Titanium has exceptional corrosion resistance due to the exterior natural oxide layer formed when exposed to oxygen. However, titanium still suffers various corrosion forms in severe operational environments, including erosion, fretting corrosion, stress-corrosion cracking, hydrogen embrittlement, pitting and crevice corrosion, and uniform corrosion.
Alloys of titanium also show corrosion-resistant properties in normal conditions, but similar to titanium metal, its alloys also corrode under actual service conditions. Scientists have developed several techniques and methods to identify the causes and types of corrosion in titanium and presented various solutions for it.
Near β titanium Alloy Thereinto, Ti-5Al-5Mo-5V-3Cr-1Zr (Ti-55531)
The corrosion-resistant properties make titanium alloys very demanding in aerospace, marine, automotive, and other sectors. Especially near β titanium alloy Thereinto, Ti-5Al-5Mo-5V-3Cr-1Zr (Ti-55531) has drawn significant attention in the aerospace industry due to its corrosion resistance, favorable ductility, and high tensile strength. However, producing complex-shaped titanium alloys is challenging due to low utilization in conventional manufacturing techniques, high cost, and long construction periods.
Additive Manufacturing (AM)
Scientists have carried out a lot of work in developing various low-cost manufacturing techniques. Additive manufacturing (AM) can produce complex geometrical components fairly easily. A special technique used in additive manufacturing is selective laser melting (SLM), which is advantageous due to high material utilization, fast molding speed, and high shaping accuracy. However, there have not been enough studies conducted on the corrosion behavior of SLM-prepared Ti-55531.
Titanium alloys’ corrosion properties are highly affected by the size and shape of their microstructures. In selective laser melting, the molten pool experiences a significant temperature differential (104–105 °C/cm) due to a brief period of laser activity. As a result, directed solidification promotes robust epitaxial grain development, creating coarse columnar β grains (C-β).
How Were Experiments Conducted?
The study discusses the effect of heat treatment on the corrosion behavior of selective laser melted Ti-5Al-5Mo-5V-3Cr-1Zr alloy.
The researchers used the electrode induction melting-gas atomization technique to prepare Ti-55531 powders. Samples of 65×25×65 mm3 dimensions were prepared using SLM. The sample’s density was measured by the Archimedes method.
Samples were further cut into vertical and horizontal planes. An electric spark cutter was used to prepare pieces of 10x10x2 mm3 for electrochemical experiments and microstructural analysis. Furthermore, a 25x10x4 mm3 sample was prepared for the weightless corrosion experiment, and 10x10x3 mm3 was prepared for microscopic morphology.
Two types of heat treatment were introduced to the samples, including αβ solution and aging treatments (αβ-STA) and supertransus triplex heat treatment (Triplex-HT).
To perform triplex heat transfer treatment, an 870 ℃ temperature was maintained for one and a half hours, then brought down to 750 ℃ in 40 minutes, maintained for one hour, and finally aging at 600 ℃ for six hours.
Similarly, the αβ-STA sample was held at 790 ℃ for one and a half hours and aged at 600 ℃ for six hours. The air cooling mechanism was used in both heat treatment types. To imitate actual service conditions, 3.5wt.% NaCl was used in the immersion experiment for 180 days.
Findings of the Study
This study discusses the heat treatment effects on the structural changes of SLMed Ti-55531 and investigates its corrosion behavior under the influence of 3.5% NaCl. The researchers have drawn few conclusions from the experimentation conducted in the study.
Recrystallization of β grains in the vertical plane sample after triplex heat treatment shows the removal of columnar β grains. The researchers achieved a noteworthy reduction in anisotropy of corrosion resistance between vertical and horizontal planes. The retention of columnar β grains is attributed to a lack of driving factor for recrystallization in αβ-STA samples.
αβ-STA specimens show better corrosion resistance than triplex heat transfer samples. Moreover, the electrochemical region's low polarization resistance (Rp) and high passive current density (lp) show higher chances of unstable passive film formation by triplex heat transfer samples, indicating their reduced corrosion resistance.
In the triplex-HT element, segregation significantly impacts the formation of passive films and their subsequent disintegration. The defect formation becomes easier due to Al-rich coarse αp grains accumulating oxygen vacancies. Pitting corrosion of Ti-55531 is caused by chloride ions that accumulate in these oxygen vacancies and defects. Hence, the presence of Al-rich coarse αp in the bi-lamellar structure proved that the corrosion resistance of the bimodal structure is better than the bi-lamellar structure.
Hanyang Zuo, Hao Deng, Lvjun Zhou, Wenbin Qiu, Ping Xu, Yongqiang Wei, Huaqiao Peng, Zuxi Xi, Jun Tang (2022) The effect of heat treatment on corrosion behavior of selective laser melted Ti-5Al-5Mo-5V-3Cr-1Zr alloy. Surface and Coatings Technology. https://www.sciencedirect.com/science/article/abs/pii/S0257897222006648