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Development of MPEA filler for dissimilar joining of steel to titanium alloy and evaluation of strength of brazed joints
Telmasre, Tushar
Telmasre, Tushar
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2021
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2023-04-14
Abstract
In the design of lightweight structures, Ti alloy to steel joining has been given considerable attention recently. Creating a robust Ti/Steel joint by various joining techniques have proven to be difficult owing to poor metallurgical compatibility and formation of brittle intermetallic phases. Thus, the major challenge in joining Ti alloys to Steel is the formation of phases such as FeTi and Fe2Ti etc. which lead to reduced joint strength. Addition of an interlayer as a diffusion barrier has shown considerable increase in joint strength by way of reducing the proportion of detrimental intermetallic phases formed. This is achieved by limiting the interaction between Ti alloy and steel during the joining process. MPEA materials, with their versatility in properties and vastness in possible elemental configurations, can make a good candidate as filler material after suitably tuning its properties. Thermodynamics based design criteria and CALPHAD techniques can be used to achieve a filler material candidate with good mechanical properties and compatibility with both Ti-alloys and Steel.
When subjected to the lap shear testing, spot joints created by brazing, resistance welding, or other techniques may fail by the plug failure mode (also called pull-out mode) or the interfacial shear failure mode. Conditions delineating these two modes are usually interpreted in literature and handbooks by comparing the product of interface shear strength and spot area to the product of base metal ultimate tensile strength, spot diameter and plate thickness. Based on the Gurson-Tvergaard-Needleman damage mechanics model, this work conducts extensive numerical simulations to assess the failure conditions with respect to key three-dimensional geometrical parameters. It is found that a small ratio of spot diameter to plate thickness prefers interfacial shear failure while a large ratio favors plug failure; other geometric parameters such as the interlayer thickness play a secondary role. Moreover, the failure map is close to predictions based on no-damage model whereas different necking conditions are precursors to these two failure modes. Modeling results are successfully compared to spot resistance welding experiments by previous researchers, where additional modeling of hardness variation throughout the heat affected zone needs to be considered.
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