TA18 (Ti-3Al-2.5V) titanium alloy is a low aluminum equivalent near α-type α+β-type titanium alloy evolved from TC4 (Ti-6Al-4V) titanium alloy. It not only has good room temperature and high temperature mechanical properties and corrosion resistance, but also has excellent cold and hot processing plasticity, formability, welding performance and corrosion resistance. TA18 titanium alloy pipes are widely used in aircraft engine hydraulic and fuel pipes Road systems, bicycle tripods and handles, golf clubs, fishing rods, casings for oil drilling and heat exchanger tubes, etc.
A certain batch of TA18 titanium alloy pipes had a large number of cracks in the process of cold rolling from Φ70mm×8mm to Φ55mm×6mm. Macroscopically observed that the cracks were distributed along the longitudinal direction of the pipe body. There were no obvious scratches on the surface of the pipe body. The cracks penetrated The pipe wall was damaged and penetrating cracks were formed, resulting in the scrapping of 60% of the batch of pipes. In order to find out the reason for the cold-rolled cracking of the TA18 alloy pipe, samples were taken from typical cracked parts and normal parts, and the chemical composition, microstructure, fracture morphology and microhardness of the structure were analyzed, and the reason for the cracking was analyzed and studied.
Three samples were taken from the cracked part of the fracture and the normal part, and the chemical composition was determined by ICP full-spectrum direct-reading spectrometer and TC-600 oxygen and nitrogen analyzer. The longitudinal and transverse metallographic samples were taken from the cracked part and the normal part respectively. The etching agent (volume ratio) was: hydrofluoric acid: nitric acid: water = 1:4:45, and the microstructure was observed with a Leica MM-6 optical microscope. Samples were taken at the cracked site, and the surface morphology of the cracked section was observed with a JEOL JSM-5610LV scanning electron microscope. Three metallographic samples were taken from the cracked part and the normal part respectively, and the hardness at 5 uniform points was tested on the metallographic sample by HMV-2T Shimadzu microhardness tester, and the test condition was 9.8N/30s.
  In the composition analysis of the cold-rolled cracked parts and normal parts of TA18 titanium alloy pipes, it was found that the content of Fe element exceeded the standard and the content of O element was close to the upper limit of the standard. The microstructure examination found that the normal part was equiaxed, the β phase was diffusely distributed in the α phase in the transverse structure of the cracked part, and the grains were found to be coarse in the longitudinal structure, which tended to transform into Widmanstatten structure. Microscopically, the cracked fracture surface is intergranular brittle fracture. The hardness test results show that the average Vickers hardness of the cracked part is 15% higher than that of the normal part.
The test results show that iron nails are added to the TA18 titanium alloy during the smelting process, which cannot be evenly distributed during the mixing process, resulting in uneven Fe content in the electrode, which eventually leads to local segregation of Fe in the smelted ingot. Due to Fe segregation, the microhardness value in this area will be about 15% higher than that of the matrix, forming a hardened block, which is the main reason for the cracking of the subsequent cold-rolled pipe.
In order to verify the accuracy of the analysis problem, the iron nails were changed to TiFe and VAlFe in the TA18 alloy ingredients, and no cracking was found in the subsequent rolling process, indicating that the problem analysis and improvement measures are effective.