Effect of laser shock peening on microstructure and hot corrosion of TC11 alloy

Highlights

• We examine the effect of LSP impacts on the microstructure, mechanical property and hot corrosion behavior of TC11

• The resistance of hot corrosion can be improved by compressive residual stresses, crystal defect and the grain refinement.

• The oxide protective layer of the LSPed specimen after hot corrosion is beneficial to improve the hot corrosion resistance.

Abstract

The effects of laser shock peening (LSP) impacts on microstructure, surface residual stress, micro-hardness and hot corrosion behavior of TC11 alloy were investigated. Results indicate that a large number of crystal defects such as dislocation cells, dislocation walls, dislocation arrays, high dense dislocation tangles and mechanical twins were gained on the surface of TC11 alloy after LSP. Furthermore, the coarse grains in the surface of the specimen have been refined to the nano-grains after multiple LSP impacts. LSP can induce high-amplitude compressive residual stresses on the surface. The maximum compressive residual stress is − 593 MPa with a 200 μm deep affected layer after 7 LSP impacts. The corrosion surface investigated by X-ray diffraction (XRD) revealed that the corrosion products mainly consist of TiO₂, Al₂O₃, NaAlO₂ and Na₂TiO₃. The oxide protective layer of the specimen treated by 7 LSP impacts is homogeneous and compact without obvious crack, spalling and pits. The dense oxide layer is beneficial to improve the hot corrosion resistance which is contributed to LSP induced crystal defects, nano-grains and high-amplitude compressive residual stresses. The minimum weight loss of the hot corrosion specimen treated by 7 LSP impacts was 1.08 mg/cm², which was much lower than that of the untreated specimen (11.87 mg/cm²).

Introduction

The higher property of materials and components (such as high fatigue resistance, high micro-hardness and high temperature wear resistance, etc. [1]) are looked forward to be used in high performance aero-engines with a thrust-weight ratio > 10 because of the rapid development of military aircraft. In order to accommodate the severe work environment such as high-cycle fatigue loading [2], high temperature wear [3] and hot corrosion [4], a large number of researchers have developed new materials with better performance and new material processing technologies [5]. The development of the aircraft industries require materials with superior mechanical and chemical properties and the applications of traditional materials used in aerospace and industrial fields is considerably restricted. In order to solve this problem, the surface strengthening technologies, such as shot peening (SP) [6], [7], surface mechanical attrition treatment (SMAT) [8] and laser shock peening (LSP) [9] have been studied and successfully applied to a lot of metallic materials. LSP is a novel surface treatment method to improve the fatigue durability, wear resistance, ultimate tensile strength and hot corrosion resistance of metallic materials and alloys.

There have been lots of efforts to investigate the surface morphologies, the microstructures in the affected layers, and the performances of materials after LSP. I. Altenberger et al. [10] found that the improvement in fatigue resistance of Ti-6Al-4 V alloy at elevated temperature was related to the high-temperature stability of highly tangled and dense dislocation substructure induced by LSP. X.D. Ren et al. [11] found that LSP affected dislocation evolution and the microstructure configuration of GH2036 alloy. After LSP, the micro-hardness of the surface was increased by 16%, and the surface topography exhibited excellent stability after thermal cycling.

Titanium and titanium alloys have been extensively used in aero-engines and industrial fields due to their high corrosion resistance, high strength to weight ratio and relatively high melting temperature [12]. However, when it goes to higher than 400 °C, titanium and titanium alloys oxidize rapidly in oxygen containing environments and undergo hot corrosion in marine environment. Majority of studies have been done on the improvement of alloys' mechanical properties and electrochemical corrosion resistance. J.Z. Lu et al. [13] found that the surface of the material generates compressive residual stress and grain refinement during LSP could improve the stress corrosion cracking (SCC) resistance. V.K. Caralapatti et al. [14] evaluated the feasibility and effects of high repetition laser shock peening (HRLSP) on corrosion resistance of Mg samples. The results showed that the corrosion rates of the laser shock peened specimens reduced by > 50% compared to the original specimen. The specimen treated with 66% overlap had the best corrosion resistance. D. Karthik et al. [15] found that the Inconel 600 treated by optimized laser shock peening without coating (LPwC) parameter has larger compressive residual stresses and affected depth with relatively lower surface roughness on the surface. The corrosion resistance of LPwC specimen increased tremendously, with a 10⁶ fold decrease in corrosion rate compared with that of original specimen. However, the influence process of multiple LSP impacts on the hot corrosion resistance of TC11 are not thoroughly explored, in particular, the effects of residual stress combined with refined micro-structure during multiple LSP impacts is still not well elucidated. Hence, the hot corrosion behavior of TC11 subjected to multiple LSP impacts is worth to be investigated.

In this paper, the influence of LSP impact times on the microstructure, micro-hardness, surface residual stress as well as hot corrosion resistance of the TC11 was systemically investigated. The aim of this study is to provide a novel way to increase hot corrosion resistance of TC11 and promote its industrial applications.