ELSEVIER Journal ofMaterials Processing Technology 63 (1997) 181-186

Machinability of Ni3AI-Based Intermetallic Compounds

Journa1of

MaterialsProcessingTechnology

Satoshi Ito, Teiji Yoshikawa, Shinichi Miyazawa and Kazuo MoriDepartment ofManufacturing Systems, Mechanical Engineering Laboratory, 1-2 Namiki, Tsukuba, lbaraki, 305 Japan

Abstract

Considerable attention is being devoted, for the last few years, to intermetallic compounds as alternative materials for possible structuralapplications. However, because the principal motivations for the study of various intermetallics have been focused on developing the materialsand analyzing the material characteristics, very poor discussions have been made for the machinability.

In this paper, the machinability of Ni3AI-based intermetallics is evaluated in turning operations. Tool life tests are first conducted toinvestigate adaptability of the tool materials by assessing the tool life curves. From the test results, CBN tools shows the longest tool lifecharacteristics among the other tested tool materials. Then the effects of the usage of coolant and the compulsory cooling of Ni3AI-basedworkpieces on improving the machinability are discussed. Although coolant secures the improvement of machinability, no significanteffects can be obtained by enforced cooling of the Ni3AI-based workpiece.

Keywords: Ni3Al-based intermetallics, machinability ofntermetallics, tool life, tool material, compulsory cooling ofthe workpiece

1. Introduction

Recently R&D for space development machines such asspaceplane and energy development instrument related nuclearfusion or coal liquefaction are being carried on, the key technologyfor them is super heat resistance material. Research on high meltingpoint materials has been carried mainly on the fine ceramics, butfor these purpose the reliable material with higher heat resistance,strength, stiffness and acid resistance are required. Nowintermetallics attracted the attention as the one of the most adequatematerials for these purpose.

Intermetallics are originally the metal/metal compounds, butcompounds with non-metallic element or semi-conductor elementare also called intermetallics and more then 2000 types ofintermetallics are reportedly developed. Within them, Ni-basedintermetallics are promising as heat-resistant materials.

But almost all intermetallics are just developed and research ofpractical applications has not been carried on yet. For practicalapplications such as the structure material for machines, materialshave to be changed to desired figure to give them the necessaryfunctions. The typical method is machining, but the machinabilityof them is not revealed yet and it is not clear whether intermetallicscan be put to practical use.

In this paper, the machinability of Ni3AI-based intermetallics isevaluated in longitudinal turning operations to get the data ofmachinability for practical applications. Tool life tests are conductedto investigate adaptability of the various type of tool materials, thenthe effects of the usage of coolant and the compulsory cooling ofworkpieces on improving the tool life and finished surface roughnessare investigated.

2. Workpiece and experiment method

Because of there brittleness, Ni3AI-based intermetallics are

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difficult to machine material. However it was reported that additionof 0.1 % boron make them more ductile[1 ,2]. So it was expectedthat boron added Ni3AI-based intermetallics had better machinabilitythan pure Ni3AI-based intermetallics, but as a first step it wasnecessary to clarified the fundamental machinability of Ni3AI-basedintermetallics. So pure ones are used as the workpieces. Chemicalcompositions of them are indicated in table I.

Table IChemical compositions of Ni3AI-based intermetallics

composition volume % composition volume %

C 0.006 Al 12.22Si 0.31 Fe 0.02Mn <0.01 (H) 0.0006P o,(m (0) 0.0014S 0.001 (N) <0.0005

Ni Bal.

Lathe, tool shank and other materials which were used in thisexperiment are shown below;Lathe: Shoun cazeneuve lathe HB500xlOO, Chuck-center supportTool shank: NIIR-44Tool insert: 12.7xI2.7x4.76mm (-5.-5,5,5.15,15,0.8)Tool material:

Carbide(Sumitomo-G3 (K30), mOE (MlO), STlOP (PlO),Toshiba-TH03 (KOI»

Cermet (Sumitomo-Tl2A (TiN»Ceramic (KobeKenametal-KYON2000 (Si3N4) )TiN-coated (Toshiba-T221 (TiN:PYD), T530 (TiCN:CYD) )Diamond-coated (DAC20)CBN (Sumitomo-BNlOO)

182 Satoshi Ito et a1.!Journal of Materials Processing Technology 63 (1977) 181-186

(a) Surface metallograph

(b) Cross section metallograph

Figure 1. Metallographs of workpiece.

From the cylindrical shape ingots just after they were casted,workpieces with diameter of IOOmm and length of 200mm werecut out with cutting wheel to be fit in the work area of turningmachine used on this research. Figure 1 shows metallographs of theworkpiece.

In figure I, it appears that workpiece structure have anisotropy.Crystals developed along the direction of cylinder axis and itsstructure looks like the bundle of thin wires along axis direction.Because when material was cooled in the ingot, crystals haddeveloped along cooling direction. Result of hardness test showsthat hardness of cylinder surface(Figure I (a» is HV237 and crosssection area(figure I (b» is harder, HV246.

3. Results and Discussions

3.1 Selection of tool material

Before machinability was evaluated, tool material should bedecided which could machine workpiece material in the turningoperation. But it was difficult to conduct the tests for all materialfor tool, so from the range of hard or heat resistance tool materials[3J,10 types of tool materials .listed in Ch.2 of [3] were selected.

Before finding the tool materials which had long tool life andevaluating the machinability of Ni3AI-based intermetallics withthose tool materials, dry cutting experiments to decide the cuttingcondition were carried on. Results of the experiments showed thathigher feed rate or larger depth of cut make the workpiece suffermicro fracture; the chips becoming of discontinuous type, andthere was a lot of exfoliation (separation) on the finished

surface.Therefore feed rate of 0.05mmlrevolution and depth of cutof 0.1 mm were selected as the cutting condition to avoid suchdamage.Under this condition, continuous chips shown in figure 2ware produced and normal cutting operation was performed.

Cutting speed of 50mlmin., feed rate of 0.05mmlrev.,depth of cut ofO.Imm, CBN(BNIOO) as the tool material,dry cutting

Figure 2. Continuous chips of Ni3Al-based intermetallics.

And figure 3 shows photograph of typical tool wear on the rakeface and flank. As feed was smaller than the corner radius of tool,flank wear was developed only on the curved surface of the corner.

(a) Wear on rake face

(b) Flank wear

Cutting speed of 50mlmin.. , feed rate of 0.05mmlrev.,depth of cut of O.1mm, CBN(BNlOO) as the tool material,dry cutting, cutting time of 30min.

Figure 3. Tool wear in dry cutting.

Satoshi Ito et aL I Journal of Materials Processing Technology 63 (1977) 181-186

O:BNIOO X:DAC20 O:STlOP 'V :TH03 f:j, :UIOE0.6• :T530 • :KYON2oo0 T :Tl2A6 + :T221 ... :G3

60.5

'" 0.4;>.<::bJ)

'0 0.3..<::"'0

]0.2 --0.:.

'" --0-<l)

~ ----0-0.1~0---<>-

00 10 20 30

Cutting time min.

Cutting speed of 50m/min., feed rate ofO.05mm/rev., depth of cut ofO.lmm, dry cutting

Figure 4. Relation between cutting time and VB(wear-land height on flank).

183

Observing the wear on the rake face, it was obvious that crater wearwas not formed and only slight wear land was developed near comeredge.In addition, there was the damaged part which seemed to beproduced by brittle fracture. One of the possible reasons wasadhesion chipping[4] which also was observed in cutting of Ni­base heat resistance alloy.

Figure 4 shows relation between cutting time and VB(wear-landheights on the flank) for the various tool materials. CBN (BNI00)showed longest tool life characteristic and TiN-coated (T530) wassecond best material. Other tested materials had worse wearresistance and in 1 or 2 minutes VB became O.3mm that was terminalflank wear in general for finishing cutting.

As described above, Ni3AI-based intermetallics were difficult tomachine material, and only CBN could keep to machine it amongthe 10 types of tool materials selected previously. So experimentsto get tool life curve (V-T curve) for CBN tool were carried on.

Figure 5 shows tool life curve for CBN with VB of 0.3mm as theterminal flank wear. This curve was a normal tool life curve whichwas shown straight line when plotted on log-log diagram with longertool life in lower cutting speed.

3.2 Effects ofthe usage ofcoolant

Conventional way to improve the machinability is the usage ofcoolant. The usage of coolant is expected to have effects of coolingthe cutting point which reduce the damages caused by hightemperature such as welded junctions, and of lubrication thatimprove wear resistance.

Cutting experiments with inert mineral oil with chlorine(NikkoSangyo Riscut A-242, density of 0.91g/cm2

, viscosity of 31cSt (at313K), oil 10.5%, chlorine 7.0%) were carried on. CBN tool wasused and 1 litter per minute of coolant was injected from the nozzlefixed near tht; tool.

Figure 6 shows the tool life curve in wet cutting by the solid lineand the same curve of the dry cutting as figure 5 shows by the dottedline. The usage of coolant made tool life increase particularly atlow cutting speed. For example, at cutting speed of 140m/min., wetcutting have twice a longer tool life than dry cutting, while at cuttingspeed of 70m/min., wet cutting have 4 times longer tool life thandry cutting.

d 200·sE

100"'0 80<l)<l)0. 60'"bJ)I:;

40'gU

2 3 4567810 20 30 50

d300·s

E200

"'0<l)<l) 1000. Dry cutting'"bJ) 80l:::.~ 60U 1 2 3 456 810 20 30 40 60 80

Tool life min.

Tool life min.

Feed rate of 0.05mm/rev., depth of cut of O.lmm,

CBN(BNloo) as the tool material, dry cutting

VB=O.3mm as the terminal flank wear

Figure 5. Tool life curve(V-T curve) for CBN in dry cutting.

Feed rate ofO.05mm/rev., depth of cut ofO.lmm,

CBN(BNloo) as the tool material,

VB=0.3mm as the terminal flank wear,

coolant : inert mineral oil with chlorine

Figure 6. Effects of usage of coolant on tool life.

184 Satoshi Ito et al. I Journal of Materials Processing Technology 63 (1977) 181-186

Figure 7 shows the relation between VB and the finished surfaceroughness in wet and dry cutting. Rmax across the feed mark weremeasured as to represent the surface roughness. There is a littlechange of surface roughness from the beginning of the cut to theend of the cut where VB became terminal flank wear of O.3mm, andthe coolant do not seems to have the effect on improving the surfaceroughness.

7

a6 °:::l.

>< 5e<:la~ 4'" 0_'".,c 3 .6. -.6. -00..c:OIl::l0

2.....,<.>

.;:!....0::l L:>. : Dry cutting : Wet cuttingen

00 I 0.2 0.3 0.4

Wear-land height VB mm

Cutting speed of 150m/min., feed rate of 0.05mm/rev.,

depth of cut of 0.1 mm, CBN(BN I00) as the tool material

Figure 7. Relation between VB and finished surface roughness.

characteristics related to the machinability such as share stress woulddecrease in low temperature and the machinability would beimproved when cutting point would be cooled compulsively.

Therefore cutting experiments were carried on with compulsivelycooled Ni3AI-based workpieces. First, workpieces were kept in thefreezing box with dry ice until temperature of the workpieces became223K. Then one workpiece was picked up and placed on the lathe,and cutting experiment was started as soon as possible. Temperatureof the cutting surface of the workpiece would rise with time, but at5 minutes later temperature of the surface measured by thermocouplewas 233K-228K.

Figure 8 shows V-T curve of the CBN tool with VB ofO.3mm asthe terminal flank wear. Compare with the room temperatureworkpieces, cooled workpieces made tool life be improved at highcutting speed, but tool life at low cutting speed varied widely andsometime it became shorter than the tool life for room temperatureworkpieces.

The main reason of this phenomena is that exfoliation chippingshown in figure 9 occurred readily after crack occurred on the flankof the tool which cut the cooled workpiece. These exfoliationchipping on the flank was not observed in room temperature cutting.The reason of this phenomena is not clear, but brittle fracture causedby the thermal strain seem to be the one of the reason, because thetemperature of the cutting point was relatively low in low speedcutting, and the tlan~ surface ofthe tool was contacted by the cooledworkpiece while the rake surface of the tool was contacted by heatedchip.

3.3 Effects oj enforced cooling ofNi3AI-based workpieces.

In general, hardness of metal decrease with increasing oftemperature. But one of the significant character of Ni3AI-basedintermetallics is reverse temperature dependency of hardness so thatyield stress increase with increasing of temperature until 873K[5,6].Considering this character, it was supposed that the hardness

Workpiece temperature: 223K (a) Rake face

...... '0

....,........, ......,,,o

Room temperature

200

100

2 3 4567810 (b) Flank

Tool life min.

Feed rate of 0.05mm/rev.,depth of cut of 0.1 mm,CBN(BNloo) as the tool material,VB=O.3mm as the terminal flank wear, dry cutting

Cutting speed of 50m/min., feed rate of 0.05mm/rev.,depth of cut of 0.1 mm, cutting time of 6min.,CBN(BNI00) as the tool material,dry cutting with compulsively cooled workpieces

Figure 8. Effects of the compulsory cooling on tool life. Figure 9. Photographs of exfoliation chipping.

Satoshi Ito et al. / Journal of Materials Processing Technology 63 (1977) 181-186 185

Figure 10. Relation between finished surface roughness and VB incooled workpiece cutting.

Cutting speed of 80m/min., feed rate of 0.05mm/rev.,depth ofcut ofO.lmm, CBN(BNlOO) as the tool material,dry cutting

(b) Wet cutting

(a) Dry cutting

(c) Dry cutting with compulsively cooled workpiece

Figure I I. Finished surface in dry, wet and cooled workpiece cutting.

0.2mm

a

/0_0

0.1Wear-land height VB

E 10:l.

K 8

'"E 6~VJVJ

4<)c:

,J:;00::l 2edlCJ 0~::l 0tI'l

Figure I I shows photographs of finished surfaces in dry, wet andcooled workpiece cutting. There were micro cracks on the surfacesin dry and wet cutting but there seem to be no crack on the surfacein cooled workpiece cutting.

From these experiments, no significant effects can be obtained onthe tool life and finished surface roughness by compulsively cooling.It is estimated that cooling only 60 degree from room temperaturemade little change of hardness of Ni3AI-based intermetallics andthe machinability was not changed significantly. But there seemedto be effect of improving surface quality.

Figure 10 shows relation between finished surface roughness andVB in cooled workRiece cutting. Rmax was 41lm at the beginning ofthe cut with almost no frank wear, and Rmax was IOllm when VBbecame 0.2mm. While the frank wear became large, surfaceroughness increased slightly. Cooled workpiece and roomtemperature workpiece had almost same surface roughness and noadvantage of the cooling was observed.

4.Conclusions

Adequate tool material to machine Ni3AI-based intermetallics wasselected from Carbide, Cermet, Ceramic, Diamond-coated, TiN­coated and CBN. With CBN which showed longest tool lifecharacteristics, the effects of the usage ofcoolant and the compulsorycooling of workpieces were investigated. The results of theseexperiments are:(I) Comparison of tool life of various tool materials for machining

of Ni3AI-based intermetallics showed that CBN was bestmaterial, TiN-coated was second best and the others(Carbide,

Cermet, Ceramic and Diamond-coated) had much lower wear­resistance than CBN.

(2) Rake face wear was not developed and flank wear was mainlydeveloped under the cutting condition of feed of 0.05mm/rev.and depth of cut of O. Imm. Shape of cutting chip is short butcontinuous type.

(3) The Usage of coolant had the effect of improving tool life. Andat low cutting speed almost four times longer tool life wereachieved compared with dry cutting. But no effect of improvingthe surface roughness could not be obtained.

(4) Cutting with cooled workpiece had no significant effect of

improving tool life and reduced tool life at low cutting speedbecause of chipping.

(5) Cutting with cooled workpiece reduced surface cracks, whilewhich were found after dry cutting, therefore improved surfacequality

186 Satoshi Ito eta!.!Journal of Materials Processing Technology 63 (1977) 181-186

References

[I] K. Aoki and O. Izumi, J. of the Japan Institute of Metals(in Japanese), 41-2 (1977) 170.

[2] K. Aoki and O. Izumi, J. of the Japan Institute of Metals(in Japanese), 43-12 (1979) 1190.

[3] H. Takeyama and N. lijima, J. of the Japan Soc. for PrecisionEngineering (in Japanese), 55-8 (1989) 1481.

[4] K. Kawai, J. of the Japan Soc. for Precision Engineering(in Japanese), 43-12 (1977) 245.

[5] T. Khan, P. Caron and S. Nara, High Temperature Aluminidesand Intermetallics, The Minerals, Metals & Materials Society,(1990) 219.

[6] O. Izumi, J. of the Japan Soc. for technology of Plasticity(in Japanese), 22-243 (1981) 364.