Effect of laser shock peening on the corrosion properties of Ti-6Al-3Nb-2Zr-1Mo alloy

https://doi.org/10.1016/j.surfcoat.2022.128425Get rights and content

Highlights

  • A large number of dislocation tangles and deformation twins were formed in LSPed Ti80 alloy.

  • LSP can effectively improve the corrosion resistance of Ti80 alloys.

  • Corrosion resistance of untreated Ti80 alloy: lamellar > bimodal > equiaxial

  • After LSP, corrosion resistance of Ti80 alloy: lamellar > equiaxial > bimodal

Abstract

The surface treatment of Ti80 alloy with different microstructures was performed by laser shock peening (LSP), and the effect of LSP on Ti80 alloy on microstructure and corrosion properties was investigated. After LSP, the surface grains of Ti80 alloy with different microstructures were all refined, microhardness was significantly increased, and compressive residual stress was generated, and both microhardness and compressive residual stress showed gradient change characteristics along the layer depth. The corrosion resistance of Ti80 alloy with different microstructures was improved in both 3.5% NaCl solution and 5MHCL solution after LSP. Before LSP, the corrosion resistance of lamellar microstructure was the best, followed by bimodal microstructure and the worst by equiaxial microstructure, while the corrosion resistance of equiaxial microstructure exceeded that of bimodal microstructure under the combined effect of volume fraction of β phase, thickness of α phase and compressive residual stress after LSP.

Introduction

Ti80 alloy is a new-developed near α type high-temperature Ti alloys with excellent outstanding performance such as high strength, corrosion resistance, superior toughness and weldability, thus, it is mainly used for structural application, especially in pressure-resistant housings of ships and bathyscaphs [1], [2]. The strong corrosion resistance of this alloy is associated with the formation of a thin (nanometer-scale) oxide film on surface which is related to surface microstructure [3]. In chlorides and sulfides containing marine environment, failure attributed to surface damage and reduced corrosion performance of Ti80 alloy still restricts the service life of Ti80 components. Based on the specific application, optimization of surface properties through surface modification techniques is required to be carried out. For example, ultrasonic shot peening (USP) [4], surface mechanical attrition treatment (SMAT) [5], ultrasonic surface rolling (USR) [6], supersonic fine particle bombardment (SFPB) [7], and laser shock peening (LSP) [8] surface modification techniques have been reported to enhance the corrosion resistance of Ti alloys.

LSP is a novel surface treatment technique, has the advantages of high pressure, high efficiency, good controllability and excellent strengthening effect, and can introduce greater compressive residual stress with less-affected surface roughness [9], [10], [11], as compared with other techniques. Luo et al. [12] found that the surface grains of Mg-Al-Mn alloy were refined, and compressive residual stress was generated at the surface after multiple LSP. This microstructural evolution led to superior corrosion resistance of Mg-Al-Mn alloy even in a high concentration corrosive solution (2.392 mol/L NaCl solution). Geng et al. [13] found that TC21 alloy possessed high compressive residual stress after LSP and underwent microstructural changes in the form of crystal defects at the surface layer during LSP, which promoted the formation of protective mixed oxides, resulting in improved hot corrosion property. Chukwuike et al. [14] found that compressive residual stress was generated at the surfaces of copper samples, resulting in an increase in surface denseness, a decrease in roughness, and consequently an increase in the corrosion resistance. Liu et al. [15] found that the corrosion resistance of 316 L stainless steel samples after LSP was better than that of the laser-welded ones. It was ascribed to that the LSPed surface owned large compressive residual stress and refined grains, as compared to the coarser and nonuniformly distributed grains of laser-welded samples. Ning et al. [16] proposed that the surface grain refinement and the generation of compressive residual stress in IN718 alloy after LSP could prevent the propagation of microcracks and corrosion pits.

Though the fact that LSP has already been successfully applied to various metallic materials and proved its ability to improve the corrosion resistance of materials, the implementation of LSP to improve the corrosion resistance of Ti alloys is still relatively rare, especially to Ti80 alloy that has not been reported yet. Besides, Ti alloys are capable of obtaining different microstructures with different plastic deformability [17]. Therefore, in this study, the LSP surface treatment was carried out on a Ti80 alloy with equiaxial, bimodal and lamellar microstructure, respectively. Then, the corrosion changes were systematically studied. The aim of this paper is to establish a correlation between the initial, LSPed microstructures and corrosion properties, and then to provide a solid basis for the implementation of LSP to improve the corrosion properties of Ti alloys.

Section snippets

Materials

The material investigated in this work was forged Ti80 alloy bar, and its nominal chemical composition is shown in Table 1. The samples were annealed in an artificial intelligence chamber resistance furnace (SGM) at 940 °C, 970 °C and 1040 °C for 1.5 h, and followed by air cooling to room temperature to obtain different initial equiaxial, bimodal and lamellar microstructure prior to LSP. As seen in Fig. 1, equiaxial microstructure consists of primary α and a small amount of transformed β that

Surface roughness

The three-dimensional surface morphology of Ti80 alloy before and after LSP is shown in Fig. 4. The surface of the specimen before LSP is uniform and flat (Fig. 4(a)), while the surface of the specimen after LSP can be obviously seen as protrusions and pits (Fig. 4(b)). It was found that microstructure had almost no effect on the roughness, and the average surface roughness Sa of the Ti80 alloy specimens before and after LSP were 0.451 and 0.501, respectively, while peak-to-valley (PV) are

Corrosion mechanisms of Ti80 alloys with different initial microstructures

According to the above analyses, the corrosion resistance of the untreated Ti80 alloys is: lamellar microstructure > bimodal microstructure > equiaxial microstructure. The volume fraction of β phase plays an important role in the corrosion properties of Ti alloys [34]. The uneven distribution of alloying elements in α and β phases and interfaces could result in the formation of potential difference, then the formation of primary electrochemical cells, and finally intergranular or pitting

Conclusion

In this study, LSP surface treatment on Ti80 alloy with equiaxial, bimodal and lamellar initial microstructure was carried out. The corrosion behaviours in 3.5% NaCl and 5MHCL solution were studied comparatively. The following conclusions can be drawn.

  • 1.

    The corrosion resistance of Ti80 alloys after LSP was improved in both in 3.5% NaCl and 5 M HCL solution.

  • 2.

    After LSP, the grains in the surface microstructure of Ti80 alloy are obviously refined and the grain boundaries increase. The increase of

CRediT authorship contribution statement

Gaoli Luo: Conceptualization, Investigation, Formal analysis, Data curation, Writing – original draft, Writing – review & editing, Visualization. Lingfeng Zhang: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Writing – review & editing, Visualization, Supervision, Project administration, Funding acquisition. Yi Xiong: Software, Validation, Investigation, Resources. Baofeng Zhang: Software, Validation, Investigation, Resources. Xuepeng Chen: Formal

Declaration of competing interest

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.

Acknowledgements

Authors want to thank the assistance from the National Natural Science Foundation of China (Nos. U1804146, and 52111530068).

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