A Novel Approach of Microstructure Refinement of TiAl in Laser Beam Welding

Abstract

Grain refinement through borides is known to be suppressed when TiAl is welded with a laser beam. As β grains do not primarily nucleate on boride at a high cooling rate, a mixture of nitrogen and argon is applied as a protecting gas for the formation of TiN during solidification. The phase transformation is changed correspondingly from Liquid → Liquid + β → β → α + β → α + γ+ β → α₂ + γ + B2 to Liquid → TiN + Liquid → β+ TiN → α + γ + TiN → α₂ + γ+ TiN. It is found that β grains prefer to nucleate heterogeneously on the suspending TiN in the melt with orientation relationship ${\text{{}111\text{}}}_{\mathrm{TiN}}//{\left\{110\right\}}_{\mathsf{\beta }}$, leading to refined β grains. α₂ colonies that were thus modified into fine non-dendritic grains. The effects of nitrogen as a shielding atmosphere on the microstructure evolution of TiAl are elaborately studied.

1. Introduction

TiAl alloys are considered to be the most competitive candidate with nickel-based superalloys in the automotive and aerospace industries because of their excellent mechanical properties in the temperature range from 600 to 1000 °C [1,2,3,4,5,6,7,8,9]. However, the as-cast TiAl alloys show a microstructure with coarse dendrites and composition segregation, leading to premature failures such as high fatigue crack growth rate, low fracture toughness, and poor ductility [10,11,12,13].

To achieve a strength and ductility balance of TiAl alloy, methods are employed to refine the grain size, such as the addition of interstitial elements such as boron [14,15,16], nitrogen [17], phase transformation via heat treatment [11,18,19], laser beam oscillation [20], and ultrasonic vibration [21]. It is found that 1.0 at.% nitrogen addition leads to remarkable grain refinement in the lamellar microstructure with an increase in yield strength by two times at room temperature. As α is the first precipitated phase in the reported alloy (Ti-48.5Al-1.5Mo), the TiN or Ti₂AlN precipitate acts as the nucleation site during the formation of the α phase [17]. Nitrogen ion was implanted into a β-solidifying γ-TiAl alloy. As Ti is a strong nitride former, several kinds of compounds were found, such as TiN, Ti₂N, Ti₃AlN, and Ti₃Al₂N₂, according to the nitrogen volume content and chemical composition [22]. With the addition of 2.5 at.% N into high Nb containing Ti-46Al-8Nb TiAl alloy, interstitial nitrogen atoms are proved to expand and stabilize the α phase region. Meanwhile, the average colony size of α phase decreases to around 5% of the size without nitrogen addition. The mechanism of α colony refinement is proposed as the diffusion of solute Al is suppressed by nitrogen at the solid–liquid interface, leading to α as the primary phase solidifies ahead of β [23].

Grain refinement by massive γ transformation requires heat treatment followed by oil/water quenching. It results in cracks and makes it difficult for manufacturing [19]. Another way of grain refinement is nucleation on borides in TiAl phase transformation. The addition of 1 wt.% boron in the alloy forms several kinds of borides, such as titanium mono-boride (TiB), diboride (TiB₂), or Ti₃B₄ as nucleation sites at the beginning of solidification. Because borides cause strong constitutional undercooling in the segregation zone, it leads to lots of nucleation sites in the interdendritic area [14]. They create an opportunity for α to nucleate heterogeneously on borides during β → α transformation, besides nucleation on β grain boundary. However, the cooling rate plays an important role in the nucleation of borides. Oehring [24] and Liu [4,10,25] have reported that heterogeneous nucleation of α grains on borides are strongly suppressed when the temperature gradient is high, especially when samples are laser beam welded or water quenched from high to room temperature.

The α grains prefer to nucleate on the β grain boundary with Burgers orientation relationship ${\text{{}110\text{}}}_{\mathsf{\beta }}{//\left(0001\right)}_{\mathsf{\alpha }}$ and $<111{>}_{\mathsf{\beta }}//<11\overline{2}0{>}_{\mathsf{\alpha }}$ during β → α transformation [1]. It is found that although α grains are of fine plate morphology, these α grains originate from the same parent β, i.e., they are in the form of textured colonies [26]. Thus, due to two factors: (1) the effect of boride for α grain refinement is strongly suppressed at a high-temperature gradient; (2) the size of α colonies is predetermined by their parent β, it is extremely important to pay attention to refine primary β grains.

It thus comes to the idea to employ nuclei which precipitate as the first solid phase from melt for β grain refinement. According to the Ti-N phase diagram, the solidification temperature of TiN is found to be in the range from 2930 °C to 3290 °C [22,27], which is significantly higher than that of β (1670 °C) [1] and brings the possibility to employ TiN as a nucleant for β grain refinement. The present work brings a novel approach that grain refinement would work despite of high cooling rate. The size of α₂ textured colonies would be reduced even though it undergoes fast cooling. It sheds light on a method for mechanical property improvement.

2. Material and Methods

The examined alloy has the chemical composition Ti-42Al-2.5Cr-1Nb-0.7Si-0.5B (at. %). The actual composition is verified by energy dispersive spectroscopy (EDS) to within 1% of the nominal value. The substrate was cast, hot isostatic pressed, and cut by electro-discharge machining into small pieces with a geometry of 30 × 30 × 2.5 mm. The panels were then cleaned and sanded with 800# and 2000# SiC paper to remove oxides and machining debris. The welding process was performed using a 10 kW IPG Ytterbium fiber laser, as shown in Figure 1. The movement of the welding head in the x-y-z coordinate system is controlled by a KUKA KR30 HA robot (Shanghai, China). The sample (Sample 1#) was butt welded under argon. For comparison, the other sample (Sample 2#) was welded with the same parameters under a mixed atmosphere with argon and nitrogen as shielding gas, with each gas taking up 50% in volume. Before and after welding, laser scans back and forth along the welding seam 10 times for preheating and post-weld heat treatment.

After welding, a visual inspection was carried out on the weld. The welded samples were cut, ground, and polished, and followed by mechanical polishing with a vibratory polisher. The microstructure was analyzed using scanning electron microscopy (SEM, Zeiss, Germany) in backscattered electron (BSE) mode, energy dispersive spectroscopy (EDS), and electron backscattered diffraction (EBSD). The phases were analyzed by micro-X-ray diffraction (mXRD) using CuKα radiation to examine the phase composition and microstructure of the nitride in alloys. The measurement size is 0.5 × 0.5 μm. After the experiment, the recorded Debye–Scheller diffraction rings are azimuthally integrated to obtain the diffraction pattern as a function of the scattering vector q: $\mathrm{q}=4\mathsf{\pi }\sin (\mathsf{\theta })/\mathsf{\lambda }$, where θ is the diffraction angle and λ is the wavelength of the X-ray beam.

3. Results and Discussion

Figure 2a shows the microstructure of the substrate. In the backscattered electron mode, γ-TiAl is dark, α₂ + γ lamellae is light gray, and B2 is the brightest phase. EDS analysis of Ti-32.78Al-2.6Cr-1.2Nb-1.05Si proves that the B2 phase is rich in Nb and Ti and deficient in Al. The Ti-Al phase diagram [1] shows the melt with the composition of Ti-42Al undergoes the following phase transformations from high to room temperature: Liquid → Liquid + β → β →α + β→α + γ+ β→α₂ + γ + B2. The arrows indicated phase, with dotted morphology and bright color, are observed in the lamellar colony interior. They are indicated as borides, with the composition of TiB, TiB_2, and Ti₃B₄, depending on their chemical composition [26,28,29]. These borides have been reported to be essential in grain refinement through the heterogeneous nucleation of α on borides [30,31].

The weld zone of the 1# sample (Figure 2b) consists of fine needle-like α₂ plates distributed in any direction. Figure 2c shows details of the microstructure, and the average width of α₂ plates is 1.12 μm. There are bright ridges located between the fine acicular α₂ plates. As analyzed by EDS, bright ridges are enriched in Cr and Nb by ejecting heavy elements into the inter-lamellar region during the β→α transition [15,32].

The welding zone of sample 2# (Figure 2d) is composed of dendrites. These dendrites are randomly orientated in the welding zone with an average length of 28 μm. Some dendrites are in the form of lots of dots because the dendrites were cut through by the exam surface. As shown in Figure 2e, the core of the dendrite is a bright color and is surrounded by dark islands. Some bright interlayers are located between the dark islands.

As shown in Figure 3a, a linear EDS scan was carried out to verify the chemical composition of dendrites. The dendritic arm is enriched with Ti and N, with around 70 at.% of Ti and 30 at.% of N. Meanwhile, Al is depleted from the dendrite with a content less than 10 at.%. For phase identification, XRD analysis was carried out. It shows in the welding zone of sample 1#, α₂ and γ phases are identified. In contrast, the welding zone of sample 2# consists of TiN, α_2, and γ (Figure 3b). Combining EDS and XRD analysis, the dendrites are defined as TiN.

From the microstructure (Figure 2b) and EBSD (Figure 4a) observation, the α₂ plates in the weld zone are distributed by chance. However, with careful inspection, it is found that lots of α₂ plates are orientated in the same direction, as they are colored the same. These α₂ plates can even be separated into groups. There is a group of α₂ plates located over 80% of the measured area (Figure 4a). As indicated in Figure 4c,f, the basal plane orientations of these α₂ plates are presented, showing they are with misorientation angles of 60° or 90° between the plates. These positions of the (0001) α₂ poles correspond well with (111)_β plane of one β grain. Β → α transformation follows Burgers orientation relationship ${\left\{0001\right\}}_{\mathsf{\alpha }}//{\left\{110\right\}}_{\mathsf{\beta }}$ and $<11\overline{2}0{>}_{\mathsf{\alpha }}//<111{>}_{\mathsf{\beta }}$. According to Gey and Humbert, misorientation angles of Burgers α variants originating from the same parent β grain can only obey angles of 0°, 10.5°, 60°, 60.8° and 90°, including parallel orientations [33]. On the other hand, when neighboring α grains obey such misorientation angles, they can be determined as α grains from the same parent β. Due to this effect, the size of the colony in the welding zone cannot be easily identified from the size of the α₂ plate. The size of the “textured colonies” should be counted.

Figure 5a is the α₂ phase texture map. All the α₂ grains are orientated in different directions. The dendrites, which are presented in black color, are located in the middle of α₂ grains. In the {0001}α₂ pole figure, basal planes of the α₂ phase are randomly distributed. The misorientation angles between neighboring α₂ grains are shown in Figure 5c. There is a limited preference for 0–2°, 8–12°, 58–62°, and 88–92°, which indicates that the α₂ grains are not stemmed from the same parent β. However, after careful examination, it was found that these α₂ grains are in six groups, as illustrated in Figure 5d,g,j,m,p,s. In each group, the basal planes of these α₂ grains are in a relationship with their neighboring TiN grains ${\left\{111\right\}}_{\mathrm{TiN}}//{\left\{0001\right\}}_{{\mathsf{\alpha }}_2}$, as shown in Figure 5e,f,h,i,k,l,n,o,q,r,t,u. Because α grains follow the Burgers orientation relationship with β ${\left\{110\right\}}_{\mathsf{\beta }}//{\left(0001\right)}_{\mathsf{\alpha }}$, it can be inferred that the primary β grain is in an orientation relationship with TiN ${\left\{111\right\}}_{\mathrm{TiN}}//{\left\{110\right\}}_{\mathsf{\beta }}$. The size of the α₂ grain group corresponds to the size of their primary β. According to the particle interval counting method proposed by ASTM E112-10 [34], the average diameter of the colonies (considered as β particles) is 35 μm. The colony size is similar to the dendrite length of TiN.

The formation of TiN is proposed as a three-step mechanism, i.e., deposition, diffusion, and reaction. In the first step, nitrogen decomposes during interaction with the laser into atomic nitrogen, which is a more effective nitrifier than molecular nitrogen. During welding, TiAl is first melted by a laser beam and driven by convection, resulting in exposure of the melt to nitrogen atoms [35]. When an excessive amount of nitrogen has dissolved in the liquid up to its solubility limit, there is an exothermic reaction with the liquid that leads to the formation of TiN:

$$\mathrm{Ti}+\frac{1}{2}{\mathrm{N}}_2=\mathrm{TiN}$$

As Ti and N present stronger chemical affinity than that of Al and N [36], atomic nitrogen absorps into the melting pool, diffuses into the melt, and reacts with Ti atoms to form Ti–N binary compounds. Several kinds of compounds could form, such as TiN, Ti₂N, and Ti₃AlN, according to the nitrogen volume content and chemical composition of the alloy. In the present work, only TiN is found. As Ti atoms diffuse towards the nitrogen-rich locations to form TiN, the Al are depleted between the nitride dendrites.

As the solidification temperature of TiN is higher than that of β. It can be inferred that TiN first precipitates from the melt. With the aid of a nitrogen atmosphere, the solidification path is modified into L → TiN + L → TiN + β → TiN + α + γ → TiN + α₂ + γ. As the temperature goes down, the β grains start to nucleate. In sample 1#, β grains homogeneously nucleate due to undercooling. Compared with sample 2#, a large amount of suspending TiN serves as heterogeneous nucleation sites for β. It can be inferred that the intake of nitrogen into the melting pool significantly increases the nucleation sites for β due to the fact that there is a large amount of TiN swimming in the melt, which is evidenced by Figure 6. An excessive amount of β nuclei brings a higher possibility for β to interact with each other as they grow, leading to β grain refinement. Thus, the nucleated β grains are significantly refined when nitrogen is applied as shielding gas.

4. Conclusions

Refined α₂ textured colonies are obtained during laser beam welding with nitrogen as shielding gas. This study shed light on a novel approach to microstructure refinement of TiAl despite of high cooling rate, such as laser welding.

(1)

When argon is employed as a shielding gas, a large amount of α₂ textured colonies is formed at a high cooling rate due to the formation of coarse β grain. Nitrogen decomposes into atomic nitrogen, diffuses into the melt, reacts with Ti, and finally forms a TiN compound.

(2)

The suspending TiN swimming in the melt serves as heterogeneous nucleation sites for β. The β grains are found to nucleate heterogeneously on the TiN with an orientation relationship ${\text{{}111\text{}}}_{\mathrm{TiN}}//{\left\{110\right\}}_{\mathsf{\beta }}$. An excessive amount of β nuclei brings a higher possibility for β to interact with each other as they grow, leading to refined β grains.

(3)

The orientation relationship of α₂ grains with their neighboring TiN grains is determined as ${\text{{}111\text{}}}_{\mathrm{TiN}}//{\left\{0001\right\}}_{{\mathsf{\alpha }}_2}$.

(4)

Due to the excellent refinement function of TiN, in the next step, a certain amount of TiN particles could be considered to add into the melt for grain refinement.