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Filler metals
Pure aluminium is not usually used for structural applications. To produce aluminium that is of adequate strength for the manufacture of structural components, it is necessary to add other elements to the aluminium. So, what elements can be added to these aluminium alloys? Does adding these elements have any effect on the material’s performance? In what applications are these alloys used?
It would be very unusual to find pure aluminium (1xxx series of alloys) chosen for structural fabrication because of their strength characteristics. Although the 1xxx series are almost pure aluminium, they will respond to strain hardening and especially so if they contain appreciable amounts of impurities such as iron and silicon. However, even in the strain-hardened condition, the 1xxx series alloys have very low strength when compared to the other series of aluminium alloys. When the 1xxx series alloys are chosen for a structural application, they are most often chosen for their superior corrosion resistance and/or their high electrical conductivity. The most common applications for the 1xxx series alloys are aluminium foil, electrical bus bars, metallising wire, and chemical tanks and piping systems.
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 structural applications.
If we consider the seven designated aluminium alloy series used for wrought alloys, we can immediately identify the main alloying elements used for producing each of the alloy series. We can then go further and examine each of these elements’ effects on aluminium. We have also added some other commonly used elements and their effects on aluminium.
Copper (Cu) 2xxx – The aluminium-copper alloys typically contain between 2 to 10% copper, with smaller additions of other elements. The copper provides substantial increases in strength and facilitates precipitation hardening. The introduction of copper to aluminium can also reduce ductility and corrosion resistance. The susceptibility to solidification cracking of aluminium-copper alloys is increased. Consequently, some of these alloys can be the most challenging aluminium alloys to weld. These alloys include some of the highest-strength heat treatable aluminium alloys. The most common applications for the 2xxx series alloys are aerospace, military vehicles and rocket fins.
Manganese (Mn) 3xxx – The addition of manganese to aluminium increases strength somewhat through solution strengthening. It also 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 cooking utensils, radiators, air conditioning condensers, evaporators, heat exchangers and associated piping systems.
Silicon (Si) 4xxx – The addition of silicon to aluminium reduces melting temperature and improves fluidity. 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.
Magnesium (Mg) 5xxx – The addition of magnesium to aluminium increases strength through solid solution strengthening and improves their strain hardening ability. These alloys are the highest strength non-heat-treatable aluminium alloys and are, therefore, used extensively for structural applications. The 5xxx series alloys are produced mainly as sheets and plates and only occasionally as extrusions. The reason for this is that these alloys strain to harden quickly and are therefore difficult and expensive to extrude. Some common applications for the 5xxx series alloys are truck and train bodies, buildings, armoured vehicles, ship and boat building, chemical tankers, pressure vessels and cryogenic tanks.
Magnesium and Silicon (Mg2Si) 6xxx – 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 and economically extruded. For this reason, the 6xxx alloys are most often found in an extensive selection of extruded shapes. These alloys form an important complementary system with the 5xxx series alloy. The 5xxx series alloy is used in the form of plates and the 6xxx are often joined to the plate in some extruded form. Some of the common applications for the 6xxx series alloys are handrails, driveshafts, automotive frame sections, bicycle frames, tubular garden furniture, scaffolding, stiffeners and braces used on trucks, boats and many other structural fabrications.
Zinc (Zn) 7xxx – The addition of zinc to aluminium (in conjunction with some other elements, primarily magnesium and/or copper) produces heat-treatable aluminium alloys of the highest strength. 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 usually fusion welded. Other alloys within this series are often fusion welded with excellent results. Some of the common applications of the 7xxx series alloys are aerospace, armoured vehicles, baseball bats and bicycle frames.
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.
Chromium (Cr) – Chromium is added to aluminium to control grain structure and to prevent grain growth in aluminium-magnesium alloys. It also helps prevent recrystallisation in aluminium-magnesium-silicon or aluminium-magnesium-zinc alloys during heat treatment. Chromium will also reduce stress corrosion susceptibility and improve toughness.
Nickel (Ni) – Nickel is added to aluminium-copper and aluminium-silicon alloys to improve hardness and strength at elevated temperatures and to reduce the coefficient of expansion.
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.
Zirconium (Zr) – Zirconium is added to aluminium to form a fine precipitate of intermetallic particles that inhibit recrystallisation.
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.
There are many aluminium alloys used in the industry today. Over 400 wrought alloys and over 200 casting alloys are currently registered with the Aluminium Association. Certainly, one of the most important considerations encountered during the welding of aluminium is the identification of the aluminium base alloy type to be welded. If the base material type of the component to be welded is not available through a reliable source, it can be difficult to select a suitable welding procedure.
There are some general guidelines as to the most probable type of aluminium used in different applications, such as those mentioned above. However, it is very important to be aware that incorrect assumptions as to the chemistry of an aluminium alloy can result in very serious effects on the weld performance. It is strongly recommended that positive identification of the type of aluminium is made and that welding procedures be developed and tested in order to verify weld performance.