Research on Ultrasonic Vibration Assisted Grinding Technology of Quartz Fiber Reinforced Ceramic Matrix Composites

Ceramic matrix composites have good mechanical properties and high temperature resistance, but it is difficult to process. In order to solve the problems of fast tool wear and low machining efficiency in the dry grinding process, ultrasonic vibration assisted grinding technology of quartz fiber reinforced ceramic matrix composites is adopted in this paper. Grinding force, machining efficiency, tool wear and surface roughness are analyzed through the comparison experiment between the conventional grinding and the ultrasonic vibration assisted grinding. The influence of the ultrasonic vibration assistance and process parameters on machining quality and tool life is further obtained. According to the findings, when compared to traditional grinding, ultrasonic vibration grinding can lower the grinding force by 25% to 58% and the surface roughness by up to 16%. The tool life and machining efficiency can be significantly increased with the help of ultrasonic vibration.


Introduction
A novel material known as quartz fiber reinforced ceramic matrix composite is based on classic ceramic material and uses quartz fiber as reinforcement [1].It has a wide range of potential applications in the aerospace industry due to its exceptional features, which include low density, high specific strength, high specific modulus, high temperature resistance, and strong wave permeability [2].Although the existing near clean forming technology can prepare the material, secondary processing is generally needed in order to meet the requirements of product shape, surface properties, surface roughness and dimensional accuracy required in actual production.However, the high strength and brittleness of quartz fiber reinforced ceramic matrix composites cause issues such rapid tool wear, inefficient processing, and significant processing damage during processing [3][4].
By applying ultrasonic vibration to the tool or the workpiece, ultrasonic vibration aided grinding, a composite machining method combining traditional grinding with ultrasonic machining, may significantly improve the machining quality of hard and brittle materials [5].In recent years, ultrasonic vibration assisted machining has been widely studied.Ding et al. [6] used diamond cup-shaped grinding wheel to conduct ultrasonic assisted grinding and conventional grinding tests on silicon carbide ceramics.The grinding force, force ratio and surface roughness of the two grinding methods were compared and analyzed, which could indicate that the ultrasonic assisted grinding can significantly reduce the grinding force compared with the conventional grinding.With the increase of the grinding amount, the difference between ultrasonic assisted grinding and conventional grinding gradually weakened, and the effect of ultrasonic assisted grinding gradually disappeared.Bahman et al. [7] used metal-bonded diamond grinding wheel to process two kinds of C/SiC ceramic matrix composites with different properties.The results show that the grinding force of ultrasonic vibration assisted machining can be reduced by more than 20% and the surface roughness Ra can be reduced by about 30%.A similar conclusion that workpiece grinding force can be reduced by 9%-20% under the action of ultrasonic vibration was obtained by Bertsche et al [8].Zhou et al. [9] studied two-dimensional ultrasonic vibration assisted grinding of ceramic matrix composites.It was determined using theoretical calculation models and grinding trials that the ultrasonic vibration aided grinding's grinding force was much lower than that of traditional grinding, and that the force rose with an increase in feed speed and grinding depth.
To minimize grinding effort and enhance surface quality following machining, ultrasonic vibration aided machining is therefore thought to be a successful machining technique for ceramic matrix composites.The porosity of ceramic matrix composite material is generally 20%~30%.After the use of coolant, the grease will be attached to the interlayer or in the pores, which is difficult to effectively remove, affecting the performance of parts.If dry machining is selected, there will be serious tool wear, low processing efficiency and even sudden burn problems during grinding.How to improve the machining quality of ceramic matrix composites under dry machining conditions is an urgent problem to be solved.
In this paper, electroplated diamond grinding heads is employed for the ultrasonic vibration aided dry grinding of ceramic matrix composites with quartz fiber reinforcement.By analyzing grinding force, processing efficiency, tool wear and surface roughness under four groups of process parameters, the influence law of ultrasonic processing and process parameters on grinding quality and tool wear was studied, which can provide a feasible method for efficient and high-quality processing.

Experiment system
In this paper, DMF180 five axis linkage machining center is selected as the grinding processing equipment, and the machining path of the workpiece is obtained based on the automatic programming software hyperMill.The experiments of conventional grinding (CG) and ultrasonic assisted grinding (UAG) under different process parameters are carried out.UAG experiment is mainly conducted by the machining system shown in Figure 1, which includes ultrasonic tool handle, dynamometer, electroplating grinding tool and fixture.The working principle of this system is that the ultrasonic power supply converts the ordinary alternating current into a high frequency electrical signal and inputs it to the primary coil, and then this electrical signal is transmitted to the ultrasonic tool handle by the primary coil.The ultrasonic transducer converts the high frequency electric energy into the mechanical energy of high frequency vibration, and the vibration is amplified and used on the tool through the amplitude transformer.Before the experiment, the frequency and amplitude of ultrasonic tool handle were measured by the electrical vortex sensor.When the suspension length of the experiment tool is 25 mm, the resonant frequency f is 19.5 kHz and the amplitude is 5 μm.This machining system is also used for CG, but without ultrasonic power.
The specific process parameters are shown in Table 1.Spindle speed n is 8000r/min, Feed speed v w , grinding depth a p and grinding width a e are variables, and 4 experimental groups are set up.The parameters of group 1 are as follows: v w =800 mm/min, a p =0.5 mm, a e =3 mm.The experiment results of CG and UAG are compared under the condition of group 1 parameter values.The feed speed, grinding depth and grinding width are respectively increased in group 2, 3 and 4 based on group 1 to explore the influence of the process parameters on UAG.Amplitude A (μm) 0、5

Tools and workpieces
The instrument utilized in this experiment is an electroplated diamond grinding head with a 10 mm diameter, and its abrasive grain size is around 150 μm.The workpieces are quartz fiber reinforced ceramic matrix composite plates, which have the fabric structure with overlapping stitching.The fiber volume fraction is 45%, the density is 1.72g/cm 3 , the length is 150 mm, the width is 50 mm, and the height is 15 mm.

Measurement method
The experiment measures the grinding force using a three-way piezoelectric crystal dynamometer made by Kistler, model number 9129AA.Through the interface board, the dynamometer is connected to the work surface.The charge amplifier can amplify the dynamometer data and transmit it to the computer through the data acquisition card.Then the grinding force signal can be analyzed and processed.Sensofar optical profiler is used to measure the workpiece surface profile.After filtering and shape removal of the measured data, two-dimensional cross-section is selected to extract the data information.Finally, the surface roughness Ra can be calculated by using MATLAB.

Grinding force
One of the crucial physical parameters in the machining of ceramic matrix composites is the grinding force, which can indicate the status of the machining and the distribution of forces throughout the machining operation.The reduction of grinding force can reduce the cracks formed in the machining of ceramic matrix composites, so as to improve the workpiece surface quality and reduce tool wear.
For ultrasonic vibration, high grinding force will reduce the effect of ultrasonic machining.Therefore, it is very important to master the factors affecting grinding force and control the force actively to improve the machining quality.
The electroplated diamond flat grinding head is used for CG and UAG.The original force signals after 10Hz low-pass filtering are shown in Figure 2. The grinding force images under the two processing modes show a periodic changing trend, which is related to the characteristics of the fiber braid structure.The force of abrasive particles is different when grinding the fibers in different directions.Under group 1: a e =3 mm, a p =0.5 mm, v w =800 mm/min, when the machining enters the stable grinding stage, the grinding force of CG is about 2~12 N, while that of UAG is about 1~5 N. It can be seen that when ultrasonic vibration is introduced into the machining system, the force fluctuation is obviously reduced and the machining stability is improved.
Figure 3 displays the average grinding forces under various processing conditions.The grinding forces of quartz fiber reinforced ceramic matrix composites under UAG are generally lower than CG.The largest tangential force reduction among them is 72%, the maximum normal force reduction is 55%, the maximum axial force reduction is 58%, and the maximum decrease in grinding force is between 25% and 58%.At the same time, within the range of parameter values, with the increase of feed speed, grinding depth and grinding width, the grinding force under the two processing modes presents an upward trend.The grinding force of CG increases from 6 N (under group 1) to 14-20 N, and the grinding force of ultrasonic vibration assisted grinding increased from 4 N (under group 1) to 7-9 N.Under larger machining parameters, ultrasonic vibration has more obvious effect on reducing grinding force.Therefore, larger parameters can be selected in UAG to improve machining efficiency on the premise of ensuring machining quality.

Processing efficiency
The machining efficiency can be expressed by the material removal rate.In the face grinding, the material removal rate can be calculated by the product of the feed rate, grinding depth and grinding width per unit time.Therefore, increasing the feed rate, the grinding depth or the grinding width can improve the material removal rate.Figure 4 displays the material removal rate and grinding force for various processing settings.Higher material removal rate can be obtained by increasing the feed speed, but meanwhile the grinding force greatly increases, resulting in poorer grinding effect.The rate of material removal significantly increases with an increase in grinding breadth, but only slightly increases with an increase in grinding force.Under group 4: v w =800mm/min, a p =1mm, a e =3mm, the maximum material removal rate of 2400 mm 3 /min can be achieved.(Group 1: vw=800 mm/min, ap=0.5 mm, ae=3 mm, group 2: vw=1400 mm/min, ap=0.5 mm, ae=3 mm, group 3: vw=800 mm/min, ap=0.5 mm, ae=5 mm, group 4: vw=800 mm/min, ap=1 mm, ae=3 mm)

Tool wear
In the grinding process, the abrasive particles at the rounded corner of the grinding tools wear more easily than other positions.The reason is that the rounded corner of the grinding tools is the first to contact the workpiece.In the surface grinding, the single particle here has the largest material removal and the highest load, so the wear condition of this position is the most serious.Figure 5 shows the morphology of the electroplated diamond surface grinding tool rounded corner when the same volume material is removed in CG and UAG.As can be seen from Figure 5, the wear characteristics of tool edge are mainly abrasive particle shedding.In CG, a large area of abrasive particle shedding occurs at the edge of the grinding head, and the cutting ability is lost.Compared with CG, UAG has a better abrasive particle shedding phenomenon, so the tool life is longer.As a kind of hard and brittle material, ceramic matrix composite is more likely to initiate and expand cracks due to high-frequency hammering of ultrasonic vibration.The abrasive particles on the grinding head may more readily remove the material because of these fissures.Moreover, the temperature of grinding arc zone is generally higher in dry machining, and the vibration makes the contact and separation between workpiece and material periodically, which is conducive to restrain grinding head plugging and avoid grinding burn phenomenon, so as to improve the tool life.

Surface roughness
Fibers, matrix, and pores make up the majority of the components of quartz fiber reinforced ceramic matrix composites.When Sensofar optical profiler is used to measure the workpiece surface profile, the measurement results will certainly be inaccurate if the pores in the material are included in the measurement due to the large size of these holes.Thus, the cross section is chosen on the fiber bundle in a single direction to prevent the impact of hole flaws on the measurement findings.For each sample, the cross section is selected at three different locations.
The workpiece surface roughness Ra value of electroplated diamond plane grinding head after CG and UAG of quartz fiber reinforced ceramic matrix composites is shown in Figure 6.Under group 1: vw=800 mm/min, ap=0.5 mm, ae=3 mm, the workpiece surface roughness Ra of UAG is slightly higher than that of CG, which is about 4.2 μm.In the range of experiment parameters, the workpiece surface roughness will be worse with the increase of feed speed, grinding depth and grinding width.Compared with CG, UAG has a small increase in roughness.Under group 2, UAG has the largest surface roughness improvement, about 16%.In this case, the feed speed vw increases from 800 mm/min to 1400 mm/min, the surface roughness of CG is 5.58 μm, and the surface roughness of UAG is 4.7 μm.
Ceramic materials will appear elastoplastic deformation under the action of abrasive particles.When the pressure exceeds the critical value, the median crack will be formed, and the lateral crack will be formed under the action of residual stress.The generation and propagation of crack is related to the positive pressure of the grinding particle.During ultrasonic machining, the reduction of grinding force will reduce the initiation and propagation of the material crack.In addition, after the introduction of ultrasonic vibration, the high-frequency ultrasonic hammer effect increases the formation of cracks and deteriorates the machining surface within a certain parameter range.Under small grinding parameters, there is no significant difference between the surface roughness of CG and UAG.When grinding parameters are increased, the surface roughness of the workpiece in CG increases more than that in UAG (Group 1: v w =800 mm/min, a p =0.5 mm, a e =3 mm, group 2: v w =1400 mm/min, a p =0.5 mm, a e =3 mm, group 3: v w =800 mm/min, a p =0.5 mm, a e =5 mm, group 4: v w =800 mm/min, a p =1 mm, a e =3 mm)

Conclusion
In this paper, ultrasonic vibration aided grinding technology was used to address the issues of low machining efficiency and significant tool wear in the grinding of quartz fiber reinforced ceramic matrix composites.The differences in grinding force, tool life, and surface roughness between this technology and CG are examined, and the impact of grinding process parameters on machining effect is investigated.The conclusions are as follows: (1) UAG can effectively reduce grinding force.In the range of the processing parameters in this paper, the grinding force of UAG is reduced by 25%-58% compared with CG.UAG can effectively

Figure 4 .
Figure 4. Relationship between grinding force and processing efficiency in UAG.

Figure 6 .
Figure 6.Workpiece surface roughness under different process parameters.

Table 1 .
Ultrasonic vibration parameters and grinding parameters.