U.S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: Manufacturing Process of Date: AC No: 33.15-1A Premium Quality Titanium Alloy Rotating Initiated By: AIR-625 Engine Components This advisory circular (AC) provides guidance and information for compliance with the provisions under title 14, Code of Federal Regulations (14 CFR), part 33 on the materials suitability and durability requirements of § 33.15, as applicable to the manufacture of titanium alloy high energy rotating parts of aircraft engines.
If you have suggestions for improving this AC, you may use the Advisory Circular Feedback Form at the end of this AC.
Daniel J. Elgas Aviation Safety Director, Policy and Standards Division, Aircraft Certification Service
AC 33.15-1A
TABLE OF CONTENTS Paragraph Page
1 Purpose1
2 Applicability1
3 Cancellation1
4 Related Material1
4.1 Title 14, Code of Federal Regulations (CFRs)1
4.2 FAA Advisory Circulars1
4.3 Industry Documents2
4.4 Additional References2
5 Definitions3
6 Background9
7 General10
8 Raw Materials and Storage11
8.1 Raw Material Requirements11
8.2 Bottom Charge Materials13
8.3 Raw Material Storage13
9 Consolidation and Melting13
9.1 Consolidation13
9.2 Primary Melting18
9.3 Intermediate and Final Melts21
9.4 Inspection22
9.5 Equipment Maintenance and Cleaning23
9.6 Housekeeping25
10 Billet Conversion, Disk Forging, and Non-Destructive Testing26
10.1 Control of Billet Conversion Process26
10.2 Control of Disk/Component Manufacture29
10.3 NDT and Criteria31
10.4 Actions Following Indication Detection35
Appendix A. Additional ReferencesA-1
Appendix B. Minimum Recommended Procedure for 3D Inclusion CharacterizationB-1
ii
AC 33.15-1A 1 PURPOSE.
This AC provides guidance and information for compliance with the provisions under Title 14, Code of Federal Regulations (14 CFR), part 33 on the materials suitability and durability requirements of § 33.15, as applicable to the manufacture of titanium alloy high energy rotating parts (life limited parts) of aircraft engines.
2 APPLICABILITY.
2.1 The guidance provided in this AC is for engine manufacturers, modifiers, foreign regulatory authorities, and Federal Aviation Administration (FAA) engine type certification engineers and their designees.
2.2 This is a guidance document. Its content is not legally binding in its own right and will not be relied upon by the Department as a separate basis for affirmative enforcement action or other administrative penalty. Conformity with the guidance document is voluntary only. Nonconformity will not affect rights and obligations under existing statutes and regulations.
2.3 This material does not change, create any additional, authorize changes in, or permit deviations from existing regulatory requirements.
3 CANCELLATION.
This AC cancels AC 33.15-1, Manufacturing Process of Premium Quality Titanium Alloy Rotating Engine Components, dated September 22, 1998.
4 RELATED MATERIAL.
The following materials are referenced in this document. You should use the latest approved edition which may be dated after the date of this AC as referenced materials are updated.
4.1 Title 14, Code of Federal Regulations (CFRs).
The following regulations are related to this AC. You can download the full text of these regulations from the Federal Register website at www.ecfr.gov.
• Section 33.15, Materials.
• Section 33.27, Turbine, compressor, fan, and turbosupercharger rotor overspeed.
• Section 33.64, Pressurized engine static parts.
• Section 33.70, Engine life-limited parts.
4.2 FAA Advisory Circulars.
The following ACs are related to the guidance in this AC. You should refer to the latest AC version for guidance, which is available on the DRS at https://drs.faa.gov.
AC 33.15-1A
• AC 21-43A, Production Under 14 CFR Part 21, Subparts F, G, K, and O, dated October 1, 2015.
• AC 21-9B, Manufacturers Reporting Failures, Malfunctions, or Defects, dated August 12, 2010.
• AC 33-2C, General Type Certification Guidelines for Turbine Engines, dated June 25, 2013.
• AC 33.15-2, Manufacturing Processes for Premium Quality Nickel Alloy for Engine Rotating Parts, dated March 22, 2017.
• AC 33.70-1, Guidance Material for Aircraft Engine Life-Limited Parts Requirements, dated May 31, 2017.
4.3 Industry Documents.
The following industry document is related to the guidance in this AC. You can obtain the document from SAE online at http://www.sae.org.
Terminology for Titanium Microstructures, SAE International Aerospace Standard, SAE AS1814, SAE International.
4.4 Additional References.
Appendix A contains additional references. These references provide further information and data used in the development of this AC. Specifically, the first reference of A5 was sponsored by the FAA, published in 2024, and provides a comprehensive review of the cold dwell fatigue (CDF) literature in addition to engine manufacturers field, rig and test specimen CDF experience. The critical topics covered in the appendix include:
• Uncontained engine failures related to melt anomalies,
• Improvements leading to reductions in melt anomaly rates,
• Improvements in melting controls,
• Uncontained engine failures related to CDF,
• Relationship between microtextured regions (MTRs) and CDF,
• MTR quantification,
• Titanium billet conversion processing,
• Relationship between hot working and MTR formation/minimization, and strain induced porosity,
• Alloy chemistry effects on MTRs and CDF,
• Temperature effects on CDF, and
• Relationship between MTRs and ultrasonic response.
AC 33.15-1A 5 DEFINITIONS.
The following definitions apply to this AC.
• Anomaly. Unusual material conditions or microstructural features that include voids, cracks, inclusions, segregation, abusive machining, etc.
• Bar. Converted material having a cross section less than or equal to 16 square inches (103 cm2), and a width less than five times the thickness.
• Beta Fleck. A region denuded in primary alpha in alpha plus beta processed titanium alloys.
• Beta Transus. The temperature at which, under equilibrium conditions, close-packed hexagonal alpha phase starts to form on cooling a titanium alloy from the high temperature body-centered cubic phase. Beta transus temperature depends on the titanium alloy composition with some elements increasing it (for example, aluminum and oxygen), and others decreasing it (for example, molybdenum and iron). In an ingot, there may be chemical segregation on a local scale and on a macroscale leading to differences in beta transus temperature within the ingot. In addition, there may be differences in chemistry associated with different types of input materials for a given alloy, resulting in a variation in the precise beta transus temperature and the rate of alpha formation as a function of temperature. As a result, it is important to have a practical beta transus temperature definition (determination) that also accounts for the rate of alpha formation as a function of temperature since processing temperatures for billet conversion, component forging, and post-forge heat treatments can often be tied to a temperature relative to the beta transus.
• Billet. Converted material having a constant round cross section greater than 16 square inches (103 cm2).
• Bottom Charge. The material placed in the vacuum arc remelting (VAR) crucible to protect the crucible during arc initiation.
• Cold Dwell Fatigue (CDF). The imposition of a hold-time at high stress, which may reduce component fatigue life at low temperatures, below approximately 400°F/200°C. The presence of (i) MTRs formed during alpha/beta billet conversion and modified during component alpha/beta forging and heat treatment or (ii) colonies formed during cooling after final component beta forging or beta heat treatment both of which may adversely impact component and test specimen fatigue lives associated with dwell cycles at maximum stress. Component and test specimen lives may be reduced by more than an order of magnitude depending on alloy class, applied stress, stress-state, stressed volume, remaining bulk residual tensile stress, temperature, hold-time and MTR or colony characteristics.
• Cold Hearth Melting (CHM). A process (i.e., electron beam melting (EBM) or plasma arc melting (PAM)) where the metal is melted and then maintained molten as it traverses a specified hearth distance, allowing inclusion elimination either by dissolution in the molten pool, or by density separation into the skull.
• Colony. An alpha colony (or simply colony) is a microstructural assemblage consisting of alternating lamellae of alpha and beta phase formed during cooling 3
AC 33.15-1A from above the beta transus. The alpha phase lamellae within each colony are morphologically aligned and share a common crystallographic orientation. The formation of the lamellar alpha is a result of precipitation with a specific orientation relationship to the parent beta phase and preferred growth along certain crystallographic directions. This means there is an inherited crystallographic orientation from the prior beta grain. Because dislocations can easily traverse alpha/beta interfaces in colony microstructures alpha colonies can act as a single unit with respect to CDF. As such, the size of these features should be controlled if a direct beta-transformed structure is utilized in a final product. Colony structures are also the precursor to MTRs in alpha/beta processed billet and forgings. The alpha lamellae are globularized into equiaxed alpha grains through hot-working and heat treatment. However, the crystallographic orientation of the grains may not change significantly. This could result in a cluster of similarly oriented alpha grains replacing what was previously an alpha colony. The impact of a colony on CDF in a final component may be greater than an identically sized MTR due to the common crystallographic orientation within the colony compared to the spread of crystallographic orientations within an MTR (MTR Intensity).
• Electrode. The consumable feedstock form for vacuum arc remelting (VAR) or the product of the primary EBM, PAM, or Skull that becomes the consumable electrode for the final VAR.
• Electrode Holder. The material that is joined to the top of the electrode, or the electrode stub, to hold the electrode and to provide the connection between the electrode and the VAR furnace electrical equipment.
• Electrode Marker. A titanium rod or other shape affixed to the consumable electrode, electrode stub, or electrode holder for VAR. The purpose of this marker is to provide visual reference for the electrode height position.
• Electrode Stub. The material that may be joined to the top of the electrode to provide the connection between the electrode and the electrode holder.
• Electron Beam Melting (EBM). The melting process using electron beams performed in a cold hearth that may be used in conjunction with VAR to meet the primary melting recommendations of this document.
• Established Procedure. A procedure that is subject to purchaser approval and is contained in a controlled document. It is also called a fixed process. It includes limits, controls, and applicable standards.
• Grain Flow. The general metal flow pattern within a billet, bar or forging as revealed by macroetching. This pattern is a result of the cumulative strain imparted on the material and the path over which that strain is imposed. The strain and strain path impact the size, shape, and internal structure of MTRs. The strain also impacts the specific macrotexture components that form during thermomechanical processing.
• Heat. The ingot and ingot product produced from the final VAR of a single consumable electrode (in the case of the multiple VAR process), or the ingot and 4
AC 33.15-1A ingot product produced from one VAR of a single EBM, PAM, or Skull consumable electrode.
• High Aluminum Defect (HAD). An aluminum-rich alpha stabilized region containing an abnormally large amount of aluminum which may extend across a large number of beta grains (also known as Type II anomalies).
• High Density Inclusion (HDI). A region with a high concentration of refractory elements, usually tungsten, molybdenum, or columbium, having a higher density than the matrix.
• High Interstitial Defect (HID). An interstitially stabilized alpha phase region of substantially higher hardness than surrounding material, arising from very high local nitrogen, oxygen, and carbon concentrations which increase the beta transus and produce the high hardness, often brittle, alpha phase. Also commonly called a Type I inclusion, low-density inclusion (LDI), or a hard alpha these features are often associated with voids and cracks. It is worth noting that Type I TiN inclusions may actually have a density higher than that of the titanium alloy.
• Hot Topping. Adjustments of process parameters during the latter stages of a melt process to minimize pipe, shrinkage porosity, and segregation.
• Inclusions. Particles of impurities or foreign materials that are present or introduced during any stage of alloy processing. Examples include, but are not necessarily limited to HADs, HDIs, and HIDs.
• Jet Engine Titanium Quality Committee (JETQC). JETQC was formed in 1990 under the auspices of the FAA, with initial membership including all the US and European Aircraft Engine Producers before subsequently growing to include a Japanese Aircraft engine producer. The purpose of JETQC was rapid dissemination to its members of titanium alloy melt related anomaly issues and data. JETQC also includes raw material suppliers that produce premium quality titanium alloy material for the JETQC engine producers.
• Macroetch. Chemical treatment of a metal surface to accentuate structural details and anomalies for visual observation.
• Macrotexture. The presence of grains with preferred crystallographic orientations within a material traditionally represented by a pole figure showing the intensity distribution of a specific crystallographic plane normal with respect to sample axes.
In contrast to microtexture, macrotexture does not imply anything about the spatial arrangement or clustering of the preferred orientations and occurs on a much larger length scale. The length scale of macrotexture may encompass the complete volume or cross-sectional area of a mill product or final component, or may vary across the cross section of a mill product or component. Because titanium alloys undergo an allotropic phase transformation during cooling, macrotexture is comprised of both deformation and transformation texture components. Deformation texture components are formed in the beta phase and alpha phase (if working at temperatures below the beta transus) during forming operations like rolling, extrusion, and forging. In this case, the change in crystal orientation may be due to 5
AC 33.15-1A dislocation motion or rigid body rotations. Crystals rotate in response to the imposed strain-path toward orientations that flow most easily and maximize the plastic work. Upon cooling, the beta phase undergoes an allotropic phase transformation and the alpha formed by this decomposition constitutes the transformation texture. The macrotexture may be further modified by solution heat treatment, which, depending on temperature, will change the fraction and strength of the deformation and transformation texture components present. Other restoration processes like recovery, recrystallization, and grain growth may further alter the specific orientations present.
• Microtexture Region (MTR). A localized region in an alpha/beta converted billet or an alpha/beta processed component forging where neighboring alpha grains and secondary alpha platelets have similar crystal orientations such that the feature may act as a larger structural unit during deformation or crack growth. MTR size, shape, density, intensity, frequency, orientation, orientation spread, neighboring region characteristics, volume fraction, and primary alpha volume fraction, are important parameters when characterizing MTR features. These features are known to impact CDF capabilities. MTRs are also sometimes referred to as “macrozones” or “primary alpha colonies.” MTR regions were identified once electron backscatter diffraction (EBSD) methods were developed which allowed correlation of specific microstructural features with their underlying crystallographic orientation.
• MTR Density. A measure of the area fraction of the alpha grains in an MTR that have a similar basal (0001) plane orientation. This factor includes contributions from both primary alpha and secondary alpha platelets.
• MTR Frequency. The number of MTR features within a volume of titanium.
• MTR Intensity. A measure of the range of variation of crystallographic orientations of all the alpha grains within an MTR that includes weighted contributions from the fraction of well-aligned alpha grains as well as the fraction of not well-aligned grains. The weighting of contributions are a function of alignment and area (or volume) fraction. Low MTR orientation spread with high MTR density would be referred to as an intense MTR. A material having intense MTRs does not imply anything regarding the macrotexture, i.e., the macrotexture could be weak or strong in a material having intense MTRs.
• MTR Neighboring Regions. The region surrounding MTR features. These could include MTR features of different character or regions of substantially randomly oriented alpha (average orientation regions). The neighborhood surrounding MTR features are critical to the potency of MTR’s impact on CDF.
• MTR Orientation. The dominant crystallographic orientation of the alpha grains within an MTR region. MTRs with the basal plane oriented nearly perpendicular to the principal stress direction are classified as “Hard MTRs.” Combinations of other orientations that enable easy basal or prismatic slip relative to a stress axis are classified as “Soft MTRs.” Both “hard” and “soft” MTRs influence CDF performance of titanium materials. The “soft” regions accumulate significant creep strain during the dwell portion of a cycle while the “hard” regions are potent crack 6
AC 33.15-1A nucleation sites that then offer very limited resistance to crack propagation. EBSD or similar methods are needed to assess crystallographic orientations in an MTR.
• MTR Orientation Spread. A measure of the average crystallographic misorientations of the similarly aligned alpha phase grains within an MTR.
• MTR Quantification. Methods to quantify MTR size, shape, density, intensity, frequency, orientation, orientation spread, neighboring region characteristics, volume fraction, and primary alpha volume fraction, are important parameters when characterizing MTR features. This characterization requires a means of discerning and measuring alpha grain orientations.
• MTR Shape. The general geometric morphology of a contiguous area of similar alpha grain orientations, which may be expressed as the aspect ratio of longest and shortest dimensions, or the ratio of the axes of an ellipse fit to the MTR in 2D crosssection.
• MTR Size. The size of a contiguous area of similar alpha grain orientations plus all other grains internal to this region often expressed as an area or equivalent diameter in 2D cross-section.
• MTR Volume Fraction. The summed volume (or area) of all MTR features, including all grains internal to the MTRs, divided by the total volume (or area) of titanium investigated.
• Non-Consumable Electrode Melting. A method of consolidating and melting titanium and its alloys using arc energy from an electrode designed to carry the melting current without being consumed. It may be a continuous or batch process and may, or may not, fully melt the input materials. It should not be considered a primary or a complete melt step, therefore the term “consolidation melt” is used.
• Non-contiguous MTR Size. The size of a region of similar alpha phase orientations that are not necessarily in direct contact with one another plus all other grains internal to this region.
• Plasma Arc Melting (PAM). A melting process using plasma torches performed in a cold hearth that may be used in conjunction with VAR to meet the primary melting recommendations of this document.
• Plate. Material converted by hot working and delivered as straight lengths of constant rectangular cross section, having a width greater than five times the thickness.
• Premium Quality (PQ). Material produced under special process and quality control requirements and used primarily for engine life-limited (critical) rotating parts. It should be noted that both within the engine industry and airframe industry, premium quality material is also produced for critical non-rotating parts. The requirements related to static premium quality material for these critical non-rotating parts may be different and are not specifically addressed in this document.
• Primary Alpha. Alpha grains that are a result of primary and secondary sub-beta transus working, and which are not transformed during final solution heat treatment.
AC 33.15-1A These grains can be spherical or elongated and can have a crystallographic orientation similar to neighboring alpha grains. The volume fraction of these grains influences mechanical properties, including CDF.
• Primary Alpha Volume Fraction. The summed volume (or area) of all primary alpha grains divided by the total volume (or area) of titanium investigated.
• Production Approval Holder (PAH). The holder of a production certificate (PC), parts manufacturer approval, or technical standard order authorization who controls the design and quality of a product or article(s). A person who has been issued a production approval by the FAA.
• Residual Stress. Stresses that are contained within the volume of a component.
These internal stresses can be created through thermomechanical processing, during quenching (e.g., quenching from solution heat treatment), joining, localized mechanical deformation (e.g., hole expansion), machining, or from surface processing (e.g. shot peening). Residual stresses are important to control. Final evaluation of life limited components should consider all manufacturing methods to which the material is exposed.
• Segregation. Volumes in the alloy product containing an abnormal content of alloying elements, which may also appear as zones of abnormal quantities of either the alpha or beta phases of titanium.
• Skull. The solidified metal in the hearth of the CHM process. During CHM, the skull is the solidified layer of metal adjacent to the hearth, not including the molten pool.
• Skull Melting. The skull melting process, used as the primary melt process, where material in the form of revert solids and chips along with virgin material is melted, under vacuum, in a water-cooled hearth prior to being tilted and poured into a mold.
Heat is provided for the melt by an arc initiated between the charge material in the hearth and a consumable electrode. The solidified remnant material remaining in the hearth after pouring is used as the consumable electrode for the subsequent melt.
• Sonic Shape. The sonic shape is the intermediate machined rectilinear or curvilinear forging shape at which the forging may be ultrasonically inspected. Shape and envelope relative to the finished component should be controlled.
• Sponge. The form of titanium or zirconium metal extracted or produced from the natural minerals containing FeTiO3 (Ilmenite), TiO2 (Titanium dioxide), or ZrO2 (Zirconium dioxide), respectively.
• Sponge Batch. The metal product of a single titanium or zirconium sponge reactor process.
• Sponge Lot. The mixed or blended sponge product containing portions of one or more sponge batches.
• Stressed Volume. The volume within a component which is at or above a specified level of stress. This is an important parameter relative to probabilistic lifing and 8
AC 33.15-1A volumetrically distributed life-limiting features, such as MTRs and melt-related anomalies.
• Supplier. Any person, including a firm, corporation, company, government entity, etc., that provides a product, article, or service, at any tier in the supply chain, that is used or consumed in the design or manufacture of, or installed on, a product or article.
• Turnings. Metal chips produced as a result of being removed by machining titanium alloy disk or billet material.
• Vacuum Arc Remelting (VAR). The VAR process used to produce ingots meeting the recommendations of this document, either as a final melt subsequent to CHM, skull melt or as multiple VAR product.
• Void (Clean). A cavity constituting a structural discontinuity related to solidification and conversion conditions of the ingot without any associated contamination.
Note: Other microstructure terms used in this document are consistent with those found in Terminology for Titanium Microstructures - Aerospace Standards (AS) 1814.
6 BACKGROUND.
6.1 The manufacture of titanium alloy forged rotating components can introduce component life limiting anomalies at all stages of material processing, from sponge processing, through melting, billet conversion, component forging/heat treatment, joining and machining. Appropriate, robust manufacturing process controls, in addition to inspection methods, can enable detection and elimination of these anomalies. Since optimum capability to detect these anomalies may vary according to their type and source in the manufacturing process, the manufacturing process should be established so that, at each stage, appropriate controls and inspections are in place to minimize the occurrence of and to maximize detection of such anomalies based on the best available technologies.
6.2 The traditional melting process for titanium alloys has been the VAR process. Triple VAR was previously recommended as a standard for critical rotating component use.
Improvements in VAR technology have resulted in a significant reduction in the occurrence of melt related inclusions since the mid-1980s. The newer CHM technology greatly reduces the risk of having HDIs surviving through the melting cycle. Cold hearth melting technology involves a CHM cycle (either PAM or EBM) followed by a final VAR cycle and has become another standard for premium quality titanium. A well-controlled CHM Plus VAR process has demonstrated the capability of reducing HID and HDI rates compared with a well-controlled triple VAR process. The presence of the hearth melt in a well-controlled CHM Plus VAR process allows a significantly longer residence time during CHM, enabling inclusions to dissolve and HDIs to density-separate compared with a well-controlled VAR process.
The manufacturing process used to produce titanium alloy forged rotating components may result in the presence of MTRs in the final part. Depending on the alloy, operating 9
AC 33.15-1A stress, stress-state, stressed volume, remaining bulk residual tensile stress, operating temperature, hold-time, etc., the presence of MTRs might result in reduced component life. The manufacturing process should be established so that, at each stage, appropriate controls are in place to minimize the severity of the MTRs. A significant amount of research has been performed related to MTRs and CDF (refer to appendix A, paragraph 5(1)), but much has yet to be learned. Initial directional guidance related to billet conversion and component forging is provided in Section 10, but it is recognized that this guidance may change as more research is conducted.
6.3 AC 21-43A, Production Under 14 CFR Part 21, Subparts F, G, K, and O, provides a means to obtain and maintain production approvals. However, these documents do not fully cover the manufacturing processes used in the manufacture of premium quality titanium alloy forged rotating components for type-certificated turbine engines. This AC, therefore, provides supplemental guidance for the establishment of manufacturing processes, in-process material and component inspections, and finished component inspections, for manufacture of premium quality titanium alloy forged rotating components, such as disks, spacers, hubs, shafts, spools and impellers.
6.4 AC 21-9B, Manufacturers Reporting Failures, Malfunctions, or Defects, details the requirements for reporting failures, malfunctions, and defects of products and articles.
These requirements apply to a holder of a type certificate (TC), amended type certificate, supplemental type certificate, parts manufacturer approval, or technical standard order authorization, or the licensee of a TC.
7 GENERAL.
7.1 Part 21 requires that PAHs must establish and maintain, as appropriate, manufacturing and process control and inspection systems which ensure that products used in type certificated engines conform to FAA-approved design data.
7.2 PAHs should also assure, through appropriate agreements with their suppliers, that effective process control documents are developed, identifying significant process control points, parameters and control limits. Measurement of these process control parameters should be made at the control points; consideration should be given to use the statistical process control.
7.3 A method of approval for changes to the process control documents, and the means of handling exceedances of any defined control limits, should be established between the PAH and the suppliers. Process operations where inclusions could be formed or entrapped, or segregation generated, or porosity induced, or MTRs formed or altered should be particularly detailed for control, monitoring, and detection.
7.4 Records of material and component inspection and disposition results, property test results, traceability of forged components to billet location, material heat, and to raw material ingredient lots making up the heat, as well as other records indicated in this 10