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Upset Resistance Welding of Aluminium to Steel

Abiri, Milad | 2021

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 55041 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Kookabi, Amir Hossein; Movahedi, Mojtaba
  7. Abstract:
  8. Aluminum and steel are the most utilized metals in industry and joining of them has many applications including heat exchangers and parts in aluminum extraction process. In this research, Upset Resistance Welding of aluminum to stainless steel was studied. In this regard, the effect of current intensity, time of welding, in-situ post-heating operation and post-heating operation in the furnace were investigated on the microstructure, mechanical properties and electrical resistance of welds. The currents of 2.25, 2.30 and 2.35 kA and times of 55, 60 and 65 cycles were applied for welding. Then in-situ post-heating conducted under currents of 1.75, 2.25 and 2.75 kA and 10 cycles on the sample with the weakest tensile strength. Finally, the post-heating operation in the furnace, at a temperature of 330 degrees and time of 6, 24 and 48 hours was performed on the sample with the weakest tensile strength. In order to observe the microstructure and joint interface, optical and scanning electron microscopes were used. Tensile, microhardness and electrical conductivity tests were performed to evaluate tensile strength, hardness and electric resistance of welds. in all samples, a flash was observed and formed on the aluminum side. enhancement of the heat input resulted in increasing of the flash size. Increase in size of the flash improved the tensile strength of the joint by removing contaminants from the joint. Microstructural studies showed that on the aluminum side of the samples, recrystallization and grain growth were the most important microstructural evolution (due to the cold worked structure of the base metal). on the austenitic stainless-steel side of the samples, in an area adjacent to the interface, dynamic recrystallization was occurred as the result of heat accumulation. In the microstructure of the surrounding areas, grain growth was the predominant metallurgical phenomenon. In addition, a thin layer (0.8 to 1.1 μm thick) of aluminum-iron intermetallic compounds was formed at the joint. Increasing the welding current and time or performing post-heat treatment led to increase of the layer thickness (up to about 2.5 μm). Increase in tensile strength of the welds (from about 65 to 85 MPa) was caused by increase in the heat input (by increasing the current or welding time). On the other side the ductility of the joint was marginally affected by increasing the heat input. Excessive heat input caused melt protrusion and no bonding was detected. Finally, post weld heat treatment increased the tensile strength of the specimens (twice compared to the as weld) by encouraging the inter-diffusion of Fe and Al atoms across the joint
  9. Keywords:
  10. Upset Resistance Welding ; Aluminum ; Stainless Steel ; Microstructure ; Mechanical Properties ; Electrical Resistivity

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