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Microstructure and Mechanical Properties of Upset Resistance Welds of Al-Mg-Si Alloy Rods

Jafari Kandovani, Ali | 2024

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 58037 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Movahedi, Mojtaba
  7. Abstract:
  8. Alloy 6061 is derived from a combination of aluminum with elements such as magnesium and silicon. Due to its high corrosion resistance, excellent machinability, and high strength, this alloy is widely used in various industries, including aerospace, automotive, and the manufacturing of mechanical parts for airplanes. Butt resistance welding is one type of resistance welding employed for joining various components, such as train rails, construction rebar, automobile rims, as well as in aerospace, petrochemical, and marine industries. Butt resistance welding is a process that can be either fusion-based or solid-state. In this process, similar or dissimilar metals are joined by bringing them to a near-melting temperature under pressure. In this study, butt resistance welding of 6061 aluminum rods with a diameter of 10 mm was examined. The effects of welding current intensity (7 to 9 kA), welding current passage time (50 to 90 cycles), and post-weld heat treatment (T6 in a furnace) on the microstructure and mechanical properties of the welds were investigated. Optical and scanning electron microscopes were used to observe the microstructure of the welded region. Tensile tests and microhardness measurements were conducted to evaluate the mechanical properties of the welds. Results showed that the size of the flash formed in the welded zone increased with higher heat input. Determinants of weld strength included: 1-Effective bonding surface 2-Recrystallization at the interface 3-Grain size 4-Morphology and size of precipitates. The effective bonding surface (e.g., cracks or oxide layers at the interface) influenced the tensile strength of the samples. No cracks or oxides were observed at the interface in any welded samples. Heat input variation did not significantly affect grain size. Since atomic bonding requires atoms to approach each other closely, the grains at the interface exhibited differing grain structure and orientation between the two ends of the rod, preventing complete atomic bonding. The weld strength, approximately 80% of the base metal, was attributed to recrystallization at the interface, which resulted in new grain structures. The tensile strength ranged from 130 MPa (minimum) to 190 MPa (maximum). Microhardness tests showed a decrease in hardness at the interface compared to the base metal. However, post-weld heat-treated samples exhibited increased hardness due to the higher volume fraction of precipitates formed after the heat treatment. For the welded samples of 6061 alloy, Mg2Si, Al2Cu, and Al3Mg2 precipitates were identified. The morphology of precipitates in the base metal was predominantly spherical, while at the interface, they appeared spherical and needle-like. In the flash region, precipitates formed at grain boundaries due to remelting. Upon continuous cooling, secondary phases formed in preferential locations in all alloys. Precipitate size did not significantly change with varying heat input, and the T6 heat treatment only increased the volume fraction of precipitates
  9. Keywords:
  10. Upset Resistance Welding ; Flash ; Tensile Strength ; T6 Heat Treatment ; Microhardness ; Aluminum Alloy 6061 ; 6xxx Series Aluminum Alloys ; Aluminum-Silicon-Magnesium Alloy

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