Effects of tool vibration on fiber fracture in rotary ultrasonic machining of C/SiC ceramic matrix composites

Abstract

The carbon fibers fracture mechanism depending on fiber orientation significantly affects the machined surface quality of C/SiC composites. The rotary ultrasonic machining (RUM) has been proved to be a beneficial method for C/SiC composites drilling with minor tearing defects at the hole exits. In contrast, the effects of tool vibration on surface topography in RUM of C/SiC composites considering fiber orientation have not been reported. In order for a unique evaluation of surface generating mechanism in RUM of C/SiC composites, several RUM experiments were conducted on 2D-C/SiC composites. The micro structural characteristics of the hole surfaces under various fiber directions, ultrasonic amplitudes and spindle speeds were analyzed. The fiber fracture mechanism in RUM of C/SiC was discovered through theoretical analysis. The results displayed that both fiber cutting direction and cutting speeds significantly affect the surface topography in RUM and CG of C/SiC composites. The tool ultrasonic vibration could contribute to the hole surface quality improvement in RUM of C/SiC composites by the fiber fracture mechanism alteration. With the ultrasonic vibration contribution, the fiber cutting direction tended towards 90° and the cutting speeds were increased. In contrast, due to the non-monotonic effect of the cutting speed on the surface roughness, only when the spindle speeds were relatively low, the higher ultrasonic amplitude apparently contributed to the hole surface quality further improvement.

Introduction

With the fast development of diverse fields from transportation and aerospace to energy, fiber reinforced ceramic matrix composites (FRCMCs), such as C/C, C/SiC, and SiC/SiC, have appeared as strategic structural materials for meeting the challenges in tougher and stronger material applications [1]. The FRCMCs have excellent physical and mechanical properties, combining ceramic characteristics of high strength, hardness and temperature resistance with high toughness owing to fibers reinforcement [5]. Compared to the corresponding pure ceramic analogues, FRCMCs are more resistant to crack propagation, resulting in being an improved safety factor of products under impact loading [6]. Due to the superior properties, FRCMCs have proven to be useful structural materials for the nuclear, energy, military, aerospace, and transportation industries [1], [2], [3], [4], [5], [6], [7].

Though a net-shape forming technology utilized in product manufacturing from FRCMCs, machining is still an essential step for the FRCMCs meeting assembly and application requirements [8]. However, FRCMCs are regarded as most difficult-to-machine materials. Due to hard ceramic matrix existence, the conventional metal cutting methods with poly crystalline diamond (PCD) cutters can induce serious tool wear, inhibiting improvement of machining efficiency [9]. Simultaneously, due to typical characteristics such as brittleness, heterogeneous and anisotropic composites, the FRCMC are susceptible to machining induced defects such as fiber pullouts, matrix fractures, interfacial debonding, fiber burring, tearing and delamination [11]. The machining efficiency improvement and the machining damage reduction of FRCMCs have therefore drawn an increasing attention from engineers and researchers. Also, various machining technologies have been introduced for machining improvement of FRCMCs including grinding [8], [11], [12], [13], [14], [15], [16], [17], [18], ultrasonic machining [19], [20], ultrasonic vibration-assisted grinding [21], [22], ultrasonic vibration-assisted filing [23], electrical discharge machining [24], laser machining [25], [26], [27], [28], [29], abrasive water jet machining [30] and rotary ultrasonic machining (RUM) [31], [32], [33], [34], [35], [36], [37].

The hole-manufacturing is one of the most substantial processing demands for FRCMCs. The RUM is considered to be a superior machining method for drilling holes in hard and brittle materials [38]. It constitutes a nontraditional hybrid machining process combining material removal mechanisms from both diamond grinding and ultrasonic machining. As presented in Fig. 1, in RUM, a rotating tool with metal-bonded diamond abrasives is executing ultrasonic vibration in the axial direction, with simultaneous feeding towards the workpiece at a constant feed rate [39]. The RUM have already been proved suitable for FRCMC drilling with a lower cutting force, lower tearing size at the hole exit and hole surface roughness reduction, compared to conventional grinding (CG) [10], [35], [40].

As it is well known, the surface generating mechanism discovery is fundamental and valuable for the application and optimization of a certain machining method. From inclusive literatures regarding the machining of carbon fiber reinforced plastics (CFRP), it could be discovered that the fiber orientation significantly affects the surface generating mechanism therefore the surface topography during machining of CFRP [41]. Similar to CFRP, the FRCMCs are also typical fiber reinforced composites. Therefore, the fiber direction should also be considered in the FRCMC machining. Actually, it has been proved that the fiber orientation indeed affects the ground surface topography of C/SiC significantly [8]. However, regarding the hole surface generating mechanism in RUM of FRCMCs, the anisotropic FRCMCs are usually treated as isotropic brittle materials. Therefore, it is necessary for the surface generating mechanism in RUM of FRCMCs to be studied by the effects consideration of fiber orientation. Also, because the tool vibration is the key characteristic of RUM, the corresponding effect on the surface generating of FRCMCs is also of a vital concern.

The primary object of this paper was for the effects of tool vibration on the surface generating in RUM of FRCMCs to be studied. In this study, the 2D-C/SiC material was selected for the RUM tests. The effects of ultrasonic amplitude, spindle speed, and fiber orientations on the fiber fracture characteristic were highlighted. Furthermore, a theoretical analysis on the fiber cutting direction was conducted for the unique surface generating mechanism in RUM of FRCMCs to be presented.