Analysis of a free machining α+β titanium alloy using conventional and ultrasonically assisted turning
Introduction
In recent decades, titanium alloys have gained widespread application in the aerospace, power-generation, biomedical and chemical industries, primarily due to a balanced set of desirable properties, such as light weight, high strength, excellent fatigue performance and resistance to an aggressive environment (Peters and Leyens, 2002). However, the main disadvantage of manufacture of titanium components is its poor machinability in conventional machining operations. The tool life is short in turning of titanium alloys due to their high strength, low thermal conductivity and chemical reactivity with tool materials at elevated temperature (Arrazola et al., 2009). In addition, a relatively low Young's modulus of titanium alloys leads to spring-back and chatter leading to poor surface quality of the finished product. Finally, during turning and drilling, long continuous chips are produced, causing their entanglement with the cutting tool and making automated machining almost impossible (Donachie, 2004).
This necessitates the use of low cutting speeds and feeds in conventional machining operations of Ti components, which lead to reduced productivity and increased component costs. The recommended cutting speeds and feed rates for finishing processes are in the range of 0.2–0.63 m/s and 0.15–0.2 mm/rev, respectively, for titanium alloys (Donachie, 2004). In addition, the time spent to remove chip entanglement from the tool and workpiece increases the production time.
In titanium cutting, chip morphology is one of the most important factors affected by machining parameters (Sima and Özel, 2010). In general, its depends strongly on several parameters, including cutting speed, feed rate, depth of cut, tool geometry and material properties of the tools and titanium workpiece. Komanduri and Brown (1981) classified the chip morphology into continuous chips, having a constant chips’ thickness, segmented chips, showing a saw-tooth like structure and discontinuous chips (fully separated segments) employing the above parameters. Mostly, segmented or discontinuous chip formation was obtained during high-speed cutting of titanium alloys (Shaw and Vyas, 1998). In segmented chips, a narrow zone of high deformation appeared between the segments, the so-called adiabatic shear-band. During the cutting process, deformation localizes in the primary shear zone and temperature increase significantly (Siemers et al., 2012).
In the past, the chip-formation process of titanium alloys was intensively studied by means of cutting experiments (Sutter and List, 2013) and simulations (Calamaz et al., 2008). Titanium alloys containing large amounts of α-phase at room temperature (α-, near-α and (α+β)-alloys) produced segmented chips for almost all cutting processes and a wide range of cutting conditions, whereas solution-treated metastable β- and β-alloys showed a cutting parameter-dependent change from continuous to segmented chip formation (Siemers et al., 2011b).
There are various ways to improve machinability of titanium alloys, e.g. by external processes such as the application of high-pressure coolant during cutting operations (Ezugwua et al., 2005), by the use of enhanced machining techniques such as ultrasonically assisted turning (UAT) (Maurotto et al., 2013) and other hybrid machining techniques (Rahman Rashid et al., 2012a, Rahman Rashid et al., 2012b) or by internal processes, e.g. by improving the machinability by alloy modification without significant changes to other material properties (Siemers et al., 2007). The improvements based on the first approach are limited. This can be explained by the high contact pressure between tools and chips in combination with low heat conductivity of titanium that prevents application of coolants directly to the process zone. Additionally, in the recent years, the increased environmental awareness and growing costs have led to a critical re-consideration of conventional cooling lubrication used in machining processes. The costs related to cutting fluids in a manufacturing operation range from 10% to 20% of the total costs of the manufactured workpiece, whereas some researchers have claimed that the costs associated with cutting fluids are higher than the cost of cutting tools (Shokrani et al., 2012). Most of these costs are primarily due to environmental requirements: handling of cutting fluids as well as their disposal must obey the strict rules of environmental protections. Therefore, green manufacturing has become attractive to industries as an alternative to conventional machining with flood supply of cutting fluid (Weinert et al., 2004).
In recent decades, a novel machining technique called ultrasonically assisted turning (UAT), in which low-energy vibro-impacts are superimposed on the movement of cutting tool, preferably in the cutting direction, has shown promise as a viable technique for machining high-strength alloys (Babitsky et al., 2004). Ultrasonically assisted machining was first introduced in the late 1960s by Skelton (Skelton, 1968). A significant improvement in surface roughness (Maurotto et al., 2012) and substantial reduction in cutting forces was observed in UAT (Muhammad et al., 2013a). Recently, a variant of UAT was proposed by Muhammad et al. (2012b) called hot ultrasonically assisted turning, combining the advantages of hot-machining with those of UAT to yield further benefits in machining intractable alloys such as Ti-15333. There is considerable research work on elliptical ultrasonic machining processes. One of the earliest works by Shamoto and Moriwaki (1994) discussed the advantages of elliptical vibration cutting with regard to reduction of cutting forces and chip thickness in machining of copper. More recently, Zhang et al. (2012) studied the effect of elliptical cutting of hardened steels showing considerable improvements compared to conventional machining. However, major disadvantages of this technique lies in kinematics of the tool motion during a vibratory cycle. The motion of the tool inevitably ends up cutting periodic grooves in the workpiece surface, ultimately affecting the surface quality of the machined workpiece. Reducing the feed rate or the vibration amplitude would improve surface quality but this would ultimately affect machining time. The developed UAT process, discussed in this paper, does not suffer from the said disadvantage of elliptical vibratory machining. Recently, analytical models were developed for ultrasonically assisted oblique turning (Nategh et al., 2012) and 2D vibrational assisted turning (Zhang et al., 2012) to investigate the benefit of this novel machining process.
Alloy modification is another technique in improving machinability, which is more difficult to perform as small changes in the chemical composition can lead to large, unexpected mechanical effects (Hussain et al., 2013). That being said, few attempts have been undertaken to develop free-machining titanium alloys. Kitayama and co-workers, added different rare-earth metals (REM) together with phosphorus (P) or sulphur (S) to achieve distribution of metal sulphide particles in a titanium alloy (Kitayama et al., 1992). Siemers and co-workers, improved machinability of Ti 6Al 4V (Ti-64) alloy by the addition of small amounts of cerium (Ce), erbium (Er), lanthanum (La) and neodymium (Nd) (Siemers et al., 2003). It was found that the addition of 0.9% Ce, La or Nd led to the formation of short chips due to softening and melting of rare-earth-metal precipitates once the temperature in the primary shear zone increase. This resulted in a decrease in adhesion between segments and chip separation during further progress of the tool. Addition of Er did not improve machinability as its melting point is too high (Siemers et al., 2011a).
Interestingly, it was observed that whenever tin (Sn) was present in the alloys, addition of REM did not improve machinability as Sn and La form intermetallics like La5Sn3, which have softening temperature above 1500 °C (Siemers et al., 2009). In Sn-containing alloys, Sn can be replaced by zirconium (Zr) ensuring that REM particles are present in the alloys. Similar attempts were made with commercially pure titanium (CP-Titanium Grade 2 and Grade 4), Ti 6Al 2Sn 4Zr 2Mo 0.08Si (Ti-6242S), Ti 15V 3Al 3Cr 3Sn (Ti-15333) and Ti 5Al 5V 5Mo 3Cr 0.5Fe (Ti5553) (Siemers et al., 2011a).
At present, up to 70% of titanium alloys produced and used are in the form of Ti-64. Due to its enhanced properties, Ti-6246 alloy will partly replace Ti-64 in aerospace engineering in the near future, especially, in aircraft engine components and airframe structures (Donachie, 2004). However, long chips obtained in turning or drilling of Ti-6246 are the main obstacles for automated machining operations. Therefore, there is an obvious need (1) to suggest new machining techniques for the production chain and (2) to develop new titanium alloys with similar properties but producing discontinuous chips in machining.
In the current work, a new variant of Ti-6246, namely, Ti 6Al 7Zr 6Mo 0.9La (designated as Ti-676-0.9La) was developed and machined with a novel machining technique – UAT – to improve machining operations and demonstrate its viability for automatic manufacture. The obtained results are compared with those for the standard Ti-6246 alloy.
Section snippets
Materials and alloy production
The standard Ti-6246 alloy was produced by the GfE-Metalle and Materialien GmbH in Nuremberg, Germany. After 2× vacuum arc remelting (VAR), the alloy was forged from diameter approximately 200 mm to diameter 75 mm in the two-phase field followed by air cooling, stress-relief annealing and stripping. To produce comparable results in the machining experiments, this alloy had been remelted once in a plasma-beam cold hearth melter (PB-CHM) followed by casting and stress-relief annealing (as described
Cutting tool and cutting conditions
In cutting tests of the studied alloys, cemented-carbide cutting inserts with a ceramic coating of titanium-aluminium-nitride on top of a layer of titanium-nitride (CP500) were used, as suggested for cutting depths ranging from 0.2 mm to 0.300 mm with a feed rate of 0.05 mm/rev to 0.25 mm/rev and cutting speeds of about 45 m/min (as specified by the manufacturer). The tool had a nose radius of 0.8 mm, a rake angle of approximately 14° and clearance angle of 0°.
The cutting conditions used in the
Experimental setup
A customized Harrison 300 lathe was used to carry out CT and UAT of both alloys as shown in Fig. 4. The lathe was modified with a customized ultrasonic transducer fixed to a cross wave-guide to superimpose ultrasonic vibration on the movement of a cutting tool, in UAT (Muhammad et al., 2012a). A schematic diagram of the experimental setup is shown in Fig. 4a. A digital signal generator was used to generate a low-current driving signal of 10 V (peak-to-peak), which was amplified for driving the
Chip morphology
In the turning experiments at various cutting conditions, chips were collected and analyzed with respect to their morphology and geometry. In machining of the standard Ti-6246 alloy, long continuous chips were developed in CT and UAT at various cutting conditions. Chips of the modified alloy (Ti-676-0.9La), in contrast, were discontinuous in both CT and UAT as shown in Fig. 7. The difference in the length of the chips is clearly visible and was apparent for various cutting conditions, even at
Conclusions
The newly developed alloy Ti-676-0.9La responded well to UAT and CT at various cutting conditions. Addition of lanthanum resulted in precipitates as inter-grain impurity improved the chip formation process both in CT and UAT. Additionally, in UAT, the vibro-impact phenomena at the tool–workpiece-interface as well as the higher temperature in process zone improved the machinability with generation of shorter chips (as compared to CT). Furthermore, a significant reduction in cutting forces was
Acknowledgements
Funding from the European Union Seventh Framework Programme (FP7/2007–2013) under grant agreement No. PITN-GA-2008-211536, project MaMiNa, is gratefully acknowledged.
The authors would also like to acknowledge Engineering and Physical Sciences Research Council (EPSRC), UK for providing FLIR SC3000 system for thermal analysis.
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