Study of the Laser Remelting of a Cold Sprayed Titanium Layer

Abstract

Cold Gas Dynamic Spray is an emerging coating technology based on the use of a supersonic gas jet to accelerate (up to 1000 m/s) and to impact a powder, with size ranging from 1 to 50 μm in diameter, on a substrate. Due to the high speed, during the impact, the powder undergoes a severe plastic deformation such that it adheres on the substrate. Thanks to this method, it is possible to produce up to fully dense metallic coatings on substrates of different materials. With this technology, different kinds of powder (metals, polymers, ceramics, composite materials and nanocrystalline powders) or their mixing can be deposited. Among the various possible powders that can be deposited by cold spray, titanium is one of the most attractive materials thanks to its potential applications. A titanium layer produced onto a softer materials (i.e. aluminium alloys) could improve both the corrosion resistance and the wear properties of the components. However, the coating made with this technique could be affected by several problems, such as porosity, high roughness and low mechanical properties. A possible solution for this issues is the use of laser remelting post-deposition treatment. The present investigation deals with the application of a continuous wave diode laser in order to change the coating properties and metallographic structure. With this aim, laser remelting of a titanium cold sprayed layer were carried out on samples of grade 2 titanium alloy, 5 mm thick, obtained by cold spray technique, by using a 220 W diode laser at different scan speed. In order to avoid the influence of the particular substrate, the laser remelting process was carried after the detachment of the coating from the substrate. After laser treatments, light and SEM microscopy were carried out to analyze the geometry of remelted zone and the evolution of its microstructure morphology. Moreover, micro-hardness measurements were made to evaluate the mechanical properties. Three different metallurgical structures corresponding to the remelted zone, the heat affected zone and base material were observed. The remelted zone showed an elliptical shape, with a depth up to 0.7 mm and a martensitic microstructure. Furthermore, in this zone, hardness higher than the base material (more than threefold) was found. In conclusion, it is possible to affirm that the laser remelting is a promising technique to improve the superficial properties of titanium cold sprayed layers

Post-Process Treatments on Supersonic Cold Sprayed Coatings: A Review

“…Similarly, a transition zone was observed between the RZ and HAZ (Figure 16f), where some acicular martensitic α grains were formed due to the high cooling rate. These microstructural features presented in different zones would in turn influence the mechanical properties of the coatings [81]. The hardness of the HAZ was lower than the one measured in the BM due to the formation of the coarser lamellae in the HAZ (Table 5).…”

“…These microstructural features presented in different zones would in turn influence the mechanical properties of the coatings [81]. The hardness of the HAZ was lower than the one measured in the BM due to the formation of the coarser lamellae in the HAZ (Table 5).…”

“…Astarita et al [81] and Rubino et al [82] employed LR to modify cold sprayed pure Ti coatings and it was found that the inter-particle porosity in the top layer of a coating was eliminated (Figure 16a). The densification of the LRed region was due to its melting and micropores escaping to the free surface with the subsequent solidification.…”

“…Surface oxides may also reduce the material's ductility and ability to resist fatigue and creep [83]. Astarita et al [81] and Rubino et al [82] also studied the effect of laser scan speed on cold sprayed Ti coatings. It was found that material vaporization occurred and surface cracks were produced when the laser scan speed was very low due to excessive heat input.…”

“…Ti [81,82] Spot diameter: 2 mm After LR, three different metallurgical zones were observed: a re-melted zone (RZ), a heat affected zone (HAZ) and the base material (BM). Spot diameter: 1 mm The laser treated Ti64 coatings with a higher laser power had a higher surface hardness.…”