390.0 / A390 (AlSi17Cu4.5Mg)
Hypereutectic alloy with ~17% silicon, extremely wear-resistant and low-expansion, for linerless engine blocks, compressors, and pumps.
The 390.0/A390 family is a hypereutectic alloy with 16–18% silicon: above the Al-Si eutectic point (~12.6%), primary silicon crystals precipitate, conferring exceptional wear resistance and a very low thermal expansion coefficient (~18 µm/m·K). Copper (4.0–5.0%) and magnesium provide precipitation-hardening strength (T6 temper). It is the reference alloy for linerless aluminum engine blocks, air-conditioning compressors, and pumps. It requires phosphorus (P) modification to refine the primary silicon crystals. We manufacture it from a primary aluminum base plus a silicon master alloy.
International Nomenclature
| Standard | Designation |
|---|---|
| AA (USA) | 390.0 / A390.0 / B390.0 |
| UNS | A03900 |
| ISO | Al-Si17Cu4Mg |
Chemical Composition
| Element | Symbol | Percentage |
|---|---|---|
| Aluminum | Al | Balance |
| Silicon | Si | 16.0–18.0% |
| Copper | Cu | 4.0–5.0% |
| Magnesium | Mg | 0.45–0.65% |
| Iron | Fe | ≤1.3% (390.0) · ≤0.50% (A390.0) |
| Manganese | Mn | ≤0.10% (390/A390) · ≤0.50% (B390) |
| Zinc | Zn | ≤0.10% (390/A390) · ≤1.5% (B390) |
| Nickel | Ni | ≤0.10% (solo B390) |
| Titanium | Ti | ≤0.20% |
| Phosphorus (refiner) | P | ~0.005–0.015% |
| Others (each) | — | ≤0.05% |
| Others (total) | — | ≤0.15% (390/A390) · ≤0.20% (B390) |
Physical Properties
| Property | Value |
|---|---|
| Density | 2.73 g/cm³ |
| Thermal Expansion Coeff. | ~18 µm/m·K |
| Melting Range | ~507–650 °C |
Mechanical Properties
| Property | Value |
|---|---|
| Tensile Strength (UTS) | ~280 MPa (T6) |
| Yield Strength | ~265 MPa (T6) |
| Elongation | ~1% (T6) |
| Hardness | 110–145 HB |
Key Characteristics
- Hypereutectic alloy (16–18% Si) with primary silicon crystals for exceptional wear resistance
- Very low thermal expansion coefficient (~18 µm/m·K) for dimensional stability
- Requires phosphorus (P) modification to refine the primary silicon crystals
- Copper (4.0–5.0%) and magnesium for high strength in the T6 temper
- Enables linerless aluminum engine blocks
- Grades 390.0, A390.0 (low Fe), and B390.0 (die casting) depending on the application
Compared to Similar Alloys
390.0 / A390 (AlSi17Cu4.5Mg) vs 336.0 (AlSi12CuNiMg)
Both are very-low-expansion Al-Si casting alloys, but they take different routes: 390.0/A390 is hypereutectic (16–18% Si, above the ~12.6% eutectic) with primary silicon crystals and high copper (4.0–5.0%) for maximum wear resistance, ideal for linerless blocks. 336.0 is eutectic (~12% Si) with nickel (2.0–3.0%) of the "Lo-Ex / Y-alloy" type, optimized for the hot strength and dimensional stability of high-performance pistons.
| Dimension | 390.0 / A390 (AlSi17Cu4.5Mg) | 336.0 (AlSi12CuNiMg) |
|---|---|---|
| Key composition / alloying | Hypereutectic: Si 16–18%, Cu 4.0–5.0%, Mg 0.45–0.65%; requires phosphorus (P) modification | Eutectic: Si 11–13%, Ni 2.0–3.0%, Cu 0.50–1.5%, Mg 0.7–1.3% |
| Si microstructure | Primary silicon crystals (hypereutectic) giving exceptional wear resistance | Eutectic silicon (~12%); nickel forms refractory intermetallics for hot strength |
| Heat treatment & strength | T6 (precipitation hardening): UTS ~280 MPa, yield ~265 MPa, elongation ~1%, 110–145 HB | T551: UTS ~248 MPa · T65: UTS ~324 MPa, yield ~296 MPa, ~125 HB |
| Thermal expansion (CTE) | ~18 µm/m·K (very low) | ~19–20 µm/m·K (low) |
| Machinability / processing | Primary Si crystals are highly abrasive: demanding machining (PCD tooling); grades 390.0, A390.0 (low Fe) and B390.0 (die casting) | Eutectic with less primary Si: more manageable machining than the hypereutectic; sand/permanent-mold casting |
| Weldability | Poor: the high copper (4.0–5.0%) makes it crack-sensitive; not recommended for structural welding | Limited: piston alloy with Cu and Ni; not intended as a weldable alloy |
| Typical application | Linerless engine blocks, cylinders, A/C compressors, pump bodies and high-wear hydraulic components | High-performance and heavy-duty diesel engine pistons, racing pistons, high-temperature compressor pistons |
390.0 / A390 (AlSi17Cu4.5Mg): Choose 390.0/A390 when you need maximum wear resistance and cylinder surfaces that run without a liner (linerless blocks, compressors, pumps), accepting more demanding machining and poor weldability from the high copper.
336.0 (AlSi12CuNiMg): Choose 336.0 when making high-performance pistons that demand hot strength and maximum dimensional stability, where the nickel (2.0–3.0%) and eutectic composition matter more than the extreme wear of primary silicon.
Industries & Applications
Automotive
Linerless engine blocks, cylinders, wear-resistant components
HVAC
Air-conditioning compressor components, rotors
Machinery
Pump bodies, pistons, high-wear hydraulic components
Special Notes
390.0/A390 is hypereutectic (16–18% Si, above the ~12.6% Al-Si eutectic): the primary silicon crystals provide the wear resistance. The grades differ by composition: 390.0 allows Fe ≤1.3%; A390.0 lowers iron to ≤0.50% for greater toughness; and B390.0 is optimized for die casting (allowing up to Zn ≤1.5% and Mn ≤0.50%, plus Ni ≤0.10%). The phosphorus addition (~0.005–0.015%) is essential to refine and disperse the primary silicon crystals. We manufacture it from a primary aluminum base plus a silicon master alloy with P modification.
Frequently Asked Questions
What is the 390.0/A390 alloy and what is it used for?
The 390.0/A390 (AlSi17Cu4.5Mg) is a hypereutectic Al-Si-Cu-Mg casting alloy with ~17% silicon, extremely wear-resistant and with very low thermal expansion. It is the reference alloy for linerless aluminum engine blocks, cylinders, air-conditioning compressors, rotors, and pump bodies. Its combination of wear resistance and dimensional stability makes it ideal for high-wear components in automotive, HVAC, and machinery.
Why is it hypereutectic and what does the 17% silicon contribute?
With 16.0–18.0% silicon, 390/A390 exceeds the Al-Si eutectic point (~12.6%), so primary silicon crystals precipitate during solidification. These hard crystals confer exceptional wear resistance and lower the thermal expansion coefficient to ~18 µm/m·K, providing high dimensional stability. This is why it enables engine blocks without cast-iron liners.
What are the key alloying elements and what does each one do?
Silicon (16.0–18.0%) forms the primary crystals that give wear resistance and low expansion. Copper (4.0–5.0%) and magnesium (0.45–0.65%) provide mechanical strength through precipitation hardening in the T6 temper. Phosphorus (~0.005–0.015%) is essential as a refiner, since it disperses and refines the primary silicon crystals to keep them from growing too coarse.
Is it heat-treatable, and what properties does it reach in the T6 temper?
Yes, thanks to the copper and magnesium it is heat-treatable by precipitation hardening. In the T6 temper it reaches a tensile strength (UTS) of ~280 MPa, a yield strength of ~265 MPa, and a hardness of 110–145 HB. Elongation is low (~1%), reflecting its stiff, wear-resistant rather than ductile character.
What is the difference between grades 390.0, A390.0, and B390.0?
All three share the hypereutectic Al-Si-Cu-Mg base but differ by composition and process. 390.0 allows iron up to ≤1.3%; A390.0 lowers iron to ≤0.50% for greater toughness; and B390.0 is optimized for die casting, allowing up to Zn ≤1.5%, Mn ≤0.50%, and Ni ≤0.10%. The choice depends on the casting process and the part's toughness requirements.
Why does it require phosphorus modification, and what happens if it is skipped?
In an untreated hypereutectic alloy, the primary silicon crystals tend to grow large and angular, which embrittles the part and hinders machining. The phosphorus addition (~0.005–0.015%) acts as a nucleation site that refines and disperses those crystals into a fine, uniform distribution. That is why P modification is an essential process step, distinct from the Na/Sr modification used in eutectic alloys such as A356.
Does Transformación Puebla manufacture 390.0/A390, and from what feedstock?
Yes, we manufacture it as a casting alloy. We start from a primary aluminum base plus a silicon master alloy to reach the 16–18% hypereutectic Si, and we apply phosphorus modification to refine the primary silicon crystals. The high silicon content and iron limits mean recycled wheel stock (A356.2) is not the right charge for this grade; it is built from primary aluminum and a master alloy.
Need 390.0 / A390 (AlSi17Cu4.5Mg)?
We manufacture this alloy in ingots, donuts, and bars to your exact specifications.