Research On Thermal Fatigue Resistance Of Cast Iron Treated By Laser Biomimetic Coupling Technique

Brake drum is the most important component in the drum braking system. According tothe braking principle, the drum suffers powerful pressure and static or kinetic friction forcefrom brake disk. The alternate heating and cooling caused by frequent braking may lead to thetemperature variety of braking surface. Correspond ingly, the alternate stresses or strain causedby the restriction to free expansion and shrinkage of materia ls can make the initiation ofthermal fatigue cracks, and the further propagation of thermal cracks will lead to the suddenbreak of drums. Cast iron has long been used for trucks as the materia ls for brake drumsbecause of its low cost and desirable properties such as good castability, excellent machiningproperty and so on. Therefore, improving thermal fatigue resista nce of cast iron materia ls hasbeen the urgent and useful problem for increasing the service life of brake drums. Somedomestic and overseas researchers had been delica ting to the studies of improving thermalfatigue resista nce of cast iron materisa ls and brake drums. However, due to the bad serviceconditions and the complexity of thermal fatigue problems, the service life of brake drum wasstill not idea l, in addition to some other potentia l researches on the late start. The phylogenyof science indica ted that nearly all great progress of science and technology had came fromhuman learning from the adaptabiligy of biology to the environment. In the same way, thestudy for enhancing thermal fatigue resista nce of cast iron had been enlightened by the cracksarresting function of the biology coupling effects.The paper is aiming to improve thermal fatigue resista nce of cast iron so as to increasethe service life of brake drum by applica ting the idea of laser biomimetic coupling treatmentto resist thermal fatigue. In this paper, through analyzing coupling mechanism of biology, thebiomimetic coupling model for mecha nical components to solve thermal fatigue problemshad been founded. The biomimetic coupling units were fabricated by laser treating on thesurface la yer of cast iron, and the effects of unit shape, base materia ls'compositions andstructures on thermal fatigue resista nce of biomimetic coupling samples were alsoinvestigated, moreover the mecha nism of action was discussed. In order to improve thetreating technology for biomimetic coupling samples, laser parameter optimization,preheating and annealing were applied in the procedure. Based on the above researches, laseralloying and laser cladd ing techniques were also used for further enhancing thermal fatigueresista nce of biomimetic samples. Furthermore, the biomimetic coupling brake drums weretrial-produced, and their car load ing experiments were carried out. The results indica te that:1. Shell nacreous la yer, pla nt lea f, dragonfly's wing and butterfly's wing have similarbiology coupling characteristics for resisting fatigue and cracking. The characteristicscan be summa rized by three aspects as following: firstly, there are hard units withdefinite shapes distributing in the soft base materia ls, secondly, the hard units have different microstructures or chemica l compositions from the soft base materia ls, andthirdly, they form an alternate soft and hard biology coupling structure. The designedbiomimetic coupling resisting thermal fatigue model based on above characteristics canguida nce the laser biomimetic coupling treatments to cast iron. The microstructure in thelaser melted zone has finer grains, which is composed of cementite, martensite andresid ual austenite. The microha rdness is increased 1.75 times compared with the basemateria ls. Only solid-sta te phase changes occur in the transformed zone, and the averagemicroha rdness is 450-500 HV. Therefore, the base materia ls and the units have formedan alternate soft and hard coupling structure at the surface la yer of biomimetic couplingsamples.2. The thermal fatigue resista nce of cast iron materia ls can be improved by laserbiomimetic coupling treatment, and it is strongly affected by the shape coupling factors.The results show that the biomimetic coupling sample with lattice units has the highestthermal fatigue resista nce, and thermal fatigue resista nce of biomimetic coupling sampleincreases with the increasing density of units, furthermore biomimetic coupling samplehas the highest resista nce to thermal fatigue when units are vertica l with the direction ofthermal fatigue crack's propagation. The carbon contents also affect the enhanceddegree of thermal fatigue resista nce between biomimetic coupling samples anduntreated samples, with carbon contents increasing, the enhanced degree increasesfurther. Laser biomimetic coupling treatment has a beneficia l effect on improvingthermal fatigue behavior of cast iron with flake, vermiform and nodular graphite. Theenhanced degree of thermal fatigue resista nce of nodular graphite cast iron is the lowest.Biomimetic coupling units are hardened and strengthened because of microstructuretransformation and grain refinement. They can resist more thermal stresses so as to haveexcellent crack initiation resista nce. When cracks propagate to biomimetic units, theydip deflection or bifurcation, and the effective driving force and stress intensity factor ofdeflected crack is always less than that of the straight-line crack under the sameconditions. Therefore, biomimetic coupling samples have better crack propagationresista nce compared with untreated samples.3. The cross section area increases with the increasing laser peak power and pulse width,while it decreases with the accelera ting laser scanning rate. The frequency and thedefocusing amount have insignifica nt effects on the size of biomimetic coupling units.The laser processing parameters sort the pulse width > the peak power > the scanningrate > the frequency > the defocusing amount by the influence degree of the crosssection area of units. Through the expanded scope experiments, the pulse width isoptimizated further to make the best laser processing parameters combination. Thecracks and holes in units caused by laser processing can be avoided by preheating at anappropriate temperature of 250°C, and the resid ual stresses in units'zone are eliminated90% through annealing. The annealing treatment makes the units'microstructure morestably and tenaciously. Preheating and annealing are all benefit for improving thermal fatigue resista nce of biomimetic coupling samples.4. When units are strengthened by laser Cr alloying technique, with the benefite laserprocessing conditions, there must be a critica l thick ness of pre-placed coating, anycoatings thicker than that will lead to the waste of alloy powders. With precond ition ofnone waste of alloy powders, increasing pre-placed coating thick ness or adding properNi powder into Cr powder is benefit for enhancing thermal fatigue resista nce of laseralloying biomimetic coupling samples. When units are strengthened by laser cladd ingtechnique, there is an optima l size of the packed groove. The oversize packed groovewill result in not meta llurgica l bonding between the deposited la yer and the basemateria ls, and even bad surface roughness of units including the deep-pit shortages. Thetoo sma ll packed groove will lead to bigger rate of dilution so as to fall short ofdeposited effect. Laser cladd ing with Fe30A, Co50 and Ni45 fluxing alloying powdersto strengthen units are all benefit for improving thermal fatigue resista nce of biomimeticcoupling samples further, and of all samples tested, the one treated by laser cladd ing ofCo50 has the best thermal fatigue resista nce.5. Biomimetic coupling brake drums are satisfied with braking requirements of vehiclescompletely. The laser biomimetic coupling treatments for brake drums can enhance theirthermal fatigue resista nce significa ntly. The sevice milea ges of the trial-producedbiomimetic coupling brake drums have reached to 70,000 kilometres and upwards. Thesevice life of biomimetic coupling brake drums is 1 times at least longer than that ofuntreated norma l brake drums.The advanced idea for resisting thermal fatigue by biomimetic coupling method in thispaper has provided an origina l research thought for solving thermal fatigue failure problemsof mecha nical components which exists widely in the industry, the agriculture and thetransportation, so it is considered that the paper has much higher scientific and learned values.The studies on thermal fatigue resista nce of cast iron materia ls treated by laser biomimeticcoupling technique have offered academic gists and experimental groundwork for theengineering and actual applica tion of biomimetic coupling brake drums. The development ofnewtype brake drum is more important for boosting up our internal independent innovation ofmotor components; moreover it can also bring marked economic benefits.