Machining Titanium Alloys with Ceramic Tools J. Kertesz ________ R.J. Pryor, D.W. Richerson and R.A. Cutler Garrett Turbine Engine Co. Cermatec, Inc.

INTRODUCTION For aerospace applications, titanium

and its alloys have been used for some time because of excellent strength-to­weight ratios and good corrosion resis­tance. 1 Despite the increasing use of titanium, the machining oftitanium alloys is still limited to speeds less than 250 sfpm, as it was 25 years ago. Cemented carbides containing 6 wt. % cobalt with a medium tungsten carbide grain size (mean size near two micrometers) are still the most successful grades used for machining titanium.2

,3

Ceramic, ceramic-composite and ce­ramic-coated tools have significantly improved productivity in machining of steels, cast irons and superalloys.

Progress in machining titanium has not kept pace with machining of other materials due to the inherent difficulties associated with titanium alloys. The poor thermal conductivity of titanium and its high chemical affinity for oxygen, nitro­gen, carbon and boron lead to high tem­peratures at the cutting tool/workpiece interface and excessive chemical attack of most cutting tool materials.2 Innova­tive concepts such as novel cutting tool geometries,4,s lubricants2 and ultra-high speed machining6 have been used with varying degrees of success, but ad­vanced materials for titanium machining

(L.ettefS, continued from previous pageJ

An additional considerable amount of further investment will also become evi­dent by considering in more detail the requirements for powder handling and treatment under protective gas atmos­phere, and clean room conditions as well as for automatic process control and quality control. All together this will easily increase the estimated total equipment cost by at least a factor of two to three, which effects a considerable impact on the cost of RS powders.

R. Ruthardt W. C. Heraeus GmbH

The authors reply: We are grateful that Mr. Ruthardt has

taken the time and effort to scrutinize the results. We agree with him in that a thorough analysis of the costs associ­ated with a novel technology, such as inert gas atomization, could incorporate numerous issues not addressed in our paper. However, the purpose of the analysis was to develop a consistent framework and methodology by which

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Figure 1. Hardness-fracture toughness vari­ation possible with ceramic cutting tool mate­rials developed for machining titanium.

have not been forthcoming. 3 Ceramics have not replaced cemented carbides due to the poor thermal conductivity of most ceramics, and their limited fracture toughness or their reactiveness with tita­nium. Improved performance was ob­served while machining Ti-6AI-4 V alloys with a ceramic cutting tool developed by Ceramatec, Inc.

EXPERIMENTAL PROCEDURES SPG 422 and RPG 42 ceramic inserts

were diamond ground without a hone. Vickers hardness measurements were made using a 100 N load on a 1360

the primary cost drivers associated with the production of inert gas atomized particulates could be identified.

Regarding the degassing stage for the nickel-base alloy, this step was indeed taken into account in the analysis and resulted in only a minor increase in en­ergy costs. Energy accounted for 2.3% of the total cost of manufacturing nickel powders (see Table VI in the original article).

Although it is completely feasible to melt aluminum using a vacuum induc­tion furnace, we agree in that there are some important differences between aluminum and nickel powder atomiza­tion. The important factors to consider include modification of facilities (i.e., explosion-resistant walls), and addi­tional safety equipment that would be necessary to process highly reactive powders. These factors were not within the scope of the analysis because "the costs associated with implementing these measures would depend on the geographic location of the plant" (from page 19).

diamond pyramid indenter on surfaces polished to a one micron finish. Fracture toughness was determined by measur­ing cracks produced by the indenter.7 All machinability testing was done in direct comparison with conventional carbide grades (Kennametal grade K 68 and Carboloy grades 883 and 895.)

RESULTS AND DISCUSSION The ceramic developed by Ceramatec

for machining or milling titanium alloys has high thermal conductivity as well as improved chemical compatibility with ti­tanium alloys as compared to conven­tional C-2 grades. By controlling the microstructure during processing, the mechanical properties of the ceramic can be controlled. Figure 1 shows the hardness-fracture toughness trade-off which occurs forthis ceramic, analogous to that seen in cemented carbides. As in cemented carbides, the proper hard­ness-toughness relationship is impor­tant for improved performance while machining titanium.

Figure 2 shows flank wear data ob­tained while machining Ti-6AI-4V with SPG 422 ceramic inserts in comparison to SPG 422 cemented carbide inserts. The depth of cut was 0.050 in. and the feed rate was 0.010 in.lrev. While no improvement was observed at conven-

The atomization gas consumption used in the analysis is shown in Table 11/ (1.87 rrr/kg). The data shown in Table I cannot be used to calculate the gas consumption because the values pre­sented in this table refer to the size and/ or capacity of the equipment, and hence do not take into consideration materials utilization. Also, it is not appropriate to foretell powder sizes and their distribu­tion solely on the basis of gas consump­tion because these variables are also strongly influenced by other parameters such as gas delivery system, melt super­heat, gas chemistry, and gas exit to metal stream flight distances.

In response to his comments re­garding additional capital investment, the results of our analysis indicate that the capital costs represent only 19.5% and 6. 1 % of the manufacturing costs for aluminum and nickel, respectively. Therefore, it is unlikely that an increase in capital expenditure would significantly modify the results. More important are the accounting assumptions used to distribute the cost of capital (Table /I).

JOURNAL OF METALS. May 1988

tional speeds (Le., < 200 sfpm) the ce­ramic removed greater than twice the volume of material at 380 sfpm for the same flank wear. The cemented car­bides failed prior to removing 0.5 in.3 of Ti-6AI-4V at cutting speeds of 800 sfpm, whereas the ceramic with its improved resistance to deformation, continued to machine well up to 1.0 in.3 Ti-6AI-4V removed. In further testing, the ceramic was run at speeds up to 1400 sfpm. Above 1000 sfpm, the ceramic ran three to seven times longer than cemented carbides for the same amount of wear. Acceptable manufacturing appears to be possible at all speeds up to 600 sfpm. Further optimization of the ceramic is needed to allow machining at speeds up to 1500 sfpm.

Titanium forms shear-localized chips at practically all speeds.aWhile the rake angle and the feed to depth of cut ratio impact chip formation, cutting speed is the most influential parameter.9 The fin­ish of the machined work piece generally improves with an increase in cutting speed.6 Results to date suggest that surface finish is not sacrificed for im­proved productivity when machining at higher speeds with the ceramic tools as compared to cemented carbides.

The ceramic shows an improvement when machining titanium, due to its good

Finally, we would like to reiterate that our objective was to provide a framework for estimating the costs associated with emerging technologies. Clearly, the model input parameters will vary with the specific facility and technology involved.

Gentlemen, I read with much interest "The Search

for 'Ductile' Ceramics" in the November 1987 issue of Journal of Metals. The author did an excellent job summarizing the latest developments in advanced ceramic materials. However, a number of ideas were presented or implied that are potentially misleading to readers unfamiliar with this class of materials.

The title of the article itself is truly an outstanding example of an attention­getting device. The mere thought of ductile ceramics is intriguing, but pseudo-ductile or tougher would have been a better description considering the information presented. Ductility im­plies an ability to plastically deform with­out fracturing under tension. This prop­erty has never been and probably never

JOURNAL OF METALS e May 1988

0.06,.....,.-.-.....-,.....,.-.-.....-,.....,.-,-,......-,.....,.-.--, • -- Ceramic ;~OO SFP

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0.01 // ,

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0.5 1.0 3 1.5 Ti-6AI-4V Removed (in )

Figure 2. Flank wear for ceramic and C-2 cemented carbide as a function of Ti-6AI-4V material removed.

thermal conductivity and adequate toughness, while other ceramics have failed.2•3.1o The ceramic can machine tita­nium at higher speeds than cemented carbides due to improved creep resis­tance and adequate chemical compati­bility with titanium alloys.

The ceramic cutting tools should be able to be substituted directly for ce­mented carbides with performance in­creases of greater than 100% at 400 sfpm and greater than 50% at 600 sfpm. More importantly, the ceramic cutting tools will allow machining of titanium alloys at speeds which are twice the conventional speeds. With further opti-

will be characteristic of ceramics to any noticeable degree. Utilization of fibers, whiskers, and dispersed particulates do nothing to enhance plasticity, they merely increase the fracture toughness of inherently brittle solids in an indirect manner.

If poor fracture toughness is consid­ered the prime disadvantage of ceram­ics, then reliability and processing costs rank second and third, respectively. Ceramic-matrix composites have drasti­cally increased the attainable Weibull modulus as compared to conventional ceramics, but there exists a wide margin between these values and those of metals. In conjunction with reliability, the processing costs of advanced ceramics are still extreme and unacceptable for many applications.

Of course, as these materials con­tinue to evolve, processing costs will inevitably decrease and pose a much smaller barrier to commercialization.

Finally, there is little doubt that in the "near" future reinforced ceramics will have developed to a point where they will

mization of the ceramic, cutting speeds similar to nickel-based superalloys are possible.

References 1. E.K. Henriksen. Machining Characteristics of the Space Age Metals. ed. by F.w. Wilson and R.W. Cox. American Society of Tool and Manufacturing Engines. Dearborn. MI (1965). pp. 47-60. 2. M. Lee. "The Failure Characteristics of Cutting Tools Machining Titanium Alloys at High Speeds". Advanced Proc· essing Methods for Titanium. ed. by D.F. Hasson and C.H. Hamilton. The Metallurgical Society of AIME. Warrendale. PA (1982). pp. 25-287. 3. J.F. Kahles. M. Field. D. Eylon and F.H. Froes. "Machining of Titanium Alloys: J. Metals (April 1985). pp. 27-35. 4. R. Komanduri. D.G. Flom and M. Lee. "Highlights of the DARPA Advanced Machining Research Program." High Speed Machining. ed. by R. Komanduri. K. Subramanian and B.F. von Turkovich. ASME. New York (1984). pp. 15-36. 5. R. Komanduri and M. Lee. "High Speed Machining of Titanium Alloys with a New Cutting Tool Insert-The Ledge Tool." ed. by R. Komanduri. K. Subramanian and B.F. von Turkovich. ASME. New York (1984). pp. 217-227. 6. D.G. Flom. R. Komanduri and M. Lee. "Review of Prior Work in High Speed Machining: Advanced Processing Methods for Titanium. ed. by D.F. HassonandC.H. Hamilton. The Metallurgical Society of AIME. Warrendale. PA (1982). pp. 227-239. 7. G.R. Anstis. "A Critical Analysis of Indentation Fracture Toughness: I. Direct Crack Measurements: J. Am. Ceram. Soc.,9 (1981). 8. R. Komanduri and B.F. von Turkovich. "New Observations on the Mechanisms of Chip formation When Machining Titanium Alloys," J. Wear. 69 (2) (1981). pp. 179-188. 9. B.F. von Turkovich and D.R. Durham. "Machining of Ti­tanium and Its Alloys: Advanced Process Methods for Titanium. ed. by D.F. Hasson and B.H. Hamilton. The Metal­lurgical Society of AIME. Warrendale. PA (1982). p. 257-10. E.M. Trent. MetaICutting.2nded .• ButtermonthsandCo .• Ltd. London (1984).

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be viable substitutes for even superal­loys. The current debate seems to cen­ter around exactly when this will occur. Many speculate widespread usage by the turn ofthe century in applications that range from cutting tools to gas turbine engine components. Considering the disadvantages already presented, it is safe to say that the metals (especially superalloys) market is safe for many years to come.

Robert M. Davis Teledyne Allvac

While the article's title may be attention getting, it merely alludes to the pursuit of an as yet unattainable property. This is not the first time that such a concept has been put forth on these pages. As Den­nis Hasson mentions in this month's Retrospect, the May 1958JOM features "Ductile Ceramics-A High Tempera­ture Possibility" by several well-re­spected authors, specifically Earl R. Parker, J.A. Pask, J. Washburn, A.E. Gorum and W Luhman.-ed.

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