Aluminum alloys for autofrettage
Work hardenable alloys are often called "not hardened by heat treatment". Such a "negativistic" and cumbersome definition sounds very unconstructive., since it does not tell anything about such a useful property of them, as the ability to harden by plastic deformation, in another way – cold deformation, and in another way – slander. In our opinion, the definition of "strain hardenable" sounds similar to the definition of "heat hardenable" and, so, much more informational and optimistic.
These alloys belong to the 1xxx series, 3xxx, 5xxx and 8xxx. Strictly speaking, all metals and alloys can be hardened by deformation, but for aluminum alloys, this term applies only to those alloys, which are not able to harden due to aging processes. These alloys obtain their condition through successive hot and cold forming operations (for example, sheet rolling) in combination with one or more intermediate and / or final anneals.
The nature of slander and hardening
Work hardening effect, which is usually called cold work or auto-work, consists in modifying the structure of the material by plastic deformation. This occurs during the manufacture of a semi-finished product during rolling or drawing operations., as well as in the operations of forming blanks or semi-finished products, for example, bending.
Work hardening is accompanied by an increase in mechanical strength and hardness, as well as a decrease in plasticity, that is, a decrease in the ability of the material to deform. The greater the degree of deformation or the rate of strain hardening, the more this effect appears.
Mechanical properties, which can be achieved by strain hardening depends on the chemical composition of the alloy. for instance, alloy 5083 (AMg4.5), which comprises 4,0 to 4,9 % magnesium, acquires greater hardness, but also more limited deformability, than an alloy 5754(analogue AMg3), which has only magnesium from 2,6 to 3,6 %.
However, a gradual increase in strength always reaches the limit, above which further deformation becomes difficult or even impossible. therefore, if the deformation processing of the alloy must be continued, then the metal must be "softened" by heat treatment - annealing.
Technological softening annealing of aluminum
The ability of the processed metal to further deformation can be restored by the type of heat treatment, which is called "annealing".
During this process, which is performed at temperatures above 300 oC, the hardness and strength of the metal decrease gradually - reduction annealing occurs. Then the strength characteristics fall more sharply - the process of recrystallization occurs – and, finally, they reach minimum values, corresponding to the mechanical properties of the annealed metal.
These processes of reduction and recrystallization annealing are accompanied by a modification of the texture and grain size of the metal. During recrystallization, the alloy is reorganized to a new grain structure.
It is worth noting, that for the same tensile strength the plasticity is higher in the metal after reduction annealing (Н2Х), than in just cold-worked metal (H1X). The state of the metal after the reduction annealing will be, so, more preferred, if maximum plastic deformability of the material is required.
Reduction annealing parameters for 5xxx alloys: temperature from 240 to 280 ºС at exposure from 1 to 4 hours, recrystallization - from 330 to 380 ºС at exposure from 0,5 to 2 hours.
For, so that the annealed metal has good ductility, it should not have a coarse grain structure. This appears in the form of the so-called "orange peel".
Annealing aluminum to fine grain
There are several conditions for obtaining fine grain annealed metal..
1) The metal must be sufficiently work-hardened, corresponding to the relative narrowing of the cross section, not less, than 15 %. If this requirement is not met, then the metal should only undergo reductive annealing (recrystallization annealing should be avoided).
2) The rise in temperature must be rapid, from 20 to 60 ºC per hour.
3) Excessively high temperatures must be avoided, higher 350 – 400 oC, and exposure times with them, i.e, no more 2 hours.
Work hardenable alloys have three main states (table below), as defined in the European standard EN 515.
- the first denotes the main state;
- the second indicates the degree of autofrettage.
Main aluminum alloys of the 5xxx series – deformation of the reinforcement – and most of their conditions, which are used in marine shipbuilding and marine infrastructure, presented here.