CBN tool wear in ductile cutting of tungsten carbide
Introduction
Tungsten carbide has been more and more widely used in industry as cutting and forming tools due to its excellent physical and mechanical properties, such as superior strength, high hardness, high fracture toughness, and high resistance to abrasion and wear. A key technical bottleneck for its industrial application limitation is with the product rapid prototyping processes. A ductile cutting technology for rapid prototyping of tungsten carbide products has been developed and reported recently [1], [2], [3]. Ductile mode cutting of tungsten carbide material can be achieved using commercial cubic boron nitride (CBN) cutting tools when undeformed chip thickness is extremely small and a ratio of the radius of tool cutting edge to undeformed chip thickness is greater than 1.
The wear behavior and mechanisms of tungsten carbide used as cutting tools and molds have been widely investigated and well known in the past decades. However, little work has been done on the material removal characteristics of tungsten carbide as a workpiece cut by hard tools, such as single-crystalline diamond, polycrystalline diamond (PCD) and cubic boron nitride. Mehan et al. studied dry sliding wear of hard materials, such as hot pressed Si3N4 and SiC, 99% dense Al2O3 and WC+6% Co (tungsten carbide), using pin-on-disc against a diamond composite in air at room temperature in 1985. The lowest wear rate was obtained with tungsten carbide among those materials [4]. Engqvist et al. evaluated the grooving wear of single-crystal tungsten carbide against a Vickers diamond indenter in single-tip scratch testing in 1999 [5]. The experimental results indicated that there was a difference in both the amount of wear and wear mechanisms between different crystallographic directions of WC. Depending on the direction of the slip planes in relation to the groove direction, the wear mechanisms changed from ductile (grooves parallel to the slip planes) to brittle (grooves perpendicular to the slip planes). Shetty et al. studied indentation fracture of a series of well-characterized WC–Co composites with a Vickers diamond pyramid indenter in 1985 [6]. The resulting crack length-indentation load data were analyzed in terms of relations’ characteristic of radial and fully developed radial/median crack geometries. An indentation fracture mechanics analysis based on the assumption of a wedge-loaded crack is shown to be consistent with the observed linear relation between the radial crack length and the indentation load and predicts a simple relation among the fracture toughness, the Palmqvist toughness and the hardness of the WC–Co alloys.
Plastic flow and fracture in WC single crystals and WC–Co materials showed that deformation resulted in the development of high compressive stress that encouraged slip in WC. Optical and transmission electron microscopy studies demonstrated that plastic flow in the carbide phase always preceded fracture [7]. Takahashi and Freise observed the slip in single crystals of tungsten monocarbide around Vickers pyramid hardness indentations in 1965. It was found that the slip plane is {1 0 0} at room temperature and the slip directions are postulated at the 〈0 0 0 1〉 and 〈1 1 0〉. Greenwood et al. in 1982 found undissociated dislocations with Burgers vectors of 〈0 0 0 1〉, 1/3 〈1 1 0〉 and 1/3 〈1 1 3〉 appeared on a variety of slip planes [8]. Fischmeister et al. modeled crack propagation in WC–Co hard metals with finite element method (FEM) in 1988. The plastic zone size in the binder phase extends only through part of the binder regions directly intersected by the crack. Void formation in the binder can be predicted by a criterion combining intense strain and a large hydrostatic-deviated stress.
Although tremendous work has been done on study of the tool wear performance in cutting of workpiece material other than tungsten carbide, study of the tool wear in ductile mode cutting of tungsten carbide work material has not been reported. As a further development of the technology for ductile mode cutting of tungsten carbide [1], [2], [3], a study of the tool wear characteristics in ductile cutting of tungsten carbide is presented in this paper. The tool wear performance and wear mechanism in ductile cutting of tungsten carbide were investigated using an optical measurement inspection system (OMIS), scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS), and the cutting forces were measured using a three-component force dynamometer.
Section snippets
Workpiece and CBN tool materials
Commercial tungsten carbide inserts (SNMM433, Sumitom) were used as the machined workpiece in this study. The grade of the commercial tungsten carbide insert was A30, or its ISO application code was P30. The tungsten carbide workpiece dimension was mm. The nominal chemical composition of the tungsten carbide is shown in Table 1. Commercial cubic boron nitride tool insert (MB730, Mitsubishi) was used as a cutting tool because of its rough and finish machining capability for
Experimental results
Tool wear experiments were carried out under different cutting speeds in face milling. The face milling is a discontinuous cutting process due to the small workpiece size. Thus, cutting distance represents the actual cutting path where the cutting tool goes through the workpiece. Fig. 3 shows the effects of the cutting distance on the tool flank wear VBmax under different cutting speeds. The cutting conditions were: feed rate of 0.01 mm/rev, depth of cut of 3 μm, and cutting speed of 148.3 m/min,
Tool wear mechanisms
Examination of the wear surfaces of the CBN tools after cutting of tungsten carbide material was carried out using a scanning electron microscope and an energy dispersive X-ray spectrometer. SEM observations on the wear surfaces of a CBN tool after cutting tungsten carbide with the speed of 370.7 m/min are shown in Fig. 7((a) side-top view, (b) top view and (c) side view). SEM photographs of Fig. 7 show that the tool wear mainly appeared on the tool flank face, and no crater wear was formed on
Conclusions
- 1.
CBN tool wear behavior in ductile mode cutting of tungsten carbide has been investigated on a high-speed milling machine tool. It has been found that the higher the cutting speed, the larger the tool flank wear VBmax and shorter the tool life.
- 2.
The cutting forces increase gradually with the increase of the cutting distance and the tool flank wear. However, the machined workpiece surface roughness shows no significant change with the progress of the tool flank wear.
- 3.
SEM and EDS examinations on the
Acknowledgements
The authors would like to express their sincere thanks to Singapore Institute of manufacturing Technology (SIMTech) for their financial support for presenting this paper at WOM 2003 conference.
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