| Moulds are important equipments and have been widely applied in plastic products and metalproducts in modern industry. Owing to the high efficiency, superior product quality and lower costof production, moulds have occupied an extremely important position in the development ofmodern economic. The level of molding tool industry has now already become one of the importantsigns of national industrial level and product manufacturing capability. The overall level ofmanufacturing moulds in our country falls far behind the advanced countries due to someunfavorable factors such as low accuracy, low complexity, low service life and long productioncycle, which limit its development. Hot working dies works in very bad conditions such as thermalstress and thermal strain which usually induces thermal fatigue failure and cracking. Because ofthermal fatigue failure, which is one of most important failure modes, hot working die steels inChina waste RMB of about several billion yuan and produce a great loss. Therefore, repairingfailure dies, prolonging their service lives, reducing costs and saving resources have beenlongstanding technical problems to producers and researchers. Researchers had been dedicating tothe studies of repairing thermal fatigue failure dies.Therefore,taking an efficient method to repairtiny thermal fatigue cracks in time and strengthen the repaired cracking have important academicvalues and good economic benefits.Some organisms such as shell, plant leaf and dragonfly wing have superior function ofresisting fatigue and cracking. It was found that they have similar structures, that is, there are hardunits with different shapes distributing in the soft base materials. The hard units and soft basematerials form an alternate soft and hard coupling structure. Biomimetic coupling modes weredesigned for the repairing and strengthening of thermal fatigue cracks by analyzing, simplifying andimitating these structures. The paper is aiming to repair failure dies for improving the thermalfatigue resistance and service life by combining laser repair technology and biomimetic couplingtheory. In this paper, the crack repairing and arresting units were fabricated by biomimetic laserremelting, and then the influences of their individual characteristics, distribution shape, size anddistribution density on the thermal fatigue resistance of repaired samples were analyzed on fatigueH13die steel. On that basis, the study of strengthening units fabricated by laser alloying wasconducted to improving the ability of resisting thermal and cracking, moreover the relativemechanism of action was discussed. The cracking of dies was repaired by laser repair with wire andthen the weld was strengthened by laser biomimetic coupling treatment. The tensile strength and thermal fatigue resistance of samples after repair and strengthening were investigated. In addition, alaser biomimetic coupling repair system was manufactured for the repair of thermal fatigue crackson the actual die and the production testing was carried out. The results indicated that:1. The biomimetic models of crack repairing and arresting constructed from couplingstructure of shell, plant leaf and dragonfly wing are accepted for the repair and strengthening of hotworking die. Biomimetic crack repairing and arresting units were fabricated on the fatigue H13diesteel with tiny thermal fatigue cracks. The units possess high strength and toughness because ofmicrostructure transformation of martensite and retained austenite as well as grain refinement. Theaverage microhardness of units is510～682HV. In the process of units formation, partial crackswere repaired. The formed units are able to resist the propagation of cracks and protect the basematerials from further destroy. When cracks propagate and encounter biomimetic units, they dipdeflection and even stop. The residual cracks on the substrate have a beneficial effect on releasingthe driving force of crack propagation in a specific range which improves the thermal fatigueresistance of samples.2. The distribution shape, distribution density and size of units have important effects on thethermal fatigue resistance of repaired samples. In this paper, it was found that, by contrasting thethermal fatigue resistance of repaired samples with spot unit, stration unit and lattice unit, the unitsdistributing in the substrate with lattice shape could generate “closed effect” and thus possess thebest ability of resisting thermal and cracking. The distribution of units with high density means thatmore cracks are repaired and the propagation space of residual cracks are restrained; units withlarge size and high strength have better blocking ability. Thermal fatigue process causes the declineof microstructure and mechanical property of units which manifests as grain coursing, carbideparticles precipitation and hardness decrease. When the microstructure became stable, the hardnessof units was still higher than that of the base materials and the units can still act as strengtheningstructure to contribute to the improvement of thermal fatigue resistance.3. The microstructure and chemical components of units strengthened by laser alloying withself-fluxing alloy powders (Co50and Fe30A) are further enhanced. And of all alloying samples, theones treated by Co50has the best oxidation resistance and thermal fatigue resistance.4. When the cracking of a die was repaired by laser welding with wire, the weld heat affectedzone is always a risk area for thermal fatigue cracks initiation and propagation. The tensile strengthand thermal fatigue resistance of the weld strengthened by laser biomimetic coupling treatment canbe significantly improved. The strengthening units distributing on the weld surface are able to resisttensile stress in the far field and delay the cracking of weld, in the meantime, the cracks startingfrom heat affected zone can be hindered by them.5. On the basis of industrial robots with6degrees of freedom and the300W Nd：YAG laser with optical fiber transmission, a set of laser biomimetic coupling flexible manufacture system wasdeveloped which can be used for the repair of a complex die. The system has functions ofprocessing in three-dimensional space, switching parameters and programmed technology.6. The optimization of laser remelting parameters was conducted. Effects of laser processingparameters on the size, hardness and surface roughness of units have been investigated. In the rangeof this paper, there is a steep increase in both the depth and surface roughness of units and adecrease in microhardness when the average peak power density uplifts; The increased effectivepeak power density caused by raising pulse frequency or decreasing scanning speed leads to a slightincrease in unit depth and a gradual drop in microhardness and surface roughness. Based on areasonable range of unit depth, microhardness and surface roughness, a parameter selection map forlaser biomimetic coupling repair was established.7. Actual failure dies for aluminum casting production were repaired and the manufacturingverification test demonstrated the effectiveness of the laser biomimetic coupling repair units inrepairing thermal fatigue cracks of a casting die and resisting their propagation as well as improvingthe die service life. |
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