new surface treatments for titanium · 2018. 4. 14. · solidified metal in the top of figure 2....

NEW SURFACE TREATMENTS

FOR TITANIUM

Robert H. Shoemaker, President

Kolene Corporation Detroit, Michigan

An objective for this paper is to examine and evaluate the present methods of removing surface contamination from titanium in light of future high production requirements. Mass construction of the F-15, F-14 and B-1 aircraft, along with other anticipated 60% to 80% titanium structures will require automated processing. The second part of this paper is concerned with the removal of the oxygen-rich alpha case resulting from elevated temperature treatment of titanium in oxidizing atmosphere. The necessity for complete cleaning prior to chemical milling is shown graphically. The etched surface condition developed in improperly cleaned titanium is illustrated. Conditioning and treatment cycles prior to effective gauge removal are tabulated. The final phase of surface treatment discussed is the most recent development. A liquid nitriding bath imparts a 0.001" (0.025 mm) to 0.002 (0.051 mm) wear resistant surface on titanium alloys. Treatment time is two hours and effectively overcomes the gall and wear problems characteristic of titanium. Photomicrographs, hardness traverse curves and profilometer wear patterns substantiate the advantages claimed for the nitriding process.

2501

2502 R. H. SHOEMAKER

Titanium Cleaning

Perfecting titanium heat treatment hot forming techniques to produce complex structures required for F-14, F-15 and B-1 aircraft has generated many unforeseen problems. A primary and intricate one is the cleaning and removal of elevated temperature oxides, protective coatings and lubricant residues at intermediate steps.

Each of the contaminants shown in Table I can occur on the surface of finished products. Titanium scales alone in alloys such as Ti 6Al-6V-2Sn are sufficient to test any cleaning process. Glassy silicate residues, while furnishing barrier films, also tend to enhance the cleaning problems. Incorporation of MoS2 and C, two chemically resistant materials capable of bonding to metal under pressure, have also increased cleaning demands.

Hot Formed Titanium Scales

Standard procedure for hot forming titanium involves precoating with a silicate type protective coating. These coatings are uniform and continuous up to 1500°F. (816°C) Beyond this temperature spherodizing and voids can occur. Glass type coatings are subject to brittle fracture under moving load conditions.

The graphite molybdenum disulfide lubricants are generally applied over the heat resistant coatings. They are more porous than the silicate type but do give some high temperature protection. Under temperature and load, binders are carbonized but the graphite and molybdenum pigments follow the forming operation.

Effective cleaning can be accomplished rapidly by solution of the oxide and silicate, with suspension of the MoS2 and C pigments.

Approaches to Titanium Cleaning

There are several choices available to clean hot formed titanium surfaces. Selection is based on the requirements placed on the finished parts. Mechanical finishing, such as shot blasting, is the most rapid when contrasted with chemical cleaning. The limitations on this process are shown in Table II.

To assume from examining this table that mechanical treatments have little place in a titanium cleaning operation would be a fallacy. Many of these factors

NEW SURFACE TREATMENTS FOR TITANIUM 2503

increase directly with the energy of cleaning and are negligible where mechanical precede chemical methods.

Various combinations of oxidizing compounds, chelates and caustic are available in water solutions for complexing and removing titanium scales. The chief disadvantages are their inability to dissolve all types of soils and the rather slow speed of reaction. Heat and pressure formed glass and carbon residues are quite resistant to the action of these compounds resulting in the retention of minute scale areas on the finished surface. These chemical solutions are adequate for treating certain titanium scales if sufficient processing time is available. Manual finishing may be required in the inspection area. Chelated compounds can also be used in secondary cleaning operations to replace acid baths.

Molten salt baths are ideally suited for meeting the requirements for high volume cleaning systems. At the elevated temperatures employed, reactions are almost instantaneous. Characteristics of molten salt b~ths that qualify them as complete metal cleaners are shown in Table III.

Molten cleaning salt temperatures may vary between a low of 400°F. (204°C) and a high of 850°F. (454°C). The former is preferable for fabricated structures and thin sheets as shown in Figure 1. Low temperature salts are also used in the mill cleaning operations shown in Table IV.

Heavy torch cut titanium sections require an 850°F. (454°C) molten salt to remove the combined oxide and solidified metal in the top of Figure 2. Forged turbine blades and extruded shapes require the same processing because of the ceramic-metal complexes developed.

Salt Bath Reactions

Molten salt reactions with the soils in Table I are identical with those expected from a standard chemical approach. The metal rich oxides are reacted to form the more soluble gas rich compounds or neutralization products as shown:

(1) 2MO + 2AO +2NaN03 = M203 + A203 + 2NaN02

(2) M203 + 2NaOH = Na2M204 + H20


Table I Nature of Contaminants on Hot Worked Surfaces

Type

1. Titanium & alloy oxides

2. Organic

3. Sintered ceramic

4. Chemical complexes

Formula Source

MO Hot work and heat treat scales

C Free graphite lubricants and binders

Si02 Protective coatings

Special pigmented lubricants

Table II Limitations on Mechanical Cleaning Systems

1. Inability to effectively clean internal sections and shielded areas.

2. Deformation of formed parts.

3. Compressive stresses developed at surface.

4. Surface contamination with corrosive material.

Table III Characteristics of Molten Salt Baths

1. Excellent solvency or fusion properties with metal oxides, glass and carbonaceous residues.

2. Wetting and penetration of discontinuities at metal-oxide interface.

3. Strong oxidizing potential to convert metal rich oxides to more soluble gas rich compounds.

4. Expansion differential promoting separation between oxide and base metal in direct proportion to salt base temperature.

5. Minimum salt-metal reaction after oxide complexing and removal.


Figure 1 Titanium Sheet Descaling in 400°F. (204°C) Molten Salt Bath.

Figure 2 Torch Cut Titanium Sections.


Graphite and carbonaceous lubricant residues can be chemically oxidized to destruction or assimilated and removed as sludge.

(3) c + 2NaN03 + 2NaOH = Na2C03 + 2NaN02 + H20

(4) NaN03 + 0.502 = NaN03

Ceramic residues are attacked by the alkaline base to yield water soluble silicates.

Pigmented lubricant residues may, if inert to the molten salt, be removed as sludge. Reactive compounds are chemically attacked as shown:

(6) MoS2 + 6NaOH + 9NaN03 = Na2Mo04 + 2Na2S04 + 9NaN02 + 3H20

Titanium Pickling

The pickling operations following molten salt conditioning can be better evaluated by considering the complete cleaning cycle involving a 400°F. (204°C) molten salt as shown in Table V.

The chemistry of molten salt conditioning which produces potassium titanates, KxTiyOz, on the surface permits the major pickling action to take place in the sulfuric acid. Surface damage can thus be prevented with good inspection practice prior to nitric-hydrofluoric pickling, since the sulfuric acid is much less reactive with titanium as shown in Figure 3. Unaffected oxide remaining can be recycled through the salt and removed before the final pickle.

Hot 35% sulfuric acid has been determined to have the maximum effect on conditioned oxides. The concentration of nitric and hydrofluoric acid is not as important as the 10 to 1 ratio. The excess nitric acid prevents the generation and absorption of free hydrogen, Equation 7, by the oxidation reaction in Equation 8.

(7) 2Ti + 6HF = 2TiF3 + 3H2

(8) 3Ti + 4HN03 + 12HF = 3TiF4 + 4NO + 8H20

Experimental work is continuing to effect the complete removal of titanium scales without the use of acids. Molten salt conditioning followed by treatment in


chelated alkaline solutions continues to show promise.

Surf ace Removal

In addition to high reaction rates, salt baths are attractive because of the chemically clean surfaces obtainable. In titanium processing this condition prevents scrap losses resulting from subsequent acid treatments to remove contaminated surface. Even 0.002" (0.051 mm) alpha case removal in strong acid can develop poor surfaces, Figure 4, when pre-cleaning is incomplete.

The acid solutions, Table VI, used to effect removal of the oxygen contaminated layer are generally mixtures of hydrofluoric, nitric and acetic acids with wetting agents and inhibitors. These solutions do tend to level and remove imperfections from clean surfaces by selective attack on hills and valleys. However, even inhibited solutions will differentiate between scale and base metal creating a reverse effect as shown in Figure 4, emphasizing the necessity for chemically clean surfaces.

Experimental investigations continue in the area of electrolytic low acid etching. The advantages expected are more uniformity since metal removal is a function of current density. It is relatively independent of acid concentration. Also the high hydrogen levels experienced with strong acid solutions under certain conditions are lacking in the electrolytic processes.

Surface Treatments for Drawing

More recently the surface from molten oxidizing salt baths has been used in the production forming of titanium. The ~se of salt baths for this purpose originated during the early experiments with beta III fastener stock. It was found that the potassium titanate formed on the surface of the metal during processing provided an excellent base for drawing lubricants. The compound not only served the purpose of lubricant retention but actually furnished the pigmentation needed to prevent metal to metal contact. For maximum uniformity the annealed wire was cleaned in molten salt, and acid etched for alpha case removal. The cleaned wire was then recycled through the salt bath, water rinsed and redrawn, or shipped to the fastener manufacturer with the coating intact.

The titanium surface resulting from molten oxidizing salt treatment is shown in Figure 5. McDonnell Douglas


.006" 1--------;1--------1------~~

• 005" t--------1-------+------+-1 Ti 6Al-4V

.004"

.002"

NO .GAUGE LOSS .001" ~--,,C---..+--AFTER 160 MINUTES AT 140F

IN 10% H2S04 , 1% CuS04

MINUTES 60 120 180

Figure 3 Comparison of Nitric-Hydrofluoric and Sulfuric Acid Pickles with Respect to Titanium Gauge Loss.

Figure 4 Acid Etch Pattern Resulting from Improperly Cleaned Surfaces.

NEW SURFACE TREATMENTS FOR TITANIUM

Table IV Origin of Titanium Scales Cleaned in 400°F. (204°C) Salt Bath

Operation

Stress relieving or aging

Annealing or creep flattening

Solution treating

Hot working

Temperature Range

900°F - 1300°F. (482°C - 704°C)

1200°F - 1650°F (649°C - 899°C)

1400°F - 1B00°F (760°C - 982°C)

800°F - 2100°F (427°C - 1149°C

Table V Typical 400°F. (204°C) Molten Salt Descaling Cycle

Cycle

Condition in Molten Salt

Rinse

Acid Pickle

Rinse

Acid Pickle

Rinse

Time

3-5 minutes

1 minute

3 minutes

1 minute

1 minute

1 minute

Temperature

400°F - 425°F (204°C - 218°C)

Cold Water

35% H2S04 at 150°F (65°C)

Cold Water

10 parts HN03 + 1 part HF at 120°F (49°C)

Hot Water

Table VI Chemical Solution for Alpha Case Removal

20%

74%

6%

0.4 oz/100 gallons

Water

Nitric Acid

Hydrofluoric Acid

Wetting Agent (FC95 by 3M)

2509


is presently processing titanium sheet in this fashion to form curved sections of aircraft ductwork. The cleaned titanium is treated in molten salt, water rinsed and coated with Everlube T-50 prior to forming.

Production Systems for Titanium Processing

All of the surface treatments described can be suitably programmed for large scale production processing. Referring to Table V, the time cycles involved in molten salt descaling are sufficiently short to allow automated work handling systems. Figure 6 illustrates the cycling for high volume turbine blade cleaning. Work baskets travel from station to station by the Meca-matic hoist system. Dwell time in salt baths, acid and chemical treatment tanks is controlled by operator input. Systems such as this are a basic requirement where large numbers of titanium sections must be processed. The efficiency of the overall operation is still dependent upon the performance of each individual treatment.

High volume titanium cleaning for the F-14 aircraft is being done in the equipment shown in Figure 7. This salt pot is a part of a modern cleaning line at the Grumman Aircraft Engineering Corporation. Although the system is not automated, thorough chemical cleaning is obtained through the efficient use of molten salt conditioning. Required acid tanks and rinses complete the line which conditions very large titanium sections. Continual control of all process chemicals is part of the quality assurance program.

Surface Hardening by Salt Bath Treatment

Molten salt baths are also under consideration for their ability to affect titanium surfaces through conversion, plating, and diffusion processes. The intent of these treatments is to enhance the wear properties of titanium surfaces while retaining their excellent corrosion and strength characteristics.

The recently developed Tiduran process produces a true diffusion effect to a depth of 0.002" (0.051 mm), Figure 8. Optimum treating time is two hours at 1480°F. (804°C) in a cyanide based molten salt.

The photomicrograph shown in Figure 8 indicates that some type of controlled alpha case structure is being formed. The surface does, however, lack the extreme brittleness, non-uniformity and cracking tendency of an oxygen stabilized surface. Also the chemistry of the


Figure 5 Potassium Titanate Surface Prepared for Lubricant Application.

. ,,,,----p--_ -B-- Monorail

( Automatic hoist Load/Unload

I

I ~ Salt bath ~~:er Sulfuric \ .-. -. ------::_~:=fD JCJl

'---- -ir~~ : ~-:b bd

--BBBEJ& . Hot Cold Nitric-HF Nitric-HF Nitric \

n., - I

Cold

) ,__,/

[nWJ]--= ~:!ffilll!!illi~ MOLTEN SALT AUTOMATIC DESCALING SYSTEM

Figure 6 Flow Sheet of Automated Salt Bath Cleaning System.

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Figure 7

Figure 8

R. H. SHOEMAKER

Salt Bath Equipment for Processing Large Titanium Sections for F-14 Aircraft.

--~..-.• ..... -- ,,.•.: Tl DURAN· ·.TREATED

·Ti 6Al·4V

500x

Diffusion Effect From Two Hour Tiduran Treatment.


salt decomposition involving CN- and CNo- would suggest the presence of other alpha stabilizing interstitials such as carbon and nitrogen. This structure has been substantiated by chemical analysis as shown in Figure 9. It is interesting to note that the percentage of dissolved nitrogen is about equal to that present in nitrided ferrous materials.

The hardness traverse curves shown in Figure 10 verify the selection of a two hour treatment time. The 55 Re surface hardness obtained is a substantial improvement over untreated titanium. Following the two hour traverse to a point of parallelism with the abscissa, a measurable diffusion zone of 0.002" (0.051 mm) is indicated.

The expected improvement in wear resistance resulting from this hardness increase has been verified by extensive laboratory and field testing. Figure 11 was developed from a laboratory tribometer test performed in the metallurgical laboratory of an aircraft eng'ine manufacturer. The area included between the horizontal base line and the profilometer trace is a direct ni~asurement of the amount of wear developed during the test.

Any improvement in wear and gall resistance at the expense of fatigue properties is not tolerable in a material which claims as its largest consumer an aircraft engine manufacturer. Consequently close scrutiny on all types of fatigue testing equipment has been made of Tiduran treated titanium test parts. Initial investigations of the structures indicated some loss in fatigue strength resulting from the stress relieving effect of the 1480°F. (804°C) treatment. Glass beading following heat treatment restored the original machin~ ing stress pattern and produced acceptable high cycle fatigue values as shown in Figure 12. Substantial improvements in these results have been obtained more recently by long time (10 hours) over-aging at l060°F. (571°C) 'and glass beading following Tiduran treatment.

Many commercial applications of salt bath nitriding of titanium are still pending in this country. Aircraft engine manufacturers are continuing to examine the effect of Tiduran treatment plus dry-film lubricants on the fretting fatigue properties of turbine blades. Wear testing of Tiduran treated titanium ball valves in naval submarines has shown promise. Titanium valves for the chemical industry, Figure 13, and the automotive industry, Figure 14, are also excellent Tiduran prospects.


4.-------..,....---,,------------...,.4 Ti 6Al-4V,Tiduran-treated

~(SCALE AT RIGHT)

3 I f---=>--""""d--_ ___::::,,,,._-+=-b.L---+~---1.1 3

~ ~ ~ ~

.DOI"

.025mm DISTANCE FROM SURFACE

.002" .003" .004" .Imm

Figure 9 Elemental Distribution of O, C and N in Tiduran Diffusion Zone.

ROCKWELL 'C'

40r----:--;'1':~::1'-.....;:::::::~:::;;;;:::::jt=:::=:::i-

.001" DISTANCE FROM SURFACE

.002" .003"

Figure 10 Tiduran Hardness Patterns.

367 PLATE TREATED------------'1-------,.....-

~LBS

u UNTREATED

36 CYLINDER TREATEDt--.::::=======~::::::::::==-

CYLINDER +PLATEt--~--=======--18=!;,~~~ TREATED WEAR OF TEST CYLINDERS (IO-' ln2 )

Figure 11 Ti 6Al-4V Wear Tests.

NEW SURFACE TREATMENTS FOR TITANIUM

Ti 8Al-1Mo-1V

70F k=2

MACHINED+ TIDURAN STRESS RELIEVED

93 ksi

TIDURAN + GLASS BEAD BLAST

Figure 12 High Cycle (107) Fatigue Strength.

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Figure 13 Tiduran Treated Titanium Valve for the Chemical Industry.

Figure 14 Tiduran Treated Automotive Valve.

2516 R. H.SHOEMAKER

Summary

Surface treatments for titanium metal will always present challenges. Some of the most difficult can be found in ordinary cleaning operations. This is primarily due to the extreme activity of the metal under processing conditions.

Molten salt baths have the capability of producing chemically clean surfaces. Acid etching and alpha case removal operations are quite dependent on these results. Salt bath oxides also furnish excellent intermediate bases for drawing and forming operations.

The finished titanium surface properties can be improved by salt bath treatment. A molten salt nitriding process is available to produce wear resistant surfaces on titanium. Fatigue properties can be enhanced by glass beading following the Tiduran treatment. Future applications for this process and all surface treatments of titanium should increase as the metal's non-aerospace applications become more diversified.

new surface treatments for titanium · 2018. 4. 14. · solidified metal in the top of figure 2....

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