Engineering

Part 9 - Aluminum 2

In 1883, American engineer Charles Bradley found that externally heated containers for electrolytic baths corroded rapidly in contact with molten salts and started heated the aluminum salts internally so that the highest temperature was inside the bath and the lowest was on its walls. 

The very high melting point of pure alumina (2,072 °C) made electrolysis impractical until 1886 when French engineer Paul Héroult and American engineer Charles Martin Hall independently solved the problem by dissolving alumina in cryolite (sodium hexafluoroaluminate, Na3AlF6) which has a melting point of only 1,000 °C. Pure aluminum metal has a melting point of only 660 °C. This discovery made possible large-scale electrolytic production methods. 

In 1888, Héroult founded Aluminium Industrie in Germany but later returned to France. The same year, Hall co-founded the Pittsburgh Reduction Company in Oberlin, Ohio, USA. The large direct current flowing from the carbon-block lined tank cathode to the carbon brick consumable anode through the mass of alumina and cryolite generated enough heat to melt the cryolite and dissolve the alumina. The oxygen was attracted to the consumable carbon anode and burned off as carbon dioxide while the molten aluminum was attracted to the cathode and settled on the carbon bottom of the tank. By September 1889, Hall's production was 385 pounds (175 kilograms) at a cost of just $0.65 per pound.

In 1888, Austrian chemist Carl Josef Bayer discovered a way of extracting alumina from bauxite directly. By sintering bauxite with alkali and leaching it with water, he found a precipitate of pure aluminum hydroxide, which decomposed to alumina by calcining (heating) to drive off the hydrogen. In 1892, he discovered that alumina, dissolved in a strong alkaline solution like sodium hydroxide (caustic soda), greatly simplified the process.

By the end of 1889, the production of a consistently high purity aluminum from electrolytic processing proved cheaper than all other methods. (Between 1889 and 1894 the price of aluminum dropped from $2 to $0.50 per pound). 

In 1903, the Wright brothers in Dayton, Ohio, could not find a engine suitable for their new aircraft. So they made their own using aluminum alloyed with copper, obtained from the new Pittsburgh Reduction Company of Ohio that later became the Aluminum Company of America. The brothers painted it black to keep their use of the metal secret.

Like many other metals, pure aluminum was too soft for practical use so metallurgists found ways to make it stronger with the addition of small amounts of alloying metals with no significant increase in weight. The first of these were developed by a German metallurgist, Alfred Wilm in 1903, who discovered that, after quenching, an aluminum alloy containing 4% copper, it would age harden (slowly harden when left at room temperature for several days). He named this duralumin.

While pure aluminum has a maximum tensile strength of 60 MPa (Mega Pascales)(8,700 psi), duralumin has a tensile strength of 450 MPa (65,000 psi). Other heat treated alloys may reach 700 MPa (101,000 psi).

Duraluminium contains between 91 and 95% aluminum, up to 5% copper, 0.9% manganese and 1.8 %magnesium with less than 0.5% iron and silicon and smaller amounts of zinc, titanium and chromium.

Alloy 7075 was developed in Japan by Sumitomo Metal in 1935. It has a composition about 91% aluminum, 5.6–6.1% zinc, 2.1–2.5% magnesium, 1.2–1.6% copper, and less than a half percent of silicon, iron, manganese, titanium, chromium, and other metals.

Alloy 7075-T6 is heat treated by homogenizing the 7075 casting at 450 °C for several hours, quenching, and then ageing at 120 °C for 24 hours. This yields the maximum ultimate tensile strength of 510–540 MPa (74,000–78,000 psi).

In 1916, Hugo Junkers built the wings and fuselage of a aircraft to demonstrate the possibility of an all duralumin airframe. This technique was used in the Ju 52 tri-motor transport aircraft build between 1931 and 1952. One of the first of these was purchased by Canadian Airways in 1932.

Anodizing (an electrolytic process to increase the thickness of the natural oxide layer on the surface of metals) was first used in 1923 to protect Duralumin seaplane parts from corrosion. (Anodizing improves resistance to corrosion and wear and provides better adhesion for paint and adhesives. Thick anodic films can absorb dyes while thin transparent films add interference effects to reflected light. While mainly used with aluminum, anodizing is also used for hafnium, magnesium, niobium, titanium, tantalum, zinc and zirconium.

Duralumin was used to build all of the rigid airships including the British built R-100, the German passenger Zeppelins and four U.S. Navy airships.

The first world war created a huge demand for aluminum. In 1916, annual world production exceeded 100,000 metric tons compared to 6,800 metric tons in 1900.In 1900, the price of aluminum was $6 per pound ($14,000 per metric ton) (in 1998 United States dollars) and by 1948 it had fallen to $1 per pound ($2,340 per metric ton). In 1919 Germany began replacing silver coins with aluminum to deal with hyperinflation. By mid-century, aluminum was an essential part of housewares, ship building, construction, railcars, trucking and military equipment. The reduced weight of transport vehicles allowed them to carry more cargo or use less fuel. Production exceeded 1,000,000 metric tons the first time in 1941, during World War II, most of this was used for aircraft production.

After the German air attacks on Britain in 1940, the Minister of Aircraft Production appealed to the public to donate any household aluminum for aircraft construction. In 1941, the Soviet Union obtained more than 300,000 metric tons of aluminum, to make aircraft and tank engines, from Britain and the USA.

Ammonal is an explosive made up of aluminum powder (produced in an inert atmosphere to prevent it oxydizing) and ammonium nitrate. Because of the large surface area of the particles, the aluminum is explosively flammable. The ammonium nitrate functions as an oxidizer and the aluminum as fuel. 

From early 1916, the British Army used the relatively cheap ammonium nitrate and aluminum as a replacement for pure TNT in bombs, shells and mines. During the Battle of Messines in World War I, several mines, containing up to 30,000 lbs (over 13.6 tonnes) of ammonal, were placed at the end of long tunnels dug beneath the German lines. These were exploded on 7 June 1917 creating 19 large craters and killing 10,000 German soldiers in one of the largest non-nuclear explosions in history.