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Journal of Materials Processing Technology, Volume 216, 2015, Pages 133–139.
Papaefthymiou1, C. Goulas2,3, E. Gavalas.
- National Technical University of Athens, School of Mining & Metallurgical Engineering, Laboratory of Physical Metallurgy, 9, Her. Polytechniou str., Zografos, GR-15780 Athens, Greece.
- Materials Innovation Institute (M2i), Mekelweg 2, 2628 CD Delft, The Netherlands.
- Delft University of Technology, Department of Materials Science and Engineering, Mekelweg 2, 2628 CD Delft, The Netherlands
- ELKEME S.A., Piraeus str. 252, GR-17778 Tavros, Athens, Greece.
Abstract
Aim of this research is first to evaluate the applicability of micro-friction stir welding (μFSW) to wrought zinc alloy sheets and then to improve the structural integrity of such joints. μFSW tool design was based on an algorithm that considers material and process limitations. Joining trials were performed at different feed rates. It is proven that joining by μFSW thin ZnTiCu sheets is possible and it offers extremely fine microstructures and β-phase distribution due to the mechanical fragmentation which is the outcome of the stirring. The β-phase particles were homogenized and precipitated inside deformed zinc grains and not at the grain boundaries, where they used to be in fusion welds. Electron microscopy showed that its size was limited to 150 nm, which is in average 13 times smaller than the size of the 2 μm that they get when sheets are TIG welded. Macroscopically, the μFSW joint mechanical properties are comparable with industrially fusion-welded material. The relative low elongation achieved, similar to fusion-welded sheets, is explained by the occurrence of three main defects: root opening, thinning and kissing bond.
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Significance statement
This paper refers to the joining in solid state of titanium zinc using micro-Friction Stir Welding (μFSW). Joining of ZnTiCu sheets is not thoroughly developed yet. Absence of literature regarding ZnTiCu behaviour in FSW made the joint evaluation even more demanding. The innovation content lies on the metallurgical particularity of the alloy examined in combination with the used joining technique. The hexagonal close packed lattice of zinc makes it difficult to be deformed due to limited possible slip planes. Titanium is used to enhance Zinc’s strength, but an intermetallic β-phase (TiZn15) forms only 2oC lower than its melting point. In the case of conventional fusion welding, β-phase precipitates at grain boundaries and causes their embrittlement. Consequently, all fusion welded Zinc sheets can easily fail brittle in the fusion zone. Aim of this research was first to evaluate the applicability of micro-friction stir welding (μFSW) to wrought Zinc alloy sheets and then to improve the structural integrity of such joints. Using micro-FSW we managed to join in solid-state 700μm (0,7 mm) thick Titan Zinc sheets, avoiding β-phase re-precipitation at grain boundaries and the embrittlement that it usually causes. The experimental procedure included tool design and welding trials. Microstructure characterization was performed by means of optical and electron microscopy. Mechanical properties were determined by means of tensile and three point bending testing. Despite the successful μFSW joints, three main defects were evident: root opening, thinning and kissing bond. As previously mentioned, an important metallurgical factor affecting directly mechanical properties is the β-phase. Thus, the response of the β-phase to the FSW process was thoroughly examined. Due to mechanical fragmentation and stirring, the β-phase distribution became even and its size was minimized positively affecting mechanical properties. It is proven that joining by μFSW thin ZnTiCu sheets is possible and it offers extremely fine microstructures and β-phase distribution due to the mechanical fragmentation which is the outcome of the stirring. The β-phase particles were homogenized and precipitated inside deformed Zinc grains and not at the grain boundaries, where they used to be in fusion welds. We emphasize that fusion methods create brittle joints and FSW can effectively overcome this problem.
