Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/16—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of other metals or alloys based thereon
  • C22F1/18—High-melting or refractory metals or alloys based thereon
  • C22F1/183—High-melting or refractory metals or alloys based thereon of titanium or alloys based thereon

Definitions

• the present invention relates to the processing of the high strength alpha-beta titanium alloys to improve the level and uniformity of their mechanical properties.
• the high strength alpha-beta titanium alloys such as Ti 6-2-4-6 (6 percent aluminum, 2 percent tin, 4 percent zirconium, 6 percent molybdenum, balance titanium) and the Ti 6-6-2 alloy (6 percent aluminum, 6 percent vanadium, 2 percent tin, balance titanium), when processed by the conventional heat treatments exhibit a broad scatter in toughness, strength and fatigue resistance.
• the most common heat treatment for the Ti 6-2-4-6 alloy comprises: solutioning for about 1 hour at 1660 F.; air cooling; precipitation heat treatment at 1100 F. for 4-8 hours; and cooling in air.
• the scatter in properties resultant from such heat treatment is directly related to microstructural differences within a given component and between components.
• the cooling rates between various sections of a given article cannot be well controlled due to difierences in forging section size.
• the thin sections tend to exhibit higher tensile and yield strengths and lower fracture toughness than thicker sections of the same forging.
• a heat treatment process for such alloys which comprises: a solution heat treatment optionally conducted concurrently with, but usually subsequent to, for ing to adjust the quantity and morphology of the alpha phase in the microstructure; a direct transfer to an isothermal media, held at a high temperature but lower than the solutioning temperature, for a time period sufficient to achieve the desired transformation products and 3,748,194 Patented July 24, 1973 phase chemistries; and a subsequent aging, at a temperature suitable for the alloy, for strenghening purposes.
• the Ti 6-2-4-6 alloy is processed by: solution heat treatment, typically 1690 F. for 2 hours; direct transfer from solution heat treatment to a molten salt bath in the general range of about 1400-1600 F., typically 1525 F. with a hold for about fifteen minutes; and subsequent aging at 950-l150 F., typically 1100 F. for 8 hours; and air cooling.
• the high strength titanium alloys are desired for current lightweight, high performance turbine powerplants where a high specific modulus and creep strength are fundamental design criteria. Of intense interest are the age-hardenable, alpha-beta titanium alloys.
• the Ti 6-6-2 alloy has been extensively investigated but, although high yield strengths are attainable with this alloy, it suffers somewhat from limited hardenability and low creep strength.
• the Ti 6-2-4-6 alloy has demonstrated high yield strengths and, in addition, has a greater hardening potential and higher creep strength than the Ti 6-6-2 alloy. Accordingly, it is the preferred alloy of current interest.
• This alloy was purchased from the producer as eight inch round billet with an actual composition (K-2407) of 6.2 percent aluminum, 2.1 percent tin, 4.2 percent zirconium, 6.1 percent molybdenum, 0.06 percent iron, 0.12 percent oxygen, 0.008 percent nitrogen, 0.007 percent hydrogen, balance titanium.
• the alpha+beta to beta transus for this heat was determined to be 1735J -10 F.
• Several billet sections were cross-worked by multiple upset and redraw operations at 1625 F. to reduce the elongated alpha particle content and create a more homogeneous billet structure. Open die pancake forgings 1.75 inches thick and 18 inches in diameter were produced at a number of forging temperatures from 1625 F. to 1800 F. All pancakes were then cut into two or more sections and the effects of solution heat treatment at temperatures from 1525 F. to 1730 F. were investigated. Aging between 950" F. to 1100 F. usually completed each processing treatment.
• cooling rate from forging temperature was studied.
• quench rate from the solution treatment temperature was investigated in substantial detail by cooling various segments in different ways including air cool, oil quench and water quench. Mechanical property measurements and micrographs were obtained both near the surface and at the center of each segment, since these locations experienced different thermal histories.
• the beta processed microstructures can be readily distinguished from the alpha-beta processed structures by their lack of primary alpha.
• the microstructures which yield the highest fracture toughness at yield strength levels between 170 and 180 k.s.i. may be defined as containing about 10 percent globular alpha (primary alpha) with a matrix of relatively coarse acicular alpha (secondary alpha) and aged beta.
• An acceptable level of tensile ductility (20% RA) is also obtained with this microstructure.
• the basic microstructure is, of course, tailored to some extent depending upon the particular goal properties desired.
• a gas turbine engine compressor disk for example, in the strength/toughness trade off as part of the alloy property optimization process, more coarse alpha is built into the alloy providing increased toughness somewhat at the expense of strength.
• the strength/toughness trade off would typically be reversed, providing increased strength even if the achievement thereof were provided somewhat at the expense of toughness.
• the fracture toughness of a given specimen is a function of the yield strength.
• the critical crack size for unstable fracture in plane strain is known to be proportional to (K /c where K is the critical plane strain stress intensity factor and 4. transformation temperature rather than by highly variable continuous cooling rates and section size sensitivity is therefore reduced.
• Alpha and beta phase transition chemistries resulting from elemental partitioning are also controlled bythe isothermal treatment so that properties may be optimized.
• the basic forging microstructure comprises substantially equiaxed primary alpha in a transformed beta matrix resultant from forging at a temperature up to 1700 F., typically 1629-1650 F., in'the case of the Ti 62-46 alloy. Forgings with coarser elongated alpha platelets which are less fragmented or exhibit less random orientation are considered rejectable, as is a lack of primary alpha.
• Solution heat treatments are conducted generally at temperatures up to about 1700 F., typically at about 1690 F. for l 2 hours, the holding time being dependent on section size but being sufficient in any event to provide relatively uniform heating.
• the solution temperature must be high enough to control the size of the beta phase since the beta subsequently controls the size of the alpha platelets.
• Solutioning is therefore conducted basically 6 ⁇ is the yield strength. It is possible, however, to modify microstructural features to increase fracture toughness at a given strength level by creating random preferred crack growth paths in the structure. These preferred crack growth paths are along alpha plate interfaces and control of the size and orientation of alpha plates is necessary to achieve fracture toughness at high strength levels in alpha-beta titanium alloys.
• the K goal was k.s.i. Vin.
• the K goal was achieved by solution heat treatment to adjust the quantity and morphology of the equiaxed alpha phase with a direct transfer to an isothermal environment providing controlled isothermal transformation of the beta phase to coarse alpha plates. Subsequent conventional aging is then performed.
• the isothermal transformation sequence allows exce lent control of the matrix phase morphology to achieve optimum microstructures and properties.
• the amount of coarse secondary alpha is determined by the isothermal below the beta transus temperature, typically about 10- 50 F. therebelow, but within about F. thereof.
• Chloride salts and the cyanides are suitable. Basically any salt is usable where the resultant surface corrosion can be accommodated.
• the microstructure and consequently, alloy properties are both a function of the temperature of the isothermal media and the holding time therein.
• the isothermal transformation treatment allows sutficient control to permit a complete transformation to coarse alpha or a mixed microstructure, both coarse and line alpha plates, if the shorter holding times or higher transformation temperatures are utilized.
• isothermal transformation is conducted within the general temperature range of the alpha transus to about 200 F. above, in the range, therefore, of about 14001600 IF., preferably 1500l575 F.
• From the isothermal media cooling may be accomplished by water quench or air cooling, the air cooling procedure involving some strength compromise.
• Aging is conventional, and usually conducted in the range of about 950-1100 F.
• the preceding table illustrates the respective room temperature tensile and fracture toughness properties of the Ti 6-2-4-6 alloy as processed by conventional means and by the processing of the present invention.
• step 1 involves a solution heat treatment high in the two-phase region to develop the small amount (5-30%, preferably about 10%) of primary alpha desired. If forging, rolling or extrusion is, in fact, done at a sufficiently high temperature, then it is indeed a solution treatment. Aging is conventional in all cases. It is the control of transformation that provides the dramatic improvement in properties. The overall process is controlled to form the proper amount of the acicular alpha phase while not producing additional equiaxed alpha, and having sufficient retained beta so that the desired strength level can be obtained upon subsequent aging.
• the isothermal transformation is conducted at a tem perature of about 1525 F. for about minutes.
• the method of processing the alloy consisting essentially of, by weight, 6 percent aluminum, 2 percent tin, 4 percent zirconium, 6 percent molybdenum, balance titanium, which comprises:
• the method of processing the alloy consisting essentially of, by weight, 6 percent aluminum, 2 percent tin, 4 percent zirconium, 6 percent molybdenum, balance titanium, which comprises:

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• Chemical & Material Sciences (AREA)
• Mechanical Engineering (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Physics & Mathematics (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Forging (AREA)
• Heat Treatment Of Nonferrous Metals Or Alloys (AREA)
• Heat Treatments In General, Especially Conveying And Cooling (AREA)
• Chemically Coating (AREA)

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Cited By (14)

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• 1971
  • 1971-10-06 US US00187037A patent/US3748194A/en not_active Expired - Lifetime
• 1972
  • 1972-07-10 CA CA146,794A patent/CA975663A/en not_active Expired
  • 1972-09-20 AU AU46901/72A patent/AU464371B2/en not_active Expired
  • 1972-09-21 FR FR7233920A patent/FR2162843A5/fr not_active Expired
  • 1972-09-24 GB GB3455572A patent/GB1369289A/en not_active Expired
  • 1972-10-02 SE SE7212694A patent/SE392128B/sv unknown
  • 1972-10-04 DE DE19722248661 patent/DE2248661A1/de active Pending
  • 1972-10-05 IT IT30112/72A patent/IT968644B/it active
  • 1972-10-06 JP JP10059072A patent/JPS5516232B2/ja not_active Expired

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