Influence of Laser Remelting on Creep Resistance in Ti-6Al-4V Alloy with Thermal Barrier Coating

Abstract

Ti-6Al-4V alloys with a thermal barrier coating (TBC) have been applied in aeronautical components as turbine blades to provide oxidation resistance and thermal protection, enabling higher operating temperatures and extending component lifespan. Research into TBCs with laser surface modification has investigated improving their mechanical and thermal properties. This study assessed the creep behavior of Ti-6Al-4V alloy with a TBC, where the coating was applied via CO₂ laser-remelted plasma spraying. Creep tests were conducted at a constant temperature and a load ranging from 500 to 700 °C at 125 MPa. The microstructure and fractography of the specimens were also investigated. The investigation also included microstructural and fractographic analyses of the specimens. The results indicate that the laser-remelted TBC provided effective thermal protection and increased oxidation resistance, with the stationary creep rate at 600 °C reduced by 50% and the creep rupture life extended by 20%. Observations revealed typical ductile fractures characterized by equiaxed dimples and a homogeneous microstructure with an equiaxed dual-phase (α+β) structure near the fracture zone.

1. Introduction

High-temperature materials have been used even more [1], including in energy generation, chemical processing, and gas turbine materials [2]. To increase the performance of structural components, these materials can reach critical conditions of temperatures and pressures [3]. In order to combine good properties at high temperatures and resistance to degradation processes over long periods, studies have been carried out to obtain new alloys [4,5,6] and, mainly, for the improvement of existing commercial alloys [7,8,9].

Creep is a phenomenon through which the material undergoes a slow and continuous deformation over time. Creep is related to plastic deformation, and it becomes significant when the material is applied at relatively high temperatures [10]. This phenomenon has been seen as a challenge to be overcome for components applied in the aeronautical industry and mathematical models to predict creep behavior [11,12,13].

Titanium alloys are widely used in aeronautical components due to their characteristics of a high strength–weight ratio [14,15,16]. Ti-6Al-4V is an α-β alloy that offers a combination of strength, toughness, and high-temperature properties [15]. Despite its high melting temperature (≈1650 °C), Ti-6Al-4V does not maintain satisfactory strength and stability above 500 °C due to the alloy oxidation [17].

Thermal barrier coatings (TBCs) provide oxidation and hot corrosion protection of components applied at high temperatures [18,19,20,21,22,23]. TBC systems have been required for components used in hot parts of gas turbine engines [24], and studies have shown that the Ti-6Al-4V alloy with TBC promotes an improvement in mechanical behavior [7]. The TBC consists of a multilayer system: a bond coat (BC) over the substrate, generally MCrAlY type (M = Ni, Co, Fe, or combinations thereof); a thermally grown oxide (TGO) produced by a BC oxidation process; and a top coat (TC), usually consisting of yttria-stabilized zirconia (YSZ), providing the thermal barrier [25,26].

Air plasma spray (APS) is a technology for applying a relatively thick coating commonly used in many industrial sectors, including aeronautics and gas turbine blades. The technique stands out for its applicability to a large range of materials and rather simple operation [24,25,26,27].

Thermal barrier coatings (TBCs) produced by plasma spray exhibit cracks and open pores, which further increase permeability in the face of reactive environmental elements such as sulfides, vanadium, and oxygen [28]. TBC laser remelting is a technique that has been studied to modify the YSZ surface by creating a densification of the top layer [29]. Studies have shown that TBC laser remelting increases resistance to isothermal and cyclic oxidation, reduces oxidation and corrosion phenomena, and also promotes increased resistance to thermal shock [28,29,30,31].

Studies are being conducted with the aim of reducing surface defects, and thus enhancing the durability of the coatings by minimizing gas infiltration through the zirconia top coat [32,33]. To achieve TBCs with complete fusion of the ceramic layer and good quality, it is essential to use a CO₂ laser with a power density between 4.0 and 8.0 J/mm² [5]. Furthermore, studies have shown that a porous layer containing microcracks, which remain beneath the laser-treated layer, facilitates the absorption of thermal stresses during the heating and cooling cycles of the material [28,29,30,31,32,33].

The laser remelting and other superficial treatments have been widely studied and applied to improve the creep lifetime of the Ti-6Al-4V alloy [7,11,12,13,34,35,36,37]. The creep behavior of the Ti–6Al–4V alloy with a metallic layer and TBC (YSZ) [7,34,35,37] by APS was investigated. The results indicated that the combination of metallic and ceramic coatings is effective in improving the mechanical resistance of the alloy on creep, and that the creep rates of the coated alloy were lower than those of the uncoated alloy in air. The plasma-sprayed coatings increased the time to rupture, and the strain at rupture was smaller than for the uncoated samples tested in air [35]. However, there is a lack of studies on the creep behavior of Ti-6Al-4V with thermal barrier coating (TBC), and those investigating the creep behavior of Ti-6Al-4V with TBC following laser surface remelting are scarce in the literature.

In this study, the creep behavior of the equiaxed Ti-6Al-4V alloy with a thermal barrier coating (TBC) deposited by air plasma spray (APS) and subjected to continuous CO₂ laser treatment was evaluated. The TBC samples were compared before and after laser remelting. Additionally, the effects of varying temperatures on mechanical performance and the fractographic analysis of the specimens were investigated.

2. Materials and Methods

Commercial Ti-6Al-4V was used for creep test samples, and these were machined with specific dimensions for the available system, according to ASTM E139 [38] and ASTM B265 [39] standards. The microstructure consists of equiaxed grains with the presence of α and β phases.

The thermal barrier coating application was carried out at General Electric (GE Celma—Petrópolis, Rio de Janeiro, Brazil) using the same procedure for repairing the turbine blade coating. Air plasma spray was used for the layers’ deposition: (1) application of the metallic layer (NiCrAlY—Bond Coat Amdry 962, Oerlikon Metco, Pfaeffikon, Switzerland) on the substrate; and (2) application of the ceramic layer (8% YSZ—Top Coat Metco 204NS-G, Oerlikon Metco) on the bond coat. The process was performed using the following equipment: Sulzer Metco 9M pistol, Sulzer Metco 9MP powder feeder, and Praxair PC-100 gas controller (Praxair, Indianapolis, IN, USA). The thicknesses of the metallic and ceramic layers were 67 ± 16 µm and 284 ± 18 µm, respectively.

Laser remelting treatment was carried out on coated samples using a continuous CO₂ laser (Synrad—ti100 series, Mukilteo, WA, USA) with a characteristic wavelength of 10.6 µm and beam diameter of 162 µm, when focused at 100 mm. A scan head (Raylase—miniscan II-14, Weßling, Germany) controlled by software (WinLase Professional, https://lanmarkcontrols.com/) was used to perform the laser-remelting process. To realize the procedure on cylindrical specimens, a mechanical device with a stepper motor driven by a microcontroller was used to precisely control the rotation. According to the previous study [7], the laser parameters used were a laser power P = 34 W, scanning velocity V = 200 mm/s, laser power per area unit E = 6.105 W/cm², and laser output energy density J = 1.05 J/mm².

Constant load creep tests were performed in air using a standard creep machine, following the ASTM E139 [38] standard. The tests were conducted at temperatures ranging from 500 to 700 °C and at a load of 125 MPa. The specimens tested included equiaxed Ti-6Al-4V with a thermal barrier coating (TBC) and Ti-6Al-4V with laser-remelted TBC, referred to as TBC and L-TBC, respectively.

Fractographic analysis was performed using scanning electron microscopy (FEI model Inspect S50, Hillsboro, OR, USA). An optical microscope (OPTON model TNM-07T-PL, Zurich, Switzerland) was used to obtain the microstructure images of titanium alloy after the metallographic procedure, which consisted of abrading with silicon carbide papers, polishing with a silica suspension, and subsequently etching with Kroll’s reagent (5 mL HNO₃, 3 mL HF, and 100 mL of distilled water), and was carried out on transverse and longitudinal sections in a region close to the fracture.

3. Results and Discussion

Figure 1 illustrates the creep curves of strain (ε) versus time (h) for TBC and L-TBC at temperatures ranging from 500 to 700 °C and a load of 125 MPa. All tests were carried out until the specimens ruptured.

Under all conditions, the creep curves exhibit typical behavior, with the majority of the creep life characterized by a constant creep rate. This behavior is likely due to a stable dislocation configuration resulting from the recovery and hardening processes.

The results of the creep tests are summarized in

Table 1. Creep data of TBC and L-TBC at 125 MPa.

Sample	T (°C)	tp (h)	$\dot{{\mathit{\epsilon }}_{\mathit{s}}}$ (1/h)	tr (h)	εf (mm/mm)
TBC	500	27.30	0.0002	409.00	0.1922
	600	0.21	0.0187	6.16	0.4092
	700	0.01	0.3050	0.25	0.2458
L-TBC	500	30.00	0.0002	598.10	0.2723
	600	0.52	0.0094	7.50	0.1990
	700	0.01	0.7893	0.21	0.4429

. In this table, ($\dot{{\epsilon }_s}$) represents the steady-state creep rate, t_p denotes the primary creep time, t_r indicates the time to rupture, and ε_f refers to the strain at fracture.

Creep phenomena are typically predicted based on the secondary stage rate, as this stage usually lasts the longest and exhibits a constant rate [10,35]. The steady-state creep rate thus reflects the material’s resistance to plastic deformation under the applied temperature and stress, with higher ($\dot{{\epsilon }_s}$) values and lower tr values indicating greater creep resistance [35]. By comparing the TBC and L-TBC samples, it can be concluded that CO₂ laser treatment of the TBC, with an energy of 1.05 J/mm², improved the creep behavior under all conditions except at 700 °C, with the most significant improvement observed at 600 °C.

The laser-remelted TBC sample at 500 °C maintained the steady-state creep rate and increased rupture time by approximately 45%. At 600 °C, the L-TBC sample demonstrated a more substantial improvement in creep resistance, reducing the secondary stage rate by 50% and extending the creep lifetime by 20%. However, at 700 °C, the trend reversed, with the TBC sample exhibiting greater creep resistance. Creep tests on Ti-6Al-4V alloy at 700 °C are known to be particularly critical, and surface defects can accelerate creep deformation under such conditions [7,35,39].

The properties of the remelted layer are significantly affected by the laser treatment parameters, which in turn influence characteristics such as surface and transverse cracks, oxygen penetration, and microstructure. These factors are directly related to creep behavior, impacting both the increase and decrease in creep life. Therefore, exploring various laser parameters and techniques to reduce surface cracks—such as preheating the part during the remelting process—could be beneficial in modifying the remelted layer’s properties and enhancing the creep life of the samples.

Moreover, it is important to note that the TBC laser-remelting process significantly enhances the oxidation resistance of the substrate. The observed improvement in the creep life of the L-TBC samples can be attributed to enhanced thermal protection and increased oxidation resistance. However, the network of surface cracks and their propagation, induced by the stresses applied during testing, can impede the overall performance.

Equation (1) is the power law creep equation that describes the creep behavior, where B is the structure-dependent constant and n is the creep stress exponent [10].

$$\stackrel{\prime }{{\epsilon }_s}=B{\mathsf{\sigma }}^n$$

(1)

Equation (2) is the power law creep equation represented by an Arrhenius, where Q_c is the activation energy for creep, B₀ is a constant that depends on the microstructure, temperature, and applied stress, n is the stress exponent, R is the universal gas constant, and T absolute temperature [10].

$$\stackrel{\prime }{{\epsilon }_s}=B_0{\mathsf{\sigma }}^n\left(\exp {\displaystyle \frac{-Q_c}{RT}}\right)$$

(2)

Figure 2 illustrates the relationship between steady-state creep rate and temperature at 125 MPa. The creep activation energies determined for TBC and L-TBC were 233.78 kJ/mol and 259.82 kJ/mol, respectively.

Creep deformation is a thermally activated process, and the values of Q_c are often found to be close to the activation energy for lattice self-diffusion [10]. This is consistent with test conditions, where temperatures exceeding approximately 0.4 Tm (melting temperature) facilitate atomic movement through enhanced diffusion [8]. For Ti-α, the self-diffusion activation energy typically ranges from 242 to 293 kJ/mol [40].

The stress exponent derived from the secondary creep rate and the activation energy for creep can provide insights into the underlying creep mechanisms. When describing steady-state creep behavior using power law relationships, the values of n and Q_c may vary across different stress and temperature regimes [10]. Reis et al. [35] analyzed the creep rate of equiaxed Ti-6Al-4V with and without a thermal barrier coating (TBC) and suggested that, at 500 and 600 °C, the creep mechanisms align with the lattice diffusion-controlled dislocation climb process in Ti-α. Reis et al. [35] reported a stress exponent of n = 4.25 and an activation energy of Q_c = 218.0 kJ/mol for equiaxed Ti-6Al-4V without treatment over the temperature range from 500 to 700 °C. Briguente et al. [37] studied creep tests on Ti-6Al-4V alloy in air with TBC (YSZ and CoNiCrAlY), as well as uncoated samples at 600 °C. Their findings indicated that the steady-state creep rate of the coated alloy was lower than that of the uncoated alloy in air. The activation energy of Q_c = 319 kJ/mol and the stress exponent n = 10.65 for the TBC suggest that the steady-state creep rate is likely controlled by dislocation climb. Freitas et al. [7] investigated the microstructure and creep behavior of laser-remelted thermal barrier coatings. They found that the stress exponent values n ranged from 3.43 to 3.88, and that the activation energy Q_c ranged from 265.5 to 271.5 kJ/mol. These values are consistent with the mechanism of low-temperature dislocation creep reported in these studies.

Figure 3 presents SEM micrographs of TBC and L-TBC samples after the creep test conducted at 125 MPa at temperatures of 600 and 700 °C. Similar types of fracture characteristics were observed in samples tested under other conditions. The fractures display a cup-and-cone morphology with a dimpled surface, indicative of ductile failure. Additionally, a shear zone with an approximate slope angle of 45° is visible at the edges of the specimens, and equiaxed dimples are observed in the higher magnification images. It is evident that the higher the testing stress, the lower the area reduction. Notably, the specimen with L-TBC2 tested at 700 °C exhibited a relatively higher area reduction (99.5%) compared to the test conducted at 600 °C.

Figure 4a displays an image of the sample before the creep test, while Figure 4b,c show optical microscopy images of a region near the fracture of the L-TBC sample tested at 500 °C and 125 MPa. Similar characteristics were observed in samples tested under other creep conditions.

The observed microstructure consists of a dual-phase composition, with the α phase (lighter) and β phase (darker), featuring a fine, equiaxed grain structure uniformly distributed, with an average grain size of 2.8 μm as per ASTM E112 (2013) [41]. It is concluded that the substrate did not experience a phase transformation, and no changes in the microstructure were noted; the equiaxed structure persisted throughout the procedures conducted in this study. While the grain size remained consistent, there was elongation of the grains in the direction of the applied stress following the creep test, as depicted in the longitudinal section image. The grain size was similar for both TBC and L-TBC samples after the creep test, and the steady-state creep rate remained unchanged.

4. Conclusions

In the present study, a continuous CO₂ laser was utilized on a TBC deposited via air plasma spray to assess the creep behavior of equiaxed Ti-6Al-4V alloy. The key findings are as follows:

• In most of the tested samples, the laser-remelting treatment improving creep behavior and increasing creep life can be associated with thermal protection and greater oxidation resistance;
• The creep resistance was more significant at 600 °C, where the L-TBC sample reduced the steady-state creep rate by 50% and increased the creep lifetime by 20%;
• The creep behavior can be described by the power law, and the creep activation energies found for TBC and L-TBC were 233.78 kJ/mol and 259.82 kJ/mol, respectively;
• The fractures show a cup-and-cone morphology and a surface formed by dimples, characteristic of ductile-type failure.

A thin equiaxed microstructure was observed, which was composed of α and β phases that were homogeneously distributed. After the creep test at 125 MPa and 500 °C, grain size remained similar for the TBC and L-TBC, where it was observed that the steady-state creep rate also remained the same.