Development of an ultrasonic-assisted brazing process for fluxless joining of cemented carbides to steel

采用钎焊技术将硬质合金与钢粘接时，使用焊剂是不可避免的。使用的焊剂不仅有害，还会在连接中造成不可忽略且通常无法完全预测的孔洞，这将显著影响粘结的强度，具体取决于它们的大小和分布。通过超声波在液体焊料中产生空化来替代助焊剂似乎是一个很有前途的解决方案。该项目的目的是开发一种超声波辅助焊接工艺，用于碳化物与钢的无助焊剂连接。与其他节点的初步工作已经显示，强度增加了1.5倍。结果表明，硬质合金中的成分可以溶解并进入液态焊料中。因此，有必要检查关于超声波穿透对脆性碳化物的机械性能和由此产生的接头的影响程度的过程控制。此外，还应开发适当的工艺控制，以最大限度地减少硬质合金在焊接过程中的影响，而不会产生无孔材料。以前被认为具有很强的抗氧化性(蓝宝石或碳化硅)的材料，或者由于贝德沃斯起球比低，形成了非常薄的钝化层(铝、钛合金)，这些钝化层被空化分解。在这方面，必须考虑钢的氧化行为或结垢行为。特别是在较高的工艺温度下，钢往往会形成强烈的、不会腐烂的氧化物。与目前的技术状况相比，这是本研究项目面临的最大挑战之一。分析不同类型钢在热处理和钎焊过程中的氧化行为是十分必要的。此外，需要澄清的是，超声波的应用以及由此产生的机械渗透和空化能在多大程度上激活表面并将其从现有氧化物中去除。

When cohesively joining carbide to steel by means of brazing technology, the use of fluxes is currently, inevitable. The fluxes that are used are not only harmful, but also cause a not negligible and usually not completely predictable porosity in the joining, which significantly affects the strength of the bond, depending on their sizes and distribution. A substitution of the flux by generating cavitation in the liquid solder by means of ultrasonic waves seems a promising solution.The aim of the project is to develop an ultrasound-assisted soldering process for a flux-free joining of carbide to steel. Preliminary works with other joints already showed an increase of the strength by 1.5 times. It was revealed that constituents of the cemented carbide can dissolve and get into the liquid solder. It is therefore necessary to examine the process control concerning how far the mechanical properties of brittle carbides and the resulting joint are affected by the penetration of the ultrasound. Furthermore, a suitable process control should be developed that minimizes the impact of the hard metal during brazing, yet results in a non-porous material. Materials that were previously considered either had a strong resistance to oxidation (sapphire or SiC) or, due to a low pilling Bedworth ratio, formed a very thin passivation layer of (Al, Ti alloys), which are broken up by cavitation. In this regard, the oxidation behaviour or scale behavior of steel must be taken into account. Especially at high process temperatures, steel tends to a strong, non-decaying oxide formation. Compared to the current state of technology, this is one of the biggest challenges of this research project. It is necessary to analyze the oxidation behavior of various types of steel during heat treatments and brazing. Furthermore, it needs to be clarified as in how far the application of ultrasound alongside the resulting mechanical penetration and the cavitations can activate the surfaces and rid it from existing oxides.