Effects Of Surface Modification And Integrity On Fatigue Behaviors Of Titaniumalloy

Titanium alloys have been used to fabricate important components, such as aircraftlanding gears, flap tracks, engine compressors and so on. However, performances of theseimportant components are closely related to the plain fatigue (PF) and fretting fatigue (FF)behaviors. In order to meet the requirements of the development of landing gears and flaptracks on the large-type passenger aircraft, two novel high strength titanium alloys (TC18andTC21) were developed independently in China. The fatigue property of high strength titaniumalloy is very sensitive to the surface integrity. The surface integrity and fatigue property areimportantly affected by the machining and surface treatments. As titanium alloys are difficultto machine, it is important to investigate the effect of surface integrity on the fatigueresistance of these high strength titanium alloys to guarantee the fatigue resistance of thenovel high strength titanium components. Shot peening (SP) is an important method toimprove the fatigue resistance of titanium alloys. High velocity oxygen fuel (HVOF) WC-Cocoating is a significant mean to increase the wear resistance of the landing gears and the flaptracks, but HVOF is harmful to the fatigue behavior of the titanium alloy. However, there hasbeen no report on the effect of the combined treatments of the SP, grit blasting with HVOFWC-Co coating on the fatigue resistance of the above high strength titanium alloys. Based onabove-mentioned problems, TC18and TC21high strength titanium alloys were selected asthe research materials. The effects of machining surface integrity on the fatigue behavior ofthose high strength titanium alloys were investigated. The effect of SP and its surface integrityon the fatigue behavior was studied. The effects of the combined treatments of grit blastingand SP with HVOF WC-17Co coating on the fatigue behavior of above novel high strengthtitanium alloys were systematically compared. In addition, there is no research into thesimilarity and difference of DLC and GLC on the FF resistance of titanium alloys and theeffect of the TiN-Ag composite film on the FF fatigue behavior. Therefore, this research isaimed to study the effect of above films on the FF behavior of titanium alloys, and in order toprovide a novel technology for improving FF resistance of titanium alloy. The maincontributions are listed as follows:(1) Cutting feed is the most obvious factor of determining the surface integrity of thetitanium alloy. Rough turning, finish turning and polishing after finish turning has differenteffects on the fatigue resistance of TC18and TC21high strength titanium alloys. That isattributed to the effects of different machining methods on surface integrity of high strengthtitanium alloy. The differences of surface roughness and surface texture characteristics produced by machining are the main factor of influencing the surface integrity and fatigueresistance of titanium alloys. TC21high strength titanium alloy with the “α+β phase”basket-weave microstructure has higher notch sensitivity than TC18high strength titaniumalloy with “equiaxial α phase+lamellar α phase+β transition phase”. Shot peening caneffectively inhibit the notch influence caused by machining grooves which are vertical tofatigue loading direction, and the fatigue resistance of the high strength titanium alloy issignificantly improved. This is attributed to the deep distribution of residual compressivestress field introduced by SP on the surface. The harmful grooves are removed and the crackinitiation and earlier propagation are delayed.(2) The fatigue resistance of the two novel high strength TC21and TC18titanium alloyscan be greatly improved by SP with optimized process parameters. This is because theresidual compressive stress introduced by SP would make the fatigue crack source migrate tothe subsurface of titanium alloy. The work hardening and microstructure refining restrain theinitiation of the fatigue crack. The residual compressive stress field would prevent or delaythe initiation of the fatigue crack. The effect of SP on improving the fatigue behavior oftitanium alloys is not monotonous with increasing SP intensity. Because SP has both thebeneficial and harmful effects on the titanium alloy surface. To SP with reasonable parameters,the beneficial effect is the governing factors to gain good surface integrity and high fatigueresistance. However, when the SP intensity is too high, the harmful factors such as the surfaceroughness increasing, delaminating, cracking play leading role and accelerate the initiation ofthe fatigue crack and the beneficial effect of SP are reduced. To the two novel high strengthTC21and TC18titanium alloys with the same strength, the optimal SP intensity of TC18ishigher than that of TC21titanium alloy due to the difference of surface notch sensitivity.Therefore, it is not proper to blindly select the same SP process specification for the materialseven if the strength was similar but the microstructure was different.(3) The fatigue resistance of TC21high strength titanium alloy is obviously reduced byHVOF WC-17Co coating. This is related to the high hardness, low toughness, large surfaceroughness, surface tensile residual stress distribution and holes defects in the WC-17Cocoating. The fatigue resistance of TC21titanium alloy is more obviously reduced by HVOFWC-17Co coating with previous grit blasting treatment. This is because the fatigue resistanceof WC-17Co coating is worse and the compressive residual stress introduced by grit blastingis relaxed during the high temperature process of HVOF, which makes the harmful effect ofnotch sensitivity induced by grit blasting more obviously. The fatigue resistance of TC21titanium alloy is reduced by the HVOF WC-17Co coating with previous SP treatment. However, the reducing extent is lower than that of the HVOF WC-17Co coating with previousgrit blasting. This is attributed to the harmful effect of the HVOF WC-17Co coating and therelaxation of the surface compressive residual stress field introduced by SP as the hightemperature of the HVOF process. But the relaxation extent is not obvious during the HVOFprocess.(4) The fatigue behavior can be obviously improved by polishing HVOF WC-17Cocoating. It is found that the fatigue resistance of the polished HVOF WC-17Co coating couldbe improve greatly by SP with optimized parameters. The fatigue resistance of the complextreated WC-17Co coating is higher than that of the TC21alloy substrate. This is because thatSP introduces the compressive residual stress with reasonable distribution and the surfaceroughness and surface defects are not obviously increased. However, when the SP intensity istoo high, the WC-17Co coating would crack and segregate with the base. Moreover, therelaxation of the compressive residual stress induced by SP would be occurred. Those factorsare harmful to improve the fatigue resistance.(5) DLC and GLC hard lubricating films produced by closed field unbalanced magnetronsputtering (CFUBMS) would improve the fretting wear (FW) and FF resistance of TC4titanium alloy. The FF resistance of DLC film is better than that of GLC film. This is becausethat DLC film has higher bonding strength and better strength-toughness comprehensivebehaviors. The bonding strength and strength-toughness properties have greater effect on theFW and FF resistance than the friction coefficient, especially for controlling FF damage whenthe fatigue factors play a dominant role.(6) The TiN-Ag composite films fabricated with hard TiN coating doped with Agelement (0.5%～20%Ag atomic fraction) are produced by ion assisted magnetron sputteringdeposition process. The TiN-Ag composite films consist of nanocrystalline TiN and Ag phase.The FW and FF resistance of titanium alloy can be improved. The FF resistance of thecomposite film is better than that of either pure TiN or Ag film. This is because that thecomposite films have better strength-toughness comprehensive behavior, friction lubricationand high bonding strength. When the atomic fraction of Ag is in range between2%and5%,the FF resistance of the composite film shows the best performance. Because the surfaceintegrity of the composite film is fine enough to prevent the initiation and early propagationof the FF cracks. The Ag-TiN composite films fabricated with Ag solid lubricating film dopedwith hard TiN phase (60%～80%Ag atomic fraction) are produced by the ion assistedmagnetron sputtering deposition process. The FW and FF resistance of titanium alloy can beimproved by the Ag-TiN composite films. The FF resistance of the composite film is also better than that of pure TiN or Ag film. This is attributed that the composite films have betterstrength-toughness comprehensive behavior, friction lubrication property, high bondingstrength and the existing residual compressive stress. However, when the atomic fraction ofAg in the composite film is larger than90%, TiN phase is difficult to form. And it fails toimprove the strength-toughness comprehensive behavior of the composite film. As a result,the FF resistance of the composite film is worse than TiN orAg film.(7) The results indicated that hard solid lubricating films (GLC and TiN-Ag compositefilms) exhibit better FF resistance than the soft solid lubricating films (Ag and Ag-TiNcomposite films) due to the greater bearing capacity and wear resistance of the hard solidlubricating films as with high bonding strength and good strength-toughness comprehensivebehavior. The surface integrity requirement is similar on improving the FW and FF resistanceof titanium alloy. To improve the FF resistance, the film needs better surfacestrength-toughness comprehensive properties than improving the FW resistance. The bondingstrength, strength-toughness comprehensive properties and friction lubricating behavior arethe main factors that determine the FF resistance of the ion assisted magnetron sputteringdeposition films. Besides, residual stress also has some effect. The FF resistance of titaniumalloy can be significantly improved only when these above factors are matched perfectly toachieve the best surface integrity.