GH4169 alloy is extensively employed in nuclear reactors, aerospace gas turbine production, and pressure containment because of its good fatigue resistance and outstanding corrosion resistance. For GH4169 alloy, the strengthening impact of γ″ on the matrix is better than that of the γ′ phase. When the temperature exceeds the limit service temperature (650 ℃), the γ″ phase, a metastable phase, is transformed into the more stable δ phase with an orthorhombic structure, decreasing the mechanical properties of GH4169 alloy. Thus, it is an urgent problem for GH4169 alloy to enhance the service temperature and microstructure stability. Presently, the common solutions are to increase the content of secondary strengthening phase γ′ and reduce the content of primary strengthening phase γ″. As a crucial component of γ′, the Al element's content will directly influence its precipitation amount. In the traditional process of casting and forging, many researchers modify the γ′ precipitated phase's amount and morphology by changing the volume fraction of Al, to achieve more stable microstructure and stronger mechanical properties. The element segregation and melting loss of adding elements are prone to occur in as-cast alloys, making it impossible to meet the desired design. In the laser deposition manufacturing (LDM) process, scholars primarily focus on the influence of Nb element on the LDMed GH4169 alloy, but there are few studies on the effect of Al element content on the evolution of microstructure of LDMed GH4169 alloy. In this research, we hope to enhance the microstructure and stability of GH4169 alloy by adding Al element using laser deposition manufacturing technology.

Conventional GH4169 powder and a certain percentage of Al powder are combined using the mechanical blending approach. Laser deposition manufacturing is used to fabricate bulk specimens. The microstructures of as-deposited and heat-treated GH4169 alloys with various Al contents are examined, and the high-temperature microstructure stabilities of alloys with various Al contents are also examined deeply. First, the added Al element is distributed uniformly in the blended powder and LDM sample, and Al element loss is identified using the inductively coupled plasma (ICP)detection. Then, the microstructures of the as-deposited and heat-treated GH4169 alloys with various Al contents are observed, and the effect of Al content on the samples is examined. Second, the heat-treated samples are subjected to a long-term high-temperature aging treatment, and each sample's high-temperature microstructure stability is assessed. Finally, the effects of the Al content on the solidification process of GH4169 alloy are observed.

This study reveals that the uniformly mixed GH4169 powder and Al powder can be attained by mechanical mixing (Fig. 2). The added Al element only indicates a small amount of loss (Table 3), and the Al element is evenly distributed in the sample without visible segregation (Fig. 3). The as-deposited GH4169 alloy's microstructure with various Al contents consists of non-uniform columnar dendrites and equiaxed grains, which changes as the Al content increases. The secondary dendrite arm gradually develops (Fig. 4), accompanied by the main columnar structure, and the 2θ angle corresponding to the (200) crystal plane also gradually increases and shifts to the right (Fig. 5). Here θ is the diffraction angle. After the solid-solution+ aging heat treatment, the lamellar structure of each sample does not disappear, the increasing Al content results in the δ phase's fragmentation (Fig. 8). After further long-term aging, the number of δ phases decreases first and then increases, and the GH4169+ 0.50Al alloy reveals better high-temperature microstructure stability (Fig. 10). The increase in Al content cannot change the alloy's solidification sequence but decreases the alloy's solidification interval (Fig. 11).

Based on the above analysis, the morphology analysis findings reveal that the as-deposited GH4169 alloys with different Al contents show the morphology of epitaxially columnar dendrites. The addition of Al inhibits the element segregation in the Laves phases and the interdendritic region. The alloy secondary dendrite morphology is gradually developed. The findings of differential scanning calorimetry (DSC) demonstrate that the melting points of NbC and the γ matrix decrease as the Al contents increase, but the alloy solidification sequence does not change. After the solid-solution+ aging heat treatment, the needle-like δ phase is precipitated, but the Laves phase is dissolved in large quantities. Furthermore, the increase in Al content results in the δ phase's fragmentation. After the long-time high-temperature aging heat treatment, the numbers of δ phases decrease first and then increase with the increase of Al content. The GH4169+ 0.50Al alloy shows better thermal stability.