Date of Award

1-1-2010

Document Type

Campus Access Dissertation

Department

Mechanical Engineering

First Advisor

Anthony Reynolds

Abstract

The joining of aluminum (Al) to magnesium (Mg) by friction stir welding resulted in the intermetallic formation inside the stir zone under all conditions of welding. The presence of brittle and low toughness intermetallic phases results in low strength and ductility of the weld joint. The strength of 6063 Al and AZ31B Mg (3.25 mm) butt weld joint is influenced by the intermetallic layer thickness and the interface characteristics. Transverse tensile strength of the butt weld joint is correlated with the interface length, interpenetrating feature thickness, microvoid coalescence on the fracture surfaces and the maximum intermetallic thickness at the interface.

Lap weld joints of AA5754 (1.5 mm) and AZ31B (1.27 mm) alloys exhibit 50% of the base metal (Mg) tensile strength. The factors that influence the shear strength of the weld joint are the infiltration height, interlayer materials, top sheet material and its flow strength. For similar welding conditions, Al(top)-Mg lap joints have demonstrated high stir zone temperature compared to Mg(top)-Al lap joints, which is attributed to high elevated temperature flow strength of Al. The results indicated that addition of pure Sn as the interlayer material and careful optimization of welding parameters, result in strength improvement up to 35% compared to the joints with no interlayer. High strength of Sn interlayer welds is due to i) formation of low hardness Mg-Sn compounds, ii) solder joint effect from Sn melting and iii) reduction in the area percent of brittle Al-Mg intermetallic compounds. The strength of the joints increases as the area percent of intermetallic compounds in the stir zone is decreased.

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