Blade is one of the important working parts of aeroengine, which requires high shape accuracy and complex service environment. The precise shape and dimensions of the blades remarkably influence the safety, efficiency, and reliability of the aeroengines. The ongoing demand for a reduction in aeroengine weight has inspired the development of complex blades with wider chord and thinner thickness [1].
TC4 titanium alloy is widely used in aeronautical blades due to its high melting point, low density, high specific strength, good intermediate temperature performance, good corrosion resistance and high heat resistance. After the processes of forging, heat treatment and machining, shot peening is implemented to shape the titanium alloy blades, to improve the service life, and to inhibit the growth of fatigue cracks [2–4]. In the shot peening process, the small shots are utilized to bombard thin metallic parts and a thin surface layer of material is plastically deformed. However, each process of manufacturing blades results in the superposition of deformation and residual stress distribution, which may lead to the blade size out of tolerance and the complex distribution of the residual stresses.
As a kind of curved thin-walled structural parts, the blade manufacturing faces significant machining difficulty and high cost. Therefore, the application of finite element method (FEM) in the field of manufacturing such thin-walled workpieces is becoming more and more widespread, especially in the shot peening process [1, 5]. Many investigators have developed the analytical and experimental methods to predict the shot peening residual stresses. Davis has established the model of residual stress and plastic strain and has pointed that the in-plane inelastic strains were zero because of compatibility when a semi-infinite surface was shot peened [6]. To investigate the influence of the peening pattern on the resulting deformed shell, Garie´py has utilized small scale tests and simple model to simulate incremental deflections as the shot stream traveled over the samples [7]. Zhang et al. have optimized the shot peening parameters of fan blades by a FEM model [1]. To compare with experimental testing, Yao et al. have proposed a FEM model to predict the deformation of fan blade after milling and shot peening [5].
However, explicitly simulating the shot peening process on the whole blade is too much time-consuming. The eigenstrain method is always introduced in FEM to achieve strain compatibility, together with stress equilibrium and traction boundary conditions [8, 9]. Salvati has studied a shot peened compressor blade by prescribing eigenstrain in 2D geometry of arbitrary shape [8]. The proposed eigenstrain modeling approach generates an efficient parametric representation of the residual stress field. Song has simulated the effect and evolution of shot peened residual stresses using eigenstrain-based finite element model of a magnesium alloy [10]. These results showed that the eigenstrain method was reasonable in residual stress and deformation prediction model.
In actual blade manufacturing, shot peening is the last process and the previous machining and milling have induced residual stresses and deformation already. Therefore, the superposition of each process should be discussed in the study of residual stress. Yang has investigated the machining distortion of aluminum alloy and titanium alloy aircraft monolithic component. The initial residual stress in the blank and cutting loads were demonstrated respectively to be the main effects of the machining distortion for aluminum alloy and titanium alloy [11]. Wang et al. have investigated the milling parameters of thin-wall surface parts using FEM, such as tool temperature, tool stress, and the elastic deformation [12]. Liu et al. have established a dynamic model to predict milling error of arbitrary point [13].
Although scholars have studied the shot peening strategies on the thin plate and different materials, few studies were focused on the influences of shot peening on the blades with complex profiles and the superposition effects of residual stresses of the entire manufacturing processes. The present research has established the model of shot peened blade mainly based on the concept of eigenstrain and the comparison with the experimental measurement. The residual stresses and deformation of the blades were discussed with no initial residual stresses, with residual stresses during milling and after dynamic milling. The effects of sequences of shot peening were also considered. This research could provide a guidance on shot peening strategies of blades in engineering.