Aluminum fretting: a little physics

Methods of pressure processing of metals – rental of, forging, stamping, molding - cast aluminum ingot is converted into semi-finished or finished article – aluminum sheet, aluminum forging, aluminum or aluminum extruded profile part. This occurs at elevated or ambient temperature, and may also include one or more intermediate heatings – annealing – aluminum or aluminum alloy to restore its ductility. In this case, there are two main changes: 1) changing the shape and 2) change in microstructure and mechanical properties.

Example: rolling foil from ingot

for instance, rolling the aluminum ingot length 5 m thick and 300 mm obtained about 200 kilometers thick aluminum foil 7 micrometers. Changing the shape of the deformation measured units. And without the numerical evaluation of the strain is clear, here they were very large, and they can not be achieved in a single pass. Typically, the foil fabrication path starts with hot rolling and cold rolling ends and annealing.

Why is aluminum plastic?

Ability to undergo large plastic deformation is one of the most useful properties of metals. Metals with a face-centered cubic lattice, which include aluminum, typically exhibit good ductility – they can easily be deformed into various complex shapes. Normally the metals composed of a plurality of individual grains or crystals, that is, they are polycrystalline. Typical grains or alumina crystal after hot and cold working, and then annealing has a diameter, let us say, 40 m, a unit cell of an atomic lattice - about 0,4 nm = 0,0004 m. So that each grain contains many millions of elementary cells - about 1015 pcs.

Dislocations in Aluminum

When casting aluminum ingots primary crystals grow from the liquid phase and the cast microstructure generally very rough. When aluminum is plastically deformed, each grain is deformed by movement of linear lattice defects in its crystal. The deformation is due slip by slip planes along the lines shift. These defects are called dislocations (drawing 1). Dislocations move along certain crystallographic planes in the crystal - the so-called "densely packed planes", that known as the slip plane. The movement of one unit produces dislocation shearing, and joint movement of hundreds of thousands of dislocations – complete deformation. deformacia-putyom-skolzeniya-dislokacyi

Figure 1

During deformation at room temperature, the number of dislocations increases and it becomes difficult to move through the atomic lattice. In this case we say, that aluminum "was peening", "got strain hardening "or even" rivet ", such as aluminum or aluminum alloy is called cold-worked. It means, that to continue deformation requires more effort, and gradually loses ductility aluminum, what, in the end, lead to the formation of cracks therein and destruction.

At this time, the following takes place at the atomic level. During deformation slip dislocations occur very actively moving dislocations and slip planes different start to react with each other, entangled with each other and form a so-called "forest" dislocations. With an increase in the dislocation density increases the yield strength of the material – somewhere is directly proportional to the square root of the dislocation density.

Recovery and recrystallization of the deformed aluminum

dislocations, that arose in aluminum work hardening, may be removed by heating the cold-worked metal to a moderately high temperature, for example, 345 ° C. This causes the aluminum to become soft again and recovers its ductility. This heating is called annealing. microstructure changes, that occur during the annealing, call return and recrystallization. During deformation at elevated temperatures usually occur recovery processes. They are called dynamic recovery and dynamic recrystallization.

Through these processes nagartovyvaetsya aluminum is not as much as at room temperature and requires deformation much lower load. Already at a temperature 200 ° C pure aluminum is almost completely lost the ability to work hardening. At moderate plastic deformation of aluminum alloys dislocation they are distributed non-uniformly, and forming cells with walls of entangled dislocations and low dislocation density inside the cells. Typically, these cells have a diameter of the order 1 micrometers. When you return, cell wall boundaries are so-called subzeren. When annealing aluminum or an aluminum alloy after a large volume of cold plastic deformation, a recrystallization process occurs with the formation of new grains (figure 2). The driving force of recrystallization is stored internal energy, which arises in the formation of dislocations.

nagartovka-vozvrat-rekristallizaciyaFigure 2

The dislocation density can be expressed in the form of their total length per unit volume of material. To this annealed material can be a value of about 1010 m-2, and for strongly cold-worked aluminum it comes to 1015 m-2.

Source: TALAT 1251