CS 25.621 Casting factors

1.Purpose. CS 25.621 is an additional rule/requirement for structural substantiation of cast parts and components. It is used in combination with a number of other paragraphs, and does not replace or negate compliance with any other paragraph of CS-25. The intent of this AMC is to provide general guidance on the use and background of "Casting Factors" as required by CS 25.621.

2.General Guidance For Use Of Casting Factors.

2.1 For the analysis or testing required by CS 25.307, the ultimate load level must include limit load multiplied by the required factor required by CS 25.619. The testing required in accordance with CS 25.621 may be used in showing compliance with CS 25.305 and CS 25.307. These factors need not be considered in the fatigue and damage tolerance evaluations required by CS 25.571.

2.2 The inspection methods prescribed by CS 25.621(c) and (d) for all production castings must be such that 100% of the castings are inspected by visual and liquid penetrant techniques, with total coverage of the surface of the casting. With regard to the required radiographic inspection, each production casting must be inspected by this technique or equivalent inspection methods; the inspection may be limited to the structurally significant internal areas and areas where defects are likely to occur.

2.3 With the establishment of consistent production, it is possible to reduce the inspection frequency of the non-visual inspections required by the rule for non-critical castings, with the acceptance of the Agency. This is usually accomplished by an accepted quality control procedure incorporating a sampling plan. (Refer to CS 25.621(d)(5).)

2.4 The static test specimen(s) should be selected on the basis of the foundry quality control inspections, in conjunction with those inspections prescribed in CS 25.621(c) and (d). An attempt should be made to select the worst casting(s) from the first batch produced to the production standard.

2.5 If applicable, the effects on material properties due to weld rework should be addressed. The extent and scope of weld rework should be detailed in the manufacturing specifications as well as on the design drawings.

3.Background.

3.1 Regulatory Background. CS 25.621 (“Casting factors”) requires classification of structural castings as either “critical” or “non-critical.” Depending on classification, the requirement specifies the accomplishment of certain inspections and tests, and the application of special factors of safety for ultimate strength and deformation.

3.2 Application of Special Factors of Safety. The application of factors of safety applied to castings is based on the fact that the casting process can be inconsistent. Casting is a method of forming an object by pouring molten metal into a mould, allowing the material to solidify inside the mould, and removing it when solidification is complete. Castings are subject to variability in mechanical properties due to this casting process, which can result in imperfections, such as voids, within the cast part. Using certain inspection techniques, for example radiographic (X-ray), it is possible to detect such imperfections above a minimum detectable size, but accurate detection depends on the dimensions of the part, the inspection equipment used, and the skill of the inspector.

3.2.1 CS 25.619 (“Special factors”) includes a requirement to apply a special factor to the factor of safety prescribed in CS 25.303 for each part of the aeroplane structure whose strength is subject to appreciable variability because of uncertainties in the manufacturing processes or inspection methods. Since the mechanical properties of a casting depend on the casting design, the design values established under CS 25.613 (“Material strength properties and material design values”) for one casting might not be applicable to another casting made to the same specification. Thus, casting factors have been necessary for castings produced by normal techniques and methodologies to ensure the structural integrity of castings in light of these uncertainties.

3.2.2 Another approach is to reduce the uncertainties in the casting manufacturing process by use of a “premium casting process” (discussed in AMC 25.621(c)(1)), which provides a means of using a casting factor of 1.0. CS 25.621 (“Casting factors”) does permit the use of a casting factor of 1.0 for critical castings, provided that: the manufacturer has established tight controls for the casting process, inspection, and testing; and the material strength properties of the casting have no more variability than equivalent wrought alloys.

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Examples of castings that may be critical are: structural attachment fittings; parts of flight control systems; control surface hinges and balance weight attachments; seat, berth, safety belt and fuel and oil tank supports and attachments; pressurised doors; and cabin pressure valves.

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1.Purpose. This AMC details an acceptable means, but not the only means, for compliance with CS 25.621 for using a casting factor of 1.0 or greater for “critical” castings used in structural applications. A premium casting process is capable of producing castings with predictable properties, thus allowing a casting factor of 1.0 to be used for these components. Three major steps, required by CS 25.621(c)(1)(i), are essential in characterising a premium casting process: qualification of the process, proof of the product, and monitoring of the process.

2.Definitions. For the purposes of this AMC, the following definitions apply:

2.1 Premium Casting Process: a casting process that produces castings characterised by a high quality and reliability

2.2 Prolongation: an integrally cast test bar or test coupon.

2.3 Test Casting: a casting produced specifically for the purpose of qualifying the casting process.

3.General. The objective of a premium casting process is to consistently produce castings with high quality and reliability. To this end, the casting process is one that is capable of consistently producing castings that include the following characteristics: Good dimensional tolerance Minimal distortion Good surface finish No cracks No cold shuts No laps Minimal shrinkage cavities No harmful entrapped oxide films Minimal porosity A high level of metallurgical cleanness Good microstructural characteristics Minimal residual internal stress Consistent mechanical properties The majority of these characteristics can be detected, evaluated, and quantified by standard non-destructive testing methods, or from destructive methods on prolongation or casting cut-up tests. However, a number of them cannot. Thus, to ensure an acceptable quality of product, the significant and critical process variables must be identified and adequately controlled.

4.A Means of Qualification of Casting Process.

4.1 To prove a premium casting process, it should be submitted to a qualification program that is specific to a foundry/material combination. The qualification program should establish the following:

(a)The capability of the casting process of producing a consistent quality of product for the specific material grade selected for the intended production component.

(b)The mechanical properties for the material produced by the process have population coefficients of variation equivalent to that of wrought products of similar composition (i.e., plate, extrusions, and bar). Usage of the population coefficient of variation from forged products does not apply. In most cases, the coefficients of variation for tensile ultimate strength and tensile yield strength less than or equal to 3.5% and 4.0% respectively is adequate to demonstrate this equivalency of mechanical properties.

(c)The casting process is capable of producing a casting with uniform properties throughout the casting or, if not uniform, with a distribution of material properties that can be predicted to an acceptable level of accuracy.

(d)The (initial) material design data for the specified material are established.

(e)The material and process specifications are clearly defined.

4.2 For each material specification, a series of test castings from a number of melts, using the appropriate production procedures of the foundry, should be manufactured. The test casting produced should undergo a standardised inspection or investigation of non-destructive inspection and cut-up testing, to determine the consistency of the casting process.

4.3 The test casting should be representative of the intended cast product(s) with regard to section thicknesses and complexity, and should expose any limitations of the casting process. In addition, the test casting should be large enough to provide mechanical test specimens from various areas, for tensile and, if applicable, compression, shear, bearing, fatigue, fracture toughness, and crack propagation tests. If the production component complies with these requirements, it may be used to qualify the process. The number of melts sampled should be statistically significant. Typically, at least 10 melts are sampled, with no more than 10 castings produced from each melt. If the material specification requires the components to be heat-treated, this should be done in no fewer than 10 heat treatment batches consisting of castings from more than one melt. Reduction of qualification tests may be considered if the casting process and the casting alloy is already well known for aerospace applications and the relevant data are available.

4.4 Each test casting should receive a non-destructive inspection program which should include as a minimum: inspection of 100% of its surface, using visual and liquid penetrant, or equivalent, inspection methods; and inspection of structurally significant internal areas and areas where defects are likely to occur, using radiographic methods or equivalent inspection methods. The specific radiographic standard to be employed is to be determined, and the margin by which the test castings exceed the minimum required standard should be recorded.

4.4.1 The program of inspection is intended to:

(a)confirm that the casting process is capable of producing a consistent quality of product, and

(b)verify compliance with the stated objectives of a premium casting process with regard to surface finish, cracks, cold shuts, laps, shrinkage cavities, and porosity, (see paragraph 3), and

(c)ensure that the areas from which the mechanical property test samples were taken were typical of the casting as a whole with respect to porosity and cleanness.

4.4.2 Guidance on non-destructive inspection techniques and methods can be obtained from national and international standards. The standard listing below is not a comprehensive list but is given as an initial reference guide. ASTM A802 Standard practice for steel castings, surface acceptance standards, visual examination. ASTM A903 Standard specification for steel castings, surface acceptance standards, magnetic particle and liquid penetrant inspection. ASTM E155 Standard Reference Radiographs for Inspection of Aluminum and Magnesium Castings. ASTM E192 Standard Reference Radiographs for Investment Steel Castings of Aerospace Applications. ASTM E433 Standard reference photographs for liquid penetrant inspection. ASTM E1030 Standard test method for radiographic examination of metallic castings. ASTM E1320 Standard Reference Radiographs for Titanium Castings. ISO 4986 Steel castings - Magnetic particle inspection ISO 4987 Steel castings - Penetrant inspection ISO 4993 Steel castings - Radiographic inspection ISO 9915 Aluminium alloy castings - Radiography testing ISO 9916 Aluminium alloy and magnesium alloy castings - Liquid penetrant inspection ISO 10049 Aluminium alloy castings - Visual method for assessing the porosity ISO 11971 Visual examination of surface quality of steel castings The test castings must show that the Foundry/Process combination is capable of producing product free of cracks, laps, and cold shuts. Ideally the test castings should be free of detectable shrinkage cavities and porosity. With regard to dimensional tolerance, distortion, and surface finish guidance for acceptance criteria can be gained from the standards cited above. Consideration that these standards are for general quality castings must be given when they are used.

4.5 All test castings should be cut up to a standardised methodology to produce the mechanical test specimens as detailed by paragraph 4.3 above. Principally, the tests are to establish the variability within the cast component, as well as to determine the variability between components from the same melt and from melt to melt. The data gathered also may be used during latter phases to identify deviations from the limits established in the process qualification and product proving programs.

4.6 All the fracture surfaces generated during the qualification program should be inspected at least visually for detrimental defects. Evidence of inclusions, oxide films, porosity or shrinkage cavities would indicate inadequate control of the casting process.

4.7 As part of the cut-up investigation, it is usually necessary to take metallographic samples for cleanness determination and microstructural characterisation.

4.8 When the process has been qualified, it should not be altered without completing comparability studies and necessary testing of differences.

5.Proof of Product

5.1 Subsequent to the qualification of the process, the production castings should be subjected to a production-proving program. Such castings should have at least one prolongation; however, large and/or complex castings may require more than one. If a number of castings are produced from a single mould with a single runner system, they may be treated as one single casting. The production-proving program should establish the following:

(a)The design values developed during the process qualification program are valid (e.g., same statistical distribution) for the production casting.

(b)The production castings have the same or less than the level of internal defects as the test castings produced during qualification.

(c)The cast components have a predictable distribution of tensile properties.

(d)The prolongation(s) is representative of the critical area(s) of the casting.

(e)The prolongation(s) consistently reflects the quality process, and material properties of the casting.

5.2 A number of (i.e., at least two) pre-production castings of each part number to be produced should be selected for testing and inspection. All of the selected castings should be non-destructively inspected in accordance with the qualification program.

(a)One of these castings should be used as a dimensional tolerance test article. The other selected casting(s) should be cut up for mechanical property testing and metallographic inspection.

(b)The casting(s) should be cut up to a standardised program to yield a number of tensile test specimens and metallographic samples. There should be sufficient cut-up tensile specimens to cover all critical (“critical” with respect to both the casting process and service loading) areas of the casting.

(c)All prolongations should be machined to give tensile specimens, and subsequently tested.

(d)The production castings should be produced to production procedures identical to those used for these pre-production castings.

5.3 On initial production, a number of castings should undergo a cut-up for mechanical property testing and metallographic inspection, similar to that performed for the pre-production casting(s). The cut-up procedure used should be standardised, although it may differ from that used for the pre-production casting(s). Tensile specimens should be obtained from the most critical areas.

(a)For the first 30 castings produced, at least 1 casting in 10 should undergo this testing program.

(b)The results from the mechanical property tests should be compared with the results obtained from the prolongations to further substantiate the correlation between prolongation(s) and the critical area(s) of the casting.

(c)In addition, if the distribution of mechanical properties derived from these tests is acceptable, when compared to the property values determined in the qualification program, the frequency of testing may be reduced. However, if the comparison is found not to be acceptable, the test program may require extension.

5.4 At no point in the production should the castings contain shrinkage cavities, cracks, cold shuts, laps, porosity, or entrapped oxide film, or have a poor surface finish, exceeding the acceptance level defined in the technical specifications.

6.Monitoring the Process.

6.1 For the product quality techniques should be employed to establish the significant/critical foundry process variables that have an impact on the quality of the product. For the product it should be shown that these variables are controlled with positive corrective action throughout production.

6.2 During production, every casting should be non-destructively inspected using the techniques and the acceptance standards employed during the qualification program.

(a)Rejections should be investigated and process corrections made as necessary.

(b)Alternative techniques may be employed if the equivalence in the acceptance levels can be demonstrated.

(c)In addition, tensile tests should be taken from the prolongations on every component produced, and the results should comply with limits developed in the process qualification and product proving programs.

(d)Additionally, as previously mentioned, a periodic casting cut-up inspection should be undertaken, with the inspection schedule as agreed upon during the proof of product program.

(e)Deviations from the limits established in the process qualification and product proving programs should be investigated and corrective action taken.

7.Modifications to the Casting Design, Material, and Process.

7.1 Additional testing may be required when alterations are made to the casting geometry, material, significant/critical process variables, process, or production foundry to verify that the alterations have not significantly changed the castings’ properties. The verification testing recommended is detailed in Table 1, below:

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