May_AMP_Digital

Overcoming Barriers

of Magnesium Ignition and Flammability

lthough weight reduction is a worthy

goal for all forms of transportation, it is

of particular importance to aerospace

applications. Today, the global aviation industry

produces roughly 2% of all human-induced

emissions and 12% of carbon dioxide generated

by all transport sources. As the number of pas-

sengers traveling by air is projected to increase

more than fivefold to reach 16 billion in 2050,

the detrimental effects of emissions from burn-

ing jet fuel pose ever greater environmental

concerns. In the interest of reducing aircraft

weight, Federal Aviation Administration (FAA)

and various aerospace specification committees

are reevaluating the existing ban on magnesium

use inside commercial aircraft cabins

[1]

Considering magnesium

for commercial aviation

In modern aircraft design, a variety of light-

weighting options are explored. For example,

significant improvements in weight savings and

durability within airframe structures may be

achieved through composite materials and

fiber-metal laminates. The latter are advanced

hybrid material systems consisting of metal lay-

ers bonded with fiber-reinforced polymer lay-

ers. Case in point: Composites and

carbo-fibers represent approximately 50% of

the Boeing 787’s primary structure, including

its fuselage and wings. This substantial in-

crease from 12% composites implemented in a

Boeing 777 to 50% in the 787 emphasizes the

growing importance of these materials. In ad-

dition to fiber composites and polymers, there

is a need for structural metals, and magne-

sium—with its specific density of 1.8 g/cm

—

is the primary candidate. Potential weight

savings are substantial when compared to an-

other light-metal option—aluminum—with its

specific density of 2.7 g/cm

Although magnesium has been widely ap-

plied in a variety of aerospace applications since

the 1930s, after reaching its peak in the 1950s,

it was gradually diminished to only residual

quantities. Now, there is renewed interest in

magnesium for components inside the aircraft

cabin, such as overhead compartments, folding

tables, and food trolleys. Passenger seats are at

the top of the list because they offer significant

opportunities for weight reduction. As an ex-

ample, an Airbus aircraft with individual seat

weights from 11 kg for certified economy class

to about 20 kg for a wide body plane, and seat

quantities from 117 in the A318 to 700 in the

A380; total aircraft seat mass ranges from 1200-

14,000 kg

[2]

. Considering that roughly 43% of

the seat weight is comprised of aluminum al-

loys, replacing them with magnesium offers a

weight reduction of 28-30%.

The high strength-to-weight ratio, along

with other unique properties exhibited by mag-

nesium alloys are, however, overshadowed by

their high surface reactivity. In particular, a lack

of stability at increased temperatures is often

seen through ignition and burning when in

contact with an open flame or another heat

source (Fig. 1). For aerospace applications,

where in-flight and post-crash fires are a con-

cern, easy ignition is detrimental to safety. To

reduce the risk of magnesium ignition, a num-

ber of design-related options are being explored

to prevent a temperature increase during pos-

sible contact with a flame. The dominant fac-

tor affecting ignition resistance, however, is

controlled by the very nature of magnesium.

Ignition vs. flammability

The easy ignition of magnesium is typically

associated with powder-like forms, commin-

uted fine chips, or magnesium dust, which ig-

nite instantly after contact with a flame or

electric spark. These characteristics are widely

explored in pyrotechnics. By contrast, bulk

forms of magnesium do not ignite easily and to

start the reaction, a metal region or its entire

volume must reach a certain temperature.

Frank Czerwinski

CanmetMATERIALS

Natural Resources

Canada

Hamilton, Ontario

ADVANCED MATERIALS & PROCESSES •

MAY 2014

The use of

magnesium in

commercial

aircraft cabins

is being

reevaluated to

save weight,

as it is the

lightest

structural

metal.

Fig. 1 —

Flames of burning Mg-3%Al alloy with extensive fumes and

temperature exceeding 3000