Effect Of Nanocrystallization In Surface Layer On Fatigue Strength Of TC4 Titanium Alloy By High Energy Shot Peening

Due to the high specific strength, excellent corrosion resistance and good high temperature performance etc., Titanium alloys have been widely used in modern aerospace, military and civilian industry, among which, Ti-6Al-4V (TC4) is the most widely used one that exceed 50% of all the applications. Materials or components often subject to repeated loads during service, though the stress levels does not exceed the strength limit materials, destroy may take place even under the elastic limit, namely, fatigue failure occurs. Therefore, to improve the fatigue strength of titanium alloy in industrial production by using of effective technology has an important meaning.Fatigue strength of metals can be improved by some ordinary process, such as shot peening or cold rolling etc.. A new technique of surface nanocrystallization (SNC) induced by high-energy-shot-peeening (HESP) has been advanced by K. LU et al years ago. The technique has the advantages of simple operation, less power consumption, high efficiency and wide application range etc. So that the main purpose of this paper is to realize the SNC of TC4 by means of HESP technology, and to study the effect of SNC on the fatigue strength of TC4.In this study, equiaxed structured and lamellae-shaped structured TC4 alloy were treated by HESP technology. The treated samples were characterized by means of X-ray diffraction, scanning electron microscopy, micro-hard testing etc. Rotation bending fatigue strength of different samples was examined subsequently. The fatigue fracture was observed and analyzed as well. Main conclusions are now listed below:(1) Parameters used in shot peening: Cr-Mo alloy steel balls of 1.0mm in diameter was used for HESP with the projectile speed of 50m/s, the duration time was 2h and 8h respectively; Cr-Mo alloy steel balls of 0.5mm in diameter was used for restoration-shot-peening (RESP) with the projectile speed of 50m/s, the duration time was 30min.(2) Certain thickness of nanostructured surface layer can be obtained be HESP technology, the grain size of which is about 20nm.(3) HESP treatment leads to the increasing of surface hardness of materials and the appearance of hardened surface layer. The longer the duration time lasts, the higher hardness gotten.(4) HESP treatment improves the fatigue strength of equiaxed structured TC4 alloy while reduces that of lamellae structured TC4 alloy. High-energy Shot Peening improve the fatigue strength of Equiaxed TC4 alloy. (5) Nano-hardened surface layer and residual compressive stress play an active role to improve the fatigue strength of materials while the surface roughness and damage exhibits negative effects on the improvement of fatigue limit. HESP causes irrreparable damage which affect the fatigue limit of materials seriously.(6) RESP improved surface roughness and partially repaired the surface damage, so that the fatigue strength of titanium alloy TC4 has been improved.