| With the development of aerospace,military industry and other large heavy equipment,large-caliber,thin-thickness metal pipe fittings have been widely used,lightweight,integration is its development of the inevitable trend,the current use of large-diameter thin-walled pipe fittings production process is mostly casting,milling and welding,the quality of the obtained pipe fittings is difficult to guarantee.The pipe fittings processed by precision plastic forming technology can not only obtain the excellent characteristics of high strength,good performance and high precision,but also have the characteristics of high production efficiency and high material rate,which is the first choice for the development of mechanization in the future.Therefore,this paper uses 6061 aluminum alloy as the research material,combined with precision plastic forming technology,to invent a new forming process for large-diameter thin-walled pipe fittings:first the thick and then reverse extrusion of the columnar blank pier to obtain thick-walled straight tubes,and then the thick-walled straight cylinders are back-extruded to obtain thin-walled pipe fittings,and the mold is designed and optimized on this basis.Combined with the optimized mold and taking the equivalence variation,the standard deviation of the equivalence variable and the standard deviation of the material flow rate as the response variables,the influence on the formed pipe fittings was discussed,and the process parameters were optimized by using this as an index,and the subsequent process experiments were guided and completed.Finally,the microstructure and mechanical properties of the pipe fittings obtained by process experiments are further observed and analyzed,and the optimal heat treatment scheme of large-diameter thin-walled pipe fittings is explored.The research content mainly includes the following aspects:(1)Combined with the actual production and experimental conditions and using Deform-3D finite element simulation software,the feasibility analysis of different processing technologies of large-diameter thin-walled pipe fittings was carried out,and the intensity of deformation and deformation uniformity were initially compared as indicators,and the best process method of large-diameter thin-walled pipe fittings was determined:first the solid blank was back-extruded into a thick-walled straight cylinder,and then the thick-walled straight cylinder was back-extruded into thin-walled pipe fittings.And for this process method,the specific process arrangement and mold design are carried out.(2)Based on the Deform-3D finite element simulation platform,the specific wall thickness of large diameter thin-walled pipe fittings was optimized and determined.Using the standard deviation of material flow rate during the pipe forming process as an indicator,the impact of uneven friction at various locations on the final formed pipe fittings was explored under actual production conditions:Under actual production conditions,with the increase of wall thickness thinning,the material flow rate increases and the sensitivity of material flow increases during the reverse extrusion process,the more uneven the material flow rate everywhere during the pipe forming process,the worse the quality of the formed pipe fittings(verticality,pipe mouth roundness,etc.).The wall thickness of 20mm is the optimal wall thickness of the pipe fittings that meet the product requirements under this process.(3)Based on the Drefom-3D finite element simulation platform,the process parameters of the two pass and three step tube forming process were optimized using the equivalent strain,equivalent strain standard deviation,and material flow rate standard deviation as response variables.The combination of the suboptimal process parameters obtained by using the Box-Behnken experimental design method and the response surface method was 410℃ billet temperature,3mm/s loading speed of pier rough plate in the pier coarse stage,and lmm/s punch loading speed in the reverse extrusion stage.The combination of the two optimal process parameters is:blank temperature 410℃ hollow punch loading speed 1mm/s.(4)Through the observation of microstructure,the influence of the process on the pipe fittings was explored,and it was found that the existence of large grains in the deformed structure also produced a large number of recrystallization,and in addition to recrystallization,there were also a large number of dislocations and subcrystals.The KAM value in this deformation tissue is as high as 1.5,the average Schmid factor is 0.41,and its continuous dynamic recrystallization and discontinuous dynamic recrystallization are easy to nucleate and the orientation is soft orientation,the critical slitting stress required for the material to start the slip system is small,the slip system is easier to start,and the pipe fittings still have good toughness and plasticity after forming.It is observed from the ODF diagram that there are more brass R-shaped textures in the deformed structure,and it can be seen from the reverse polar diagram that it forms a fiber texture of ED//<112>,which conforms to the characteristics of reverse extrusion forming process.(5)By setting different solution temperature and aging time,the T6 heat treatment process scheme most suitable for large-diameter thin-walled pipe fittings was explored:when the solution temperature was 535℃,the holding time was 3h,the artificial aging temperature was 175℃ and the holding time was 16 h,the mechanical properties of the thin-walled pipe fittings reached the best,the yield and tensile strength reached 273.57MPa and 316.7SMPa,respectively,and the elongation was 12,1%,When it is necessary for pipe fittings to have good toughness and plasticity,a heat treatment scheme with a solid solution temperature of 510℃,a holding time of 3h,an artificial aging temperature of 175℃,and a holding time of 8h is selected.The elongation of the pipe fittings reaches 24.6%,and the yield and tensile strengths are 208.03MPa and 258.92MPa,respectively. |
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