Laser spot welding was used for the dissimilar joining of aluminum alloy to steel in the keyhole mode. The results showed that the defocusing amount and laser power had significant influence on the weld formation. The aluminum alloy was prone to the formation of welding defects, such as porosity and cracks in the fusion zone, which resulted in an extremely instable welding process and poor joint strength. In order to improve the weld quality, a copper heat sink was placed under the aluminum alloy, which effectively absorbed the heat of the aluminum alloy and reduced the previous reported welding defects. The influence of laser power, defocusing amount, welding time, and shielding gas on the joint characteristics was also investigated. The microstructure analysis showed that a conelike fusion zone and sparsely distributed Fe-Al intermetallic compounds were formed. Hence, a combination of metallurgical bonding and mechanical interlocking was achieved in the laser keyhole spot welds, enhancing the joint mechanical properties. The joint strength of the laser keyhole spot welds reached 620 N/mm, which was comparable to that obtained in resistance spot welds.
Nanostructured Al2O3-13wt.%TiO2 ceramic coatings were prepared on the TiAl alloy surface by using the plasma spraying and plasma spraying-laser remelting composite technology, respectively. Hot corrosion behaviors of these two coatings in 75%Na2SO4+25%NaCl (mass fraction) fused salts under 850 °C were discussed. Moreover, phases and microstructures of specimens after the corrosion were analyzed by X-ray diffractometer (XRD) and scanning electro microscope (SEM). Effects of laser remelting on hot corrosion resistance of coatings were explored. Results demonstrated that hot corrosion of ceramic coating prepared by plasma spraying is serious, but the hot corrosion resistance can be improved effectively after laser remelting. Laser remelting specimens show the higher hot corrosion resistance for two reasons. On the one hand, laser remelting forms a uniform and dense remelting layer by eliminating the lamellar structures and most pores of the coating layer, thus decreasing the corrosion diffusion channels during hot corrosion. On the other hand, laser remelting changes the metastable-phase γ-Al2O3 into stable-phaseα-Al2O3.
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