Because magnesium is one-third lighter than aluminum, it is very appealing to automotive engineers. More and more automakers use using magnesium sheets and castings in front-end engine compartment applications, replacing what is currently sheet steel. However, because of the lack of elasticity in magnesium, at times traditional thread-forming screws are not functional.
Because magnesium is one-third lighter than aluminum, it is very appealing to automotive engineers. Manufacturers of laptop computers, cell phones, power tools, and lawn and garden equipment are also eager to use the material.
“Magnesium usage will not only increase in the automotive arena, but will be used more extensively by OEMs,” predicts Ken Gomes, vice president of marketing and engineering at Research Engineering & Manufacturing Inc. (REM, Middletown, RI), which specializes in designing fasteners. “In North America, there are cars and trucks being designed using magnesium sheets and castings in the front-end engine compartment area, replacing what is currently sheet steel.”
REM has reacted to this trend by developing the Magtite 2000 thread-forming screw. Because of the lack of elasticity in magnesium, at times traditional thread-forming screws are not functional.
“For these situations, Magtite 2000 is [ideal] because it creates a clean internal thread in the magnesium without creating debris due to the internal thread material crumbling or breaking away,” Gomes points out. “Magtite 2000 screws are ideal for use in internal powertrain attachments and electrical applications.”
However, using threaded fasteners in magnesium presents several challenges to engineers. “Problems with magnesium include galvanic corrosion concerns and its reactivity to other materials,” says Tim Bartlett, director of product development for Avdel products at Acument Global Technologies Inc. (Troy, MI). “This can require use of various types of special platings to isolate the fastener from magnesium, depending on the mechanical strength needed for the application.”
For several years, Taptite and Taptite 2000 steel thread-rolling screws, neutral hardened, have been used in simple attachment applications. “One of the biggest suppliers of magnesium automotive components to the North American automotive industry has informed us that in approximately 95 percent of their attachments in magnesium applications, these screws are specified,” says Gomes. “A major North American automotive company has been using M6 Taptite screws to assemble components to their magnesium instrument panel tubes.”
Manufacturing engineers have had success using these screws in magnesium applications because there is no remarkable thermo cycling involved and the applications are not in a corrosive environment. “Using thread-rolling screws can present a problem, as the resulting internal thread may not be sound enough to withstand multiple insertion and removal cycles,” warns Gomes. “Some debris in the internal thread can be generated.
“There are applications where multiple reuse of the fastener and debris generation are concerns,” adds Gomes. “For these types of applications, there are a few screws designed specifically for magnesium”
For example, Magtite 2000 screws have a special broad radius flank thread that tends to form a thread by compressing the magnesium. This creates an internal thread that is clean with little or no debris formed.
Thread-rolling screws designed for steel or aluminum roll form the internal thread, utilizing the elastic response of the steel or aluminum nut member. The nut material is easily displaced and the internal thread is formed.
“Magnesium does not possess this elastic response,” Gomes points out. “Instead of being fully volumetrically displaced, the material may crumble, creating an amount of magnesium chips or debris.
“For applications that are exposed to a corrosive environment, the concern is galvanic corrosion between the steel screw and the magnesium component,” says Gomes. “In spite of this issue, steel screws are still used. The challenge is to create a barrier between the steel screw and the magnesium.
“One popular method is to apply a plastic coating around the screw head and down along portion of the screw shank,” notes Gomes. “This solution has been used successfully for several years. Various configurations of aluminum washers have been used under the screw head as a corrosion barrier.”