Heat Treatment Effects on Powder Metallurgy Inconel 718 TIG Welded Joints

Literature Overview

This paper by Zhao Yunmei, Zhao Hongze, Wu Jie, Tian Xiaosheng, and Xu Lei, published in Chinese Journal of Materials Research in 2023 (Vol. 37, Issue 3, pp. 184–192), investigates the effects of post-weld heat treatment on the microstructure and mechanical properties of TIG-welded powder metallurgy (PM) Inconel 718 alloy plates. The PM Inconel 718 was produced by VIGA (Vacuum Induction Melting Inert Gas Atomization) followed by HIP (Hot Isostatic Pressing). The authors examined the weld joint microstructure using SEM and EBSD, measured microhardness distributions, and evaluated the effects of solution aging, homogenization, and post-weld HIP treatments. The study was supported by the Chinese Academy of Sciences YSBR Program (YSBR-025). The classification TG441.8 and keywords (Inconel 718, powder metallurgy, welding, heat treatment, microstructure, mechanical properties) indicate this is a high-temperature alloy welding metallurgy study with aerospace and power generation applications.

Base Metal Microstructure and Weldability

The base metal exhibited fine equiaxed grains with a grain size of approximately 28 μm, and the tensile strength approached the requirements for the same grade of wrought superalloy. This confirms that the VIGA-HIP processing route successfully produces PM Inconel 718 with microstructural characteristics comparable to wrought material. The absence of macroscopic porosity and inclusions in the weld joint indicates good weldability of the PM Inconel 718 alloy under TIG welding conditions.

Heat Treatment Effects on Microstructure and Properties

The study evaluated three post-weld heat treatment routes and their effects on the weld joint:

The most critical finding is that the Laves phase (Fe₂Nb-based intermetallic) present in the weld metal and HAZ is substantially dissolved after homogenization treatment, resulting in a more uniform microstructure and significantly improved ductility. The post-weld HIP treatment effectively eliminates microvoids formed during welding, enhancing the stability and reliability of mechanical properties.

Fracture Mechanism and Failure Analysis

The fracture analysis revealed that during tensile testing, the weld joint preferentially fails at the interface between Laves phase and the matrix. Microvoids nucleate at these interfaces, coalesce to form microcracks, and ultimately propagate to cause macroscopic fracture. This failure mechanism is characteristic of Inconel 718 weld joints where Laves phase segregation occurs due to the non-equilibrium solidification of the weld metal.

The Laves phase formation is a well-known challenge in Inconel 718 welding. The weld metal solidifies rapidly from the liquid state, and the Nb, Mo, and Ti alloying elements partition to the last-solidifying interdendritic regions, exceeding their solubility limits and forming Laves phase. This phase is brittle and acts as a crack initiation site under tensile loading. The homogenization treatment at 1120°C provides sufficient thermal energy to dissolve the Laves phase and redistribute the alloying elements uniformly throughout the microstructure.

Engineering Practice and Standards Compliance

Inconel 718 is governed by ASTM B637 (wrought), ASTM B447 (cast), and AMS 5663 (aerospace wrought). The PM Inconel 718 described in this study, produced by VIGA-HIP, represents a specialized processing route for applications requiring superior properties, particularly in the aerospace and power generation sectors where thin-wall components with high strength and fatigue resistance are required.

For welding PM Inconel 718, the following practices are recommended based on this study and industry experience:

• Use of matching filler metal (ERNiCrMo-3 per AWS A5.14) with controlled composition to minimize Laves phase formation
• Low heat input TIG welding with interpass temperature controlled below 150°C
• Post-weld solution heat treatment (1065°C/1h + 720°C/8h + 620°C/6h) to precipitate strengthening γ' and δ phases
• Homogenization treatment (1120°C) when superior ductility and uniformity are required
• Post-weld HIP when microvoid-free weld joints are essential for fatigue-critical applications
• Microstructural examination of the weld joint to verify Laves phase dissolution and grain boundary integrity

Study Insights and Reflections

This paper addresses a highly relevant topic for the aerospace and power generation industries, where Inconel 718 components are subjected to extreme temperatures and cyclic loading. The PM processing route (VIGA-HIP) offers the advantage of producing material with fine, uniform grain structure and minimal porosity, which is particularly beneficial for welding applications where the base metal quality directly influences weld joint performance.

The finding that homogenization treatment substantially dissolves Laves phase and improves ductility is practically significant. In conventional wrought Inconel 718 welding, the presence of Laves phase in the weld metal is often accepted as an unavoidable consequence of the welding thermal cycle, with only partial mitigation through post-weld heat treatment. The PM Inconel 718, with its more uniform initial composition, may be more amenable to complete Laves phase dissolution, resulting in weld joints with properties closer to the base metal.

The post-weld HIP treatment finding is also noteworthy. While HIP is commonly applied to PM fabrication, its application to weld joints is less common due to cost and equipment constraints. However, for critical applications such as turbine blades, compressor disks, and nuclear reactor components, the elimination of microvoids through post-weld HIP may be justified by the improved fatigue life and property stability. The fracture mechanism analysis—showing failure at Laves phase-matrix interfaces—provides clear metallurgical justification for the homogenization treatment and underscores the importance of microstructural control in high-temperature alloy welding.

This study represents a significant contribution to the understanding of PM superalloy welding metallurgy and provides a clear roadmap for achieving weld joints with properties equivalent to the base metal through appropriate post-weld heat treatment sequences.