Metallographic structure analysis of LPDC castings, α-Al、 Modification control of eutectic silicon, iron phase, and needle shaped silicon

Metallographic structure analysis of LPDC castings, α-Al、 Modification control of eutectic silicon, iron phase, and needle shaped silicon
Metallography is the fundamental basis for determining the effects of melting, deterioration, and heat treatment. Many factories only focus on tensile strength and hardness, without conducting metallographic analysis; Eutectic silicon is coarse, needle like iron phase, and segregation, which can cause fatigue and elongation of castings to continue to deteriorate under the premise of qualified hardness, and failure under compression and vibration conditions.
1、 Standard metallographic structure determination criteria (A356)
Ideal organization: α - Al matrix+uniformly spheroidized/short rod-shaped eutectic silicon, with a small amount of fine π - iron phase; Eutectic silicon with a size of ≤ 10 μ m, evenly distributed, and without large areas of coarse or blocky silicon.
Metamorphic treatment: Sr strontium metamorphism, with an addition amount of 0.015-0.03%; Sr is insufficient, and silicon appears in coarse plate-like form; Excessive Sr produces pores and needle shaped Sr inclusions.
Iron phase control: Fe content control<0.12%; High Fe content generates needle shaped β - Al5FeSi, which fractures the matrix and is the main cause of brittle fracture in castings; When Fe is well controlled, it is mainly in the form of Chinese character shaped π phase, with low harm.
Grain refinement: AlTiB refining agent, added at a dosage of 0.01-0.02%, refines alpha aluminum grains, reduces the tendency for shrinkage cracks, and improves mechanical stability.
Xinfeng Mechanical Metallographic Sampling is fixed near the hot spot and gate, synchronously comparing the changes in microstructure before and after metamorphism and heat treatment, and locating melting anomalies.
2、 Key points for sampling, sample preparation, corrosion, and rating
Sampling location priority: at the root of the gate, at the junction of the T-bar, and at the thick walled hot spot, these locations are most prone to segregation and coarsening.
Sample preparation: rough grinding → fine grinding → polishing, without scratches; Corrosion is carried out using a 0.5% HF aqueous solution, with a corrosion time of 8-12 seconds. Excessive corrosion results in unclear silicon morphology.
Rating dimensions: eutectic silicon morphology, silicon size, iron phase type and size, grain size, porosity, and oxide inclusions.
Metallographic changes after T6 heat treatment: flaky silicon spheroidization, rounded silicon particles; The deteriorated coarse silicon cannot be completely repaired by heat treatment.
3、 Differences in metallographic control among overseas customers
Southeast Asian customers only conduct problem tracing in metallography, without daily sampling. If the iron phase is too high and the tensile strength is qualified, it can be released.
The North American Tier 1 drawings specify the metallographic rating chart, with upper limits on the length and proportion of β - needle like iron phases, and records of Sr modification and grain refinement included in PPAP.
European customers attach great importance to fatigue performance, and the frequency of metallographic sampling is high. Even if the mechanics are qualified, large areas of β - iron phase will require adjustments to the melting formula.
Middle Eastern engineering machinery castings are subjected to alternating loads for a long time, and metallography is a mandatory item for type testing, with strict control over coarse silicon and long needle like iron phases.
4、 Metallographic defect chain
Insufficient Sr addition → plate-like coarse eutectic silicon → low elongation and easy brittle fracture;
Excessive Fe → long needle shaped β - iron phase → fracture of aluminum matrix, resulting in a sharp drop in fatigue performance;
Insufficient AlTiB → coarse grains → increased tendency for shrinkage cracks and more looseness;
Excessive Sr → Sr hydrogen binding → Multiple pores and increased X-ray defects.
5、 Long tail traffic layout ideas
Low pressure casting metallurgy, strontium modification, β - iron phase, eutectic silicon spheroidization, aimed at melting and materials engineers.
keywords LPDC A356 metallography strontium modification、β-Al5FeSi needle phase aluminum casting；
Problem class: Sr addition rate for A356 modification;
Comparison category: β iron phase vs π iron phase aluminum casting.
Suitable for material metallography and melting process special pages.
6、 Common misconceptions in the industry
Misconception 1: If the hardness is qualified, there is no problem with metallographic examination. The hardness under needle like iron phase can meet the standard, but the elongation and fatigue are poor.
Misconception 2: Heat treatment can turn all coarse silicon into spherical shapes. However, if the original coarse silicon is severe, the improvement of heat treatment is limited.
Misconception 3: The more strontium added, the better the deterioration. Excessive Sr can induce pores and increase the risk of inclusions.
FAQ
Q: What is the range of A356 strontium Sr addition?
A：0.015–0.03%。
Q: What is the harmful iron phase?
A: Needle shaped β - Al5FeSi.
Q: What is the amount of AlTiB refining agent added?
A：0.01–0.02%。
Q: Common reagents for metallographic corrosion?
A: 0.5% HF aqueous solution.
Q: Ideal eutectic silicon form?
A: Spherical, short rod-shaped, size ≤ 10 μ m.
Q: The hazards of beta iron phase?
A: Splitting the matrix reduces elongation and fatigue life.
Q: Sampling priority location?
A: Hot section, gate root, and rib junction.
Q: What defects can excessive Sr bring?
A: Pores and strontium inclusions.