Heat Treating 356 Aluminum Alloy to T6 Using an Electric Kiln

Because the heat-treating temperature for aluminum is close to its melting point, it is important to protect parts placed near the heating elements. Radiant heat from the elements can create localized overheating. To reduce this risk, use a sheet-metal shield for any part positioned within 3 inches of the elements.

1. Solution Heat Treatment

The first step is solution heat treating. The purpose of this stage is to raise the aluminum to a temperature at which the alloying elements go into a uniform solid solution.

• Temperature range: 980°F to 1000°F
• Typical soak time: approximately 6 to 12 hours, depending on the mass of the part

In general, the solution process begins at around 6 hours, with additional time required for heavier sections. Excess time typically does not create major treatment issues, but insufficient time may result in the center of the part not reaching the same condition as the outer areas. Some users report good results at 980°F for 8 hours.

2. Quenching

After solution treatment, the part is quenched in boiling water.

• Quench temperature: 212°F

Using boiling water maintains a stable quench temperature and should produce a T4 condition. If lower-temperature water is used, distortion may increase.

3. Aging

The second step is aging. Aging schedules can vary, but for hobby-level applications, the following options are commonly used:

• 1 hour at 400°F
• 5 hours at 350°F
• 8 hours at 325°F

No quench is required after aging.

Additional Notes

In many cases, 356 alloy does not require heat treatment. However, heat treating can improve hardness, reduce brittleness, and produce cleaner machining characteristics in cast aluminum parts.

In aluminum casting, achieving high-quality, defect-free components depends heavily on controlling dissolved gases, especially hydrogen. Unlike many other metals, molten aluminum has a strong tendency to absorb hydrogen from open flame in the furnace, moisture in the surrounding environment, including humidity in the air, wet tools, or contaminated charge materials. If not properly managed, this dissolved hydrogen can lead to porosity, which significantly weakens the final casting.

Degassing is the process used to remove these unwanted gases from molten aluminum before it solidifies. As the metal cools, its ability to hold hydrogen decreases, causing the gas to come out of solution and form bubbles. These bubbles become trapped within the solidifying metal, resulting in internal voids, reduced mechanical strength, and poor surface finish.

Several methods are used for degassing aluminum, with inert gas purging being one of the most common. In this technique, gases such as argon or nitrogen are bubbled through the molten aluminum using a lance. The inert gas forms tiny bubbles that attract and carry dissolved hydrogen to the surface, where it escapes into the atmosphere. Fluxes may also be used to assist in removing impurities and improving overall melt cleanliness.

Effective degassing not only reduces porosity but also improves mechanical properties, enhances pressure tightness, and ensures better machinability of cast components.

In summary, degassing is not just a routine step in aluminum casting—it is a vital process that directly impacts product quality. Proper control of melt conditions, combined with the right degassing technique, ensures that the final casting meets the desired standards of strength, durability, and integrity.

1. Degassing Tablets or Pool Shock

These are usually based on compounds that release chlorine or similar gases when plunged into molten aluminum. Common examples include:

• Hexachloroethane (C₂Cl₆) tablets
• Calcium Hypochlorite (Ca(ClO)₂)  Pool Shock
  • “Caution: releases chlorine gas; you must have adequate ventilation.”
  • Two teaspoons per 10 to 12 lbs. Wrap in aluminum foil and push to the bottom of the crucible, one slow stir to spread the hypochlorite.

How they work:
When added to molten aluminum, they decompose and release chlorine bubbles that pull dissolved hydrogen out of the melt.

Important warning:

• These produce toxic fumes (chlorine gas)
• Must be used outdoors with proper ventilation
• Recommended minimum safety gear
  • No laces on leather shoes
  • Face shield or safety glasses
  • Cotton or wool clothes (no synthetics)

Discover how to make greensand for casting with the right sand, bentonite, and water ratios. Improve mold strength, finish, and casting results.

Reliable Greensand

A reliable greensand mold starts with the right balance: typically 87–90% fine silica sand, 10–13% bentonite clay, and 2–6% water.

The type of bentonite affects the mold's strength and how easily it releases from the metal.

Western (Sodium) Bentonite: Can swell dramatically (often 10–15× its dry volume when fully hydrated). It offers high dry and hot strength, making it excellent for steel or heavy iron castings where erosion resistance is critical.

Southern (Calcium) Bentonite: Swells significantly less and provides good workability and reliable green strength, making it popular for non-ferrous castings like aluminum and brass because it allows for easier "shakeout" (removing the casting from the sand).

Mixed Blends: Many hobbyists and foundries use a blend, such as a 4:1 or 2:1 ratio of Southern to Western bentonite, to balance high green strength with enough heat resistance.

Sand grain size also plays a major role—

• Finer sands, like 120 mesh, improve surface finish but reduce permeability.
• Coarser sands vent gases better but produce a rougher casting.

Mixing and "Mulling" Tips

Proper mulling ensures each grain is uniformly coated, which is what gives greensand its strength, plasticity, and ability to hold detail. Simply stirring the ingredients is usually insufficient; they must be "mulled" to coat each sand grain in a thin layer of clay.

1. Dry Mix First: Combine the dry sand and powdered bentonite thoroughly before adding any liquid to prevent the clay from clumping.
2. Add Water Gradually: Use a spray bottle to mist the sand while mixing.
3. The "Squeeze Test": Take a handful of sand and squeeze it into a ball. It should break cleanly in half with sharp edges and not crumble or stick excessively to your hand.
4. Resting Period: Allow the mixture to sit in a sealed container for a few hours or overnight. This gives the clay time to fully hydrate and expand.

Quick summary

What is the best greensand mix ratio?
 A typical mix is 87–90% sand, 10–13% bentonite clay, and 2–6% water.

How do you know if greensand is mixed correctly?

Use the squeeze test—proper greensand holds shape, breaks cleanly, and isn’t sticky or crumbly; it should have sharp edges and not stick to your hands.

Items are available in the “Foundry Supplies” section of our store.

• We offer pre-measured bentonite for 50 lb. and 100 lb. sand batches, along with ready-to-use greensand mixes available.
• 120 mesh Greensand—this is an excellent sand for detail.

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