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metal science

Material science — metals

The physical structure of wood (and to some extent plastics) provides a tactile feedback loop that reflects/mirrors the micro structure of the material. This is not the case with metals. To understand the "feel" of metal requires a cerebral empathy with atomic structure.

Depending on the metal, individual metal atoms pack themselves into one of three common geometrically aligned lattices as shown below.

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HCP (hexagonal close packed) - zinc, magnesium

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FCC (face-centred cubic) - aluminum, copper, nickel

These alignments/lattices often span millions of atoms to form one "grain," and millions of grains comprise the metal of the average furniture component. The three-dimensional orientation of each grain is at random angles to its neighbours (diagram, left), producing disordered alignments of atoms at the grain boundaries (diagram, right).

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(left) Three-dimensional model of metal atoms
(right) Two-dimensional diagram showing disordered atoms (white) at the boundaries of ordered lattices (black)

When a stress is applied to the atomic lattice it will "slip" along certain geometrical planes that vary in size, number and importance, depending if the alignment is HCP, FCC or BCC. The lattice will also contain flaws (missing atoms, misaligned rows) that will travel freely through the lattice when stress is applied, which explains metal ductility. However, continual stress will rid the lattice of flaws (by pushing them to the grain boundary) to produce a more rigid/strong metal, which explains another metal property - work-hardening.

The outer electrons of metallic atoms are loosely held by the nucleus and exist in a fluid-like state shared by all the atoms of the metal. This "migration" of electrons explains the high thermal and electrical conductivity properties of metals.

Common furniture metals can be grouped into two main classifications - ferrous and non-ferrous, from the Greek word ferro, which means iron. Steel and stainless steel are typical ferrous iron alloys; aluminium is a typical non-ferrous metal.

Ferrous

Steel is the strongest and least expensive of common furniture materials (see material science chart) and an alloy (mixture) of iron, carbon and small quantities of other elements. Unlike other metals, the alignment of iron atoms is temperature dependent: BCC below 910 °C, FCC between 910 °C and 1400 °C and BCC (again) above 1400 °C. This unique property can be manipulated by metalurgists and manufacturers (usually through the application of heat and rapid cooling) to create steel grains of varying size and structure. Steel alloys with smaller grains will be stronger (less ductile), because the grain dislocations have a shorter distance to travel before being stopped at the grain boundaries.

The numbering system established by the American Iron and Steel Insitute (AISI) defines the common steels used for furniture profiles (tube, angle, strip, sheet, etc.) as low carbon AISI 1008, 1010 and 1020 - the last two digits indicate the carbon content.

Stainless steel, the Rolls Royce of ferrous steel, costs five to eight times more than carbon steel. On the plus side, using less of it gets the job done and without the need for toxic plating or finishing procedures. It is available in three basic molecular alignments: ferritic (BCC), austenitic (FCC) and martensitic (several). For fabricating furniture components, the workhorse alloy is austenitic AISI 304 - typically containing 74 percent iron, 18 percent chromium and 18 percent nickel.

Non-ferrous

Only iron and silicon are more plentiful than aluminum in the Earth's crust. Aluminum is about double the cost of steel but forms a stable protective oxide surface that replaces the time-consuming finishing procedures needed for steel furniture components. Environmentally, extracting aluminum from ore consumes a large amount of electrical energy; fortunately, aluminum can be recycled easily, using only about five percent of the original energy required.

In its pure form, aluminum is a soft, ductile FCC lattice metal that needs to be alloyed with metals such as mangnesium with its slip-resistant HCP lattice structure. Aluminum can be strengthened by work-hardening and heat treatment. The workhorse alloy for furniture production is 6061 (alloy numbers designated by the Aluminum Association) for general fabrication, including welding. 6063 is used for extrusions.

For more information research Aluminum by Design an exhibition organized by the Carnegie Museum of Art that traced the history of aluminum from its first use as a precious metal in the nineteenth century to its evolving and enduring role in everyday life.

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