Principle: Molten titanium is poured into prefabricated molds and solidified to form near-net-shape components, typically using investment casting (lost-wax casting) for complex geometries.
Internal Structure
Cast titanium (including commercially pure titanium and alloys like GR.5) has a coarse, directional microstructure dominated by columnar or equiaxed grains, with a high degree of segregation of alloying elements (e.g., Al and V in GR.5) at grain boundaries. The as-cast structure often contains casting defects such as shrinkage porosity, gas pores (H₂, N₂), and non-metallic inclusions (TiO₂). For α+β alloys like GR.5, the as-cast microstructure is primarily lamellar α+β (Widmanstätten structure), with no significant grain refinement or texture development.
Density
The theoretical density of titanium is 4.51 g/cm³, but cast titanium has a relative density of 95–98% due to inherent porosity and shrinkage defects. Severe shrinkage cavities or large pores can further reduce density to below 95%, leading to stress concentration and performance degradation.
Mechanical Properties
Strength and Ductility: Cast titanium has low room-temperature tensile strength (e.g., as-cast GR.5 has a tensile strength of 700–750 MPa, ~15% lower than annealed forged GR.5) and poor ductility (elongation of 5–8%, less than half of forged grades) due to coarse grains and segregation. Its yield strength is also low (600–650 MPa) with significant anisotropy caused by directional solidification.
Toughness and Fatigue: Fracture toughness of as-cast GR.5 is only 30–40 MPa·m¹/² (vs. 60 MPa·m¹/² for forged material), and fatigue strength (10⁷ cycles) is 200–250 MPa (a 37–50% reduction from forged grades), as defects act as crack initiation sites.
High-Temperature Performance: The lamellar structure provides moderate creep resistance at 300–400°C, but overall high-temperature strength is inferior to wrought titanium due to low densification.
Principle: Titanium billets are subjected to high-temperature (below β-transus for α+β alloys) plastic deformation via hammer or press forging, breaking coarse cast grains and forming a deformed microstructure.
Internal Structure
Forged titanium has a refined, deformed α+β microstructure with directional grain flow (fibrous texture) along the forging direction. The as-forged structure eliminates casting defects (porosity, segregation) and breaks down coarse lamellar grains into equiaxed or bimodal α+β grains (depending on forging temperature and cooling rate). For GR.5, annealing after forging produces a uniform equiaxed α+β structure with grain sizes of 5–10 μm (vs. 50–100 μm for as-cast material).
Density
Forging eliminates internal pores and compacts the material, resulting in a relative density of ≥99.5%, close to the theoretical density of titanium. The densification ensures no internal voids that could cause stress concentration.
Mechanical Properties
Strength and Ductility: Forged and annealed GR.5 has a tensile strength of 860–900 MPa, yield strength of 800 MPa, and elongation of 10–15%, representing a balanced combination of high strength and ductility. The directional grain flow leads to moderate anisotropy (strength along the forging direction is 5–10% higher than the transverse direction).
Toughness and Fatigue: Fracture toughness reaches 55–65 MPa·m¹/², and 10⁷-cycle fatigue strength is 350–400 MPa, significantly higher than cast titanium, due to grain refinement and defect elimination.
High-Temperature Performance: The bimodal microstructure of forged GR.5 provides excellent creep resistance at 300–400°C (creep strain <0.1% at 200 MPa for 1000 h), outperforming cast and rolled grades.
Principle: Titanium ingots or billets are processed via hot/cold rolling to produce sheets, plates, or strips, with plastic deformation occurring along the rolling direction to form a flattened, textured microstructure.
Internal Structure
Hot-rolled titanium has a recrystallized equiaxed α+β microstructure with grains elongated along the rolling direction (forming a rolling texture). Cold-rolled titanium (before annealing) has a deformed, work-hardened structure with high dislocation density; annealing after cold rolling refines grains to 3–5 μm (finer than forged titanium). For GR.5 sheets, the rolled structure has a strong {0001} basal texture, leading to significant anisotropy in formability and mechanical properties.
Density
Rolling achieves full densification with a relative density of ≥99.8%, as the continuous compressive deformation eliminates residual porosity and ensures uniform material packing. Cold rolling further improves density by reducing intergranular gaps.
Mechanical Properties
Strength and Ductility: Annealed hot-rolled GR.5 sheets have a tensile strength of 850–880 MPa, yield strength of 780–800 MPa, and elongation of 12–18% (higher ductility than forged titanium due to finer grains). Cold-rolled (unannealed) GR.5 has ultra-high strength (tensile strength >1000 MPa) but low ductility (elongation <5%) due to work hardening.
Toughness and Fatigue: Fracture toughness of rolled GR.5 is 50–60 MPa·m¹/² (slightly lower than forged grades due to texture-induced anisotropy), while fatigue strength is 380–420 MPa (higher than forged material due to finer grain size and smooth surface finish).
Formability: Rolled sheets have excellent cold formability (e.g., bending, stamping) along the rolling direction, but formability is poor in the transverse direction due to strong texture, limiting their use in complex-shaped components.
Principle: Titanium powder (produced via gas atomization or hydride-dehydride (HDH) methods) is compacted and sintered at high temperatures to form fully dense components, enabling near-net-shape manufacturing and microstructure control.
Internal Structure
PM titanium has a uniform, fine-grained equiaxed microstructure (grain size 2–5 μm) with no directional texture or segregation, as powder particles are fully recrystallized during sintering. For PM GR.5, the microstructure is a homogeneous α+β matrix with uniformly distributed fine β-phase particles. However, residual pores (≤1% volume fraction) and minor oxide inclusions (from powder surface oxidation) may remain at grain boundaries.
Density
The density of PM titanium depends on sintering parameters: vacuum sintered PM titanium has a relative density of 98–99.5%, while hot isostatic pressing (HIP) post-treatment can increase density to ≥99.8%, matching wrought titanium levels. HDH powder (irregular shape) yields lower density than gas-atomized powder (spherical shape) due to poor packing efficiency.
Mechanical Properties
Strength and Ductility: Sintered PM GR.5 has a tensile strength of 800–850 MPa, yield strength of 750–780 MPa, and elongation of 8–12% (slightly lower than wrought grades due to residual porosity). HIP-treated PM GR.5 achieves tensile strength of 850–900 MPa and elongation of 10–15%, comparable to forged titanium.
Toughness and Fatigue: Fracture toughness of PM GR.5 is 45–55 MPa·m¹/² (lower than wrought material due to oxide inclusions), and fatigue strength is 300–350 MPa (improved to 380–400 MPa with HIP). The fine-grained structure gives PM titanium excellent wear resistance, exceeding that of forged grades.
Cost and Customization: PM enables the production of complex components with minimal material waste, but powder oxidation and porosity limit its use in high-fatigue aerospace applications, making it suitable for industrial and medical components (e.g., orthopedic implants) with moderate performance requirements.
