Effect Of Stress And Galvanic Factors On The Durability Of Aluminum Alloys

For the aircraft structure,when parking on the ground the main damage is corrosion,while when flying in the air the main damage is fatigue.So,corrosion and fatigue alternate is the main course during the whole service life of aircraft.Aluminum alloy is widely used in the aviation industry.In those conditions,environment factors,stress factors and galvanic factors all have the significant influence on the aluminum alloy structure's strength and using life.There are much researchers pay attention on the fatigue life of aluminum alloy structure that affect by those factors.While,there are few study pay attention on the unite effects of those factors for aluminum alloys,so the unite theory are not clear.When design the corrosion/fatigue spectrum,there are also many factors in the spectrum such as corrosion style,stress corrosion,heat shock,environment factors,size effect,corrosion-fatigue alternating frequency,stress factors and over load factor.So,in this paper,we study the effect law of stress,environment and galvanic factors of the fatigue life of aluminum alloy.Investigate those factors influences during design the corrosion/fatigue spectrum and build the corrosion/fatigue spectrum for Marine industrial atmospheric environment area and Marine industrial atmospheric environment area.The main conclusions show as follow:?1?The Cl^-concentration is the primary factor for galvanic corrosion of AA5050/S45.In distilled water,at the beginning of test,S45 acted as cathode while AA5050 is anode that the corroion is accelerated.With the extending of test,the potential of AA 5050 increased because of better passivation,and the potential will higher than S45,at this time,the galvanic electrode become reverse.In the NaCl aqueous solution,AA5050 always acted as the anode that suffered accelerated corrosion and with the increased of Cl^-concentration,the corrosion degree incrased.When the solution pH changed from 4⁷,galvanic corrosion degree are not much different.While,when the solution pH decreased to 2,galvanic corrosion current increased 5 times and the corrosion pattern change from pitting corrosion to general corrosion.?2?In 3.5%NaCl solution,stress reduce the electrochemical potential and the passive film stability,so,the AA5050/S40CrNiMoA galvanic corrosion increased.With the increased of stress level,the corrosion degree increased.Galvanic factor promote the mechanical performance degradation of AA5050,and combine with the stress factor,mechanical performance degradation accelerated.When the stress is no more than 75%of its yield strength,the galvanic factor is the main influence mechanical performance.The potention different of AA2E12/SS00Cr18N10N is larger than AA5050/S45,so the galvanic corrosion degree is higher.Galvanic factor lead to fatigue life of AA2E12 decreased 2/3 and this is belong to galvanic-stress united effect and the grain boundary area rich copper p hase which lead to the higher intergranular corrosion sensitivity.?3?The pre-corrosion way has important impact for the fatigue life of AA7B04,and the influence order is:salt spray corrosion>alternate immersion corrosion>thermal-humidity+salt spray corrosion.This is belonging to the corrosion mechanism difference of aluminum alloy in those solutions.From orthogonal test,the influence order of environment factors for the fatigue life of aluminum alloy is obtained,that NaCl concentration>corrosion time>corrosion temperature>solution pH.In those factors,Cl^-is the primary factor that not only decided corrosion mechanism but also have the influence to pH,temperature and corrosion time.When the Cl^-concentration is low,aluminum alloy occurs general corrosion and the influence of solution pH is non-significant.When the Cl^-concentration is high,aluminum alloy occurs pitting corrosion,and with the decreased of pH or increased of temperature and corrosion time,pitting corrosion increased that lead to fatigue life decreased.Through the results of orthogonal test,the relationship between environment factors and corrosion current density are established,then the relationship between corrosion current density and fatigue life are get.From those relationship,we could forecast the fatigue life of aluminum alloy that exposed in the outside environment?the main damage is pitting corrosion?.?4?In NaCl solution,fatigue life of aluminum alloy appears nonlinear regularity with the corrosion time.The fatigue resistance decrease with the extending of corrosion time,and this is belonging to the corrosion product could prevent the corrosion process.Thermal shock at149?would reduce the inherent fatigue resistance of aluminum alloy,but because of thermal shock would increase the corrosion resistance of corrosion products?Al?OH?₃ change to AlOOH?,fatigue life reduces little with the extend of thermal shock time.At the same time,suffered thermal shock,after pre-corrosion,fatigue life of aluminum alloy is higher than no thermal shock condition.?5?Fatigue life of aluminum alloy appears nonlinear regularity with stress,and this is because of mechanical-chemistry effect promoted the corrosion pit and nonlinear cracks development during pre-corrosion process.In the same corrosion condition,large size specimen has higher fatigue life than small size specimen,and this phenomenon is entirely different to the conventional fatigue.This is due to the low crack propagation rate and large propagation trip of large specimen.?6?After the same time corrosion,pre-corrosion/fatigue alternating frequency has significant influence on the fatigue life of AA7B04.When alternating frequency is 6 times,4 times,and 2times,the fatigue life of AA7B04 is about longer 136%?84.4%and 56.4%than alternating frequency 1 time.This is due to the corrosion factor has different proportion in different alternating frequency conditions.On the one hand,corrosion would promote crack initiation,on the other hand,corrosion also would promote crack tip passivation.At the same time,corrosion products would promote crack close and delay the fatigue crack propagation.While,long time continuous corrosion,large corrosion pits would appear and crack initiation life is significantly reduced?7?After pre-corrosion,fatigue life using different load spectrum would appear different results.The fatigue life order that under different spectrum is that standard spectrum>constant amplitude spectrum.This is due to the different fatigue crack propagation speed under different spectrum.From the orthogonal test,we get the result that in fatigue process,the load factors order is that:stress>R>overload amplitude>overload cycles.The higher of the stress,the easier of the crack initiation and propagation and the lower of R,the bigger of the crack propagation driving force.With the increased of overload amplitude,the pressure stress in crack tip increased and overload effect slow down crack propagation increased significantly.?8?After corrosion in some coastal industry environment area and inland industry environment region,fatigue life of AA7B04 occur different reduced.This is due to the surface corrosion damage of aluminum alloy that lead to the fatigue crack initiation.In the coastal industry environment area,Cl^-concentration in atmosphere is high,and would lead aluminum alloy to occur pitting corrosion,intergranular corrosion and exfoliation corrosion.Those kinds corrosions could cause fatigue crack initiation and reduce the fatigue resistance of aluminum alloy.While in the inland industry environment region,Cl^-concentration in atmosphere is low and only occurs little pitting corrosion for aluminum alloys.?9?The solution“0.05%NaCl+0.35%?NH₄?₂SO₄+0.5%H₂O₂”is used to simulate inland industry environment and when the acceleration coefficient is 146.00,the corrosion morphology,corrosion mechanism and the fatigue life are basically the same between laboratory and outside environment for AA7B04.The solution“5%NaCl”used to simulate coastal industry environment and when the acceleration coefficient is 97.33,the corrosion morphology,corrosion mechanism and the fatigue life are basically the same between laboratory and outside environment for AA7B04.Base on those research results,the load/environment spectrum for simulate inland industry environment region and coastal industry environment area is build and those spectrums are suitable aluminum alloy opening structure.