Direct Drive Welding
There are two principal FRW methods: direct-drive welding and inertia-drive welding. Direct-drive FRW, sometimes called conventional friction welding, uses a motor running at constant speed to input energy to the weld.
Process of Direct Drive Welding:
In direct-drive FRW, a machine resembling a lathe is equipped with a brake and clutch, a means of applying and controlling axial pressure, and a weld-cycle timer and displacement controller. The operation of a direct-drive machine consists of a friction phase where heat is generated, a stopping phase where the rotation is terminated, and a forging phase where the pressure is applied to join the pieces.
The relationships among the process variables:
The time required to stop the spindle is also an important variable because it affects the weld temperature and the timing of the forging force.
PLOT OF SELECTED PARAMETERS VERSUS TIME RELATIVE TO THE THREE PHASES OF THE DIRECTDRIVE FRW PROCESS
- The forging phase starts at the instant when higher pressure (that is, a larger forging force) is applied in the weld cycle. Thus, the forging phase actually starts somewhere in the stopping phase.
- In general, the larger forging force can be applied (case 1) while the spindle is decelerating in the stopping phase (case 2) after the spindle has stopped rotating at the end of the stopping phase.
- The difference in the two applications of the larger forging force is the presence of a second friction peak. In case 1, the torque will rise again to reach a second peak before dropping.
- This produces a torsional force. In case 2, especially when the stopping phase is very short due to rapid braking, frictional torque does not rise but actually starts to decrease at the onset of the forging phase.
- In this case, there is no torsional force, and forging is affected only by the upsetting force.
The friction force is generally applied gradually to the weld to help overcome the initial contact-torque peak.
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