Precision casting, such as lost wax casting, is widely used in high-end fields such as aerospace, automotive, and medical devices due to its advantages of near-net shape, high precision, and strong adaptability to complex structures. To determine whether its quality meets the standard, a comprehensive evaluation should be conducted from multiple dimensions, including appearance, dimensional accuracy, internal quality, mechanical properties, surface quality, material composition, and special properties, to ensure that the casting meets the design requirements and the needs of the usage scenario. The specific judgment methods are as follows:

1. Appearance quality: preliminary screening of visual defects

The appearance serves as a "visual impression" of the casting quality, and the following defects require special inspection:

Common defect types include: porosity (round holes on the surface or subsurface), sand holes (pits containing sand particles), cold shuts (gaps where the casting joints are not fully fused), cracks (linear or net-like fractures), fins and burrs (excess metal caused by poor mold fit), and shrinkage (surface depressions), etc.

Inspection method: Visual inspection (with magnifying glass) and fluorescent penetrant inspection (for surface fine cracks) are employed. For instance, fluorescent penetrant can penetrate into cracks, and after cleaning and ultraviolet irradiation, the cracks will exhibit fluorescence, facilitating identification.

Judgment criteria: According to industry standards (such as GB/T 14408 "Cast Carbon Steel Parts for General Engineering Purposes"), fatal defects (such as penetrating cracks, large-sized pores) are directly judged as non-conforming; non-fatal defects (such as minor sand holes) need to be evaluated in conjunction with the allowable range of the usage scenario (e.g., seals are not allowed to have any leakage defects).

II. Dimensional accuracy: the core indicator of near net shape forming

The core advantage of precision casting lies in its high dimensional accuracy, which needs to be verified through the following methods:

Measuring tools: Calipers, micrometers, and height gauges for conventional dimensions; coordinate measuring machines (CMM) for complex surfaces or 3D dimensions, capable of achieving accuracy within ±0.02mm.

Tolerance standard: Refer to GB/T 6414 "Dimensional Tolerances and Machining Allowances for Castings" or ISO 8062. Precision castings typically achieve CT7-CT9 level tolerances (with CT7 level tolerance ranging from ±0.15 to ±0.3mm, depending on the casting size).

Key dimension verification: Focus on inspecting dimensions related to assembly, such as reference surfaces, mating holes, and threads, to ensure consistency with the design drawings. For example, the deviation of the blade profile dimensions of an aeroengine blade needs to be controlled within ±0.05mm, otherwise it will affect the airflow efficiency.

III. Internal quality: Non-destructive testing of hidden defects

Internal defects (such as shrinkage cavities, inclusions, and internal cracks) cannot be observed through appearance, and non-destructive testing techniques are required:

X-ray flaw detection: X-rays are used to penetrate castings, and defects such as shrinkage cavities, slag inclusions, and porosity are identified through the resulting images. For example, shrinkage cavities inside the casting will appear as dark areas on the X-ray film, which must meet the grade requirements of GB/T 5677 "Radiographic Testing of Steel Castings" (such as Grade I with no defects, and Grade II with a small number of minor defects allowed).

Ultrasonic testing: Internal cracks or delamination are determined through ultrasonic reflection signals. It is suitable for thick-walled castings and can detect defects with a depth of over 2mm.

Magnetic particle testing: For ferromagnetic materials (such as carbon steel), it utilizes magnetic particle aggregation to reveal surface or near-surface cracks, with a sensitivity reaching up to 0.1mm depth.

IV. Mechanical properties: Guarantee of reliability in use

The mechanical properties directly determine the load-bearing capacity of castings, which needs to be verified through experiments:

Tensile test: Samples are taken for testing according to GB/T 228 "Metallic Materials - Tensile Testing" to measure tensile strength, yield strength, and elongation. For example, the tensile strength of 304 stainless steel castings must be ≥520MPa, and the elongation must be ≥40%.

Hardness testing: Utilize Rockwell hardness (HRC) or Brinell hardness (HB) to assess whether the surface hardness conforms to design specifications (e.g., the hardness of mold castings must exceed HRC50).

Impact toughness: The fracture resistance of castings under impact loading is evaluated according to GB/T 229 "Metallic Materials - Charpy Pendulum Impact Test", which is particularly applicable to castings used in low-temperature environments (such as LNG equipment components).

V. Surface quality: Combination of functionality and aesthetics

The surface quality of precision castings affects assembly accuracy and corrosion resistance:

Roughness: The Ra value is measured using a roughness tester. Precision castings typically require a Ra value of 1.6~6.3μm (lost wax casting can achieve a Ra value of 0.8μm). If the surface roughness is too high, it can increase wear or lead to sealing failure.

Cleanliness: Inspect the surface for any residual molding sand, oxide scale, or oil stains, which must be removed through processes such as sandblasting and acid pickling to ensure a clean surface.

VI. Material composition: the fundamental guarantee of performance

The material composition directly affects the physicochemical properties of the casting:

Detection method: Rapid analysis of metal element content (such as C, Si, Mn, Cr, etc.) is conducted using a direct reading spectrometer, or chemical analysis method is employed to determine the composition.

Standard requirements: It must comply with the material standards specified in the design (such as GB/T 1220 "Stainless Steel Bars" and GB/T 3190 "Chemical Composition of Wrought Aluminum and Aluminum Alloys"). For example, the Cr content of 316L stainless steel castings must be ≥16%, Ni≥10%, and Mo≥2%.

VII. Special performance: verification of contextualized requirements

For specific usage scenarios, special performance testing is required:

Corrosion resistance: The salt spray test (GB/T 10125) evaluates the corrosion resistance of castings in a salt spray environment. For example, medical device castings must pass a 48-hour salt spray test without rusting.

Tightness: Pressure testing (such as hydraulic pressure, pneumatic pressure) is used to inspect whether castings leak, which is applicable to sealing components such as valves and pump bodies.

Wear resistance: The wear resistance of casting surfaces is evaluated through wear tests, applicable to moving parts such as gears and bearings.

summary

The assessment of precision casting quality compliance is a systematic project that requires a multi-dimensional inspection encompassing appearance, dimensions, internal structure, mechanics, surface quality, material properties, and special performance characteristics. It also involves referencing industry standards and design requirements. Only when all indicators meet the specifications can the casting be ensured to be safe and reliable in practical applications. Furthermore, process control during production (such as wax mold making, shell baking, and pouring temperature) directly impacts quality. Therefore, it is necessary to simultaneously inspect process records to ensure casting quality from the source.