Large strain extrusion machining of pure titanium at cryogenic temperature: experimental and simulation insights into microstructure and mechanical properties evolution

Abstract

Mastering the relationship between microstructure evolution and macro-mechanical properties is conducive to more convenient and accurate preparation of high-quality commercially pure titanium (CP Ti) products, reducing production costs, in line with the concept of green manufacturing. Cryogenic temperature large strain extrusion machining (CT-LSEM) is a promising processing method to fabricate materials with ultrafine grains (UFG). The microstructure evolution and mechanical properties of UFG CP Ti after CT-LSEM at different chip compression ratios λ in a low-temperature environment (~ − 50 °C) were systematically investigated using hardness testing, tensile testing, and electron backscatter diffraction (EBSD) techniques. The results showed that with the increase in λ, the grain size of CP Ti decreased from the initial grain size (31 µm) to 0.70 µm, the hardness increased from 165 to 226.4 HV, and the yield strength increased from 275 to 518 MPa. A finite element model was established to describe the microstructure and mechanical properties evolution of CP Ti under CT-LSEM. During the CT-LSEM process, the grain size and hardness of CP Ti were predicted, with maximum errors of 3.2% and 7.4%, respectively. Combined with the EBSD results, the microstructure evolution was characterized, with the dislocation density increasing from \(10.06 \times 10^{14} {\text{m}}^{ - 2}\) (λ = 1) to \(11.54\times {10}^{14} {\text{m}}^{-2}\) (λ = 1.5) and twins were observed, including {10–12}, {11–22} and {11–21} twins. The properties and microstructural changes of chips at low temperature (~ − 50 °C) and room temperature (~ 25 °C) were compared. The strengthening mechanisms in the CT-LSEM process—fine grain strengthening, dislocation strengthening, and twinning strengthening—were systematically discussed.

Graphical abstract