“Metastable β-titanium alloy is the strongest titanium alloy after precipitation hardening. In this paper, an arc additive process is used to prepare Ti–3Al–8V–6Cr–4Mo–4Zr alloy, and its mechanical properties and microstructure after heat treatment are studied. After solution treatment, the alloy is mainly composed of body-centered cubic β phase, which has high plasticity, but the strength is average, and the yield strength is only 860MPa. But after post-aging treatment, extremely high strength can be achieved, and its yield strength (>1600MPa) almost doubles.”
Titanium alloy is a strong candidate material for additive manufacturing (AM). Most previous studies focused on the reaction of the most commonly used structural titanium alloy Ti-6Al-4V to AM, including understanding its crystal phase, microstructure and mechanical properties . However, from a metallurgical point of view, acicular α’martensite will inevitably be formed when Ti-6Al-4V alloy is cooled. In contrast, metastable β-titanium alloy usually does not form α’during rapid cooling. Martensite, therefore, metastable beta alloys provide greater options for heat treatment, and will have broader application prospects in AM manufacturing. Metastable beta alloys are easy to undergo age hardening to control the volume fraction and morphological characteristics of the alpha phase, so that not only higher strength can be achieved, but also mechanical properties can be effectively controlled. Although expensive, metastable beta alloys are still the most ideal candidate materials.
For Ti–3Al–8V–6Cr–4Mo–4Zr alloy, M.J. Bermingham et al. first used arc additive manufacturing, and then performed heat treatment and mechanical property characterization. Solution treatment is carried out after processing to eliminate any undesirable metastable phases that may be formed during additive manufacturing. Related papers Related papers were published on Materials Science & Engineering A with the title “High strength heat-treatable β-titanium alloy for additive manufacturing”.
XRD spectra of AM Ti–3Al–8V–6Cr–4Mo–4Zr under heat treatment conditions
Low-temperature pre-aging is of great significance to the formation of intragranular strengthening phase α phase in the subsequent double aging process. The solute atom coordination in the β phase is related to the formation of a finer enhanced α precipitation phase through a single-step aging treatment.
After the double aging treatment, the intensity of the detected α phase peak increased significantly due to the formation of the strengthened intragranular α precipitation and the growth of the existing native α. During the aging process, the two different parameters of α and α are formed, and different α-phase Bragg diffraction peaks are formed during the aging process, and there are two sets of peaks appearing in the XRD spectrum. However, the strength increases with the increase of temperature, indicating that the volume fraction of the primary α phase increases at higher temperatures. The higher 2θ peak angle of α observed during the aging process indicates that the crystal lattice size is smaller.
In addition, there are some other small peaks in the XRD spectrum of the double aging treatment at the temperature of 500℃ to 550℃. The temperature range of these peaks is the same as the previously observed Ti–3Al–8V–6Cr–4Mo–4Zr alloy aging During the process, the intermetallic phase formed on the β grain boundary is consistent, which can be considered as TiCr2 phase or Tix-Zrx intermetallic compound. The difference in the lattice parameters of different forms of α can be attributed to the primary α phase formed at a higher temperature, so that enhanced diffusion can form a composition closer to equilibrium. At the same time, the composition of the α phase formed during the low temperature aging process is limited by the lower diffusion rate, which will further deviate from the equilibrium.
SEM micrographs of α precipitation phase at various double aging temperatures. The age hardening of metastable β-titanium alloy occurs through the precipitation of α phase, and the precipitation of α phase can be controlled by a suitably designed heat treatment system.
Ti–3Al–8V–6Cr–4Mo–4Zr, as a commercial alloy, produces the main β-phase microstructure under solution treatment conditions. In addition, the manufactured Ti–3Al–8V–6Cr–4Mo–4Zr has a yield of 861MPa Strength, and has extremely strong toughness. Under the initial aging condition of 300℃/24 hours, the yield strength increased slightly to 910MPa, and the plasticity decreased to about 45%. With continuous aging at high temperatures, that is, dual-phase aging, the α phase is significantly precipitated, which has an important impact on the properties of the alloy. After the final aging at 450°C/24 hours, the yield strength was significantly increased to 1622MPa. This is almost twice the yield strength of the alloy under β-annealing conditions, which is much higher than Ti-6Al-4V made with wire arc additives (see Figure 3 for comparison). Aging at 500℃, 525℃ or 550℃ for 24 hours, the strength is 1385-1210MPa, and the ductility is further balanced between 21-27%. The strengthening of α-precipitates in aged Ti–3Al–8V–6Cr–4Mo–4Zr is closely related to their dispersibility and size. The precipitated phase acts as a strong obstacle to the movement of dislocations by hindering the movement of dislocations in the crystal lattice. To strengthen. A smaller precipitation interval reduces the mean free path of dislocations and increases strain hardening, but it also leads to a decrease in ductility. Therefore, in order to maintain sufficient ductility, 500°C-550°C may provide a more suitable balance of mechanical properties for the alloy.
The characteristics observed in the two cases are similar, indicating that the Ti–3Al–8V–6Cr–4Mo–4Zr after solution treatment does not fail through shear and fracture. Metastable β alloys such as Ti–3Al–8V–6Cr–4Mo–4Zr are considered to be truly hardenable, and the size and fraction of alpha precipitates can be precisely controlled by solution and aging heat treatments to optimize performance for subsequent heat treatments Provides a broader choice space. In the manufacturing process of AM parts, the ability to precisely control the size and distribution of precipitates is also advantageous.
In most of the existing studies, the alloy strength level is between 600-900MPa, which is equivalent to Ti–3Al–8V–6Cr–4Mo–4Zr after solution treatment. However, Ti–3Al–8V–6Cr–4Mo–4Zr can significantly increase the strength through α-precipitation, so related research on optimizing the properties of metastable β-Ti alloys through precipitation hardening has great potential.
In this paper, Ti–3Al–8V–6Cr–4Mo–4Zr alloy is produced by the arc additive manufacturing process and has undergone related heat treatment. The results show that the metastable β-Ti alloy is a promising additive manufacturing material. The β-phase strength in the solution-treated Ti–3Al–8V–6Cr–4Mo–4Zr alloy is average, but the plasticity is higher. Phase aging can promote the formation of α precipitated phase. The hardness and strength of α precipitated phase have a wide range, depending on the aging temperature and time.