Intermittent grinding of ceramic matrix composites (CMCs) utilizing a developed segmented wheel

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Abstract

Ceramic matrix composites (CMCs) are relatively new and promising materials for many high-technology engineering applications in harsh and severe environments due to their superior properties. Despite being mostly at the development phase, CMCs have some very successful applications in several high technology fields. However, in spite of all advantages, the employment of CMCs has been impeded by their high machining and finishing costs. Many recently developed CMCs are very difficult to machine with conventional machining technology, and improvement of the existing machining process is required and crucial. The main objectives for overcoming these technological constraints are reducing high grinding forces and tool wear, improving surface integrity and increasing material removal rate. To overcome the existing technological constraints in the grinding of CMCs, a specially designed segmented wheel has been developed. Reducing the static cutting edges via segmenting the wheel, which automatically leads to reduction of momentarily engaging cutting edges, results in a reduction of rubbing and plowing regimes. Consequently, the specific grinding energy decreases. Experimental results illustrate the high performance of the presented method. A significant reduction in normal and tangential grinding forces and an increase in G-ratio have been achieved.

Introduction

Ceramic matrix composites (CMCs) are relatively new and promising materials for many high-technology engineering applications in harsh and severe environments due to their superior properties, including high strength at elevated temperatures, low thermal conductivity, corrosion resistance, excellent wear resistance, good frictional behavior, high fracture toughness, high strength to weight ratio and low density. These properties provide considerable lifetime increases over conventional metal and/or ceramics components [1].

Like most ceramic components, CMCs are fabricated using sintering technique. CMC components generally require a close dimensional tolerance and complex shape. However, sintering is accompanied by shrinkage of components. Hence using another machining and finishing process after sintering is inevitable. The hardness of sintered CMCs generally limits the material removal processes to machining processes with geometrically undefined cutting edges. Grinding with a diamond wheel is the most commonly used process to achieve dimensional accuracy and required surface finish of ceramic and CMC parts. The cost for such a machining process can be as much as 60–80% and sometimes even up to 90% of the overall manufacturing cost [2], [3]. Hence, in spite of all aforementioned advantages, the employment of CMCs has been impeded by their high machining and finishing costs. A range of new ceramic matrix composites with excellent material properties is also being developed every day for various engineering applications. Many of these new materials are very difficult to machine with conventional machining technology, and improvement of the existing machining process is required and crucial.

There are few reports about the grinding of these new composites. CMC grinding is generally characterized with excessive grinding forces and temperature [4], [5], [6]. Tashiro et al. [5] ground a C/C–SiC composite in a dry condition with various tool materials (cemented carbide K10, WA120 and D180V). They concluded that the porous vitrified bonded diamond wheel was the most proper tool in the experiment. However, even the diamond wheel produced some grooves and left uncut SiC areas on the workpiece, demonstrating that the material is very difficult to machine [5]. Li et al. [4] and Weinert et al. [6], while utilizing the face grinding process for machining CMCs, have also concluded that due to the hardness of ceramic matrix, which is almost comparable to conventional abrasives, diamond abrasives are the optimum choice for machining CMC composites. Li et al. [4] have introduced rotary ultrasonic machining (RUM) into face grinding of CMC materials for the first time. They found that compared with conventional diamond face grinding process, the cutting force can be reduced significantly (about 50%) and material removal rate can be improved (about 10%) with RUM.

Analysis of the topography of the grinding wheel has demonstrated that only a very small portion of the static cutting edges are involved in the cutting action while the majority of edges are engaging in rubbing and plowing regimes [7], [8]. Hence, the major fraction of the grinding energy is induced by the tribomechanical systems [9]. Reducing the static cutting edges via segmenting the grinding wheel automatically leads to reduction of momentarily engaging cutting edges. Thus, it results in a reduction of rubbing and plowing regimes and therefore a decrease in specific grinding energy. Utilizing this methodology, Tawakoli [10], [11], [12] developed a segmented grinding wheel (T-Tool profile). Higher wheel life, lower grinding temperatures and significantly decreased grinding forces (about 50%) are the main advantages of T-Tool profile wheel [10], [11], [12]. Reduced power, wheel wear, thermal damage and thermal stress as well as improved surface finish have also been reported by various researchers as the benefits of intermittent grinding [13], [14]. Kim et al. [15] developed a segmental wheel in which the grooves, in order to provide structural integrity of the wheel, were filled in with highly porous abrasive material. Due to the very fast wear of the high porous sections, the wheel behaves in such a way as it is discontinuous. It has been reported that utilizing such a wheel in grinding materials with low hardness and high toughness, e.g. aluminum, copper and stainless steel prohibits wheel loading and poor surface finish and increases the grinding performance significantly [15]. Recently, Nguyen and Zhang [16]. have introduced a segmented grinding wheel and succeeded to enhance the grindability with a lower specific energy and less application of coolant.

This paper investigates the feasibility of intermittent grinding (IG) with a segmented wheel on two CMC materials. Grinding forces, surface roughness, surface profile, elastic deformation and tool wear are compared for grinding of CMC with segmented and normal diamond wheels for two types of CMC materials. The main effects of cutting conditions (cutting speed, feed speed and depth of cut) on CMC grinding are also systematically studied.

Section snippets

Experimental set-up and procedures

Fig. 1 illustrates the experimental set-up. The grinding forces were measured by a dynamometer, which was mounted beneath the workpiece and above the machine table. An A/D converter transforms the produced electrical (analog) signals by the dynamometer into numerical (digital) signals. The grinding forces are finally displayed and saved on the computer by a self-written software. The grinding wheel wear and elastic deformation were measured by a sensing measurement system, which was integrated

Experiments

The experimental equipment consists of the following:

  • Machine tool: Elb Micro-Cut AC8 CNC universal surface grinding machine.

  • Surface roughness and profile tester: Hommel-Werke model T-8000.

  • Dynamometer: Kistler piezoelectric dynamometer model 9255B.

  • Nano sensing device: Z-Nano HP by Novo Blum GmbH.

Table 1 summarizes settings of main machining parameters. Two different ceramic matrix composites, CMC-I and CMC-II, were used as the workpiece material in the grinding investigation. The CMC-I is a

Experimental results and discussion

In all the carried out experiments normal grinding forces were higher than tangential grinding forces and both normal and tangential forces were reduced by the intermittent grinding process (with employment of the T-Tool wheel).

Fig. 2, Fig. 3 compare the grinding forces and surface roughness produced by the intermittent grinding process with the conventional grinding process under different cutting speeds, feed speeds and depth of cuts. It should be noted that the scatter in the measured

Conclusion

The grinding process is influenced directly by the macro-topography of the wheel. Reducing the static cutting edge density by segmenting the wheel decreases the number of kinematic and momentarily engaging cutting edges. Hence, the mean chip cross-sectional area and chip size increase and consequently the grinding forces and temperatures reduce. Lower grinding forces and temperatures cause higher G-ratio by the intermittent grinding process.

The finer surface roughness obtained by the

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