CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructural Evolution in the Partially Melted Zone of Inconel 718 TIG Welding
Literature Overview
This study, published in Acta Metallurgica Sinica in 2014 by Ye Xin, Hua Xueming, Wang Min, and Lou Songnian from Shanghai Jiao Tong University, investigates the microstructural transformations occurring in the partially melted zone (PMZ) during gas tungsten arc welding (GTAW/TIG) of Inconel 718 superalloy. Inconel 718 is a precipitation-strengthened nickel-based alloy widely employed in high-temperature, high-stress applications such as aerospace turbine components, hydrogenation reactor internals, and pressure vessel cladding layers. Understanding the PMZ behavior is critical because this zone, though not fully melted, experiences peak temperatures approaching the solidus and undergoes significant grain coarsening, precipitate dissolution, and phase instability—factors that can severely degrade mechanical integrity in service.
Core Technical Findings
The research identifies several key microstructural phenomena in the PMZ:
- Grain boundary precipitation dissolution and re-precipitation: The original γ' (Ni₃(Al,Ti)) and γ'' (Ni₃Nb) strengthening phases that provide room-temperature strength are partially dissolved at peak temperatures exceeding approximately 1100°C near the fusion line. Upon cooling, incomplete re-precipitation occurs due to rapid thermal cycling, resulting in a zone depleted of coherent strengthening precipitates.
- δ phase (Ni₃Nb) formation: In regions where Nb is locally enriched, the equilibrium δ phase may form or grow at grain boundaries, particularly where cooling rates are moderate. This phase is thermodynamically stable but can act as crack initiation sites under thermal cycling.
- Laves phase (Mo,Fe)₆Si₃ precipitation: In Nb-rich or Mo-rich regions of the PMZ, brittle Laves phase may nucleate, especially at higher welding currents or lower travel speeds. This phase is detrimental to ductility and creep resistance.
- Grain coarsening: Recrystallization and grain growth occur in the PMZ where temperatures exceed the recrystallization threshold (~565°C for Inconel 718). The resulting coarse grains near the fusion boundary reduce yield strength and promote intergranular fracture.
Process Parameters and Their Influence
| Parameter | Typical Range | Effect on PMZ |
|---|---|---|
| Welding current | 80–200 A | Higher current → wider PMZ, greater precipitate dissolution |
| Travel speed | 300–800 mm/min | Lower speed → higher heat input, wider PMZ |
| Heat input | 1.5–4.5 kJ/mm | Directly proportional to PMZ width |
| Shielding gas | Ar (99.99%) or He/Ar mix | He increases penetration, narrows PMZ |
| Interpass temperature | ≤ 150°C | Exceeding this promotes δ phase and Laves phase |
Engineering Practice Integration
In bimetal pressure vessel fabrication, Inconel 718 overlay layers are frequently applied to carbon steel or low-alloy steel substrates for hydrogen service or high-temperature corrosion resistance. The PMZ concerns in this literature directly translate to weld overlay scenarios:
- Hot-wire TIG overlay of Inconel 718 on carbon steel: The substrate PMZ experiences even more severe thermal cycling. Post-weld heat treatment (PWHT) at 720°C for 8 hours with water quenching is typically required to restore precipitate distribution.
- Multi-pass overlay considerations: Each subsequent pass re-heats the previous PMZ. Cumulative thermal exposure can lead to excessive δ phase formation and grain boundary embrittlement.
- Inspection implications: The PMZ is a critical region for hydrogen-assisted cracking (HAC) susceptibility in Inconel 718, particularly under cyclic loading or in corrosive environments. Ultrasonic testing (UT) and phased array ultrasonic testing (PAUT) should be directed at this region.
Key Reflections
The fundamental challenge with Inconel 718 is its sensitivity to thermal history. The alloy's mechanical properties are entirely dependent on the proper distribution and volume fraction of γ' and γ'' precipitates. Any thermal exposure that disrupts this microstructure—whether from welding, PWHT, or service conditions—requires careful management. In engineering practice, I have observed that the PMZ often becomes the weakest link in Inconel 718 welded joints, particularly under creep conditions above 650°C. The study reinforces the necessity of strict interpass temperature control and proper post-weld aging treatment to restore mechanical properties in the PMZ.
Concluding Remarks
This literature provides essential foundational knowledge for engineers working with Inconel 718 in cladding and overlay applications. The identification of precipitate dissolution, δ phase formation, Laves phase precipitation, and grain coarsening in the PMZ enables targeted process optimization and quality control strategies. For pressure vessel fabrication involving Inconel 718 overlay layers, the implications are profound: weld procedure qualification must include microstructural examination of the PMZ, and post-weld aging schedules must be validated to ensure full precipitate restoration before service entry.