Elsevier

Journal of Manufacturing Processes

Volume 60, December 2020, Pages 188-199
Journal of Manufacturing Processes

Influences of machining parameters on tool performance when high-speed ultrasonic vibration cutting titanium alloys

https://doi.org/10.1016/j.jmapro.2020.10.053Get rights and content

Abstract

Tool performance is a key factor in evaluating machining processes. To improve machining productivity and part quality, researchers have conducted numerous studies on improving tool performance, such as tool design, coatings, functional micro textures, cooling/lubrication conditions, cutting parameter optimization, and intermittent cutting. Focusing on materials that are difficult to cut (i.e., Ti alloys), this paper explores a high-speed ultrasonic vibration cutting method that combines intermittent cutting, cooling, and lubrication. Through theoretical analysis and experiments on tool wear, cutting force, temperature, etc., the influence of machining parameters on tool performance is investigated. The results show that a large separation effect coupled with good cooling and lubrication conditions is key to improving tool performance. Among these, the feedrate and phase shift resulting from the rotary (spindle) speed are the core machining parameters. On this basis, the choice of machining parameters is summarized to provide a reference for the high-efficient machining of Ti alloys for scholars and engineers. First, cooling and lubrication conditions, such as dry machining and fluids, are determined. The duty cycle is then set from 0.5 to 0.6 via a relatively small feedrate value (i.e. 0.005 mm/r) and a π phase shift. Finally, the cutting speed and depth of cut are chosen according to the requirements of machining efficiency and cost.

Introduction

Tool performance is a key factor in evaluating machining processes and limits machining productivity and part quality [1,2]. In particular, for typical difficult-to-cut materials, such as Ti and Ni alloys, rapid tool wear during machining directly influences the surface quality, which is a crucial parameter in the aviation and aerospace fields [3]. Various factors, such as friction, heat generation/distribution, cutting fluid at the cutting contact interface, material elastoplastic deformation, and chemical reactions, can cause premature failure and rapid tool wear during machining [4,5]. In addition to these strong interfacial effects, an increase in the cutting force and temperature leads to worse surface quality, especially when cutting with a worn cutting edge under dry-cut conditions [6,7]. A large cutting force and temperature lead to an increase in the depth of deformation of the machined surface or subsurface with increasing tool wear [8]. Under these severe conditions, the adhesion effect between the cutting contact interfaces is enhanced by thermomechanical loads that result in cavities on the machined surface as the carbide particles tear up [9]. Moreover, cavities and some micro-void defects can easily cause stress concentration and thereby influence the subsequent mechanical properties of critical structural components [10]. Therefore, methods for improving tool performance are crucial.

Researchers have attempted to improve tool performance through tool design, cutting parameter optimization, cooling and lubrication improvement, and special machining processes. With regard to tool design, textured tools have been designed and fabricated to machine Ti alloys [11]. Better tribological behavior and wear performance of cutting tools have been realized with a friction reduction of 16 %–39 % [12]. Furthermore, functional coatings have been applied to generate thermal contact resistance between cutting contact interfaces, thereby effectively reducing the cutting force and improving the antifriction effect when machining both H13 hardened steel and Inconel 718 [13,14]. Some tool shape modifications, such as honed and chamfered cutting edges [15] and cutting edge microgeometry [16], can reduce the cutting force and tool wear and facilitate a longer tool life. With regard to cutting parameter optimization, statistical approaches with Taguchi-based predictions have been widely used to predict and evaluate progressive tool wear [17] and surface roughness [18]. Tool wear and surface integrity have also been investigated in terms of the influence of cutting parameters based on a mechanistic cutting force model [19] or machine tool vibration [20]. Concerning cooling and lubrication improvement, many studies have attempted to reduce the cutting force or temperature. Laser-assisted machining is an innovative method that reduces the cutting force by heating and softening the workpiece, thereby reducing tool wear by up to 50 % [21]. Minimum quantity lubrication (MQL) [22,23] and nanofluids [24] are also very popular and efficient lubrication methods that improve tool life by reducing both the cutting force and cutting temperature to realize an appropriate machining balance. The large amounts of cutting heat generated from cutting Ti and Ni alloys with a high heat capacity and low thermal conductivity can be removed via a high-pressure coolant [25,26] or cryogenic cooling [27]. With regard to special machining processes, an intermittent cutting method efficiently reduces the cutting temperature and prolongs tool life [28,29]. Ultrasonic vibration cutting, a typical intermittent cutting method, can realize an extremely low cutting force and tool flank wear (12 %–25 % of that in conventional cutting), thereby enhancing the tool life by 4–8 times [30]; this is mainly attributed to the lower abrasive wear compared to that in conventional cutting [31]. This benefit is obtained through periodic tool-workpiece separation during the cutting process [32]. When using lubrication methods such as MQL in the ultrasonic vibration-assisted milling process [33], tool wear properties can be significantly improved by suppressing the generation of microcracks and friction traces on the tool surface and simultaneously improving the machining surface quality of Ti alloys [34,35]. In addition, to obtain an ultra-precision surface quality of Ti alloys, a selective laser melting method has also been applied [36]. When an optimal laser beam scanning strategy is determined, the milling surface quality can be improved with the help of selective laser melting [37]. Overall, such methods are applied during machining to realize optimal machining conditions according to the specific processing requirements.

For difficult-to-cut materials, such as Ti alloys, high-efficiency machining is limited by heat accumulation between the cutting contact interfaces, which results in rapid tool wear and poor surface quality. To solve this problem, the high-speed ultrasonic vibration cutting (HUVC) method, a type of wave-motion cutting (tool motion trajectory) method, has been proposed [38]. This method, in combination with intermittent ultrasonic cutting and a high-pressure coolant, can effectively enhance tool life by up to six times at a high cutting speed (400 m/min) [39]. Moreover, the wave-motion cutting trajectory can suppress cutting vibrations and enhance the cutting stability during the machining of low-stiffness components [40]. These advantages are attributable to high-speed periodic tool-workpiece separation [41]. However, although previous studies demonstrated the effectiveness of this method, they did not clarify the relationships among the relevant parameters in the cutting process. Therefore, this study evaluates tool performance as an index of the proposed HUVC method and investigates the influences of relevant machining parameters from the perspective of both theoretical and practical engineering.

Section snippets

Theory analysis

Unlike traditional ultrasonic vibration cutting, vibrations occur in the vertical direction relative to the cutting direction in HUVC or wave-motion cutting. As shown in Fig. 1, vibrations occur along the feed direction (Y-axis) while cutting is performed along the circumferential tangential direction (Z-axis). HUVC is a finishing process and thus the tool nose is treated as a circle with a tool nose radius. The cutting tool wave-motion trajectory can be expressed as:xt=apyt=Asin2πFt+vftzt=vct

Method and experiments

Machining experiments are set up on a CNC-controlled HASS-SL40 machine center, as shown in Fig. 6. Triangular type cemented carbide tooltip TCMT110204 from Zhuzhou cemented carbide cutting tools Co., LTD and cemented carbide, CBN tools from SANDVIK Co., LTD are set at the specified tool position of the vibrator for each cutting. The tool conditions and cutting parameters are set in Table 1. However, not all the parameters will be used in each experiments. The parameter is chosen in an optimal

Dry machining

To save time and cost during the dry machining process, triangular-type cemented carbide tooltip TCMT110204 from Zhuzhou Cemented Carbide Cutting Tools Co., LTD were chosen. Fig. 7 shows a comparison of the flank face wear and tool life. The tool life is enhanced when the duty cycle is decreased from 1 to 0.55 in HUVC. The longest tool life is achieved when Dc = 0.55, which approaches the minimum value in a real machining process. Although Fig. 3 shows that the duty cycle can reach even smaller

Conclusions

This study aims to reveal the deep relationships between the machining parameters and tool performance in a HUVC process. It can be concluded that the advantages are obtained under the conditions of large separation effect and sufficient cooling and lubrication, where the feedrate and phase shift resulting from the spindle rotary speed are important influential parameters. Generally, within the effective cutting speed range, by controlling the duty cycle value between 0.5 and 0.6, a relatively

Declaration of Competing Interest

We would like to submit the enclosed manuscript entitled “Influences of machining parameters on tool performance when high-speed ultrasonic vibration cutting titanium alloys”, which we wish to be considered for publication in “Journal of Manufacturing Processes”. The authors are from Beihang University and Civil Aviation University of China.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 91960203, 52005023) and China Defense Industrial Technology Development Program (Grant No.JCKY2018601C209).

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