Friction stir welding of dissimilar sheet materials

运输行业对提高燃油效率的需求日益增长，这促使人们通过增加铝、镁合金和复合材料等轻质材料的利用来减轻重量。然而，在组件制造过程中，主要的挑战是这些材料的焊接，特别是当遇到不同的组合时。即使是类似的合金连接也不是微不足道的，由于弧焊过程中的裂纹，凝固孔隙和严重的变形。涉及纤维增强聚合物复合材料的接头通常需要紧固件或粘合剂，这比焊接更昂贵、更耗时。近年来，搅拌摩擦焊作为一种固态焊接技术出现，与熔焊相比具有许多优点和新的性能。固态焊接的主要特点是，通过将材料保持在熔点以下，可以避免许多传统问题，如开裂和气孔。搅拌摩擦焊技术使用一个圆柱形的工具，在末端有一个销状的突起，沿着要连接的材料穿过，可以是搭接焊或对接连接结构。本提案探讨了搅拌摩擦焊接的一种变化，在这种变化中，点焊可以使用缩回到圆柱形工具中的销钉进行。只有少数研究报道了这种所谓的“重新填充搅拌摩擦点焊”的方法，但已经证明，与传统的搅拌摩擦点焊相比，优越的接头强度可以实现。该工艺可以应用于不同的材料组合，本工作的主要重点是将铝与纤维增强复合材料板连接起来。目的是确定在这些材料的重新填充搅拌摩擦点焊过程中控制粘合的因素，并确定提供更高连接强度的工艺参数或可能的夹层。一个专门的填充搅拌摩擦焊接系统将用于研究薄板点焊接头形成的机制。在反应、混合和机械联锁方面的粘合细节将与焊接参数进行比较。先进的电子显微镜技术将用于评估温度和应力对键形成的作用。将使用重叠剪切测试以及使用宏观和纳米压痕技术的仪器压痕测试来研究接头的性能。这将使整体接头强度与焊接过程中形成的微观特征和相相相关。拟议的工作将产生促进在工业中广泛实施这项技术所需的知识。由于时间、温度、压力和变形控制着接缝强度，因此需要了解各个参数的作用。对界面键合的深入研究将有助于确定哪些参数对接头强度起主导作用并做出最重要的贡献。所提出的工作将允许为该技术开发加工图，该技术也可能适用于其他固态连接工艺，包括传统的搅拌摩擦焊接。这些图以后可以被加拿大工业利用，以便通过将加工参数连接到提供最高强度的所需微观结构来快速开发连接困难组合的解决方案。这将有助于运输行业利用先进的固态连接技术，以提高其能力并降低制造成本。

There has been a growing need for improved fuel efficiency in the transportation industries, and this has motivated weight reductions through increased utilization of light-weight materials such as aluminum and magnesium alloys, and composites. However, during component fabrication the main challenge is welding of these materials, particularly when dissimilar combinations are encountered. Even similar alloy joining is not trivial, due to cracking, solidification porosity, and severe distortion during arc welding. Joints involving fiber reinforced polymer composites typically require fasteners or adhesives, which are more costly and time-consuming than welding. In recent years, the process of friction stir welding has emerged as a solid state welding technique which offers many advantages and new capabilities compared to fusion welding. The main feature of solid state welding is that many of the traditional problems like cracking and porosity are avoided by keeping materials below the melting point. The friction stir welding technique uses a cylindrical shaped tool with a pin shaped protrusion at the end, which is traversed along the materials to be joined, either in a lap weld or butt joint configuration. This proposal examines a variation of friction stir welding in which spot welding can be conducted using a pin that retracts into the cylindrical tool. Only a few studies have been reported on this so-called ‘refill friction stir spot welding’ approach, however it has been demonstrated that superior joint strength can be achieved compared to traditional friction stir spot welding. The process may be applied to dissimilar material combinations, and the main focus of this work is joining aluminum to fiber reinforced composite sheets. The objective will be to determine factors which control bonding during refill friction stir spot welding of these materials, and to identify processing parameters or possible interlays that provide higher joint strengths. A specialized refill friction stir welding system will be used to study the mechanisms of joint formation in thin sheet spot welds. The details of bonding in terms of reactions, intermixing, and mechanical interlocking will be compared to the welding parameters. Advanced electron microscopy techniques will be applied to evaluate the role of temperature and stress on bond formation. The properties of the joints will be investigated using overlap shear testing, as well as instrumented indentation testing using both macro-scale and nano-indentation techniques. This will allow overall joint strengths to be correlated with microscopic features and phases which form during welding. The proposed work will generate knowledge that is required in order to promote widespread implementation of this technology in industry. Since time, temperature, pressure and deformation control the joint strength, the role of each parameter needs to be understood. A thorough study of the interfacial bonding will help to identify which parameters dominate and contribute most significantly to joint strengths. The proposed work will allow processing maps to be developed for this technique which may also have applicability to other solid state joining processes, including conventional friction stir welding. These maps can be later utilized by the Canadian industries in order to rapidly develop solutions for joining difficult combinations by connecting processing parameters to the desired microstructures which provide highest strength. This will help transportations industries utilize advanced solid state joining technologies in order to increase their capabilities and reduce fabrication costs.