What are alloy hardeners and why do we add them to pure or primary aluminium?
By the very term alloying hardeners, to can imagine why the first series of alloying ‘ingredients’ were added to pure aluminium.
Adding alloys or a combination of alloys began because of several discoveries on the use of primary aluminium for industrial applications. Aluminium in its pure form has low strength, limited conductivity, ductility and viscosity (required for die casting).
Pure aluminium has a yield strength of only 15MpA when compared to wrought Iron (around 150Mpa) you can see how it is limited for some applications, therefore adding ‘hardeners’ can modify the behaviour of aluminium .
These alloys can be categorised as “major” alloying additions. Sadly some of the effects were positive however some not so and other alloying elements we used to counter or cancel these detrimental effects this can be known as “minor” alloying additions.
Used in combination the addition of alloying elements to aluminium is the principal method used to produce a selection of different materials that can be used in a wide assortment of packaging, structural, aerospace and automotive applications.
And so the Metallurgist is born.
When we refer to an aluminium series we know specifically the alloying ‘major’ addtions based upon the series quoted. For example
1xxx series – Primary / pure aluminium >99.00%
2xxx Series – With the addition of Copper (Cu)
3xxx Series – With the additions of Manganese (Mn)
4xxx Series – With the addition of Silicon (Si)
5xxx Series – With the addition of Magnesium (Mg)
6xxx Series – With the addition of Mg and Si
7xxx Series – With the Addition of Zinc (Zn)
The principles of the addition of these elements to aluminium are as follows.
2xxx Series (Copper Additions Cu)
Typically the aluminium Copper (AlCU) alloys contain between 2% and 10% copper and sometimes containing other minor elements to give additional properties to the final product.
The copper facilitates precipitation hardening and adds substantial strength to the aluminium alloy. The addition of Cu reduced ductility of the alloy and also reduces corrosion resistance.
Aluminium / Copper alloys are more prone to solidification cracking with an increase in the Cu content within the alloy.
It is not unheard of for large cast AlCu slabs to crack even after several hours have past since casting.
Welding of Cu alloys is difficult but Al Cu alloys are some of the highestst strength aluminium alloys known with applications ranging from defence (military vehicles) to aerospace.
3xxx Series (Manganese Additions Mn)
The addition of manganese to aluminium increases strength through solution strengthening and improves strain hardening while not appreciably reducing ductility or corrosion resistance.
These are moderate strength non heat-treatable materials that retain strength at elevated temperatures and are seldom used for major structural applications. The most common applications for the 3xxx series alloys are can bodies, cooking utensils, radiators, air conditioning condensers, evaporators, heat exchangers and associated piping systems.
4xxx Series (Silicon Additions Si)
The addition of silicon to aluminium reduces melting temperature and improves fluidity. Silicon unlike many other additions soes not form an intermetallic compound with aluminium.
Silicon alone in aluminium produces a non heat-treatable alloy; however, in combination with magnesium it produces a precipitation hardening heat-treatable alloy. Consequently, there are both heat-treatable and non heat-treatable alloys within the 4xxx series. Silicon additions to aluminium are commonly used for the manufacturing of castings. The most common applications for the 4xxx series alloys are filler wires for fusion welding and brazing of aluminium.
5xxx Series (Magnesium Mg)
Magnesium is added to aluminium to increase strength through solid solution strengthening and increases the strain hardening ability.
Al Mg alloys are the strongest of the non heat treatable alloys and are therefore used in many structural and construction applications.
5xxx series aluminium is generally cast as a flat rolled product (sheet or plate) for the workability and strengthening mechanism and rarely cast as an extrusion billet due to the work hardening during the extrusion process and the difficulty that brings.
Common applications for the 5xxx series alloys are Can End Stock (CES), truck and train bodies, construction and buildings, defence / armoured vehicles, ship building, chemical tankers, SCUBA, pressure and cryogenic tanks.
6xxx Series (Magnesium and Silicon Mg / Si)
The addition of magnesium and silicon to aluminium produces the compound magnesium-silicide (Mg2Si).
The formation of this compound provides the 6xxx series their heat-treatability. The 6xxx series alloys are easily formed and ductile and therefore often used in extruded profile shapes.
These alloys form an important complementary system with the 5xxx series magnesium alloys.
The 5xxx series alloy used in the form of plate with the extruded profile form often joined to the plate.
Common applications for the 6xxx series alloy include, construction materials (windowframes) automotive parts (crash protection, safety and components), scaffolding, handrails, drive shafts, bicycle frames, ladders and lawn furniture.
7xxx Series (Zinc Zn)
The addition of zinc to aluminium (in conjunction with some other elements, primarily magnesium and/or copper) produces a heat-treatable alloys exhibiting some of the highest strengths.
The zinc substantially increases strength and permits precipitation hardening.
Some of these alloys can be susceptible to stress corrosion cracking and for this reason are not easily welded.
Because of the high strength of Zn alloys they are often used in military applications. Some other common applications include aerospace, vehicle panels, baseball bats and bicycle frames.
Below are some other elements added to aluminium to change its mechanical or physical parameters.
Iron (Fe)
Iron is the most common impurity found in aluminium and is intentionally added to some pure (1xxx series) alloys to provide a slight increase in strength. Common Fe levels in primary metal range from 0.07 to 0.20wt%.
Titanium (Ti)
Titanium is added to aluminium primarily as a grain refiner. The grain refining effect of titanium is enhanced if boron is present in the melt or if it is added as a master alloy containing boron largely combined as TiB2. Titanium is a common addition to aluminium weld filler wire as it refines the weld structure and helps to prevent weld cracking.
Chromium (Cr)
Chromium is added to control grain structure, to prevent grain growth in aluminium-magnesium alloys, and to prevent recrystallization in aluminium-magnesium-silicon or aluminium-magnesium-zinc alloys during heat treatment.
Chromium will also reduce stress corrosion susceptibility and improves toughness.
Nickel (Ni)
Nickel is added to aluminium-copper and to aluminium-silicon alloys to improve hardness and strength at elevated temperatures and to reduce the coefficient of expansion.
Zirconium (Zr) – Zirconium is added to aluminium to form a fine precipitate of intermetallic particles that inhibit recrystallization.
Lithium (Li)
The addition of lithium to aluminium can substantially increase strength and, Young’s modulus, provide precipitation hardening and decreases density.
Lead (Pb) and Bismuth (Bi) – Lead and bismuth are added to aluminium to assist in chip formation and improve machinability. These free machining alloys are often not weldable because the lead and bismuth produce low melting constituents and can produce poor mechanical properties and/or high crack sensitivity on solidification. Very much restricted in use due to health related implications.
Summary
The aluminium association has over 400 registered wrought alloys in circulation and over 200 casting alloys each with its own benefits and mechanical behaviour.
Careful selection of the alloy is required for any application and each will come with its pro’s and cons and castability, rollability and extrudability.
Thanks for reading this short 2 minute blog and please feel free to get in touch and find out more. The blog is written based upon academic and vocational training and through research papers presented in global forums and available in the public domain. I have taken advise from present and former technical colleagues and with my own insights from experience in the field and in the casthouse.
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