METALS HISTORY The Discovery of Strong Aluminum

Charles R. Simcoe ow did aluminum, a metal that is light Interest in Wilm’s alloy spread throughout Watseka, Ill. H in weight (only one-third as heavy as the metal-making world. Samples of the alloy steel) and in its unalloyed form is were obtained, and studies were undertaken on The discovery among the weakest of all metals, become the the hardening mechanism at the United States structural material of modern airplanes? This Bureau of Standards in Washington, D.C. The of age story started in the very early 1900s, the in- research was performed by Paul Merica, hardening of fancy of the aluminum industry. At this time, Howard Scott, and R.G. Waltenberg, and they aluminum was the aluminum market was mostly in cooking would become famous as the next major con- a seminal utensils, pots, and pans. The leap from such tributors to the amazing story of age harden- small, mundane beginnings to building Zep- ing. They found that nature had held a secret event in the pelins in a few years, Douglas DC-3s in 25 on a method of hardening metals, and that history of years, and Boeing 747s in 60 years is a remark- Wilm’s alloy was just a single example of a uni- metals. able feat of engineering. versal behavior that was undiscovered since an- Read the The first character in this drama was a Ger- cient man had learned to make alloys during complete man engineer by the name of Alfred Wilm. He the age of bronze. Paul Dyer Merica was the began a study to replace the heavy brass alloy, senior member of the research team. article at which was the time-honored jacket material for Based on the results of their research, the www.metals- cartridges, with an aluminum alloy for weight team concluded: history. savings. Wilm, being familiar with heat treat- • Age hardening required an alloy where the blogspot.com. ing of steel, attempted to combine both alloying second metal was soluble in the base and heat treating in his research. No heat treat- metal at an elevated temperature, but was ment known at that time could harden metals, considerably less soluble at low except for steel. temperatures. Wilm was working with an aluminum-cop- • The alloy samples had to be heated at this per-manganese (Al-Cu-Mg) alloy, water- higher temperature to take the second quenching it from a high-temperature molten metal into solution and then salt bath. The strength of his heat-treated sam- water-quenched to a low temperature to ples was not sufficiently high to interest the maintain a supersaturated solution; at this army. He next tried adding 0.5% Mg to his alloy. point the metal alloy was relatively soft. After heating and water-quenching, Wilm re- • Aging represented the precipitation of the ported that he gave a sample to his assistant, second metal or a compound between the Fritz Jablouski, for hardness testing. Because it two metals; this occurred at room was a Saturday afternoon, Jablouski wanted to temperature in the case of Wilm’s alloy, leave the laboratory early. Wilm urged him to although aging was accelerated by heating take at least one reading and then finish on to a higher temperature. Monday. This reading showed only a small in- This knowledge immediately set off a crease in hardness due to the Mg addition. worldwide search for other alloys that could However, on Monday, the hardness was consid- obey the conclusions of Merica, Scott, and Wal- erably higher. Wilm and Jablonski had observed tenberg. Numerous alloy compositions were a room-temperature hardening (or strengthen- found in base metals of aluminum, cobalt, cop- ing) with time in a metal alloy, something never per, gold, iron, lead, magnesium, nickel, silver, before observed by metals makers. Wilm’s heat platinum, zinc, and others. treatment was called age hardening. Aluminum Company of America (Alcoa), America’s only aluminum company, developed a new higher strength alloy in the 1930s. The major change was to increase the magnesium level to 1.5% from the 0.5% of Wilm’s alloy. This increased the design strength to 50,000 psi from 40,000 psi. In addition, it was found that some moderate cold working, such as stretch- Paul Dyer Merica ing or rolling sheet material immediately after led the team that water quenching and then aging, could further discovered the increase the design strength to 57,000 psi. This principles governing precipitation The DC-3, the first successful commercial alloy was the major construction material for hardening. all-aluminum airplane (American Airlines). the first commercially successful passenger

ADVANCED MATERIALS & PROCESSES • AUGUST 2011 35 plane, the Douglas DC-3 (1935), and for the nearly 300,000 A better understanding of metal behavior during plas- planes built in the U.S. during World War II. tic deformation began in the late 1920s and the1930s with the work of a number of European research scientists. Other systems They developed the concept of a crystal lattice defect that Another precipitation-hardening alloy system of great they called a dislocation. industrial importance is based on adding small amounts of Dislocations were segments of a row of atoms that did magnesium and silicon to aluminum. This alloy system is not line up with the atoms on the next row as they do in a the structural material for a great tonnage of ordinary en- perfect crystal. The scientists found that dislocations could gineering applications. move if enough stress was applied to the metal, and that While the conclusions of Merica, Waltenberg, and Scott the movement or gliding of dislocations could account for were sufficient to send metals workers off on the successful plastic deformation in metals. Also, dislocations could in- search for precipitation-hardening alloys in both aluminum teract with one another and with grain boundaries; they and other metals; they could multiply by various means to explain the shape of the were not precise enough plastic deformation curves. to explain to metals sci- The concept of dislocations as the instrument of plas- entists how the harden- tic deformation was also useful in explaining the strength- ing occurred. It was ening of alloys by precipitation hardening. If dislocations generally believed that at moving under an applied stress produced plastic behavior, some early stage in the then anything restricting this movement increased the precipitation of the sec- strength. The method by which precipitates accomplished ond material, a set of this result became the challenge for many metal re- particles of very small searchers over the period from 1935 to the 1960s. size (perhaps on the Two researchers in Europe, A. Guinier and G. D. Pre- The Destiny Research Module for the Space atomic scale) provided a ston, independently identified the first stage of the move- Laboratory (Boeing); various strong aluminum alloys were used in its keying action to plastic ment of atoms to sites in the lattice in preparation for construction. deformation. forming the final precipitate. These sites were rich in cop- per atoms in an aluminum-copper alloy, but still retained the crystal structure of aluminum. The sites of this first stage in precipitation are called G-P zones in honor of Guinier and Preston. Other researchers found that with longer aging times, a more advanced stage in progression toward the final precipitate occurred. This precipitate showed a new crystal lattice that was not yet the crystal structure of the final precipitate. As long as these particles were constrained to con- form to the crystal lattice of aluminum (a condition called coherency), they provided additional strengthening. When they grew larger and farther apart with additional aging and formed their own boundary be- tween them and the aluminum, the strength decreased. Wilm’s discovery of age hardening is one of those seminal events in technical history. He didn’t invent an alloy, he dis- covered a new universe. This is a rare oc- currence experienced by only a few researchers in all of science.

For more information: Charles R. Simcoe has done research at Battelle Memorial Insti- tute, Armour Research Foundation, and in industry. He retired as lecturer in metallurgy at the State University of New York at Buffalo. He can be reached at crsimcoe@ www.clemex.com yahoo.com.

36 ADVANCED MATERIALS & PROCESSES • AUGUST 2011