Material Removal Behavior and Surface Integrity in Grinding of Ultrafine-Grained WC-Co Materials

YUAN Yi-gao(原一高)，CHE Jun-hua(車俊華)，WANG Yan-kun(王焱坤)，SUN Wei-quan(孫衛(wèi)權(quán))，BAI Jia-sheng(白佳聲)，ZHU Xin-fa(祝新發(fā))

1 College of Mechanical Engineering，Donghua University，Shanghai201620，China

2 Shanghai ToolWorks Co.，Ltd.，Shanghai200093，China

Material Removal Behavior and Surface Integrity in Grinding of Ultrafine-Grained WC-Co Materials

YUAN Yi-gao(原一高)1*，CHE Jun-hua(車俊華)1，WANG Yan-kun(王焱坤)2，SUN Wei-quan(孫衛(wèi)權(quán))2，BAI Jia-sheng(白佳聲)2，ZHU Xin-fa(祝新發(fā))2

1 College of Mechanical Engineering，Donghua University，Shanghai201620，China

2 Shanghai ToolWorks Co.，Ltd.，Shanghai200093，China

Due to the excellent combination of wear resistance and fracture toughness，the ultrafine-grained WC-Co composites can significantly im prove the durability and reliability of industrial tools.However，the grinding of ultrafine-grained WC-Co remains a challenge.In order to provide an experimental basis for im proving grinding quality of ultrafine-grained WC-Co，a series of surface grinding experiments on ultrafine-grained WC-Co hardmetals were conducted by diamond wheel under various grinding conditions，and thematerial removal behavior and surface integrity in grinding of ultrafine-grained WC-Co materials were characterized by means of scanning electron m icroscopy(SEM)，X-ray m icrostress analyzer and surface roughness analyzer in this paper.The results indicate that the material removal behavior in grinding of ultrafine-grained WC-Co materials is determ ined not only by the abrasive grain size on the wheel，but also by the depth of cut.The roughness values of ground surface increase w ith increasing grit size of diamond wheel，and increase initially，then decrease with increase in depth of cut.Grinding causes the residual compressive stress in the surface layer of ground cemented carbides under various grinding conditions;the magnitudeof residualsurface stress increaseswith increasing grit size of diamond wheel，and isn't changed obviously along w ith the change of depth of cut.

ultrafine-grained WC-Co cemented carbides;grinding; removalmechanisms;surface roughness;residual stress

Introduction

WC-Co cemented carbides have been used extensively for cutting tool materials in the manufacturing industry，ow ing to their high hardness and wear resistance.However，the wear resistance and fracture toughness of conventionalmicrostructured WC-Comaterials are inversely related to each other.The wear resistance is often improved at the expense of the fracture toughness，and vice versa［12］.But this lim its the lifetime of many WC-Co tools.Because the mechanical properties of the WC-Co materials are primarily dependent on microstructural parameters，including the volume fraction of the binder phase，mean carbide grain size，mean free path of the binder phase and contiguity of the carbide phase，various researchers studied the relation between themicrostructure and mechanical properties of theWC-Co hardmetals.It was found that the ultrafine-grained WC-Co composites offer superior combinations of wear resistance and fracture toughness compared w ith the conventionalm icrostructured WC-Co［34］，and may offer strong potential to significantly improve the durability and reliability of industrial tools.

Cemented carbides are difficult to machine due to their exceptional mechanical properties.Hence， grinding w ith diamond wheels to date is still the method of choice in the industry for machining various WC-Co materials.Unfortunately， high hardness， fracture toughness， and resistance to abrasivewear of ultrafine-grained WC-Co materials make them extremely difficult to grind，the ground surfaces of materials are easily to be caused various types of grinding damage，and these damages involving surface/subsurface m icrocracks，chipping and residual stresses，etc.，which are detrimental to the engineering performance of WC-Co tools.Therefore，the researches for improving grinding quality of ultrafine-grained WC-Co materials have attracted w idespread attention in themetalworking industry in the recent years.

The nature of the grinding damage and ground surface integrity depends to a large extenton themechanism ofmaterial removal［5］.For improving performance and reliability of ultrafine-grained WC-Co tools，it would be essential to understand the material removal behavior and evaluate the significance of the process parameters on the quality of surface produced.Previously，many researchers have studied the grinding characteristics and material removal behavior in grinding of cemented carbides w ith coarse WC grains of a few m icrons in size under various operating conditions，such as dry grinding［6-7］，wetgrinding［8］，precision grinding［910］and ultraprecision grinding［11］.Detailed studies of the ground carbide surfaces revealed that the WC grains were cracked and pulverized by the diamond abrasive grains.It was also shown that parts of the carbide grainswere pulled out，leaving pits，or plastically deformed by the abrasive grains.The relatively soft metal binder was smeared out over the surface w ith the pulverized WC grains and partly removed from the surface together w ith theWC grain fragments.

Although there have been considerable researches reported over the last two decades on diamond grinding of conventional m icrostructured cemented carbides，few researches have been carried out on the grinding characteristics，especially on the surface integrity of ultrafine-grained WC-Comaterials so far.In fact，the mechanical properties of the carbide materials significantly influence the machining characteristics and the surface integrity of the materials［12］.In the present study，a series of grinding experiments were undertaken to study the material removal behavior and surface integrity of ultrafinegrained WC-Co materials during surface grinding，the purpose of this work is to provide experimental basis for improving grinding quality of ultrafine-grained WC-Co cemented carbides.

1 Experimental

1.1 M aterial and sam p le preparation

Thematerial used in this investigation was ultrafine-grained WC-Co alloys containing 10%cobalt by weightw ith a nom inal 0.4μm grain size.The preparation of samples was carried out as follows:the WC-10Co m ixed powder wasmilled in alcohol for 48 h in a ballmill.Themilled powderwas dried at95℃and then cold pressed at200 MPa into green compacts of 21 mm× 6.5mm×5.25mm in dimensions.Finally，the green compactswere sintered in vacuum at 1 420℃ for 1 h.Apart from this，before the grinding experiments，the samples were heat-treated at900℃ for 8 h in high vacuum with subsequent slow cooling to relieve the sintering-induced residual stress.The summary of thematerial structuresand themechanical properties provided by themanufacturer is listed in Table 1.

1.2 Grinding procedure

The grinding experiments were performed using an M 7120 grindingmachine for plane surface grinding.Three resin-bonded diamond straight wheels w ith different grit size were used in grinding.The average sizes of the abrasive grains on thewheels were 109(D-coarse)，53(D-medium)，and 18(D-fine)μm，respectively.The dimensions of the wheels were 250 mm in diameter，12 mm in w idth，and 8 mm in the thickness of the diamond layer.Water-oil emulsion was used as a coolantduring the grinding process to avoid burning out and thermal damage.The coarse grain-sized diamond grinding wheel was most commonly used in the production of WC-Co tools，hence we mainly focus on the D-coarse grinding wheel and the others are used to evaluate the effect of wheel parameters.During the grinding experiments，various parameterswere used as listed in Table 2.

Grinding was carried on the21mm×6.5mm surface of the specimens.The specimenswere ground w ithin one single pass.Three repeats were conducted on each grinding condition to exam ine repeatability.After grinding，the ground specimenswere cleaned sequentially w ith acetone(to dissolve the wax)and alcohol in an ultrasonic bath at least 30 m in.

1.3 Characterization of the ground carbide surfaces

After the grinding tests，the ground specimens were characterized to determ ine mechanisms of material removal，residual stress，and surface roughness.

To identify the material removal behavior and machining damage，the ground specimens were analyzed using a scanning electron microscopy(SEM)(JSM-5600LV，Japan)w ith a secondary electron detector，a backscatter detector and also an X-ray microstress analyzer.

The ground surfaces were investigated using an X-ray microstress analyzer(LXRD，Canada)tomeasure the grindinginduced residual stresses by utilizing the sin2ψmethod.The stresses in the ground surface of samples were measured using the(301)reflection of WC w ith Cu Ka radiation.The full w idth halfmean peak position was determ ined for a range ofψ angles(-25°，-16.78°，-5.85°，0°，5.85°，16.78°，and 25°).The residual stressmeasurementswere repeated six times on each sample to improve accuracy and obtain themean value.

The roughness of the ground surfaces was examined by means of surface roughness analyzer(JB-4C，China).Arithmetic mean roughness(Ra) was used to characterize surface roughness.The roughness measurements perpendicular to the grinding direction were repeated six times on each sample and themean value was obtained.

2 Results

2.1 M orphology of the ground WC-Co surface

After grinding，the specimens were studied using the backscatter detector to increase the contrast between the cobalt phase and the WC grains in order to reveal the cracks on the ground surfaces.Scanning electron micrographs of the ground surfaces after grinding using diamond wheels w ith different grit size are shown in Fig.1.It can be observed that themorphologyat the ground surfaces varies w ith diamond grit sizes on the wheels.When using D-fine grinding wheel，the grooves on the ground surface are narrow and shallow，and sides of uplift are lower(Fig.1(a)).In contrast，there are some cracks and rupture fragments on the ground surface，and valleys and ridges of the grooves are rough after grinding w ith D-medium wheel (Fig.1(b)).Moreover，when using the D-coarsewheel，there are large numbers of grinding debris and rupture fragments on the ground surface(Fig.1(c)).These suggest that thematerial removal behavior in grinding of ultrafine-grained WC-Co materials is sequentially transformed from scratching，plow ing to cracking and brittle fracturew ith the increasing of grit size of grinding wheels.

In Fig.2，SEM micrographs of the ground surfaces after grinding using different depth of cut，ap，are shown.Figure 3 shows the energy dispersive X-ray spectroscopy (EDS) measurements of cobalt contents distribution on the ground surfaces as mentioned in Fig.2.It can be found that the morphology and cobalt contents distribution at the ground surfaces are changed with the increase of depth of cut.When ap=5μm，the finished WC-Co surface after grinding is noted to have grooves running along the grinding direction.Partially exposed WC grains are observed in the groove valleys，and the ridges along the sides of the grooves are found to have high cobalt content(as shown in Figs.2(a)and 3(a)).When ap= 10μm，more fragments，cracks and pits are found on the ground surface(Fig.2(b)).However，when ap=20μm，only a little shallower grooves and smaller debris on the ground surface are found(Fig.2(c)).Also，EDSmeasurement shows that cobalt contents on ground surface are basically uniform，which approximately reaches its nominal Co content as in the bulk of the specimen(as shown in Fig.3(c)).These suggest that during grinding of ultrafine-grained WC-Co materials at higher depth of cut，the WC grains are pulverized by the diamond abrasive grains，and the relatively softmetal binder is smeared out over the surface w ith the pulverized WC grains.

According to Figs.1 -3，it can be concluded that the material removal behavior in grinding of ultrafine-grained cemented carbides is determined by not only the ratio between the grain size of theworkpiece and the abrasive grain size on the wheel，but also the depth of cut.

2.2 Surface roughness

The variation of the surface roughnessw ith the grit sizes of wheels is shown in Fig.4.One can clearly see that the roughness values of ground surfaces increasew ith the increasing grit sizeson wheels.The roughness of ground surfaces is plotted as a function of depth of cut as shown in Fig.5.It can be seen that surface roughness is increased w ith the increase in depth of cut initially and then decreased w ith further increase in the depth of cut.That is，when ap＜15μm，roughness values of groundsurfaces linearly increase approximately w ith the increasing of depth of cut.However，when ap≥15μm，the roughness values of ground surfaces decrease w ith the depth of cut increases.

2.3 Grinding-induced residual stresses

Grinding-induced residual stress，i.e.grinding residual stress，is considered to be the difference between the total residual stress after grinding asmeasured w ith X-ray diffraction (XRD)and the residual stress before grinding.Figures6 and 7 show the effects of grit size and depth of cut on grinding residual stress，respectively.It can be seen that samples surface after being ground is under compressive stress state under different grinding conditions，and the magnitude of grinding residual stress decreases w ith the decreasing of grits size on wheels and does not change obviously with the depth of cut.It indicates that the grit sizes on wheels dom inate the grinding residual stress in grinding of ultrafine-grained WC-Comaterials，and the depth of cut seems to have no significant effect on the grinding residual stress.

Figure 8 shows the XRD spectra of sample surface beforeand after grinding w ith D-coarse grinding wheel and aP=20 μm.It can be seen that them icrostructure of ground surface has no obvious change compared w ith that before grinding.

3 Discussion

Because of their high hardness and low fracture toughness，WC-Co cemented carbides belong to the category of so-called hard and brittle materials as ceramics.In grinding of hard and brittle materials，the material removal mechanisms can be usually classified into two categories:brittle fracture and plastic deformation［13-14］.In the brittle fracture，material removal is accomplished through void and crack nucleation and propagation，chipping or crushing.Plastic deformation is similar to the chip formation in the grinding of metals，which involves scratching，plow ing，and chip formation.Thematerial is removed in the form of severely sheared chips as obtained in machining of metals.The results of the previous studies revealed that thematerial removalmode，brittle or ductile，was substantially influenced by the chip thickness or the grain load in the grinding of hard and brittlematerials［1516］.When the value of the maximum undeformed chip thickness was below the critical value，there would be a reduction in the degree of brittleness of the material removal mechanism.If the depth of cutwas large enough to cause cracks，a chip removal w ill be due to the fracture of material.Consequently，the maximum undeformed chip thickness can be used as an important quantitative indicator to characterize the mode of material removal.

In the case of surface grinding process，the maximum undeformed chip thickness，tm，can be expressed as［17］:

where r represents the chip w idth-to-thickness ratio，m is the number of active grits per unit area of the wheel periphery，Vwis theworkpiece feed rate，Vsis thewheel velocity，and D is the wheel diameter.The value of r is usually in the range of 10-20 and is assumed to be equal to 10 in this paper.The value of m can be obtained by a simple geometric relationship，derived by Xu et al.［18］as follows:

where dgis the equivalent spherical diameter of diamond particle，v is the volume fraction of diamond in the grinding wheel，and f is the fraction of diamond particles thatactively cut in grinding.The grinding wheels used in the present study have a density of 100，or in other words，the volume fraction v is 0.25.To obtain the value of m，it is assumed thatonly one-half of the diamond particles on the wheel surface are actively engaged in cutting，or the value of f is equal to 0.5.The equivalent spherical diameter of diamond grit(dg)is given by［19］:

where M is the mesh size used in the grading sieve.In the present study，mesh sizes of 140，270，and 800 were used，respectively.The values of tmcan be calculated，after substituting all these parameters in Eq.(1)，as shown in Tables 3 and 4.

The critical maximum undeformed chip thickness，tc，for a brittle-ductile transition in grinding is given below［20］:

whereβ=0.15，assum ing that the depth of cut is equal to themachine in feed;E is the Young's modulus;H is the hardness;and K is the fracture toughness.For the ultrafinegrained WC-Co materials used in this investigation，the calculated result of tcis approximately 1.39μm.

It can be seen clearly from Tables 3 and 4 that in the grinding of ultrafine-grained WC-Co materials， the maximum undeformed chip thickness is increased w ith the increase in depth of cut at fixed grit size of wheel，and decreased with the reduction of grit sizes of wheels at fixed depth of cut.Under the same depth of cut，the values of tmin grinding are smaller than the threshold maximum undeformed chip thickness tcfor the D-fine and D-medium grinding wheels(in Table 3)，so thematerials are removed in plastic deformation，such as scratching，plow ing，and form ing chip，as shown in Figs.1(a)and(b).Since the value of tmexceeds tcfor D-coarsewheel，material removal is accomplished in brittlemode，such as cracks，pulverization，as shown in Fig.1(c).Sim ilarly，it can be also explained that thematerial removal behavior in grinding of ultrafinegrained WC-Co materials is sequentially transformed from plastic deformation mode to brittlemode w ith the increasing of depth of cut.These suggest that the material removal behavior in grinding of ultrafine-grained WC-Comaterials is influenced by not only the abrasive grain size on the wheel，but also the depth of cuts.

The surface roughness generated during the grinding process depends upon the material removal behavior.The surface roughnessmodelw ritten in term sof undeformed chip thickness is［19］:

where Rais the center line average value of surface roughness，and q is the ratio of wheel speed to workpiece speed.In the present study，under the same depth of cut，according to Eq.(5)，values of the roughness of groundsurface increasew ith the increase in tm;as a result，increase of diamond grit size on wheel w ill cause an increase in surface roughness(as shown in Fig.4).In addition to the wheel parameters，the depth of cut also significantly affects the surface roughness.It can be seen from Fig.5 that surface roughness increases initially and then rapid ly decreases w ith the increase in depth of cut.The initial increase is due to the increase in the maximum chip thicknessw ith the increase in depth of cut，which results in an increase in surface roughness.The decrease in surface roughness beyond certain value of depth of cut could be due to smear out ofmetal binder over the surface at high depth of cut(as shown in Fig.2(c))，resulting in the im provement in the surface finish.

The changes in stress state significantly affect the mechanical behavior of the materials.XRD measurement indicates that the m icrostructure of ground surface of ultrafine-grained WC-Co materials has no obvious change compared w ith that of before grinding(as shown in Fig.8).An experimental investigation of grinding tem peratures in grinding of cemented carbide revealed that the maximum temperature at the grinding zone was below 100℃，even though the temperatures increased w ith increasing wheel speed or depth of cut［7］.Hence，the residual stresses on ground surfaces should be induced by non-uniform plastic deformation near the workpiece surface in wet grinding.Under the same depth of cut，the increase diamond grit sizes on wheels w ill lead to a larger maximum undeformed chip thickness thus generating larger non-uniform plastic deform ation near the workpiece surface.As a result，grinding residual stresses w ill be increased with the increase of the diamond grit sizes on wheels.In the employment of XRD for exam ination of the residual stress in the surface of the ground carbides，the X-ray penetration depth in theWC was calculated to be about3.9μm［8］.Thus，under the same diamond grit size on wheel，on one hand，increase the depth of cut w ill result in a larger maximum undeformed chip thickness thus generating larger grinding compressive residual stress;on the other hand，increase the depth of cut w ill lead to crack and pulverize of the surface carbide grains thus relieving the residual stress also.Because of relaxation of stressby cracking and pulverizing in theWC grains，there is no clear relationship between the residual stress and the depth of cut in grinding of ultrafine-grained WC-Co materials.

4 Conclusions

In the present study，an experimental study was conducted to reveal thematerial removal behavior and surface integrity of ultrafine-grained WC-Co cemented carbides during surface grinding.The effects of the diamond grain size on the wheels and depth of cut on the material removal behavior，surface roughness，and grinding residual stress were discussed.The follow ing conclusions are obtained:

1) the material removal behavior in the grinding of ultrafine-grained WC-Comaterials is determ ined notonly by the ratio between the grain size of the work piece and the abrasive grain size on the wheel，but also by the depth of cut;

2)the roughness of the ground surface increases w ith increase of diamond grit size on wheel，and increases initially and then decreasesw ith increase in depth of cut;

3)the grinding residual stress w ill increase w ith the increase of the diamond grit size on wheels，and there is barely relationship between the residual stress and the depth of cut.

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TG580.1 Document code:A

1672-5220(2015)02-0219-06

date:2014-05-16

National Science and Technology Major Project，China(No.2012ZX04003031)

* Correspondence should be addressed to YUAN Yi-gao，E-mail:yuanyg@dhu.edu.cn