Heat treatment is one of the most powerful tools available to the aluminium casting engineer — and one of the most frequently misapplied. The ability to nearly double the yield strength of a gravity die cast LM25 component through a controlled thermal cycle, without changing its weight or geometry, is a genuine engineering advantage that no other post-cast process can match. But heat treatment is not a universal solution, it is not free of risk, and specifying the wrong temper condition — or applying the right temper incorrectly — produces results that range from negligible improvement to catastrophic distortion.
Understanding what heat treatment actually does to aluminium microstructure, what the T5 and T6 designations mean in practice, and when each is the appropriate specification is foundational knowledge for anyone involved in aluminium casting design, procurement, or production.
What Heat Treatment Does to Aluminium — The Metallurgical Basis
The heat treatability of aluminium casting alloys depends on the presence of alloying elements that have higher solubility in solid aluminium at elevated temperatures than at room temperature. For the casting alloys most commonly used in gravity die casting — primarily LM25 with its magnesium addition — the relevant element is magnesium, which combines with silicon in the alloy to form magnesium silicide precipitates during the ageing stage of heat treatment.
In the as-cast condition, LM25 contains magnesium and silicon distributed throughout the microstructure in various forms — some in solid solution within the aluminium grains, some as coarse intermetallic compounds that formed during solidification. The as-cast mechanical properties reflect this heterogeneous microstructure: modest strength, reasonable ductility, but significant variability from location to location within the casting depending on local solidification conditions.
Heat treatment imposes order on this microstructure through a controlled sequence of thermal steps. The solution treatment step dissolves the alloying elements into a uniform solid solution. The quench step locks this uniform solution in place by cooling too rapidly for the dissolved elements to precipitate. The ageing step then allows controlled precipitation of fine, coherent particles that obstruct dislocation movement — the physical mechanism that produces the strength increase. The size, distribution, and coherency of these precipitates determine the mechanical properties of the final temper condition.
T6 — Solution Treatment, Quench, and Artificial Ageing
The T6 temper is the full precipitation hardening sequence — solution treatment, quench, and artificial ageing — and it produces the highest strength and hardness achievable in heat-treatable aluminium casting alloys. For LM25, the T6 sequence involves solution treatment at approximately 530 to 540 degrees Celsius for a period sufficient to dissolve the alloying elements into solid solution — typically four to eight hours depending on section thickness and furnace loading. The casting is then water quenched — immersed rapidly in water at controlled temperature — to freeze the solid solution state before any precipitation can occur. Artificial ageing follows at 150 to 175 degrees Celsius for four to twelve hours, during which the fine strengthening precipitates form throughout the microstructure.
The mechanical property improvement in LM25 T6 is substantial. Tensile strength rises from approximately 140 to 160 MPa in the as-cast condition to 230 to 280 MPa in T6. Yield strength increases from approximately 70 MPa to 200 to 220 MPa. Elongation in T6 is typically two to five percent — lower than the as-cast condition because the strengthening precipitates that increase strength also reduce ductility. This trade-off between strength and ductility is fundamental to precipitation hardening and must be understood when specifying T6 for components that require both high strength and damage tolerance.
The quench step in T6 is the most process-sensitive operation. Too slow a quench allows precipitation to begin before the casting reaches the ageing temperature — producing a coarser, less effective precipitate distribution and lower final properties. Too aggressive a quench — cold water at very low temperature — generates large thermal gradients within the casting that produce residual stresses and distortion, particularly in castings with varying section thickness. Quench water temperature is a controlled process parameter, typically maintained between 60 and 80 degrees Celsius for castings prone to distortion, accepting a modest reduction in peak properties in exchange for dimensional stability.
T5 — Cooling from Elevated Temperature and Artificial Ageing
The T5 temper omits the solution treatment and quench steps entirely. The casting is cooled from the elevated temperature of the casting process — either air cooled or controlled-atmosphere cooled from die ejection temperature — and then artificially aged. The ageing step produces some precipitation strengthening from whatever alloying elements are in solid solution after the casting process, but without the homogenising effect of solution treatment, the improvement is significantly more modest than T6.
T5 is the appropriate specification when the casting geometry is too complex or section-variable to survive the quench step without unacceptable distortion, when the dimensional tolerances of the component are too tight to accommodate the distortion that T6 quenching would introduce, or when the application requires a moderate strength improvement over the as-cast condition without the full investment of T6 processing. For components where T6 strength is not required — where the as-cast mechanical properties are marginal rather than inadequate — T5 provides a useful intermediate condition at lower processing cost and risk.
The practical distinction matters for purchasing and specification: a component specified as LM25 T5 has not received solution treatment, and its properties will be lower and more variable than LM25 T6. Confusing the two designations in a procurement specification — or accepting T5 when T6 was required — is a quality failure that may not manifest until the component is in service under load.
When Heat Treatment Cannot Be Applied
Not all aluminium casting alloys are heat-treatable, and not all casting processes produce castings that can be heat treated successfully. LM6 — the near-eutectic Al-12Si alloy — does not respond to T6 heat treatment because it lacks the magnesium content necessary for precipitation hardening. LM24 — the Al-Si-Cu alloy — can be given a T5 stabilising treatment but is generally not T6 treated in gravity die casting applications because the copper content makes it susceptible to hot cracking during quenching.
High-pressure die castings, regardless of alloy, typically cannot be T6 heat treated because the entrapped gas porosity inherent to the HPDC process expands during solution treatment at high temperature, causing surface blistering and internal void growth that destroys both the cosmetic and structural integrity of the casting. Vacuum-assisted HPDC and squeeze casting processes reduce porosity sufficiently to allow heat treatment, but standard HPDC is incompatible with T6.
Process Control — Where Heat Treatment Goes Wrong
The most common heat treatment failures in foundry practice are not metallurgical — they are process control failures. Overloaded furnaces where castings shield each other from the furnace atmosphere, producing non-uniform temperatures and inconsistent solution treatment. Thermocouples that have drifted from calibration, indicating correct temperature when the furnace is running 15 degrees low — enough to dramatically reduce solution treatment effectiveness. Ageing ovens without adequate temperature uniformity across their volume, producing property gradients in batch-treated castings depending on their position in the oven. Each of these failures is invisible to standard mechanical property testing unless samples are taken from multiple locations and multiple positions in the furnace load — a practice that few small foundries maintain. The result is heat-treated castings with as-cast properties that pass inspection because the test sample happened to come from a well-treated location.