PROCEDURES AND PRECAUTIONS IN MACHINING TITANIUM …€¦ · Intergranular attack or preferential etching resulting from electrochemical machining (ECM) or chemical milling (CHM)

PROCEDURES AND PRECAUTIONS IN MACHINING TITANIUM ALLOYS

Norman Zlatin and Michael Field Metcut Research Associates Inc.

Cincinnati, Ohio

Machinability of Titanium Alloys

Titanium alloys have unique machining properties. While the cutting forces are only slightly higher than in machining steels, there are other characteristics that make these alloys more difficult to machine than steels of equivalent hardnesses. For example, the chip-tool contact area in turning a titanium alloy is only about one-third to one-half as great as that for turning a steel. Also, the thermal conductivity of titanium alloys is

.-about one-sixth of that of steels. This combination of a small contact area and the low thermal conductivity results in very high cutting temperatures. 'At a cutting speed of 100 ft. /min., the temperature developed at the cutting edge of a carbide tool is 1000° F when cutting a steel, while on the titanium alloy, the temperature reaches 1300° F. Hence, the cutting speeds on titanium alloys must be lower in order to maintain a tool-chip temperature below that which results in short tool life.

Another characteristic of titanium that must be reckoned with is the fact that titanium chips have a strorig tendency to weld to the cutting edge, particularly after the .tool starts to wear. When cutting a steel, the cutting forces will generally increase about 25 to 50 percent as the tool dulls. However, in the case of turning titanium, the forces perpendicular to the workpiece may increase three to four times as a result of a build up of titanium

489

490 N. ZLATIN AND M. FIELD

on the wearland of the tool. Because of this higher thrust force and the low elastic modulus of titanium, the deflection of the workpiece can be a serious problem. Even though these peculiar machining characteristics exist for the titanium alloys, they can be machined at reasonable production rates provided specific machining conditions are used.

The aforementioned machining characteristics pertain to all of the titanium alloys. However, the machining conditions vary considerably for the different titanium alloys. One should recognize that there are several general categories into which most of the titanium alloys can be grouped. These are (1) commercially pure; (2) alpha; (3) alpha-beta; and (4) beta types. The commercially pure can be machined at speeds even higher than those used on an alloy steel having a hardness of 300 BHN. As shown in Table I, the alpha alloy required from one to 2-1 /2 times the machining time compared to a 4340 steel having a hardness of 300 BHN depending on the machining operation involved. (l) In the case of the alpha-beta alloy, the times can range from 1. 7 to 3. 3. The beta alloys having a hardness of 400 BHN are the most difficult to machine of the titanium alloys. For example, in drilling, not only are the cutting speeds low, but light feeds must also be used. Hence, the drilling time on these alloys may be as much as ten times greater than that on the 4340 steel.

Machining conditions should be selected which either circumvent or minimize the adverse effects of the machining characteristics of titanium alloys in order to obtain satisfactory tool values at acceptable production rates. Rigidity of the setup (particularly of the cutter) is especially important. The overhang of the tool must be kept to a minimum. Note in Figure 1 how much the tool life decreased in peripheral end milling when the end mill with a 4" flute length was substituted for one having a 2" flute length. A further increase in flute length to 611 resulted in a negligible tool life even at a much lighter depth of cut.

Since the chips tend to weld to the cutting edge of the tool, it is important in face milling with carbide tools that a setup be used which will minimize the welding effect. In conventional milling, the cutter encounters the maximum thickness of the chip as it leaves the workpiece. Hence, at that point, the welded bond between the chip and the tool is the greatest. As a result, the tool life is poor since the chip is pulled off the cutting edge when the tooth re-enters the workpiece on the next pass. When this


TABLE I(I)

MACHINING TIME RATIOS FOR VARIOUS TYPES OF TITANIUM ALLOYS COMPARED TO AISI 4340 STEEL AT 300 BHN

Turning Face Milling Titanium Alloy Carbide Tool Carbide Tool

Commercially pure 175 BHN 0. 7: I l. 4: l

Alpha Ti-BAI-I Mo- l V 300 BHN I. 4: I 2. 5: I

Alpha-Beta Ti-6Al-4V 365 BHN 2. 5: I 3. 3: I

Beta Ti-13V-11Cr-3Al 400 BHN

...l w > < 0: ....

"' 0: 0 :.

"" 0

"' w :t u 3 w

"" :::; ...l 0 0 ...

JOO

250

zoo

150

100

5:0

PERIPHERAL END MILLING TITANIUM 6Al-4V BETA FORGED HI BHN

I 0: I

EFFECT OF CUTTING SPEED AND FLUTE LENGTH

CUTTER: I" DIA. M4Z HSS 4- FLUTE END MILL HELIX ANGLE: 30° FLUTE LENGTH: SEE BELOW RADIAL RAKE: 10° PER. CLEARANCE: 7° CA: 45° x • 060" END CLEARANCE: 3°

F EEO: . 002. IN. /TOOTH

~~~;: g; ~~;i 0S~~ 1 ~ELOW --+----f-----4f----

SETUP: CLIMB MILLING CUTTING FLUID: SOLUBLE OIL (l:lO) TOOL LIFE ENO POINT:

. 012" UNIFORM WEAR 0

.020" LOCALIZED WE:AR ---+-----

FLUTE LENGTH: 2" . 125" DEPTH OF CUT

50 FLUTE LENGTH: 6'' L· 060" DrTH OF CUT

100 IZ5 150 17? zoo 225

CUTTING SPEED - FEET/MINUTE

Fig. 1.

Drilling HSS Tool

0. 7: I •

1:1

I. 7: I

10: I

491


happens, there is a strong possibility that chipping will occur at the cutting edge of the tool. As shown in Figure 2, the tool life was very poor in conventional milling. Merely by changing the relative position of the cutter with respect to the workpiece so that a climb milling setup was obtained, the cutter life improved considerably. With conventional milling, the tool life was negligible at all cutting speeds, while with climb milling, the tool life was 120 inches of work traveled per tooth at a cutting speed of 200 ft. /min.

In general, the grades of high speed steel, such as M33, Tl5, and the M40 series should be used in machining the titanium alloys. All of these grades of high speed steel contain cobalt. The production rates are appreciably greater with these premium grades of high speed steel. In those operations where carbide tools can be used, even higher production rates can be obtained. A comparison of tool life curves for high speed steel and carbide tools is presented in Figure 3 in turning one of the titanium alloys. Note that the cutting speed for a 30 minute tool life with the carbide was 150 ft. /min. as compared to 55 ft. /min. with a premium grade of high speed steel.

Machining recommendations for several operations on the Ti-8Al-1Mo-l V (alpha), Ti-6AI-4V (alpha-beta), and Ti-13V-11Cr-3Al (beta) alloys are listed in Tableu.( 2 , 3, 4)

Surface Integrity

When machining titanium alloy components that are to be subjected to high stresses in service, it is necessary to select machining conditions which provide good surface integrity. Surface integrity has been defined as the inherent or enhanced condition of a surface produced by machining. (5, 6) There are two aspects of surface integrity. The first is the surface roughness or surface finish or surface topography, and the second is the metallurgical alterations of the surface layer. This paper will review the metallurgical alterations produced in machining and their effects since this subject has not been given much attention in the past.

Metallurgical Alterations

It has been found that the surface resulting from a given machining operation may contain various metallurgical alterations.


:i: r 0 0 r :J w > "' a: h

FACE MILLING TITANIUM AI.LOY 6AL-4V

SOLUTION TREATED AND AGED ~6~ BHN

EFFECT OF CUTTING SPEF.0 AND SETUP

CUTTER: 4" DIA. S!NGI.E TOOTH FACE MILL WITH C-l. (88 H CAHAi DE

AR: 'i" NEG. RR: 'i" NEG. CA: 4'i"

FE-:ED: .OO'i IN/TOOTH DEPTH OF CUT: 060" WIDTH OF CUT: l."

F.CF:A: 4'i" C LF.AR ANC F.: i:,•

NR: . om" -+------1

I C:..O CUTTING FLUID: DRY 1------1-TOOl. l.IFE END POINT· .UJI'.," UNIFORM WEAR -I .OlO"LOCAL!l.EDWF.AR

'" t---------+----------1---+---l------l------l 0

~ I 00 l-----+----1-----1- CUMR MILLING -0

" "' 0

"' ~ 7'i l-----+----l~----1--u ~

w "' J ..l 0 0 r ::==TKt_

~ coNVENr~~AL I ~ MILLING

,("l I

100 ,l 'iO lOO 300

CUTTING SPEED - FEET/MJNUTF:

Fig. 2.

493

494

60

' "' 50

"' .... ::> z :i

"' 40

"' J ..l

8 .... JO

20

10

TURNING RETA III TITANIUM. AGED, HI BHN EFFECT OF TOOL MATERIAL AND CUTTING SPEED

TOOL MATERIAL: SEE BELOW TOOL GEOMETRY:

HSS

FEED: . 010 IN. /REV.

N. ZLATIN AND M. FIELD

~:= I~: ~: ~~~: I - ~~~~~N~FF~~;~: ·~~~~BLE OIL ---<,f----+-----i

~

TOOL LIFE END POINT: , 015" UNIFORM WEAR .030" LOCALIZED WEAR

M42 HSS

0

-\ 0

50 75 100 125 150

CUTTING SPEED - FEET/MINUTE

Fig. 3,

175


TABLE II

RECOMMENDED CONDITIONS FOR MACHINING

Ti-8Al-1Mo-1V ANNEALED 310 BHN(2)

Tool Cut Speed OEeration Material fL /min. Feed

Turning C-2 Carbide 250 . 005 in. /rev. Face Mill Tl5 HSS 90 . 005 in. /tooth

Face Mill C-2 Carbide 410 . 005 in. /tooth

Per. EndMill M2 HSS (3 I 4" dia. ) 150 . 004 in. /tooth

End Mill M2 HSS (3/4" dia.) 97 . 003 in. /tooth Drilling TIS HSS (1/4"dia.) 45 . 005 in. /rev. Reaming M2 HSS (. 272" dia.) 70 . 009 in. /rev. Tapping Ml HSS (5/16-24NF) 17

Ti-6A1-4V STA 365 BHN(3)

Tool Cut Speed OEeration Material ft. /min. Feed

Turning C-2 Carbide 1 75 . 009 in. /rev. Face Mill M43 HSS 80 . 005 in. /tooth Face Mill C-2 Carbide 200 . 005 in. /tooth Per. End Mill Mj-1 HSS (1 11 dia. ) 180 . 003 in. /tooth End Mill M41 HSS (3/ 4" dia.) 95 . 002 in. /tooth Drilling M41 HSS (1/4" dia.) 40 . 005 in. /rev. Reaming M33 HSS (. 272" dia.) 65 .009 in./rev. Tapping Ml HSS (5/16-24NF) 15

Ti-13V-11Cr-3Al STA 400 BHN(4 )

495

Cutting Fluid

(b) (b) (a) (b) ( c) (d) (d) (d)

Cutting Fluid

(a) ( c) (a) (c) (b) ( c) (b) (d)

Tool Cut Speed Cutting 0Eeration Material ft. /min. Feed

Turning C -·2 Carbide 100 . 007 in. /rev. Face Mill M41 HSS 25 . 007 in. /tooth Face Mill C-2 Carbide 75 . 004 in. /tooth Per. End Mill M41 HSS (3/4" dia.) 60 • 002 in. /tooth End Mill M41 HSS (3/ 4" dia.) 50 . 002 in. /tooth Drilling M41 HSS (1/4" dia.) 25 .001 in./rev. Reaming M41 HSS (. 272 11 dia.) 35 . 005 in. /rev. Tapping MIO HSS (5/16-24NF) 9

(a) Dry (c) Chemical Emulsion (d) Chlorinated Oil (b) Soluble Oil

Fluid

(d) (d) (d) (d) (b) (d) (d) (d)


In the case of machining of titanium, these possible alterations include:

Plastic deformation

Phase transformations

Tears, laps, and crevice-like defects associated with the built-up edge produced in machining

Plastically deformed debris resulting from grinding

Mic roe racking

Microhardness alterations

Redeposited metal from thermal operations such as electrical discharge machining (EDM) and laser beam machining (LBM)

Recast layer resulting from thermal operations

Intergranular attack or preferential etching resulting from electrochemical machining (ECM) or chemical milling (CHM)

Absorption of products of reaction from the machining process

The causes of these surface alterations are the following:

High temperature or high temperature gradients produced in the machining operation

Hot or cold work produced in the machining operation

Chemical reactions and subsequent absorption of products of reaction into the surface layer

It is possible when employing a given machining operation, such as milling or grinding, to minimize surface alterations or to provide major alterations. Usually, machining paramP.ters can be selected that are gentle, which in turn minimize alterations, or that are abusive, which in turn produce major alterations. In a chip removal operation, such as milling or turning, gentle conditions are those which provide long tool life and which in turn employ sharp cutters. Maximum surface alterations, and hence abusive conditions, would be those caused by using a dull cutter. In surface grinding, gentle conditions are obtained by using soft grinding wheels, low wheel speeds, very light down feeds, and application of a satisfactory cutting fluid. When electrical discharge machining (EDM), roughing conditions


con3isting of high current and low frequency tend to produce greater am()unts of surface alterations and redeposited metal.

497

In electrochemical machining (ECM), "standard" or proper conditions tend to produce less surface alterations than "offstandard" or improper conditions. The "off-standard" conditions may promote intergranular attack or preferential etching which are highly detrimental to both surface finish and fatigue strength.

Typical photomicrographs of surface characteristics produced by surface grinding of Ti-6Al-4V beta rolled, 32 Re, are shown in Figure 4, (7) Three types of grinding conditions were employed: gentle, conventional, and abusive, Gentle grinding conditions produced no microstructural or microhardness changes. Conventional and abusive grinding show evidence of plastic deformation at the surface plus some tendency toward tearing; Both conventional and abusive grinding produce surface softening due to localized surface heating. The gentle and abusive grinding conditions are dcsc ribed in Figure 5.

Residual Stress and Distortion

It has been found that every machining operation produces a distinctive residual stress in the surface layer. The machining parameters which tend to be more abusive also tend to induce greater amounts of residual stress within the surface. The residual stress profile produced by surface grinding and peripheral end milling of beta rolled Ti-6Al-4V are shown in Figure 5. (7) The abusive grind produced a high and deep residual ten.5ile stress layer while the· gentle grind produced a shallow tensile layer. In contrast, the abusive peripheral end milling produced a small tensile layer while the gentle milling produced a very shallow compressive layer. The distortion of the workpiece has been found to be proportional to the area under the residual stress curves. Note that the gentle grinding utilized a soft silicon carbide wheel with a low wheel speed, a very light down feed, and potassium nitrite cutting fluid. The major factors leading to abusive grinding of the titanium were the use of hard aluminum oxide wheel at a high wheel speed, a high down feed, while grinding dry. In the milling operation, the major factor between the gentle and abusive milling was the tool wearland. The sharp tool with the . 003 in. maximum wearland was the predominant parameter in minimizing the residua 1 stress.

(a) Gentle Conditions (b) Conventional Conditions (c) Abusive Conditions

~ 40 <Jl <Jl 35 Ill c

'O ... Cl!

30

:r:

Gentle grinding produced no visible microstructure hardness, although some degree of surface roughness is evident. Conventional and abusive grinding shows evidence of plastic deformation at the surface plus some tendency toward tearing. Both conventional and abusive grinding produced surface softening due to localized surface heating. Gentle grinding did not show this effect.

Surface Finish: 35AA Surface Finish: 45AA Surface Finish: 65AA

--- G~n le

-- 'r.nn• rP....,,.,. '"~l

- -·- Abu! ive

·r, ~:

. 004 . OvB . 012 v . 020

Depth Beneath Surface, Inches Fig. 4.

lOOOX

SURFACE CHARACTERISTICS OF TITANIUM 6Al-4V, BETA ROLLED (32 Re)

PRODUCED BY SURFACE GRINDING

z N ..... > -t z > z 0

~

::!! m ..... 0


>80

>60

z 0 iii z "' E- +40

iii '20

"' ui "' "' "' .... "' .J <(

" Q iii "' "' -ZO

-40 z 0 iii "' "' "' Jl. ::; -60 0 u

-80

RESIDUAL SURFACE STRESSES IN BETA ROLLED TITANIUM 6Al-4V, JZ &

PRODUCED BY VARIOUS MACHINING METHODS

GRINDING CONDITIONS

~ ~ Grinding Wheel C60HV A46MV Wheel Speed, ft./min. 2.000 6000 Down Feed, in. /pass "LS" • ooz Cross Feed, in. /pau . 050 . 050 Table Speed, ft. /min, 40 40 Grinding Fluid KNOz D•Y

(I •ZO)

ABUSIVE GRIND

I

PERIPHERAL END MILLING CONDITIONS - CARBIDE CUTTER

Cutting Tool Depth of Speed Feed Cutting Wea rland Finish Cut

(ft. /min,) (in, /tooth) Fluid ~ (in.)

Gent. 100 . 008 Sol. Oil .003 max. . 005

A bus. 150 . 008 D•y . 018-.0W . 030

• 002. . 004 . 006 . 008 . 010

DEPTH BELOW SURFACE - INCHES

Fig. 5.

499


Electrical discharge machining (EDM) tends to produce a high but shallow tensile stress when employing either "roughing" or "finishing" conditions. Electrochemical machining (ECM) generally produces a surface with very little if any residual stress. In all cases, it has been found that the distortion produced in machining is proportional to the integrated area under the residual stress curve.

Mechanical Properties

The major mechanical property affected by the type of machi.ning operation and its severity is the high cycle fatigue strength. The effect of the type of machining operation and m::i.chining parameters on fatigue strength of beta rolled Ti-6Al-4V is shown in Figure 6. (7) Gentle peripheral end milling and gentle surface grinding produced high endurance limits of 66 to 62 ksi, whereas abusive milling and abusive grinding resulted in an enormous drop in endurance limit of 32 to 13 ksi. A chemically milled surface also resulted in a reduced endurance limit to 51 ksi. A summary of the high cycle fatigue behavior of four typP-s of titanium is given in Figures 7, 8, 9, and 10. (7 • 8 • 9) Here are shown the 107 cycle endurance limits of specimens using cantilever bending at zero mean stress, 1800 cycles/minute, at room temperature. Figure 7 indicates the endurance limit for beta rolled Ti-6Al-4V under five different machining operations: surface grinding, hand grinding, end cut end milling, peripheral cut end milling, and chemi.cal milling (CHM). The endurance limit as well as the surface roughness in microinches AA is given for both gentle and abusive conditions for each operation. Note the wide range in endurance limit as a function of both the type of operation as well as the range of parameters within a given operation. Also note the lack of .association between surface roughness and endurance limit.

The fatigue characteristics of Ti-6Al-4V annealed are given in Figure 8; Ti-6Al-6V-2Sn in Figure 9; and Ti-6Al-2Sn-4Zr-2Mo in Figure 1 O. Peening can be used as a post-machining operation to improve the endurance limit of most surfaces produced by both conventional and nonconventional machining operations. Data showing the improvement possible by peening of surface ground and electrochemically machined specimens are given in Figure 9.

L

c J )

PROCEDURES AND PRECAUllONS IN MACHINING TITANIUM ALLOYS

~r-

_)

l<fiGUE CHARACTERISTICS OF BETA ROLLED TITANIUM 6Al-4V, 3Z Re

METAL REMOVAL PROCESSES: SURFACE GRlNDlNG, PERIPHERAL END MILLING, CHEMICAL MILLING

.; "' "' 0:

!;; Cl z ;::: < z 0:

"' ,... .J <

MODE: CANTILEVER BENDING, ZERO MEAN STRESS TEMPERATURE: 75° F

ENO UR. CONDITION LIMIT

KS!

GENTLE MILL 66 GENTLE GRIND 6Z

CHEM. MILL Sl

40 1--+t+++----+--~+-:--..H':--. "'+"'~=---+--11-++++I ++JI I --1-:--:-.:...:...!._..!.!..1 ABUSIVE MILL 3Z

ABUSIVE GRIND 13

CYCLES TO FAILURE

Fig. 6.

Ti-6Al-4V BETA ROLLED, lZ Re

Surface Roushness, AA

SURFACE Gentle I 62 35 GRIND ~13 65

HAND 57 80

GRIND Ab .. •i.ve 30 80

END MILL- 64 67

END CUT 77 84

r

END MILL- .... 66 41 PERIPHERAL CUT A 32 59

CHEMICAL 51 20 MILLING n«. -"

,., 165

I I I I

zo 40 .60 80

ENDURANCE LIMIT, KSI

Fig. 7.

SURF. FINISH ----,;;:-

41

" lD

S9

6S

501

502

Ti-6Al-4V ANNEALED, 35 Re

SURFACE GRIND

ECM FRONTAL 60

ECM TREPAN 40

' I I

zo 40 60 BO


Fig. 8.

Ti-6Al-oV-lSn (STA, 42 Re-)

SURFACE Gentle "' GRIND Conv. I lO

Abusive lO

SURFACE G-,.,11 .. BJ GRIND+ PEEN • 50

HAND Gentle I ?7 GRIND Abusive lb7

END MILL- Gentle I 7l PERIPHERAL CUT Abusive -. 45

ECM I " Off-Standard • 47

ECM + IB5 PEEN _c- ''"

' I I I

lO 40 60 BO


Fig. 9.

Ti- 6Al-2Sn-4Zr-ZMo (STA, J6 Re)

6B

END MI LL- l£;;;Jik:======::;:======:::J Bl PERIPHERAL CUT "'"A"'b"''"'"'v•._ _____ _. 47

zo 40 60 BO

ENDURANCE LIMIT, KS!

Fig. 10.

N. ZLATIN AND M. FIELD

Surface Roughneu, AA

14

161

Surface Roughnes!I, AA

4 l

44 70

4)

55

14 14

ZB 39

II 145

4B IZO

I

Surface Roughness, AA

39 41

IZO

l6 77

PROCEDURES AND PRECAUTIONS IN MACHINING TITANIUM ALLOYS 503

It has thus been illustrated that the machining process selection and control are necessary and practical methods for maintaining high surface integrity together with good productivity in the manufacture of high strength structural titanium hardware.

Acknowledgements

The data incorporated in this report were taken from programs sponsored by The Manufacturing Technology Division, Air Force Materials Laboratory, Wright-Patterson Air Force, Dayton, Ohio under USAF contracts: AF 33 (615)-1385, AF 33 (615)-3467, AF 33 (600)-42349, F 33 (615)-68-C-1003, and F 33 (615)-70-C-1589.

References

1) Vaughn, R. L. & Zlatin, N. "Producibility Aspects of Aerospace Products with Regard to Machinability", ASTME, Paper No. MR67-729, 1967.

2) Zlatin, N,, et al, "Final Report on Machina~ility_of Materials", USAF Report #AFML-TR- 65 -444, January, 1966, Metcut Research Associates Inc., Cincinnati, Ohio.

3) Zlatin, N., et al, "Machining of New Materials", USAF Report #AFML-TR-67-339, October, 1967, Metcut Research Associates Inc., Cincinnati, Ohio.

4) Zlatin, N., et al, 11 Final Report on Machining of Refractory Materials", USAF Report# ASD-TDR-63-581, July, 1963, Metcut Research Associates Inc., Cincinnati, Ohio.

5) Field, Michael and Kahles, J. F., "The Surface Integrity of Machined-and-Ground High Strength Steels", DMIC Report 210, October 1964, pp. 54-77.

6) Field, Michael and Kahles, J. F., "Review of Surface Integrity of Machined Components", presented at Annual CIRP Meeting in Warsaw, Poland, September, 1971.

7) Koster, W. P., et al, "Surface Integrity of Machined Structural Components", USAF Report AFML-TR-70-11, March, 1970, Metcut Research Associates Inc, Cincinnati, Ohio.


8) Koster, W. P., et al, "Manufacturing Methods for Surface Integrity of Machined Structural Components", USAF Report AFML-TR-71-258, February 1972, Metcut Research Associates Inc., Cincinnati, Ohio.

9) Bellows, Guy, "Surface Integrity of Electrochemical Machining", ASME Paper No. 70-GT-111, 1970, American Society of Mechanical Engineers, New York, New York.

Documents

PROCEDURES AND PRECAUTIONS IN MACHINING TITANIUM …€¦ · Intergranular attack or preferential etching resulting from electrochemical machining (ECM) or chemical milling (CHM)