Investigation On Preparation And Thermo-mechanical Behavior Of Laser Cladding Double Gradient Coating For Copper Crystallizer

Copper crystallizer is a core component of continuous casting equipment,and its performance and service life are critical to the stable operation during the continuous casting.The preparation of coating on the surface of the crystallizer copper plate is one of the important ways to prolong the service life of the crystallizer.However,the premature failure of the crystallizer coatings which are widely used is mainly caused by local thermal cracking or peeling of the coating,wear and deformation of the copper plate,etc..Based on this,the current coating cannot meet the performance requirements and environmental requirements of high-efficiency continuous casting production.Laser cladding technology is considered as an emerging environment-friendly technology for preparing coatings with metallurgical bonding and high quality on the mold copper surface.However,due to the high thermal conductivity and the high reflectivity of copper alloys to laser,it is still difficult to realize large area preparation of coatings with good interfacial compatibility,reliable metallurgical bonding,free of cracks and other defects on copper substrate.The coating prepared on the surface of copper plate will affect the heat transfer and stress state of the copper cyrstallizer.Therefore,researches to understand the influences of the coating itself on the thermo-mechnical behavior of the crystallizer are the prerequisite for coating preparation on the copper crystallizer surface.The effects of coating material,coating thickness and design of coating structure with unequal thickness on the heat transfer and thermal stress of the mold copper plate were studied.The results show that the surface of the working layer had uneven temperature and stress distribution.The highest surface temperature and thermal stress occur near the mould meniscus.The temperature on the coating surface increases significantly with the thicker of the coating and the lower of the thermal conductivity of the coating material.The maximum value of the equivalent thermal stress appear on the surface of the coating near the mould meniscus.The thermal stress on the surface was higher than that on the bonding surface between the coating and substrate on the same thickness section.The equivalent stress decrease significantly with the decrease of height below the meniscus.According to the distribution characteristics of the surface temperature field and stress field of the working copper plate and the influence of the coating structure design,combined with the different failure modes of different regions on the surface of the crystallizer,a coating design idea is proposed to prepare the coating of which the hardness and thickness are gradiently increased along the direction of the mold blanking,so that each region of the hot surface of copper plate can obtain the same service life.The coating designed will gain better heat conduction in the upper region of copper plate and higher wear resistance in the lower part.In order to reduce the cracking risk of the coating during the multi-layer and multi-passlaser laser cladding process,the influences of the cladding process path and the thickness of the single-layer on the stress of the samples were studied.And the stress field of multi-layer laser cladding process was analyzed by numerical simulation with life-and-death cell method.The results show that the residual stress of the laser cladding layer is tensile stress,and the residual stress along the longitudinal direction of the cladding layer is much larger than that in the direction of the vertical direction.The residual stress on the back of the specimen is also tensile stress.The residual stresses in coating and back surface of substrate are both increased obviously when increasing the thickness of the laser cladding single-layer.Though the pre-deformation of the substrate with a crown of about 2 mm before laser cladding has no obvious effect on the residual stress of the coating,but significantly reduces the residual stress on the back of the specimen.In the five kinds of schemes about multi-layer and multi-pass laser cladding path,the residual stress of the laser cladding coating is the lowest in the case where sub-regional preparation of laser cladding coatings and the scanning paths of the cladding layers in each region intersect vertically.The plate has an upward warping deformation along the length of the center line after the completion and sufficiently cooled of the cladding.The longitudinal plastic deformation was the main cause to produce cracking during the laser cladding process.By designing the material,process and coating structure of the coating on the surface of copper alloy by laser cladding,the problem that it is difficult to prepare a reliable metallurgical interfacial bonding and no defect on the surface of copper alloy by laser cladding is solved.A superior defect-free and reliable metallurgically bonded coating can be obtained by laser remelting a 0.4 mm thickness of pure nickel plating on the surface of the copper alloy at room temperature with a fiber-coupled-output semiconductor laser equipment.The remelting parameters were as below:laser power 4200 W,the scanning speed 10mm/s,and overlap ratio 30%.The preset of nickel plating and application of semiconductor laser greatly increase the absorption rate of laser by copper substrate.The preset nickel plating converted from?-Ni before remelting to re-melted?Ni,Cu?solid solution to ensure a good metallurgical bonding.In addition,the hardness of the laser remelting layer was about 135 HV_0.05,slightly higher than the hardness of the CuCrZr matrix.The smooth transition of the hardness was beneficial to alleviate the residual stress at the fusion interface.The cobalt-based transition layer and the working layer are sequentially prepared on the laser remelting primer layer by laser cladding synchronous powder feeding,to obtain a defect-free gradient composite coating.The gradient of the composition and hardness of the coating mitigate the thermal stress during the laser cladding preparation process and avoid the cracking in the layer.At the same time,the obtained gradient composite laser cladding coating had good resistance to thermal fatigue and good thermal stability at high temperatures.Its pin-disk friction and wear performance at both room temperature and high temperature is much higher than that of NiCo plating for crystallizer,and its relative wear resistance is more than 10 times that of the NiCo plating.The wear mechanism of the laser cladding coating was characterized by micro-cutting"rough"-like abrasive wear.Based on the above coating design idea,new coating preparation process and optimized cladding path,the continuous casting copper crystallizer with laser cladding coating is made and verified by engineering.Its over-steel capacity has increased from 50,000 tons of NiCo plating cyrstallizers widely used in the current steel industry to 180,000 tons,significantly reducing production costs.