Aluminium annealing

Types of aluminium annealing

Aluminium annealing have several types that differ in obective [1]:

• full annealing
• partial annealing
• stress-relief annealing
• stabilizing annealing
• controlled-atmosphere annealing.

Annealing times and temperatures depend on:

• alloy type
• initial structure and temper.

Full annealing

• The softest, most ductile, and most workable condition of both non-heat-treatable and heat-treatable aluminium alloys
• Temper designation: O temper

Typical annealing conditions used for some alloys in common use are shown in the table of Figure 1.  

Figure 1 – Typical full annealing treatments for some common wrought aluminium alloys [1]

The treatments, which are listed in the table, anneal the material to the O temper. They are typical for various sizes and methods of manufacture. So, they may not exactly describe optimum treatments for specific products.

The instructions in the table of Figure 1

The instruction (a):

• Time in the furnace need not be longer than necessary to bring all partsof the load to annealing temperature.
• Cooling rate is unimportant.

The instruction (b):

• These treatments are intended to remove the effect of solution treatment.
• Included cooling at a rate of about 30 ⁰C/h from the annealing temperature to 260 ⁰C.
• Rate of subsequent cooling is unimpotrant.
• Treatment at 345 ⁰C followed by uncontrolled cooling, may be used to remove the effects of cold work or to partly remove the effects of heat treatment.

The instruction (c):

• These treatments are intended to remove the effects of solution treatment.
• Include cooling at an uncontrolled rate to 205 ⁰C or less, followed by reheating to 230 ⁰C for 4 h .
• Treatment at 345 ⁰C followed by uncontrolled cooling, may be used to remove the effects of cold work or to partly remove the effects of heat treatment.

The instruction (d):

• Cooling rate to 205 ⁰C or below is less than or equal to 30 ⁰C/h.

Partial annealing

• Another name is recovery annealing
• Partial annealing is the heat treatment of cold worked, non-heat-treatable wrought alloys to obtain intermediate mechanical properties for H2 tempers)
• Temperature used are below those that produce extensive recrystallization.
• Incomplete softening is accomlished by substructural changes in dislocation density and rearrangementsinto cellural patterns.

Figure 2 shows changes in yield strength as functions of temperature and time for sheet of two non-heat-treatable alloys (1100 and 5052). Initialy these alloys were in the highly cold-worked condition (H18 temper).

Figure 2 – Isothermal annealing curves for alloys 1100-H18 and 5052-H18 [1]

1. Heat Treating / Aluminum and aluminium alloys // ed. J.R. Davis – ASM International Handbook – 1993