Novel Brazing Filler Metals using High Entropy Alloys

铜焊是连接材料的重要工艺。它速度快，强度高，在高温永久连接方法中是独一无二的，可以使连接的材料基本保持不变；因此，它可以进行复杂的连接，并连接不同和难以焊接的材料(例如，金属与陶瓷和高Al/Ti含量的镍高温合金)。它的工作原理是在被连接的部件之间引入一种特殊的合金，称为铜焊填充金属(BFM)。组件的热处理用于熔化和固化BFM，形成粘结。这些BFMS是专门为不同类型的焊接情况而设计的，可以有许多不同的成分。钎焊是许多先进应用的关键技术，包括航空航天和核部门，但它有局限性。随着服务要求的提高和贱金属的精炼，必须开发新的BFMS。今天的铜焊技术面临的一些具体问题包括：1)扩大可连接材料的范围(包括更高温度的材料，粘接金属与陶瓷，以及对不能经受现有钎焊合金的材料的工艺温度的降低；例如，功能陶瓷和高强度7000系列铝合金)，将以新的方式利用现有和先进的材料，开辟一整套新技术。2)高温钎焊使用诸如硼或硅的添加剂来降低BFM熔点。它们在这方面做得很好，但也在接头区域引入了脆性金属间化合物相，限制了机械性能。3)在实践中，焊接参数是根据特定应用的基础上通过实验试验和错误来确定的。对铜焊工艺有更深的基本了解将使这项工作更有效率，从而使设计铜焊条件成为可能。这个项目建立了对解决这些挑战的理解。一种新型合金--高熵合金(HEAs)最近在合金设计中脱颖而出。在这些合金中，许多元素的组合数量相似，而不是典型的主溶剂元素与少量添加其他元素来调整性能的方法。一些HEA报告了BFMS所需的特性；例如，能够添加大量元素来控制熔点或润湿和流动行为，而不会导致脆性相，并且多组分的性质可以调节不同材料之间的连接转变。然而，目前人们对HEAs的物理冶金机理了解较少，在钎焊方面的应用也非常有限。在这项工作中，我们系统地研究了HEAs作为BFMS的设计、理解和使用。这既增加了我们对这一耐人寻味的新合金类别的基本理解，也提供了允许为工业设计新产品的知识和技能。我们将生成的铜焊过程的数据和计算机模型为开发铜焊参数提供了设计方法和数据，目前是在个案的基础上完成的。该项目首次汇集了英国学术界和工业界关于铜焊的研究，并将成为关注铜焊兴趣的焦点。在我们工业伙伴的帮助下，我们将通过两个合金开发的案例研究来展示这项工作的成果：i)降低航空发动机的BFM成本；当前的合金含有大量的Au，因此具有适当性能的贵金属无金属BFM将降低成本。ii)Fusion BFM；为了建立先进的聚变反应堆设计，必须将反应堆内部的钨块连接到用于冷却剂的铜管。目前使用熔点为&325℃的BFMS来实现这一点，这限制了操作温度。新的BFM将提高聚变反应堆部件的性能，并提供更大的设计灵活性。

Brazing is an important process for joining materials. It is quick and permits high strength, and is unique among high-temperature permanent joining methods in leaving the materials being joined largely unchanged; hence it can make complex joints and join dissimilar and difficult to weld materials (e.g. metals to ceramics and high Al/Ti content nickel superalloys respectively). It works by having a specific alloy, called a Brazing Filler Metal (BFM), introduced between the parts to be joined. Thermal treatment of the assembly is used to melt and solidify the BFM, forming a bond. These BFMs are designed specifically for different types of bonding situation, and can have many different compositions.Brazing is a key technology for many advanced applications, including the aerospace and nuclear sectors, but it has limitations. As the service requirements become more demanding, and base metals are refined, new BFMs must be developed. Some specific problems facing brazing technology today include:1) Widening the spectrum of materials that can be joined (including higher temperature materials, bonding metals to ceramics, and also lower process temperatures for materials that cannot survive those of existing brazing alloys; functional ceramics and high strength 7000 series aluminium alloys, for example), would open up a whole host of novel technologies, using both existing and advanced materials in new ways2) High temperature brazing uses additions such as boron or silicon to suppress the BFM melting point. They do this well, but also introduce brittle intermetallic phases in the joint region, limiting mechanical performance.3) In practice, the parameters for brazing are determined on an application-specific basis, by experimental trial and error. Greater fundamental understanding of the brazing process will render this more efficient, permitting the brazing conditions to be designed.This project builds the understanding to address such challenges.A new type of alloy, High Entropy Alloys (HEAs) has recently come to the fore for alloy design. In these alloys, similar amounts of many elements are combined, rather than the typical approach of main solvent element with small additions of other elements to adjust the properties. Some HEAs have reported properties desirable for BFMs; e.g. the ability to add large amounts of elements to control melting point or wetting and flow behaviour without inducing brittle phases, and the multicomponent nature could mediate the transition in a joint between dissimilar materials. However, the physical metallurgy of HEAs is still relatively poorly understood, and their use in brazing has only been explored to a very limited extent.In this work we are investigating systematically the design, understanding and use of HEAs as BFMs. This both adds to our fundamental understanding of this intriguing new class of alloys, and provides the knowledge and skills to permit the design of new products for industry. The data and computer models of the brazing process we will generate give the design methods and data for the development of brazing parameters, which is currently done on a case-by-case basis.The project brings together the UK academic and industrial community on brazing for the first time, and will act as a focus for brazing interest. Aided by our industrial partners we will demonstrate the outcome of this work by two example case studies of alloy development:I) Reduced cost BFM for aero engines; current alloys contain significant amounts of Au and so a noble metal-free BFM, with appropriate performance, would reduce costs.II) Fusion BFM; to build advanced fusion reactor designs, it is necessary to join tungsten blocks on the reactor interior to copper pipes for coolant. This is currently done with BFMs with melting points <325degC; this limits operating temperatures. A new BFM would improve the performance and give more design flexibility for fusion reactor components.