Wear mechanism map of uncoated HSS tools during drilling die-cast magnesium alloy

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Available online at www.sciencedirect.com

Wear 265 (2008) 685–691

Wear mechanism map of uncoated HSS tools duringdrilling die-cast magnesium alloy

J. Wang a,∗, Y.B. Liu b, J. An b, L.M. Wang a

a State Key Laboratory of Rare Earth Resources and Application, Changchun Institute of Applied Chemistry,Chinese Academy of Sciences, Changchun 130022, PR China

b Key Laboratory of Automobile Materials, Ministry of Education, Department of Materials Science and Engineering,Nanling Campus of Jilin University, Changchun 130025, PR China

Received 17 January 2007; received in revised form 20 November 2007; accepted 14 December 2007Available online 12 February 2008

bstract

A wear mechanism map of uncoated high-speed steel (HSS) tools was constructed under the conditions of dry-drilling die-cast magnesium

lloys. Three wear mechanisms appear in the map based on the microanalysis of drilled HSS tools by SEM, including adhesive wear, abrasive wearnd diffusion wear. In the map, there exists a minor wear region which is called “safety zone”. This wear mechanism map will be a good referenceor choosing suitable drilling parameters when drilling die-cast magnesium alloys.

2007 Elsevier B.V. All rights reserved.

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eywords: HSS tool; Die-cast magnesium alloy; Drilling; Wear mechanism ma

. Introduction

Tool machining is the radical process of friction and wear.ool wear during cutting not only decreases the service life ofutting tools, but also leads to increased roughness of cuttingurfaces of work pieces [1]. An optimal machining process isne where the maximum material removal rate is obtained withhe minimal amount of tool wear. This can be attained throughhe appropriate choice of machining conditions [2]. The wear

echanism map is a good means of selecting suitable machiningonditions to the machinist, which can reflect wear rates andear mechanisms under different operating conditions in oneap, and shows the transformation relation of one dominantear mechanism to another. It is the most forceful implement toear resistant design at present.The concept of creating wear maps were made as early

s 1941, but until 1987, Lim et al. constructed the first wear

echanism map which possesses certain useable function and

heoretical significance in tribology research field. The diagramas plotted for the unlubricated sliding of steels [3,4]. Based on

∗ Corresponding author. Tel.: +86 431 8526 2836; fax: +86 431 8526 2447.E-mail address: [email protected] (J. Wang).

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043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2007.12.009

fety zone

he same methodology, Antoniou and Subramanian constructedwear mechanism map for the unlubricated sliding wear of

luminum and its alloy on steel [5]. The more detailed andore refined wear mechanism maps for aluminum alloys pro-

osed later by Liu et al. [6]. Through the systematic studiesf cutting tools, Lim, Liu and their cooperators constructed theear mechanism maps of several cutting tool’s materials dur-

ng cutting steels, which used feed rates and cutting speeds aswo axes, respectively [7–9]. In the early 2000s, Zhang et al.1] constructed the wear mechanism maps of uncoated HSSools drilling die-cast aluminum alloy. These maps explainedhe tribological characteristics of cutting tools and relative wear

echanisms in different regions under dry sliding conditions.As the lightest structural metals, magnesium alloys are used

n a wide range of applications in the automotive, electron-cs, aerospace industries. Magnesium alloys are considered ashe third kind of structural metallic materials afterwards theteels and aluminum alloys [10–12]. With the increasinglyevelopment of the application, the machinability of magne-ium alloys became the interesting research direction to the

achinist gradually. It is generally thought that magnesium

lloys have good machinability, but the systematic researchelated with the machinability of magnesium alloys were rarelyeported, especially the study on the wear characteristic and

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686 J. Wang et al. / Wear 265 (2008) 685–691

Table 1Chemical composition of the die-cast magnesium alloy AZ91

Element Wt.%

Al 8.93Zn 0.63Mn 0.195Si 0.0377Fe 0.0015Cu 0.0025NO

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umpoint in the diagram represents the cutting tool’s wear rate cor-responding to one drilling parameter. It can be seen from thediagram that the data points with similar wear rates assemble ina certain region. In this diagram, four regions are separated by

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echanism of tools during cutting magnesium alloys [13,14].n this research the wear mechanism map of uncoated HSSools during dry-drilling the die-cast magnesium alloy AZ91s constructed, and the safety zone is identified in which theear rate of tools would be minimum. Drilling is an impor-

ant secondary processing method of the die-cast magnesiumlloys, so the wear mechanism map of drilled tools was studiedn this paper [15]. But the wear mechanism map is possibleo use for other form of machining, to predict the generalrend of tool wear (not the rates), such as the approximateocation of the safety zone or the lower-wear regions [16].hese maps will be good references for choosing suitable pro-essing parameters for uncoated HSS tools cutting magnesiumlloys.

. Experimental details

The chemical composition of the die-cast magnesium alloyZ91 is listed in Table 1. The alloy was melted in a crucible

esistance furnace under condition of shielding atmosphere ofF6 and CO2, mixing ratio was 1:100 (vSF6 : vCO2 ). Then thelloy was pressure die cast in a DCC630M cold chamber magne-ium die casting machine, and the specimen with the dimensionf 15 mm × 20 mm × 380 mm. The pressure in chamber was 4.9Pa, the injection speed was 6 m/s, the dwell time was 3 s, die-

ast temperature was 700 ◦C, and the mold temperature was at50–200 ◦C for the test parameters. The mechanical propertiesf the die-cast magnesium alloy AZ91 are listed in Table 2. Fig. 1hows the microstructure of the die-cast magnesium alloy AZ91sed in the experiment.

The drilling tests were carried out on the VMC-VS-1100 ver-ical machining centre, and without use of any cooling methodsnd lubrications at the drilling process. Uncoated HSS drill tolls

W18Cr4V) with a diameter of 5 mm were used for the drillingests. The hardness of the HSS drill tools adopted in the tests is3–65 HRC. The cutting speeds varied from 1000 to 8000 rpmnd the feed rates varied from 0.05 to 0.3 mm/rev. The hole

able 2echanical properties of the die-cast magnesium alloy AZ91

ensile strength (MPa) 220ield strength (MPa) 173longation (%) 2rinell hardness (HB) 65

Fig. 1. Optical metallograph of die-cast magnesium alloy AZ91.

n the specimen was through, and after drilling 180 holes onhe specimen, the flank wear VB of the drilled tool was mea-ured by FDI JX11 universal tool microscope. The position forhe flank wear measurement is shown in Fig. 2. A stereoscopic

icroscope was used to observe the chip and the drilled tools.canning electron microscopy with energy spectrum was usedor worn surface analysis.

The two-dimensional map of flank wear of HSS tools wasonstructed by employing the cutting speed as abscissa, the feedate as ordinate. The best expression of wear rate is a dimension-ess quantity on the wear map, which is the wear rate of a unitutting length. The value of the measured wear rate was nor-alized using a formula of log10(VB/Lm), where Lm is cutting

istance. After normalization, the dimensionless wear rate wasbtained.

. Results and discussion

.1. The map of flank wear rate of uncoated HSS drilledools

Fig. 3 shows the change trend map of flank wear rate ofncoated HSS tools which were used for drilling the die-castagnesium alloy AZ91 under dry-drilling operations. Every

Fig. 2. Flank wear measurement of HSS drill tools.

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mdut−7.9, generated the stable built up edge (BUE). Due to the pro-tective effect of the built up edge on tool wear, wear rate of thisregion is minor. The region B with the wear rate from −7.3 to

Fig. 3. Map of flank wear rate of the HSS drilled tools.

oundaries with a step length of 0.3 based on the change of wearate, morphology analysis of the drilled tools by stereoscopicicroscope, and the chip form in different regions. The values

f step interval include: −6.7∼−7.0, −7.0∼−7.3, −7.3∼−7.6nd −7.6∼−7.9. According to the four step intervals, the wearate map was demarcated and connecting line of similar wearate points act as boundary. The boundaries are expressed inroken line in the map.

There is a region in the centre of Fig. 3, in which the wearate changes from −7.6 to −7.9. This region is called safetyone, indicating the lowest flank wear rates of HSS tools in thisange of processing. It is the optimal machining parameter inhe safety zone with an acceptable wear rate. The periphery ofhe safety zone is a region with higher wear rate range of −7.3o −7.6, and is called the lower-wear region. This region hasigger scope than the safety zone and is also optional machiningange.

.2. The wear mechanisms map of uncoated HSS drilledools

The microanalysis of flank worn surface for uncoated HSSrilled tools indicates that the dominant wear mechanism is dif-erent in every separated region by wear rate. Fig. 4 shows the

Fig. 4. Wear mechanism map of flank wears of HSS drilled tools.

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echanisms map of the flank wear for HSS drilled tools duringry drilling. In this diagram, the different letters and patterns aresed to express the different wear mechanisms. The region A inhis map, it is the safety zone with the wear rate from −7.6 to

ig. 5. Wear characteristics of built up edge in cutting speed of 2000 rpm, feedate of 0.1 mm/rev: (a) SEM micrograph of the built up edge on the HSS drilledool; (b) optical photograph of the built up edge; (c) shape of chip generated inhis region.

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7.6, the wear mechanism is mainly adhesive wear. The regionsand D with the wear rate from −7.0 to −7.3, the wear mecha-

ism are mainly abrasive wear. The region E with the wear raterom −6.7 to −7.0, the wear mechanism is mainly diffusionear. The formation of built up edge is due to the chip bondingn the surface of cutting tool, the wear mechanism of region As can also be considered as adhesive wear. Therefore, there areve regions separated in the diagram, but the wear mechanismsave three forms: adhesive wear, abrasive wear and diffusionear.

.3. Analysis of the wear characteristics of each wearechanism

In region A (Fig. 4) there is stable built up edges generated sohe wear of the drilled tools is reduced. The SEM microstructuref the worn flank surface of a drilled tool under a cutting speedf 2000 rpm, a feed rate of 0.1 mm/rev is shown in Fig. 5(a). It

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ig. 6. Wear characteristics of adhesive wear in cutting speed of 5000 rpm, feed rateEM photograph of the machined surface; (c) energy analyses of the grain on the ma

5 (2008) 685–691

s shown that there is a little metal wedge block adhered on theoundary of the main cutting edge. The little wedge block is builtp edge. The built up edges were removed and inlaid, and thenade into metallographic samples. The microstructure of the

uilt up edge is shown in Fig. 5(b). The built up edge exhibitsulti-lamellar structure as a result of chip accumulating over

he drilled tool. Furthermore, it hardness is very high after sev-ral deformation strengthening. The microhardness of the builtp edge is 158HV by measuring the metallographic samples,nd the value is an average value of six measured results. Aspseudo cutting edge, it is able to replace the cutting edge to

rill the magnesium alloy, so its existence reduces the tool wear.n addition to this, built up edge also increases the rake anglef the cutting tool, lead to the magnesium alloy chip break-

ng easily. The chip form generated in this region is shown inig. 5(c).

In region B (Fig. 4), the wear mechanism of flank face isainly adhesive wear. The SEM microstructure of the worn

of 0.2 mm/rev: (a) SEM photograph of flank wear of the HSS drilled tool; (b)chined surface; (d) shape of chip generated in this region.

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Fig. 7. Wear characteristics of abrasive wear in cutting speed of 1000 rpm, feedrate of 0.2 mm/rev: (a) SEM photograph of flank wear of the HSS drilled tool;(b) energy analyses of the grain on the flank wear of the HSS drilled tool; (c)

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ank surface of a drilled tool under a cutting speed of 5000 rpm,feed rate of 0.2 mm/rev is shown in Fig. 6(a). A relatively

moother surface with parallel plough ridges is observed andome adhesive pits on the plough ridges are shown by arrowsn Fig. 6(a). The surface of the drilled hole was analyzed byEM and the result is shown in Fig. 6(b). It can be seen thearticle which shattered from cutting tool’s flank face adheredn the hole’s surface during drilling process. The componentsf the particle shown by arrow in the photograph were analyzedy the energy spectrum and the result is shown in Fig. 6(c).nergy spectrum analysis shows that the main component of

he particle is Fe, indicating the adhering particle from the cut-ing tool. The chip generated in this region is a massive chipwing to extrusion, as shown in Fig. 6(d). The massive chipometimes accumulates at the flute, and not beneficial to chipemoval.

Abrasive wear is the major wear mechanism in regionsand D (Fig. 4). The SEM microstructure of the worn

ank surface of a drilled tool under a cutting speed of000 rpm, a feed rate of 0.2 mm/rev is shown in Fig. 7(a).he worn flank surface reveals obvious plow groove. The par-

icle as shown in Fig. 7(a) by the arrow generated deepercratches and cracks on the flank face of the drilled tool.nergy spectrum analysis of the particle is shown in Fig. 7(b),hich indicates that the particle is carbide. This is because

he duration of drilling process has been prolonged as aesult of the low cutting speeds and small feed rates athese two regions. Therefore, high temperature generated athe tool–work interface due to the contact time increas-ng. And it made the carbides in HSS matrix dissociateut and scratch the tool face. Most of the chip gener-ted in this region is strip-type helical chip, as shown inig. 7(c).

In region E (Fig. 4) the dominant wear mechanism isiffusion, which is controlled by temperature. High temper-tures could be generated at the tool–work interface withhe increasing cutting speeds and feed rates, resulting inapid wear of drilling tools. All the chips accumulate at theake face and flute of drilling tools, the appearance of therilled tool is shown in Fig. 8(a). The shape of the chips ishown in Fig. 8(b), from the shape we can see that the chipdhered together owing to heat and extrusion and formed aong massive chip. After removing the adhered chip on therilled tool, the SEM microstructure of the worn flank sur-ace of the drilled tool under a cutting speed of 8000 rpm,

feed rate of 0.3 mm/rev is shown in Fig. 8(c). SEM photohows that severely plastic deformation took place on theorn surface and more cracks and large quantities of debrisenerated on it. The component in the region near main cut-ing edge on the flank face was analyzed by linear scanning.he result shows that deficient carbon and rich magnesiumppeared there, as shown in Fig. 8(d). This indicates thatiffusion took place due to the high temperature generated

t the tool–work interface under high cutting speeds andarge feed rates. In this region, tool wear increases rapidlyo that the drilling parameters in this region should not bedopted.

shape of chip generated in this region.

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ig. 8. Wear characteristics of diffusion wear in cutting speed of 8000 rpm, feedn this region; (c) SEM photograph of flank wear of the HSS drilled tool; (d) lin

. Conclusions

. In the wear rate map, five zones are separated based on thechange of wear rate and morphology analysis.

. There are three types of wear mechanisms in separatedregions in the map based on the SEM observation of the

worn flank surface. They are adhesive wear, abrasive wearand diffusion wear, respectively.

. Wear mechanism map is constructed according to the prin-ciples and methods of constructing wear mechanism. In this

f 0.3 mm/rev: (a) appearance of the HSS drilled tool; (b) shape of chip generatedanning component analyses of flank wear of the HSS drilled tool.

wear mechanism map, the main wear mechanisms of regionsA and B are adhesive wear, regions C and D are abrasivewear and region E is diffusion wear.

. In the map, there exists a minor wear region which is called“safety zone”. The drilling parameters in this region are theoptimal machining parameter. This wear mechanism map

will be a good reference for choosing suitable drilling param-eters (based on the criterion of tool wear) when drillingmagnesium alloys, with the aim of guaranteeing the produc-tion efficiency and lower the cost of the manufacturing.

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cknowledgement

This research is supported by the Key Technologies “Fifteen”&D Program No. 2001BA311A07-2.

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