ANALYSIS ON INTERNAL SURFACE PITS OF EXTRUDED PURE TITANIUM TUBES

The internal surface pits usually occur in the extruded pure titanium tube blanks.The composition and phases of the secondary phase of the cracked copper canning surface and the titanium tube blanks after etch cleaning were analyzed by OM,EDS and XRD.Furthermore,the reasons were also investigated.Results show that the secondary phase was Cu3Ti on the contacting interface between copper canning and pure titanium tubes.Under the bad lubricant condition,the extrusion ratio increased from 14.2 to 22.9,leading to bigger friction heat and deformation heat;as a result,the local temperature increased to above 850 ℃,and in a short time less than 10 s the Ti-Cu liquid phase reaction happened,resulting in the copper canning tearing and dropping.The shearing copper canning had thick wall and lower running speed than titanium tube blank.Its irregular shape caused lots of spot-liked or strip-liked pits on the internal surface of titanium tubes.In conclusion,the thermal extrusion parameters should be improved to reduce the friction heat and deformation heat.