How to determine the qualified metallographic structure of castings

How to determine the qualified metallographic structure of castings
The metallographic determination of aluminum alloy castings depends on three factors: grain size, secondary dendrite arm spacing, and defect level. The grain size of wheel hub components is often required to be between 6-8 levels.
Grain size reflects cooling conditions
The grain size is evaluated according to GB/T 6394, and the higher the grade, the finer the grain size. Wheel hub components often require grades 6-8.
The grain size is coarse, and the strength and fatigue performance decrease. Insufficient cooling speed is the main cause, which can be caused by high mold temperature or insufficient cooling water circuit.
Fine crystals require sufficient nucleation particles and fast cooling rates. Both deterioration treatment and cold iron can improve it.
Secondary dendrite arm spacing
The secondary dendrite arm spacing (SDAS) directly reflects the local cooling rate. The smaller the spacing, the better the mechanical properties.
The SDAS of aluminum alloy castings is often controlled between 40-80 μ m. Thin walled areas may be less than 40 μ m, while thick hot junctions may exceed 100 μ m.
The location where SDAS exceeds the standard is often also a high-risk area for shrinkage and loosening. Both often appear simultaneously.
Determination of defect level
Gold phase can observe pinholes, shrinkage porosity, and inclusions. Pinhole degree is rated according to the standard chart, and airtight components are often required to be at level 1-2.
If the needle hole degree exceeds level 3, the pass rate of the airtight test significantly decreases.
Impurities can become the source of cracks. For fatigue parts, the size and distribution of inclusions are more important than the quantity.
The sampling location is crucial
Metallographic sampling should cover three areas: thick wall, thin wall, and hot spot. Only taking one location cannot represent the whole.
Wheel hub components are often taken at the root of the spokes, the middle of the wheel rim, and the center of the wheel hub.
The sampling direction also affects the results. The evaluation results along the direction of grain elongation and perpendicular direction may be different.
Sample preparation and corrosion
The sample needs to be ground to the mirror surface and then polished. Scratches can mask the real tissue.
Keller reagent is commonly used for corrosive liquids, with a corrosion time of 10-20 seconds. Excessive corrosion and blackening of the tissue make it difficult to discern.
Xinfeng Mold suggests that customers conduct a metallographic examination during the T0 stage to establish an organizational baseline for the structural component mold project.
Corresponding to the process
The grain size is coarse, first check the pouring temperature and mold temperature. Excessive temperature leads to a decrease in nucleation rate.
SDAS exceeds the standard, check the local cooling rate. Insufficient water supply or excessive coating will reduce the cooling rate.
There are many pinholes, check the gas content and exhaust of the aluminum liquid. Both often have problems at the same time.
Correspondence between Metallography and Mechanical Properties
Tensile strength and elongation are routine testing items that mutually confirm with the metallographic structure. Fine grain size and small SDAS usually result in better strength and plasticity.
When the mechanical properties are unqualified but the metallographic structure is normal, the heat treatment process and sampling location should be rechecked.
How to keep the report on file
Metallographic photographs should be labeled with magnification, sampling location, and corrosion conditions. Without this information, the report cannot be reproduced.
Prepare the metallographic sample as soon as possible after sampling. If left for too long, the surface will oxidize, which will affect the interpretation of the structure.
Metallographic interpretation is recommended to be completed independently by two people, and any discrepancies should be rechecked to reduce subjective errors.
The metallographic report should specify the ratio of corrosive agents, and the display effect of different ratios of microstructures varies greatly.
Frequently Asked Questions
Q: What is the required grain size?
Answer: Wheel hub components often require grades 6-8.
Q: How much does SDAS control?
Answer: Usually 40-80 μ m, lower in thin-walled areas.
Q: What level of pinhole degree is qualified?
Answer: Airtight components often require 1-2 levels.
Q: How many samples should be taken?
Answer: At least three locations, covering thick walls, thin walls, and hot joints.
Q: What reagents are used for corrosion?
Answer: Commonly used Keller reagent, corrode for 10-20 seconds.
Q: How to change the coarse grain size?
Answer: Reduce pouring temperature, increase cooling rate, and strengthen deterioration treatment.
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