Analysis Of The Causes Of Cold-Rolled Cracking in TA18 Titanium Alloy Tubes
Aug 13, 2025
TA18 (Ti-3Al-2.5V) titanium alloy is a low-aluminum equivalent, near-alpha-type, + -type titanium alloy derived from TC4 (Ti-6Al-4V) titanium alloy. It not only exhibits excellent room-temperature and high-temperature mechanical properties and corrosion resistance, but also boasts excellent cold and hot working plasticity, formability, weldability, and corrosion resistance. TA18 titanium alloy tubing is widely used in aircraft engine hydraulic and fuel piping systems, bicycle tripods and handlebars, golf clubs, fishing rods, oil drilling casing, and heat exchanger tubes.
A batch of TA18 titanium alloy tubing developed numerous cracks during the cold rolling process from a cross-section of 70 mm × 8 mm to 55 mm × 6 mm. Macroscopic observation revealed that the cracks were locally distributed along the longitudinal direction of the tubes, with no visible scratches on the surface. The cracks penetrated the tube wall, forming through-type cracks, resulting in 60% scrapping of the batch. To identify the cause of cold-rolled cracking in TA18 alloy pipes, samples were taken from typical cracked and normal locations for analysis of chemical composition, microstructure, fracture morphology, and microhardness. The cause of the cracking was also investigated.
Three samples were taken from the cracked and normal locations, respectively, and their chemical composition was determined using an ICP direct-reading spectrometer and a TC-600 oxygen and nitrogen analyzer. Longitudinal and transverse metallographic specimens were taken from the cracked and normal locations, respectively, using an etchant (volume ratio) of hydrofluoric acid:nitric acid:water=1:4:45. The microstructure was observed using a Leica MM-6 optical microscope. Samples were taken from the cracked locations, and the surface morphology of the cracked sections was observed using a JEOL JSM-5610LV scanning electron microscope. Three metallographic specimens were taken from the cracked and normal locations, and hardness was measured at five uniform points using a Shimadzu HMV-2T microhardness tester at 9.8 N/30 s. Composition analysis of both cracked and normal cold-rolled TA18 titanium alloy tubes revealed excessive Fe content and near-standard O content. Microstructural examination revealed an equiaxed structure in the normal section. In the cracked section, phase was dispersed within the phase in the transverse microstructure, while coarse grains were observed in the longitudinal microstructure, showing a tendency toward Widmanstätten transition. Microscopic observation of the cracked section revealed intergranular brittle fracture. Hardness testing revealed an average Vickers hardness of 15% higher in the cracked section than in the normal section.
Test results indicate that the addition of iron nails during the smelting process of the TA18 titanium alloy prevented uniform distribution during the mixing process, resulting in uneven Fe content in the electrode and, ultimately, localized Fe segregation in the smelted ingot. This Fe segregation resulted in microhardness values in this area approximately 15% higher than that of the matrix, forming a hardened mass that was the primary cause of subsequent cracking in the cold-rolled tube. In order to verify the accuracy of the problem analysis, the iron nails were replaced with TiFe and VAlFe in the TA18 alloy ingredients. No cracking was found in the subsequent rolling process, indicating that the problem analysis and improvement measures were effective.
German-imported precision titanium tube production line (annual production capacity: 30,000 tons);
Japanese-technology titanium foil rolling line (thinnest to 6μm);
The MES system enables digital control and management of the entire production process, achieving product dimensional accuracy of ±0.01μm.
