Analysis of ultrasonic-assisted drilling of Ti6Al4V
Introduction
The widespread use of titanium alloys both in structural and corrosion-resistant applications is well known. There is growing interest concerning the process ability of titanium alloys since they exhibit a good compromise between density and yield strength and also have good creep and fatigue resistance at mid temperatures. The Ti6Al4V alloy is inside the α+β phase alloys, and it is most widely used among the different titanium alloys employed in aerospace industry [1]. Their characteristics allow lightweight structures to be achieved at temperatures above 600 °C [2].
Nowadays, there has been a growing interest and tendency to employ more friendly processing techniques. In the machining process, minimum use of lubricant and dry machining are good solutions for reducing the wastage, but the lack or reduction of cutting fluid tends to derive into problems associated with heat generation and chip removal. These problems become more prominent when dealing with titanium. Low thermal conductivity and good thermal resistance make machining of titanium, especially drilling [3], [4]. The fact being that temperature is the major wear factor on coated tools being tested on dry drilling experiments [5], the US-assisted drilling of Ti6Al4V alloy is a prospective alternative to fluid-assisted cutting where achieved temperatures will be lower than those achieved by conventional drilling.
The use of ultrasonic vibration in different manufacturing processes is well documented for more than 50 years [6]. Ultrasonic machining has been mainly applied on brittle materials, and although removal rates are not high, ultrasonic technology suits very well this type of material. Recently, ultrasonic vibration has been applied as a process assisting conventional machining operations (turning and drilling) instead of the vibro-impact regime of the ultrasonic movement being the main cutting mechanism. This technique is called ultrasonic machining (USM) or rotary ultrasonic machining (RUM) [7]. Process assistance involves applying the ultrasonic technology in the machining of non-brittle and difficult-to-cut materials [8]. Assisted ultrasonic machining has been proven to be an efficient technique for improving the machinability of several aeronautic materials such as aluminum [9], [10] or Inconel 718 [11]. Chip breaking, burr generation, workpiece roughness, tool life or torque and cutting forces are some parameters studied with vibration applied in conventional cutting processes [12], [13], [14].
Although some researchers have observed chip fragmentation in materials such as inconel [11], [14] or aluminum [15], [16] when ultrasonic vibration was applied in the drilling process, some others did not address the chip-breaking effect either in drilling [8] or turning [17]. However, the mechanism that produced chip segmentation has not been well explained. Regarding chip segmentation and serrated chip formation, catastrophic shear failure and adiabatic shear forming mechanisms are considered the main causes [18]. At this point, difficulties associated with the determination of appropriate constitutive equations [19], [20] and the establishment of adequate failure modes [21] of titanium alloys in simpler laboratory and orthogonal cutting tests limit the understanding of more complex material behaviors encountered in drilling operations.
There might be two reasons for chip breaking when vibration is superimposed on the drilling process. The first one is purely geometrical: due to the periodic nature of tool vibration and its spin, chip breaking is dependent on tool vibration amplitude calculated on the phase shift between vibratory motion and tool spinning frequency. Therefore, the vibration amplitude A which is necessary to achieve segmented chips is given according to [22]where A indicates vibration amplitude, f feed per revolution and Wf the number of vibration cycles per tool revolution. Fig. 1 shows the chip-breaking area (above each set of points) and non-chip-breaking area (below) and curves generated with feeds ranging from 10 to 100 μm have been drawn.
In ultrasonic-assisted machining, it is almost necessary to work in a resonant vibration state of the tool if high amplitudes have to be achieved [23]. When vibro-impact regimes are reached, there is a non-linear type force acting on the tool and the system tends to have unstable resonant states. Auto-resonant control strategies are used to tune phase shift in addition to frequency by means of a closed-loop control system [24], [25].
The second reason for segmented chip formation is the strain–stress state of the material. The analysis and study of chip formation in Ti6Al4V has been long reported [26], but it is still not clear which conditions generate serrated chip. Several theories have been formulated to explain the non-homogeneous chip formation assuming different crack initiation criteria and different crack initiation regions. The first theory addressed the catastrophic shear instability in machining due to slope of the true stress–true strain curve reaching of zero. However, several of the modern theories are based on adiabatic shear theory, a more prominent thermal softening than the strain hardening effect of the material, or crack initiation due to surface irregularities [27]. In the case of Ti6Al4V, segmented chip formation occurs as a consequence of adiabatic shear leading to a large strain concentration in a narrow band [26]. Due to the low thermal conductivity, all the heat generated concentrates on the shear band. If temperature in the shear band is high enough, heat generation also might increase due to the possibility of allotropic transformation in titanium [18]. Recently, the use of simulation by finite element method (FEM)-based software permits the prediction and recreation of severely deformed shear bands [19] and serrated chip morphology [20]. In this case, the results obtained are directly dependent on the employed material's flow stress, but aspects such as surface cracks, phase transformations, discontinuities and allotropic transformations are not yet taken into account.
Regarding tool wear, research indicates that ultrasonic-assisted machining yields longer tool lives. There is evidence of maximum vibration amplitude over which tool life shortens as a consequence of the impact regime reached [14]. Similar working mechanism exists in modulation-assisted machining (MAM) [28] for particulate powder production. This research group has also investigated the use of vibrating tools in drilling and turning operations where tools with high amplitudes in the range 100–200 μm were employed with notorious improvements in tool life especially concerning deep drilling operations. This apparent contradiction might be due to the huge difference in the number of impact-cutting cycles between the tool and the workpiece on both techniques; while in ultrasonic-assisted machining, working frequencies are of the order of 20 kHz, in the case of MAM, these are of the order of 100 Hz.
This study aims to analyze the effect ultrasonic assistance has on the drilling of Ti6Al4V alloy. To our knowledge, no study has been reported in the literature on applying ultrasonic assistance to the drilling process in order to achieve more favorable cutting conditions. Here, aspects such as measurement of force, high-speed imaging of chip formation and temperature measurement of the drill will be studied in order to analyze the material behaviors.
Section snippets
Experimental investigation
The experimental investigation of this work has been divided into several aspects beginning with the construction of an ultrasonic vibration device and then the monitoring of the drilling process, including its feed force and temperature measurements. Titanium alloy Ti6Al4V was drilled in the aged condition, with mechanical properties σu 1100 MPa, hardness 41 HRC and modulus of elasticity 114 GPa.
Discussion of results
Our results appear to describe the results in some points as confusing compared to those found in literature. Temperature increment during the cutting process accelerates the diffusion of the work material into the tool, decreases the hardness of the tool making it more prone to abrasion and wear, and promotes thermal softening either in the work or in the tool. From one side, temperature increments are directly related to reductions in tool life [37, p. 515], whereas from the other, some
Conclusion
After constructing a US-assisted workpiece holder, drilling of Ti6Al4V was carried out and different parameters were monitored.
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In situ chip formation was analyzed and no difference was observed with regard to chip geometry. Similarly, when new tools were employed in drilling, burr formation was null.
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When ultrasonic-assisted drilling was applied, the feed force decreased by 10–20% on average, and the decrease in force was more notorious as the vibration amplitude was higher.
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Tool tip temperature
Acknowledgements
This research was sponsored by the Basque Government Project Advance Manufacturing Technologies and coordinated by the marGUNE Cooperative Research Center. Special thanks to marGUNE researches working on US, especially O. Gonzalo and R. Alberdi. Thanks are also addressed to Prof. Girot, for his valuable suggestions.
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