均质化大锻件构筑成形过程中界面演化机理研究

Heavy forgings, whose quality have a significant influence on operational reliability of equipments, are key components of major projects. However, there always exist some serious defects such as macrosegregation, center porosity and coarse grain in present heavy forgings manufactured by large steel ingots, which may severely impair the operating safety. Jumping out of the traditional concept that a heavy forging must be made by a large steel ingot, we first put forward a novel technology of fabricating a large high-quality forging by building continuous casting billets with much better quality and lower cost. The process is as follows, after surface machining, cleaning, the slabs are stacked in order and vacuum packaged, then the whole package is forged with hold-pressure pressing and muti-direction forging under high temperature until the interfaces are welded together perfectly. In brief, the core concept of this technology is breaking up the whole into parts and building up the whole with small parts. By simulation and experimental study on the additive forging process, the present work will reveal the mechanisms of void healing in the bond interface at the condition of high temperature, high pressure, and large deformation. Meanwhile, it will also illustrate the rules of refining and homogenizating microstructure by controlling the recrystallization and mass flow. Furthermore, the self-diffusion and interdiffusion behavior of atoms on the bond interface will be explored under theinteraction effect of alloying elements. Finally, the research direction of the novel discipline about additive forging for heavy forgings will be explored. On the basis of these fundamental study, through the process of experimentally manufacturing nuclear rotor and support ring, this project focuses on the development of original technology for additive forging, which is being used to reduce or even eliminate the defects of the macrosegregation and shrinkage cavity, and solve the homogenization and densification problems which have baffled the heavy industry for a long time.

大锻件是重大装备的核心部件，其质量直接影响装备的运行可靠性。然而，当前采用大钢锭制备的大锻件常存在严重的偏析、疏松、粗晶等缺陷，影响性能稳定。本项目突破了大锻件“以大制大”思想局限，首次提出构筑成形新思路，利用品质优、成本低的连铸坯作为基元，经表面加工、清洁处理、堆垛成形和真空封装后，在高温下施以保压锻造、多向锻造为特点的变形工艺，充分愈合界面，实现界面与基体完全一致的“无痕”连接，其核心思想是“化整为零、以小制大”，巧妙地避免了金属凝固过程的尺寸效应。项目拟通过构筑成形过程的模拟与实验研究，揭示高温、高压、大变形作用下界面区域的孔洞愈合机制，阐明再结晶和质量流控制下的组织细化、均匀化规律，探索合金元素交互作用下界面原子的自扩散、互扩散行为，拓展金属构筑成形新学科方向。在此基础上，本项目通过研制核电支承环锻件，减轻直至消除宏观偏析和缩孔疏松，解决困扰行业多年的大锻件均质化和致密化难题。

大锻件是重大装备的核心部件，在国家安全、国民经济中发挥不可或缺的作用。大锻件常采用百吨级铸锭制备，由于金属凝固过程的尺寸效应，大铸锭存在严重的宏观偏析、缩孔疏松等缺陷，严重影响锻件质量，这已成为世界性难题。中科院金属所在国际上率先发明金属构筑成形技术，该技术以多块小尺寸均质化板坯作为基元，通过表面活化 、真空封装、高温形变等手段，使构筑界面与基体完全一致，进而获得大锻件所需均质化母材，实现了“以小制大”的新型制造。本项目采用实验与模拟相结合的方法，对不同材料体系构筑界面结合行为进行了系统研究，发现构筑表面氧化和粗糙度对界面结合具有较大的影响；不同体系的合金界面组织演化机制存在差异，界面再结晶促进界面组织均匀化；由于真空条件的限制，构筑连接界面不可避免会残存界面氧化物，通过高温大变形和长时保温处理可实现界面氧化物的调控分解，界面氧化物在高温保温过程中与基体发生固相反应，直至完全消除，实现界面的完全愈合。在此研究基础上，构建了大锻件构筑成形的新理论、新方法，揭示了高温、高压、大变形作用下界面氧化物分解演化及孔洞愈合机制，阐明了组织细化、均匀化的演化规律。成功实现金属构筑成形的工业应用，解决大锻件的组织性能均匀化问题。项目完成后，已发表32篇相关学术论文，其中29篇 SCI 收录；参加多次国际国内学术会议并作邀请报告；授权28项发明专利，其中3项为国际专利。依托本项目的研究成果，在伊莱特能源装备股份有限公司建立金属构筑成形示范线，完成不同规格海上风电法兰、示范快堆φ15.6m不锈钢支承环、715mm大口径一体化压力管等巨型构件的生产制造，并交付使用，显著提高了材料利用率，为企业降低制造成本20%以上。研究成果入选“壮丽70年共和国发展成就巡礼”；获“中国专利金奖”；入选国家“十三五科技创新成就展”，获《新闻联播》、《人民日报》报道。