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Microstructure modification for improved mechanical properties in A356 aluminum using friction stir processing.

机译：通过摩擦搅拌工艺对微观结构进行了改进，以改善A356铝的机械性能。

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摘要

Friction Stir Welding (FSW) is a revolutionary solid-state joining technology patented by The Welding Institute (UK) in 1991. This technique uses a rapidly rotating, non-consumable high strength tool steel-in that extends from a cylindrical shoulder to generate frictional heating at the point of welding and to induce intense plastic deformation of work-piece material. The rotating tool assembly is traversed along the joint line, forcing the plasticized material to forge behind to form a solid phase joint. The rotation of the tool produces intense plastic deformation due to stirring action leading to significant microstructural refinement in a single step.; The microstructural refinement obtained after FSW has given rise to the concept of friction stir processing (FSP), wherein, instead of joining, the technique is used to locally modify the microstructure. In this case instead of joining two plates, FSP is used on a monolithic sheet and advantage is taken of the severe plastic deformation caused during the process. In recent years, cast alloy microstructure, in relation to both crack initiation and propagation, has gained much attention considering that advanced casting techniques such as squeeze casting, thixocasting, rheocasting, etc. give rise to significantly fewer defects than conventional cast structures. Moreover, new developments in conventional casting technologies continuously lower defect levels, especially in critically loaded areas. This research work was initiated to develop FSP as a tool for locally modifying the microstructures in a various aluminum alloy and studying the change in mechanical properties in the modified regions. Al-7wt. %Si-Mg alloys with Mg contents in the range of 0.25 to 0.65 wt.% (A356, A357 and C355 alloys) are widely used to cast high-strength components in the aerospace and automobile industries because they offer a combination of high strength to weight ratio with good casting characteristics. Al-Si alloys were selected for the current study because of the above facts.; During FSP, microstructural refinement is induced in the stir zone due severe plastic deformation around the rotating pin, leading to a reduction in the number of small porosities along with reduced aspect ratios of second phase particles. The weld integrity, microstructural refinement and hence the mechanical properties are significantly influenced by the processing parameters and tool geometry. During this study, critical processing parameters were determined and a process window was established to obtain sound welds. Multiple tool designs and processing parameters were studied to study the influence of these parameters on mechanical properties in the stir zone. A correct combination of tool design and processing parameters was determined to achieve optimum tensile properties.; An in depth understanding of the FSP microstructure was achieved through the use of optical, scanning and transmission electron microscopy. It was seen that after FSP, there is a homogenous distribution of the secondary particles in the aluminum matrix; a significant reduction is the porosity volume fraction and a development of finer grain structure in place of a cast microstructure. The influence of heat treatment on the FSP microstructure was also studied and it was found that homogenization treatment during the casting process leads to abnormal grain growth in a FSP microstructure. However, this abnormal grain growth does not adversely affect the mechanical properties in case of FSP A356. (Abstract shortened by UMI.)
机译：搅拌摩擦焊（FSW）是一种革命性的固态连接技术，1991年获得英国焊接协会（The Welding Institute（UK））的专利。该技术使用快速旋转的，非消耗性的高强度工具钢，该工具钢从圆柱肩部伸出以产生摩擦在焊接时加热并引起工件材料强烈的塑性变形。旋转工具组件沿连接线移动，迫使增塑材料向后锻造以形成固相连接。工具的旋转由于搅拌作用而产生强烈的塑性变形，从而在一个步骤中显着改善了微结构。在FSW之后获得的微观结构细化产生了摩擦搅拌处理（FSP）的概念，其中代替接合，该技术用于局部修改微观结构。在这种情况下，不是将两个板连接在一起，而是在整块板上使用FSP，并利用了过程中引起的严重塑性变形。近年来，考虑到先进的铸造技术（如挤压铸造，触变铸造，流变铸造等）与常规铸造组织相比，产生的缺陷明显减少，因此与裂纹萌生和扩展相关的铸造合金显微组织已引起了很多关注。此外，常规铸造技术的新发展不断降低缺陷水平，特别是在临界载荷区域。开展这项研究工作的目的是开发FSP，将其作为一种工具来局部修改各种铝合金中的微观结构，并研究修改区域中机械性能的变化。 Al-7wt。 Mg含量在0.25至0.65 wt。％范围内的％Si-Mg合金（A356，A357和C355合金）被广泛用于铸造航空航天和汽车工业中的高强度组件，因为它们提供了高强度的结合。重量比良好的铸造特性。由于上述事实，选择了Al-Si合金用于本研究。在FSP期间，由于旋转销周围的严重塑性变形，在搅拌区引起了微观结构的细化，导致小孔隙数量的减少以及第二相颗粒的长宽比降低。加工参数和工具几何形状会显着影响焊接的完整性，微观结构的细化以及因此而产生的机械性能。在这项研究过程中，确定了关键的工艺参数并建立了一个工艺窗口以获得良好的焊接效果。研究了多种工具设计和加工参数，以研究这些参数对搅拌区机械性能的影响。确定工具设计和加工参数的正确组合以实现最佳拉伸性能。通过使用光学，扫描和透射电子显微镜可以深入了解FSP的微观结构。可以看出，经过FSP处理后，铝基体中的次级粒子分布均匀。孔隙体积分数的显着降低和代替铸造组织的细小晶粒组织的发展。还研究了热处理对FSP显微组织的影响，发现铸造过程中的均质化处理会导致FSP显微组织异常晶粒长大。但是，对于FSP A356，这种异常的晶粒长大不会对机械性能产生不利影响。 （摘要由UMI缩短。）

著录项

  • 作者

    Sharma, Siddharth Ramkrishna.;

  • 作者单位

    University of Missouri - Rolla.;

  • 授予单位 University of Missouri - Rolla.;
  • 学科 Engineering Metallurgy.
  • 学位 Ph.D.
  • 年度 2005
  • 页码 123 p.
  • 总页数 123
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 冶金工业;
  • 关键词

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