Alloy Composition Designations
The Aluminum Association has designed a numbering system for general classification of cast aluminum alloys.
- Pure aluminum, as produced from the Hall-Héroult process, can achieve a 99.7% purity, but his has little value to industrial applications.
- Commercially pure aluminum is 99.5% Aluminum and is soft and weak.
- Pure aluminum has a modulus (stiffness) that is 1/3 of steel.
- The thermal expansion for aluminum is 2x that of ferrous materials.
- Aluminum is generally considered “resistant” to corrosion.
- Aluminum alloys have been designed that are specific to the casting process being used to form the parts.
- Cast alloys include 300, 400 and 500 series materials.
- The most common cast alloys are 300 series.
- The most common High Pressure Die Casting (HPDC) alloys (380, 383, 384) use Si, Cu, Fe, Mn and carefully limit Mg.
- The most common aluminum alloy for sand casting, gravity casting and low pressure casting is the 356 and 357 (6% and 7% Si respectively).
90% of aluminum castings are Al-Si alloys. The most significant reason for alloying Silicon with Aluminum is to improve the cast-ability by making the molten metal more “fluid like”. As a teaching and training aid, many university lab experiments have been conducted to assess fluidity of an alloy by using a “spirol” mold. There is no specific engineering standard for this test, however the basic physics of the system shown in the illustration have been studied for many years. By comparing actual casting condition from a laboratory experiment to analytical models, many simulation software programs have been validated. Since the early 2ooo time frame, most companies involved with casting molten metal are using and trusting results from the most commonly used computer modeling software applications that utilize finite-difference or finite-element modeling practices. As of this writing, the finite element based software applications have excellent correlation to specific alloy compositions in their data base. Therefore, empirical studies to assess gating, alloy composition and metal temperature are rarely performed today.
Industry standards for aluminum casting alloys have been governed by many different documents for many years. In general, the Aluminum Association (AA) is looked at to establish the main alloy category designations. However, because the AA membership is made of aluminum producers, the acceptance ranges for individual elements are generally considered “wide”. To reduce variability in casting performance, most OEM and Tier 1’s control alloy compositions “tighter” than the AA standard. As a consequence, there are quite a few specifications for the same general alloy. For instance, the A380 alloy has the following possible documents to apply for quality governance:
- AA alloy: A380.0
- ASTM: SC84C per ASTM B85
- SAE: 308 per J452
- UNS: A13800
- FCA: Grade D per MS-50125
When speaking in general terms, most casting professionals describe alloy in the AA terms. However, when specific tolerances are being established for a product application, the OEM specification, such as FCA MS.50125, governs the manufacturing control plan acceptance criteria. Therefore, it’s not considered unusual to find variations of the AA designations appearing on drawings and references in publications. It’s a good practice to keep a cross reference chart readily available.
The Aluminum Association Casting Temper Designation System (see above chart) uses letters and numbers to indicate the major types of thermal treatments applicable to engineered castings:
- F = as-cast
- O = annealed
- T4 = solution treated and aged
- T5 = precipitation hardened
- T6 = solution heat treated, quenched, and precipitation hardened
- T7 = solution heat treated, quenched, and over-aged
The temper designation is presented immediately following the alloy designation. Thus, for a 356.0 alloy casting that has been solution heat treated, quenched, and artificially aged, the full alloy and temper designation would be shown as 356.0-T6. Examples of registered temper variations are A357.0-T61, 242.0-T571, and 355.0-T71. Other variations of temper designations are permitted by the Aluminum Association Temper Designation System (see above chart), the most common being the use of “P” added to a standard temper designation (e.g., T6P) indicating a producer variation of the standard processing treatment.