CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Dynamic Mechanical Properties of TIG Welded Joints of Ti6321 Alloy
Introduction and Material Background
Ti6321 (Ti-6Al-3Sn-2Zr-1Mo) is a near-α titanium alloy widely used in aerospace structural components, particularly in compressor disks, fan blades, and airframe structures. The alloy combines excellent room-temperature strength with good creep resistance and fatigue performance at elevated temperatures. However, welding Ti6321 using conventional TIG (GTAW) processes presents significant challenges due to the alloy's susceptibility to hot cracking, phase instability, and limited solid solubility of intermetallic-forming elements. This study investigates the weld metal and HAZ microstructure evolution and evaluates the dynamic mechanical properties (strain rate sensitivity, fracture toughness) of TIG welded joints.
Microstructural Analysis
The microstructure of the TIG welded joint exhibits distinct zones with varying characteristics:
| Zone | Microstructure | Grain Size | Key Features |
|---|---|---|---|
| Weld Metal (WM) | Widmanstätten α + small β | 5-15 μm | Coarse acicular α, some equiaxed α |
| Coarse Grain HAZ (CGHAZ) | Widmanstätten α + residual β | 20-50 μm | Prior β grains coarsened significantly |
| Fine Grain HAZ (FGHAZ) | Equiaxed α + small β | 10-25 μm | Moderate grain growth |
| Base Metal | Equiaxed α + β | 15-30 μm | Homogeneous, fine-grained |
The weld metal microstructure is predominantly acicular (lamellar) α phase formed by the rapid cooling of the β phase upon solidification and subsequent transformation. The cooling rate in the TIG weld zone typically ranges from 10-50°C/s, which is significantly slower than electron beam or laser welding but still fast enough to suppress complete equilibrium transformation. The CGHAZ exhibits the most concerning microstructural features, with prior β grains growing to 20-50 μm due to the peak temperature reaching 1050-1200°C, which exceeds the β-transus temperature (approximately 1020°C for Ti6321).
Dynamic Mechanical Property Evaluation
The dynamic mechanical properties are assessed using Split Hopkinson Pressure Bar (SHPB) tests at strain rates ranging from 0.001 to 1000 s⁻¹:
- Yield strength at quasi-static strain rate (0.001 s⁻¹): approximately 850-900 MPa for weld metal, 950-1000 MPa for base metal
- Yield strength at high strain rate (1000 s⁻¹): approximately 950-1000 MPa for weld metal, 1050-1100 MPa for base metal
- Strain rate sensitivity coefficient (n): 0.02-0.04, indicating moderate rate sensitivity
- Fracture toughness (KIC): approximately 45-55 MPa·m^0.5 for weld metal versus 70-85 MPa·m^0.5 for base metal
The weld metal exhibits lower fracture toughness compared to the base metal, primarily due to the coarse acicular α morphology and the presence of residual stresses. The strain rate sensitivity is relatively low, indicating that the material does not benefit significantly from high strain rate loading in terms of ductility enhancement. This is a critical consideration for pressure vessel applications where impact loading or seismic events may impose high strain rate conditions.
Welding Process Optimization
The study evaluates the effect of welding parameters on microstructure and properties:
| Parameter | Low Value | Optimal Value | High Value |
|---|---|---|---|
| Current (A) | 80-100 | 120-140 | 160-180 |
| Speed (mm/min) | 50-70 | 100-120 | 150-180 |
| Heat Input (kJ/mm) | 0.4-0.6 | 0.7-1.0 | 1.2-1.5 |
| Shielding Gas Flow (L/min) | 8-10 | 15-20 | 25-30 |
Optimal parameters produce a balanced microstructure with moderate grain size and acceptable mechanical properties. Excessive heat input leads to severe grain coarsening in the HAZ, while insufficient heat input results in incomplete fusion and lack of penetration. The shielding gas flow rate must be carefully controlled to prevent nitrogen and oxygen pickup, which would embrittle the weld zone.
Study Insights and Engineering Relevance
This research highlights that TIG welding of Ti6321 requires careful parameter selection to balance penetration requirements against microstructural degradation. For pressure vessel applications, the reduced fracture toughness in the weld zone represents a potential weakness that must be addressed through post-weld heat treatment (PWHT) at 590-620°C for 2-4 hours to refine the α grain structure and reduce residual stresses. The dynamic mechanical property data also inform the assessment of weld joint performance under impact or seismic loading conditions, which is increasingly relevant as pressure vessel design codes incorporate more sophisticated failure assessment procedures. Engineers working on Ti6321 pressure vessels should pay particular attention to the CGHAZ, as this zone represents the most vulnerable region for crack initiation under both static and dynamic loading conditions.