US3481799A - Processing titanium and titanium alloy products - Google Patents

Description

Dec. 2, 1969 D. L. DAY ET AL 3,481,799

PROCESSING TITANIUM AND TITANIUM ALLOY PRODUCTS Filed July 19, 1966 Fig.3

/N VE N TORS. Dwayne L. Day Albert A. Haskell, Jr:

WW 4 THE/R ATTORNEYS United States Patent 3,481,799 PROCESSING TITANIUM AND TITANIUM ALLOY PRODUCTS Dwayne L. Day, Wintersville, Ohio, and Albert A. Haskell, Jr., Belmar, N.J., assignors to Titanium Metals Corporation of America, New York, N.Y.

Filed July 19, 1966, Ser. No. 566,412 Int. Cl. C22f 1/18 U.S. Cl. 148-115 9 Claims ABSTRACT OF THE DISCLOSURE Method of treating a body of titanium or alpha or alpha-beta type titanium base alloys comprising forging the body to an extent to achieve a significant reduction in cross sectional area at a temperature below the beta transes of the metal, heating the body at a temperature above the beta transes of the metal for a period sufficient to obtain complete recrystallization and reforging the body to an extent to obtain a significant reduction in cross sectional area at a temperature between the beta transes of the metal and about 1600 F.

This invention relates to the processing of titanium and titanium alloy products to refine the grain structure thereof and more particularly to improve the forgeability of said products.

Mill processing of titanium and titanium alloy ingots involves subjecting the metal body to a series of forging and annealing steps to create a billet or other forged product having the desired shape and size. The forging temperature and the amount of deformation accomplished in a forging step are important factors in producing forged products, and forging temperatures are generally selected so that the metal is sufiiciently plastic to permit relatively easy deformation to the desired size and shape. However, the forging temperature should not be so high as to cause excessive oxidation and scaling of the metal surface. Ordinarily, forging is accomplished at a temperature below the beta transes of the titanium or titanium alloy being forged to avoid the coarse grain structure which tends to result at and above this critical temperature. The formation of coarse grain structure at temperatures above the beta transes of the metal cannot be Wholly counteracted and increases when metal is held for an excessively long time above its beta transes. Therefore, forging temperatures as high as necessary to provide the required plasticity for efiicient forging have been employed but have been kept below the beta transes of the metal when possible. Forging at relatively lower temperatures is economically desirable; but as the temperature is lowered, the plasticity of the metal decreases, resulting in a more difficult forging operation. Additionally, forging at relatively lower temperatures causes cracking and splitting of the metal body being forged.

Forged mill products of titanium and titanium alloys are often sold in the form of bars or billets which are worked to the desired product by additional forging steps performed by the purchaser.

Cracking of titanium and titanium alloy mill products during forging has been observed to occur at the boundaries of large, unrefined as-cast grains revealed by macroetching. In the case of products which have been forged from ingots at temperatures close to or above a 3,481,799 Ice Patented Dec. 2, 1969 the beta transes of the metal, these large grains may be transformed beta grains particularly if the alloy is of the mixed alpha-beta type. It is, therefore, necessary that the bars and billets possess reasonably fine grain structure and be forgeable at relatively low temperatures so that when further processed either at the mill or :by a purchaser they may be reduced to the desired size and shape without cracking or splitting.

Our invention provides a method for processing unalloyed titanium and alpha and alpha-beta titanium alloy ingots to provide a forged product having a refined grain structure which may be further forged at relatively low temperatures without cracking or splitting. Refined grain structure and improved forgeability are accomplished by a series of steps in which the ingot is first forged from a temperature and with a deformation sufiicient to create strain energy in the metal body which will permit subsequent recrystallization of the beta grains therein. The forged body is then heat treated at a temperature above the beta transes of the metal to recrystallize the beta grains. This heat treatment is termed a beta anneal and is of sufiicient duration to effect recrystallization throughout the body. The body is then reforged from a temperature requiring a substantial amount of deformation to occur below the beta transes to break up and distort the alpha networks surrounding the equiaxed transformed beta structure formed by the beta anneal. Our process may be used to treat unalloyed titanium and alpha and alpha-beta type titanium alloys having suflicient plasticity at temperatures below the beta transes of the metal to permit the reforging step to be performed in a manner to obtain a significant amount of deformation below the beta transes. The major portion of the deformation obtained during reforging must be below the beta transes to prevent formation of undesirable coarse beta grains in the metal and to distort the alpha networks in the transformed beta structure.

The temperature from which the first forging step is initiated and the amount of deformation accomplished by this step must be such that strain energy is imparted to the body and will vary with the size of the ingot and the size and type of forging equipment used. It is critical, however, that a significant amount of deformation be accomplished at a low enough temperature below the beta transes of the metal in order to insure creation of sufficient strain energy in the body to permit subsequent recrystallization. Generally, it will be advantageous to recrystallization. Generally, it will be advantageous to begin the initial forging at a temperature above the beta transes since deformation is more easily accomplished at the higher temperatures and to perform a significant portion of the reduction in the alpha-beta field due to the normal heat loss during forging. It is possible, however, to carry out the initial forging step entirely in the alphabeta field if desired and if metal plasticity permits. The reforging step may also be started at a temperature above the beta transes, but it is essential that a significant amount of the deformation obtained during reforging take place below the beta transes to insure distortion of the alpha networks and to minimize coarse beta grain structure.

A titanium or titanium alloy body processed in accordance with our invention may be further forged at advantageous relatively low forging temperatures without cracking or splitting. It is believed that this enhanced forgeability is due to the finer grain structure resulting from the combination of initial forging of the body followed by recrystallization and additional hot deformation.

In the accompanying drawings:

FIG. 1 shows the etched surface of a sectioned titanium alloy billet after forging, 1.3x magnification;

FIG. 2 shows the same section after recrystallization by a beta anneal, 1.3 X magnification; and

FIG. 3 shows the etched surface of a sectioned titanium alloy billet which has been forged, beta annealed and reforged to break up alpha networks in the transformed beta structure, l magnification.

An aqueous hydrofluoric acid etching solution was used to bring out the grain structure shown in FIGS. 13. 7

Referring in more detail to the drawings, the section shown in FIG. 1 shows the structure obtained by processing a i 6" square section from an as-cast ingot of Ti-6Al-4V alloy by hammer forging from 1900 F. to 5" square reduction), and then hammer forging from 1750 F. to 3 /2" square reduction). The section shown in FIG. 2 is a section from the same billet as that from which the section in FIG. 1 was taken which was similarly forged from 6" square to 3 /2 square. Afterv forging the section of FIG. 2 was beta annealed at 1950" F. for one hour followed by air cooling to recrystallize the beta grains. The material is shown in the annealed condition.

The section shown in FIG. 3 was produced from the.

same Ti-6Al-4V ingot as the sections shown in FIGS. 1 and 2. The billet was processed from the as-cast ingot by hammer forging from 2050 F. to 5" square (30% reduction) and then from 1750 F. to 3 /2" square (50% reduction). The billet was then beta annealed at 1950 F. for one hour followed by air cooling to recrystallize the beta grains. After recrystallization the bilet was harnmer forged from 1750 F. to 2 /2" square (50% reduction) to break up the alpha networks in the prior transformed beta structure caused by the recrystallization to provide fine grain structure.

The effect on the subsequent forgeability of a product processed by the forging, heat treating and reforging process of our invention is readily apparent from the following example in which test disks forged from cylinders taken from a billet which had been forged from an ingot are compared with test disks forged from cylinders taken from a billet processed in accordance with our invention. A pair of disks were upset forged from a temperature of 1650 F. from a pair of 2 inch diameter cylinders taken from a billet which was previously forged from a Ti-6Al-4V ingot. The disks exhibited severe edge cracking. The second pair of disks were also upset forged from a temperature of 1650 F. from 2 inch diameter cylinders. However, the cylinders from which the second pair of disks were forged were taken from a Ti-6Al-4V billet which had been processed by first reducing a billet forged from an ingot by 42% from a temperature of 1800 F., beta annealing at 1950 F. for two hours and then reducing 62% from 1750 F. The beta transes of Ti-6Al-4V alloy will vary slightly with variations in chemistry but will be between about 1800 F. and l840. F.; and, therefore, the intermediate heating was well above the beta transes of the alloy and the majority of the forging and reforging was carried out below the beta transes. The disks forged from the cylinders taken from the billet processed in accordance with our invention showed no cracking. The lack of cracking is attributed to the improved grain structure created by treating the metal in accordance with our invention.

We have found that sufficient hot working must be imparted to a coarse grained titanium or titanium alloy body at a temperature below the beta transes to impart the required strain energy to the metal for subsequent recrystallization. The amount of hot working which will create the required strain energy will vary with the working temperature and the size of the body being reduced.

Generally speaking, the lower the deformation temperature and the greater the amount of'redu'ction, the lower will be the temperature necessary to cause recrystallization. Once the body has received the required hot working, the coarse grained macrostructure can be completely refined to a fine-grained, recrystallized structure by annealing at a temperature above the beta transes in the single phase beta field. Although these recrystallized grains are much smaller than those in the original body of metal, they are equiaxed transformed beta grains surrounded by relatively non-ductile alpha networks and the metal body is not forgeable at the low temperatures used in some forging operations. This is particularly noticeable in alpha-beta alloys such as Ti-6Al-4V and in alpha alloys such as Ti-5Al-2.5Sn in which less ductile oxygenor aluminum-rich alpha may be .concentrated at the transformed beta grain boundaries. To improve forgeability, the material must be again hot worked, i. e., reforged. to break up the alpha networks in this equiaxed transformed beta structure. This 'is perfor med by forging at at temperature below thebeta' transes; but-the starting temperature for this reforging step is not critical aslong as'a significant amount of deformation occurs below the beta transes. The amount of deformation necessary' be low the beta transes varies, and it is possible to take the material directly from the beta annealing'furna'ce'and forge without any deliberate cooling. Since the metal cools during forging. a substantial amount of deformation occurs below the beta transes and the alpha networks in the equiaxed transformed beta structure are effectively broken up. The post-recrystallization hot working, that is the reforging step, also may be advantageously employed when processing unalloyed titanium to improve subsequent low temperature forgeability, but it is more important when processing the alpha-beta and alpha type titanium alloys. v

The final phase'of the initial forging step is advantageously carried out at a relatively low temperature, which will generally be between about 1600 F. and the beta transes of the metal and preferably in the lower part of this range if the plasticity of the, metal permits. For. unalloyed titanium and many of the alphaand alpha-beta type titanium base alloys, a convenient and advantageous temperature range is from about 1600 F. to about 1750 F. Within this temperature range, the metal will be plastic enough to be readily deformed. .It is important in the initial forging step that sufficient plastic deformation be imparted to the metal body to put strain energy into the metal. The amount of reduction in cross sectional area of the metal body will vary with the size of the body, and it has been found that, a reduction in cross sectional area of about 30% will generally be sufiicient.

After the initial forging step, the metal body isbeta annealed at a temperature above its beta transes to accomplish recrystallization. The precise temperature and time at'temperature are not critical so long as the body is above the beta transes for sufficient time toeffect complete recrystallization. However, it willbe recognized by those skilled in the art that thetemperature and time should be kept at a minimum because of the tendency for titanium and titanium alloys to become oxidized and severely scaled as the temperature and time are increased. Additionally, undesirablebeta grain growth increases as the temperature and time are increased. It

has been found that heating the metal body at a temperature between its beta transes and about. F. above its beta transes for. a maximum period .of about two hours after the whole body is at temperature will provide recrystallization without excessive oxidation, scal ing and beta grain growth. v

After the beta anneal, the metal body is reforged to distort and break up the alpha netw rks in the equiaxed transformed beta structure formed by heating above the beta transes. This can best be accomplished by forging to produce an appreciable reduction in'cross section, preferably about 30%. A significant amount of the finish forging should be accomplished at a temperature below the beta transes although the forging may be initiated at a temperature above the beta transes.

The initial forging step according to this invention requires a significant amount of hot reduction at a temperature below the beta transes to create the necessary strain energy to permit the desired recrystallization. This effect is illustrated in the table which shows the effect of a two hour beta anneal at 1900 F. on samples taken from 4% inch and 5 inch square billets of Ti-6Al-4V alloys press forged from 6 inch cubes of as-cast metal.

Center From the table it can be seen that forging from 1750 F. which is below the beta transes of the alloy imparted sufficient strain energy throughout the sample to obtain the desired recrystallization by a subsequentbeta anneal. When forging was initiated from 1900" and 2050 F. which are respectively about 100 F. and 250 F. above the beta transes of the alloy recrystallization was only obtained at the surface of the samplerwhich had been reduced 50%. A 30% reduction froth 1900 F. and z0so F. did not permit sufficient working below the beta transes to create the necessary strain energy whereas the temperature of the surface of the samples was below the beta transes for a sufficient portionof the 50% reduction to create strain energy in the surface portion of the samples.

The following non-limiting examples illustrate specific embodiments of the invention:

EXAMPLE 1 A billet of a Ti-6Al-4V alloy having a characteristic large grain size generally of the macrostructure illustrated in FIG. 1 was forged from a temperature of 1750 F. to provide a reduction in cross sectional area of 50%. The beta transes of this alloy was about 1825 F. The forged billet was then beta annealed for 'two hours at 1900 F. and air cooled to room temperature. After the beta anneal, the billet was reforged from a temperature of 1750 F. until a reduction in cross sectional area of 50% was obtained. A sliced section of the reforged billet when etched showed a completely recrystallized fine grain macrostructure. The billet was substantially reduced in cross section by further forging without edge cracking.

EXAMPLE 2 A billet of a Ti-6Al-4V alloy havinga characteristic large grain size generally of the macrostructure shown in FIG. 1 was forged from a temperature of 1850 F. to produce a reduction in cross sectional area of 30%. The beta transes of this alloy was about 1900 F. The forged billet was then beta annealed at 1950 F. for one hour and cooled to room temperature. After annealing, the billet was reforged from a temperature of 1850 F. until a reduction in cross sectional area of 30% was obtained. A sliced section of the reforged billet when etched showed a completely recrystallized, fine grain macrostructure. When further forged from a temperature below the beta transes, the billet was substantially reduced in cross section without edge cracking.

EXAMPLE 3 A billet of Ti-SAl-lMO-IV alloy having a characteristic large grain size generally of the macrostructure shown in FIG. 1 was forged from a temperature of 1850 F. to provide a reduction in cross sectional area of 45%. The beta transes of the alloy was about 1900 F. The forged billet was then beta annealed at 1950" F. for one hour and cooled to room temperature. After annealing, the billet was reforged from a temperature of 1875 F. until a reduction in cross sectional area of 40% was obtained. A sliced section of the reforged billet when etched revealed a completely fine grained, recrystallized macrostructure. When further forged, the billet was substantially reduced in cross section without edge cracking.

EXAMPLE 4 A billet of commercially pure titanium having a characteristic large grain size generally of the macrostructure shown in FIG. 1 was forged from a temperature of 1650 F. to achieve a reduction in cross sectional area of 40%. The beta transes of the metal was about 1725 F. The billet was then beta annealed for two hours at 1800 F. and cooled to room temperature. After annealing, the 'billet was reforged from a temperature of 1650 F. until a reduction in cross sectional area of 50% was obtained. A sliced section of the reforged billet when etched showed a fine grained, completely recrystallized macrostructure. When further forged from a temperature below the beta transes, the billet was substantially reduced in cross section without edge cracking.

After processing according to this invention by initial forging, beta annealing to effect recrystallization and reforging, the metal body may be forged in any desired manner and at normal forging temperatures to produce a desired product. Forgeability is substantially enhanced in that it may be carried out at a reasonably low temperature without the danger of cracking and splitting. Thus, unalloyed titanium and titanium base alloy products produced according to this invention will be found to possess improved and extremely desirable forgeability characteristics.

While we have shown and described preferred embodiments of our invention, it may be otherwise embodied within the scope of the appended claims.

We claim:

1. A method for refining the grain structure of a metal body selected from the group consisting of unalloyed titanium metal and titanium alloy metals of the alpha and alpha-beta types and thereby improving the forgeability of said body comprising forging said metal body to produce a significant reduction in cross sectional area at a temperature below the beta transes of said metal to impart strain energy to said metal, heat treating said forged metal body at a temperature above the beta transes of said metal to effect recrystallization of beta grains throughout said body and reforging said metal body to produce a significant reduction in cross sectional area at a temperature between about 1600 F. and the beta transes of said metal to distort and break up alpha networks in the equiaxed transformed beta structure formed by said heat treating.

2. The method as described in claim 1 wherein said forging of said metal body is accomplished at a temperature between about 1600 F. and the beta transes of said metal.

3. The method as described in claim 1 wherein said forging of said metal body is accomplished at a temperature between about 1600 F. and about 1750 F.

4. The method as described in claim 1 wherein said heat treating is carried out at a temperature between the beta transes of said metal and about F. above said beta transes.

5. The method as described in claim 1 wherein said heat treating is carried out for a period of from one to two hours after said metal body is completely heated above the beta transes of said metal.

6. The method as described in claim 1 wherein said forging and said reforging of said metal body reduces the cross sectional area of said body about 30%.

7 7. The method as described in claim 1 wherein said forging -is initiated at a temperature above the beta transes of said metal.

. 8. The method as described in claim 1 wherein said reforging is initiated at a temperature above the beta 5 8 References Cited it UNITED STATES PATENTS- 2,968,586 1/1961 Vordahl map-11.5 3,394,036 7/1968 Parris 148-115 L. DEWAYNE RUTLEDGE', Primary Examiner w. w. STALLARD, Assistant Examiner 1

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