The basic process of precision investment casting usually includes steps such as making wax patterns, coating multiple layers of refractory materials, dewaxing, pouring metal, and removing the shell after cooling. Therefore, quality inspection needs to be carried out at each step or after the final product is completed. Testing the quality of precision investment castings requires a combination of multiple methods and technologies, covering the entire process control from raw materials to finished products. The following are systematic inspection steps and methods:
1. Pre-inspection of wax patterns and shells of castings
In the process of investment casting, the quality of the wax pattern is critical. It is necessary to check whether the wax pattern has defects in the early stage to avoid problems in subsequent casting. In addition, whether the dewaxing process is complete and whether the shell has cracks may also affect the quality of the final casting. There are also factors such as temperature control and cooling rate during the pouring process, which will also affect the internal structure of the casting and the formation of defects.
Wax pattern inspection: Make sure that the wax pattern has no deformation, cracks or bubbles, and the size meets the design drawings (using 3D scanning or projector).
Shell integrity: Check the uniformity of the refractory coating and the strength of the shell after sintering to avoid cracks or peeling.
2. Appearance inspection
Appearance inspection is definitely the basis. For example, whether there are defects on the surface, such as pores, cracks, shrinkage holes or burrs. It may be observed with the naked eye, or with a magnifying glass or microscope to check the subtle places. However, some defects may be internal, and other methods are needed at this time.
Visual inspection: Observe surface defects (pores, cracks, burrs, cold shuts, etc.) with the naked eye or a magnifying glass.
Surface roughness: Use a roughness meter to measure whether it meets the process requirements.
Fluorescent penetration testing: For surface opening defects (such as microcracks), spray fluorescent agent and observe with ultraviolet light.
3. Casting dimensional accuracy detection
Precision casting has high requirements for dimensional accuracy, so tools such as calipers, micrometers, and three-dimensional coordinate measuring machines (CMM) are needed to measure whether the key dimensions meet the requirements of the drawings. However, the shrinkage rate after casting must be considered. The shrinkage rates of different materials are different, which needs to be taken into account when designing the mold, and whether it is within the allowable tolerance range during testing.
Traditional tools: calipers, micrometers, height gauges, etc. measure key dimensions.
Advanced equipment: Coordinate measuring machine (CMM) or laser scanning, obtain 3D data and compare with CAD model.
Shrinkage compensation: verify whether the actual shrinkage rate meets the material characteristics (such as the shrinkage rate of nickel-based alloy is about 2%).
4. Parameter control before pouring (temperature, material composition)
Process parameter monitoring: real-time recording of melting temperature, pouring speed, cooling rate, etc.
Statistical process control (SPC): analyze production data and prevent batch defects.
Standard reference: based on ASTM, ISO or customer specifications (such as ASTM E3 metallographic preparation, ISO 8062 dimensional tolerance).
5. Internal defect detection (non-destructive testing, X-ray, ultrasonic, etc.)
X-ray testing (RT): detect internal pores, shrinkage, inclusions, etc., suitable for complex structures.
Ultrasonic testing (UT): detect deep cracks or delamination, requiring coupling agent.
Industrial CT scanning: three-dimensional tomography, accurate positioning of internal defects (suitable for high-value castings).
6. Material performance testing, mechanical properties, metallographic analysis
Then comes material performance testing, such as chemical composition analysis, which requires a spectrometer or X-ray fluorescence spectrometer (XRF) to determine whether the material composition meets the standards. Mechanical performance testing, such as tensile strength and hardness testing, requires sampling for testing, such as cutting samples from castings, using a universal testing machine to test tensile strength, and using a hardness tester to measure hardness. For example, the specific application scenarios of X-ray and ultrasonic testing, or the limitations of certain testing methods. For example, penetrant testing can only detect defects with surface openings, while X-rays can see inside, but the technical requirements for operators are higher, and the equipment cost may be higher.
Internal defect detection may require the use of non-destructive testing technologies, such as X-ray testing, ultrasonic testing, or penetrant testing. X-rays can see whether there are pores, inclusions, shrinkage, etc. inside; ultrasonics can detect internal cracks or delamination; penetrant testing may be used for defects with surface openings, such as cracks.
Metallographic analysis is also important, observing the microstructure of the metal through a microscope to see the grain size, whether there are inclusions, shrinkage or other structural defects. This requires cutting the sample, polishing and corrosion, and then observing. Some testing methods may require the destruction of samples, in which case sampling testing may be required rather than full testing.
Chemical composition analysis: Spectrometer (OES) or XRF quickly detects elemental composition.
Mechanical properties test:
Tensile test: Determine tensile strength and elongation (according to ASTM E8 standard).
Hardness test: Brinell/Rockwell hardness tester detects surface hardness.
Metallographic analysis: Cut the sample and observe the microstructure (grain size, inclusions, shrinkage, etc.).
7. Air tightness test
Air tightness test: If the casting is used in a pressure-bearing occasion, an air tightness test is required, such as air pressure or water pressure test, to check for leaks.
Air tightness test: Pressurization (water pressure/air pressure) to detect whether the pressure-bearing casting leaks.
Assembly verification: Test assembly with mating parts to ensure dimensional matching (such as impeller and shaft matching).
8. Process control and standard compliance
There are differences in the testing methods for castings of different materials. For example, the testing focus of aluminum alloys and high-temperature alloys may be different.
Different levels of complexity of castings require different inspection strategies. For example, thin-walled parts are more prone to cold shuts and may require more X-ray inspections.
9. Surface treatment inspection
It is necessary to check the surface treatment of the casting, such as whether the polishing and coating meet the requirements, or whether there is residual stress and whether annealing is required.
10. Conformity inspection (compliance with drawings and standards)
11. Other special inspections
Residual stress detection: X-ray diffraction or ultrasonic method to evaluate whether annealing is required.
Corrosion resistance test: Salt spray test (such as ASTM B117) to evaluate surface coating performance.
Assembly test: Ensure that the casting can fit correctly with other parts.
Equipment calibration: Regularly calibrate the inspection tools to ensure data accuracy.
Personnel training: Operators need to be familiar with the standards and equipment operations to avoid human errors.
Finally, environmental factors need to be considered, such as lighting conditions during inspection, accuracy of optical measurement equipment, temperature and humidity, etc., to see if they will affect certain test results.
Through the above multi-dimensional testing, the quality of precision investment castings can be fully controlled to ensure that they meet the high-precision requirements of aerospace, medical equipment and other fields.
In short, the quality of precision investment castings requires multi-faceted considerations, combined with a variety of testing methods, from raw materials to production processes to finished products, each link must be strictly controlled to ensure that the quality of the final product meets the requirements. Each step requires corresponding tools and methods, and records and traceability are required so that problems can be traced back to production batches or links when they are found. At the same time, personnel training and calibration of testing equipment are also important to ensure the accuracy of test results.
Ningbo Suijin Machinery Technology Co., Ltd. relies on years of technical accumulation and innovation capabilities in the casting field and focuses on the research and development and application of intelligent casting technology. In terms of precision investment casting, advanced digital technology is used to realize the full process intelligence from product design, process planning, production and manufacturing to quality control, providing high-quality precision castings for high-end equipment manufacturing industries such as aerospace and gas turbines.