structural titanium alloy (Ti±6Al±4V) was machined with an abrasive waterjet (AWJ) to investigate its machinability under varying cutting conditions. Machinability ...
1709
Machinability of titanium alloy (Ti±6Al±4V) by abrasive waterjets Y W Seo1 **, M Ramulu2 * and D Kim3 1 Inje University, Kyungnam, Korea 2 Department of Mechanical Engineering, University of Washington, Seattle, Washington, USA 3 Washington State University, Vancouver, Washington, USA
Abstract: Titanium alloy is known as one of the di cult-to-machine materials using conventional machining processes, although it has superior formability. In the present study, a widely used aircraft structural titanium alloy (Ti±6Al±4V) was machined with an abrasive waterjet (AWJ) to investigate its machinability under varying cutting conditions. Machinability was evaluated in terms of kerf geometry, cut surface quality and microstructural integrity. Quality of the machined surface and microstructure features were examined using surface pro®lometry, scanning electron microscopy and energy dispersive X-ray (EDX) analysis. The surface roughness was ranged several micrometres near the jet entrance region and was observed to increase at the jet exit of the workpiece. Scanning electron microscopy (SEM) analysis of the surface microstructure revealed that the mechanism of material removal was a combination of scooping induced ductile shear and ploughing actions of the abrasive particles. EDX analysis showed garnet particle embedment in titanium throughout the cutting depth, and the particle size was estimated to be several tens of micrometres. Keywords: abrasive waterjet machining, titanium alloy (Ti±6Al±4V), surface ®nish, cutting conditions
1
INTRODUCTION
Titanium alloys have many unique mechanical and physical properties such as a high strength±weight ratio, high temperature strength and exceptional corrosion resistance. These are the major reasons that titanium alloys are widely used, particularly in aerospace engine components as well as for structural members. Other applications of titanium alloys include the chemical, electronic and biomedical industries [1]. Among several alloying types of titanium, Ti±6Al±4V is the most widely used structural alloy in the aerospace industry, providing 45 per cent of the industry’s total titanium alloy consumption [2]. The superior mechanical properties that are bene®cial for service conditions also limit the machinability of the titanium alloy. Since the ®rst studies on the machining of titanium alloys in the 1950s, tremendous e ort has been focused on the development of e cient machining methods for surface integrity and longer tool life [1, 3, 4]. When machining titanium, the high strength induces great The MS was received on 28 January 2003 and was accepted after revision for publication on 5 September 2003. *Corresponding author: Department of Mechanical Engineering, University of Washington, Box 352600, Seattle, WA 98195, USA. **Visiting scholar, University of Washington, Seattle, Washington, USA. B01903 # IMechE 2003
cutting stress at a localized tool±workpiece contact and leads to a high temperature rise. The machining-induced thermal energy is not e ectively dissipated due to the adverse thermal properties of titanium. Therefore, the poor machinability of titanium alloys are mainly due to the high temperature rise and rapid tool wear during machining, which limits the cutting speed and tool life [5, 6]. Among the variety of cutting tools currently available, straight grade (WC/Co) cemented carbide tools are recommended to be the most suitable for continuous cutting and high-speed steel tools for interrupted cutting under the application of cutting ¯uid [3]. At an elevated temperature, the titanium is chemically active with almost all tool materials; hence, the tool wear is accelerated and results in a short tool life. Also, inhomogeneous shear localization due to the serrated and unstable chip formation causes the resultant cutting forces to ¯uctuate. This, combined with the low elastic modulus of titanium, causes chatter and excessive de¯ection of the machined parts [7]. However, the titanium alloys are still classi®ed as one of the extremely di cult-to-machine materials using conventional machining processes [3, 7±9]. As new advanced engineering materials are introduced, new machining processes that are technologically and economically viable are also being developed. Abrasive Proc. Instn Mech. Engrs Vol. 217 Part B: J. Engineering Manufacture
1710
Y W SEO, M RAMULU AND D KIM
Table 1
AWJ cutting variables and levels Variable levels
Cutting variables
`Low’ (L)
`Medium’ (M)
`High’ (H)
Pressure (MPa) Grit mesh number (average diameter in mm) Stando (mm) Traverse speed (mm/s)
138 100 (150) 4.0 0.7
172 80 (180) 2.5 1.6
207 50 (300) 1.0 2.4
Abrasive mass ¯owrate ˆ 10 g/s (constant).
waterjet (AWJ) machining is one of them [10, 11]. In AWJ machining, the workpiece material is removed by the action of high-speed water mixed with abrasive particles. A high-speed waterjet transfers kinetic energy to the abrasive particles and the mixture impinges on to the workpiece. Numerous abrasive particles in the mixture serve primarily as an erosive medium, continuously acting as a micromachining process resulting in highspeed cutting. Since the AWJ machining technology was ®rst commercialized a couple of decades ago, rapid industrial applications enabled the technology to machine a wide range of engineering materials [10, 12±15]. The AWJ process has the capability of cutting complicated shapes, comparably adequate surface ®nishes with close tolerances, minimal process temperature generation and low process-induced deformation of the workpiece [16]. E ects of cutting parameters on surface qualities generated by AWJ machining of various materials have been studied and related models have also been proposed [17, 18]. The results of the studies showed that the AWJ cutting process is signi®cantly a ected by the variation of process parameters; however, the degree of parametric in¯uence depends on both the magnitude of parametric variation and machinability of the workpiece material. Despite the potential for machining metals and composites with the AWJ, limited work has been performed in utilizing machining of titanium alloys [19]. In the current experimental study, a series of abrasive waterjet machining tests were performed to investigate the surface quality and suggest the optimal machining conditions of titanium alloy Ti±6Al±4V. The variations of cutting condition include pressure, abrasive size, stando distance (SOD) and traverse speed, while the abrasive ¯owrate is kept constant. After machining, the cut surfaces were observed under pro®lometry to analyse the surface ®nish from the jet entry down to the exit of the workpiece. In addition to the surface analysis, scanning electron microscopy (SEM) analysis was performed to study the mechanisms of material removal
Table 2
characteristics of machined surfaces. Also, the possible embedment of the abrasive particle during machining was examined under the energy dispersive X-ray (EDX) system. 2
EXPERIMENTAL CONDITIONS
2.1 Design of the experiments The experimental parametric machining conditions were planned to investigate the main e ects, i.e. linear and two variable interaction e ects using the design of experiment technique. The cutting tests were performed using a threelevel, four-factor, nine-run experimental design based on the preliminary results obtained from the analysis of previous studies [20, 21]. All cutting was performed with an abrasive ¯owrate of 10 grams/s, which was designated as the best ¯owrate [19]. The three-level design consists of low, medium and high levels with nine-run set experiments each and four independent variables including: jet pressure, stando distance, traverse speed and grit size. Details of the AWJ cutting variables and levels used are listed in Table 1. Three sets of nine runs for each experiment were performed to produce a fully crossed experimental design array. Three variable levels of `low’, `medium’ and `high’ were grouped based on the productivity aspects. 2.2 Experimental set-up All the experiments were performed with a PowerJet model driven by a Model 20-35 waterjet pump. A gravity feed abrasive hopper and model 22-40PFX workpiece table were equipped with this unit. The ori®ce assembly consisted of two parts: a 0.3 mm diameter sapphire jewel that transforms high-pressure water into a highspeed collimated jet and a 44 mm long, 1.0 mm internal diameter carbide nozzle insert, for the e ectively
Mechanical properties of the specimen materials (Ti±6Al±4V)
Composition C Fe Si Al V Ti Percentage
