T651, T6, and T7351: A Practical Guide to Aluminum Temper Designations
By NOX METALS
Aluminum temper designations are not just quality codes - they define the mechanical properties, residual stress state, and machinability of the plate you receive. Specifying the wrong temper can result in warping during machining, dimensional instability, or a material that fails incoming inspection. This guide explains the ANSI temper designation system and covers the tempers most relevant to machined plate applications.
The ANSI Temper Designation System
Aluminum temper designations in the United States follow ANSI H35.1, a standardized system that describes the sequence of mechanical and thermal processing applied to the material after casting. The designation consists of a letter (F, O, H, or T) followed by one or more digits that further specify the processing. Understanding the letter prefix immediately tells you the class of treatment: F means as-fabricated, O means annealed, H means work hardened, and T means thermally treated.
F - As-Fabricated
The F temper designates material in the condition as it came from the mill process - no special thermal treatment or work hardening has been applied after the primary forming operation. Properties in F temper vary and are not controlled to specific minimums in most specifications. F temper plate is rarely specified for structural or precision machined applications. It may appear as a transient state during processing before final heat treatment.
O - Annealed
The O temper designates fully annealed material - the softest and most ductile condition achievable for a given alloy. For heat-treatable alloys like 6061 and 7075, O temper means the material has been solution heat treated and then slowly cooled to relieve all strengthening from prior treatment. O temper is used when maximum formability is required (bending, hydroforming) and strength is not the primary requirement. It is not used for structural machined parts.
H Series - Work Hardened (5000 Series)
The H series applies to non-heat-treatable alloys like the 5000 series (5052, 5083, 5086). These alloys cannot be strengthened by heat treatment - they are strengthened by cold working (rolling). H32 means strain hardened and then stabilized to one-quarter hard. H116 and H321 are specific tempers for marine-grade 5083 that meet both mechanical and corrosion resistance requirements. For heat-treatable alloys like 6061 and 7075, the H tempers do not apply.
T4 and T451 - Solution Heat Treated, Naturally Aged
T4 temper means the alloy was solution heat treated (heated to dissolve alloying elements into solid solution, then rapidly quenched) and then allowed to naturally age at room temperature to a stable condition. T451 adds stress relief by stretching after quench. Natural aging develops partial strength and good ductility - properties are typically lower than the peak-aged T6 condition. T4 is sometimes used when further forming is required after delivery, since the material is softer and more workable.
T6 and T651 - Peak Aged
T6 is the most common temper for 6061 and 7075 in bar, extrusion, and thin plate form. It consists of solution heat treatment followed by artificial aging at elevated temperature to maximum strength (peak age). T651 is T6 with an added stress relief step: after quenching, the plate is stretched 1 to 3 percent to relieve residual quench stresses before aging. The 51 suffix is critical for thick plate used in machined applications - without stress relief, the residual stresses locked into the plate will be released during machining, causing warping and dimensional drift.
T7 Series - Overaged
T7 designates solution heat-treated material that is overaged or stabilized. The additional digits identify particular conditions, not a universal percentage change in strength. In 7075 and 7050 plate, the specified overaged temper is important to corrosion performance. Use the complete alloy and temper requirement on the drawing.
T7351 vs T7451 - Stress Relief in T7
T7351 and T7451 both use stretching for stress relief; T7351 does not mean compression. The preceding digits identify different aging conditions. 7075-T7351 and 7050-T7451 are both plate designations. Select the complete alloy, temper and specification required by the drawing rather than substituting one suffix based on thickness alone.
Temper Reference Table
| Temper | Treatment Description | Typical Application |
|---|---|---|
| F | As-fabricated, no additional treatment | Transient state only |
| O | Fully annealed, maximum ductility | Sheet forming, bending |
| H32 | Strain hardened + stabilized (1/4 hard) | 5000 series plate, marine |
| H116 | Strain hardened, corrosion resistant | 5083/5086 marine plate |
| H321 | Strain hardened + stabilized for marine | 5083 marine, pressure vessels |
| T4 | Solution heat treated + natural age | Pre-form condition, limited structural |
| T451 | T4 + stress relief stretch | T4 with lower residual stress |
| T6 | Solution heat treated + artificial age (peak) | Extrusions, bars, thin plate |
| T651 | T6 + stress relief stretch | 6061 and 7075 plate |
| T73 | Solution heat treatment + overaging | 7075 with improved SCC resistance |
| T7351 | T73 + stress relief stretch | 7075 plate |
| T7451 | T74 + stress relief stretch | 7050 plate |
Order the complete alloy and temper required by the drawing. T651, T7351 and T7451 all include stress relief by stretching, but describe different material conditions. Verify the product form and specification against the mill certificate; a thickness cutoff does not authorize a temper substitution.
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