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Wrought Aluminium Alloys

Leonardo da Vinci, Helsinki Award 2006, Design and execution: Gerold Fink, Austria. Click to open PDF document about this award

Leonardo Da Vinci Helsinki Award 2006

Leonardo da Vinci, Helsinki Award 2006, Design and execution: Gerold Fink, Austria. Click to open PDF document about this award

Bronze medal for an outstanding project promoting and supporting the LifeLong Learning EU policy. Award Berlin 2007

 
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Alloying and Property Examples

For many aluminium products, the pure metal cannot be used. For example for casting applications, the addition of alloying elements is required to improve castability. For wrought apllications, the addition of alloying elements improves mostly the mechanical properties i.e. higher strengths can be obtained, but also other properties are affected. The alloys of the 8 series can have very different mechanical, physical and corrosion properties, and consequently very different behaviour during service, surface treatments, welding, forming etc.

These differences are achieved through one of two ways:

  1. the addition of different types, quantities and combinations of alloying elements

  2. through differences in processing (temper differences)

This is illustrated for some properties in the next diagrams as an introduction to the later sections where the alloy influences will be explained in more detail.

Effect of Mg content on mechanical properties of 5xxx series alloys

Why does yield strength increase with alloy element addition? 

The yield strength of a metal is the maximum stress the material can support without plastic deformation. The addition of alloying elements increase the strength of an alloy by either solute hardening or precipitation hardening. The alloy elements, either as solute species or in the form of alloy (often intermetallic) phases, strengthen the metal by forming stress barriers against the movement of dislocations. The more of these barriers, the more energy required for dislocation movement and hence the harder it is to plastically deform the metal: hardness and strength have increased.

Stress-strain diagrams for various series’ alloys

Why is the stress-strain behaviour alloy dependent? 

Certain alloys are stronger then others - resulting in a higher possible load before plastic deformation occurs - because different additions of alloy elements allow different strengthening mechanisms through work hardening (non-heat treatable) and/or thermal treatment (heat treatable alloys). Strong/hard alloys however break after less elongation than the softer alloys; they are more brittle. Hence, formability is usually better for softer, less strong alloys.

Trends in corrosion resistance as a function of the alloy series

Alloy series Corrosion resistance Mechanical strength
1xxx
2xxx
3xxx
5xxx
6xxx
7xxx
7xxx with Cu

Why is the corrosion resistance alloy dependent? 

Generally the addition of alloying elements has a negative effect on the corrosion resistance. Aside from disrupting the protective native oxide film on aluminium, the alloy element precipitations (often intermetallic particles) can be more anodic or cathodic than the surrounding aluminium matrix. This makes the metal electrochemically more reactive, and hence more susceptible to corrosion. Certain elements, for example copper in the 2xxx alloy series, are quite detrimental in that respect.

Why is the fatigue resistance alloy dependent? 

Fatigue is the process of failure under cyclic loading. The fatigue resistance is a complex material property for it combines the effects of strength and ductility of the metal, both associated to the alloy content. Strength determines the (minimum cyclic loading) stress required for crack initiation and ductility determines the rate of crack growth to a critical dimension.

Why is the anodising ability alloy dependent? 

Anodising is an electrolytic surface treatment during which aluminium is anodically oxidised to form a thick, hard, corrosion resistant oxide layer on its surface. Alloy (often intermetallic) precipitates can during anodising either be inert (to be occluded into the oxide layer), or dissolve in the acidic electrolyte, or evolve oxygen gas (creating gas filled holes in the oxide), and/or be anodised themselves (disturbing locally the porous oxide structure). Certain solute species, such as solute copper, migrate under the high electric field during anodising, to become enriched at the metal/oxide interface, weakening the oxide properties.

Why is the weldability alloy dependent? 

Weldability depends for most types of welding on the high temperature behaviour of the alloy. Generally the heat treatable alloys, especially the 2xxx series alloys, can have problems with loss of properties after welding.

Important note

Aside from the alloying elements intentionally added to the molten metal for the improvement of various functional (mostly mechanical) or other properties (for example, grain refiners during solidification), there are always trace elements present from the ore, or from the electrolysis process of the aluminium, that can not be easily removed, for example Fe, Pb, Bi,….

Although initially present in the primary aluminium at trace levels, the amounts of these elements may increase in recycled aluminium. These elements may also influence certain properties, for example the corrosion susceptibility, fatigue and fracture behaviour. As a result, special attention needs to be focussed on the behaviour of these trace elements during recycling and their possible influence on the properties of recycled aluminium.

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