Elsevier

Materials & Design

Volume 102, 15 July 2016, Pages 297-302
Materials & Design

Origin of stray-grain formation and epitaxy loss at substrate during laser surface remelting of single-crystal nickel-base superalloys

https://doi.org/10.1016/j.matdes.2016.04.051Get rights and content

Highlights

  • The mechanism of stray-grain formation at substrate of laser-processing single-crystal superalloys was first investigated.

  • The relationship between stray-grain formation and microsegregation in the substrate was first found.

  • A basic precondition, the homogenized treatment of substrate, for successful single-crystal laser processing was revealed.

Abstract

The stray-grain formation and epitaxy loss at the substrate during laser surface remelting of single-crystal nickel-base superalloys, which immediately lead to the break of the single-crystal growth, were investigated by means of modeling and experiments. Results indicate that the stray-grain formation and epitaxy loss at the substrate are attributed to the increased trend of the formation of equiaxed grains resulting from the composition segregation at the substrate interdendritic region. Accordingly, a solution treatment for the substrates prior to laser processing can effectively avoid the SG formation and epitaxy loss, which reveals a basic precondition, homogenized solution treatment of substrates, for successful single-crystal laser processing.

Introduction

Nickel-base single-crystal (SX) superalloys are unique high-temperature materials used as turbine blades and vanes due to their excellent mechanical properties at elevated temperatures [1], [2]. Nevertheless, many types of damage, e.g., blade tip erosion, are unavoidable under high temperature conditions. This means that the repair of damaged SX components is necessary because of the extremely high replacement costs. In addition, it is also necessary to repair the casting defects (such as surface pores) and crystalline imperfections within the single-crystal components resulting from the casting process [3], [4], [5]. Recently, the laser additive manufacturing (LAM) process, a near-net-shaping laser-deposited process, has exhibited a significant impact on the fabrication of a variety of alloys such as Ni-base superalloys [6], [7], [8], [9], [10]. Furthermore, since the LAM process allows rapid and accurate addition of controlled amounts of material to required locations with a low heat input, it also has potential for rapid forming and precision repair of SX components [4], [5], [11], [12], [13], [14], [15], [16], [17], [18], [19].

Successful SX LAM needs to ensure that the columnar dendrites epitaxially grow from the SX substrate along one of the six 〈001〉 preferred crystal directions [20]. However, the SX growth may be broken by the stray grain (SG) formation in the constitutional supercooling (CS) region ahead of the solid/liquid interface, i.e. columnar-to-equiaxed transition (CET), especially when the solidification is close to the top of melt pool. Therefore, many researchers have focused on the understanding of the CET [4], [5], [13], [14], [15], [16], [17], [18], [19]. However, in addition to the CET at the melt-pool top, there exists a remarkable phenomenon, epitaxy loss at the substrate, which immediately results in the break of the SX nature. In previous research, the epitaxy loss at substrate is generally attributed to the SG formation resulting from (1) the difference of crystallographic structures between deposit and substrate [18], [21], [22] and (2) certain undesirable particles, e.g., topologically closed-packed (TCP) phases, in substrate [21], [22], [23]. However, the mechanism of SG formation and resulting epitaxy loss at the substrate has not yet been taken into account seriously because these misoriented grains will be suppressed by other dendrites growing with a preferred crystal direction [21], [22]. More importantly, SGs forming at the substrate and resulting epitaxy loss will be retained in the repaired components because such SGs cannot be fully remelted. Therefore, in-depth understanding regarding the origin of the SG formation at the substrate is necessary for successful SX laser processing.

The aim of this paper is to elucidate the mechanism of the SG formation at the substrate, which immediately leads to the break of the SX growth. For this purpose, the microstructures produced on the substrates with different conditions were compared with those calculated by microstructure selection model to reveal the origin of SG formation at substrate.

Section snippets

Experimental

In this paper, laser remelting experiments were performed on the different SX substrates with or without standard solution treatment to reveal the origin of SG formation at substrate for following several reasons. Firstly, because the LAM and laser remelting possess a similar solidification process, which is especially true at melt-pool bottom, the latter can be regarded as the LAM process without powder feeding, i.e., a simplification of the LAM process [19]. Secondly, experiments by a process

Results and discussion

Fig. 1 shows the microstructures of the different SRR99 SX substrates. In Fig. 1a, the dendritic morphology of the as-cast substrate (without standard solution treatment) is clearly visible due to the composition segregation. In addition, a limited amount of γ-γ′ eutectic islands occurs in some interdendritic regions. For presenting the phases in the as-cast substrate more clearly, the high-magnification SEM image of the microstructure is shown in Fig. 1c. One can see that numerous cubical γ′

Summary and conclusions

Laser remelting experiments, simplifications of the LAM process, were performed on the different SX substrates with or without a solution treatment to reveal the origin of the SG formation and epitaxy loss at the substrate, which immediately leads to the break of the SX nature. By comparing the CET positions calculated by the microstructure selection model with the experimentally observed ones, the SG formation at the substrate is attributed to the increased trend of the CET resulting from the

Acknowledgements

This work was supported by the National High Technology Research and Development Program of China (Grant No. 2014AA041701). The authors thank Drs. ZHANG Shu-Quan and TIAN Xiang-Jun for the laser processing.

References (27)

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