Effect Of Combined Plasma Chromizing And Shot Peening On The Fatigue Behavior Of TC4 Alloy And 2Cr13 Steel

The coupling of fatigue and wear failure is important hidden danger that threaten aircraft engine and industrial fans,especially for the fretting fatigue?FF?damage which often exists in the contact surface of fastener connection structures and the interface region of the dovetail portion of the blade and the disk.The FF damage is the"cancer"of industrial equipment,involving three failure modes,i.e.,fatigue,wear and corrosion?including oxidation?at the same time.The plasma surface alloying technology can effectively improve the wear and corrosion resistance,but is often not conducive to fatigue resistance.While shot peening treatment can remarkably increase the fatigue resistance of metal materials,but the improvement to wear resistance is slight.Therefore,the duplex of plasma surface alloying technology and shot peening becomes an important way to solve the FF damage.Cr element is very beneficial to improve wear and corrosion resistances,however,there are few studies about the effect of plasma chromizing combining with shot peening on the fatigue properties of titanium alloy and stainless steel.Besides,the crystal structure of titanium alloy and stainless steel have large difference,thus the effects of crystal structure on the microstructure of modified layer and fatigue properties are one main concern.Based on the above background,with TC4 alloy and 2Cr13 stainless steel as the research object,this paper investigates the effect of Cr-alloyed layer,Cr-Zr-alloyed layer and their combining with shot peening post-treatment on the plain fatigue?PF?,FF and wear behavior of titanium alloy and stainless steel.The main work and research results are as follows:?1?The formation process of Cr-alloyed layer and its microstructure was studied.And the result showed that the Cr-alloyed layer formed via Cr atom diffusing into titanium alloy at800?mainly consisted of Cr deposit layer,TiCr₂ layer,Ti₄Cr layer and Cr-Ti solid-solution layer.The Cr-Ti solid-solution layer stemmed from the phase transformation of??Ti,Cr?phase via the eutectoid transformation??Ti,Cr??Ti₄Cr+??Ti,Cr?during furnace cooling to room temperature.The hardness of Cr-alloyed layer was about 670 HK;the TiCr₂ layer had the highest hardness?780 HK?,while the Ti₄Cr layer had lower hardness?390 HK?;the hardness of Cr-Ti solid-solution layer was in the range of 430 HK⁵40 HK,and changes with the content of Cr element and the amount of Ti₄Cr phase.The Cr-alloyed layer had poor surface toughness,while the Cr-Ti solid-solution layer had good toughness.?2?The effects of plasma chromizing and Cr-Ti solid-solution layer?removed the Cr deposit layer,TiCr₂ layer and Ti₄Cr layer?on the PF behavior of TC4 alloy were comparatively investigated.The result indicated that plasma chromizing treatment reduced the PF properties.Because the composition and mechanical properties?elastic modulus and hardness?of adjacent sub-layer were great difference,cracks easily initiated from the interfaces of sub-layers.The fatigue resistance of the specimen with Cr-Ti solid-solution layer was higher than that of the plasma-chromized specimen,but still lower than that of the TC4 base material?BM?because of the degradation of mechanical performance resulted from the high temperature during plasma chromizing process.?3?Shot peening post-treatment could significantly improve the fatigue resistance of the plasma-chromized TC4 alloy,which is better than that of the shot peened TC4 alloy.This is mainly because shot peening post-treatment had introduced high residual compressive stress in the surface layer of plasma-chromized TC4 alloy.High residual compressive stress could effectively delay the fatigue crack initiation and early development.The Cr-Ti solid-solution layer combining with shot peening post-treatment,its PF resistance is great higher than that of the plasma-chromized treatment combining with shot peening post-treatment.This is mainly because that the Cr-Ti solid-solution layer had good toughness and slightly higher hardness than that of TC4 alloy,and thus had great potential of deformation reinforcement.So shot peening treatment could introduce higher residual compressive stress in the Cr-Ti solid-solution layer.?4?The effects of plasma chromizing,Cr-Ti solid-solution layer and combining with shot peening on the FF behavior of TC4 alloy are comparatively studied.The result showed that plasma chromizing treatment could remarkably improve the wear resistance,but decreased the FF resistance significantly because of the poor surface toughness and fatigue resistance of the Cr-alloyed layer.Although the Cr-Ti solid-solution layer only slightly improved the wear resistance,but its FF resistance was higher than that of the plasma-chromized specimen due to the good toughness and fatigue resistance of Cr-Ti solid-solution layer.Shot peening could improve the FF resistance of plasma-chromized TC4 alloy,and its FF life was better than that of the shot-peened TC4 alloy.Because shot peening could improve the surface toughness and fatigue resistance of the plasma-chromized TC4 alloy;meanwhile,the good wear resistance of plasma-chromized TC4 alloy combining with shot peening post-treatment played a synergistic effect.The FF resistance of Cr-Ti solid-solution layer combining with shot peening was great higher than that of plasma-chromized treatment combining with shot peening.Because the fatigue resistance of Cr-Ti solid-solution layer was higher after shot peening treatment,and its wear resistance also obtains improve at some extent.?5?In order to further improve the strength and toughness of the Cr-Ti solid-solution layer,the Cr-Zr-alloyed layer is prepared on TC4 alloy through the pre-zirconizing and subsequent chromizing treatment.The Cr-Zr-alloyed layer mainly consists of Cr deposit layer,TiCr₂ layer,Ti₄Cr layer and Cr-Zr-Ti solid-solution layer.The Cr element content of Cr-Zr-Ti solid-solution layer decreased obviously,and the precipitation amount of Ti₄Cr soft phase also declined,so the Cr-Zr-Ti solid-solution layer had significantly higher hardness than that of the Cr-Ti solid-solution layer.Meanwhile,the Cr-Zr-Ti solid-solution layer retained good toughness.The Cr-Zr-Ti solid-solution layer could improve the PF and FF resistance of TC4alloy without shot peening treatment.Because the Cr-Zr-Ti solid-solution layer had good match of hardness and toughness and good coordination deformation ability as well as a certain residual compressive stress.?6?Cr-Zr-Ti solid-solution layer combining with shot peening post-treatment not only could further improve the PF resistance,but also could significantly increase the FF resistance of TC4 alloy.And the improvement effect of Cr-Zr-Ti solid-solution layer combining with shot peening post-treatment is better than that of Cr-Ti solid-solution layer combining with shot peening post-treatment.Because the Cr-Zr-Ti solid-solution layer had good match of hardness and toughness,and thus can obtain greater residual compressive stress field distribution.Residual compressive stress field played a leading role in block the crack propagation.Moreover,the shot peening treatment increased the hardness of Cr-Zr-Ti solid-solution layer as well as its wear resistance,which was beneficial to improve FF resistance.?7?Preliminary study showed that Cr-Zr-Ti solid-solution layer also could improve the PF and FF resistance of TC4 titanium alloy at 350?.This has achieved the goals that improved PF and FF resistance by employing surface alloying,and breakthrough the difficulty to improve the FF resistance at elevated temperature.However,the fatigue life of PF and FF of Cr-Zr-Ti solid-solution layer at elevated temperature is clearly lower than that of at room temperature.This is attributed to the decrease of the formation resistance of reside slip bands,and thus crack easily initiated from the surface.?8?Three kinds of Cr-alloyed layer were prepared on 2Cr13 stainless steel by employing plasma chromizing treatment at 850?,750?and 670?,respectively.The microstructure of the Cr-alloyed layer prepared at 850?and 750?were similar,and mainly contained Cr deposit layer,chromium iron carbide layer and Cr-Fe solid-solution layer.While the Cr-alloyed layer prepared at 670?mainly consisted of Cr deposit layer and chromium iron carbide thin layer.The three plasma chromizing treatments could significantly improve the wear resistance,but severely decrease the fatigue resistance of 2Cr13 stainless steel.This is due to the temperature of plasma-chromized treatments was in the range of sensitization temperature of the stainless steel,which led to serious degradation of the mechanical performance.Even the shot peening post-treatment also failed to recover its fatigue resistance.Moreover,the Cr-alloyed layer on 2Cr13 stainless steel had poor toughness,and there was clearly cracking or spalling off after shot peening post-treatment.Thus the residual compressive stress introduced in the surface layer is not enough to offset the negative effect resulted from the crack of Cr-alloyed layer.