1. Overall Weldability of Titanium Materials
Commercially Pure (CP) Titanium (GR.1/GR.2/GR.3)
CP titanium (single α-phase) has excellent weldability. Its low alloying content minimizes the formation of brittle intermetallic phases during welding, and its high thermal conductivity (relative to titanium alloys) helps distribute heat evenly, reducing localized overheating. Common welding methods (GTAW/TIG, PAW/plasma arc welding, LBW/laser beam welding) are all applicable, and with proper shielding, weld joints with high integrity can be achieved.
α+β Titanium Alloys (e.g., GR.5/Ti-6Al-4V)
GR.5 has moderate weldability. The presence of aluminum (α-stabilizer) and vanadium (β-stabilizer) introduces challenges such as phase segregation and grain coarsening in the weld zone. However, it can still be reliably welded with strict process control (e.g., low heat input, precise shielding).
β Titanium Alloys (e.g., Ti-15V-3Cr-3Sn-3Al)
β alloys have good weldability due to their stable β-phase at room temperature and lower sensitivity to heat input, but they are less commonly welded in industrial applications compared to CP titanium and GR.5.
2. Crack Susceptibility of Titanium Welds
Lack of Solidification Cracking
Titanium has a wide freezing range, but its weld metal solidifies in a single-phase (α or β) manner, avoiding the formation of low-melting eutectic phases at grain boundaries (the primary cause of solidification cracking). This makes titanium immune to solidification cracks even with high heat input.
Hydrogen-Induced Cold Cracking (HICC)
This is the most common cracking type in titanium welds. Hydrogen can enter the weld and HAZ from moisture in the shielding gas, contaminated filler metal, or ambient air. At temperatures below 250°C (482°F), hydrogen combines with titanium to form brittle hydride (TiH₂) precipitates along grain boundaries. These hydrides create stress concentrations, leading to cold cracking during post-weld cooling or subsequent service (especially under tensile loads). CP titanium and GR.5 are both susceptible to HICC if shielding is inadequate.
Stress Cracking
Residual stresses from welding (caused by uneven thermal expansion/contraction) can induce stress cracking in the HAZ, particularly for thick-section components or welds with high restraint. GR.5's HAZ is prone to grain coarsening, which reduces ductility and makes it more susceptible to stress cracking under residual tensile stress.
3. Changes in Mechanical Properties After Welding
Strength Variations
CP Titanium: The weld metal and HAZ of CP titanium typically have slightly higher strength than the BM but lower ductility. The HAZ undergoes grain coarsening due to welding heat, increasing tensile strength by 5–10% (e.g., GR.2 BM tensile strength of 485 MPa vs. weld joint strength of 510–530 MPa) but reducing elongation by 10–15% (from 25% to 20–22%).
GR.5 Titanium Alloy: The as-welded GR.5 weld metal has a martensitic α' phase (formed by rapid cooling of the β-phase during welding), which increases tensile strength to ~1000 MPa (higher than the annealed BM's 860 MPa) but drastically reduces ductility (elongation drops from 12% to 5–8%). The HAZ of GR.5 experiences grain coarsening and phase transformation, with yield strength decreasing by 5–10% compared to the BM due to softened microstructures.
Ductility and Toughness Reduction
For all titanium grades, welding causes a significant drop in ductility and toughness in the weld zone. The HAZ's coarse grains and the weld metal's non-equilibrium microstructure (e.g., α' martensite in GR.5) act as crack initiation sites, lowering fracture toughness by 20–30% (e.g., GR.5 BM fracture toughness of 60 MPa·m¹/² vs. weld joint toughness of 40–45 MPa·m¹/²). CP titanium's weld joint elongation decreases by 20–25% due to grain coarsening in the HAZ.
Fatigue Performance Degradation
Weld joints are the weakest link for fatigue resistance. The combination of residual stresses, microstructural inhomogeneity, and potential porosity/inclusions reduces the fatigue strength of titanium welds by 30–50% compared to the BM. For example, annealed GR.5 BM has a 10⁷-cycle fatigue strength of 400 MPa, while its as-welded joint fatigue strength drops to 180–250 MPa. Post-weld heat treatment (e.g., stress-relief annealing or recrystallization annealing) can partially restore fatigue performance by reducing residual stresses and refining microstructures.
Corrosion Resistance Changes
Improperly shielded titanium welds may have oxygen/nitrogen contamination in the HAZ, which reduces corrosion resistance in aggressive media (e.g., seawater, acids). With full shielding, however, the corrosion resistance of the weld joint is comparable to the BM.
