Effects of laser shock peening on the hot corrosion behaviour of the selective laser melted Ti6Al4V titanium alloy

Highlights

• Effect of LSP on hot corrosion behavior of the SLMed Ti6Al4V alloy was studied.

• The SLM-LSPed specimen has lower mass gain after hot corrosion.

• LSP generated high-density dislocations and nano-twins.

• LSP generated the grain boundary effect in the SLMed specimen.

• LSP efficiently improved hot corrosion resistance of the SLMed Ti6Al4V alloy.

Abstract

The effects of laser shock peening (LSP) on the hot corrosion behaviour of the selective laser melted (SLMed) Ti6Al4V titanium alloy were investigated. The results indicated that the SLMed specimen exhibited a significantly lower hot corrosion resistance in comparison to the SLM-LSPed specimen. In addition, high-density dislocations and a large number of nano-twins were generated after LSP treatment, which contributed to the grain boundary effect in the surface layer. The coupling effect of LSP-induced grain boundary effect and plastic deformation effectively improved the hot corrosion resistance of the SLMed Ti6Al4V.

Introduction

Ti6Al4V titanium alloy has been widely used in aeronautic, astronautic and marine fields, due to its outstanding comprehensive performance (high specific strength, good ductility, and excellent corrosion resistance) [1,2]. However, because of the high melting point of titanium alloys, the equipment for smelting and casting are costly, and the operation is complicated. The poor thermal conductivity in the chip cutting process also makes it difficult to fabricate the titanium alloy components by traditional technologies [3].

Currently, selective laser melting (SLM) as one of the additive manufacturing (AM) technologies, uses high-energy laser to selectively melt metal powder to produce high-density materials, which has become the rapidly developing advanced manufacturing technology. Compared with traditional processing technologies, SLM has the advantages of short manufacturing cycle, high material utilization, high precision, good surface quality, good density of formed parts, and strong ability to prepare complex structural parts [[4], [5], [6]]. Due to the above advantages, this promising technology has becoming an important technology in the field of Ti6Al4V titanium alloy processing [7].

Due to the above unique advantages, the selective laser melted (SLMed) Ti6Al4V alloy is considered as prospective high-temperature structural materials (typically above 400 ℃) used in aero-engines and gas turbines [[8], [9], [10]]. At present, a great deal of efforts have been developed to improving the high temperature oxidation resistance of Ti6Al4V titanium alloys [[11], [12], [13]]. However, the service environments are generally more complex in practical applications. Some salts such as Na₂SO₄ and NaCl might cover the surface at elevated temperature, which will destroy the protective oxide layers and accelerate the failure of alloys. This process is the so-called “hot corrosion” [[14], [15], [16]]. Sodium is mainly derived from the marine atmosphere (sea salts usually contain Na₂SO₄ and NaCl). Meanwhile, it can also be found in the industrial atmospheric pollutants and fuel. During combustion, Na₂SO₄ can form from sodium and sulfur, the latter being present in the fuel. Furthermore, in combustion gases, NaCl itself will react with sulfur oxide and oxygen, forming Na₂SO₄. Hot corrosion is generally classified into two forms of attack: high-temperature hot corrosion (800 °C – 950 °C, type I), and low-temperature hot corrosion (600 °C – 750 °C, type II) [17,18]. Generally, Type II hot corrosion is even more corrosive than type I. Unlike oxidation, this form of corrosion can destroy materials at an unpredictable rate. Consequently, the load-carrying ability of the component is reduced, which eventually leads to its catastrophic failure.

As we know, the anti-corrosion performance of materials highly depends upon the surface state [19]. Laser shock peening (LSP) is considered as a promising surface treatment technology, wherein a high-peak power density laser beam acts on the material surface to create ultra-high energy (GW), pressure (GPa), strain rate (>10⁶ s⁻¹) and short duration (ns) shock wave [20]. Subsequently, as the peak pressure exceeds the dynamic yield strength of the material, severe plastic deformation are generated by the ultrastrong laser shock wave (LSW), thereby modifying near-surface microstructure, inducing compressive residual stress and improving mechanical properties, such as micro-hardness [20], fatigue life [21], wear [22] and corrosion resistance [23]. Cao et al. [24] studied the effect of LSP on the hot corrosion resistance of the nickel-based superalloy (GH202) and demonstrated that the loss weight of the LSP-treated specimen was much lower than that of the untreated specimen. Tong et al. [25] also found that there is a dense and homogeneous oxide protective layer without obvious cracks on the surface of the LSPed specimen, which can improve the hot corrosion resistance of TC11 alloy. Recently, Chen et al. [26] reported that the LSPed specimen has more excellent high-temperature oxidation resistance with lower mass gain after oxidation for 100 h at 900 °C in comparison to the unLSPed specimen. Guo et al. [13] also researched the oxidation behaviour of laser additive manufactured (LAMed) Ti6Al4V titanium alloy before and after LSP and found that the dense Al₂O₃ induced by LSP lead to the improved oxidation resistance. Therefore, LSP treatment is feasible to enhance the hot corrosion resistance of the traditional and LAMed metallic material. Nevertheless, few researches have focused on the effect of LSP on the hot corrosion resistance of the SLMed Ti6Al4V titanium alloy. Under the thermal/mechanical effects, the relationship between the microstructural evolution and hot corrosion behaviour of the SLMed Ti6Al4V titanium alloy treated by LSP and corresponding corrosion mechanism seldom were given full attention, which is crucial to enhance the hot corrosion resistance.

Hence, in this paper, the effects of LSP on the hot corrosion behaviour of the SLMed Ti6Al4V alloy in the salt mixture at 400, 500, 600 and 700 ℃ for 50 h were examined. Hot corrosion kinetics was measured by the mass change, and the surface and cross-sectional morphologies of the corrosion scale (CS) were characterized by scanning electron microscopy (SEM) method. Furthermore, LSP-induced microstructural evolution was also investigated by electron back scatter diffraction (EBSD) and transmission electron microscopy (TEM) methods. Finally, the dominant mechanism of the improved hot corrosion resistance of the SLMed Ti6Al4V titanium alloy by LSP was revealed.