Preparation of Dense TiAl Intermetallics by Cold Spraying the Precursor–Hot Isostatic Pressing

Abstract

In this study, based on the element powder metallurgy method, a new hybrid method is proposed, which firstly prepares TiAl-based deposit precursors by the cold spraying of mixed Ti and Al powders and then combines this with hot isostatic pressing to achieve the preparation of TiAl-based alloys. This paper explores the effects of deposition parameters on deposition efficiency and coating composition and investigates the evolution of the microstructure and properties of TiAl-based alloys by different hot isostatic pressing parameters. The results show that the prepared TiAl deposits are dense and free of microstructural defects; a high deposition efficiency (75%) and small deviation of coating composition (3 at %) are obtained under the spraying parameters of 5 MPa, 500 °C. The TiAl-based alloy with a dense microstructure can be prepared by controlling the parameters such as temperature, pressure, and heating rate of subsequent hot isostatic pressing.

1. Introduction

TiAl-based intermetallic compounds can serve for a long time in the temperature range of 650–850 °C due to their high ratio of strength, excellent oxidation resistance, and creep resistance. They are regarded as ideal materials to replace traditional nickel-based superalloys to achieve a lightweight and high-temperature resistance engine and improve engine thrust–weight ratio [1,2,3,4]. When the aluminum content is between 43 and 48 at %, the γ-TiAl alloy is composed of the γ-TiAl phase and a small amount of the α₂-Ti₃Al phase at room temperature, which is considered to be the most practical application prospect in the field of aviation high-temperature application. Although there are several processing methods for preparing TiAl-based intermetallic compounds, it is difficult to form TiAl components by traditional forging and casting, or the processing technique is complex [5,6]. The prominent problem is that cracks are easy to occur in TiAl components formed by laser additive manufacturing [7,8]. There are problems of poor performance stability, interlayer segregation, and high cost of materials and equipment in electron beam additive manufacturing [9,10,11]. Hot isostatic pressing (HIP) forming has the outstanding problems of difficulty in the design, processing, and removal of shape control molds [12].

In order to avoid the above problems, Novoselova [13] proposed a new method for preparing TiAl intermetallic compounds. In this method, Ti/Al mechanically mixed powders were deposited by cold spraying technology, and then the deposits were alloyed by heat treatment. Compared with other methods, cold spraying is an emerging rapid prototyping technology [14,15], which is usually used for the deposition of metal coatings. It is a solid-state deposition technology that drives powder particles (generally 5–50 μm in diameter) into the de Laval nozzle by gas (helium, nitrogen, or air, etc.) with a certain pressure, so that the sprayed particles reach supersonic speed and impact the substrate to produce strong plastic deformation, and then deposit on the substrate [16]. Due to the low heating temperature of the cold spraying process, the raw material powder remains solid throughout the deposition process, which can effectively avoid phase transition or oxidation caused by high temperature and temperature changes [17,18]. In addition, the particles can form close contact under high local pressure to form strong atomic bonds [19], so the prepared metal coating has low porosity. Therefore, it is a convenient and effective method for the preparation of TiAl alloy billets.

Novoselova et al. [20] prepared TiAl intermetallic compounds by cold spraying combined with heat treatment, but the porosity of the final alloy sample exceeded 10 percent. Compared with traditional heat treatment, hot isostatic pressing technology refers to the use of inert gases such as argon as a pressure transmitting medium in a closed container under high-temperature and high-pressure conditions, so that products can be sintered and densified [21].

In this paper, a new composite preparation method of a dense and high-performance TiAl alloy by cold spraying and hot isostatic pressing was proposed. The advantage of this method is that a dense precursor is efficiently prepared using the mixed elemental powder. After HIP treatment, intermetallic compounds are formed in situ, and the almost completely dense precursors do not shrink significantly during HIP.

Firstly, the mechanically mixed powder of Ti and Al was selected to deposit TiAl mechanical alloy precursors with different Ti/Al ratios. The effects of different cold spraying parameters on powder deposition efficiency and coating composition were studied. The aim was to establish a spraying process window of high-density and high-efficiency deposition and to accurately control the chemical composition of TiAl deposits. Then, the cold sprayed preforms with an appropriate composition were transformed into intermetallic compounds by hot isostatic pressing. The effect of treatment conditions on the microstructure and properties of the TiAl alloy was investigated.

2. Materials and Methods

2.1. Materials

The substrate material was an aluminum plate with a size of 100 × 100 × 4 mm³, which was composed of 95.9 wt % Al and 4.1 wt % Fe. The pure Al powder used as the feedstock was inert gas atomized spherical powder (Hunan Ningxiang Jiweixin Metal Powder Co., Ltd., Ningxiang, China), and the pure Ti powder is irregular powder (Baoji Baotai Metal Products Co., Ltd., Baoji, China). The average particle sizes of the powder were 25.7 µm and 27.4 µm, respectively. The SEM surface morphologies of the powders are shown in Figure 1.

2.2. Coating Preparation

Before spraying, the two feedstock powders were placed into a plastic bottle in proportion and uniformly mixed for 30 min, and six mixed powders with different Ti/Al ratios were prepared. Before spraying, the substrate was treated by degreasing and sandblasting. The coating was prepared by a PCS-800 cold spraying system from Plasma Giken, Saitama, Japan. Nitrogen was used as the accelerating gas. The spraying pressure was 5 MPa and the spraying temperature was 400, 450, and 500 °C, respectively. The specific spraying parameters are shown in

Table 1. Cold spraying process parameters and the spraying system.

Composition (at%)	Gas Temperature (°C)	Gas Pressure (MPa)	Carrier Gas	Gun Traverse Speed (mm/s)	Powder Feed Rate (r/min)
Ti-43Al/Ti-48Al/Ti-53Al	400	5	N2	200	8
Ti-43Al/Ti-48Al/Ti-53Al	450
Ti-43Al/Ti-48Al/Ti-53Al	500

. Three kinds of preforms, Ti-43 at % Al(Ti-43Al), Ti-48 at % Al(Ti-48Al), and Ti-53 at % Al(Ti-53Al), with a thickness of approximately 6 mm, were prepared by deposition.

2.3. Hot Isostatic Pressing Treatment

After spraying, the Ti-48Al coating was selected and the substrate was removed using a wire cutting machine. The sample was processed into a block sample and treated in a hot isostatic pressing furnace (QIH-15L, Quintus Technologies, Vasteras, Sweden). The temperature–pressure curve of the hot isostatic pressing process is shown in Figure 2. HIP process 1: the initial pressure was 35 MPa; the temperature was raised to 600 °C from room temperature at a heating rate of 20 °C/min, the pressure increased to 80 MPa, and the temperature and pressure were kept for 0.5 h. Then, the temperature was raised to 1300 °C at the same heating rate; the pressure correspondingly increased to 160 MPa and the holding time was 2 h. Then, cooling occurred with a furnace. HIP process 2: the initial pressure was 35 MPa; then, the temperature was raised to 1393 °C from room temperature at heating rate of 20 °C/min, the pressure increased to 200 MPa, and the temperature and pressure were kept for 1 h. Then, the temperature decreased to 700 °C at a cooling rate of 10 °C/min; finally, cooling occurred with a furnace.

2.4. Characterization

The morphology of the original powder and the particle distribution of the as-sprayed sample were characterized by field emission scanning electron microscopy (Nova NanoSEM 430, FEI, Hillsboro, OR, USA), and the elemental distribution of the as-sprayed samples was analyzed by an energy spectrum analyzer (Ultim Max 65, Oxford Instruments, Oxford, UK). The metallographic structure of the sample after hot isostatic pressing was characterized by an optical microscope (DMI5000M, Leica, Wetzlar, Germany). The composition of the preform was detected by chemical titration analysis. The porosity of the coating was measured by the ImageJ (version 1.51j8) image analysis method. The microhardness of the samples at room temperature was measured by the Vickers hardness tester (FM-810, Future-Tech Corp., Kawasaki, Japan). The test load was 200 gf and the loading time was 15 s.

3. Results and Discussion

3.1. Cold Sprayed Samples

3.1.1. Microstructure

Ti-43Al, Ti-48Al, and Ti-53Al coatings were prepared at different gas temperatures; the SEM-BSE diagram of the coating is shown in Figure 2. The Ti-48Al sample was selected to take the EDS energy spectrum scanning map. As shown in Figure 3, it can be proved that the dark color region in Figure 3 corresponds to Al and the light color region corresponds to Ti. With the increase in Al content in the mixed powder, the proportion of Al in the coating also increases accordingly. The two powders in the coating are obviously deformed, and the two powders are evenly distributed in the coating, which is beneficial to the element diffusion and reaction in the subsequent hot isostatic pressing process. At the same time, all TiAl mixed coatings are dense, and there are no obvious crack defects in the coatings. The particles undergo initial deformation during deposition, and the subsequent particles will continuously impact the previously deposited particles and further deform them. The accumulated strong deformation enhances the combination of particles and forms a dense coating. The average porosity of all coatings is approximately 0.8%, and a small amount of small black pores mainly exist at the Ti/Al particle interface.

3.1.2. Deposition Efficiency

The cold spray deposition efficiency of binary mixed powders at different spray temperatures is shown in Figure 4. From the diagram, it can be seen that the mixed powder has a high deposition efficiency, in the range of 60%–75%; at the same time, the deposition efficiency of the mixed powder increases with the increase in the spraying gas temperature. Relevant research shows that the increase in gas temperature will lead to the increase in gas velocity, which will increase the flight velocity of particles [22]. At the same time, the increase in gas temperature can soften the particles, promote the deformation of the particles, and promote the metallurgical bonding between the particles. Therefore, as the gas temperature increases, the deposition efficiency of the mixed powder increases.

3.1.3. Coating Composition

The composition of the binary mixed coating prepared at different spraying temperatures is shown in Figure 5. It can be seen from the figure that the Al content in the mixed coating sprayed at different temperature is higher than that in the corresponding original powder, and the composition deviation range is between 3 and 5 at %. At higher gas temperatures, the composition deviation of the coating is smaller. In different coatings, the change of Ti content may be due to the different deposition efficiency of Ti and Al powders during cold spraying [23]. Compared with Al, the density of Ti is higher (Al is 2.70 g/cm³ and Ti is 4.51 g/cm³), which will reduce the maximum particle velocity. The yield strength of Ti is higher, which will increase the critical speed required for deformation. Ti powder cannot be fully plastically deformed because it does not reach its critical speed. This lack of deformation will lead to a decrease in the deposition efficiency of Ti, thereby changing the composition of the deposit, resulting in a deviation between the coating composition and the original powder composition. Increasing the gas temperature, the flight speed of the powder increases, and the deposition efficiency of both particles increases. Al is easy to deform, and the deposition efficiency increases more than Ti, so the Al content in the coating is higher than that in the powder. At the gas temperature of 500 °C, the deformation degree of Ti increases, and the deposition efficiency increases, so that the composition deviation between the deposited body and the original powder decreases.

It can be seen that during the cold spraying process, the powder has higher deposition efficiency at a higher gas temperature (500 °C), and the obtained coating composition is close to the feedstock mixed powder composition.

3.2. Samples after Hot Isostatic Pressing

3.2.1. Microstructure

The metallographic structure of the cold sprayed Ti-48Al preform after hot isostatic pressing is shown in Figure 6, and the porosity and microhardness values obtained after measurement are shown in

Table 2. Porosity and microhardness of Ti-48Al deposit after hot isostatic pressing.

	Process 1	Process 2
Porosity/%	31.35	0.49
Microhardness/HV0.2	284	571

. It can be seen from the results that there are obvious differences in the microstructure of the TiAl alloy after two different hot isostatic pressing processes. The metallographic image of the sample treated by process 1 exhibits more pores, and the porous sample is obtained. The pore size is 100–200 μm, the porosity is 31.35%, and the hardness value is 284 HV_0.2. The metallographic image of the sample treated by process 2 has no obvious pores and the dense TiAl sample is obtained, and the hardness value is 571 HV_0.2.

The reaction mechanism of Ti and Al powders is commonly recognized as the diffusion couple mechanism. The entire reaction process is divided into two stages: low-temperature diffusion and high-temperature diffusion. The diffusion reaction process is as follows [24]:

6Ti + 6Al→4Ti + 2TiAl₃

(1)

4Ti + 2TiAl₃→Ti₃Al + TiAl + 2TiAl₂

(2)

Ti₃Al + TiAl + 2TiAl₂→6TiAl

(3)

Reaction (1) occurs in the low-temperature diffusion stage, and reaction (2) and reaction (3) occur in the high-temperature diffusion stage. During hot isostatic pressing, the mechanical alloy undergoes element diffusion and phase transformation. In the process of process 1, the low-temperature solid-state diffusion reaction below the Al melting point is fully carried out at 600 °C for 0.5 h. The reaction product is the TiAl₃ phase. Because its nucleation free energy barrier is significantly lower than that of other intermetallic compounds, TiAl₃ will first nucleate [25]. Since the diffusion rate of Al in TiAl₃ is one order of magnitude higher than that of Ti, Kirkendall pores will be generated. The temperature continues to rise, reaching the melting point of Al, and solid–liquid diffusion reaction occurs. The unreacted Al melts into liquid, which is wrapped on the surface of the unmelted Ti particles and reacts with it to form the TiAl₃ phase. The Kirkendall pores produced by the solid–liquid reaction are larger than the Kirkendall pores produced by the solid–solid transition. The large pores are due to the faster movement of liquid Al, resulting in a faster reaction and significant porosity. When the temperature continues to rise, the reaction between the four solid phases of α-Ti, TiAl₃, Ti₃Al, and TiAl begins to occur. The diffusion path of Al begins to diffuse into pure Ti through the transition phase TiAl₃. Since Ti is a secondary element in TiAl₃, the diffusion rate is lower than that of Al, which will also lead to the generation of Kirkendall pores. The yield strength of the TiAl alloy will be strongly reduced above the critical temperature. Under the action of high temperature and high pressure, plastic deformation and creep behavior will occur in the pore area of the alloy. Then, under the action of diffusion behavior, metallurgical bonding will occur on the surface of the collapse area, thus improving the density of the material. The sample treated by process 1 has a porous structure, indicating that the densification behavior has not yet begun or has begun but not yet ended at 1300 °C. Therefore, many large pores can be seen, resulting in a low microhardness value of the alloy. In the process of process 2, the temperature is directly increased to 1393 °C, the solid-state reaction time is insufficient and the Kirkendall pores are less. At the same time, the pressure is increased to 200 MPa. Under this temperature and pressure, the densification behavior is fully carried out. Therefore, under the action of uniform pressure of hot isostatic pressing, the compression deformation and agglomeration of the hole can be observed. The particles around the hole undergo plastic deformation and move into the hole, so a dense TiAl alloy is prepared.

3.2.2. XRD Patterns

The XRD patterns of cold sprayed Ti-48Al preform before and after hot isostatic pressing are shown in Figure 7. The diffraction patterns of the as-sprayed samples show that there are no other peaks except for the peaks of pure Al and Ti. The results show that the sprayed deposits are only composed of pure Ti and Al phases, and there are no other intermetallic compounds or oxides, or the amount of both is lower than the detection limit of XRD, indicating that there is no obvious phase change or oxidation during the spraying process, which proves that the deposition mechanism of the spraying process is solid deposition at low temperature. It can also be seen from the EDS spectrum of the sprayed samples that the distribution of Ti and Al has obvious boundaries, so there is no obvious reaction. The experimental results of Kong et al. [26] also show that the as-deposited coating was mainly Ti and Al without observable phase transformation during cold spraying.

The diffraction patterns of the samples treated by hot isostatic pressing show that there are only γ-TiAl and α2-Ti₃Al phases in both samples, and there are no elemental phases and other intermetallic compounds. It shows that Ti and Al react completely during hot isostatic pressing, and the intermediate phase TiAl₃ is also completely consumed.

3.2.3. Shape Change

The macro-morphology of cold sprayed Ti-48Al preform before and after hot isostatic pressing is shown in Figure 8. It can be seen that after the treatment of process 1, the sample undergoes obvious expansion and certain deformation. The sample size is 30 × 20 × 6.44 mm³ before treatment, and 34 × 22 × 6.5 mm³ after treatment, and the volumetric expansion ratio is 25.8%. The expansion is caused by the Kirkendall pores generated by the diffusion of Al during the Ti and Al reaction, and the bending may be caused by the continuous concentration of stress inside the sample during the sintering process. This may also be related to the placement direction of the sample during the sintering process. Because the sample is placed vertically, it will also cause bending due to its own gravity. After the process 2, the sample undergoes a certain shrinkage, with no obvious deformation, and the shape accuracy is more than 96%. In this process, the processing pressure was increased, the low-temperature reaction time was reduced, and the sample placement direction was changed to horizontal placement, so no bending and expansion were observed.

4. Conclusions

In this study, the TiAl alloy with a dense microstructure was successfully prepared by a cold spraying–hot isostatic pressing hybrid method. TiAl preforms were firstly prepared by cold spraying technology using Ti and Al mixed powders. Ti-43Al, Ti-48Al, and Ti-53Al (at %) preforms were deposited at three temperatures of 400, 450, and 500 °C under 5 MPa spraying pressure. Ti-48Al preforms were subjected to hot isostatic pressing post-treatment, and the effects of different hot isostatic pressing parameters on the microstructure and properties of TiAl preforms were studied. The conclusions are as follows:

(1)

The microstructure of the cold sprayed TiAl preform prepared at all temperatures is dense (porosity < 1%), and there is no obvious defect. The Ti and Al particles in the coating are evenly distributed and fully deformed, which is beneficial to the diffusion and reaction of elements in the subsequent hot isostatic pressing treatment.

(2)

Comparing the deposition efficiency and coating composition of mixed powder at different gas temperatures, it can be seen that the deposition efficiency of mixed powder is higher than 60%, and higher powder deposition efficiency and smaller coating composition deviation can be obtained at a higher cold spraying gas temperature (500 °C).

(3)

The two-phase γ-TiAl alloy with a dense microstructure was successfully prepared by reasonably adjusting the parameters of temperature, pressure, and heating and cooling modes of the subsequent hot isostatic pressing process. The shape accuracy after treatment was more than 96%. This experiment proved the feasibility of preparing dense TiAl intermetallic compounds by cold spraying–hot isostatic pressing.