Dissimilar friction welding of titanium alloy and stainless steel and its welding principle elucidation for spinal fixation application

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In the last step, we have found that high shear strain rate between two materials and high temperature rising rate would cause the formation of harmful mechanically mixed layers during friction welding (FW) of Ti-6Al-4V (Ti64) and SUS316L, hence significantly deteriorating the joint quality. Rotation speed reduction (to 100 rpm) and liquid CO2 cooling were then adopted to reduce the shear strain rate and temperature rising rate. As a result, the mechanically mixed layer formation was effectively suppressed, and the joint quality was successfully improved.Based on these findings, an ultralow rotation speed with a proper high friction pressure were adopted in this step in order to eliminate the liquid CO2 cooling process. The ultralow rotation speed of 40 rpm provided a low temperature rising rate and a low peripheral velocity corresponding to a low shear strain rate between faying surfaces, hence suppressing the formation of harmful mechanically mixed layers and associated cracks. The proper high friction pressure of 450 MPa produced a proper welding temperature that produced a thin and strong　intermetallic compound layer while avoiding interface cracking. Furthermore, the proper high friction pressure worked to guarantee a sufficient faying surface deformation on SUS316L to sufficiently reduce surface roughness, break oxide layers and create fresh surfaces. Finally, a Ti64/SUS316L FW joint with superior quality was successfully fabricated in a simpler way using ultralow rotation speed and proper high friction pressure, without using liquid CO2 cooling.