【作者】 杨涛；

【导师】 崔国荣；

【作者基本信息】 哈尔滨工业大学 ， 材料与化工（专业学位）， 2023， 硕士

【摘要】 镁合金以其较高的比强度、比刚度等突出性能被称为二十一世纪的“绿色结构材料”。添加稀土元素强化镁合金是提高镁合金强韧化的重要途径,在航空航天等追求轻量化的尖端领域得到一定的应用。目前常见形变热处理工艺方案中,难以获得细晶强化、弥散强化相叠加的优势组织,能够显著细化组织的剧烈塑性变形工艺,如轧制、挤压等往往难以抑制高温大变形中的部分析出相粗化,而容易控制沉淀相析出尺寸的固溶时效热处理方案,则会在固溶的高温阶段造成晶粒明显粗化。因此,探索在细晶强化基础上的细小弥散沉淀相析出控制机制,充分发挥细晶强化和沉淀相强化的叠加效应是提高稀土镁合金强韧化的关键瓶颈。相比常见的多向锻造,单向镦粗工艺能显著缩短高温变形时间、抑制热变形期间的沉淀析出。因此,本文以WE43稀土镁合金单向热镦粗及锻造前后热处理工艺为主要技术路线,通过引入预冷变形、预时效等工艺,研究不同工艺参数对晶粒演化、沉淀析出的影响规律及其对拉伸性能的影响机制。首先,以铸态WE43为初始材料,选择固溶、热镦、时效的工艺路线,通过调整固溶时间、热镦变形温度、变形量和时效时间等变量,研究了工艺参数对固溶镦粗的组织演化影响规律,并讨论了连续与不连续时效对析出相分布特征的影响机制。其中热镦粗使得平均晶粒尺寸从150μm减小20μm,抗拉强度达到270MPa且断后延伸率超过22%。峰时效处理后达到408MPa的抗拉强度和9%的断后延伸率。连续时效处理形成的析出相分布均匀,而不连续时效可在前次时效形成的无析出带外缘进一步析出更小尺寸的沉淀相,这一现象可有效提升沉淀强化效果。其次,在固溶后、热镦前引入室温预冷变形工序。适量冷变形引入孪晶分割粗大晶粒,应变诱导冷变形后和热镦粗期间在孪晶界上形成小尺寸第二相颗粒的链状分布。充分利用了孪晶促进动态再结晶DRX行为显著地细化了热镦粗后的显微组织,平均晶粒尺寸达到10μm以下,抗拉强度达到280MPa断后延伸率达到23%。且峰时效处理后达到了428MPa抗拉强度和12%的断后延伸率。最后,在基础形变热处理工艺上叠加了预时效处理,发现了预时效处理细化WE43合金热镦粗后的晶粒尺寸,并抑制新的动态再结晶晶粒较快长大的关键作用。但是,也发现了热镦粗前合金中预先析出的部分沉淀在晶粒内部析出的同时也不可避免的在晶界上强烈析出,并严重聚集粗化。造成了热镦粗及时效处理之后晶界上粗大(长度达500nm)且呈薄壳状的第二相析出状态,强烈地抵消了细晶强化提高的组织性能改善。更多还原

【Abstract】 Magnesium alloys are known as "green structural materials" in the 21 st century for their excellent specific strength and specific stiffness.Adding rare earth elements to strengthen magnesium alloys is an important way to improve the strength and toughness of magnesium alloys,and it has been applied in cutting-edge fields such as aerospace and aerospace that pursue light weight.In the current common deformation heat treatment process schemes,it is difficult to obtain the superior structure of superposition of fine-grain strengthening and dispersion strengthening phases,and severe plastic deformation processes that can significantly refine the structure,such as rolling and extrusion,are often difficult to suppress part of the high-temperature large deformation.The phase coarsening is analyzed,and the solution aging heat treatment scheme,which is easy to control the size of the precipitated phase,will cause the crystal grains to be significantly coarsened in the high temperature stage of solid solution.Therefore,exploring the precipitation control mechanism of fine dispersed precipitates on the basis of fine-grain strengthening and giving full play to the superimposed effect of fine-grain strengthening and precipitated phase strengthening are the key bottlenecks for improving the toughness of rare earth magnesium alloys.Compared with common multiple forgings,the unidirectional upsetting process can significantly shorten the high-temperature deformation time and suppress precipitation during hot deformation.Therefore,this paper takes WE43 rare earth magnesium alloy unidirectional hot upsetting and heat treatment before and after forging as the main technical route,and introduces pre-cooling treatment,pre-aging and other processes to study the influence of different process parameters on grain evolution,precipitation and precipitation.Mechanisms affecting tensile properties.First of all,with the as-cast WE43 as the initial material,the process route of solid solution,hot heading and aging was selected,and by adjusting the variables such as solid solution time,hot heading deformation temperature,deformation amount and aging time,the effect of process parameters on solid solution upsetting was studied.The influence law of microstructure evolution,and the influence mechanism of continuous and discontinuous aging on the distribution characteristics of precipitated phases are discussed.Among them,thermal upsetting reduces the average grain size from 150μm to 20μm,the tensile strength reaches 270 MPa and the elongation after fracture exceeds 22%.A tensile strength of 408 MPa and an elongation after fracture of 9% were achieved after continuous peak aging.The precipitates formed by the continuous aging treatment are evenly distributed,while the discontinuous aging can further precipitate smaller-sized precipitates on the outer edge of the no-precipitation zone formed by the previous aging.This phenomenon can effectively improve the effect of precipitation strengthening.Secondly,a pre-cooling deformation process at room temperature is introduced after solid solution and before hot heading.An appropriate amount of cold deformation introduces twins to split coarse grains,and a chain-like distribution of small-sized second-phase particles is formed on the twin boundaries after strain-induced cold deformation and during hot upsetting.Making full use of twin crystals to promote dynamic recrystallization DRX behavior significantly refines the microstructure after hot upsetting,the average grain size reaches below 10μm,and the tensile strength reaches 280 MPa,and the elongation after fracture reaches23%.And after peak aging treatment,the tensile strength reached 428 MPa and the elongation after fracture was 12%.Finally,pre-aging treatment was superimposed on the basic deformation heat treatment process,and the key role of pre-aging treatment in refining the grain size of WE43 alloy after hot upset and inhibiting the rapid growth of new dynamic recrystallization grains was found.However,it was also found that the pre-precipitated part of the precipitation in the alloy before hot forging precipitated in the grains,and at the same time,it inevitably precipitated strongly at the grain boundary,and seriously aggregated and coarsened.After thermal upsetting and aging treatment,the coarse(length up to 500nm)and shell-like second phase precipitation state on the grain boundary is caused,which strongly offsets the improvement of the microstructure and performance of the fine grain strengthening.更多还原