Laser strengthening of U71Mn rail steel was performed under different parameters using a narrow line-shaped CO2 laser beam formed by newly developed binary optics. The microstructure, microhardness and wear resistance of the strengthened layers were investigated. Laser strengthening of PD3 rail steel was performed using a normal circle-shaped beam, and the contact fatigue resistance of samples obtained by optimized laser processing parameters was investigated. The research results showed that a layer with fine martensite was produced on the surface of the rail steels after laser strengthening. The wear resistance to sliding friction was improved over 1.3-2.5 times than that of original steel U71Mn. And the rolling contact fatigue property was improved over 4-5 times than that of steel PD3. The results also indicated that the laser strengthening processing parameters should be selected not to melt and overheat the surface.
This paper presents the research on direct laser deposition of Inconel 738 alloys on directionally solidified (DS) Ni-base supperalloy substrate in order to strengthen or repair the DS gas turbine blades. The results indicate that cracks occur very easily during the deposition process and cracks in laser deposited Inconel 738 on DS Ni-base supperalloy substrate are mostly thermal cracks which may originate on the interface between deposited layers and DS substrate, and develop to multi-deposited layers. The low melting temperature point eutectics between the grain boundaries of DS Ni-base superalloy substrate are the main sources of thermal cracks. Strict control of the heat input of the deposition process can dramatically decrease the cracking tendency. Layers with the DS characteristics are achieved by direct laser deposition on Ni-base superalloy substrate with good shaping and free of cracks by optimizing the deposition technique and laser parameters. Compared with the substrate, the microstructure of the layers is much finer, the average primary spacing of the dendrites is about 5μm. The microhardness of Inconel 738 multi-deposited layers is very uniform, indicating the structure of the layers is homogeneous. The results demonstrate the feasibility and great foreground in repairing and fabricating local parts on DS Ni-base superalloy turbine blades by direct laser fabrication technology.