< ALUMINIUM 1
ALUMINIUM 2
NO ALLOYING ELEMENT LM0, 1XXX, ASTM 150
Primarily used for electrical conductivity purposes and food processing utensils, high purity aluminium is not a widely used art founding metal, mainly due to it’s inherently low tensile strength. High purity wrought aluminium is sometimes found in the form of round wire or square bar which is often used as armature material affixed to modelling posts to support small scale sculptures in clay. This metal is easily bent to shape and manipulated by hand, whilst still retaining an adequate strength to support a modest amount of clay or other modelling medium. When welding is done, joints in pure aluminium can be filled using a 1080A or 1080B type filler rod. If the artwork is destined for an ANODISED finish, the use of high purity aluminium for casting may be considered, this is because purer grades are better suited to an anodising process than heavily alloyed derivatives (especially the more common silicon bearing versions). High purity aluminium is classed as being non-heat treatable.
COPPER LM12, 2XXX, ASTM 222
Copper additions greatly improve the STRENGTH and HARDNESS of aluminium, though this is at the expense of castability. There is an increased potential for casting faults to arise if this grade of alloy is used. In addition to casting difficulties, corrosion resistance is lessened and weldability is also poor – as such this grade is not normally specified for making cast or fabricated artworks. When used, aluminium grades alloyed with copper are usually both HEAT TREATED and aged hardened.
MANGANESE 3XXX
Present to some degree in nearly all aluminium alloys, the addition of this element in excess of 0.5% improves DUCTILITY, an especially useful a quality for wrought materials that are used in FORMING processes (pressing etc). The addition of manganese to a cast alloy improves resistance to contraction and shrinkage faults. Manganese rich alloys are not heat treatable.
SILICON LM20 (ASTM A413), LM25 (ASTM A380), 4XXX, 6XXX
Silicon is one of the most important alloying elements found in aluminium materials, present to some extent in all nearly all alloy grades, but especially in those noted in the heading (LM20 for example contains some 10-13% silicon). Silicon greatly improves the casting characteristics of aluminium, producing a free flowing CHARGE which is resistant to many of the more common aluminium casting faults, especially hot tearing. The alloys usually selected for casting art and design works contain rather less silicon than LM20; typical of these is LM25 (ASTM A380), alloyed with between 6.5% and 7.5% silicon.
Silicon rich aluminium (especially ISO 4043A & 4047A), is often used as a general purpose aluminium filler rod for welding. Rods of these compositions are suitable for joining the majority of aluminium alloy grades, with the exception of those containing magnesium (see below), where the use of a specific filler rod is necessary. 6XXX series alloys contain both silicon and magnesium additions and are heat treatable.
MAGNESIUM LM5 (ASTM 514), 5XXX, 6XXX
Magnesium, like manganese and silicon, is present to some degree in most aluminium alloys. In small amounts this element improves the MECHANICAL PROPERTIES of the metal. Magnesium grades are especially suitable for welding, by either a TIG (AC) or MIG welding process, as they are resistant to weld cracking, a potential fault in some other grades. The filler wire used for building up welded joints in magnesium bearing grades is usually an ISO 5056A or similar rod. Magnesium rich alloys with a low silicon content are often preferred for decorative and architectural use, especially if an anodised or bright POLISH finish is specified. Low silicon bearing magnesium alloy (5XXX series) is not heat treatable, but silicon/magnesium alloy combinations (6XXX series) are heat treatable.
ZINC LM31 (ASTM 712), 7XXX
Alloying with zinc produces an aluminium product in the medium to high TENSILE STRENGTH range, providing an excellent strength to weight ratio. With additional alloying elements such as copper and magnesium, a zinc bearing aluminium also offers excellent heat treatment and age hardening characteristics. Casts formed in LM31 are usually allowed to age harden for up to three weeks before use, if the attainment of a full metallurgical strength is critical.
OTHER ALLOYING ELEMENTS LMX, 8XXX
Other elements are present in various grades of aluminium, typically iron, which is often present in die casting alloys, and nickel which is sometimes added to the copper bearing alloy to further improve it’s tensile strength. For wrought aluminium alloys, special grades are usually prefixed by the number ‘8’. These grades may or may not be heat treatable according to their exact composition. Special alloys are usually of only of very limited interest to art and design founders and fabricators.
SELECTING AN ALUMINIUM ALLOY >
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