Journal of Materials Processing Technology 192–193 (2007) 159–165

A study on ultrasonic vibration cutting of low alloy steel Chandra Nath, M. Rahman ∗ , S.S.K. Andrew Department of Mechanical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore

Abstract High quality mirror surface machining of hard and brittle materials such as glass, ceramics, tungsten, Ni-based and Ti-based alloys has become more important in recent advanced technological applications; as these materials are widely used as aspheric shapes in optical and electronic devices, aerospace industries, household works etc. Much research has been conducted covering various cutting parameters to optimize machining conditions of such difficult-to-machine materials. Low alloy steel (DF2), a hardened steel, is a workhorse material in all industries, especially in chemical process industries due to its lower cost, good mechanical properties, weldability, and corrosion resistance at moderate temperatures. High quality surface finish is essential in most of the cases and this cannot be obtained by conventional turning (CT) process. Ultrasonic vibration cutting (UVC) has been applied to study the tool wear, cutting forces, chip formation and surface roughness under specified cutting conditions over a range. This paper investigates the effects of UVC in cutting of DF2 material, on abrasive wear and on the transition of low to high tool wear rate in the machining of the same material. Finally, it compares the UVC with CT in machining of DF2 for different cutting parameters. It is observed with photography that UVC process results in better surface finishes as compared to CT, and UVC requires lower cutting forces than in the CT process. It is also found that UVC has lower tool flank wear rate compared to CT under all cutting conditions. © 2007 Elsevier B.V. All rights reserved. Keywords: Ultrasonic vibration cutting; Low alloy steel; Abrasive wear; Tool flank wear; Tool wear acceleration; Surface finish

1. Introduction Machining of high performance hard and brittle materials such as Ti-based and Ni-based alloys, glass, ceramics and hardened steels is very difficult and cumbersome in conventional turning (CT) processes [1–3]. Moreover, CT does not provide all the benefits in technological aspects such as mirror surface finish, low cutting force, low tool flank rate, long tool life, low tool wear ratio and so on [2]. Ultrasonic vibration cutting (UVC), a precision machining technology, has been used in the machining of these hard materials which show exceptional cutting strength and low thermal conductivity, and resistance to abrasion, corrosion, pressure and high temperature [3]. In addition, vibration cutting is a promising machining method for hard cutting because of its high cutting stability [1,4]. This technique is widely used to produce high quality mirror-surface finished products [4] in recent advanced technological applications to meet all aspects of growing markets. This technique results in cleaner handling, a noticeable reduction of deep cracks in the material, the reduction of machining steps and the elimination of electrode machining which in turn reduces the cutting cost, avoids wastage and improves product quality [1,3]. ∗

Corresponding author. Tel.: +65 6516 2168; fax: +65 6779 1459. E-mail address: [email protected] (M. Rahman).

0924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2007.04.047

UVC has been conducted efficiently among different combinations of tool-workpiece materials. However, machining of low alloy steel (DF2) with cubic boron nitride (CBN) has not yet been tried. High-strength low-alloy steel (HSLA steels) [9] containing alloying elements such as C, Cr, Mn, W, V, Wr, Mo, etc. between 1 and 5%, has better mechanical properties than conventional carbon steels [5–7]. For many low-alloy steels, the primary function of alloying elements is to improve hardenability in order to optimize mechanical properties and toughness after heat treatment. Moreover, this material shows high weldability and corrosion-resistance properties at moderate temperatures [5–7]. All these properties mentioned above have made this alloy steel a superior workhorse material in all industries, especially in chemical process industries. In addition, lower cost is another special advantage of this hardened steel [7]. High-quality mirror-surface finish is highly demanded in most of the cases and this cannot be achieved by conventional machining processes. Moreover, any complex and aspheric shapes in hard and brittle materials are very difficult to get using conventional methods. As discussed earlier, the UVC method can meet that demand very efficiently. In this paper, ultrasonic vibration cutting has been applied to investigate the tool flank wear, cutting forces, chip formation and surface finish over a range of cutting conditions. This paper also investigates the effects of UVC in cutting of DF2 material,

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on abrasive wear and on the transition of low to high tool wear rate in the machining of the same material. Finally, it compares the UVC with CT in machining of DF2 for different cutting parameters. From the experiment results, it is observed from microscopic photography that the UVC process results in better surface finish and requires lower cutting forces as compared to CT processes. Also, it is found that the UVC technique has lower tool flank wear rate under all cutting conditions. 2. Experimental procedure A CNC lathe was used to conduct the experiments. A Sonic impulse SB-150 was mounted on the machine in order to provide high ultrasonic frequency of about 19 kHz and vibration amplitude of about 15 ␮m using its electrostrictive transducer element. With these parameters, the maximum vibrating speed of the tool tip can be calculated as V = 2πfa = 107.4 m/min. In order to maintain separating type vibration cutting, the cutting speeds in all operations have been chosen to be less than that vibrating speed. Although three principle vibration directions may be provided in the tool tip by ultrasonic transducer [8], tangential vibration cutting direction (also called perpendicular direction) has been applied in these experiments as shown in Figs. 1 and 2. The available power supplied for that machine was AC 100 V with 50–60 Hz frequency.

Fig. 3. Tool holder used in experiments.

A specially designed ultrasonic tool holder as shown in Fig. 3 was used in Sonic Impulse 150 instrument so that it can withstand the impact of vibration during operations. Triangular-shaped insert was mounted on that tool holder and a workpiece of 600 mm length and 120 mm diameter was used in these tests. Table 1 shows the elemental compositions of the workpiece material. The workpiece was hardened at a temperature range of 800–840 ◦ C and soft annealed at 765 ◦ C to achieve 220 HB. The experimental conditions used in these tests are shown in Table 2. The length of cut was varied accordingly to ensure equal volume of material removal. The depth of cut was set with 0.2 mm and the tool rake angle was positive 100 for all cutting tests. In regular intervals of one-pass, machining has been stopped in order to measure and observe all output parameters such as cutting forces, chip formation, flank wear, surface roughness and so on. Table 1 Percentage of alloying elements used in low alloy steel (DF2) C Cr Wr W Mn V

Fig. 1. Principle vibration directions of ultrasonic vibration cutting [8].

Fig. 2. A schematic diagram of relative movements of the workpiece and cutting tool in orthogonal ultrasonic vibration cutting [9] (t1 , undeformed chip thickness; t2 , chip thickness; Lc , tool–chip interface length; φ, shear angle).

0.90 0.50 0.50 1.10 1.20 0.10

Table 2 Experimental conditions Tool Material Rake angle Relief angle Major cutting edge angle Nose radius

CBN +10◦ 11◦ 60◦ 0.4 mm

Workpiece Material Diameter Length

Low alloy steel 120 mm 600 m

Cutting conditions Depth of cut Feed rate Cutting speed

0.20 mm 0.1, 0.2 mm/rev 50, 70, 90 m/min

Vibration conditions Frequency Amplitude Vibrating speed

19 kHz 15 ␮m 107.4 m/min

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Burr formed on the workpiece surface was removed after each cutting. 3. Results and discussions 3.1. Investigation of cutting force In all tests, three components of cutting force, namely tangential, axial and radial components were measured with a Kistler 3-components dynamometer. The force signals were recorded via a Graphtec chart recorder for analysis. In all cases, it can be seen that the UVC process requires approximately 50% cutting forces in all cutting speeds at the same feed rate of either 0.1 mm/rev or 0.2 mm/rev. Another observation was that the main cutting force (i.e. tangential force) was the highest in all cases followed by axial (feed) force and lastly, radial force was the smallest of all force components. Fig. 4 shows the tangential cutting forces against three cutting speeds at two different feed rates for CT and UVC techniques. Similarly, Fig. 5 shows the axial forces and Fig. 6 shows the radial forces. 3.2. Observation of chip formation The photographs in Fig. 7 show the type of chips produced in three different cutting speeds for both conventional and ultrasonic vibration turning. It can be seen that under the same respective cutting conditions, comparatively longer and thicker continuous chips are produced for CT than for UVC. In addition, the CT process requires more cutting force to remove material

Fig. 6. Radial force vs. cutting speed at different feed rates.

which was already discussed in Section 3.1. This means that the tool cutting edge experiences a longer period of surface contact with the chip, thus subjecting it to higher temperatures and friction. All these may explain the significantly higher flank wear for usual cutting (i.e. CT). The chips were collected and the thickness of those chips was approx 0.2 mm in CT whereas UVC produces approx 0.11 mm-thick chips. The following photographs in Fig. 8 show the type of chips that were produced at two different feed rates under the same cutting speed of 90 m/min. It is observed that higher feed rates produced chips of both higher volume and uneven edges. These types of chips always affect the surface roughness of the workpiece. It is also observed that the UVC process produces comparatively sharper fine chips that have less influence on the workpiece surface. Hence, UVC promises better surface finish than conventional machining processes. 3.3. Observation of surface roughness A hand-held surface analyzer, namely, Surtronic 10 was used to measure the surface roughness. For all cutting conditions, it is generally observed that the UVC method results lower roughness values from conventional method. Thus, it indeed produces a better surface finish as compared to the convectional method. In general, roughness values increase with the increase of cutting speed and feed rate

Fig. 4. Tangential force vs. cutting speed at different feed rates.

Fig. 5. Axial force vs. cutting speed at different feed rates.

Fig. 7. Photograph of chips produced under both CT and UVC in different cutting conditions.

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Fig. 10. Surface roughness values vs. cutting speeds at a feed rate of 0.1 mm/rev for both CT and UVC methods.

Fig. 8. Photograph of chips formed during CT and UVC at 90 m/min with feed rate of (a) 0.1 mm/rev and (b) 0.2 mm/rev.

increases. Fig. 9 shows the roughness curved-lines produced at three different cutting speeds, 50, 70 and 90 m/min. It is observed that roughness values increase with increasing values of cutting speed. The effect of varying feed rates on roughness was also observed and compared for the ultrasonic vibration cutting. Comparison of Figs. 10 and 11 reveals that the worst surface finish occurs when the feed rate is increased from 0.1 mm/rev to 0.2 mm/rev. It is generally true that surface finish degrades with increased feed rate. At lower feed rate of 0.1 mm/rev, the Ra values do not exceed 2 ␮m across the specimens. This indicates that good surface finish can be achieved with vibration cutting at lower feed rates. The variation of surface roughness is also related to the chip formation. Most of the tests produced long continuous chips as shown in Section 3.2. In the case of CT, because the

Fig. 9. The roughness curved line at three different cutting speeds in both CT and UVC techniques.

tool-edge and workpiece are always in contact, the machined surface deteriorates with the conditions of frictional heat, built-up edge, workpiece displacement and natural frequency, etc. of the machine tool due to cutting force. In contrast, because the builtup edge rarely occurs in UVC method, the finishing obtained is much better than CT method as shown in Figs. 10–12. The close-up photograph with BUE was also taken as shown in Fig. 12. The BUE increases the effective rake angle of the tool, gives low cutting force, reduces vibration and makes swarf flow easily. BUE protects the cutting edge initially, but whenever it becomes larger, it can suddenly break away, taking some of the cutting edge with it thereby causing a poor surface finish. That occurs due to the same reason mentioned in the previous paragraph of Section 3.3. But in the case of UVC, because the built-up edge hardly occurs, at the end of processing of the specimen, it can obtain geometric roughness. The best surface finish was obtained by ultrasonic vibration cutting at 0.1 mm/rev with roughness values averaging 0.8 ␮m. 3.4. Investigations of tool flank wear Tool flank wear was measured after each cut using the Toolmaker’s Microscope. One notable problem in the CT process is a short tool life due to tool flank wear. Figs. 13 and 14 show the photographs of the tool flank wear effect after 20 min of operation for both methods at a cutting speed of 90 m/min with a feed rate of 0.1 mm/rev and 0.2 mm/rev, respectively. In both cases, it can be easily seen that the higher tool wear was produced in CT processes than in UVC processes. Similarly,

Fig. 11. Surface roughness values vs. cutting speeds at a feed rate of 0.2 mm/rev for both CT and UVC methods.

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Fig. 12. Photograph of workpiece surface after operations at feed rate of 0.1 mm/rev [(a) and (b)] and 0.2 mm/rev [(c) and (d)] for both CT and UVC methods.

Figs. 15 and 16 compare tool flank wear produced for these both methods under same conditions based on experimental data measured by the Toolmaker’s Microscope. They reveal that tool flank wear increases if cutting time is prolonged in both methods. Also tool wear in the CT process is very high which reduces tool life and surface finish. It also limits the repeatability of the tool which in turn increases the manufacturing costs of the products. In contrast, for ultrasonic vibration cutting, there was insignificant wear as the cutting speed and feed rate are increased. From the test results, it can be observed that in the range of cutting speeds between 50 m/min and 90 m/min and feed rates

between 0.1 mm/rev and 0.2 mm/rev, a reasonably higher tool life is obtained for the UVC process. Moreover, it exhibits a rather slow increase in flank wear. The results point that abrasion and adhesion mechanisms are the main reasons for the flank wear of CBN tools. Breakage of the cutting edge caused by BUE is one reason why flank wear is enhanced subsequently. The unfavorable flow of continuous chips of the cutting edge also contributes to the wear. This wear is worse if flaking occurs at the end of the cutting edge. We see from Figs. 15 and 16 that cutting at a higher feed rate gives the highest flank wear across all the six different cutting

Fig. 13. Microscopic photograph of tool wear after 20 min at a cutting speed of 90 m/min with feed rate 0.1 mm/rev: (a) CT process, (b) UVC process.

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Fig. 14. Microscopic photograph of tool wear after 20 min at a cutting speed of 90 m/min with feed rate 0.2 mm/rev: (a) CT process, (b) UVC process.

Fig. 15. Tool wears at three cutting speeds for both CT and UVC processes with a feed rate of 0.1 mm/rev (a, b, c are for cutting speeds 50, 70, 90 m/min, respectively). Subscripts c: conventional cutting and u: UVC method.

conditions. In both figures, we can also observe that tool wear reduces by approximately one-fifth when the UVC process is used to cut the workpiece under all cutting conditions. Another observation is that tool wear starts at 5 min for the UVC method. Before this time period, no tool wear was seen in the UVC-used tool. Two possible reasons of this lower rate of tool wear are that firstly, UVC always requires lower cutting force as explained in Section 3.1 and secondly, it is a discontinuous cutting process. In a discontinuous process, the tool-workpiece gets time to cool during the separation between the tool and workpiece which helps to reduce the tool wear. In continuous cutting processes, tool wear is always higher due to the continuous friction between the tool and workpiece which increases the temperature in the tool-workpiece interface and then elastic deformation occurs. Finally, these factors help to a large tool wear in CT processes. 3.5. Investigation into the effects of UVC on abrasive wear and tool wear acceleration

Fig. 16. Tool wears at three cutting speeds for both CT and UVC processes with a feed rate of 0.2 mm/rev (a, b, c are for cutting speeds 50, 70, 90 m/min, respectively). Subscripts c: conventional cutting and u: UVC method.

There are many possible wear mechanisms in any material removal process. These are mainly adhesion, abrasion, erosion, chemical, diffusion and plastic deformation wear. Generally, more than a single mechanism occurs simultaneously during the cutting operations. Among all of these wears, abrasive wear influences more than flank wear. The wear rate of abrasive wear is at least one to two orders of magnitude larger than those of other mechanisms. Whenever there is a possibility of abrasive wear, it is the most important problem to be solved. In Section 3.4, we observed that more tool wear is produced in the CT process than in the UVC process. This is first due to the continuous friction at the tool-workpiece interface. In Figs. 15 and 16, we observed that tool wear is almost one-fifth in the UVC process as compared to the CT process. Since tool wear is mainly caused by abrasive wear, the UVC process perceives less abrasive wear

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which reduces tool wear. Figs. 13 and 14 also show the photographs of tool wear where the UVC causes decreased wear than the CT process. Thus it can be summarized that the effects of abrasive wear in UVC process is lower than in CT process. Acceleration of tool wear rate from a lower to a higher rate is called tool wear acceleration. From both Figs. 15 and 16, it is found that tool wear rate increases very slowly in the UVC process than that in the CT process after a certain time. This lower tool wear rate in the UVC process is due to the discontinuous process. In contrast, this rate is higher in the CT process due to the continuous cutting operations which causes a rise in temperature and different wear mechanisms and finally, elastic and plastic deformations to occur. Therefore, the UVC process shows significantly low tool wear rate than the CT process.

to tangle around the tool workpiece causing potential danger to the operator, and may adversely affect the surface finish. 4. The data also shows that the feed rate of 0.2 mm/rev gives rise to the poorest surface finish, highest tool wear as well as high cutting forces. This trend is consistent for both the ultrasonic vibration cutting and the convectional method. 5. It is found that tool wear acceleration in the UVC technique is very low compared to the CT techniques. In addition, this wear rate increases very slowly as compared to the CT methods. This is mainly due to the intermittent cutting mechanism of the UVC method.

4. Conclusions

[1] M. Xiao, K. Sato, S. Karube, T. Soutome, The effect of tool nose radius in ultrasonic vibration cutting of hard metal, Int. J. Machine Tools Manuf. 43 (2003) 1375–1382. [2] M. Xiao, Q.M. Wang, K. Sato, S. Karube, T. Soutome, H. Xu, The effect of tool geometry on regenerative instability in ultrasonic vibration cutting, Int. J. Machine Tools Manuf. (2005) 1–8. [3] L.C. Hock, Ultrasonic cutting of brittle materials, Department of Mechanical Engineering, National University of Singapore, 1992–1993. [4] A.V. Mitrofanov, N. Ahmed, V.I. Babitsky, V.V. Silberschmidt, Effect of lubrication and cutting parameters on ultrasonically assisted turning of Inconel 718, J. Mater. Process. Technol. 162–163 (2005) 649–654. [5] Steels–Carbon Steels, Mild Steel, Carbon-Manganese Steels, Alloys Steels, Low-Alloy Steels and Micro-Alloy Steels, Abstracted from IMMA Handbook of Engineering Materials, 5th Edition, http://www.azom.com/ details.asp?ArticleID=2537. [6] Steel alloys, Steels, Metallurgical Consultants, http://www. materialsengineer.com/E-steels.htm. [7] Carbon and (Low-) Alloy Steels, http://httd.njuct.edu.cn/MatWeb/ material/m fe.htm. [8] V.I. Babitsky, A.N. Kalashnikov, A. Meadows, A.A.H.P. Wijesundara, Ultrasonically assisted turning of aviation materials, J. Mater. Process. Technol. 132 (2003) 157–167. [9] A.V. Mitrofanov, V.I. Babitsky, V.V. Silberschmidt, Thermomechanical finite element simulations of ultrasonically assisted turning, Comput. Mater. Sci. 32 (2005) 463–471.

Based on this experimental study of the various performance parameters such as the tool failure mechanisms, flank wear, surface roughness, cutting forces and chip formation; the following conclusions can be made: 1. From these experimental results, it can be said that the UVC technique demonstrates outstanding capabilities. It achieves nano-finished surface qualities (Ra < 1 ␮m). Ultrasonic technology sets new standards for machining time, contour accuracy, and surface quality. This increases the durability and productivity of modern high-tech products. It therefore has good potential for industries which require higher quality machining. 2. UVC method shows low tool flank wear due to lower abrasive wear effects as compared to CT processes. 3. UVC process produces better surface finish at the lowest feed rate. However, cutting at all low speeds together with low feed rates produces long continuous chips that can amount to more than 1 m in length. This is undesirable as they tend

References