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ADVANCED MATERIALS & PROCESSES •
NOVEMBER-DECEMBER 2014
Metallurgy Lane,
authored by
ASM life member
Charles R. Simcoe
,
is a yearlong series
dedicated to the early
history of the U.S. metals
and materials industries
along with key
milestones and
developments.
Aluminum: The Light Metal—Part III
Alcoa’s aluminum monopoly continued throughout the 1920s and 1930s—
a serious problem when World War II demands far exceeded production capacity.
A
lcoa acquired the laboratory of the Alu-
minum Casting Co. in 1920 as payment for
the debt owed on aluminum purchases
made during World War I. Two metallurgists with
aluminum experience came with the laboratory—
Zay Jeffries and Robert Archer. During WWI, Jef-
fries worked on aluminum casting problems,
mainly with ordinance fuses and the Liberty Air-
craft Engine. Jeffries and Archer continued to make
major contributions to the field of cast and forged
aluminum alloys throughout the 1920s, working in
the Cleveland laboratory.
With two lab facilities dedicated to research
and development throughout the 1920s and 1930s,
more than 20 new alloys were added to Alcoa’s
product line. The first new sheet alloy—called
17S—was based on the German alloy, Duralumin.
It made its debut in the 1920s and was used to
build the first commercial all-metal passenger air-
plane in the U.S., the Ford Trimotor, with roughly
200 manufactured in the late 1920s and early
1930s. Production ceased as the Great Depression
deepened and the 10-15 passenger design became
cost prohibitive for commercial service.
The precipitation hardened 17S alloy also
lacked sufficient corrosion resistance in a salt spray
atmosphere. This problem was solved by a new
process that bonded a more corrosion resistant
layer of pure aluminum to both sides of the 17S
sheet metal. These two layers make up about 10%
of total sheet thickness. This product—named Al-
clad—is still used in aluminum alloy applications
exposed to corrosive atmospheres.
Alloy development
In the 1930s, Alcoa developed a higher strength
alloy called 24S. The major change from 17S to 24S
involved boosting the magnesium level from 0.5%
to 1.5%. This increased the design strength of 24S
to 50,000 psi, from 40,000. In addition, moderate
cold working, such as stretching or rolling the
sheet material immediately after water quenching
and then aging it, further increased 24S design
strength to 57,000 psi. All of these properties could
be produced with Alfred Wilm’s original room
temperature treatment called
natural aging.*
The
new alloy was used to construct the first commer-
cially successful passenger plane, the Douglas
DC-3 in 1935.
Another precipitation hardening alloy system
developed by Alcoa in the 1930s adds 1% magne-
sium, 0.6% silicon, and 0.3% copper to aluminum.
This alloy—called 61S (now 6061)—is the struc-
tural material for a great tonnage of ordinary engi-
neering applications. A number of alloys based on
6061 contain additional alloying elements and are
widely available as well. These alloys are known for
ease of fabrication, corrosion resistance, and low
cost compared to high-strength aircraft alloys.
They feature design strengths of 35,000 to 50,000
psi and have excellent characteristics for general
industrial applications, such as trucks, buses, rail
cars, trailer tanks, storage tanks, building construc-
tion, and light aircraft. Some of the numerous mill
products made of these alloys include sheet metal,
forgings, extrusions, bar, tubing, pipe, and wire.
Aluminum for WWII
Alloy 24S was the aluminum used for nearly all
of the 300,000 planes built in the U.S. during World
War II. The quantity of aluminum needed for this
vast undertaking greatly exceeded the capacity of
Alcoa, the only aluminum manufacturer in the
country. In 1939, the U.S. produced 148,000 tons
Alcoa’s 17S sheet alloy was based on the German
alloy, Duralumin, and was used to build the first
commercial all-metal passenger airplane in the U.S.,
the Ford Trimotor. Shown here is the Ford 5-AT-B
“City of Columbus” flown by Charles Lindbergh.
Public domain image.