Nghiên cứu chuyên sâu về tính chất nhiệt và điện của Ti–6Al

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j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j m a t p r o t e c

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 Department of Mechanical & Aeronautical Engineering, University of California, Davis, One Shields Avenue, Davis, CA 95616, USA b 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-

trical properties on the EDM productivity. In the study, temperature measurements have

Keywords:

been made for Ti–6Al–4V workpieces with various duty factors to clarify the essential causes

EDM

of difﬁculty in machining titanium alloys and observe the optimal duty factor in terms of

Temperature stability

productivity and quality.

Duty factor

Titanium alloys

1. Introduction

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.

∗ Corresponding author. Tel.: +1 530 554904; fax: +1 530 7524158.

E-mail address: zgwang@ucdavis.edu (Z. Wang).

EDM technology has developed rapidly and become indis- pensable 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 fab- rications, 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 com- ponents. Up to now, EDMs have been successfully used to machine hard materials that pose problems for traditional mechanical cutting, yet aerospace materials, namely titanium In aerospace industry, titanium alloys have been widely used because of their low weight, high strength or high temper- atures 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 efﬁ- ciency 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 tita- nium alloys (Rahman et al., 2006) with traditional mechanical cutting. The energy-based technique, such as electrical dis- charge machine (EDM), has continued to advance and gain

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then the workpiece temperature is measured at different duty factors, and the effect of materials thermal and electric prop- erties on the EDM productivity is investigated.

Effects of material properties on EDM

2. processes

Fig. 1 – Material usage in aero-gas turbines (Ezugwu et al., 2003).

Fig. 2 – Die-sinker EDM usage by application (Anon., 2006b).

Since the EDM process is both a thermal and an electrical pro- cess, 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 mate- rial 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 conduc- tivities may complicate the EDMing of Ti–6Al–4V as listed in Table 1.

and nickel-based alloys, are rarely machined using the EDM process. Their low thermal conductivity and high strength at elevated temperatures complicate mechanical cutting pro- cesses, which makes achieving a high quality product difﬁcult. By applying modern EDM technology, however, these materi- als may be able to be EDMed effectively and efﬁciently and eventually increase the application of EDMs in the aerospace industry.

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 tempera- ture 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 resis- tivity, 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 dissi- pate 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 temper- ature increases.

Chen et al. (1999) investigated the machining character- istics 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 machin- ing 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, To account for this, EDMing conditions need to be care- fully adjusted to ensure that adequate heat dissipation occurs and no workpiece temperature increase is observed. The most

26.3

NAK 80 steel Ti–6Al–4V

207 114

1350 1630

178

42.6 6.7

Table 1 – Material properties comparison between NAK 80 steel and Ti–6Al–4V (MPDM, 2007) Material Elastic modulus (GPa) Melting point (◦C) Electrical resistivity ((cid:2)(cid:2) cm) Thermal conductivity (W/m K)

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Fig. 4 – The tool-gap-workpiece’s electrical resistance representation. Fig. 3 – Electrical resistivity and thermal conductivity vs. temperature for Ti–6Al–4V (Zhang et al., 2001; Anon., 2006a).

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, how- ever, 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.

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 “over- whelm” the workpiece’s ability to dissipate heat and result in a gradual rise in workpiece temperature. As mentioned ear- lier, 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 sur- face quality, the optimal duty factor must be found for a given material with poor material properties with respect to the EDM process.

Fig. 4 represents the tool-gap-workpiece’s equivalent resis- tance. 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 resis- tivity 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 inter- nal 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 instrumen- tation. 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.

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 ﬁnish- ing 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 machin- ing by monitoring the voltage cycles using an on-machine oscilloscope, the presence of instability, or inconsistent volt- age 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 contin- uous 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- 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 mate- rial removal rate. The material removal rate or machining

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Fig. 5 – NI thermocouple DAQ # 9211A (NIPS, 2007).

Fig. 6 – Ti–6Al–4V temperature measurement experimental setup.

Fig. 7 – Thermocouple and EDMing test location.

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 set- ting. 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 exper- iments, using a high discharge ON time often results in EDMing instability and makes it difﬁcult to maintain con- stant discharge and achieve good machining speed. Using a smaller discharge ON time, however, allows for a more sta- ble 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 experi- mentation. 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 approxi- mately 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

the cooler fan speed, which may cause some slight ﬂuctu- ation 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 pre- vent high voltage transferred to the thermocouple, because thermocouple used in this study could not withstand high voltage.

4. Experimental results and discussion

Polarity Current (A) Voltage (V) Servo voltage (V) Discharge ON time ((cid:2)s)

– 60 120 85 20

Table 2 – Constant EDMing parameters used for experimentation EDM parameter Value

After conducting each experiment, some important differ- ences in temperature behavior were observed. Fig. 8 displays the temperature data versus time while using a 3% duty fac- tor. 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

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Fig. 8 – Temperature data vs. time at duty factor of 3%. 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%.

nal temperature and in turn, raise the workpiece’s electrical resistivity.

The experimental results for all the tests conducted are dis- played 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. 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 fol- lowing parts. Fig. 11 displays the temperature data versus time while using a 50% duty factor. An obvious increase in work- piece 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-

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Fig. 13 – SEM pictures of machined workpiece surfaces at different duty factors.

workpiece surfaces in Fig. 13. For a 3% and 7% duty fac- tor, individual discharge craters can be observed and are for the most part uniform over the EDMed surface. For a 20% and 50% duty factor, however, blending of the individ- ual discharge craters can be observed, which is most likely caused by elevated material temperatures that do not allow sufﬁcient time for the discharge craters to solidify and cool before the next discharge occurs. This also corresponds to a decrease in machining speed for higher duty factors as seen in Fig. 12.

workpiece temperature. At low duty factors, the temperature is approximately constant. Above duty factors of approxi- mately 7%, the temperature starts to increase dramatically during EDMing and a noticeable decrease in material removal rate is observed beyond a duty factor of approximately 20%. While a duty factor of 20% yields the fastest material removal rate, the elevated temperature present at this duty factor causes elevated levels of arcing and slight damage to the electrode surface. This damage to the electrode surface is undesirable for most EDMing operations, therefore, an opti- mal duty factor is observed at approximately 7%, which is the highest duty factor percentage possible without caus- ing the workpiece temperature to increase steadily. Although only an increase of a few degrees Celsius is plotted, the actual temperature difference at the discharge location would be considerably larger and dramatically change the work- piece’s electrical resistivity. As mentioned earlier, an increase in resistivity would result in poor machining results due to non-productive heat accumulation in the workpiece and as observed, this is proven experimentally.

Moreover, the surface proﬁle measurement device, Zygo Newview 5000, was used to observe the surface proﬁle of the electrodes bottom surfaces to get the EDMed surfaces in Fig. 13. At lower duty factors of 3% and 7%, the surface proﬁles are more uniform than those at higher duty fac- tors as shown in Fig. 14. The reason is that higher duty factors result in higher temperature, which causes elevated levels of arcing and slight damage to the electrode bot- tom surface. Since the damage to the electrode surface is undesirable for most EDMing operations, this observa- tion reiterates that the optimal duty factor is at around 7%. To further examine the effect of an increase in work- piece temperature, SEM pictures were taken of four EDMed

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r e f e r e n c e s

Fig. 14 – Surface proﬁles of electrodes used to get workpiece surfaces in Fig. 13.

5. Conclusions

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In this paper, a basic study of titanium alloy Ti–6Al–4V and its material properties’ relation to machinability is conducted. Keeping in mind that EDMing is an electro-thermal process of material removal, the important material properties with respect to EDMing are observed and compared with com- monly EDMed materials such as steels. Due to Ti–6Al–4V’s poor electrical and thermal properties, namely the electrical resistivity and thermal conductivity, the energy application during EDMing needs to be controlled in order to prevent rapid heat generation in the workpiece. Temperature measure- ments have been made for Ti–6Al–4V workpieces with various duty factors to observe the optimal duty factor in terms of productivity and quality. The duty factor is a vital EDMing con- dition parameter that is user-deﬁned and is an easy means of changing the energy application to the workpiece. The result- ing temperature measurements show that as the duty factor increases, the internal workpiece temperature also increases. Beyond a certain duty factor, the temperature begins to steadily increase, which causes poor EDMing productivity and quality. Some ﬂuctuation is present in the collection of tem- perature data, represented by low frequency oscillation. This is mainly attributed to cycling of the dielectric ﬂuid cooling unit, which automatically adjusts its fan speed. While the oscilla- tion was present for all experiments, obvious changes in tem- perature could still be observed. Finally, the optimal duty fac- tor in terms of productivity and quality is found at around 7%.