The effects of the welding conditions on the initial friction process of continuous drive friction welding are investigated using 20-Mild steel. The results show that the friction interface will sequentially experience three typical friction behaviours of abrasion, slide and stick before the initial peak torque. The torque in slide stage shows a platform feature, which transforms to stick stage with a rapidly increasing feature of the torque. As the friction pressure increases from 15 to over 60 MPa, the initial joining location (i.e. corona bond) shifts from one-fourth of the specimen radius (1/4R) towards 1/2R. The maximum surface temperatures show initial slow rising, platform and rapid rising features before the highest temperature appears, which is corresponding to the interface friction status of abrasion, slide or stick. The initial joining location virtually coincides with the location where the maximum temperature is attained.

The rotary friction welding of AA6061-T6 was conducted and the influence of rotation speed was investigated on friction behavior and on radially non-uniform mechanical properties of AA6061-T6 rotary friction welded (RFWed) joint. The results show that, as the rotation speed increases from 500 to 2100 rpm, the curves of friction time and friction work present like ‘V- curve’ with a minimum value at 900 rpm and the ratio of friction work in the first stage to that in the whole process increases and then decreases after reaching a maximum value at 900 rpm. The local strength of the joint correlates positively to the thickness of the deformation layer and radially uniform joint is just achieved at 900 rpm. In addition, the maximum local strength of 295 MPa, corresponding to the joint efficiency of 88%, achieves in the peripheral region at 900 rpm.