C355 aluminum is a heat-treatable casting alloy with about 5% silicon, 1.0 to 1.5% copper, and 0.4 to 0.6% magnesium, with iron held to 0.20% or less. The copper gives it more strength than A356 and better performance at moderately elevated temperatures, while the low iron keeps it ductile and pressure tight. Sand-cast C355-T6 must reach at least 36 ksi tensile and 25 ksi yield strength under ASTM B26.
C355 is the alloy engineers move to when A356 doesn’t carry enough load, especially in housings and brackets that run warm or hold pressure. It casts well in both sand and permanent mold, and in the T61 temper permanent mold castings reach noticeably higher strength than A356-T6.
This guide is for engineers and procurement managers sourcing C355 castings for heavy truck, engine, pump, and other load-bearing applications. It covers composition, properties by temper, how C355 compares with A356, where it’s used, and what to include when you specify it. For a side-by-side look at every alloy LeClaire pours, see our aluminum casting alloys guide.
What is C355 aluminum made of?
C355 is a higher-purity version of 355 alloy. The “C” prefix marks tighter limits on impurities, mainly iron, which improves ductility and strength in the finished casting.
- Silicon (4.5 to 5.5%): Gives the alloy the fluidity to fill complex molds and cores.
- Copper (1.0 to 1.5%): Adds strength and helps the alloy hold properties at moderately elevated temperatures.
- Magnesium (0.40 to 0.60%): Combines with silicon during heat treatment to form the precipitates that harden the alloy.
- Iron (0.20% max): Kept low to limit brittle iron-rich phases. Standard 355 allows considerably more iron.
- Titanium (0.20% max), manganese (0.10% max), and zinc (0.10% max): Controlled as minor elements and residuals.
What are the mechanical properties of C355-T6 and C355-T61?
C355 has to be heat treated to reach its useful properties. T6 is the common temper for sand castings, and T61 is the common temper for permanent mold castings.
- C355-T6, sand cast (ASTM B26 minimum): 36 ksi (248 MPa) tensile, 25 ksi (172 MPa) yield, 2.5% elongation.
- C355-T61, permanent mold (typical): about 44 ksi (310 MPa) tensile, about 34 ksi (230 MPa) yield, roughly 3% elongation.
- A356-T6, sand cast (ASTM B26 minimum, for reference): 34 ksi (234 MPa) tensile, 24 ksi (165 MPa) yield, 3.5% elongation.
Specification minimums apply to separately cast test bars. Specimens cut from an actual casting typically need to average at least 75% of the tensile and yield values and 25% of the elongation value, so agree on where properties will be measured before tooling is built.
Permanent mold C355 outperforms sand-cast C355 because the metal mold cools the casting faster and produces a finer grain structure. LeClaire runs both processes and handles heat treating in-house, which keeps temper control under the same roof as the pour. Our heat treatment process guide explains what T6 and T61 involve.
C355 vs A356: which alloy should you specify?
Specify C355 when the part needs more strength or will run warm. Specify A356 when ductility, corrosion resistance, and ease of casting matter more than the last few ksi of strength.
- Strength: C355 is stronger in comparable tempers, and the gap widens in T61 permanent mold castings.
- Elevated temperature: The copper in C355 helps it hold strength better in parts that sit near engines, transmissions, or other heat sources.
- Ductility: A356 has higher elongation, which helps in parts that see impact or need to deform before failing.
- Corrosion resistance: A356 resists corrosion better. Copper lowers corrosion resistance, so C355 parts in wet or salted environments may need paint or another coating.
- Castability: Both are aluminum-silicon alloys with good fluidity. A356’s higher silicon makes it slightly easier to pour into thin sections.
- Pressure tightness: Both are good choices for parts that must hold fluid or air pressure.
For the full A356 picture, including tempers and the 356 and B356 variants, see our A356 aluminum guide.
Where is C355 aluminum used in heavy truck and industrial parts?
C355 shows up where a cast aluminum part carries structural load, holds pressure, or sits near heat. Heavy truck programs use it to move more brackets and housings from iron to aluminum without giving up the strength the duty cycle demands.
- Heavy truck: Transmission and drivetrain housings, mounting brackets, and air and fluid system components where weight savings add payload.
- Engine components: Crankcases, cylinder heads, covers, and housings that run at operating temperature.
- Valves and pumps: Pump bodies and valve housings that need pressure tightness and more strength than A356 provides.
- Structural fittings: Load-bearing castings where engineers want an aluminum-silicon alloy’s castability with added strength.
How do you specify C355 castings?
A complete C355 callout covers alloy, temper, process, and how properties will be verified. Missing any of these is the most common cause of quote revisions and first-article delays.
- Call out alloy and temper. State C355-T6 or C355-T61 on the drawing, not just “355.”
- Name the casting process. Sand and permanent mold reach different properties, so the process affects whether your targets are realistic.
- Define where properties are measured. Separately cast test bars and coupons cut from the casting give different results.
- List pressure or leak test requirements. Include test pressure, medium, and allowable leak rate for housings and pump bodies.
- Identify machined features and datums. A single source for casting and machining can set datums during tool design so the part machines cleanly.
Frequently asked questions
What does the C in C355 mean?
A letter prefix in an Aluminum Association casting designation marks a variation of the base alloy. For C355, the variation is tighter impurity limits, mainly lower iron, than standard 355. The result is better ductility and strength in the finished casting.
Is C355 stronger than A356?
Yes. In comparable tempers C355 has higher tensile and yield strength than A356, and C355-T61 permanent mold castings show the largest margin. A356 keeps the edge in elongation and corrosion resistance.
What is the difference between C355-T6 and C355-T61?
Both tempers are solution heat treated, quenched, and artificially aged. T61 uses a different aging practice and is the temper typically specified for C355 permanent mold castings, where it reaches higher strength than sand-cast T6.
Is C355 aluminum corrosion resistant?
C355 has fair corrosion resistance for general industrial use. Its copper content makes it less corrosion resistant than A356 or 535, so parts exposed to road salt, standing water, or saltwater often get a protective coating.
Can C355 be sand cast and permanent mold cast?
Yes. C355 casts well in both sand casting and permanent mold casting. Permanent mold delivers higher properties and a smoother finish, while sand casting suits larger parts, complex cores, and lower volumes.
Get a quote on your C355 castings
C355 aluminum gives you more strength than A356 and better performance near heat while keeping the castability and pressure tightness of an aluminum-silicon alloy. LeClaire Manufacturing pours C355 in sand and permanent mold, heat treats and machines in-house, and reviews every new part for castability before tooling is built. Request a quote to have your C355 part reviewed.