Ti6Al4V/SiC Metal Matrix Composites Additively Manufactured by Direct Laser Deposition

Additive manufacturing (AM) or 3D printing is a fabrication method with which is possible to obtain 3D parts with complex and custom geometries in a short period by depositing material on a layer-by-layer basis [1]. Therefore, AM enables the creation of designs that cannot be manufactured via conventional methods and it offers the advantage of producing the least amount of waste material [2]. Raw ceramic and metallic materials apply to the 3D printing technology to give rise to metal matrix composites (MMCs), which are currently highly investigated [3]. One of the most commonly used AM techniques is direct laser deposition (DLD), where a high-energy laser acts as a source to melt materials in powder or wire form following a layer-by-layer model that is built from a CAD file with the desired part design [4]. During this process, the creation of a molten pool takes place, and the selected fabrication parameters influence its size and shape, modifying and improving the resulting microstructure of the material [5]. In addition, DLD originates less heat damage and distortion to the part, and the high level of control over the laser leads to exact energy values supplied in specific substrate areas.

The variation in the DLD processing parameters, such as laser power energy and laser scanning speed, has an influence on the shape and dimensions of the molten pool and in the heat-affected zone (HAZ), thus these parameters affect the morphology, microstructure, and mechanical properties of the final composite material [6, 7, 8]. Both the laser power and the scanning speed can result in layer re-melting, different cooling rates, and modifications in the grain size [8]. Therefore, the manufacturing parameters can be tuned to achieve specific material properties.

There are several alloys available in the market to be utilized in additive manufacturing, and Ti6Al4V (an α–β type alloy) is the most used material in AM [9, 10]. It represents 70% of titanium alloys, due to its excellent properties like light weight, good structural stability, low thermal conductivity, high elastic limit, great fatigue and corrosion resistance and high biocompatibility and osseointegration. Hence, Ti6Al4V is widely used not only in components fabrication in the aerospace and transport industry [11], but also, in biomedicine, where it is the majority component for the manufacture of hip prostheses. Even though the titanium alloy used in the additive manufacturing process costs more than the forging of the traditional process, the cost of each component can be reduced by 50% without compromising the mechanical properties [12].

Nevertheless, the main disadvantage that this alloy presents is the low resistance to wear due to its high coefficient of friction. This leads to the material failure in applications like hip implants, where the formation of abrasive particles that cause toxicity, inflammation and pain in the patient occur. Consequently, few studies have been performed to improve Ti6Al4V characteristics, which involves surface treatments and coating depositions [13, 14, 15]. All these aspects must be studied for the specific case of titanium matrices.

Titanium matrix composites (TMCs) based on Ti6Al4V as the metal matrix reinforced with ceramic materials, which have a great potential to improve mechanical properties like wear resistance, are currently being investigated [16, 17]. Several reinforcement materials can be introduced in the titanium matrix, such as boron nitride, alumina, or silicon carbide. In particular, in other metal matrices, SiC has great chemical and mechanical properties, like high thermal shock, wear and corrosion resistance and high hardness [18]. Previous research has shown that SiC particles improved the composite strength as a consequence of an increase in hardness [19]. Kang et al. [20] reinforced porous titanium materials for medical applications with SiC by conventional powder metallurgy methods and demonstrated that just a 3 wt% of SiC increased the hardness by 170%. Aigbodion and Hassan [21] studied the microstructure and properties of alloys of Al–Si–Fe reinforced with 5–25 wt% of SiC particles, and have determined that the higher the SiCp content, the greater the hardness, the elastic limit, and the maximum tensile strength of the material. The interaction between the SiC reinforcement and the metal matrix is also of high importance. Riquelme et al. [22] have used laser cladding to reinforce an aluminum matrix (Al–Si) with SiC particles, and have evaluated the matrix–reinforcement reactivity, which can be reduced with the addition of alloying elements such as Si or Ti, and the effects on tribological properties in fabricated composites.

Ti6Al4V metallic matrix composites (reinforced with TiC, TiB, TiN between others) used for aircraft components have even higher hardness, better strength-to-weight ratio and wear resistance than the unreinforced material, especially at high temperatures [23]. The manufacture of these composites by additive manufacturing has been successfully carried out in several research studies [24], although their widespread use in aerospace applications has some way to go.

One of the main problems that occur in composite materials reinforced with ceramic particles is the poor matrix–reinforcement interaction, which depends on the wettability (surface chemistry) of the ceramic particle when in contact with the metal in the molten state. The poorer the particle–matrix interfacial bonding, the worse the properties of the composite; this is because the ceramic particles will tend to detach from the matrix, resulting in a low wear resistance [25]. Sivakumar et al. [26] used SiC nanoparticles in concentrations of 0–15 wt% to reinforce Ti6Al4V metal matrices, increasing both the hardness and resistance and also the wettability and the adhesion forces when adding 5 wt% content of SiC. Kloosterman et al. [27] examined the interface between Ti6Al4V matrix and SiC injected particles after laser embedding, and determined that the degree of dissolution of the particles in the matrix influences the mechanical properties of the composite material.

The influence of the reinforcement content in the microstructure and mechanical properties of TMCs has been analyzed by a few authors. Li et al. [28] fabricated gradient Ti–SiC coatings on Ti64 substrate using pure Ti and increasing percentages of SiC powder. They started from pure Ti without SiC and increased the SiC percentage by 10 vol% on each new layer deposited until a final proportion of 90 vol% SiC + 10 vol% Ti. They concluded that different reaction products were formed at each layer as a result of the in-situ reactions between Ti and SiC and that the Vickers microhardness increased gradually from the Ti64 substrate with the increase of SiC volume fraction. Li et al. [29] also manufactured two cladding layers of 90%Ti + 10%SiC and 80%Ti + 20%SiC on Ti6Al4V substrates and have reported that TiC and Ti₅Si₃ phases were formed, leading to a higher Vickers microhardness of the composite layer compared to that of the substrate. Li et al. [30] in whose work TiCp is selected in concentrations ranging from 5 to 50 wt% for Ti6Al4V matrices, where an increase in microhardness of up to 94% compared to the material unreinforced has been observed. Zhang et al. [31] have characterized the interfacial reaction that takes place between matrix and reinforcement in composite materials of Ti43Al9V reinforced with SiC fibers by SEM and TEM, identifying three types of reaction products formed between the C of SiC and the titanium matrix, such as TiC.

Other fabrication techniques such as selective laser melting (SLM) have been used to create Ti6Al4V with similar reinforcement contents. Krakhmalev et al. [32] fabricated pure Ti composites reinforced with 20, 30, and 40 wt% SiCp coatings on Ti6Al4V substrates with SLM and have studied the microstructure phase formation. The composites had higher hardness and higher abrasive wear resistance.

However, the influence of the DLD process parameters like the scanning speed in the microstructure, tribological and mechanical properties for Ti6Al4V/SiCp composites has not been deeply studied yet. Yang et al. [33] have used a hybrid additive manufacturing method of laser cladding and layer deposition manufacturing (LCLD) to manufacture Ti6Al4V/SiC fiber composites and studied and optimized the processing parameters. Shojaei et al.  [34] analyzed the effect of applying a Ti/SiC MMC coating on a Ti64 substrate for enhancing the hypervelocity impact resistance. Nevertheless, these authors did not fully characterize the fabricated composites nor made a microstructural evaluation. Also, some mechanical properties like microhardness and nanohardness were not made. Das et al.  [35, 36] modified the surface properties of pure Ti by adding SiC particles at different laser power values and scanning speeds, creating two single layers of Ti–SiC composite coatings that exhibited higher hardness than the substrate. The researchers concluded that the composite fabricated with 400 W and 10 mm/s demonstrated the highest hardness, which is attributed to the formation of reaction products.

Moreover, just a few bibliographic studies show evidence of the reactivity that occurs between the SiC particles and the titanium from Ti6Al4V, and the work has been done just for low reinforcement percentages. This matrix–reinforcement reactivity leads to the formation of reaction products that can change the material properties and its stability. Furthermore, the effect of the variation of the percentage and size of SiC particles in the final material of Ti6Al4V/SiCp and its properties should be further analyzed.

This paper aims to determine the effect of the scanning speed of DLD additive manufacturing of Ti6Al4V/SiC metal matrix composites, in the morphology, microstructure, and micro and nanohardness properties of the fabricated samples. The role of the different SiC particle sizes and content in the matrix has been researched. The possible reaction mechanisms between the SiC and the titanium matrix have been proposed together with their effect on the microstructure. In addition, the relation of the process parameters with these mechanisms and their contribution to the sample microhardness have been analyzed.

Thin wall samples (length = 60 mm and height = 15 mm) of Ti6Al4V/SiC (Fig. 1a) were manufactured by DLD using 316 L sheets (100 × 100 × 20 in mm) as the 150 °C preheated support base. Ti6Al4V commercial powder has been used, and the effect of the SiC size has been evaluated. As received titanium and SiC particles are shown in Fig. 2a–c, and the particle size distribution for each material is shown in Fig. 2c–f. In all cases, the particle size distribution was small (most Ti6Al4V particles were between 15 and 39 μm, the SiC F240 particles were between 43 and 83 μm, and the SIC F360 were between 20 and 38 μm.

The laser used was a high-power diode laser (1300 W) (ROFIN DL013S) with a wavelength between 808 and 940 nm. The carrying gas was argon (4.5 atm pressure and 0.05 L s^− 1 flow rate). The powder and the laser focus were 12.5 mm below the nozzle tip. The laser spot had a rectangular shape with 1.25 mm × 0.6 mm of size. The Ti6Al4V and SiC particles were in two different hoppers that worked simultaneously and were sprayed coaxially with the laser beam through a Fraunhofer IWS COAX 8 coaxial nozzle. The powders were sprayed at the Ti6Al4V:SiCp ratios shown in Table 1. The powder feed ratio was 5 g/min and the laser focus overlapped with the powder focus. In addition, this system is placed in an ABB IRB2400 robot. A hot plate connected to a temperature control system (150 °C) was used for substrate support.

The effect of using laser scanning speeds between 5 and 25 mm/s has been analyzed. Table 2 shows the set of samples fabricated with their corresponding fabrication conditions. To obtain the height increase per layer for each laser parameter used, a single layer with a length of 60 mm was deposited for each condition before the full thin wall fabrication (Fig. 1b). In this way, single layer heights were measured using a caliper with a precision of ± 0.01 mm, and these values determined the height increase per layer for the manufacturing process. Once this height increment per layer was established, thin-wall samples with a length of 60 mm made of Ti6Al4V/SiC were built layer by layer using the parameters shown in Table 2, as is shown in Fig. 1c. The samples height chosen was 15 mm. The number of layers depended on the height increment per layer required to reach the established height of the sample. In all cases, the laser power gradually decreases during the deposition process to avoid heat build-up. Some tests were performed at different laser powers to optimize the manufacturing parameters. Laser power values below 300 W could not melt the titanium particles. Increasing these values provided AM structures that were not homogeneously built. At the laser power value of 650 W, a high-quality first layer was deposited but maintaining this value in the following layers led to the melting of the manufactured samples. To avoid an excessive heat input, the laser power was gradually reduced from the initial 650 W to a final value of 100 W used for the last layer. Therefore, the power reduction per layer depended on the height of the samples and the number of deposited layers. Three samples were manufactured for each condition for reproducibility.

X-ray diffraction (XRD) was performed using a Panalytical X’Pert PRO diffractometer and analyzed the patterns by X’Pert HighScore Plus software. The radiation source was the Cu Kα. (λ = 1.5406 Å) operating at 45 kV and 300 mA. The 2θ scanning range used was from 20° to 95° with a step of 0.02° and an acquisition time of 20 s.

To evaluate the actual proportion of SiC particles incorporated into each sample, an X-ray fluorescence (XRF) analysis was performed using a Panalytical MagiX XRF equipped with a 4 kW X-ray generator and a rhodium anode.

Samples were metallography prepared on their cross-section to measure their microhardness and study their microstructure. For it, the samples were mounted in an electrically conductive resin, wet grounded using a sequence of abrasive silicon carbide (120–4000 grit), and finally polished using a 1 μm diamond with ethylene glycol as a lubricant.

The percentage of SiC particles was measured with an image analysis software (Leica Application Suite) on the captured images obtained with a light optical microscope (OM; Leica DMR). Microstructures were evaluated by scanning electron microscope (SEM; Hitachi S-3400N) equipped with an energy dispersive X-ray spectrometer (EDX).

Vickers microhardness (HV_0.1 and 10 s) values were measured by using a Shimadzu microhardness tester on the cross-section of the specimens. The microhardness measurements were made from the substrate to the top of the sample with distances of 1500 μm between each indentation.

The nanohardness of the different phases of the composite DLD samples was measured using a Berkovich indenter (MTS nanoindenter). Nanoindentation matrices of 15 × 15 were made by applying 5 gf for 10 s, to analyze the nanohardness distribution. In addition, 10 × 10 nanoindentation matrices were made to evaluate the hardness of the individual phases in the material by applying 1 gf for 10 s.

The hardening of the titanium matrix in Ti6Al4V/SiC composites is associated with (i) the scanning speed used during the sample fabrication process; (ii) the amount and size of SiC particles; and (iii) the amount and distribution of reaction products in the composite matrix. Regardless of these factors, the increase in the scanning speed, which reduces the heat input and increases the cooling rates, resulted in finer microstructures with higher microhardness.

On the other hand, the microstructural analysis revealed that microstructures were similar across the samples in most manufacturing conditions. The greatest differences occurred for unreinforced matrices and the smallest reinforcement size at low scanning speeds. This indicates that the cooling rate was homogeneous throughout the manufactured sample, what has been achieved by the reduction of the laser power during the manufacture, and the increase of the temperature of the platform. The variation in the microstructures of the composite materials is related to the difference in the heat input as a function of the laser scanning speed. The microstructural transformations will be influenced by the scanning speed.

When the molten pool is formed during the manufacturing process, the titanium matrix in liquid state dissolves the SiC particles, resulting in an enrichment in C and of the molten matrix. This mechanism is more pronounced when the temperature is higher and the contact time between the molten matrix and the particles is longer. Reducing the scanning speed increases the heat input and the interaction time, so lower scanning speed promote the dissolution of the SiCp.

According to Das et al. [36], the formation of reaction layers takes place in SiC/Ti materials as a result of the reactivity between Ti and C (preferably rather than between Ti and Si). They reported that the solidification begins at the SiC surface after the partial dissolution of the particles and that Si combines with Ti in the molten pool to form Ti₅Si₃.

Near the SiC particle, the carbon content is high, and the alloy is in the hypereutectic zone of the Ti–C phases diagram. When the TiC rich layer on the surface particle is smaller, primary TiC dendrites near the SiCp are formed. The reactions that occur during the solidification and the cooling process are the following:

The reaction continues so that L1 solidifies in β-Ti and eutectic TiC:

On the other hand, the silicon content in the titanium after the reaction layer is very high. As discussed above, Si is a β eutectoid element that leads to Ti₅Si₃ when combined with Ti at high temperatures. Primary Ti₅Si₃ is formed at the grain boundaries in the titanium matrix following the reaction [29]:

During the cooling times of the liquid phase, the β-Ti₅Si₃ eutectoid microconstituent forms, which will turn into a during the solid cooling. As the cooling of the parts occurs in metastable conditions, the transformation of the Ti₅Si₃ compound will not happen, according to Fiore et al. [37].

The reactivity between SiC particles and the Ti matrix results in a decrease in the SiC particles percentage and lower microhardness; however, the lower the heat input, the lower the reactivity and the higher the microhardness.

This approach implies that the TiC dendrites zone near the partially dissolved SiC particles in the Ti/20SiC F360-20 sample is smaller than in the sample fabricated using 5 mm/s, because the C rich zone is smaller due to the lower heat input. In the same way, the silicon content is lower due to the lower dissolution of the SiC particles, so, strip-like eutectic Ti₅Si₃ can be observed in the α-Ti phase limits.

The increment in the SiC size in the Ti/20 SiC F240 samples increases the microhardness, mainly at low scanning speeds because, although there is a reactivity between the particles and the titanium matrix, the size of the particles after the partial dissolution is higher. The increment in the scanning speed resulted in less reactivity since the net energy input is lower, so, for this reason, the difference in the microhardness in samples fabricated at high scanning speeds of 25 mm/s and with a variation in the particle size is lower. The increase in the SiC size has another effect on the microstructure: higher contents of Si and C are present in the matrix due to the higher reactivity at the surface of the SiC particles. For this content of Si and C, the formation of titanium carbides and silicides takes place, independently of the laser scanning speed and the zone of the 3D sample. A possible reaction mechanism may follow reactions (1) and (4) to form TiC and primary Ti₅Si₃, followed by reactions (2), (3) and (4).

Finally, the increment in the percentage of large SiC particles (F240) results in a higher microhardness independently of the chosen process parameters. This is due to the partial dissolution of SiC particles, they are well distributed in the matrix and keep their hardening effect. TiC also provides a hardening effect, thus, the higher the TiC proportion, the higher the microhardness. In addition, the molten pool temperature and the subcooling increase given the higher absorptivity of the SiC particles which increases the heat absorption. Therefore, the thermal conductivity increases, and this results in the formation of equiaxial TiC dendrites.

In the present work, the influence of DLD additive manufacturing process parameters, such as laser scanning speed, in the morphology, microstructure, and microhardness of the resulting Ti6Al4V/SiCp composite materials has been analyzed. Similarly, the effect of both size and concentration of SiC reinforcement introduced into the titanium matrix has been studied. It can be concluded that: