Friction stir alloying of immiscible magnesium alloy and mild steel with aluminium and carbon nanotubes as additives / Sufian Raja

Sufian , Raja (2022) Friction stir alloying of immiscible magnesium alloy and mild steel with aluminium and carbon nanotubes as additives / Sufian Raja. PhD thesis, Universiti Malaya.

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Abstract

With growing environmental concerns, the need for more energy-efficient vehicles is bigger today than ever, and producing lighter vehicle support structures is a promising way to increase efficiency. Magnesium alloys welded with steel propose the possibility of weight-efficient structures, with the stronger steel in critical locations and magnesium alloy in locations where a stronger material (usually heavier) is not required. Friction stir alloying (FSA), as a solid-state process, is shown to be a promising method for joining dissimilar/ immiscible metals by using additives to improve joint properties. Dissimilar materials joining AZ31-mild steel using Al-Mg additives and AZ61 magnesium alloy-mild steel using Al-CNT additives were successfully produced by the friction stir alloying process. Al-Mg powder additives were injected in a gap between AZ31 and the mild steel specimen’s butt prior to welding. The experiments were performed for different weight percentages of Al-Mg powder additives at welding speeds of 25 mm/min, 50 mm/min and 100 mm/min with a constant tool rotational speed of 500 rpm. The effect of powder additives and welding speed on tensile strength, microhardness, characterisation across welding interface and fracture morphology were investigated. Tensile test results showed significant enhancement of tensile strength of 150 MPa for 10% Al and Mg (balance) powder additives welded joint compared to the tensile strength of 125 MPa obtained for welded joint without powder additives. The loss of aluminium in the alloy is compensated by Al-Mg powder addition during welding under a suitable heat input condition identified by varying welding speeds. Microstructural analysis revealed that the Al-Mg powder was well mixed and dispersed at the interface of the joint at a welding speed of 50 mm/min. Intermetallic compounds detected in the welding interface contributed to the welding strength. In other material combinations, another promising magnesium alloy, AZ61 used to join with mild steel with Al-CNT additives by the friction stir alloying. Before welding, Al-CNT additives were too injected in a gap between the AZ61 and steel specimens. The experimentation was performed using different welding speeds while tool rotational speed was kept constant for varying weight percentages of carbon nanotubes (CNT) in Al powder. The influence of additives and welding speed on tensile strength and characterisation across the welding interface was studied. Tensile strength of joint with 3% CNT improved up to 222 Mpa. In terms of improving joint strength qualities, CNT has a significant impact. Al-rich Intermetallic compounds were found at the welding contact, while the formation of Al2MgC2 carbide was detected, which contributed to CNT strengthening effects all to improved welding strength.

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