FABRICATION OF TITANIUM= *RICHALLOYS Mechanical Properties of Some Wrought Alloys E. I. Larsen, E. F. Swazy, L. S . Busch, and R. H . Freyer P . R . Mallory & Company, Inc., Indianapolis, Ind.
T h i s paper presents the melting procedure and a discussion of a furnace used for the production of small (1 to 2 pounds) titanium and titanium alloy ingots. The fabrication of the billets into specimens suitable for the determination of tensile strength, elongation, resistance to oxidation, hardness a t elevated temperatures, and resistivity is also discussed. A 40-kw. resistance-type furnace was used to melt and cast either titanium powder or sponge into ingots approximately 1.5 inches in diameter by 6 inches long, Melting and casting were done in graphite in an inert atmosphere. After a slight surface scalping, the ingots were hot forged to 0.5- to 0.625-inch square rods from which appropriate test specimens were machined. __ Alloys containing up to 5% iron, cobalt, nickel, chromium, molybdenum, manganese, vanadium, copper, indium,
T
HE properties of ductile titanium metal are well known and
are rightfully considered outstanding. Certainly, then, the properties of titanium-base alloys, with their probable enhanced properties of strength-to-weight ratio, corrosion resistance, impact, and fatigue strength would be of great interest to metallurgists, designers, and consumers alike. The alloys described in this paper were prepared by melting and casting in graphite; hence, all the alloys contain some carbon, the source of which was the crucible. The remaining alloy ingredients were mixed with titanium metal, and this mixture constituted the melting charge. In general, the alloys contained 98 to 90% titanium, up to 0.6% carbon, and up to 8% of one or more of the metals copper, boron, aluminum, indium, silicon, vanadium, chromium, tungsten, manganese, iron, cobalt, and nickel. The melting apparatus and procedure, working of the ingot, and properties of some of the alloys are described.
Melting Apparatus Titanium rapidly absorbs hydrogen, oxygen, and nitrogen a t elevated temperatures, and their extreme embrittling effect makes it mandatory that melting be accomplished in a pure, inert atmosphere. Argon was selected as alloys prepared in it contain less than haIf the amount of nitrogen as those melted in helium. The argon was purified by passing itsovertitanium sponge a t 900' C. prior to its introduction into the melting chamber; the hot titanium removed the greater part of the oxygen, nitrogen, and water vapor. The furnace is a resistance type in which the heat source is a split graphite tube utilizing currents up to 3400 amperes a t 10 volts. The details of the furnace are shown in Figures 1,2, and 3. Two types of titanium melting stock-namely, sponge titanium made by the Du Pont Company and titanium powder made by the U. S. Bureau of Mines-were used in the preparation of alloys. Both of these raw materials were found suitable for melting stock.
silicon, boron, aluminum, manganese-silicon, manganesealuminum, and aluminum-chromium have been made. Of the alloys investigated to date, those containing boron, vanadium, tungsten, iron, manganese-silicon, and aluminum-chromium appear to have the highest mechanical properties. The following values are typical of those obtainable with this group of binary or ternary alloys: Ultimate tensile strength, Ib./sq. in. 150,000-180,000 Proportional limit, lb./sq. in., 90,00~105,000 Yield strength (0.2 % offset), Ib./sq. in., 120,000-145,000 Modulus of elasticity, Ib./sq. in. 16.6-17.8 XlOB Elongation in 2 inches, q~2.5-13 The mechanical properties of titanium containing from 0.3 to 1.9% carbon are given.
Both materials contain some magnesium, the major portion of which is volatilized and eliminated in the melting process, and small amounts of iron, chlorine, nitrogen, oxygen, and carbon. Some of the additive metals used were not pure as it was neither expedient nor possible to buy or prepare many elements in their pure form. The approximate purity of the materials is shown in Table I. Melting stock was prepared by mixing the alloying ingredients with titanium powder or sponge and compacting the mixture in a 115/le-inch diameter die under a hydraulic press. Titanium powder is easily mixed with the additives in a ball mill but titanium sponge, which is in the form of 0.25- to 3-inch diameter lumps, is somewhat more difficult to mix. T o accomplish this, the additives were first pressed into small pellets; these pellets were then interspersed among the titanium sponge lumps and pressed into a cohesive compact. Some elements, particularly in massive form, dissolve slowly in titanium so that i t is preferable to add the alloying ingredients as metal powders to ensure complete dissolution in the shortest time a t a reasonable temperature. This practice precludes the pickup of excessive carbon from the crucible and the reduction of ductil-
Table I. Alloying Metal Copper
Boron
Aluminum Indium Silicon Ziroonium Vanadium Chromium Molybdenum Tungsten Manganese Iron Cobalt Nickel
237
Purity and Form of Alloying Metals Metal Purity, ?' & Form 99.9 94 99 9 99.9 99.5
...
95 98-99 P9,9 99.9 98 99.5 99.0 99.0
Shot Powder Powder or 25 wire Powder Powder Sponge Lump Powder (electrolytic) Powder Powder Powder Powder Powder Powder
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 42, No. 2
Figure 1. Titanium Melting Furnace (Left to r i g h t ) Argon cylinder and flowmeter, vacuum gage, argon purifying furnace, furnace with graphite mold and shields i n place, and furnace hell and step-up transformer (40 liw.)
itg associated with high carbon contents. The recovery of t h added metals has been GO t,n 1 0 0 ~ with o the alloying inetal riot found in t h e ingot commonly found adhering to the crucible wiill.
Melting Procedure The mold in the assembled furnace n.as so placed that the crucible spout was just inside the top of the mold to prevent splattering of the melt during pouring. The closed furnace was purged with purified argon a t a rate of 6 cubic feet per hour for 0.5 hour, and a t that, time the melting cycle was started. A typical melting cycle log is given in Table 11. Temperature was measured with an opt,ical pyrometer sighted on the charge through a hole in the top of the furnace cover. Tapping of the crucible w s accomplished by inimereirig il tungsten rod in the melt, stirring for 15 to 30 seconds, and then pushing the rod through the crucible spout, and pushing out R titanium plug existing in a mushy statc. With a f l o of ~ metal thus established, the crucible empties except for just enough
Table TI. Time, Minutes
metal to act as a plug for the next melt. The use of othei plug materials has been inconvenient as well as a source of undesirable contamination. byhen the charge is allowed to remain molten for approximately l minute, determined by viewing it through the sighting hole, the titanium alloys absorb 0.3 to 0.6Yo carbon from the crucible. Increasing the time Mhile molten to 2 minutes increases the carbon content to a t least 0.7%. The ingots produced by the present equipment and process are 1.5 inch diameter by 5 to G inches long, averaging 1.6 pounds each [Jsually the ingots may be removed from the graphite mold \+itkinlit hrwilting the mold. Those ingots which do rtirk are
Log of RIellting Cycle Current, Amperes
Voltage
Temperature. OC
Figure 2. rooling time,
1'/2
hours
Furnace Components
( L e f t to r i g h t ) Graphite mold, graphite melting pot, graphite resistor, molybdenum reflector, and quartz shield with molyhdeniim reflertnr
on top
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 19.50
239
Table 111. Properties of Hot Forged Titanium Containing Various Amounts of Carbon
Carbon 0.47 0.77 1.49 1.91
Composition by Analyais, % ' Nitrogen Titanium Balance 0.048 Balance 0.082 0.028 Balance 0.038 Balance
Hardness, Rockwell A 64.5 60.5 63.5 64
Ultimpte Tensile Strength, Lb./Sq. Inch 119,000 100,500 104,600 108,000
Table IV.
Proportional Limit, Lb./Sq. Inch 89,000 60,000 70,000 74,000
Modulus of Elasticity Lb./Sq. Indh I6 X 106 17 X 108 1 8 . 5 X 108 8 1 . 5 X 106
Elongation, yo in 2 Inches 15.6 11.0 9.4 3.0
Resistivity, Ohm-Cm. 63 X 10-5 6 6 . 5 X 10-6 6 2 . 5 X 10-6 7 2 . 5 X 10-8
Hardness at 600' C . , Rockwell
4
6
9 14 14
Composition by Analysis,
%
5 05 0 33 Balance 0 61 0 33 Balance 4.03 0 60 Ralance
cu C Ti
B
C TI AI
C
Ti
3 30 0 42 0 06 Balance 0.99 0.47 Balance 4.22 0.48 Balance
? ! I
N E
Ti
sj Ti
8'Ti a
Hardness, Rockwell A 65.5
F
=
hot forged; Q
=
0.85R 0.085
properties of Hot Forged Titanium Alloys
c
Ultimate Tensile Strength, Lb./Sq Inch 123,000
Increase in Weight (16 Hr. at 9000 C.), Gram/Sq. Inch 0.14 0.OB
Proportional Limit Lb./Sq. inch 81,000
Modulus of Elasticity Lb./Sq. In& 1 8 . 6 X 108
Elongation, % in 2 Inches 11
Resistivity, Ohm-Cm. 6 8 X 10-6
Hardness at 6OO0C., Rockwell A 27
Increase ixi Weight (16 Hr. at 9000 C . ) . Gram/Sq. Incb 0.11
x
10-
13
0 I.?
IO-@
17
a . 14
11
0.104
66
128,000
84,000
1 9 . 5 X 10'
11
59
65
118,300
76,000
1 8 . 1 X 108
'1 1
90.5 X
66
122,300
77,000
1 7 . 0 X 108
15.7
9 7 . 5 x 10-6
6 6 . 5 (F)"
121,500
60,000
1 8 . 2 X 10'
12.5
7 5 . 5 x 10-
67 (Q)' 70
113,500 156,000
... 16.5 X 108
15.6 1.5
1 6 . 5 X 108
7.8
93,000
... 86.5
x
10-6
139 X 10-8
0
0.02
32 0
0.196
32 (Q) 33
0.210
...
hot forged and water quenched from 1000° C.
fused to the top of the mold which, during melting, attains the highest temperature. The sides of the ingot are usually discolored due to the liberation of gases, probably oxygen and nitrogen, from the mold proper when the melt strikes the mold. The top of the ingot retains a bright, metallic luster. Ingot soundness is a prerequisite for subsequent hot or cold working. Experience has shown that shrinkage of the ingot top generally indicates a sound ingot. It is therefore necessary to control the cooling rate and direction of solidification. This control was established by varying the mold length and its position relative to the melting crucible. Although it has not been observed, it is apparent that the temperature gradients over the mold length permit normal solidification.
Fabrication of Ingot into Rod and Sheet Preparatory to forging, the ingots were lathe scalped to a depth of 0.06 to 0.125 inch on the diameter and 0.125 to 0.5 inch on the top. The scalped ingots were then heated in a gas-fired furnace to 800" to 950' C. and forged to 0.5-inch square bars, a reduction in area of 75 to 85%. Though a superficial, tenacious oxidation product formed, on none of the alloys did i t impair the properties nor cause forging difficulties, even when intricate shapes were forged. The 1.25 inch diameter by 5.5-inch alloy ingots have been hammer forged into 0.5-inch square bars 25 to 30 inches long: 0.375-inch-thick plate for hot rolling; drop forged into conveyer belt links, and wrenches; and upset into mushroom shapes. A drop-forged conveyer link is shown in Figure 4. Forged bars have been alternately annealed and swaged to 0.045-inch-diameter wire and sheet has been rolled to a thickness of 0.020 inch. The sheet was made by hot rolling at 850" C. from 0.375 to 0.125 inch thick, rolling to 0.057 inch thick a t 500' C. and cold rolling to 0.020 inch thick. The machinability of the alloys is comparable to that of 18-8
stainless steel and the same tool angles, feeds, and cutting speeds are satisfactory. Titanium alloys have been found very difficult to drill and tap by ordinary practices, but little work has been directed toward this problem. If heavy tool cuts are taken both tool and work become very hot and, if oil is used as a cutting lubricant, precautions against fire are prescribed. I n the absence, however, of foreign flammable substances, titanium rhips will not support combustion.
Properties of Titanium-Carbon Alloys It was previously stated that all of the titanium alloys under discussion contain carbon. It seems proper, then, that the properties of straight titanium-carbon alloys be investigated Those properties determined were: 1. Ultimate tensile strength 2. Proportional limit 3. Elongation 4. Electrical resistivity 5. Resistance to oxidation at 900" C. 6. Hardness at temperatures up to 600" C. Tensile properties were determined on a standard B S T M 0.25O-inch-diameter, threaded-end test bar. The gage length was 2 inches. Figure 5 illustrates the general effects of increasing carbon on the properties of titanium. The modulus of elasticity increasesthere is no room temperature hardness correlation although the hardness a t elevated temperatures increases, as does the oxidation resistance. Generally, tensile properties increase with increasing carbon content, whereas ductility decreases. Appraisal of the nitrogen contents of the titanium-carbon alloys given in Table 111reveals no correlation with physical properties. As a hypothesis it is possible that the effects of the quantities of nitrogen present are sufficiently small to be overshadowed by the carbon contents of the alloys.
240
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 4.
Vol. 42, No. 2
Drop-Forged Titanium Conveyer Belt Link
( R i g h t ) T r i m m e d a n d cleaned; ( l e f t ) as forged showing flashing; forged at 870' C. under 1600-pound board h a m m e r
of the 0.4% carbon-balance titanium alloy after hot forgiug and quenching from 1000' C. in water. Figure 3.
Crucible, Resistor, and Mold Assembly
Properties of Some Alloys ef Titadam The melted and wrought alloys of titanium presented here were chosen on the basis of results previously obtained on exploratory alloys prepared by powder metallurgy methods. Those investigated contained 0.37, to 0.6% carbon and up to 7% of the following metals: copper, chromium, boron, tungsten, aluminum, manganese, indium, iron, silicon, cobalt, vanadium, and nickel.
Microscopic examination of the quenched alloys of titanium and carbon show a Widmanstatten structure, probably mixed alpha and beta titanium, due to the change from body-centered cubic to hexagonal close-packed structure on cooling from above 885" C. Figures 5 and 6 show typical microstructures a t 200 X
Table V. Composition by Analysis,
%
w
Hardness, Rockwell
5.0 0.39 Balance
C
Ti
V
5.75
C Na Ti
C Fe
x2
Ti
Mn C Ti
Co C Ti Ni
C
S Z
Ti
hf n
ETi
a
F
-
0.37 0.08 Balance 4.95 0 44 0 1 Balance 4.47 0.40 Balance 2.90 0.56 Balance 4.25 0.57 0.04 Balance 1.50 1.33 0.40 Balance hot forged; Q =
A
C1tima t e Tensile Strength, Lb./Sq. Inch
Properties of Hot Forged Titanium Alloys Proportional Limit Lb./Sq. inch 114,000
71
165,000
68 (F)a
153,000
82,000
7 5 . 5 (Q)
173,000
110,000
Modulus of Elasticity,
Lb./Sq. Inch 17.5 X 106
Elongation, % in 2 Inches 12.5
Resistirit y , Ohm-Cm. 61 X 10-8
20
900" C.), Grams/Bq. Inch 0.03
13
0.348
-4
7.8
15
x
106
4.7
x 10-6 ...
18
x
106
12.5
64 X 10-6
19
i 27
1 5 . 5 X 106
1 5
...
24
...
x
10-5
10
0.16
84 X 10-6
12
0 12
17
0 20
17 (F)
3.03
91.6
1~0,000
77.5 (Q)
217,000
78
190,000
68
128,500
71,000
1 6 . 5 X 106
0
63
123,000
54,000
2 0 . 3 X 106
4.7
63 5
x
10-5
79
150,000
95,000
16.6
2.4
89
x
10-
92,000
