CLADDING TECHNOLOGY SHANXI CO., LTDHeat Treatment Influence on TIG Welded Powder Metallurgy Inconel 718 Alloy
Literature Overview
This study, published in the Chinese Journal of Materials Research in 2023 by Zhao Yunmei and colleagues from the Institute of Metal Research, Chinese Academy of Sciences, and the University of Science and Technology of China, investigates the effects of heat treatment on the microstructure and mechanical properties of TIG welded joints of powder metallurgy (PM) Inconel 718 superalloy. The research was supported by the CAS Youth Team Program (YSBR-025) and addresses a significant challenge in the fabrication of high-performance superalloy components, where welding and heat treatment must be carefully coordinated to achieve the desired mechanical properties.
Material Background and Welding Challenges
Inconel 718 is a nickel-based superalloy strengthened by the precipitation of γ′ (Ni₃(Al,Ti)) and δ (Ni₃Nb) phases, offering exceptional strength, fatigue resistance, and creep performance at elevated temperatures. Powder metallurgy Inconel 718 exhibits a finer and more uniform microstructure compared to cast or wrought variants, due to the atomization and consolidation process, which results in superior mechanical properties and reduced anisotropy. However, the welding of PM Inconel 718 presents unique challenges, including the risk of hot cracking, sensitization, and degradation of the precipitation-hardened microstructure in the weld zone and HAZ.
TIG welding is the preferred method for welding Inconel 718 components due to its low dilution, precise heat input control, and the ability to use inert gas shielding effectively. However, the thermal cycle of TIG welding can dissolve the strengthening precipitates in the HAZ, leading to a soft zone with reduced strength and creep resistance. Post-weld heat treatment (PWHT) is therefore essential to restore the microstructure and mechanical properties of the welded joint.
Heat Treatment Procedures and Microstructural Evolution
The study examines several heat treatment sequences, including solution treatment followed by aging, and the effects of different aging temperatures and durations on the microstructure and mechanical properties of the TIG welded PM Inconel 718 joints. The standard heat treatment for Inconel 718 involves solution treatment at 1065 °C followed by double aging at 720 °C and 620 °C. However, for welded joints, the heat treatment parameters may need to be modified to account for the altered microstructure and residual stresses in the weld zone.
The solution treatment at 1065 °C dissolves the γ′ and δ precipitates, creating a supersaturated solid solution. Upon subsequent aging, the strengthening phases re-precipitate, restoring the mechanical properties. The aging temperature and duration are critical parameters, as they control the size, distribution, and volume fraction of the precipitates. Over-aging leads to coarsening of the γ′ precipitates and a reduction in strength, while under-aging results in incomplete precipitation and suboptimal properties.
| Heat Treatment Step | Temperature (°C) | Duration (h) | Purpose |
|---|---|---|---|
| Solution treatment | 1065 | 1–2 | Dissolve precipitates, homogenize |
| First aging | 720 | 8 | Precipitate γ′ and δ |
| Second aging | 620 | 8 | Refine γ′, enhance strength |
| Alternative single aging | 720 | 8–16 | Simplified treatment, slightly lower strength |
The microstructural analysis of the TIG welded joints after heat treatment reveals distinct zones: the weld metal, the HAZ, and the base metal. In the weld metal, the microstructure is characterized by columnar dendrites with interdendritic segregation of Nb and Ti, which affect the precipitation behavior during aging. The HAZ exhibits a gradient of microstructural transformation, with the region closest to the weld showing complete dissolution and re-precipitation, while the region farther from the weld retains some of the original PM microstructure.
Mechanical Properties and Performance Evaluation
The mechanical properties of the TIG welded PM Inconel 718 joints were evaluated through tensile testing, hardness profiling, and microstructural examination. The results demonstrate that the heat treatment sequence has a profound effect on the strength and toughness of the welded joint. The solution-treated and double-aged joints exhibited the highest tensile strength and yield strength, with values approaching those of the base metal. The as-welded joints showed significant softening in the HAZ, with hardness reductions of 20 to 30% compared to the base metal.
The hardness profile across the cross-section of the heat-treated joints reveals a more uniform distribution compared to the as-welded condition, with the HAZ hardness restored to within 10% of the base metal value. This uniformity is critical for applications involving cyclic or creep loading, as localized soft zones can serve as initiation sites for fatigue or creep cracks.
| Condition | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Hardness (HV) |
|---|---|---|---|---|
| Base metal (PM) | 1350–1450 | 895–965 | 20–25 | 380–420 |
| As-welded | 1100–1200 | 750–850 | 15–20 | 320–360 |
| Solution + double aged | 1300–1400 | 870–940 | 18–23 | 370–410 |
| Solution + single aged | 1250–1350 | 830–900 | 17–22 | 360–400 |
Engineering Practice and Welding Procedure Development
In engineering practice, the welding and heat treatment of PM Inconel 718 components requires careful planning and execution. The welding procedure specification must define the welding parameters, including current, voltage, travel speed, and shielding gas composition, to minimize the thermal input and reduce the extent of HAZ softening. The heat treatment procedure must be designed to restore the mechanical properties while avoiding excessive grain growth or precipitate coarsening.
A critical consideration in the welding of PM Inconel 718 is the control of interpass temperature, which should be maintained below 150 °C to minimize the thermal input and reduce the risk of hot cracking. The use of preheating is generally avoided, as it can promote grain growth and reduce the effectiveness of the subsequent heat treatment. Post-weld stress relief is often performed at 620 °C for 4 to 8 hours to reduce residual stresses without significantly affecting the precipitate structure.
Study Insights and Implications
This research provides valuable insights into the interaction between TIG welding and heat treatment in PM Inconel 718 superalloy. The key finding is that the solution treatment and double aging sequence is essential for restoring the mechanical properties of the welded joint to near-base-metal levels, while the as-welded condition exhibits significant property degradation in the HAZ. The study also highlights the importance of understanding the microstructural evolution in each zone of the welded joint, as the precipitation behavior varies between the weld metal, HAZ, and base metal. For engineers fabricating PM Inconel 718 components, the study underscores the necessity of integrating welding and heat treatment as a unified process, rather than treating them as separate operations. The results also suggest that the powder metallurgy route offers advantages in terms of microstructural uniformity and mechanical property consistency, which translate into improved weldability and post-weld heat treatment response.