不同表面改性強(qiáng)化處理對(duì)TC4鈦合金表面完整性及疲勞性能的影響

不同表面改性強(qiáng)化處理對(duì)TC4鈦合金表面完整性及疲勞性能的影響INFLUENCE OF DIFFERENT SURFACE MODIFICATION TREATMENTS ON SURFACE INTEGRITY AND FATIGUE PERFORMANCE OF TC4 TITANIUM ALLOY

對(duì) TC4 鈦合金進(jìn)行了噴丸強(qiáng)化、激光沖擊強(qiáng)化和低塑性拋光 3 種表面強(qiáng)化改性處理, 測(cè)定了不同表面改性處理下的表面粗糙度、顯微硬度、殘余應(yīng)力及微觀組織, 研究了不同表面改性處理下的旋轉(zhuǎn)彎曲疲勞性能, 利用 SEM 觀察分析了疲勞斷口特征, 提出了表面強(qiáng)化改性機(jī)理和效果評(píng)價(jià)方法. 結(jié)果表明, 噴丸強(qiáng)化、激光沖擊強(qiáng)化和低塑性拋光 3 種表面強(qiáng)化改性處理后, TC4 鈦合金的旋轉(zhuǎn)彎曲疲勞壽命提高, 疲勞強(qiáng)度也大幅度提升, 而且疲勞裂紋位于表面強(qiáng)化層下的次表層; 對(duì)于未表面強(qiáng)化改性處理的 TC4 鈦合金, 疲勞裂紋位于表面. 基于位錯(cuò)理論, 對(duì)次表層裂紋萌生抗力和疲勞強(qiáng)度進(jìn)行了分析并給出了定量分析模型.

TC4 titanium alloy is usually used to manufacture engine blades, blings or blisks and fatigue is the main failure of these components due to its high strength, good corrosion resistance and light weight. In engineering applications, three typical surface modification processes as shot peening (SP), laser shock peening (LSP) and low plasticity burnishing (LPB) were employed to improve fatigue performance. In this work, SP, LSP and LSB were taken to enhance surface layer of TC4 titanium alloy. The surface integrity of specimens including surface roughness, microhardness, residual stresses and microstructure was investigated to obtain the effects of modification on surface layer by different methods. The rotation bending fatigue performance was tested at room temperature and fatigue fracture surfaces were analyzed by SEM. Fatigue life was compared at the same stress 760 MPa with the referece machinced specimen. Fatigue strength was determined by stair method for 1×107 cyc. The results show that both the rotation bending fatigue life and fatigue strength of TC4 titanium alloy are increased by these surface modification processes. The fatigue life prolonging factor (FLPF) for SPed specimens is 20.4, and FLPF for LSPed specimens and LPBed specimens is 89.6 and 99, respectively. Meanwhile, fatigue strength improvement percentage (FSIP) for SPed, LSPed and LPBed specimens is 36.3%, 37.8% and 38.8%, respectively. Moreover, the fatigue cracks initiate beneath surface enhanced layer for surface-modified specimens, while they are located at surfaces for un-surface-enhanced ones. Based on dislocation theory, the subsurface cracks initiation resistance and fatigue strength for surface-enhanced specimens were analysied. Finally, surface modification mechanism was discussedand some quantitative analysis method on surface modification effects was proposed. For surface-enhanced smooth specimens, the FSIP limit is 40% based on proposed analysis model and it is verified in this work by different surface layer enhancement processes (36.3% for SPed specimens, 37.8% for LSPed and 38.8% for LPBed specimens are near to 40%). Fatigue total life including initiation and propagation is a complex problem, and therefore it is difficult to give accurate life prediction and analysis, especially for small crack growth, although some invesitigations on total fatigue life can be roughly estimated based on Basquin relation for stress fatigue life or Coffin and Marson equtionfor strain fatigue life which have not any physical meaning or any mechanism.

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