Engine blocks are the "hearts" of vehicles—they hold pistons, crankshafts, and coolant, enduring temperatures up to 200°C and pressures of 10+ MPa. For decades, cast aluminum alloys (like A356. A380) have been the top choice for engine blocks: they’re lightweight (30% lighter than cast iron), improve fuel efficiency, and handle high heat. But there’s a persistent problem that haunts manufacturers: porosity defects—tiny air bubbles trapped inside the cast metal.
Here’s why porosity matters: A single 1mm air bubble in the cylinder wall can weaken the metal by 20%, leading to cracks under pressure. Worse, porosity can cause oil or coolant leaks—forcing manufacturers to scrap 10-15% of engine blocks per batch. That’s a huge cost: a mid-sized factory making 10.000 engine blocks a year loses $500.000+ to porosity-related scrap. The good news? Porosity isn’t inevitable. By fixing key steps in the casting process, you can cut defects to 3% or lower. This article breaks down the most effective preventive measures, with real-world examples from automotive manufacturers.
First: What Causes Porosity in Cast Aluminum Engine Blocks?
Before fixing porosity, you need to know where it comes from. The main culprits are:
Moisture in raw materials: Aluminum ingots or scrap with even 0.1% moisture will release steam during melting—this steam becomes bubbles in the final cast.
Trapped air during melting: Stirring the molten aluminum too vigorously, or not removing air properly, lets bubbles get trapped.
Poor mold venting: If the mold doesn’t have enough channels to let air escape during pouring, air gets squeezed into the metal.
Incorrect pouring temperature: Pouring too cold (below 650°C for A356) makes the metal solidify too fast, trapping bubbles before they rise to the surface.
Now, let’s dive into how to fix each of these.
Key Preventive Measures for Porosity Defects
Porosity prevention is a "full-process" job—you can’t fix it with just one step. Here are the 5 most critical actions, proven to work in real factories:
1. Dry Raw Materials Thoroughly (Stop Steam Before It Starts)
Moisture in aluminum ingots, scrap, or additives (like silicon) is the #1 cause of porosity. Even a small amount of water turns to steam at melting temperatures (600-700°C), and that steam becomes tiny bubbles in the cast.
The fix is simple but strict:
Pre-dry ingots/scrap: Put raw materials in a drying oven at 120-150°C for 4-6 hours. For scrap aluminum (which often picks up moisture from storage), extend drying to 8 hours.
Store dry materials properly: Keep dried ingots in a sealed, climate-controlled area—no more leaving them outside in rain or humidity.
Dry additives: Silicon or copper additives (used to strengthen A356 alloy) should also be dried at 100°C for 2 hours before adding to molten aluminum.
A Chinese automotive supplier learned this the hard way: In 2021. they skipped drying scrap aluminum (stored outside in rainy weather) and saw porosity rates jump to 18%. After adding a drying oven and strict storage rules, porosity dropped to 4% in 2 months.
2. Remove Air During Melting (Use Inert Gas or Refining Agents)
Once the aluminum is melted, you need to "scrub out" trapped air. Two methods work best for engine block casting:
a. Inert Gas Degassing (Most Effective for High-Volume Production)
This uses argon or nitrogen (inert gases that don’t react with aluminum) to bubble through the molten metal. The gas bubbles pick up tiny air bubbles and carry them to the surface, where they escape.
Key steps for success:
Use argon (better than nitrogen—it’s denser, so it traps more air) at a flow rate of 1-2 L/min per 100 kg of molten aluminum.
Lower a rotating degassing lance (with small holes) to the bottom of the melting furnace—this ensures gas bubbles spread evenly, not just at the top.
Degas for 5-8 minutes—too short, and air remains; too long, and you cool the metal too much.
A German engine manufacturer switched to argon degassing in 2022. Their porosity rate fell from 12% to 3%, and they saved $300.000 a year in scrap.
b. Refining Agents (Great for Smaller Factories)
If you don’t have a degassing lance, use solid refining agents (like sodium aluminum fluoride or calcium chloride). These agents melt in the aluminum, release small gas bubbles, and absorb impurities that trap air.
How to use them:
Add 0.5-1% of the agent by weight (e.g., 500g for 100 kg of aluminum) to the molten metal.
Stir gently (too hard traps more air!) for 2-3 minutes, then let the metal sit for 10 minutes to let bubbles rise.
A Mexican factory using A380 alloy for small engine blocks uses this method—they cut porosity from 10% to 5% with just $500 in monthly refining agent costs.
3. Optimize Mold Venting (Let Air Escape Before It Gets Trapped)
Engine blocks have complex shapes—webbed walls, cylinder bores, and oil passages—all of which can trap air during pouring. A mold with poor venting is a guaranteed way to get porosity.
Here’s how to fix mold design:
Add vent grooves: Cut thin grooves (0.2-0.3mm wide, 5-10mm long) in the mold at "air traps"—like the top of cylinder bores or the ends of oil passages. These grooves let air escape as metal fills the mold.
Use vent plugs: For deep, narrow areas (like bolt holes), insert porous ceramic vent plugs. Air flows through the plug’s tiny holes, while molten aluminum can’t—preventing bubbles in hard-to-reach spots.
Tilt the mold: Pour the metal into a slightly tilted mold (5-10° angle) instead of a vertical one. Tilting lets air escape from the top as the metal fills the mold slowly, instead of being squeezed into the metal.
A U.S. manufacturer of pickup truck engine blocks redesigned their molds with 12 extra vent grooves and 8 vent plugs. Their porosity rate dropped from 14% to 2%, and they haven’t had a porosity-related leak in 18 months.
4. Control Pouring Temperature and Speed (Don’t Let Metal Solidify Too Fast)
Pouring molten aluminum too cold or too fast is a common mistake that traps bubbles. For engine block alloys like A356. the "sweet spot" is precise:
Temperature: Pour at 670-690°C (use a pyrometer to check—don’t guess!). Below 670°C, the metal thickens quickly, trapping air before it rises. Above 690°C, the metal shrinks more during cooling, creating shrinkage porosity (another type of defect).
Speed: Pour at 0.5-1 m/s (slow enough to let air escape, fast enough to keep the metal from cooling mid-pour). Use a controlled pouring system (like a ceramic launder) to avoid splashing—splashing traps air in the metal.
A Thai factory was pouring A356 at 650°C (too cold) and saw 16% porosity. They raised the temperature to 680°C and slowed the speed to 0.7 m/s—porosity dropped to 4% in 1 week.
5. Inspect Early (Catch Porosity Before It Becomes a Problem)
Even with perfect processes, it’s smart to check for porosity early—before you machine the engine block (which wastes time and money). Use non-destructive testing (NDT) methods:
Ultrasonic testing (UT): Sends sound waves through the metal—bubbles reflect the waves, showing up as dots on a screen. Great for checking thick areas like the cylinder wall.
X-ray testing: Takes images of the cast metal—porosity shows up as dark spots. Use this for critical areas like the crankshaft housing.
Test 10-15% of each batch—if you find porosity, adjust the process (e.g., add more degassing time) before making more blocks. A South Korean manufacturer uses UT on every 5th engine block—they catch 90% of porosity defects early, saving $200.000 a year in wasted machining.
Real-World Success: A Japanese Automaker’s Porosity Fix
Let’s wrap up with a full example: A Japanese automaker making hybrid car engine blocks struggled with 15% porosity in 2020. They implemented all 5 measures:
Dried ingots at 140°C for 6 hours and stored them in a humidity-controlled warehouse.
Added argon degassing (1.5 L/min per 100 kg) for 7 minutes.
Redesigned molds with 15 vent grooves and 10 vent plugs.
Set pouring temperature to 680°C and speed to 0.8 m/s.
Used UT testing on 20% of blocks.
By 2021. their porosity rate dropped to 2.5%. They saved $600.000 in scrap, and their engine blocks now have a 5-year warranty (up from 3 years) because of improved durability.
“The key wasn’t one big fix—it was small, consistent changes to every step,” said their casting manager. “Porosity used to be our biggest headache; now it’s a non-issue.”
Conclusion
Porosity defects in cast aluminum engine blocks are frustrating, but they’re not unavoidable. By drying raw materials, removing air during melting, optimizing mold venting, controlling pouring conditions, and inspecting early, you can cut porosity to 3% or lower. The upfront effort (like buying a degassing lance or redesigning molds) pays off fast—less scrap, fewer leaks, and more durable engines.
For manufacturers, this isn’t just about saving money—it’s about building trust. An engine block without porosity lasts longer, performs better, and keeps customers happy. And in the competitive automotive industry, that’s the difference between winning and losing.
