Effect Of Humidity On The Ultrasonic Fatigue Behavior Of Cast Aluminum Alloys

Lightweight design is becoming a major method to improve the fuel efficiency in modern vehicle industry.Cast aluminum alloys have taken the place of cast iron to manufacture vehicle engine's component because of its high strength/weight ratio,excellent heat conduction ability and good corrosion resistant.Components in vehicle's engine like cylinder head,engine block as well as connecting rods always subject to more than 108 alternative stress cycles during their service life,so a better understanding of material's fatigue behavior becomes more and more important,especially in the very high cycle regime.Conventional fatigue test limited to 107 alternative stress cycles due to the loading frequency,so it can not satisfy the durability requirement in vehicle engine's components design.Ultrasonic fatigue test technique is using piezoelectric transducer which converts 20 kHz electrical signal into 20 kHz mechanical signal to stimulate the fatigue specimen into resonant state,in this way the testing time for fatigue data generation can be reduced dramatically.However,for cast aluminum alloys,using ultrasonic fatigue technique generally results in higher fatigue life if compared to the conventional fatigue technique under same stress amplitude,and academia attributed this difference to the environment humidity which is detrimental to fatigue life.The frequency of ultrasonic fatigue test is much higher than the frequency of conventional fatigue test which turns out that more harmful humidity effect on the conventional fatigue crack propagation.As a result of that,cast aluminum alloys generally have higher fatigue lives if ultrasonic fatigue technique is applied.In the present study,conventional and ultrasonic fatigue tests were conducted in various environments(dry air,moist air(RH?50%),saturated humidity and distilled water)aimed on the humidity effect to the fatigue life and crack propagation rate of Al-Si cast aluminum alloys.The main conclusions are as follows:(1)Ultrasonic and conventional fatigue test of AS7GU-T64 and A356-T6 were carried out under various humidity levels in order to find out the mechanism of humidity effect on the ultrasonic fatigue lives of cast aluminum alloys.It was observed that environment humidity can significant reduce the ultrasonic fatigue lives of these two cast aluminum alloys,increasing environment humidity resulted in decreased ultrasonic fatigue life.Also,ultrasonic fracture surfaces presented lots of fatigue facets in the crack propagation path.According to the OM,SEM and EBSD examination,the mechanism of facets formation was firstly introduced.Because the cast defects were pretty low for the specimens,fatigue cracks initiated from surface where PSBs occurred.The crack propagation rate in the initiation area can be extremely slow when the newly initiated crack starts to propagate.Under the condition of that,cyclic plastic deformation accumulated within the crack tip plastic zone and new facets may form if the grain is properly oriented(crystallographic slip planes close to the maximum shear stress direction).(2)Established signal collection platform and realized high-accuracy acquisition of feedback signal of ultrasonic fatigue system.Proposed a method using ultrasonic nonlinearity parameter rel? to monitor specimen ultrasonic fatigue crack initiation life and crack propagation life.On the basis of the variation of ultrasonic nonlinearity parameter,once the slop of ultrasonic nonlinearity parameter achieved a critical value,the ultrasonic fatigue crack began to propagate unstablely.Ultrasonic nonlinearity parameter is much more sensitive than the resonant frequency to deduce the fatigue damage like crack initiation and propagation.Ultrasonic fatigue tests under same stress amplitude and different environment humidity levels were conducted and revealed that environment humidity not only accelerated fatigue crack propagation which reduced fatigue crack propagation life but also shorted stress cycles for fatigue crack initiation.(3)Conducted A356-T6 ultrasonic and conventional fatigue crack propagation tests under various humidity levels: It was observed that increasing environmental humidity corresponded to accelerated crack propagation rate and the peak aging cast aluminum alloy was less sensitive to the humidity if compared to the overaging cast aluminum alloy.The factor of fP / in Wei's model was replaced by fP / to reduce the humidity effect and the crack propagation rate prediction model for peak aging cast aluminum alloy was firstly established.The predicted and tested crack growth rate in 50% humidity environment correlated very well and proved that the modified model is much better than its original form.(4)The governing factor which determines the ultrasonic fatigue life humidity sensitivity of cast aluminum alloys was investigated.Integrated all Ford Motor Company cast aluminum alloys ultrasonic fatigue test data and divided C448,AS7 GU,E319 and A356 cast aluminum alloys into seven groups according to the heat treatment,casting method and tensile properties differences.It was observed that casting porosity and secondary dendrite arm spacing are not the governing factors.The peak aging cast aluminum alloys had higher UFR values,while over-aging and under-aging cast aluminum alloys had lower UFR values.It was concluded that in terms of same Cu percentage,yield strength is the governing factor and higher yield strength corresponded to larger UFR value.