Microstructures and physical properties of laser amorphous reinforced composite coatings

1 Introduction

Titanium and its alloys are employed in industry because of their remarkable characteristics: high strength to weight ratio, excellent corrosion resistance and good performance [1–4]. Amorphous alloys are multi-component and approximate deep-eutectic alloys that offer extraordinarily high hardness, elastic limit and corrosion resistance. Laser surface processing has been used to produce amorphous reinforced composite coatings on metal substrate, leading to improvement of surface performance of the substrate [5, 6].

In the past few years, efforts have always focused on the microstructure analysis of martensites and autensite in laser clad coatings, while less work has been carried out on the formation mechanism of ledeburites in laser alloying composite coatings. In this study, the formation mechanism of ledeburites in laser alloying was extensively researched, and it was confirmed that laser alloying of the Al-Fe-Co-C mixed powders on the important aeronautical material TA2 titanium alloy can form the amorphous reinforced coating, and the ledeburites were included in such a coating. The Al-Fe-Co-C powders are interesting in the scheme of research concerned with laser alloying of hard ceramics and amorphous reinforced composite coatings. Moreover, during the laser alloying process, a large quantity of elements, such as Fe and Si entered into the molten pool from the substrate due to the dilution effect, which changed the microstructure performance of laser alloying coatings, and a series of amorphous alloying with high glass forming ability in Fe- and Si-based alloy systems were produced in such a coating [7]. In this study, the microstructures and physical properties of amorphous reinforced laser alloying composite coatings on a TA2 titanium alloy were investigated in detail.

2 Experimental

The materials used in this experiment, the TA2 samples, size of 10 mm×10 mm×10 mm, were abraded with abrasive paper prior to the coating operation. Chemical compositions of the TA2 alloy: 0.30Fe, 0.10Si, 0.10C, 0.05N, 0.25O, 0.0015H, and balance Ti. The pre-placed powders of Co (≥99.5% purity, 50∼150 μm), Fe (≥99.5% purity, 50∼150 μm), Al (≥99.5% purity, 50∼150 μm), C (≥99.5% purity, 150∼250 μm), and TiC (≥98.5%, 50∼100 μm) were used for the laser alloying. The thickness of the pre-placed coating was approximately 0.8 mm. A 5 kW continuous CO₂ laser (HL-T5000) was employed to melt the surface of samples. During alloying process, argon gas at a pressure of 0.4 MPa was fed through a nozzle which was coaxial with the laser beam. Process parameters of laser alloying: laser power P=1 kW, scanning velocity V=3∼7.5 mm/s and the laser beam diameter D=4 mm. Too high power can burn out a portion of the pre-placed powders, and also increase the dilution rate of the substrate to the coating, influencing the quality of the coating, so 1 kW has been selected in this study. An overlap of 35% between successive tracks was selected. The compositions (wt%) of the pre-placed powders: 20Co-45Al-5Fe-30TiC (sample 1) and 20Co-45Al-5Fe-30C (sample 2).

Metallographic samples were prepared using standard mechanical polishing procedures and then etched in a solution of HF, HNO₃ and H₂O in a volume ratio of 2:1:10 to reveal the growth morphologies of the compounds in laser alloying coatings. The microstructural morphologies of the coatings were analyzed by means of a QUANTA200 scanning electron microscope (SEM) and a JEM-2100 high resolution transmission electron microscope (HRTEM). The phase constituent of the coatings was determined by X-ray diffraction (XRD) using a D/MAX-RC equipment. Differential thermal analysis was performed using a DZH8DZ3320A differential thermal analyzer.

3 Results and analysis

As shown in Figure 1A, the microstructure of the laser alloying composite coating in sample 1 was austenite γ-Co solid solution and was in cell and cell-dendrite morphology. Owing to a rapid cooling rate of molten pool, a small amount of elements, such as Fe and Si, had no time to precipitate from the liquid and solution in γ-Co to form super-solution, which caused the solution strengthening [8]. Ti-Co binary intermetallic alloy has a uniform and dense microstructure consisting of predominantly the primary dendrites and minor amount of interdendritic eutectic-like structure (see Figure 1B).

Figure 1

SEM micrographs of the composite coating in samples 1 (A, B) and 2 (C, D): (A) microstructure, (B) the TiC morphology, (C) overview cross-section, and (D) microstructure.

As shown in Figure 1C, the fine and compact microstructure coating was obtained in sample 2. It was interesting to note that ledeburites were produced in such a coating (see Figure 1D). In fact, during the solidification process of the molten pool, the ledeburites were produced in a liquid iron-carbon alloy. However, due to the different carbon content, the produced solid alloy consisted of ledeburites and the other compositions. Before the temperature decreased to the eutectic temperature, austenites and cementites precipitated. When the temperature decreased to approximately 1147°C, the eutectic transformation occurred in the retained liquid alloying, leading to the formation of ledeburites [9, 10]. After the alloying process, ledeburites and first cementites were produced in such a composite coating.

As shown in Figure 2, there were Ti₃Al, TiAl, Co-Ti intermetallics, TiC, γ-Co, and α-Ti in the coatings of samples 1 and 2. According to XRD result, it was known that during the alloying process, Ti reacted with Al in a molten pool, leading to the formation of Ti₃Al and TiAl. Due to the low Fe content, the diffraction peaks of Fe and its compounds were not found in this XRD pattern. As shown in the XRD diagram, it was interesting to note that a high α-Ti diffraction peak was present in the coating of sample 2. In fact, the temperature during the β-α transformation in the Ti alloy decreased from about 882°C to 850°C. In this process, when the cooling rate was >200°C/s, the martensite transformation completed without the diffusion, and the α-Ti acicular martensite was produced. It was considered that more C content in sample 2 led to higher TiC being produced during the laser alloying process, and in such a coating, the growth of TiC was retarded by γ-Co to a certain extent, favoring the formation of block-shape precipitates [11]. A great number of TiC absorbed more energy from the laser, which increased the super-cooling degree, forming a high α-Ti diffraction peak.

Figure 2

XRD diagrams of the composite coatings in samples 1 and 2.

Moreover, it was also noted that the TiC diffraction peak in sample 1 was significantly higher than that of sample 2. In fact, due to the dilution effect, a lot of Ti entered into the molten pool from the TA2 substrate, then C reacted with Ti, leading to the formation of TiC in a molten pool. Due to the dilution effect, these reactions mainly occurred in the bottom of molten pool. Thus, under the action of a great super-cooling degree, a portion of TiC did not have enough time to float onto the coating surface, so its diffraction was lower compared with that of sample 1. The broad diffraction peaks that appeared at 2θ=35°∼43° indicated that the amorphous phases were produced in such coatings. The atomic (C) packing model distortion in the short program range led the solid/liquid interfacial energy to increase, which promoted the production of amorphous phases [12]. Moreover, due to the rapid solidification rate of the laser molten pool, the Fe- and Si-based amorphous phases were produced in such a coating [13].

As shown in Figure 3A, the micro-crack was produced in the coating of sample 2. In fact, the production of the ledeburites increased the micro-hardness and brittleness of the coating, forming the micro-crack. As mentioned previously, during the solidification time of the molten pool, cementites precipitated along the crystal boundary of austenites, leading to the formation of the second cementites. When the temperature decreased to approximately 727°C, the austenites changed to pearlites (see Figure 3B). Figure 3C and D were the test location, and its corresponding electron diffraction pattern, respectively. As shown in Figure 3C, the rod-shape morphologies of ledeburites were present. As shown in Figure 3D, two small spots in the direction of the white line of the Selected Area Electron Diffraction (SAED) pattern revealed that fine stick-shape crystals were produced in the coating [14], which corresponded with the SEM result of Figure 3B. Moreover, an obvious diffraction ring was present in the SAED pattern, which proved the production of the amorphous phases.

Figure 3

SEM micrographs of the composite coating in samples 2 (A, B), and TEM of ledeburites and its electron diffraction pattern (C, D).

The high stress and a great number of the dislocation and stacking faults existed in TiC crystals. The sliding motion was produced in such a dislocation and stacking fault under the action of the laser alloying technique, leading to the formation of the dislocation accumulation, which increased the micro-hardness and brittleness of the coating, so the micro-crack was produced (see Figure 4A). Due to the shock heating and chilling effect of the laser rapid melting, the large stress was produced. Thus, the stress caused by the laser alloying process superimposed with the stress which was caused by ceramics, resulted in the formations of the stress fringes (see Figure 4B).

Figure 4

TEM morphologies of the micro-crack (A) and stress fringes (B), HREM morphologies of the coating (C), and DSC curve of alloying powder (D).

It was confirmed that the irregular amorphous spots were present in the diffraction pattern of the coating (see Figure 4C). The HRTEM image indicated that the Ti₃Al high-resolution lattice was produced, which corresponded to its (3 1 2) crystal plane.

In order to research the reaction of the alloy powders during the temperature increasing process, differential scanning calorimeter (DSC) analysis was performed. As shown in Figure 4D, the sharp exothermic peak was present in 1181°C, indicating that the exothermic reaction occurred at this temperature. In fact, during the alloying process, the low-melting eutectics were firstly produced in molten pool, and according to the DSC test result, it was known that with increase of the temperature C melted and dissolved into the molten pool, which then reacted with Ti and Fe at 1181°C, leading to the formation of TiC and α-Fe.

4 Conclusions

In summary, an amorphous reinforced coating was fabricated on the important aeronautical material TA2 alloy by laser alloying of Co-Al-Fe-TiC mixed powders. Laser alloying of Al-Fe-Co-C mixed powders on TA2 alloy can also form amorphous reinforced coating, which included the ledeburites. More TiC ceramics were produced in the Al-Fe-Co-C laser alloying coating, which increased the super-cooling degree of the laser molten pool, favoring the formation of ledeburites. The eutectic transformation occurred in the retained liquid alloys, leading to the formation of ledeburites. During the alloying process, low-melting eutectics were produced, which promoted the formation of amorphous phases. With the increase of temperature, C melted and dissolved into the molten pool, which then reacted with Ti and Fe, forming TiC and α-Fe.