AC 33.15-1A material from the abnormally discolored wafer should not be used for premium quality applications.
9.4.2 Ingot Inspection.
9.4.2.1 All electrodes and final ingots should be inspected for color and surface condition and processed as detailed in 9.3.1.2, Electrode Cleaning/Descale/Crown. Ingots exhibiting abnormal surface condition (excessive oxidation) indicative of furnace coolant or air leaks, should not be released for premium grade applications, and additional material should not be processed in the affected furnace until the leak source has been identified and repaired.
9.4.2.2 Exposed pipe should not be permitted on final ingots. The location of internal pipe should be determined by an inspection technique or by process history to allow appropriate inspection and disposition of the pipeaffected material during or following the subsequent conversion/cropping process.
9.5 Equipment Maintenance and Cleaning.
TIG (Tungsten Inert Gas) welds, original or repair, are considered undesirable on equipment surfaces in contact with sponge, raw materials, or other in-process material.
9.5.1 Blending.
Equipment used for blending should be maintained in areas isolated from potential sources of contamination likely to cause HDIs or HIDs (e.g., carbide tools, welding operations, etc.). All processing equipment should be regularly cleaned and inspected to established procedures. Procedures should specify methods of cleaning and frequency.
9.5.2 Compaction.
Compaction equipment (e.g., tooling, handling, etc.) should be maintained in a clean contamination-free environment. Tools should be regularly cleaned and inspected to established procedures.
9.5.3 Electrode Fabrication.
Procedures should be established for thoroughly cleaning and inspecting the interior of the welding chamber, fixtures, welding equipment, etc., to assure all surfaces are free of all contaminants known to cause HDIs and HIDs. Procedures should be specific to what is cleaned, how, at what frequency, and how it is inspected. At a minimum, chambers should be visually inspected before the electrode is welded.
9.5.4 VAR Furnaces.
Furnace Cleaning.
Procedures should be established for thoroughly cleaning and inspecting the interiors of all melt furnaces. Base plates, stubs, covers and seals should be free of all contaminants known to cause HIDs and HDIs.
AC 33.15-1A Procedures should specify cleaning frequency, method of cleaning, and methods of inspection.
9.5.4.1 Crucible Cleaning.
Procedures should be established to clean and inspect the inside surfaces of crucibles before each melt. Examples include tumbling with some hard medium, wire brushing, water jets, etc. The system chosen should be capable of removing any surface contaminant which could cause an HID or HDI. Each crucible should be inspected to a written procedure prior to release for assembly into a furnace melt sequence. After cleaning and inspection, crucibles should be stored in a manner to minimize possible contamination, and a final visual inspection is prudent prior to sealing the furnace for melting.
9.5.4.2 Leak-Up Rate Checks.
After the furnace assembly is sealed, but prior to melting, the system should be checked to established procedures to ensure that the requirements for leak-up rate will be met. Results should be recorded.
9.5.5 CHM Furnaces.
9.5.5.1 Furnace Cleaning.
Procedures should be established by the CHM source for thoroughly cleaning and inspecting the interior of the melt furnace, the hearth, barriers, spray shields, and all associated equipment used on the interior of the furnace prior to use, to assure that they are free of all contaminants known to cause HDIs and HIDs. Procedures should be specific as to what is cleaned, how it is cleaned, and the frequency of cleaning. At a minimum, cleaning should be done when the melt chamber is exposed to air, between alloy type or alloy quality changes, and after a contaminated melt.
9.5.5.2 Ingot Mold Cleaning.
Procedures should be established to ensure that the mold and base/puller assembly is thoroughly cleaned and inspected prior to each melt.
9.5.5.3 Feed Mechanism.
Procedures should be established to ensure that all raw material feed mechanisms are cleaned and inspected to preclude contamination prior to melting.
9.5.5.4 Skull Maintenance.
Procedures should be established to assure that skulls that are going to be reused are cleaned and not contaminated during removal, storage or reassembly. Only skulls of like alloy and quality from a non-contaminated melt should be reused. The skulls should have traceability to previous 24
AC 33.15-1A melts and should be inspected prior to reuse to assure that no loose contaminants are present.
9.5.6 Skull Furnaces.
9.5.6.1 Furnace Cleaning.
The skull source should establish procedures for thoroughly cleaning and inspecting the interior of the melt furnace, the hearth casting chute, protective chamber cover, electrode holder furnace roof, mold and any other associated equipment used on the interior of the furnace prior to use, to assure that they are free of all contaminants known to cause HDIs and HIDs. Procedures should specify what is cleaned, how it is cleaned, and the frequency of cleaning. Cleaning should be performed between every heat.
9.5.6.2 Skull Maintenance.
Care should be used, and procedures should be established to assure that skulls to be used as consumable electrodes for a following skull heat are cleaned and not contaminated during removal or storage.
9.5.6.3 Leak-Up Rate Checks.
After the furnace assembly is sealed, but before melting, the system should be checked to establish procedures to ensure that requirements for leak-up rate can be met. Results should be recorded.
9.6 Housekeeping.
It is important to ensure a high standard of housekeeping in all areas associated with the premium quality titanium manufacturing processes.
9.6.1 Raw Materials and Melting Facilities Process Flow.
The layout of raw materials areas and melt shops should be designed to minimize the potential cross-contamination of the in-process premium quality material with inclusion-forming material generated from other processes related to the production facilities.
9.6.2 Raw Materials and Melting Facilities Cleanliness.
The raw materials areas and melt shops should be maintained with a high standard of cleanliness to prevent cross-contamination of in-process premium quality material with inclusion-forming material associated with poor housekeeping.
9.6.3 Internal and Customer Product Hazard Reviews.
Both internal and customer product hazard reviews should be conducted to ensure all employees remain vigilant against the possibility that a reduction in housekeeping standards might lead to an increase in the occurrence of melt-related inclusions and the associated risks with the premium quality titanium material in aircraft engines.
AC 33.15-1A 10 BILLET CONVERSION, DISK FORGING, AND NDT.
10.1 Control of Billet Conversion Process.
Specific detailed procedures should exist for the conversion of ingot to billet and bar products. These include but are not necessarily limited to:
10.1.1 Billet Conversion.
Ingot to billet conversion should be performed using controlled, consistent, and detailed procedures.
10.1.1.1 Forge Parameter Control.
Documented procedures to control ingot and billet products should be developed and used. These procedures include, but are not limited to:
• forge furnace temperatures/atmospheres,
• soak times,
• amount of draft,
• reduction schedules/sequences,
• quench time, media, and methods,
• die type,
• single or double end procedures,
• off-die procedures, and
• traceability of material during loading and withdrawal into and from pre-heat furnace.
10.1.1.2 Forge Practice.
Forge practices which preclude the formation of strain induced porosity (SIP) or clean voids and contamination associated with material cutting should be developed. In addition to the potential of creating SIP during billet forging, it should be noted that billet forging may also act to seal residual ingot microporosity.
10.1.1.3 Cropping.
Any ingot conversion should incorporate a crop of the ingot extremities to remove undesirable end material. Limits for crop lengths from the ingot top and bottom should be established to account for the hot topping procedures and bottom charge used for the final melting.
10.1.1.4 MTR – Billet Forge Processing.
Sub-beta transus billet forge practices for alpha/beta processed and heattreated alloys should be developed to minimize MTR formation, size, 26
AC 33.15-1A frequency, and intensity by controlling the following processing parameters below (refer to the first bullet of appendix A, paragraph 5.
1. Beta Recrystallization. Initial beta work combined with a subsequent alpha/beta pre-strain and a beta recrystallization step will set-up a smaller recrystallized prior beta grain size. A smaller prior beta grain size will minimize the size of the subsequent MTRs. A minimum deformation ratio is necessary in this temperature domain to break down the solidification structure and obtain a uniform, relatively small prior beta grain size. Minimizing the time at temperature for the beta recrystallization step results in a smaller recrystallized prior beta grain size.
2. Post-beta-recrystallization Cooling. Cooling after beta recrystallization results in the formation of alpha colonies; these colonies are the origin of MTRs. A faster cooling rate minimizes the alpha colony size and the thickness of the lamellae within the colonies. Smaller colonies and thinner alpha plates increase the ability of MTRs to be broken-up during subsequent billet and component forging processing. Metal temperature, transfer time and temperature of the water tank prior to quenching along with section size should be controlled to maximize the cooling rate.
3. Final Alpha/beta Work. The degree of sub-beta-transus work imposed on the billet following the last quench from the beta phase field is important in reducing MTRs through the breaking up of colonies.
Increased levels of final alpha/beta work generally lead to a reduction in MTR size and a decrease in MTR intensity.
4. Temperature of Final Alpha/beta Work. A higher final alpha/beta work temperature decreases the MTR intensity. The temperature for initial and reheating sequences should be controlled to mitigate variations in bar length, reduction per pass, and bites per pass with associated surface chilling and adiabatic heating.
5. Reheat Time for Final Alpha/beta Work. Longer reheat times between the final alpha/beta forge operations (provided the prior forge operation imparted a sufficient level of strain) may decrease the MTR intensity. This should be balanced with the need to retain a refined, spheroidized primary alpha structure.
6. Type of Press Work. Billet forging work should be conducted using a press that is capable of imparting strain through the entire billet crosssection enabling refinement of the microstructure, including MTRs, through to the center of the billet.
7. Redundant Work for MTR Break-up. A combination of upset and draw should be conducted to improve work in the billet material to reduce MTRs. Working in multiple directions imparts strain on all MTR orientations and subsequently reduces the MTR size and 27
AC 33.15-1A intensity. In addition, intermediate billet cross-section shape should be considered to ensure uniformity of microstructure.
8. Billet End Effects. It is known that forging and conversion of cast ingots to billets results in strain variation between the near steady-state region and ingot/billet ends. The variation of strain within the ends of ingots and billets will influence the evolution of MTRs. Strain nonuniformity during initial ingot upsetting and processing can result in specific zones of undesirable MTR conditions. Any ingot conversion should incorporate a crop of the ingot extremities to remove undesirable end material. Limits for crop lengths from the ingot top and bottom should be established to account for initial upset strain variations, the hot topping procedures and bottom charge used for the final melting. It is also believed that the level and the intensity of MTRs may be increased at the end of individual billets due to the potential for less uniform deformation conditions during billet conversion. When cropping billet ends, the potential for increased MTR size and intensity should be considered.
10.1.1.5 There are many attributes of MTRs that can be quantified as noted in appendix A, paragraph 5(1), including the following:
• size,
• shape,
• density,
• intensity,
• frequency,
• orientation,
• orientation spread,
• neighboring region characteristics, and
• volume fraction.
10.1.1.6 Records.
Reduction sizes, sequences, and temperature records should be maintained.
10.1.1.7 Traceability.
Traceability of billet to exact ingot location should be maintained. The identity and orientation of the billet within an ingot is important for material-related problem-solving and risk reduction.
AC 33.15-1A
10.2 Control of Disk/Component Manufacture.
10.2.1 Forging/Heat Treatment Process.
Specific detailed procedures should exist for the production of finished rotating components from billet product. These should include but not necessarily be limited to:
10.2.1.1 Forging Controls.
Controlled, consistent, and detailed procedures should be established for the control of the forging press/hammer processing.
10.2.1.2 Thermal Controls.
Documented procedures for control of forge/soaking furnace temperature and times should be established.
10.2.1.3 Heat Treatment Controls.
Documented procedures should be established for the control of heat treatment furnaces, atmospheres, times, and temperatures.
10.2.1.4 Quenching.
Quench practice and cooling rates should be controlled to ensure freedom from quench related cracking.
10.2.1.5 Records.
Forging and heat treatment records should be maintained, including traceability of forgings to exact billet location.
10.2.2 MTR – Forge Processing and Heat Treatment.
Forging and heat treatment parameters for alpha/beta processed and heat-treated alloys should be developed to minimize MTR size, frequency, and intensity by controlling the following processing parameters:
10.2.2.1 MTR levels in billet used to manufacture parts.
Billet that has been processed to minimize MTR size, frequency and intensity is less susceptible to retaining MTRs in forgings.
10.2.2.2 Alpha/beta versus beta forging practice.
Alpha/beta forged parts may retain MTRs from the billet processing, whereas beta-forged parts erase MTRs from the billet processing. Beta forging followed by cooling through the beta transus can lead to the generation of colonies that may adversely impact CDF capability of the material.
10.2.2.3 Forge strain, strain rate and temperature.
A sufficient level of strain and associated strain rate and temperature will reduce remnant MTRs carried over from billet for alpha/beta forged parts.
Strain path should also be considered relative to modification of location- 29
AC 33.15-1A specific MTR characteristics within the final part. Note that high levels of strain along a consistent strain path may increase the macrotexture within the forging and that alpha/beta forge practice should avoid excessive adiabatic heating that may locally reduce primary alpha volume fraction in the overheated region leading to the formation of a beta fleck. Note, increased levels of strain at higher rates and lower temperatures may lead to SIP formation.
10.2.2.4 Number of re-heats.
Time at temperature associated with multiple re-heats should be minimized to retain a finer primary alpha grain size.
10.2.2.5 Heat treatment temperature for alpha/beta processed alloys.
Selecting a heat treatment temperature closer to the beta transus for alpha/beta-processed alloys will reduce the primary alpha volume fraction and size, resulting in a reduction in MTR size and intensity. Note, if the selected temperature is too close to the beta transus, areas of remaining local ingot segregation of beta stabilizing elements may result in the formation of beta fleck.
10.2.2.6 Post-solution cooling rate for alpha/beta-processed alloys.
Increasing the post-solution cooling rate for alpha/beta-processed alloys will reduce the primary alpha volume fraction and grain size and will reduce the secondary alpha plate thickness as well as increase the likelihood that the secondary alpha plate morphology will change from a colony to a basketweave morphology.
10.2.2.7 Post-forge cooling rate for beta-processed alloys.
The post-forge cooling rate should be maximized to ensure refined alpha colonies and thinner alpha lamellae.
10.2.2.8 Post-solution cooling rate for beta-heat treated alloys.
The post-solution cooling rate should be maximized to ensure refined alpha colonies and basketweave morphology with thinner alpha lamellae.
10.2.3 MTR/Microstructural Features.
The above forge processing and heat treatment parameters will impact the following MTR and other microstructural features that may influence the component CDF life:
• MTR size,
• MTR shape,
• MTR density,
• MTR intensity,
• MTR frequency, 30
AC 33.15-1A
• MTR orientation,
• MTR orientation spread,
• MTR volume fraction,
• MTR neighboring region characteristics,
• colony size for beta processed alloys,
• primary alpha grain volume fraction for alpha/beta processed alloys, and
• primary alpha grain size for alpha/beta processed alloys.
10.2.4 Bulk Residual Stress.
Manufactured-in residual stresses are additive to application stress which make them important relative to component life. Bulk residual stress present after heat treatment will be a function of:
• Size and geometry of heat-treated forging,
• Heat treatment solution temperature,
• Cooling method and transfer time, where applicable, post-solution heat treatment,
• Aging/stress relief temperature and time, and
• Alloy class. Creep-resistant near-alpha alloys have the greatest susceptibility to retaining residual stress with beta alloys having the least susceptibility to retaining residual stress.
10.2.5 MTR – Alloy Class.
It should be noted that different alloy classes are believed to have differing tendencies to form MTRs, and so processes will need to be adjusted between alloy classes in order to minimize MTR formation, size, volume fraction and maximize MTR crystal orientation spread. The alloy classes believed to be susceptible to CDF are:
• Near-alpha alloys such as Ti-6242, Ti-811, Alloy 834, Alloy 685, Alloy 829.
• Alpha plus beta alloys such as Ti-64.
Alloys Ti-17 and Ti-6246 are considered to have low susceptibility to CDF.
10.3 Non-Destructive Testing and Criteria.
10.3.1 Background.
10.3.1.1 Non-Destructive Testing (NDT) of all material used for turbine engine rotating hardware is critical. Common NDT techniques are ultrasonic testing (UT), eddy current inspection (ECI), fluorescent penetrant inspection (FPI), and macroetch. Each of these methods can support the identification of material anomalies such as melt-related features (e.g.
inclusions or segregation) and conversion issues (e.g. MTRs). UT is useful 31
AC 33.15-1A for volumetric inspection to ensure the quality of titanium billet, bar, and final forgings, while ECI, FPI and macroetch are surface inspection methods.
10.3.1.2 Ultrasonic inspection relies on the transmission of elastic waves within the material. Discontinuities in the material with large differences in density or elastic properties, like cracks, voids, or inclusions, will result in discrete backscattered reflections of appreciable amplitude indicating the presence of an undesirable defect. These signals are relatively straightforward to detect in material with fine, uniform grain size without any MTRs (or with MTRs much smaller than the incidence wavelength), but the presence of MTRs diminishes the inspectability of bar, billet, and finished components.
10.3.1.3 Two distinct phenomena are observed depending on the size, shape, and orientation of the major axis of the MTR relative to the incident ultrasound. Significant attenuation, leading to decreased signal, occurs when the wavelength is much smaller than the MTR size in the direction of wave propagation, whereas significant backscattering, leading to enhanced noise, occurs when the wavelength is of the order of the MTR size. These phenomena are a result of the elastic anisotropy of the alpha phase. Enhanced attenuation occurs as a result of variations in the local wave velocity inside different MTRs that distort the coherent incident wavefront.
Additionally, the shape of the MTRs resulting from metal flow during the forging process may cause the wave to reflect, refract, or otherwise travel in a path away from the source. The additional backscattering occurs because of the elastic (impedance) mismatch at MTR boundaries. Either of these phenomena decrease the signal-to-noise ratio making it more difficult to detect real flaws. In some instances, when the MTR size is large relative to the wavelength and depending on the attributes of the specific MTR and its neighborhood, geometric scattering can occur resulting in a single, large, discrete reflection to be received. In the same way that MTRs can impede the inspectability of titanium alloys, the presence of MTRs can also be qualitatively inferred from ultrasonic inspection. MTRs can produce both an increased average noise response, quantified by the average root-mean-square of the backscattered signal content, as well as single high amplitude reflections. Depending on size and aspect ratio relative to the incident sound wave, MTRs can also cause a measurable increase in attenuation or backscattering, and hence interrogation in multiple directions can be useful in determining preferred MTR orientation. The use of these effects requires correlation of the ultrasonic parameters with levels of MTR severity, along with consideration of interactions between sound wavelength and microstructure.
AC 33.15-1A
10.3.1.4 Finally, the presence of macrotexture may also be inferred from the measurement of ultrasonic wave velocity. Because wave speed is proportional to elastic modulus, the velocity is highest along the C-axis of the unit cell, and slowest perpendicular to it.
10.3.2 NDT.
To obtain further confidence that titanium rotating parts are free from potentially detrimental anomalies, NDT should be performed at appropriate stages throughout billet/bar and component manufacture. NDT requirements will typically include but not necessarily be limited to the following:
10.3.2.1 Billet/Bar.
• Ultrasonic.
Immersion ultrasonic testing of billets that are less than or equal to 10 inches in diameter should be performed using a system with demonstrated inspection sensitivity equivalent to, or better than, a #2 flat bottom hole (FBH) (0.031 in.) at all billet depths. The UT inspection should be performed in accordance with SAE Document AMS 2628 sections 3 and 4 or an equivalent Phased Array accepted procedure. Immersion ultrasonic testing of billets that are greater than 10 inches in diameter should be performed using a system with demonstrated inspection sensitivity equivalent to, or better than, a #3 FBH at all billet depths. The UT inspection should be performed in accordance with SAE Document AMS 2628 sections 3 and 4 or an equivalent Phased Array accepted procedure.
• Macroetch.
Macroetch inspection of billet/bar ends adjacent to ultrasonic crops should be performed to assure freedom of the remaining material from segregation or other anomalies.
10.3.2.2 Forging.
Ultrasonic.
Immersion ultrasonic testing of forgings should be performed to a sensitivity consistent with the lifing analysis for the component. The inspection should be applied as needed to achieve the required sensitivity, noting that specific regions of a component may require enhanced inspection sensitivity capabilities consistent with part requirements.
• Macroetch.
Sonic shape or any other intermediate machined forging shape may be macroetched for examination for the presence of surface-connected segregation or other anomalies.
AC 33.15-1A
10.3.2.3 Finish Machined Component.
• Etch.
Finish machined, or near-finish machined components should have their surfaces blue etch anodized and inspected to assure freedom from segregates or other anomalies, including abusive machining. Alternate suitable macroetch procedures, although not preferred, may be utilized when agreed with the PAH. Performing an etch as close to the final shape as possible is desired to validate no segregates intersect the finish machined component surface.
• Fluorescent Penetrant Inspection.
Finish machined components should be fluorescent penetrant inspected.
10.3.3 Ultrasonic Testing (UT) Criteria.
The criteria for UT of billet/bar and forgings should consider the operational and control requirements that materials/ultrasonic inspection systems must achieve. The criteria should include but not necessarily be limited to the following:
10.3.3.1 System Variability.
Materials systems from billet/bar through forging, heat treatment, and machining should be engineered such that materials-generated ultrasonic noise or boundary (surface finish) ultrasound transmission characteristics do not interfere with the detectability of potentially detrimental anomalies.
Periodic system reliability/repeatability studies should be conducted to assure instrumentation and standards are functioning properly and within calibration.
10.3.3.2 Test Blocks.
Test blocks used to verify operation of the ultrasonic inspection system should be representative of the billet/bar or forging being inspected, including MTRs and grain flow to the extent possible, and should contain appropriate ultrasonic reference standards (flat bottom holes or side drilled holes). The ultrasonic reference standards should be correlated with potentially detrimental anomalies.
10.3.4 Statistical Assessment.
Statistical assessment of factors responsible for material/ultrasonic system variability should be conducted.
10.3.5 Acceptance Limit.
The acceptance limit should be set appropriately above the noise level to minimize false calls while maintaining detection sensitivity consistent with design requirements.
10.3.6 Records.