6 %. Nevertheless, the amount of used energy is decreased by 15 % and the process time is reduced by 35 %. The reasons for this are higher material removal rates, due to the fact that the cutting time per edge and revolution can be reduced to a minimum. The heat-up phase and the cutting temperatures are small. By increasing the cutting parameters, e. g. cutting speed or feed, it is possible to improve the material removal rate. This enables a higher energy productivity and lower process time. The milling tool was used for both milling strategies. For trochoidal milling the tool has sufficient potential to increase the cutting pa-rameters. For the conventional cutting process, however, the tool works at the load limit. 7 ACKNOWLEDGEMENT

We would like to thank the DFG for supporting the research on the internally cooled turning tool within the CRC 1026 Sustainable Manufacturing. For their experimental work, we would like to thank Philipp Mantzke, Robert Anger and Erik Holubek. Additionally we would like to thank Sarah Fürstmann for the correction of the translation.

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