Heat treatment is one of the most powerful tools available to aluminium casting producers — capable of transforming the mechanical properties of a casting by a factor of two or more relative to the as-cast condition, without changing its geometry. Yet the temper designations that describe different heat treatment conditions — T5, T6, T7, and their variants — are frequently misunderstood or mis-specified in casting design and purchasing. Understanding what each treatment does at the metallurgical level, what properties it produces, and when it is the right choice for a given application is essential knowledge for casting engineers and procurement teams alike.
The Metallurgical Foundation: Precipitation Hardening
The heat treatability of aluminium casting alloys depends on the precipitation hardening mechanism — a sequence of controlled heating steps that dissolve alloying elements into solid solution and then precipitate them as fine, dispersed particles that impede dislocation movement and increase strength. The alloys that respond to heat treatment — principally the AlSiMg alloys like LM25 and A356, and to a lesser extent the AlSiCu alloys — contain magnesium and/or copper in quantities that are more soluble at elevated temperature than at room temperature. This solubility difference is the thermodynamic engine of precipitation hardening. The three standard tempers — T5, T6, T7 — represent different points on the precipitation trajectory, producing different combinations of strength, ductility, and dimensional stability.
T5: Artificial Ageing Only
The T5 temper involves only a single low-temperature ageing step — typically 150–180°C for two to eight hours — applied directly to a casting after it has been removed from the die. There is no solution heat treatment step: the casting is not heated to high temperature to dissolve precipitates first. T5 relies on the supersaturation of solute that occurs during rapid die cooling — particularly in thin-walled sections of permanent mould castings where cooling rates are high — to provide a starting state from which some precipitation hardening can occur during ageing. The resulting property improvement over the F (as-cast) condition is moderate: tensile strength typically increases by 15–25%, with some reduction in elongation. T5 is used when dimensional stability and residual stress management are more important than maximum strength — the lower treatment temperature minimises distortion — and when the casting geometry and die cooling configuration produce sufficient natural supersaturation to make ageing worthwhile. It is particularly common for low-pressure die castings and some gravity die castings with favourable geometry.
T6: Full Solution Treatment and Artificial Ageing
T6 is the workhorse temper for aluminium casting alloys where maximum strength is required. The treatment consists of two sequential steps: solution heat treatment at 500–540°C (the exact temperature depending on alloy) for several hours, followed by rapid quenching — typically into water at 60–80°C — and then artificial ageing at 150–180°C for six to twelve hours. The solution treatment dissolves all soluble phases into a homogeneous solid solution. The rapid quench traps this supersaturated condition at room temperature. The ageing step then causes fine precipitates — principally Mg2Si in LM25/A356 — to nucleate and grow to the size range where they most effectively obstruct dislocation movement. T6 produces the highest strength of the standard tempers: for LM25/A356, typical T6 tensile strength is 260–310 MPa with elongation of 3–8% depending on alloy cleanliness and casting soundness. It is the standard specification for automotive structural castings, suspension components, and any application where fatigue life or impact resistance at high stress levels is required.
T7: Over-Ageing for Dimensional Stability
T7 follows the same solution treatment and quench as T6, but uses a higher ageing temperature or longer ageing time that deliberately over-ages the precipitate structure past the peak hardness condition. This produces slightly lower strength than T6 — typically 10–15% reduction in tensile strength — but with improved dimensional stability and reduced susceptibility to stress corrosion cracking. The over-aged precipitate structure in T7 is coarser and less coherent with the aluminium matrix than in T6, which makes it less effective at blocking dislocations but also less prone to the stress concentration mechanisms that can lead to stress corrosion under sustained tensile loading in aggressive environments. T7 is specified for castings used in marine environments, chemical plant, or under sustained loading in potentially corrosive conditions — applications where the long-term dimensional stability and corrosion resistance of the over-aged condition outweigh the modest strength premium of T6.
Choosing the Right Temper: A Practical Guide
The choice between T5, T6, and T7 should be driven by the application requirements rather than by cost alone — though cost is a real factor, since T6 involves furnace time, quench facilities, and the risk of distortion during high-temperature processing. T5 is appropriate for lightly-loaded structural components, covers, and housings where moderate strength improvement over as-cast is sufficient and dimensional consistency is paramount. T6 is appropriate for safety-critical and structural components — suspension parts, powertrain housings, load-bearing brackets — where maximum strength and fatigue life are required. T7 is appropriate for components operating under sustained stress in corrosive environments. For automotive supply, the temper will almost always be specified on the customer drawing; the foundry's role is to ensure that the heat treatment cycle actually achieved matches what the drawing specifies — which requires process control, periodic coupon testing, and furnace calibration records rather than assumption.