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Resistance Spot Welding of 5xxx Aluminum Alloy to Aluminum Clad Steel Sheet

Rahimi Gazvar Oliya, Saeed | 2020

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 52941 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Movahedi, Mojtaba
  7. Abstract:
  8. Due to the growing trends in application of aluminum alloys for automotive body production, the need for development of successful dissimilar joints between steels and aluminum alloys is growing too. The main process for joining these metals in automotive industry is resistance spot welding (RSW). However, there are some challenges in RSW of the aluminum to steel such as different physical properties of two metals, formation of brittle Al/Fe intermetallic compounds (IMCs) and presence of porosities at the weld interface which adversely affect the weld properties. In order to overcome these limitations, several methods have been proposed. Al interlayer is one of these methods. The use of the aluminum interlayer can impede the formation of IMCs and increase joint strength. In the present work, aluminum cladded steel sheet was welded to Al-Mg aluminum alloy using resistance spot welding. First, the aluminum was pre-bonded on the steel sheet by the roll bonding process and then the aluminum cladded steel sheet was resistance spot welded to the Al-Mg sheet. Effect of the welding current, the aluminum clad thickness and the thickness of Al-Mg base metal were studied on the microstructure and mechanical properties of the joints. The weld nugget shape, variation of the grain size and the formation of IMCs at the weld interface as well as the tensile-shear and cross-tension strengths of welded joints were evaluated. Results showed that the steel sheet was not melted during welding. However, the Al-1050 clad layer and Al-5083 base sheet were melted and formed the weld nugget. With the enhancement of welding current, the electrode indented more into the aluminum base sheet and temperature gradient (G) inside the weld nugget increased. The higher temperature gradient in weld metal caused the weld nugget microstructure to be changed from the columnar to equi-axed grains. Also, the Al/Fe intermetallics were formed in the interface of weld nugget and the steel base sheet. With increasing welding current, the thickness of the IMCs was also increased. Moreover, increasing the thickness of aluminum clad layer from 0.3 to 1 mm resulted in decrease of intermetallic compounds thickness from 3.1 to 2.5 µm. The results of mechanical tests showed that the thickness of the base metal has an effective role in enhancement of the joint strength. Also by changing the thickness of Al-5083 from 1 to 2 mm, due to lower electrodes indentation, the peak load in the tensile-shear test increased more than 40 %. On the other hand, in the tensile-shear test, by reducing the thickness of the aluminum clad, the same weld strength can be achieved in lower welding current. In the cross-tension test, when the thickness of the Al-5083 increased the weld strength enhanced more than 90 %. When the thickness of the aluminum layer was reduced, weld strength decreased due to formation of thick intermetallic layers. Moreover, by using of aluminum clad layer the ductility ratio (the ratio of cross-tension strength to tensile-shear strength) at the highest shear strength was 0.7 which was significantly higher than the previous studies. Also, the study on the cross section of the sample fracture surfaces showed that with increasing the thickness of the aluminum layer, intermetallic compounds did not play a detrimental role in the weld strength
  9. Keywords:
  10. Roll Bonding ; Resistance Spot Welding ; Intermetallic Compounds ; Interlayer ; Aluminum Alloy Series 5xxx ; Welding Dissimilar Metals ; Tensile-shear Strength ; Cross Tention Strength

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