Teng Wu

Laser cladding technology is a surface modification technology emerging in recent years, and it is widely used in the marine engineering field. Since the structural steels used in marine engineering are mostly carbon steels, their wear resistance and corrosion resistance are poor. In this paper, 316 L stainless steel/TiC composite coatings with different Y₂O₃ addition amounts were fabricated on Q355B steel surface using the laser cladding technology, and the phase composition, microstructure, microhardness, wear resistance, and corrosion resistance of the coatings were investigated with an X-ray diffractometer (XRD), a scanning electron microscope (SEM), a digital microhardness tester, a friction and wear tester, and an electrochemical workstation. When Y₂O₃ is added, the crystals in the bottom part of the coating are basically broken and the organisation becomes more dense. When the Y₂O₃ addition amount is 2%, the hardness is highest, the frictional coefficient curve of the coating exhibits a stable descending trend, and the coating mass loss is the lowest. When Y₂O₃ is added at 1%, the dynamic potential polarisation curve shows high corrosion potential and low corrosion current density and exhibited the best performance in EIS. The added Y₂O₃ evidently improves the wear resistance and corrosion resistance of 316 L stainless steel/TiC composite coatings, and this provides a new possibility that a composite coating modified with rare earth oxide is used for repair of marine engineering structures.

To explore the influence of different welding modes on the properties of 316L thin-plate welded joints, a new type of laser arc compound gun head similar to a coaxial one was used in this experiment. A high-speed camera was used to record the welding process and analyze the droplet splash behavior of the molten pool. The microstructure, microhardness change, and tensile test results of welded joints under different welding modes were analyzed. The results showed that laser welding (LW) is more prone to molten pool splash than hybrid laser arc welding (HLAW). The HLAW pool area was significantly increased compared with that of LW. The HLAW joint microstructure was more uniform than that of LW, which can improve the microhardness of welded joints. HLAW improved the tensile properties of the joint, with the maximum tensile strength of the joint increasing from 433 to 533 MPa. This test can provide guidance for the HLAW process.

The high cooling rate and temperature gradient caused by the rapid heating and cooling characteristics of laser welding (LW) leads to excessive thermal stress and even cracks in welded joints. In order to solve these problems, a dynamic preheating method that uses hybrid laser arc welding to add an auxiliary heat source (arc) to LW was proposed. The finite element model was deployed to investigate the effect of dynamic preheating on the thermal behavior of LW. The accuracy of the heat transfer model was verified experimentally. Hardness and tensile testing of the welded joint were conducted. The results show that using the appropriate current leads to a significantly reduced cooling rate and temperature gradient, which are conducive to improving the hardness and mechanical properties of welded joints. The yield strength of welded joints with a 20 A current for dynamic preheating is increased from 477.0 to 564.3 MPa compared with that of LW. Therefore, the use of dynamic preheating to reduce the temperature gradient is helpful in reducing thermal stress and improving the tensile properties of the joint. These results can provide new ideas for welding processes.

The effect of 60Si2Mn substrate preheating on the forming quality and mechanical properties of cobalt-based tungsten carbide composite coating was investigated. Substrate preheating was divided into four classes (room temperature, 150 °C, 250 °C, and 350 °C). The morphology, microstructure, and distribution of elements of the coating were analyzed using a two-color laser handheld 3D scanner, a scanning electron microscope (SEM), and an energy dispersive X-ray spectrometer (EDX), respectively. The hardness and wear properties of the cladding layer were characterized through a microhardness tester and a friction wear experiment. The research results show that the substrate preheating temperature is directly proportional to the height of the composite coating. The solidification characteristics of the Stellite 6/WC cladding layer structure are not obviously changed at substrate preheating temperatures of room temperature, 150 °C, and 250 °C. The solidified structure is even more complex at a substrate preheating temperature of 350 °C. At this moment, the microstructure of the cladding layer is mainly various blocky, petaloid, and flower-like precipitates. The hardness and wear properties of the cladding layer are optimal at a substrate preheating temperature of 350 °C in terms of mechanical properties.