Microstructure-property development and defect characterization of Al-X friction welds for automotive applications

Zhou, Nan (2020). Microstructure-property development and defect characterization of Al-X friction welds for automotive applications. University of Birmingham. Ph.D.

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Abstract

Friction Welding (FW) has broad application prospects in the fields of automotive industry. In the industrial production as well as big amounts of literatures, various defects can have a significantly influence on the quality of the FW joints. However, the formation mechanism of the defects in the welds as well as the effect regulation on the properties of the welds have not been systematically studied. Therefore, the present work has been focused on the relationship between the microstructure and mechanical properties in friction welding joints, which covered ordinary butt joints of friction stir welding (FSW), friction stir lap welding (FSLW) as well as linear friction welding (LFW), especially on the characterization of various defects in these friction welds and their influence regulations on the mechanical properties.

FSW butt welds for AA5083-H112 were prepared at various welding parameters. The microstructural and property development throughout the welds were evaluated, with a special focus on the kissing bond defect, which can have a substantial effect on the microstructure and properties of the FSW joints. It was found that the length of the kissing bond had a reasonably linear relation with heat input (HI), which depends mostly on the tool rotation speed as well as feed rate. Moreover, with the increasing length of the kissing bond, the tensile strength and elongation increased first and then decreased, however, the yield strength had no obvious trend. Besides, better fatigue results were shown for the weld with shorter kissing bond. It was also found that the kissing bond can affect the fracture type, which were strongly correlated with the tensile strength and fatigue life.

FSLW of a wrought aluminium alloy (AA6061-T6) to a casting aluminium (as-cast A356) sheets was prepared, with the investigation on the microstructure evolution, second-phase distribution and microscale defects. In the microstructure, kissing bond and another kind of defect named hook were detected. It was found that the kissing bond did not significantly affect the shear behaviour of the joints. However, the hook defect, increased in height and range with the increasing tool rotation speed, was found to have a remarkable influence on the shear behaviour of the joints.

LFW of AA5083 welds and dissimilar AA5083-Cu C101 welds were prepared using different forging pressures, to assess the impact of the defects, as well as the forging pressure on the mechanical properties and microstructural development. In the LFW of AA5083, discontinuous lack of connection defects were detected on the welding interface, where crack source formed on the in the stretching process. With the increase of the forging force, which led to the increase in heat input, the size of the defect was significantly decreased, resulting in the increase of the mechanical properties. In dissimilar AA5083 Cu C101 LFW joints, the LFW process can lead to the formation of an intermetallic phase across the weld interface, which affected the mechanical properties based on the morphology and extent of the intermetallic phases. In general, high forging pressure resulted in the formation of a layered intermetallic phase (identified as Al2CuMg), which was the most detrimental to the mechanical properties, compared with a scattered distribution of fragmented Cu-particles and intermetallics across the joint interface in lower to intermediate forging pressure.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):