Laser cladding has been used to repair damaged axles worldwide because of its low dilution rate, high metallurgical bonding strength, and controllable coating thickness. However, the heat input during the cladding process induces martensite transformation in the heat-affected zone (HAZ) of the steel substrate. The low plasticity and fracture toughness of martensite reduce the performance and service life of the axle. Therefore, reducing and eliminating the brittle and hard characteristics of martensite in HAZ have become a research focus. In this study, considering the deposition characteristics of no heat effect on the substrate of cold-spraying, the cold-spraying-laser cladding sequential coupling technology is preliminarily used to prepare a composite structure composed of a laser-cladded Fe314 coating and a Ni30 cold-sprayed intermediate layer on the axle steel EA4T. This study aims to explore the influence of the cold-spraying interlayer on the microstructure and properties of laser-clad axle steel.
Nd∶YAG IPG-4000 laser and cold-spraying systems were used to prepare laser cladding and cold sprayed coatings, respectively. First, a cold-sprayed Ni30 coating with a thickness of approximately 1 mm was prepared on the EA4T axle steel substrate, and then a Fe314 coating was laser-cladded on the cold-sprayed coating, the oxide film of which was removed by grinding. Simultaneously, a single laser-cladding Fe314 coating was prepared using the same process parameters for comparison. An optical microscope and a scanning electron microscope were used to observe the coating, HAZ, and micro-shear fracture morphologies. The distribution and content of the chemical elements in the samples were studied using an energy dispersive spectrometer equipped with a scanning electron microscope. The Vickers hardness of the coatings was tested using a digital microhardness tester with the loading of 200 g and the holding time of 15 s. A micro-shear test was performed on a mechanical testing machine.
The cold-sprayed Ni30 coating is composed of extensively deformed Fe particles and unevenly distributed Ni particles. The pores and interfaces are clarified in the coating, and the coating is mechanically bonded to the substrate [Fig. 5(a)]. The average hardness of the cold-sprayed Ni30 coating is (229.89±11.80)HV, and the hardness of the substrate is (234.63±7.60)HV. The clad zone of the composite coating and that of single cladding coating are similar: owing to the effects of the temperature gradient G and solidification rate R, the grain morphology transfers from the plane crystal to the columnar crystal, and then to dendrites at the cladding layer bottom from the upward interface (Fig. 8), and the microstructures of both HAZs are martensite. The addition of the Ni30 layer has no effect on the morphologies of the cladding layer and HAZ, although it has a dilution effect on the laser cladding Fe314 layer in terms of the decrease in Fe content and increase in Ni content. This occurs at different alloy element contents at the interface after mutual diffusion. In addition, the absorption of laser heat by the cold- sprayed Ni30 reduces the area of the HAZ by 13.39% (Fig. 10). The laser heat causes the extensive plastic deformed Ni30 particle interface to melt, and the mechanical bonding of the cold-sprayed Ni30 coating with axle steel is changed to metallurgical bonding, increasing its shear strength from 35.9 MPa to 224.4 MPa. The average hardness of the clad zone of the composite sample is 275.1 HV, which is lower 56 HV than that of the single cladding, and the shear strength of the clad was slightly lower 25 MPa than that of the single cladding, and the cutting rate of the cross section increases by 53% (Fig. 13). Owing to the lower laser heat input into the HAZ of the composite cladding, the shear strength of the HAZ for the composite layer is lower (233.2 MPa) than that of the single cladding (Fig. 13). Finally, the fracture surfaces of the cladding and HAZ zone for the composite sample are covered with dimples and dense tear edges, suggesting a quasi-cleavage fracture (Fig. 14).
The cold-sprayed Ni30 interlayer has no effect on the microstructure of the cladding or HAZ zones, although the area of HAZ decreases by 13.39%. The interface of the as-sprayed Ni30 intermediate layer is dissolved via laser heating.
The clad hardness of the composite sample is reduced to 275.1 HV, which is lower than the hardness value (331.8 HV) of the single cladding. The hardness of HAZ for the composite sample is reduced from 478.3 HV to 458.2 HV for the single cladding sample.
The shear strength of the cold-sprayed Ni30 interlayer and substrate for the composite sample is 224.4 MPa, which is lower than that (339.6 MPa) of the single cladding interface. The strength of the HAZ of the composite coating is lower by 233.2 MPa than that of the single cladding layer because of the lower heat input absorption of the cold-sprayed Ni30 interlayer.
