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Aluminum oxide nanopowder has been fabricated and characterized successfully to improve the properties of friction stir welded aluminum/titanium joints, which pose a great challenge due to the great difference in mechanical properties. Intermetallic compounds (IMCs) of large sizes formed at the welding interface are a significant problem that deteriorates joint strength due to the uncontrollable thermal effect generated in the joint. Therefore, friction stir processing was integrated with friction stir welding, in which a dissimilar lap joint of AA6061-T6 and Ti6Al4V was reinforced with γ-Al 2 O 3 nanoparticles of about 8 nm in diameter. First, several attempts were conducted to determine the most effective parameters in the welding process of the aluminum/titanium reinforced joint and then the effects of γ-Al 2 O 3 nanopowder on microstructure, mechanical properties, and weldability were analyzed and compared to a welded joint without reinforcing powder. Sound joints have been successfully produced using welding speed of 25 mm/min, and tool rotational speed of 600 rpm. Lap shear fracture load and displacement of the Al/Ti joint increased from 3.5 KN and 1.25 mm to 3.7 KN and 2.45 mm, respectively, while the hardness of the reinforced joint increased from 384 HV to 391 HV at the Al/Ti weld interface with a remarkable improvement at the Al and Ti sides. • Al/Ti FSWed joint was reinforced with aluminum oxide nanoparticles (γ-Al 2 O 3 NPs). • The effect of γ-Al 2 O 3 NPs on microstructure, mechanical properties, and strengthening of the joints was discussed. • γ-Al 2 O 3 NPs refine the grains at Al/Ti interface and reduce the thickness of Al/Ti intermetallic compounds. • γ-Al 2 O 3 NPs improve the tensile strenght, ductility, and hardness of the Al/Ti joint. [ABSTRACT FROM AUTHOR]

• The embrittlement X factor for weld metal is 45, which is three times more than the recommended value (≤15). • It is apprehended that the fractured metal part could be susceptible to temper embrittlement. • The Charpy impact energy absorbed by the fractured metal is 13 J, which is lower than impact energy absorbed by the base metal 300 J. • The scanning electron microscopy demonstrated intergranular crack propagation, discontinuous cracking along grain boundaries, and blunted cracks at grain interiors. • The fractography is performed on the fracture surface of the impact test specimens and it indicated heavily cavitated intergranular facets along the grain boundaries and deep secondary cracks on the fracture surface. • These findings suggested that the severe cracking observed in the fire tube at the U-bend is likely due to temper embrittlement (TE) induced by exposure of the fire tube material to elevated temperatures. Overheating, corrosive degradation, hydrogen embrittlement, creep, erosion, and corrosion are the cause of most failure problems in fire tubes. Herein, we have evaluated the macrostructural, microstructural, and mechanical properties at welded U-bend of the fractured fire tube to analyze the failure of the SA 387 grade 22 steel heater treater fire tube that has been under continuous use for 18 years at an oil refinery. We have found that the embrittlement factor at U-bend proved to be the most significant factor in its failure. The embrittlement X factor for weld metal is 45, which is three times more than the recommended value (≤15). It is apprehended that the fractured metal part could be susceptible to temper embrittlement. There is a significant higher hardness value of weld metal as compared to base metal, which is due to metallic phase transition at high temperature. The Charpy impact energy absorbed by the fractured metal is 13 J, which is lower than impact energy absorbed by the base metal 300 J. The macroscopic examination reveals that the crack initiated on the emulsion side and propagated radially in welded U-bend before extending further into the base metal in the axial direction. The scanning electron microscopy demonstrated intergranular crack propagation, discontinuous cracking along grain boundaries, and blunted cracks at grain interiors. The fractography is performed on the fracture surface of the impact test specimens, and it indicated heavily cavitated intergranular facets along the grain boundaries and deep secondary cracks on the fracture surface. Energy disperse spectroscopy identified the carbides formation at the U-bend cracking parts, which counter confirm the microstructural phase transition. These findings suggested that the severe cracking observed in the fire tube at the U-bend is likely due to temper embrittlement (TE) induced by exposure of the fire tube material to elevated temperatures. [ABSTRACT FROM AUTHOR]