The temper designation on an aluminium casting specification — T6, T5, F, or any of the other letter-number combinations that appear after the alloy designation — is not a shorthand for "heat treated" versus "not heat treated." It is a precise description of the thermal processing history the casting has received, which determines its microstructure, and therefore its mechanical properties. Understanding what each temper designation actually specifies, why different applications require different temper conditions, and what the thermal processing operations behind each designation entail — is foundational knowledge for anyone writing, interpreting, or fulfilling aluminium casting specifications.
The Designation System — What the Letters and Numbers Represent
The aluminium temper designation system is standardised by the Aluminium Association — the US-based industry body whose designations are used globally, with minor regional variants in some markets. The system uses a letter prefix — F, O, H, or T — to indicate the broad category of condition, followed by numbers that specify the particular treatment within that category. For casting applications, F and T are the relevant prefixes — H designations cover strain-hardened wrought alloys that are not produced by casting, and O (annealed) is rarely specified for castings.
F is the simplest designation: it means as-fabricated, which in the casting context means as-cast — the alloy in whatever condition results directly from the solidification and cooling process without any subsequent thermal treatment. The casting is in whatever microstructural state its solidification history produced, with the alloy elements distributed in the various phases — solid solution, intermetallics, and precipitates — that the solidification and cooling rate determined. F condition properties are variable because they reflect the as-cast microstructure, which depends on section thickness, die temperature, and cooling rate in ways that vary within a casting and across castings in a production batch.
The T designations cover the various forms of thermal treatment — collectively termed precipitation hardening or age hardening — that are applied to heat-treatable aluminium alloys to improve their mechanical properties above the as-cast condition. The number following T specifies the sequence of thermal operations applied, and understanding what each sequence does to the alloy microstructure is the key to understanding what properties each temper condition delivers.
T1 — Naturally Aged After an Elevated-Temperature Shaping Process
T1 designates material that has been cooled from an elevated-temperature shaping process — in casting terms, cooled from the die at casting temperature — and then naturally aged at room temperature. Natural ageing for most aluminium casting alloys means allowing the supersaturated solid solution that results from relatively rapid cooling in the die to gradually precipitate its strengthening phases at room temperature over a period of days to weeks.
T1 properties are modest compared to artificially aged conditions — the room-temperature precipitation process is slow and produces coarser, less coherent precipitates than the controlled artificial ageing cycle of T5 or T6. T1 is rarely explicitly specified for aluminium casting applications; when it appears in a specification, it typically describes the condition of a casting that has been air-cooled from the die without any subsequent thermal treatment, which has then naturally aged to a stable condition over time. For alloys that respond well to natural ageing — particularly those with significant copper content — T1 properties may be adequate for lightly loaded structural applications, but for the alloys that dominate gravity die casting — LM6 and LM25 — T1 properties are generally inferior to both T5 and T6 and T1 is not a meaningful engineering specification for most applications.
T2 — Annealed (Castings Only)
T2 designates cast products that have been annealed after casting to improve ductility and dimensional stability. The annealing process heats the casting to a temperature that allows the intermetallic phases formed during solidification to coarsen and spheroidise, reducing their stress concentration effect on the matrix and improving the alloy's elongation and toughness relative to the as-cast condition — typically at the expense of strength and hardness.
T2 is specified for casting applications where maximum ductility and toughness are more important than maximum strength — applications where the casting must absorb impact energy without fracture, or where residual stresses from the casting process must be reduced to improve dimensional stability during subsequent machining. Complex, thick-section castings that cool non-uniformly from the die and develop significant residual stress are candidates for T2 annealing before precision machining — the stress relief that annealing provides prevents the dimensional distortion that stress release during machining produces in high-residual-stress castings.
T4 — Solution Heat Treated and Naturally Aged to a Stable Condition
T4 designates material that has received solution heat treatment — heating to a temperature that dissolves the alloying elements into a uniform solid solution — followed by quenching and then natural ageing at room temperature to a stable condition. T4 provides better properties than T1 because the solution treatment produces a more uniform and more fully dissolved solid solution than the as-cast condition, and the subsequent natural ageing precipitation is more effective from this better starting condition.
T4 is occasionally specified for aluminium casting applications where moderate strength improvement over the as-cast condition is required but the full artificial ageing cycle of T6 is not practical — for example, when casting geometry makes rapid quenching from solution treatment temperature impractical, or when the application requires better ductility than T6 provides at the expense of some strength. T4 elongation values are typically higher than T6 because the natural ageing precipitates are coarser and less effective at inhibiting crack propagation than the fine T6 artificial ageing precipitates. The higher elongation makes T4 castings better able to absorb impact energy, which is the basis for its selection in applications where impact resistance is the critical mechanical property.
T5 — Cooled from an Elevated-Temperature Shaping Process and Then Artificially Aged
T5 designates material cooled from the elevated-temperature shaping process — in casting, cooled from the die — and then artificially aged without a separate solution treatment step. Artificial ageing means holding the casting at an elevated temperature — typically 150 to 180 degrees Celsius for most aluminium casting alloys — for a defined period that causes the strengthening precipitates to form from whatever dissolved alloying elements are in solid solution after the casting process.
The practical significance of T5 is that it eliminates the solution treatment and quench steps of T6, reducing the thermal processing cost and the dimensional distortion risk that the T6 quench step introduces for complex geometries. The properties achievable in T5 are lower than T6 — because the as-cast solid solution is less uniform and more dilute than the solution-treated and quenched condition — but they are meaningfully better than the as-cast F condition for alloys where some alloying elements are in solution after the casting process. T5 is the appropriate temper specification when moderate property improvement over F condition is required at lower processing cost and distortion risk than T6, and when the application does not require the full mechanical properties that T6 delivers.
T6 — Solution Heat Treated, Quenched, and Artificially Aged
T6 is the temper condition that delivers the maximum mechanical properties from heat-treatable aluminium casting alloys and the one that is most widely specified for structural casting applications. The T6 sequence has three distinct steps: solution heat treatment at a temperature where the alloying elements dissolve into solid solution; rapid quenching that freezes this uniform solid solution state; and artificial ageing at a lower temperature where the fine, coherent precipitates that provide maximum strengthening form throughout the matrix.
The properties achievable in LM25 T6 — tensile strength of 230 to 280 MPa, yield strength of 200 to 220 MPa, elongation of 2 to 5 percent — represent approximately double the strength of the as-cast condition, achieved through the microstructural mechanism of fine precipitate formation that obstructs dislocation movement. T6 is the specification for structural automotive castings, pressure-containing components where yield strength determines the safe operating pressure, and precision engineering components where dimensional stability and fatigue resistance at elevated strength levels are simultaneously required.
The compromise associated with T6 is distortion. The rapid quench from solution treatment temperature generates thermal gradients in the casting that produce residual stresses — stresses that may cause dimensional distortion during the quench itself, during subsequent ageing, or during machining when material removal releases the stored stress. Managing T6 quench distortion — through controlled quench water temperature, quench fixtures for complex geometries, and machining sequencing that minimises distortion from stress relief — is the primary process management challenge in T6 production and the reason why T5 is sometimes preferred for geometrically complex castings where dimensional stability is more critical than maximum mechanical properties.
T7 — Solution Heat Treated, Quenched, and Over-Aged to Provide Improved Properties
T7 designates solution heat treated and quenched material that has been artificially aged beyond the peak strength condition — to a condition called over-ageing — to provide improved resistance to stress corrosion cracking and improved dimensional stability at elevated service temperatures. The over-ageing that T7 applies produces coarser, less coherent precipitates than the T6 peak ageing condition, which reduces strength modestly relative to T6 while providing the microstructural coarsening that improves resistance to stress corrosion and elevated-temperature property retention.
T7 is specified for aluminium casting applications in environments where stress corrosion is a concern — marine environments, chemical exposure, and highly stressed components in corrosive atmospheres where the combination of residual stress and corrosive exposure would cause T6 material to crack prematurely through the stress corrosion mechanism. For most general industrial and automotive casting applications, T6 remains the appropriate specification — T7 is a specialist temper for specialist environments rather than a general alternative to T6.
Selecting the Right Temper — The Decision Framework
The temper selection decision for a specific casting application reduces to three questions. First, does the application's mechanical property requirement justify the cost and complexity of heat treatment — or are F condition properties adequate? If F is adequate, no heat treatment is required. Second, if heat treatment is required, can the casting geometry tolerate the T6 quench distortion, or is T5's more modest property improvement at lower distortion risk the appropriate trade-off? Third, is the application in an environment that requires T7's stress corrosion resistance, or is T6 peak strength the right specification? These three questions, answered in sequence for each casting application, produce the correct temper specification more reliably than selecting from a reference table without understanding the physical basis for each condition.