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- Type of Document: M.Sc. Thesis
- Language: Farsi
- Document No: 46867 (07)
- University: Sharif University of Technology
- Department: Materials Science and Engineering
- Advisor(s): Kokabi, Amir Hossein; Movahhedi, Mojtaba
- Abstract:
- Friction stir spot welding (FSSW), as a solid state welding process, can be used instead of resistance spot welding (RSW) for dissimilar joining of aluminum and steel sheets due to its advantages over the RSW process including the non-preferential melting of aluminum as well as the formation of thinner intermetallic phases at the joint interface. In the present study, it has been tried out to investigate the effect of rotational speed, dwell time and pin length on the tensile-shear strength of the Al-5083/St-12 friction stir spot welded joints. The pin diameter was 5 mm while the pin lengths of 2.8 and 3.2 were used to weld the samples. Rotational speeds of 800, 900 and 1100 rpm were applied in association with the dwell times of 5 to 15 s for each of the pin lengths. Thermal history was recorded during the joining process. Interfacial microstructural observation, formation of intermetallic compounds (IMCs) at the joint interface and the fracture locations were studied using stereo, optical and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). The results indicated stronger FSSW joints for the welds made by the tool with the pin length of 3.2 mm. For the pin length of 2.8 mm, there were optimum dwell times (10 and 12 s for the rotational speeds of 1100 and 900 rpm, respectively) to reach the maximum failure loads while the joint strength continuosly increased with the enhancement of dwell time when rotational speed of 800 rpm was applied. The increasing trend of the strength as a function of the dwell time was related to the formation of a thin and non-continuous intermetallic (IM) layer at the joint interface. However, the decreasing trend was attributed to the formation of a relatively thick IM layer at the joint interface as well as the grain growth of aluminum at the exit-hole periphery where the final fracture occurred. From the joint strength point of view, IM reaction layer thickness of 2.3 µm was found to be a critical thickness for the welds made by the pin length of 2.8 mm. Furthermore, for a given dwell time, stronger joints were achieved by application of lower rotational speeds. On the other hand, increasing of the rotational speed and dwell time were both resulted in stronger joints when using the tool with the pin length of 3.2 mm. Microstructural observations revealed that the strength of welds was improved with the increment of intermetallic layer thickness up to 5 µm. Furthermore, the obtained strength of FSSW joints for the welds made by the tool with the pin length of 3.2 mm was related the combination of two major mechanisms, i.e. metallurgical bonding and mechanical locking
- Keywords:
- Intermetallic Compounds ; St-12 Steel ; Aluminum Alloy 5083 ; Friction Stir Spot Welding ; Tensile-shear Strength
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