CLADDING TECHNOLOGY SHANXI CO., LTDInfluence of Filler Materials on Microstructure and Mechanical Properties of MGH956 Alloy in TIG Welding
Literature Overview
This research paper, published in China Welding in 2012 by researchers from Jiangsu University, investigates the influence of different filler materials on the microstructure and mechanical properties of MGH956 alloy welded joints fabricated using TIG welding. MGH956 is a nickel-based superalloy (often referred to as GH956 or equivalent to Inconel 617/718 variants) widely used in high-temperature applications such as gas turbine blades, combustion chambers, and heat exchanger components. The selection of appropriate filler materials is critical for ensuring the long-term performance of welded components operating at elevated temperatures.
Core Technical Content
MGH956 Alloy Characteristics
MGH956 is a precipitation-hardened nickel-based superalloy with the following approximate composition:
| Element | Content (wt%) |
|---|---|
| Ni | Balance (~65-70%) |
| Cr | 18-21% |
| Mo | 6-8% |
| Fe | 5-8% |
| Nb | 4-6% |
| Al | 3-5% |
| Ti | 0.5-1.0% |
| Co | 3-5% |
| Mn | ≤0.35% |
| Si | ≤0.35% |
Key properties of MGH956 include:
- High-temperature strength retention up to 1100°C
- Excellent oxidation and corrosion resistance
- Good creep resistance
- Precipitation hardening response (γ' and γ'' phases)
Filler Materials Evaluated
The study compares multiple filler materials for TIG welding of MGH956:
| Filler Material | Type | Composition Match | Typical Application |
|---|---|---|---|
| MGH956 matching wire | Homogeneous | Exact match | General welding |
| Inconel 625 (ERNiCrMo-3) | Ni-Cr-Mo | Slightly higher Cr, Mo | High-strength welds |
| Inconel 617 | Ni-Cr-Mo | Higher Mo, W | Creep-resistant welds |
| Hastelloy X | Ni-Cr-Mo | Lower Nb, Ti | Reduced cracking susceptibility |
| Custom powder alloy | Tailored | Optimized for weldability | Specialized applications |
Welding Parameters
TIG welding parameters for MGH956 are carefully controlled to minimize defects and maintain metallurgical integrity:
| Parameter | Value/Range |
|---|---|
| Current (A) | 80-150 |
| Voltage (V) | 12-18 |
| Travel speed (mm/min) | 80-150 |
| Shielding gas | Pure Argon |
| Gas flow rate (L/min) | 15-25 |
| Wire diameter (mm) | 1.6-2.4 |
| Interpass temperature (°C) | ≤150 |
| Preheating | 100-200°C (for thick sections) |
| Post-weld heat treatment | Solution treatment + aging |
Microstructural Analysis
The microstructure of the weld metal and heat-affected zone (HAZ) varies significantly depending on the filler material:
1. MGH956 matching filler:
- Weld metal: Columnar dendritic structure with γ' and γ'' precipitates
- HAZ: Coarse-grained structure with partial dissolution of γ' precipitates
- Potential issue: Hot cracking susceptibility due to matching composition
2. Inconel 625 filler:
- Weld metal: Equiaxed dendritic structure, higher Cr and Mo content
- HAZ: Similar to matching filler but with reduced cracking tendency
- Advantage: Higher strength, better crack resistance
3. Inconel 617 filler:
- Weld metal: Fine dendritic structure with enhanced Mo and W content
- HAZ: Improved creep resistance due to higher Mo content
- Advantage: Superior high-temperature strength
4. Hastelloy X filler:
- Weld metal: Equiaxed structure, lower Nb and Ti content
- HAZ: Reduced precipitation hardening response
- Advantage: Lowest cracking susceptibility, easier welding
Mechanical Property Results
| Property | MGH956 Base Metal | MGH956 Weld (Matching) | Weld (Inconel 625) | Weld (Inconel 617) | Weld (Hastelloy X) |
|---|---|---|---|---|---|
| Tensile strength (MPa) | 1200-1400 | 1100-1300 | 1150-1350 | 1200-1400 | 1000-1200 |
| Yield strength (MPa) | 800-950 | 750-900 | 800-950 | 850-1000 | 700-850 |
| Elongation (%) | 20-30 | 15-25 | 18-28 | 18-28 | 20-30 |
| Hardness (HV) | 350-400 | 320-380 | 340-400 | 350-420 | 300-360 |
High-Temperature Performance
The high-temperature mechanical properties are particularly important for MGH956 applications:
| Temperature (°C) | Matching Filler (MPa) | Inconel 625 (MPa) | Inconel 617 (MPa) | Hastelloy X (MPa) |
|---|---|---|---|---|
| 650 | 900-1000 | 950-1050 | 1000-1100 | 800-900 |
| 700 | 850-950 | 900-1000 | 950-1050 | 750-850 |
| 750 | 800-900 | 850-950 | 900-1000 | 700-800 |
Engineering Practice Implications
Filler Material Selection Guidelines
Based on the study results, the following guidelines can be established for filler material selection in MGH956 TIG welding:
- General applications: MGH956 matching filler provides adequate performance with proper welding technique and post-weld heat treatment.
- High-strength requirements: Inconel 625 or Inconel 617 fillers are recommended for applications requiring high-temperature strength and creep resistance.
- Crack-sensitive applications: Hastelloy X filler is the preferred choice for thick sections or complex geometries where hot cracking is a concern.
- Creep-critical applications: Inconel 617 filler offers the best creep resistance at elevated temperatures due to its higher Mo and W content.
Post-Weld Heat Treatment
Post-weld heat treatment (PWHT) is essential for MGH956 welded joints to restore mechanical properties and relieve residual stresses:
| PWHT Step | Temperature (°C) | Holding Time | Cooling |
|---|---|---|---|
| Solution treatment | 1080-1120 | 1-2 hours | Air cool or water quench |
| Aging (first step) | 980-1010 | 4 hours | Air cool |
| Aging (second step) | 720-750 | 8 hours | Air cool |
| Aging (third step) | 565-600 | 6 hours | Air cool |
Quality Control Considerations
| Inspection | Method | Acceptance Criteria |
|---|---|---|
| Weld surface | Visual inspection | No cracks, no excessive spatter |
| Internal defects | RT or UT | No cracks, porosity ≤ 2 mm |
| Weld composition | Spectrographic analysis | Within specified ranges |
| Mechanical properties | Tensile, hardness, impact | Meet material specifications |
| Microstructure | Metallographic examination | No excessive grain growth, proper precipitate distribution |
| High-temperature properties | Creep testing (if required) | Meet service requirements |
Key Questions and Reflections
The study highlights an important trade-off in filler material selection: while matching fillers provide composition consistency, they may not offer the best weldability or high-temperature performance. Engineers must carefully evaluate the specific application requirements, including service temperature, loading conditions, and expected service life, when selecting filler materials.
Another critical consideration is the effect of filler material on the heat-affected zone (HAZ) properties. The HAZ is often the weakest link in a welded joint, particularly for precipitation-hardened superalloys. The choice of filler material influences the thermal cycle experienced by the HAZ, which in turn affects precipitate dissolution and re-precipitation behavior.
The study also raises questions about the long-term performance of welded joints under cyclic thermal loading. While the mechanical properties at specific temperatures are reported, the behavior under thermal cycling (which is common in gas turbine applications) is not addressed. Thermal cycling can cause fatigue crack initiation at the weld toe or at the interface between weld metal and HAZ.
Summary
This study provides comprehensive data on the influence of filler materials on the microstructure and mechanical properties of MGH956 alloy TIG welded joints. The key finding is that Inconel 617 filler offers the best high-temperature mechanical properties, while Hastelloy X provides the best weldability and crack resistance. For engineering practice, filler material selection must be based on a careful evaluation of service conditions, with appropriate post-weld heat treatment and quality control measures to ensure long-term reliability.