Wednesday, September 10, 2008

Friction Welding

Unlike the other processes covered in this book friction welding is a solid phase pressure welding process where no actual melting of the parent metal takes place. The earliest version of the process utilised equipment similar to a lathe where one component was held stationary and the other held in a rotating chuck (Fig. 8.11). Rubbing the two faces together produces suf- ficient heat that local plastic zones are formed and an end load applied to the components causes this plasticised metal to be extruded from the joint,carrying with it any contaminants, oxides, etc. Thus two atomically clean metal surfaces are brought together under pressure and an intermetallic bond is formed. The heat generated is confined to the interface, heat input is low and the hot work applied to the weld area results in grain refinement. This rapid, easily controlled and easily mechanised process has been used extensively in the automotive industry for items such as differential casings, half shafts and bi-metallic valves. Since the introduction of this conventional rotating method of friction welding many developments have taken place such as stud welding, friction surfacing, linear and radial friction welding, taper plug welding and friction stir welding.
One very important characteristic of friction welding is its ability to weld alloys and combinations of alloys previously regarded as unweldable. It is possible to make dissimilar metal joints, joining steel, copper and aluminium to themselves and to each other and to successfully weld alloys such as the 2.5% copper–Al 2618 and the AlZnMgCu alloy 7075 without hot cracking. The primary reason for this is that no melting takes place and thus no brittle intermetallic phases are formed.