Effect of faying surface gradient in friction welding

Summary

SCM440H steel bar to S45C steel bar welds (both of 25 mm dia.)(Part I) and SCM440H steel pipe (with an OD of 25 mm and ID of 14 mm) to S45C steel bar (of 25 mm dia.) welds (Part II) were produced, and the effect of the faying surface gradient on the properties of the friction-welded joints was investigated. The friction welding conditions were a rotational speed of 3000 rpm, preheating pressure of 20 MPa, friction pressure of 50 MPa, upset pressure of 120 MPa, and burn-off length control of 4.0 mm. Friction welding was performed with base metals with and without gradients to the faying surfaces. The results obtained may be summarised as follows.

Part I: At faying surface gradients up to 8.6/100 in the SCM base metal and up to 12.0/100 in the S45C base metal, the joints sustain tensile fracture in the S45C base metal. At any higher gradients, however, tensile fracture occurs in the weld interface. The fracture initiation point in the case of weld interface fracture is located at the periphery of the weld interface. Fractographic observations suggest that the fracture initiation zone is a dimple fracture surface containing numerous fine oxide particles in the bases of the dimples. The defect producing this type of fracture cannot be detected by ultrasonic flaw detection using the water immersion technique.

Part II: In the SCM base metal, the joints sustain tensile fracture in the HAZ of the SCM steel at a gradient up to 2.8/100. At any higher gradient, however, fracture occurs by a combination of weld interface fracture and HAZ fracture of the SCM steel. In the S45C base metal, fracture occurs in the HAZ of the SCM base metal at gradients up to 2.4/100. At gradients up to 6.0/100, the fracture behaviour found is much like that affecting SCM steel joints with a gradient. At any higher gradient, however, the joints sustain weld interface fracture.