The use condition of a part refers to its position on the aircraft or engine, its importance, working conditions, ease of assembly and disassembly, and the complexity of the part. The working conditions of the part also include factors like force magnitude, vibration, operating temperature, and corrosion level. Starting from the basic principle that forgings serve the parts and meet the conditions of their use, the technical requirements for forgings should include two aspects: the shape, size, and surface condition of the forging, and the structure and mechanical performance of the forging.
To meet the use requirements of forgings, the key is to correctly select the raw materials for the forgings and strictly control the production process and quality of the raw materials. Then, by rationally formulating the production process for the forgings, effective quality control can be implemented.
The material selection for forgings is usually determined by product design and specified on the part drawing. In addition to the basic properties of the material, such as yield strength, tensile strength, plasticity, and fracture toughness, for aircraft, more meaningful parameters are the specific strength and stiffness of the material. Additionally, the physical properties, processability (forgeability, hardenability, machinability, weldability, etc.), and economy of the material must also be considered.
1. Technical Requirements for Forging Raw Materials
Choosing the most reliable raw materials is a prerequisite for ensuring the quality of forgings. The main factors determining the quality of raw materials are the melting, casting, and semi-finished product processing of the material. The technical requirements for raw materials used for aerospace forgings can be summarized as follows:
- Chemical Composition: The content of alloy elements, harmful impurities, gases, and residual elements in the material should meet the technical standards and relevant technical conditions or technical agreements for aerospace raw materials. Efforts should be made to control the content of harmful elements, gases, and residual elements in the material. The uniformity of the distribution of alloy elements must meet certain requirements.
- Melting Process: Ultra-high strength steels, titanium alloys, and superalloys are produced using a vacuum self-consumable remelting process. Titanium alloys and superalloys require at least two vacuum self-consumable remelts. Alloy structural steels, stainless and heat-resistant steels are produced using electric arc furnaces, combined with electro slag remelting or other superior melting methods. Aluminum alloys are typically melted using flame furnaces, resistance furnaces, and induction furnaces. High-quality aluminum alloys also require a series of process measures to strictly control impurity content and diversify the heat treatment state of the material.
- Material Types, Surface Quality, and Dimensional Tolerances: Depending on the production process and quality requirements of the forgings, material types, and specifications include ingots, bars (rolled, forged, extruded), billets, flat materials, cakes (rings), etc. When forgings have strict streamlined distribution requirements, care should be taken to select the flow direction of the raw materials to coordinate with the specified streamlined distribution of the forgings. Surface defects in raw materials, such as cracks, folds, scars, and heavy skin, can lead to defects on the surface of forgings, so they should be limited. Dimensional tolerances of raw materials significantly impact the precision forming of forgings, so clear requirements should be set.
- Forging Ratio of the Material: It’s essential to ensure that the material undergoes a sufficient degree of deformation. The forging ratio should be specified within an appropriate range to ensure adequate deformation of the material, reducing or eliminating the casting structure in the material. For large aerospace forgings, the forging ratio of raw materials is generally required to be greater than 6-8.
- Mechanical Properties: The mechanical properties of raw materials include mechanical properties at room temperature and high temperatures, such as strength indices, plasticity indices, impact toughness, hardness, fracture toughness, endurance strength, creep limit, fatigue properties, and stress-corrosion resistance. Depending on the forging and its application, these properties should be specified separately and clearly stated in the technical requirements for raw materials. Some mechanical properties of large-spec raw materials may be slightly lower, which should be considered during material selection.
- High-magnification Structure: This refers to the requirements for the microstructure, grain size (for steel), and purity of the material in its final heat-treated state. The structure of the material has a decisive impact on its properties. Abnormal structures in the raw material, such as excessive ferrite phase in austenitic and martensitic stainless steels, eutectic compound phases in other steels, and aluminum and magnesium alloys, continuous coarse β grain boundaries in titanium alloy structures, low melting point phases in superalloys, carbide segregation, banding structures, and other microscopic structural defects, as well as excessively coarse intrinsic grain size in steel and low purity, not only seriously affect the properties of forgings but also increase the defect rate of forgings. Therefore, clear requirements should be set for the high-magnification structure of raw materials and should be specified in the relevant technical standards.
- Low-magnification Structure: This is used to inspect and limit various low-magnification metallurgical defects in raw materials. Metallurgical defects in raw materials, such as white spots, white bands, shrinkage cavities, residual shrinkage, bubbles, voids, peeling, delamination, cracks, inclusions, slag inclusions, foreign metal inclusions, point deviation, pinholes, severe dendritic crystals, carbide aggregation (segregation), oxide films, and spots in titanium alloys, seriously affect the properties and enforceability of forgings and should be strictly limited. They should be handled according to relevant technical standards.
- Supply State of Raw Materials: This refers to the supply state of raw materials before they are used in forging production, including requirements for whether preliminary heat treatment is needed and whether the surface of the raw material needs to be peeled or processed to a certain roughness.
- Processability of the Material: This refers to the forgeability, hardenability, machinability, and weldability of the raw material. The enforceability of raw materials has a significant impact on forging formation and forging quality. Forgeability is often measured by two indicators: plasticity and deformation resistance. Hot upsetting tests are one way to express its forgeability. The hardenability, machinability, and weldability of the material are essential technological properties that forgings must have during their processing into parts. The processability requirements of the material should be clearly stated in the relevant technical standards.
- Special Inspection Item Provisions: Raw materials used for critical aerospace forgings, such as bars, cake (ring) billets, etc., should undergo ultrasonic flaw detection to prevent or avoid bringing metallurgical defects that are not detected in destructive tests into the forgings. The ultrasonic flaw detection method and standards, as well as the materials to be inspected, should all be specified in the relevant technical conditions.
- Re-test Provisions: If the test results of various inspection items of raw materials do not meet the requirements, the issue of re-testing them should be treated cautiously, based on the circumstances. For items that are determined to be non-compliant due to issues with sample processing (including heat treatment of samples), inaccurate test methods, or solid evidence showing that non-compliance was not caused by material defects (such as mechanical properties, chemical composition, supply state hardness, hot upsetting tests, etc.), re-testing is allowed. Metallurgical defects of raw materials at low magnification are generally not allowed to be re-tested (except as specified). If the raw material supply factory has the conditions to screen for metallurgical defects in raw materials using ultrasonic flaw detection or other effective methods, and to re-batch separately, the raw material receiving factory can consider acceptance and testing. However, under any circumstances, white spots and white bands in steel, once discovered, should be scrapped for the entire batch.
- Test Methods: The test methods used to check each technical requirement of raw materials should be clearly specified and should comply with the provisions of relevant national standards (GB), metallurgical standards (YB), aerospace standards (HB), or other authoritative test method standards.
- Other Technical Requirements: For materials used for critical forgings, head management or batch number management should be implemented. Based on technical justification, and according to the requirements of the requester, stricter technical requirements can be imposed on raw materials, such as narrowing the range of chemical composition, stricter low and high magnification structures, non-textured steel, higher mechanical properties, stricter surface quality (depth of the peeled layer, etc.), or other quality requirements.
2. Technical requirements for shape, size, and surface condition of forging
On the premise of reliable raw material quality, one of the tasks of forging is to obtain the desired shape, size, and surface condition of the forgings. This is to meet the requirements of parts processing and usage conditions and to comply with the stipulations of the part drawings.
The shape and size of the forgings should conform to the requirements of the part’s external contour and dimensions, and they should be as similar or close to them as possible. The primary basis for forging design is the part drawing. However, consideration should also be given to the characteristics of the forging process, the additional machining allowances, the process material, special allowances for Class I forging inspection, as well as elements like the structural elements of the forgings, surface form, and position tolerances, and dimensional tolerances. The forging drawing serves as the primary basis for production and acceptance of forgings.
The surface condition of the forging is an important criterion for evaluating the quality of the forging. The surface condition of the non-machined surface of the forging will also directly impact the performance of the part during use. The starting point for setting technical requirements for the surface condition of forgings is to ensure that the surface integrity of the forgings meets certain specified standards.
3. Technical requirements for the structure and mechanical properties of forgings
Ensuring that a part meets operational conditions primarily hinges upon the microstructure and mechanical properties of the forged item. The chief means of ensuring that the microstructure and mechanical properties of forgings are up to par is by correctly formulating and strictly controlling the forging process.
The microstructure of a forged item encompasses both low-magnification and high-magnification structures. The low-magnification structure is used to inspect the flow line distribution of the forging and to identify various metallurgical defects within it. The high-magnification structure includes the microstructure of the forging in its final heat-treated state, grain size, and purity, among other aspects.
The mechanical properties of the forging vary depending on its intended use. Mechanical properties at room temperature, such as strength, plasticity, impact toughness, hardness, fatigue strength, and fracture toughness, will differ depending on the material and the application of the forging. For components operating at high temperatures, there are additional requirements for high-temperature transient tensile properties, endurance performance, creep resistance, thermal fatigue performance, and thermal stability. The mechanical property indices of the forging should also be consistent with the mechanical property indices of the raw material. For large forgings made from large-size raw materials, the mechanical property indices can be appropriately reduced as stipulated. The mechanical property criteria for various types of forgings are defined by relevant technical standards. Additional test items required for pilot batch forgings should be specified in the dedicated technical conditions.
4. Other technical requirements for forgings
Other technical requirements for forgings include provisions for heat treatment, special inspection items, and testing methods.
Heat treatment of forgings varies based on the material. Forgings undergo either preparatory heat treatment or final heat treatment. Steel forgings are generally supplied in a preparatory heat-treated state, with the final heat treatment taking place during component processing. Forgings made of high-temperature alloys, titanium alloys, and aluminum alloys are mostly supplied in a final heat-treated state.
The protocols for preparatory and final heat treatments are indicated in the forging drawings or dedicated technical documents.
Special inspections refer to ultrasonic flaw detection and other non-destructive testing (NDT) of the forgings. Given the complex shape of forgings, ultrasonic testing is reserved for critical forgings, such as Class I and II forgings. The areas to be inspected should not only consider the parts under significant stress but also take into account the manufacturing characteristics and the weak links prone to defects. The specific areas for testing can be jointly decided upon by the design, metallurgy, and process departments, and marked on the product or forging drawings. Other NDT methods, like magnetic particle inspection and fluorescent penetrant inspection, should be conducted 100% on parts made from Class I and II forgings. For components made from Class III and IV forgings, the inspection frequency and sample size can be tailored as necessary, and for specific metallurgical flaws like the alpha case in titanium alloys, advanced techniques such as blue anodization are preferable, with documentation in relevant technical papers.
Testing methods for forgings: The methods to inspect various technical requirements of forgings must be clearly defined and stated in the technical standards of the forging.
5. Technical standards for forgings
The technical standards for forgings are formulated based on the technical requirements proposed for the forgings under specific operational conditions. In terms of the legal efficacy and scope of control, the technical standards for aerospace forgings in our country are categorized as follows:
(1) HB (Aerospace Industry Department Standards) and YB (Metallurgical Industry Department Standards, approved by the Aerospace Industry Department).
(2) Q/6s (Aerospace Industry Material Research Institute Standards).
(3) Corporate or factory enterprise standards.