Sunday, March 06, 2011

Titanium Limitations in Aircraft Repair

 Titanium is entering main-stream usage. The Boeing 787 is 18% by weight titanium. To maintain aircraft that contain titanium it helps to know the material's quirks and limitations...and titanium has a few big ones... such as titanium's fire hazard; Titanium catches fire before it melts - unusual for a metal.

 Fire Hazard:
"There have been over 140 known instances of titanium fires in aircraft turbine engines in flight and in ground tests" 1.

Fire damage to titanium and titanium alloys becomes critical above 1000 degrees F due to the absorption of oxygen and nitrogen from the air which causes surface hardening to a point of brittleness. An overtemperatured condition is indicated by the formation of an oxide coating and can be easily detected by a light green to white color. If this indication is apparent following fire damage to titanium aircraft parts, the affected parts will be removed and replaced with serviceable parts.  T.O. 1-1A-9 page 5-6

"The application of titanium in the engine design should be directed primarily to minimizing the probability of uncontained titanium fires, i.e., fires that penetrate the engine casing" 1.

1.  FAA AC33.4 "Design Considerations Concerning the Use of Titanium in Aircraft Turbine Engines.

Hydrogen Embrittlement
Hydrogen-embrittlement is a major problem with titanium and titanium alloys. Hydrogen is readily absorbed from pickling, cleaning and scale removal solution at room temperature and from the atmosphere at elevated temperatures. Hydrogen embrittlement in the basically pure and alpha alloys is evident by a reduction in ductility and a slight increase in strength. This is associated with a decrease in impact strength at temperatures below 200 degrees F. and a shift in the temperature range where the change form ductile to brittle occurs.

With alpha-beta alloys, embrittlement is found at slow speeds of testing and under constant or "sustained" loads as demonstrated by tests on notched specimens. This type of embrittlement, which is similar to the embrittlement of steel, only becomes evident above a certain strength level. Solution heat treating and aging the alpha-beta alloys to high strength levels increases sensitivity to hydrogen embrittlement.2.

Cadmium Plate Caution

Cadmium plated self-locking nuts shall not be used in contact with titanium and titanium alloy bolts, screws, or studs in application where the operating temperature exceed 450 degrees F. Cadmium plated clamps, fixtures, and structures per Aeronautical-Design-Standard-ADS-13F-HDBK. Note, when considering localized cadmium embrittlement of titanium, consider that friction can sometimes cause this heating effect.

Boeing-Design-Manual-BDM-1054 states "The use of cadmium plated titanium components is not allowed. Cadmium plated components which come in contact with titanium are not allowed, except for hydraulic systems where cadmium plated steel fittings may be coupled to titanium fittings and cadmium plated steel or titanium nuts on titanium or steel bolts.

MIL-S-5002 prohibits all contact between titanium and cadmium on military programs." Cadmium plated clamps, fixtures, and\ jigs should not be used for the fabrication or assembly of titanium components or structures.

Cadmium plated self-locking nuts shall not be used in contact with titanium and titanium alloy bolts, screws or studs.  MIL-HDBK-1599A. 

Titanium parts shall not be cadmium or silver plated. Cadmium plated clamps, tools, fixtures, and jigs shall not be used for fabrication or assembly of titanium components or structures.

 "Solid Cadmium Embrittlement: Titanium Alloys", p 409 in Corrosion vol 236, no. 10, Oct 1970 by D.N. Fager and W.F. Spurr

"Solid Cadmium Cracking of Titanium Alloys", p 192 in Corrosion vol 20, no. 5, May 1973 by D.A. Meyn

Silver Plate Caution
Silver-plated self-locking nuts shall not be used in contact with titanium and titanium alloy bolts, screws, or studs in application where the operating temperatures exceed 600 degrees F. Per MIL-STD-1515A "Fastener Systems for Aerospace Applications."

 Silver brazing of titanium parts should be avoided for elevated temperature applications." ADS-13F-HDBK at temperatures exceeding 230°C (450°F). The warning on cadmium and silver is most likely because it was found that when cadmium or silver plated fasteners were pressed or smeared into the titanium surface at or near the yield of titanium that embrittlement of the titanium and cracking resulted. This became known as cadmium-embrittlement or Solid-Metal-Embrittlement (SME). Any barrier that prevents direct contact (such as a dry film lubricant) can prevent cadmium embrittlement. In most applications, the likelihood of SME is quite low or non-existent since the cadmium must be smeared into the surface while titanium is in tension well above 50% of its yield strength.

Skydrol Caution
Titanium can be embrittled by accumulations of Skydrol-hydraulic-fluid (BMS3-11) at temperatures above 270 degrees F. Per Boeing-Design-Manual-BDM-1054.

Alcohol Caution
Titanium can be embrittled by methyl alcohol and anhydrous ethyl alcohol at room temperature. Per Boeing Design Manual BDM-1054. The most damaging failure of a pressure vessel in the Apollo Program occurred in October 1966 when a main propellant tank ruptured inside an Apollo service module and caused extensive damage and serious loss of hardware. The failure occurred while the pressure vessel, made of titanium alloy 6A1-4V, was filled with methanol and was pressurized. The failure mode was stress-corrosion cracking. Accelerated Crack Propagation of Titanium by Methanol, APOLLO EXPERIENCE REPORT THE PROBLEM OF STRESS-CORROSION CRACKING by Robert E. Johnson Manned Spacecraft Center Houston, Texas

Solder Caution
Titanium can be embrittled by silver, zinc, lead and lead alloys at elevated temperatures. Per Boeing Design Manual BDM-1054.

High Temperature / Fire Exposure Caution
Titanium should not be used at temperatures above 1050 degrees F 565.6 C) as it has an unusually high attraction for carbon, oxygen, nitrogen, and hydrogen above this temperature.  This makes the titanium brittle. 
"An overtemperatured condition is indicated by the formation of an oxide coating and can be easily detected by a light green to white color. If this indication is apparent following fire damage to titanium aircraft parts, the affected parts will be removed and replaced with serviceable parts."Aerospace Metals-General Data and Usage Factors, Engineering Series for Aircraft Repair 26 February 1999, U.S.Air Force.

Working with titanium requiring the application of heat in excess of 800 degrees F., must be performed in a closely controlled atmosphere. The absorption of small amounts of oxygen or nitrogen makes vast changes in the mechanical properties. In gaseous oxygen, a partial pressure of about 50 psi is sufficient to ignite a fresh titanium surface over the temperature range from -250 degrees F to room temperature or higher.

Salt Caution
Titanium is susceptible to stress-corrosion-cracking by sodium chloride or chloride solutions at elevated temperatures. If you are using titanium parts above 450 degrees F (232.2 C), then use a nonchlorinated 
solvent and avoid leaving fingerprints.

"An American turbine engine manufacturer recently published a service letter alerting operators that wrapping stainless steel tube assemblies with a chloride-based material, such as neoprene tubing and fibreglass tape to prevent chafing, has resulted in premature tube failure. A chloride-based material breaks down from the presence of high engine temperatures and attracts moisture, resulting in the formation of salts which are highly corrosive to stainless steel tubes. After a period of time, stress cracking develops resulting in failure of the tubes. Additional investigation along the same lines by a foreign engine manufacturer revealed that titanium is also affected by the chemical reaction between chloride-based materials when operating in temperatures in excess of 150 degrees C (302 degrees F).

A related problem is the use of chloride-based packaging material, such as PVC sheeting (plasticized polyvinyl chloride) as a packaging material. This can result in chloride-based residue being left on the component, possibly leading to the sort of failure described above.

In summary, operators are reminded to follow the engine manufacturer's publications in installing stainless steel engine air, oil and fuel tubes and warned against using chloride-based materials on any stainless steel or titanium components, whether installed on the engine or held in storage. " AAC 1-13 Australian-Government-Civil-Aviation-Authority

Mercury Caution
Under certain conditions when in contact with cadmium, silver, mercury, or their compounds, titanium may become brittle. Refer to MIL-S-5002 and MIL-STD-1568 for restrictions concerning applications with titanium when in contact with these metals or their compounds. Silver will cause cracking in many titanium alloys at temperatures above 650 degrees F. 

Liquid Oxygen Caution
The use of titanium in contact with liquid oxygen should be avoided since the presence of a fresh surface, caused by cracking or abrasion, may initiate a violent reaction. Per Boeing Design Manual BDM-1054

Wear and Galling Caution
Titanium-galls very easy. It has been described as a "gummy" metal, strong but soft. Titanium threaded fasteners may require anti-seize. The loss of Sikorsky S-92A ship number CHI91 due to galling of titanium studs is an example of how galling is a serious concern.  Conversion coatings, such as Tiodize can be applied to titanium fasteners to prevent galling. For example the Titanium interference fit bolts in the F-14 wings would gall if driven into the hole bare. Tiodize coating is used to prevent such galling. Bare titanium should not be used for components having sliding surfaces. Pined joints subject to rotation, vibration, or repeated loads must be bushed with unplated aluminum-nickel-bronze or CRES bushings.

Crevise Corrosion Caution
Titanium is susceptable to crevise-corrosion in chloride (salt) solutions at elevated temperatures. Different heat treatments and alloys vary. "Care should be taken to ensure that cleaning fluids and other chemicals are not used on titanium assemblies where entrapment can occur. Substances which are known to be contaminants and which can produce stress corrosion cracking at various temperatures include hydrochloric acid, trichlorethylene, carbon tetrachloride, chlorinated cutting oils, all chlorides, freons, and methyl alcohol." ADS-13F-HDBK

Galvanic Corrosion Caution
Titanium is similar to Monel (nickel-copper alloy) and stainless steel and galvanic reactions generally will not occur when coupled with these materials. Less noble materials, such as aluminum, carbon steel, and magnesium alloys may suffer galvanic corrosion when coupled to titanium.

Welding Caution
Titanium welding must be done in an inert atmosphere. Cracked titanium bicycle frames are a good example of how lax attention to welding details results in fatigue cracks years down the road. Here is a write-up from a fatigue failure of a titanium duct on a Lockheed Tristar:

Although welding of commercially pure titanium normally results in a slight local hardness  increase, a well executed weld should only produce an increase in the range 10 to 25 HV. The weld  at the duct fracture location exhibited a much greater hardness increase (45 HV) and would  therefore be expected to have had reduced ductility, impairing the fatigue characteristics of the duct.   A difference greater than 30 HV compared with the parent material with an associated loss of  ductility can indicate that gas contamination has occurred, leading to weld embrittlement. Gas  contamination and embrittlement occurs when the weld pool is not sufficiently shielded from  atmospheric gases such as oxygen, nitrogen and hydrogen. The blue/purple tint to the weld area  adjacent to the fracture is evidence of elevated temperature oxidation... AAIB Bulletin No: 6/99 Ref: EW/C98/9/5 Category: 1.1

This report seems to imply that a local hardness test close to the weld may be a good test for excessive embrittlement.

When titanium is heated to 500 degrees C. (930F), it absorbs oxygen, hydrogen, nitrogen, and carbon. These atoms enter the titanium and make it brittle. Evidence of titanium weld contamination is readily apparent as a discoloration of the weld surface.  This discoloration is caused by oxidation and starts at about 900 degrees F. Heating to temperatures above 1000 degrees F. under oxidizing conditions results in severe surface oxidation and brittleness.

General Welding Principles:
  • Not every good welder can weld titanium - requires discipline.
  • Cleanliness. You do need to be manic about cleanliness. Solvents must be very fresh and always stored in sealed containers. Purge gasses need to be pure. Avoid  rubber or plastic hoses in handling the gasses. The permeability is too high and you will pick up oxygen and moisture. Use Lint-free gloves after cleaning so as to avoid contaminating the surface with perspiration.
  • Protect the backside. Wherever the titanium is heated, brittle alpha-case can form.
  • The presence of blue or white oxide is an indicator that contamination has occurred.
  • A bright silver color (mercury color) is desired.

High quality industrial and aerospace welding of titanium is done in a hermetic welding chamber which maintaines the atmosphere of Argon with less than 20-ppm O2 and 20-ppm moisture.

Hydrogen Migration to Weld at Elevated Temperature Caution
Metallurgical examination of the duct fracture surfaces showed that it had failed due to cracking from multiple origins on the duct inner surface, adjacent to the weld. Hydride formations were present and the metallurgical report concluded that the failure was similar to that described in Boeing Service Bulletin 747-36A2074...This states "At duct operating temperatures of 300 to 350 degrees Fahrenheit, hydrogen in the titanium duct material tends to migrate towards areas of high stress, and then during cooling, hydrides form. These hydrides have an embrittling effect on the duct material and may contribute to crack initiation...Studies indicate that stress relieving the ducts eliminates the residual stress and local stress concentrations which stops the migration of hydrogen to the circumferential welds." Airframe cycles on duct: 14,698. Ref. AAIB EW/A92/6/1 Boeing 747-283B, G-VOYG


R.J.H. Wanhill
National Aerospace Laboratory

S.A. Barter, S.P. Lynch and D.R. Gerrard
Defence Science and Technology Organization
Fishermans Bend, Victoria