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

在最后一步中，我们发现在Ti-6Al-4V （Ti64）与SUS316L的摩擦焊接（FW）过程中，两种材料之间的高剪切应变速率和高升温速率会导致有害的机械混合层的形成，从而使接头质量显著恶化。然后采用减速（至100 rpm）和液态CO2冷却来降低剪切应变速率和升温速率。有效地抑制了机械混合层的形成，成功地提高了接头质量。基于这些发现，在这一步中采用了超低转速和适当的高摩擦压力，以消除液态CO2冷却过程。40 rpm的超低转速提供了较低的温升速率和较低的外围速度，对应于受热面之间较低的剪切应变速率，从而抑制了有害机械混合层和相关裂纹的形成。适当的高摩擦压力（450mpa）可产生合适的焊接温度，在避免界面开裂的情况下形成薄而坚固的金属间化合物层。此外，适当的高摩擦压力可以保证SUS316L表面有足够的变形，从而充分降低表面粗糙度，打破氧化层，形成新的表面。最后，在不使用液态CO2冷却的情况下，采用超低转速和适当的高摩擦压力，以更简单的方法成功制备了质量优良的Ti64/SUS316L FW接头。

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.