HEAT TREATMENT OF ALUMINIUM ALLOYS
- Aug 24, 2017
- 3 min read
Aluminium alloys of widely varying compositions and strength are used in aircraft structures, and they each attain their optimum properties by either cold-work or heat treatment process.
The heat treatment process may consist of solution treatment followed by natural ageing, or solution treatment followed by precipitation treatment.
Heat treatment is sometimes necessary to soften materials for manipulation purposes, and this is done either by annealing or by solution treatment, depending on the extension of the proposed manipulation.
When material has been annealed , it is essential that its specified properties should subsequently be restored by the application of the full recommended heat treatment process. Solution heat treatment of artificially aged alloys must be followed by precipitation treatment.
3 basic reasons for heat treating aluminium alloys are to :
o Increase their strength
o Improve their corrosion resistance
o Improve their workability
Increased strength and improved resistance to corrosion are achieved by :
a. Solution treatment followed by natural age hardening
b. Precipitation treatment which is applied to specific materials which have
been previously solution treated.
c. This process is commonly known as artificial ageing
To improve workability, the annealing process is applied
TYPES OF HEAT TREATMENT FOR ALUMINIUM ALLOYS
Solution heat treatment
To improve mechanical properties by putting alloying elements into solution
Precipitation treatment (artificially aging)
-Provide hardening by precipitation of constituents from solid solution
Annealing
-To soften strain-hardened and heat treated alloy components
-To relieve stresses and to stabilize properties and dimensions
SOLUTION HEAT TREATMENT
-Solution heat treatment consist of heating the material to a prescribed temperature below the melting point generally in the temperature range 4950 C + 100 C depending on material specification.
-This causes an amount of the alloying element to go into solid solution with the aluminium.
-Solid solution means that the alloying elements are evenly dispersed throughout the material even though the material is in its solid state.
-After cooling fine particles of Aluminium/Copper alloy are thrown out of solid solution to produce a structure which gradually reaches its maximum strength and hardness; generally over a period of between 3 to 5 days.
COLD WORKING OF SOLUTION HEAT TREATED MATERIAL
-When solution heat treated material has to be manipulated, it is desirable that the work be completed within two hours of quenching. (Otherwise it will start to precipitate and hardens)
-Natural ageing (refrigeration) can be suspended (in room temp) by storing the material at low temperature – immediately after solution treatment.
-Clad sheet should be heat treated a maximum of three times
NATURAL AGEING PROCESS (REFRIGERATION PROCESS)
-The natural ageing process of aluminium alloys can be suspended by refrigeration (to delay natural ageing)
-The solution treated material should be stored in a refrigerator immediately following solution treatment and maintained at a suitable temperature
-Any manipulation required must be completed within two hours of the material being removed from refrigeration.
-Should the time allowed for manipulation elapse or in the case on expiry of the maximum storage period; further heat treatment will be necessary
PRECIPITATION HEAT TREATMENT
-The process of artificially aging an alloy after heat treating to increase its strength
is called precipitation heat treatment (artificially aging)
-Artificially aging is to speed up age hardening
-The alloy is heated to a specific temperature for several hours
-Followed by slowly cooling in air.
-Certain alloying elements, tend to precipitate out of the solid solution and
form particles throughout the material.
-These microscopic particles give additional strength to the material.
-Temperature and time vary with material specification
ANNEALING
-The annealing process softens the materials for manipulation purposes
-The annealing process consists of heating the material to a specific temperature
followed by cooling in air at room temperature.
-The process normally calls for a temperature about 1000 C to 2000 C below that
specified for solution treatment.
-The soaking period for all gauges of sheet material is usually about 1 hour
ØTo improve workability, the annealing process is applied
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