Study On The Coupling Characteristics Of Compression Creep And Low Cycle Fatigue Of Cast Aluminum And It's Application

Because of the advantages of good casting performance,good thermal conductivity,low density and high specific strength,the aluminum alloy material has been widely used in the manufacture of cylinder head in internal combustion engine.The cylinder head is one of the components with the most complex structure and highest thermal load in the internal combustion engine.At present,with the improvement in demand of the design precision for the internal combustion engine and the introduction of the turbocharger and the lightweight design technology,the low-cycle fatigue failure of the cylinder head becomes more and more serious.The main low cycle fatigue load of the cylinder head is the low frequency thermal stress load,and the compressive creep damage is also caused by the high temperature compressive stress during the work.The life cycle of the cylinder head is jointly determined by the low cycle fatigue damage and the compression creep damage.In this paper,the problem of low cycle fatigue failure of cylinder head in engine start and stop conditions was studied from three aspects: material performance test,life prediction theory analysis and numerical simulation of cylinder head.The main purpose is to accurately evaluate the low cycle fatigue life of the cast aluminum alloy cylinder head,to form its life evaluation method and obtain the relevant parameters.Therefore,the compressive creep properties and low-cycle fatigue properties of cast aluminum alloy at high temperature were experimentally studied in this paper.The compression creep and low-cycle fatigue properties of the cast aluminum alloy were obtained at high temperature,and the fatigue fracture analysis method was used to investigate the damage creep mechanism and fracture mode of the material under high temperature conditions.The prediction model of compression creep-fatigue life of the material was established based on the energy law and damage mechanics,and the relevant parameters were obtained.Finally,the fatigue life of aluminum alloy cylinder head under typical low cycle fatigue loading condition(start and stop cycle)was calculated and evaluated.The static load of cylinder head with bowl type combustion chamber and its low cycle fatigue Life distribution characteristics were also investigated.The main research contents and conclusions of this paper are as follows:(1)The experimental study on high temperature compression creep behavior and damage mechanism of cast aluminum alloyThe high temperature compression creep experiments of the material were designed and conducted.The results show that the steady creep rate of the cast aluminum alloy was different under constant tensile / compressive load at the same temperature,and the difference increased with the increasing stress.The main reason is the difference in the number and development type of the micro-damage such as holes and cracks under tensile and compressive stress.The specific mechanism is that the creep effect under high temperature can restrain the evolution of existing holes and microcracks.And these micro-damage still have some compressive stress-carrying capacity under compressive creep load,so the material's tensile and compressive creep rate exists larger difference.(2)The experimental investigation on the response properties and damage mechanism of the cast aluminum alloy under high temperature low cycle fatigue and compression creep-fatigue.The low-cycle fatigue and creep-fatigue tests of cast aluminum alloy at high temperature were carried out.The results show that high temperature could lead to continuous softening of the cast aluminum alloy under cyclic loading,and the cyclic softening rate of the cast aluminum alloy increased sharply with the increase of temperature.The cyclic softening of the material was relieved after the compressive creep load was coupled in the fatigue load cycle.The compressive creep strain contributed to the cyclic stability of the material at high temperature.In addition,compressive creep did not affect the cyclic hardening behavior of the material,but caused the moving up of the yield center and cyclic stress-strain curves of the material.At high temperature,the fracture mode of the material under fatigue load and creep-fatigue load was that the eutectic silicon particles first broke and formed many tiny cracks or holes.These tiny cracks then propagated in the aluminum matrix and were interconnected with adjacent cracks,leading to fracture,and many fine dimple structures and tearing ribs formed in the event of ultimate fracture.(3)The study on Creep-Fatigue Life Prediction Model of MaterialsBased on the energy law and the damage mechanics analysis method,the low cycle fatigue life prediction model of cast aluminum alloy was established.The model parameters were fitted based on the experimental results.The theoretical calculation results were in good agreement with the test results of cast aluminum alloy.(4)The calculation and evaluation of low cycle thermal fatigue life of cast aluminum alloy cylinder headAiming at the characteristics of low cycle fatigue load of engine cylinder head under start-stop condition,a calculation method of low-cycle fatigue life was proposed.The thermal load and low cycle fatigue life of a four-cylinder combined-type cylinder head with bowl-type chamber were calculated and evaluated.The results show that the second cylinder and the third cylinder bore larger low-cycle thermal load than the first cylinder and the fourth cylinder,and the third cylinder suffered the largest stress load.The lowest fatigue life area of each cylinder was located in the edge region of the exhaust valve.The fatigue life of the third cylinder of the cylinder head was the lowest,and the maximum number of start and stop times was 7354 cycles.