Analysis and Solutions for Shrinkage Porosity Defects in Aluminum Alloy Casting

Analysis and Solutions for Shrinkage Porosity Defects in Aluminum Alloy Casting

I. Understanding: The Nature and Differences of Shrinkage Porosity

Before addressing the problem, it's important to clarify the core differences between the two types of defects. Both essentially involve insufficient shrinkage compensation during solidification of the molten metal, but their manifestations and severity differ, requiring specific solutions.

Defect Type Appearance Characteristics Location Primary Hazard
Shrinkage Porosity: A single or a few large holes with irregular shapes and rough inner walls (often appearing honeycomb or dendritic). These holes are often located in the "hot spots" of the casting (i.e., the last areas to solidify, such as thick wall protrusions and rib intersections). They directly undermine the structural integrity of the casting, leading to stress concentration and susceptibility to fracture when subjected to load, making them unsuitable for structural or pressure-bearing parts.
Shrinkage Porosity: A large number of small, dispersed holes, typically between 0.1 and 1 mm in diameter, difficult to detect with the naked eye (requiring nondestructive testing). They are often located within the thick walls of the casting, at the bottom of the riser, or near the chiller. While they do not directly cause macroscopic fractures, they can reduce the density of the casting, affect sealing and thermal conductivity, and weaken fatigue performance.

II. Core Cause Analysis: Four Key Factors Contribute to Shrinkage Porosity "Severely Affected Areas"

In aluminum alloy casting, from alloy melting to casting formation, parameter deviations at every stage can induce shrinkage and porosity. Alloy composition, process parameters, gating system design, and solidification control are the four core influencing factors, and they are also the aspects most easily overlooked by export companies.

1. Alloy Composition: Poor Flowability is an "Inherent Hidden Danger"
The composition of aluminum alloys directly determines their flowability and shrinkage, and is an "inherent factor" for shrinkage and porosity:

Insufficient flowability: If the Si content in the alloy is too low (such as pure aluminum or low-silicon alloys), or if impurities (such as Fe and Mn) exceed the specified limit, the molten metal will fill the mold slowly and solidify quickly. This can lead to localized solidification and formation of cavities before the feeding metal arrives. For example, ADC12 alloy (Si content 9.5%-12%) has much better flowability than A356 alloy (Si content 6.5%-7.5%). However, under the same process, A356 is more susceptible to shrinkage. Abnormal shrinkage: Excessive levels of elements like Mg and Cu in an alloy increase the molten metal's "bulk shrinkage" (the change in volume from liquid to solid). If the feeding system cannot match this shrinkage, shrinkage cavities will form in the final solidification zone. For example, 6-series aluminum alloys with a Mg content exceeding 1% have shrinkage 15%-20% higher than standard ADC12 alloys, requiring enhanced feeding design.

2. Process Parameters: The "Imbalance Trap" of Temperature and Speed
Pouring temperature, mold temperature, and pouring speed are the most easily adjusted parameters in the Casting Process and are also the "acquired key variables" that contribute to shrinkage porosity and shrinkage cavities:
Excessive pouring temperature: Exceeding the alloy's liquidus by more than 50°C (e.g., pouring temperatures exceeding 750°C for ADC12) prolongs the solidification time of the molten metal, leading to an expansion of the hot zone within the casting and increased volumetric shrinkage. This can cause the molten metal in the riser to solidify prematurely, preventing effective feeding of the casting and ultimately resulting in shrinkage cavities. Pouring temperature is too low: If the pouring temperature is below 20°C below the liquidus (e.g., A356 pouring temperature below 680°C), the fluidity of the molten metal decreases dramatically. This can lead to premature solidification during the filling process, resulting in "under-pouring" or scattered shrinkage in thick-walled areas (where the molten metal is not fully fused).

Uneven mold temperature: If the mold temperature is too high in a certain area (e.g., a blockage in the cooling water channel causes the mold temperature in a thick-walled area to exceed 300°C), the solidification rate in that area of ​​the casting will slow down, creating a "delayed hot spot." If the temperature is too low in a certain area (e.g., using the chiller without preheating it), the molten metal will solidify rapidly near the chiller, forming a "false shell." This prevents internal shrinkage from being compensated, inducing shrinkage.

3. Gating System Design: Failure of the Feeding "Channel"
The gating system (including gates, risers, and runners) is the core of "guiding molten metal filling and feeding solidification shrinkage." Design flaws can directly lead to inadequate feeding:
Improper riser design: The riser serves as a "reservoir" for feeding. If the riser is too small (not meeting the principle of "riser modulus ≥ 1.2 times the casting's hot spot modulus"), positioned away from the hot spot (e.g., a riser located in a thin-walled area unable to cover a thicker hot spot), or the riser neck is too thin (prematurely solidifying and cutting off the feeding channel), the hot spot in the casting will not be fed.
Incorrect gate placement: If the gate is located in a thin-walled area, the molten metal will first fill the thin wall and solidify rapidly, leaving the thick wall at the "far end" and difficult for the feeding metal to reach. Alternatively, if there are too few gates, the molten metal will travel a long distance within the mold, resulting in significant temperature loss and premature solidification at the end, leading to shrinkage porosity. Improper runner cross-section: A narrow runner (e.g., a cross-sectional area less than 1.5 times the gate area) increases resistance to molten metal flow, resulting in slow filling and rapid temperature drop. This is especially true for Large Castings, where shrinkage porosity is more likely to form in areas away from the gate.

4. Solidification Control: "Sequential Solidification" Violation
The core principle of aluminum alloy casting is "sequential solidification"—that is, the casting solidifies gradually from areas away from the riser (thin walls) toward the riser (thick walls), ensuring consistent "liquid metal feeding" during shrinkage. If this sequence is violated, shrinkage porosity is inevitable.

Improper Chill Use: The function of a chill is to "accelerate local solidification." If the chimney is not placed near a hot spot (e.g., not near a thick wall protrusion), solidification in the hot spot will lag. Alternatively, if the chimney is too small and poorly contacted with the casting, it will not effectively cool the casting. Instead, a "secondary hot spot" will form near the chimney, inducing shrinkage porosity. Irrational mold cooling water channel layout: Cooling channels that are too far from thick-walled areas of the casting (e.g., exceeding 20mm) or with too small a diameter (<10mm) will slow cooling in thick areas, making them the last to solidify. If channels are too close to thin-walled areas, the thin walls will solidify first, leaving the thicker walls without a source of shrinkage.

III. Practical Solutions: Approaching the Problem Step by Step, from Prevention to Repair

To address the above issues, a strategy should be developed from three perspectives: "Pre-production Prevention - In-Production Control - Post-Production Repair." Especially for export castings that must meet international standards such as ASTM and ISO, strengthening the "prevention" stage is crucial (repair costs far exceed prevention).

1. Alloying: Optimizing Composition to Improve "Innate Flowability"

Select appropriate alloys for different casting requirements: ADC12 is preferred for structural parts (excellent flowability and low shrinkage), while A356 is preferred for high-strength components (although additional optimization for feeding is required). Strictly control impurity content (Fe ≤ 0.8%, Mn ≤ 0.5%). Adding "Flow Improvers": Adding 0.1%-0.3% RE (rare earth) elements or 0.05%-0.1% Sr (strontium) during smelting can refine grain size, improve molten metal fluidity, and reduce shrinkage. (Note that excessive Sr content can increase casting brittleness, so the content must be strictly controlled.)

2. Processing: Precise Temperature Control to Match the "Solidification Rhythm"

Pouring temperature: Targeted temperature is determined by alloy type. A temperature range of 680-720°C is recommended for ADC12 and 700-740°C for A356. An infrared thermometer should be used to monitor the molten metal temperature in real time to avoid fluctuations exceeding ±10°C.
Mold temperature: Utilize a "zoned temperature control" strategy, maintaining the mold temperature in thin-walled areas at 180-220°C and in thick-walled hot zones at 220-260°C (achieved by adjusting the cooling water flow rate). This ensures sequential solidification from thin to thick walls. Pouring speed: Adjust according to the complexity of the casting. For simple parts, keep it within 5-8 m/s; for complex parts (multiple ribs, thin walls), keep it within 3-5 m/s. Avoid excessive speeds that may cause air entrainment (indirectly inducing shrinkage), or excessive speeds that may cause premature solidification.

3. Pouring system: Restructure the "feeding channel" to precisely cover the hot spot.

Riser design: Follow the "modulus principle"—riser modulus (M) = casting hot spot modulus (M) × 1.2-1.5. For example, if the thickness of the casting's hot spot is 20 mm (M = 20/6 = 3.3), the riser thickness must be ≥ 40 mm (M = 4.0). Furthermore, the riser should be located directly above (or to the side of) the hot spot to ensure that the feeding channel reaches the final solidification zone. Gate Optimization: Use bottom-pouring or stepped gates (to avoid temperature unevenness caused by top-pouring). The gate should be located close to the thick-walled hot spot (e.g., at the base of a thick-walled protrusion) to minimize the distance the molten metal flows.

Chiller Matching: Place an external chiller (made of cast iron or copper, with a thickness of 0.8-1.0 times the thickness of the hot spot) near the hot spot of the casting (e.g., at the junction of thick and thin walls) to accelerate the cooling of the hot spot and prevent delayed solidification. If the hot spot is internal to the casting, use an internal chiller (made of a material compatible with the aluminum alloy, such as pure aluminum rod).

4. Detection and Repair: Eliminate Export Quality Leaks

Pre-production Simulation: Use casting simulation software (e.g., MAGMAsoft, ProCAST) to perform "flow field + solidification simulation" to identify the location of the hot spot and areas of insufficient shrinkage feeding in advance, allowing for pre-production optimization of the mold and gating system (which can reduce defect rates by over 30%). Post-production Inspection: All exported castings must undergo 100% non-destructive testing (NDT). Shrinkage cavities cannot be identified using radiographic testing (RT) or ultrasonic testing (UT), while shrinkage porosity must be screened using penetrant testing (PT) or eddy current testing (ET) to ensure non-conformities are detected before shipment.
Defect Repair: For small shrinkage porosity (not in critical stress areas), argon arc welding can be used (using wire of the same material, followed by stress relief Heat Treatment). If the shrinkage porosity exceeds 5% of the casting's volume or is located in a critical stress area, the casting must be scrapped immediately to avoid further after-sales risks after export.

IV. Special Recommendations for Export Scenario: Meeting Overseas Customers' Quality Needs

Overseas customers (especially those in Europe, America, Japan, and South Korea) have much higher requirements for casting density than domestic customers. In addition to the above technical measures, two additional points should be noted:
Issuing a "Defect Analysis Report": If a customer reports a defect, a report (in both Chinese and English) must be provided, including the defect location, cause analysis, corrective measures, and re-inspection data, demonstrating professionalism and responsibility.
Advanced Standard Alignment: Based on customer needs, align international standards (such as ASTM B26/B26M-22 "Aluminum Alloy Sand Casting Standard" and ISO 9001:2015 quality management system) with these requirements. Incorporating standard requirements into the process design phase (for example, ASTM requires a shrinkage area of ​​≤0.5% for structural parts) can reduce quality disputes at the source.

Conclusion
Shrinkage and porosity are not "unsolvable casting problems" but rather "process deviations that can be avoided through systematic control." For export companies, addressing these defects is not only a means to improve product qualification rates but also a key to building trust in overseas markets and enhancing brand competitiveness. From subtle adjustments to the alloy composition, to the precise design of the pouring system, to strict control of the testing process, the optimization of each link can lay the foundation for "zero-defect export castings."