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The Diction of Friction

June 26, 2010

From Webster: fric•tion: the rubbing of one body against another; the force that resists relative motion between two bodies in contact.

Engineers in almost every field work to avoid friction by use of proper design techniques or by applying oils and lubricants. We’ve all seen the TV commercials talking about how a certain oil product and/or additive will reduce the friction inside your car’s engine.

Conversely, properly harnessed, friction can be your friend. Rubs your hands together and soon enough they start to warm-up. The same concept applies when we friction weld two metals together. Friction welding has been around for many years, and used to weld many different products including; various automotive parts, hydraulic cylinder rods, aircraft engine components, military components, and many different bi-metal combinations. Friction also doesn’t just have to be round components, it can extend to welding square cross-sections with linear friction welding, or a newer addition to the friction welding family – friction stir welding.

So what does friction have to do with the friction welding process anyway? Well, not much. When two parts are to be welded, one is rotated and then they are forced together. When the parts initially contact each other they are cold and have a relatively high coefficient of friction. As the parts heat up, the process enters a phase were the coefficient of friction decreases and small areas of the two parts start to bond. However, the one part is still rotating, so these bonds are immediately broken. As the temperature increases and the parts reach a plastic state at the interface, the coefficient of friction is reduce to a minimal level and the process reaches a steady state.

Now contrary to what one would think with friction welding, increasing the rotation speed, all other parameters being equal, more often than not does result in an increase the amount of heat generated. Increasing the speed tends to cause the heat generated to be pushed out into the flash, thus reducing he amount of heat allowed to soak back into the part behind the interface. It is often found that lowering the rotation speed produces a higher quality joint.

So the next time you are warming your hands-up outside on a cold winter day, better watch. You might weld your hands together.