Journal of Materials Processing Technology 157–158 (2004) 422–426

Wear of PVD-coated solid carbide end mills in dry high-speed cutting

M. Sokovica,∗, J. Kopaca, L.A. Dobrzanskib, M. Adamiakb

a Faculty of Mechanical Engineering, University of Ljubljana, Aˇskerceva 6, SI-1000 Ljubljana, Sloveniab Faculty of Mechanical Engineering, The Silesian University of Technology, Konarskiego 18a, 44-100 Gliwice, Poland

Abstract

The aim of the present research of wear behaviour on the PVD-coated solid carbide end mills in dry high-speed cutting (HSC) is to findthe main causes for unpredicted tool life. The contribution presents some results of characteristic wear types. The high cutting temperature,which is a result of high-speed cutting enhance diffusion and oxidation process. Diffusion processes between the chip and the top rake surfaceof the cutting edge result in crater wear, and oxidation reactions with the environment (air cooling in dry cutting) induce scaling of the cuttingedge. The characteristic wear of such tools (so-called “central wear”) can be used to define the tool life in the case of high-speed milling ofalloyed tool steel X63CrMoV5.1 (hardness >47 HRC).© 2004 Elsevier B.V. All rights reserved.

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eywords:Tool wear; PVD-coated end mills; Dry high-speed cutting

. Introduction

The applied stress and the temperature on the tool cuttingdge is mainly influenced by the cutting speed and feed rate.he cutting speed relates to the thermo chemical stability of

he tools material and the feed rate to its mechanical strengthr fracture toughness. Increased speed generates more heatissipated to the tool and consequent high temperatures in-uce dissolution of the tool material – or chemical wear –nd lead to eventual failure by cratering. Increased feed rate

nduces locally higher mechanical stresses on the tool, whichauses plastic deformation, chipping or fracture of the tooldge[1,2].

In conventional machining abrasive wear is dominant. Inigh-speed machining the cutting speed is increased at lower

eed rates — chemical wear becomes the dominant feature.ccording to the limitations in improving the mechanicalroperties of cutting tools, further possibilities seem to ex-

st to improve tool performance on the basis of reducinghermo chemical wear. This is possible by choosing inno-ative PVD coatings, including the ternary TiAlN, which the

of both PVD-coated high-speed and cemented carbide[3,4]. This is particularly beneficial to modern machintechniques as a dry high-speed milling, turning and dri[5,6].

Contemporary coated high-speed cutting tools showdifferent wears behaviour depending on the base maused (high-speed steel or micro-grained cemented carthe type of coating (single-layer or multi-layer) and the tof application (dry cutting, hard machining or high-spmachining). The hard coatings applied in the PVD pro(TiAlN or TiAlN/TiN) show the best wear behaviour wiuniform progress of wear that is caused by the fact thatool in these cutting processes actually slides on the wpiece surface — the wear mechanisms are completelyferent in comparing to conventional cutting; the changewear pattern induce changeable tribological contact[7,8].

2. Experimental

The purpose of experimental work was to study thewear behaviour of different solid carbide end mills dur

rocess of diffusion wear slowed down as much as possible,

nd extended the range of cutting parameters and applications

∗ Corresponding author.E-mail address:mirko.sokovic@fs.uni-lj.si (M. Sokovic).

high-speed machining in tool-making production (Fig. 1) andalso to find out the possibilities for determining the criteriafor estimation of the end mills tool life. Dry hard milling testswere executed on a DMC 63-V modern CNC-milling centre.

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924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserveoi:10.1016/j.jmatprotec.2004.09.066

M. Sokovic et al. / Journal of Materials Processing Technology 157–158 (2004) 422–426 423

Fig. 1. High-speed machining of the die for hot forging of aluminiumalloys.

Because of performing the machinability tests at the practi-cal dies is rather too expensive, therefore the tests were con-ducted on plates (310 mm× 310 mm× 40 mm) made fromalloyed tool steel DIN X63CrMoV5.1, hardened and tem-pered to hardness of 47–48 HRC. Tests were performed insuch a way that every tool has passed the same surface ofthe workpiece (103 mm× 310 mm) under the same cuttingconditions. The time of cutting was ca. 35 min.

In high-speed milling operations, generally special ballnosed solid carbide end mills are used. In our experimentfour typical two-flute cutters 6 mm in diameter (Fig. 2a) weretested. The tools were obtained from four different commer-cial market leaders (A–D) for comparison and differ withrespect to the quality of micro-grained cemented carbide sub-strate and their coatings system. The type and composition ofcoatings (regarding to a quantitative EDS analysis) is showin Table 1.

F ective p re

Table 1Composition of coatings

Producer Composition Type

A 0.96% Ti, 0.04% Al Single layerB 0.55% Ti, 0.45% Al Single layerC 0.52% Ti, 0.48% Al + WC/C Two-layer dry lubricating

coatingD 0.36% Ti, 0.64% Al/TiN Multi-layers (nano-layer

system)

All of used tools were coated with 3�m thick coating.

TiAlN coating has a higher thermal stability than TiN.TiAlN is resistant to oxidation at temperatures up to 800◦C.The aluminium in the coating material reacts with the oxy-gen from the air (air cooling in high-speed machining) andforms a passivating Al2O3 layer, which prevents a further ox-idation of the coating. The properties of coating can be evenoptimised by variation of the aluminium content[1]. It is areason why TiAlN is the most popular coating in HSC appli-cations.

FromFig. 2b it is possible to calculate the effective con-tact between the tool and the workpiece, the effective cuttingspeed and theoretical roughness of machining surface.

The optimal cutting parameters (vc = 100 m/min, f=420 mm/min,ap = 0.3 mm,ae = 0.3 mm) used in this investi-gation were obtained in the previous work[9]. Optimisationof the applied cutting parameters is made with respect to:

• lower cutting forces,• better surface quality and thus• better dimensional and shape accuracy of the workpieces

at relatively low tool wear[10].

ig. 2. The geometry of typical two-flute solid carbide end mills (a), effdge and position of central wear (c).

arameters of cutting in high-speed milling (b), SEM micrograph of won cutting

424 M. Sokovic et al. / Journal of Materials Processing Technology 157–158 (2004) 422–426

The flank or central wear level of the tool (Fig. 2c)was measured after a single tool cut off the appointed partof workpiece surface (one part of engrave), using a tool-maker’s microscope. The tests were repeated two times un-der the same conditions; however, since the purpose ofour investigation was not to estimate the reliability of thetool life but rather to present the wear behaviour in high-speed machining in tool-making production, the discus-sion in the paper is based only on a single representativeexample.

In addition, the worn end mills were examined using aPhilips XL-40 SEM–EDS instrument.

3. Results and discussion

The tool wear observed in the investigation of high-speedmilling of alloyed tool steel X38CrMoV5.1 (47–48 HRC) inused working conditions is very specific. According to theoptical and metallographic observation of wear behaviouron the cutting edges of the ball nosed end mill cutters weestablished two types of wear occurred in most cases: flankwear at the cutting edges (cannot be easily compared withthe flank wear of twist drill) and central wear of the centre(the top) of the mill.

In the early stage of the tool wear process it could be de-fi endm ni gesa ing isd . Af-t shedt city.Wt pro-

ill cutte

duction), such circumstances cause a phenomenon similar tothe formation of a BUE (built-up edge). Due to changes in thegeometrical shape of the tool tip, the wear of the tool and con-sequently the surface roughness are increased[7–9]. Centralwear is therefore an unfavourable phenomenon, which low-ers the tool life and has a negative influence on the surfaceroughness[11].

To get a better insight into the wear of cutting tools at theend of the wear tests for all cutting edges, SEM micrographswere taken and for particular edges also a quantitative EDSanalysis was made.Figs. 3–5show the typical wear behaviourof the tested cutters coated by different coatings after 35 minof high-speed milling.

Fig. 3 shows the SEM micrographs of the worn tool ofproducer A (single-layer of TiAlN). It is possible to noteuniform, not so strong flank wear with beginning of centralwear.Fig. 4shows the similar SEM micrograph of the worntool of producer B (different composition of TiAlN) and EDSanalysis of the top point of the worn tool; removal of hardcoating occurred on the exact centre of the tool tip. Presenceof some elements (Fe, Cr, Ni) from the workpiece is evidentfrom the EDS spectra.

The results of test were shown (Fig. 5, left) dry lubri-cating coating of producer C (TiAlN + WC/C) is not pre-ferred to use in high-speed milling of alloyed tool steel;the strong central wear is distinctive. Both the flank andr pris-i erredm xideh opy[

eT D)h ndi-t

ned like deformation of the top point of the solid carbideills (so-called central wear), which mostly finishes as a

nterruption (pull out) of the central part of the cutting edt the top of the tools. Because the hard protective coatamaged the tool wear resistance is significantly reduced

er analysing the problem of the central wear, we establihat it is caused by the influence of the feed rate velo

hen the cutter linear motion along engraves (seeFig. 1) isoo small (less than 1 m/min in the case of small engrave

Fig. 3. SEM micrographs of worn end m

r after 35 min high-speed milling, producer A.

ake surfaces exhibited typical sliding wear features comng grooves and dispersed material transfer. The transf

aterial was predominantly high-temperature iron oematite Fe2O3 as determined by SEM–EDS spectrosc

9].On the other hand (Fig. 5, right), it was found that th

iAlN/TiN multi-layers nano-coating system (produceras very high wear resistance in the given cutting co

ions.

M. Sokovic et al. / Journal of Materials Processing Technology 157–158 (2004) 422–426 425

Fig. 4. SEM micrograph and EDS analysis of worn end mill cutter after 35 min high-speed milling, producer B.

Fig. 5. SEM micrographs of worn end mill cutters, producers C and D (wear land 35 min trial).

4. Conclusions

In high-speed machining of steel with increasing cut-ting speed, the contact zone temperature will rise suffi-ciently to come up to or even exceed the heat resistivityof the cutting material. This leads to a substantial reduc-tion of tool life — a limiting factor is diffusion type ofwear.

The results of previous research of high-speed milling onimproved and hardened steels[4,6,7,9]show the following:when the protective coating is removed from the tool tip, afurther wearing process, accompanied by increased cuttingresistance and elevated temperatures, is demonstrated as thesudden chipping of the cutting edges and catastrophic failureof the tool. The end mill cutters, coated with dry lubricating

coating TiAlN + WC/C (producer C), have no place in dryhigh-speed milling.

In further research work we will try to define the param-eters value of the central wear, which can be used to definethe tool life in the case of dry high-speed milling of hardenedalloyed tool steels.

References

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