Microstructural characterization of resistance welded joints in amorphous Zr-based alloys

Author(s)

Carolina Soares

• Document type: Master's Dissertation
• Press: São Carlos.
• Institution: Universidade de São Paulo (USP). Escola de Engenharia de São Carlos (EESC/SBD)
• Defense date: 2020-02-14

Examining board members

Marcelo Falcão de Oliveira; Mauricio Angeloni; Francisco Gil Coury

• Advisor: Marcelo Falcão de Oliveira

Abstract

Some metal alloys when solidified at critical cooling rates have an amorphous structure, unlike traditional metals whose structure is crystalline. The first amorphous metal alloys developed had micrometer thickness, as they required cooling rates higher than 105 K/s. With the advancement of the research, alloys with greater tendency to glass formation were developed and thus the bulk metallic glasses (BMG), whose amorphous thickness is greater than 1 mm, could be produced. These materials currently have structural, electronic, medical, and aerospace and defense applications. However, there are still limitations to its use, one of them being its low weldability. In BMG welding, the mechanical strength of the joint is impaired by the formation of crystals due to the thermal input. Thus, for the expansion of the application of VMM, the commercial alloy Vitreloy® 105 was chosen as the focus of the study of the influence of welding parameters on the microstructure of their welded joints. The welding was done on a commercial butt resistance welding machine. The microstructural characterization techniques used were X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy combined with energy-dispersive X-ray spectroscopy. Four types of microstructures were identified according to the current density applied in welding, indicating the joint crystallization sequence. In crystallization the first phase to be formed is a tetragonal phase polymorphic with the matrix, then a C14-type Laves phase composed of all alloying elements is formed. With the increase of the thermal input the joint becomes totally crystalline and besides the already mentioned phases also forms a C16 Laves phase and an unidentified phase. The joints also had their metallurgical bonding evaluated by bending tests followed by their fractographic analysis. The union was complete only in cases where joints were melted, however, in these cases the joint became quite brittle and completely crystalline. In cases where the crystallization was only partial there was no effective expulsion of the oxidized interface layer, thus, the union was prevented in a large area. It was also possible to observe from the fracture surface that bad heat distribution occurred during welding. Therefore, as critical points for the improvement of the metallurgical union are the increase of the pressure of compression, during the welding, and uniform distribution of temperature along the joint. (AU)

• FAPESP's process: 17/10052-9 - Microstructural characterization of welded joints in Zr-based amorphous metallic alloys
• Grantee: Carolina Soares
• Support Opportunities: Scholarships in Brazil - Master