Study On Laser Bionic Repair Of Thermal Fatigue Cracks In Ductile Iron Brake Discs

Ductile iron with good wear resistance,heat conductivity,and castability has become a popular cast metal material that is widely used to manufacture train brake discs which is an important component of the braking system of the train.When the vehicle brakes,severe friction occurs between the brake disc and the friction pad,which converts the kinetic energy of the train into heat energy,the temperature of the brake disc rises,and then it returns to normal temperature after braking.Therefore,during the service life of the brake disc,its temperature will inevitably undergo an alternating change of rising-decreasing,which will cause it to be subjected to cyclic thermal stress for a long time,and eventually cause thermal fatigue cracks on the surface and cause its failure.Thermal fatigue crack is one of the main failure forms of brake disc.In order to ensure the safety of driving,the failure brake disc has to be replaced,which not only increases the operation and maintenance cost of the vehicle,but also causes a large amount of waste of resources.Therefore,it is of great research value to remanufacture the failure brake disc,repair the thermal fatigue cracks on its surface and extend its service life.However,due to the existence of graphite in nodular cast iron,serious chill defect,the brittle-hard phases and micro cracks are easily occur in the repair zone after remelting,which causes great difficulties in repairing the cracks.Traditionally welding process often be used to repaired the thermal fatigue cracks.Although thermal fatigue cracks can be healed to a certain extent,the objective problems still cannot be ignored,such as cumbersome repair process,high repair cost,complicated process,long repair cycle,residual thermal stress in the heat affected zone of the weld,etc.Therefore,the problem to be solved is to explore an effective method to repair thermal fatigue cracks.In nature,some biological tissues,such as shells,plant leaves and dragonfly wings,have good anti-crack and anti-fatigue functions.It is found that these structures with excellent fatigue resistance have similar characteristics,that is,the combination of soft phase?parent?and hard unit alternately,thus forming the coupling structure of"soft and hard"phase.By simplifying the coupling structure and imitating it with laser technology,the thermal fatigue crack can be repaired and the thermal fatigue resistance of the material can be improved.Under this inspiration,this paper puts forward a new idea of thermal fatigue crack repair of brake disc that is to use laser technology to fabricate bionic crack stop element as hard phase to repair the crack intermittently.In this paper,the bionic crack blocked unit was prepared by laser remelting repair technology to repair the thermal fatigue crack,and orthogonal test was designed to optimize the repair parameters.The size and structure of the units with different laser energy density and the regularity of the thermal fatigue resistance of each bionic repair sample were investigated,and the anti-fatigue mechanism of the cracking of the unit was discussed.And then,the bionic anti-crack composite structural units were prepared by laser additive technology to repair the thermal fatigue cracking area of large size.The influence of different cladding alloy powders?Fe30A and Co50?on the structure,hardness and thermal fatigue resistance of bionic repair sample was analyzed by comparison.The results showed that the biomimetic crack stop unit with highly refined grain size,dense structure,higher strength,hardness and plasticity than the matrix can effectively heal the thermal fatigue crack and prevent it from further expansion,thus greatly extending the fatigue life of the biomimetic repair sample.When the laser energy density is 213.3 J/mm²,the biomimetic fusing unit had the maximum effective depth to completely bridge the cracks.At this time,its average microhardness was the highest,which was 749(1_0.2,and the tensile strength of the biomimetic repair sample also reached the maximum.Under the same number of thermal cycles,the crack width increment was the smallest and the crack propagation rate was 77.56%lower,which had the best thermal fatigue resistance.In the cladding layer,the structures of bionic crack arrest composite structure unit LC1 and LC2,which cladding material is Fe30A and Co50 respectively,were composed of dendritic,boride and carbide.After repairing the cracks by LC2 unit,the crack width and length increments were 90.5%and 78.9%respectively in LC1 unit repair samples under the same conditions,which showed the better thermal fatigue resistance.In addition,the unit spacing also had certain influence on the thermal fatigue resistance of biomimetic repair specimens:under the same thermal cycles,the increment of crack width and the growth rate gradually increase with the increase of unit spacing.The mechanism of thermal fatigue resistance of biomimetic repair specimens can be summarized as followed:?1?Blocking effect of unit.Units with higher strength and toughness could bridge the cracks in the process of forming,the stress concentration effect caused by the thermal fatigue crack could be eliminated,and the crack propagation could be effectively prevented.?2?Improvement mechanism of stress state.The dynamic source of thermal fatigue crack growth was the cyclic thermal stress of material caused by cyclic thermal strain in cold and hot cycling environment.Intermittent repair of cracks by means of a unit could relax the thermal stress due to reduced local restraints and thus improved the stress state.