Fasteners for magnesium reduce shear, increase compression.
By Larry Pickett
Textron Fastening Systems
Edited by Kenneth Korane
Engineers increasingly recognize the physical and mechanical attributes of magnesium that make it such a good fit in so many applications. With one-quarter the mass of steel and two-thirds that of aluminum, magnesium has an excellent strength-to-weight ratio. It also has good fatigue strength, dimensional stability, and sound and vibrationdamping qualities.
Magnesium's good flow characteristics permit complex castings with thin walls, low porosity, good finish, and tight dimensional controls. And it is typically easier and less expensive to process and machine, compared to steel and aluminum.
That's why, for instance, the automotive industry's use of magnesium has more than doubled since 1990, growing from an average of 3 lb per North American car to over 7 lb in 2001. Applications are expected to grow 10 to 12% annually, with Ford Motor Co. projecting 250 lb in every car in its fleet within two decades.
Current magnesium automotive components include instrument panels, seat structures, antilock-brake mounts, and bumper support beams. And temperature-resistant magnesium alloys are finding their way into gearboxes, clutches, transmission housings, and as valve and cam covers.
Magnesium has drawbacks, however, particularly when using conventional fasteners to join or mount parts. The nut-and-bolt approach is costly because it requires thicker joints and wider flanges than other methods. Machine screws using drilled and tapped holes, or thread-cutting screws, permit leaner and less-expensive part designs. But these methods pose other problems when used with magnesium parts.
The metal's low ductility and deformability leads to slivering, chipped threads, and fracturing when mated with threaded fasteners. Removing and reinserting fasteners often destroys threads and creates chips, debris, and contamination.
The key to fastening magnesium lies in reducing stresses in the threads. Traditional threadforming fasteners have 60° flank angles and narrowly spaced threads. They work well with steel and aluminum parts, but not with low-ductile materials. When driven into magnesium, standard threadforming fasteners create excess debris. They also easily exceed the material's ductility limits, which damages or fractures the formed threads.
A different type of threadforming fastener overcomes these problems. Called Mag-Form fasteners, from Textron Fastening Systems, these steel fasteners feature a 105° threadflank angle and lobe-shaped threads with a wide, nonstandard pitch. The broad flank angle and pitch reduce shear stresses while increasing compression, letting magnesium deform when the fastener is inserted. In essence, they roll-form the threads, rather than cutting them from the metal. This compressive action forms strong threads in the base material while generating little debris. The larger flank angle also lets the fastener be removed and reinserted many times for service and repair.
Mag-Form fasteners have essentially the same clamp-load capability as machine screws and conventional threadforming screws. And size for size, ultimate tensile strength and load-carrying capability are also about the same. However, magnesium is prone to creep. Because thread designs are not the same, there are minor differences in retained clamp loads over time.
The fasteners offer other advantages over more-traditional methods for fastening magnesium. Standard machine screws often require locking mechanisms because looser tolerances make them susceptible to vibration and loosening. Threadforming fasteners have metal-to-metal contact between threads that resist vibration.
Traditional thread-cutting screws must be longer and have deeper pilot holes. They are also difficult to reuse, due to their asymmetrical thread-cutting points. Thread cutters usually extend three to four threads, which the parts must accommodate to fully engage the threads that follow. Installing these fasteners generates chips so there must be sufficient space to accommodate debris, particularly in a blind hole. Otherwise the fastener binds against the chips. None of this is necessary on Mag-Form threadforming fasteners.
Although Mag-Form fasteners were designed specifically for magnesium, they may also be used in aluminum and zinc components. In fact, some OEMs successfully use the same fastener on all three types of castings, thus reducing parts inventory.
The fasteners are available in any standard head or drive style. Sizes range from M1.6 to M16, and sizes down to M1.0 are under development to address magnesium's growing use in the business-equipment industry. Recommended length is at least 2.5 diameter and costs are on par with standard thread-cutting fasteners.