To improve the weldability of 6061 aluminum alloy and improve the mechanical properties of welded joints, ultrasonic was introduced into the welding process. The microstructure changes of welded joints under different ultrasonic power were studied, and their effects on the mechanical properties of welded joints were analyzed. The grain size was calculated, and the distribution of precipitated phases was observed by the EBSD technique. The results show that the cavitation and acoustic flow produced by ultrasonic vibration can refine the microstructure of welded joint, reduce the grain size by nearly 50%. It promotes the transition of alloy elements to weld and eliminates the segregation of the strengthening phase to the grain boundary, thus improving the mechanical properties of the welded joint.

High-strength low-alloy (HSLA) steel is used in important steel structural members because of its strength and plastic toughness. Q960 steel is HSLA steel obtained by adding an appropriate amount of alloy elements and quenching and tempering treatment on the basis of ordinary low-carbon steel. This kind of steel has strong hardenability due to the alloy elements added. Cold cracks, embrittlement and softening of the heat-affected zone easily occur after welding. In particular, the low-temperature impact toughness cannot meet the requirements and limits its use. In this paper, self-shielded welding is used to adjust the content of aluminum in flux-cored wire. The relationship between weld metal (WM) microstructure and strength and properties was studied by tensile test and impact test, and the influence mechanism of Al content on weld metal microstructure and properties was analyzed. The results show that when the content of Al is 0.21%, the impact energy at 0 °C~−60 °C is the best, the tensile strength can reach 1035 MPA and the number of pores is small. The size of inclusions in WM is mostly less than 1.0 μm Al2O3 spherical oxide. It can become the center of acicular ferrite (AF) and increase the nucleation probability. However, with the increase of Al content, large irregular AlN inclusions are produced, which reduces the tensile strength and impact energy of the welded joint.

In this study, a WC-reinforced Ni-based surfacing layer was prepared on Q235 steel plate by plasma arc welding. The effects of nano-Y2O3 with different contents (0 wt.%, 0.4 wt.%, 0.8 wt.%, 1.2 wt.%, and 1.6 wt.%) on the microstructure, phase composition, microhardness, and wear resistance of the surfacing layer were studied by scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), microhardness test, and pin-on-disk test. The results show that the phase composition of the surfacing layer was γ-Ni, FeNi3 solid solution, WC, W2C, M23C6, M6C, Cr7C3, and other carbides. When the addition of nano-Y2O3 was 1.2 wt.%, it has a good improvement on microstructure grain refinement and carbide hard-phase increase. Compared with other contents, 1.2 wt.% nano-Y2O3 surfacing layer has the highest microhardness and the lowest friction coefficient and wear loss. At this time, the wear mechanism is abrasive wear accompanied by slight adhesive wear.

In this paper, Fe-Cr-C-N-Al-Y wear-resistant surfacing alloy was prepared by open arc surfacing with self-shielded flux-cored wire. The wear properties of the surfacing alloy were analyzed by MLS-23 rubber wheel wet sand wear tester. The phase composition and microstructure of the surfacing alloy were analyzed by XRD, SEM, EBSD and TEM, and the strengthening mechanism and wear mechanism of the alloy were discussed. The results show that the microstructure of Fe-Cr-C-N-Al-Y surfacing alloy is composed of M + γ-Fe + M7C3 + AlN. When the content of nano-Y2O3 is 0.456wt. %, the formability of the surfacing alloy is the best and the wetting angle is the smallest, which is 50.8°, AlN and M7C3 precipitate the most, and the microstructure grain is the smallest. At this time, the hardness value of the surfacing alloy is up to 62.3 HRC, which is 11.8 % higher than that of the unmodified surfacing alloy. The minimum wear weight loss is 0.125 g, and the wear resistance is increased by 41.86 %. The wear mechanism of surfacing alloy is mainly plastic deformation mechanism, and the material removal process is micro cutting and furrow wear.

In this paper, Fe-Cr-C-N-Al-Y wear-resistant surfacing alloy was prepared by open-arc surfacing with self-shielded flux-cored wire. The wear properties of the surfacing alloy were analyzed using an MLS-23 rubber wheel wet sand wear tester. The phase composition and microstructure of the surfacing alloy were analyzed using X-ray diffraction, scanning electron microscope, electron backscatter diffraction and transmission electron microscope; the strengthening mechanism and wear mechanism of the alloy were discussed. The results show that the microstructure of Fe-Cr-C-N-Al-Y surfacing alloy is composed of M + γ-Fe + M7C3 + AlN. When the content of nano-Y2O3 is 0.456 wt.%, the formability of the surfacing alloy is the best, and the wetting angle is the smallest, which is 50.8°; AlN and M7C3 precipitate the most, and the microstructure grain is the smallest. At this time, the hardness value of the surfacing alloy is up to 62.3 HRC, which is 11.8% higher than that of the unmodified surfacing alloy. The minimum wear weight loss is 0.125 g, and the wear resistance is increased by 41.86%. The wear mechanism of the surfacing alloy is mainly a plastic deformation mechanism, and the material removal processes are microcutting and furrow wear. This study provides theoretical guidance for refining primary carbides and improving the wear resistance of high chromium cast iron.