A fundamental study on Ti–6Al–4V’s thermal and electrical properties and their relation to EDM productivity

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(BQ) In the study, temperature measurements have been made for Ti–6Al–4V workpieces with various duty factors to clarify the essential causes of difficulty in machining titanium alloys and observe the optimal duty factor in terms of productivity and quality.

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Nội dung Text: A fundamental study on Ti–6Al–4V’s thermal and electrical properties and their relation to EDM productivity

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 2 ( 2 0 0 8 ) 583–589 journal homepage: www.elsevier.com/locate/jmatprotec A fundamental study on Ti–6Al–4V’s thermal and electrical properties and their relation to EDM productivity Peter Fonda a , Zhigang Wang a,∗ , Kazuo Yamazaki a , Yuji Akutsu b a b Department of Mechanical & Aeronautical Engineering, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA Sodick Corporate Headquarters, 1605 N. Penny Lane, Schaumburg, IL 60173, USA a r t i c l e i n f o a b s t r a c t Article history: There are strong needs for productive/quality machining strategies of notoriously “difﬁcult- Received 21 June 2007 to-machine” aerospace materials. The current means of machining these materials is Received in revised form dominated by mechanical cutting methods, which are costly due to high tooling costs, 15 August 2007 poor surface quality and limitations in the workpiece features and operations that can be Accepted 24 September 2007 machined. The newest EDM technology may be able to circumvent problems encountered in mechanical machining methods. In this paper, the EDM technology has been used to machine titanium alloy Ti–6Al–4V to investigate the effect of Ti–6Al–4V’s thermal and elec- Keywords: trical properties on the EDM productivity. In the study, temperature measurements have EDM been made for Ti–6Al–4V workpieces with various duty factors to clarify the essential causes Temperature stability of difﬁculty in machining titanium alloys and observe the optimal duty factor in terms of Duty factor productivity and quality. Titanium alloys 1. Introduction In aerospace industry, titanium alloys have been widely used because of their low weight, high strength or high temperatures stability. For example, a typical Boeing 747 aircraft contains approximately 40,000 pounds of titanium, a little over 10% of its total weight (Guitrau, 2006). Titanium alloys have also seen an increase in demand due to the increased efﬁciency and higher operating temperatures of aero-gas turbine engines. Dating back to the 1960s, the use of both titanium alloys has increased while the use of steel and aluminum has decreased as shown in Fig. 1. With the increased application of titanium alloys, the ability to produce parts products with high productivity and good quality becomes challenging. Owing to their poor machinability, it is very difﬁcult to machine titanium alloys (Rahman et al., 2006) with traditional mechanical cutting. The energy-based technique, such as electrical discharge machine (EDM), has continued to advance and gain ∗ Corresponding author. Tel.: +1 530 554904; fax: +1 530 7524158. E-mail address: zgwang@ucdavis.edu (Z. Wang). 0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.09.060 © 2007 Elsevier B.V. All rights reserved. favor as an alternative to traditional machining methods. Different from traditional machining methods, the material removal in the EDM process is achieved through melting and vaporization. EDM technology has developed rapidly and become indispensable in manufacturing applications such as die and mold machining, micro-machining, prototyping, etc (Kunieda et al., 2005). Among all EDM processes, die-sinker EDM is widely used. Fig. 2 shows that die-sinker applications are typically dominated by plastic injection and various other mold fabrications, the key to continued use and expansion of the EDM process’s capabilities are new applications for growing industries such as the aerospace industry. As shown in Fig. 2, there is no large percentage of EDM applications focused on either the production or modiﬁcation of aerospace components. Up to now, EDMs have been successfully used to machine hard materials that pose problems for traditional mechanical cutting, yet aerospace materials, namely titanium 584 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 2 ( 2 0 0 8 ) 583–589 then the workpiece temperature is measured at different duty factors, and the effect of materials thermal and electric properties on the EDM productivity is investigated. 2. Effects of material properties on EDM processes Fig. 1 – Material usage in aero-gas turbines (Ezugwu et al., 2003). Fig. 2 – Die-sinker EDM usage by application (Anon., 2006b). and nickel-based alloys, are rarely machined using the EDM process. Their low thermal conductivity and high strength at elevated temperatures complicate mechanical cutting processes, which makes achieving a high quality product difﬁcult. By applying modern EDM technology, however, these materials may be able to be EDMed effectively and efﬁciently and eventually increase the application of EDMs in the aerospace industry. Chen et al. (1999) investigated the machining characteristics of Ti–6A1–4V with kerosene and distilled water as the dielectrics. They found that higher material removal rate and lower electrode wear ratio when machining in distilled water. After that, Lin et al. (2000) used the combination of EDM and ultrasonic machining to improve the machining efﬁciency. Up to now, limited work has been done to investigate the machining performance of titanium alloys with the EDM process. In this study, the EDM process is used to machine Ti–6Al–4V, which is the most widely used titanium alloy. The essential causes of difﬁculty in machining Ti–6Al–4V is to be clariﬁed, Since the EDM process is both a thermal and an electrical process, little or no attention must be paid to consider a material’s mechanical properties and their effect on the EDM process. With mechanical machining methods, a material’s modulus of elasticity, hardness, etc. play an important role in determining its machinability. Thermal and electrical properties, however, play a vital role in determining a material’s EDM machinability, and how efﬁciently the material can be heated up, vaporized and cooled down. In comparison to a common steel material with a relatively low electrical resistivity, Ti–6Al–4V has a resistivity on the order of ﬁve times larger than steel. Titanium alloys have relatively high melting temperature, low thermal conductivities and high electrical resistivities when compared to other commonly EDMed materials. Thus, the EDM process becomes very sensitive to the energy applied to the workpiece. While the EDMing strategy for steel materials is well-known and a desired productivity and/or quality can be achieved by adjusting the discharge settings in a known method, for titanium alloys, this same strategy cannot be applied due to drastically different material properties. By directly comparing the material properties of commonly machined, easy-to-EDM materials with titanium alloys, the difference between their melting points, electrical resistivities and thermal conductivities may complicate the EDMing of Ti–6Al–4V as listed in Table 1. Also, it is important to note that a material’s electrical resistivity is highly dependent on the temperature. As an EDMing operation progresses, a constant amount of heat will be applied for every discharge pulse. Although the temperature of the actual discharge spark is much higher than the melting temperature of any metallic material (∼3000 ◦ C), the rate at which the material is removed heavily depends on how quickly it can absorb and dissipate this heat. Due to Ti–6Al–4V’s low thermal conductivity and high electrical resistivity, it is ﬁrst of all, difﬁcult for the workpiece to conduct the electrical power which is applied to it and second, once the energy is absorbed by the workpiece, it is difﬁcult to dissipate the heat without causing a steady increase of workpiece temperature. As observed in Fig. 3, titanium alloys’ electrical resistivity and thermal conductivity increase as their temperature increases. To account for this, EDMing conditions need to be carefully adjusted to ensure that adequate heat dissipation occurs and no workpiece temperature increase is observed. The most Table 1 – Material properties comparison between NAK 80 steel and Ti–6Al–4V (MPDM, 2007) Material NAK 80 steel Ti–6Al–4V Elastic modulus (GPa) 207 114 Melting point (◦ C) 1350 1630 Electrical resistivity (␮ cm) 26.3 178 Thermal conductivity (W/m K) 42.6 6.7 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 2 ( 2 0 0 8 ) 583–589 585 Fig. 3 – Electrical resistivity and thermal conductivity vs. temperature for Ti–6Al–4V (Zhang et al., 2001; Anon., 2006a). Fig. 4 – The tool-gap-workpiece’s electrical resistance representation. vital factor inﬂuencing that is the duty factor %, which is deﬁned as the discharge ON time divided by the sum of the discharge ON and OFF times. If a high duty factor is applied to Ti–6Al–4V, the high-energy application will result in rapid heat generation. This rapid heat generation theoretically will “overwhelm” the workpiece’s ability to dissipate heat and result in a gradual rise in workpiece temperature. As mentioned earlier, a material’s electrical resistivity is highly dependent on its temperature, thus an increase in the workpiece temperature results in an increase in its resistivity. With a constant amount of energy continuously being applied, the non-productive heat generation continues to compound. To avoid such an EDMing circumstance that will result in low productivity and poor surface quality, the optimal duty factor must be found for a given material with poor material properties with respect to the EDM process. While there are numerous EDM machining parameters that need to be taken into account when developing a set of machining operations for roughing, semi-ﬁnishing and ﬁnishing of workpieces. A few electrical parameters play a vital role in determining machining stability, as well as the temperature stability of the workpiece. The main objective of this paper is to optimize the duty factor for a given common used voltage and current for rough machining of Ti–6Al–4V. As mentioned previously, the duty factor largely contributes to EDMing stability and the resulting machined surface quality. While EDMing stability can be visually observed during machining by monitoring the voltage cycles using an on-machine oscilloscope, the presence of instability, or inconsistent voltage cycles, is a sign of arcing. While EDM discharging and arcing are very similar, the main difference between them is the duration of the current ﬂow. Discharging is achieved by quickly extinguishing the current ﬂow after dielectric breakdown occurs. Arcing can be characterized as a continuous ﬂow of electric current from one conductive surface to another, which results in high levels of heat generation in both conductive surfaces. Without allowing the dielectric ﬂuid to recover its resistivity, arcing causes an elevation in temperature in both conducting surfaces, thus an increase in the workpiece resistivity. The presence of arcing during an EDM operation is often a result of high energy and/or a high system electrical resistivity. Upon the next duty cycle, the ele- vated workpiece resistivity causes the electrode to approach closer and closer to the workpiece, in order for dielectric breakdown to occur. Beyond some crucial distance, however, there is not an adequate discharge gap present between the electrode and workpiece, preventing discharging to occur and resulting in only arcing. It is also important to observe that the use of different electrode materials plays a role in when discharging occurs and how to choose an optimal duty factor. Fig. 4 represents the tool-gap-workpiece’s equivalent resistance. The dielectric ﬂuid resistance, k2 , is relatively constant during EDMing, it has been discussed previously that the workpiece resistance, k3 , will change with its temperature. Depending on the electrode resistance, however, the duty factor % can be adjusted. A copper electrode used in this study naturally has a lower electrical resistance than a typical graphite electrode, resulting in a lower total system resistivity and a more efﬁcient energy transfer to the workpiece. Due to this efﬁcient energy transfer in the electrode, the duty factor % must be lower to prevent high heat buildup in the workpiece. 3. Experimental setup In this study, the widely titanium alloy Ti–6Al–4V is chosen as the workpiece material. In order to observe how the duty factor affects the EDMing results, it is necessary to measure the internal workpiece’s temperature during a machining operation. To accomplish this, a K-type thermocouple is inserted into a small hole on a rectangular Ti–6Al–4V workpiece, which allows the thermocouple tip to be mounted as close as possible to the machining surface without causing damage to the instrumentation. A national instruments (NI) data acquisition module as shown in Fig. 5 was used along with Labview 8.2 to collect the temperature data from the workpiece and the dielectric ﬂuid temperature, which was used as a reference value for comparison. Along with workpiece temperature measurement, the material removal rate was also measured in order to observe the effect of increasing or decreasing the duty factor on material removal rate. The material removal rate or machining 586 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 2 ( 2 0 0 8 ) 583–589 Fig. 5 – NI thermocouple DAQ # 9211A (NIPS, 2007). speed is deﬁned as electrode bottom surface area × actual machined depth/machining time. Duty factor settings ranged from 3.5 to 80% and a 5 mm diameter copper electrode was used with a typical negative polarity rough condition setting. To easily change the duty factor, the discharge ON time is held constant and the discharge OFF time is modiﬁed to achieve the desired duty factor percentage. The reason is that we try to eliminate the fact that changing the ON time will also affect the machining speed. The discharge ON time was chosen based on the size of electrode being used. For a small diameter electrode, as used in our experiments, using a high discharge ON time often results in EDMing instability and makes it difﬁcult to maintain constant discharge and achieve good machining speed. Using a smaller discharge ON time, however, allows for a more stable EDMing operation and results in higher productivity for small-sized electrodes. The user-deﬁned polarity parameter during EDMing usually deﬁnes the polarity of the electrode; therefore in this paper, a negative polarity is selected and deﬁnes the electrode’s polarity. Table 2 lists the important EDMing parameters that were held constant during experimentation. Fig. 6 displays a typical experimental setup used during temperature measurement testing. The workpiece is mounted in a vise and the electrode is positioned precisely in the same position on the workpiece for every test. The thermocouple measuring the dielectric ﬂuid temperature is positioned approximately 50 mm away from the machining position. The dielectric ﬂuid temperature is held approximately constant by using the ﬂuid cooler equipped on the machine. It is important to note, however, that the ﬂuid cooler is controlled automatically and when a temperature decrease/increase is observed, it will automatically adjust Table 2 – Constant EDMing parameters used for experimentation EDM parameter Polarity Current (A) Voltage (V) Servo voltage (V) Discharge ON time (␮s) Value – 60 120 85 20 Fig. 6 – Ti–6Al–4V temperature measurement experimental setup. Fig. 7 – Thermocouple and EDMing test location. the cooler fan speed, which may cause some slight ﬂuctuation or cycling in the ﬂuid temperature output data. It is important to note that the maximum workpiece temperature measured in this study is the temperature of the workpiece approximately 7 mm away from the discharging surface, as shown in Fig. 7. This distance is selected so as to prevent high voltage transferred to the thermocouple, because thermocouple used in this study could not withstand high voltage. 4. Experimental results and discussion After conducting each experiment, some important differences in temperature behavior were observed. Fig. 8 displays the temperature data versus time while using a 3% duty factor. Throughout the duration of the test, there are no visible signs of a temperature increase while EDMing progresses. The maximum workpiece temperature is approximately 28.3 ◦ C j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 2 ( 2 0 0 8 ) 583–589 Fig. 8 – Temperature data vs. time at duty factor of 3%. 587 Fig. 10 – Temperature data vs. time at a duty factor of 20%. Fig. 9 – Temperature data vs. time at a duty factor of 7%. Fig. 11 – Temperature data vs. time at a duty factor of 50%. and does not ﬂuctuate more than 0.7◦ for the duration of the test. While there is some low frequency oscillation observed in both the workpiece and dielectric ﬂuid temperature, this may be attributed to the cycling of the ﬂuid cooler fan. Fig. 9 displays the time versus temperature data while using a 7% duty factor. Similar results are found for using a duty factor of 7% when compared to a duty factor of 3%, with only a small change in temperature, less than 1◦ during the duration of the test. Fig. 10 displays the temperature data versus time when using a 20% duty factor. Compared to Figs. 8 and 9 where a 3% and 7% duty factor are used, respectively, obvious signs of temperature increase as the test progressed is evident. While this duty factor results in the highest machining speed of all duty factor settings tested, the EDMed surface proﬁle showed signs of excessive arcing, which will be explained in the following parts. Fig. 11 displays the temperature data versus time while using a 50% duty factor. An obvious increase in workpiece temperature is observed as well as a steady increase in temperature as the test progressed. The maximum workpiece temperature is observed to be approximately 39.7 ◦ C, which is over 10◦ higher than the previously displayed test with a duty factor of 3%. From these preliminary results, a change in duty factor can dramatically change the workpiece inter- nal temperature and in turn, raise the workpiece’s electrical resistivity. The experimental results for all the tests conducted are displayed in Fig. 12. As discussed earlier, an increase in duty factor beyond some optimal value results in a steady increase in Fig. 12 – Workpiece temperature and machining speed vs. duty factor percentage.