Influence of Mg and Si Content in Aluminium Alloys on Severe Plastic Deformation Behaviour during Solid-State Coating Deposition using Friction Surfacing

动态再结晶对摩擦固相连接技术的工艺特性和材料流动有重要影响。除了一般的材料性能，如热容量和高温强度外，动态微观组织过程，如位错运动、晶界迁移、亚结构的形成或相的沉淀，对作用的流动应力有很强的影响。在摩擦堆焊或类似的固态连接技术中，这种微观结构机制与材料行为的相关性至今尚未得到系统的研究。合金元素含量的微小变化，例如在铝合金中，需要对迄今为止通过经验或统计方法建立的工艺参数进行重大调整。在摩擦表面（FS）过程中，由涂层材料制成的螺柱绕其纵轴旋转，被压在基材上。经过短暂的加热阶段（< 2秒），螺柱材料粘附在基材表面，通过剪切软化的螺柱材料来调节旋转相对运动。当附加的平移运动叠加时，将塑化的螺柱材料从螺柱上剪切下来并作为涂层沉积到基板上。该工艺所需的热量仅由塑性变形产生。对于铝合金的FS，应用高达40001 /min的转速，工艺温度达到熔化温度的80%左右，冷却速度为30k /s。虽然应变和应变率只能从剪切层的尺寸来估计，但显然变形条件是极端的。关于这种恶劣条件下的动态再结晶和流动应力的知识非常有限，只有少数关于gleeble试验和高温高压扭转实验的出版物提供了一些线索。在这个项目范围内，有6种定制铝合金由FS加工。每一种合金的不同之处在于其镁或硅的含量，允许直接比较，从而研究这些合金元素对材料行为的影响。Si含量将提高到17.5 wt%，在加工过程中提供不可溶解的硬相，这将进一步影响变形和再结晶机制。除了检查工艺力和涂层几何形状外，还将进行XRD， EBSD和TEM研究塑性变形的微观结构机制，并与FS期间的材料行为相关联。FS通常导致非常低的晶粒尺寸和硬相球化。获得的涂层的机械性能将通过本项目范围内的微力学测试进行评估，这与FS工艺通过严重塑性变形生成涂层的潜在工业应用有关。

Dynamic recrystallization has a major influence on process characteristics and material flow in friction-based solid state joining techniques. In addition to general material properties, a.o. heat capacity and high temperature strength, dynamic microstructural processes, e.g. dislocation movement, grain boundary migration, formation of substructures or precipitation of phases, have a strong effect on the acting flow stresses. The correlation of such microstructural mechanisms and the material behaviour during Friction Surfacing or similar solid state joining techniques has not been systematically investigated up to today. Small changes in the content of alloying elements, e.g. in Aluminium alloys, require significant adaptations of the process parameters, which are to date established by empirical or statistical approaches.During Friction Surfacing (FS), a stud made from the coating material, rotating around its longitudinal axis, is pressed onto the substrate. After a short heating phase (< 2 s), the stud material adheres to the substrate surface and the rotational relative motion is accommodated by shearing the softened stud material. When an additional translational motion is superimposed, the plastified stud material is sheared off the stud and deposited onto the substrate as a coating layer. The heat required for the process is solely generated from plastic deformation.For FS of Aluminium alloys, rotational speeds up to 4000 1/min are applied, process temperatures reach approximately 80% of the melting temperature and cooling rates range at 30 K/s. Although strain and strain rates can only be estimated from the dimensions of the shear layer, obviously the deformation conditions are extreme. The available knowledge of dynamic recrystallization and flow stresses under such severe conditions is very limited, and only few publications on Gleeble-tests and high-pressure-torsion experiments at high temperatures provide some clues.In the scope of this project 6 custom-made Aluminium alloys are processed by FS. Each of these alloys only differs in its content of Mg or Si, allowing a direct comparison and therewith the investigation of the effects of those alloying elements on the material behaviour. The Si content will be raised up to 17.5 wt%, providing undissolvable hard phases during processing, which will further influence the deformation and recrystallization mechanisms. Besides examining process forces and coating geometry, XRD, EBSD and TEM investigations of the microstructural mechanisms of plastic deformation will be carried out, and correlated with the material behaviour during FS.FS typically results in very low grain sizes and spheroidization of hard phases. The mechanical properties of the obtained coatings, which are relevant for a potential industrial application of the FS process to generate coatings via severe plastic deformation, will be evaluated through micromechanical tests in the scope of this project.