Balance of strength and ductility for laser directed energy deposited Ta-containing Ti6Al4V via solution and aging treatment

The tensile properties of laser directed energy deposited (LDEDed) titanium (Ti) alloys are usually characterized by high strength and low ductility, which seriously limits their wide application prospects. In this study, a novel Ti6Al4V4Ta alloy is prepared using LDED technique and achieved excellent strength (UTS ∼ 990 MPa) and ductility (elongation ∼ 16%) through solution treatment at 930 °C and aging at 550 °C. The high strength is mainly attributed to the ultrafine micron-scale α s while the good ductility results from equiaxed α p, the increased β phase and weakened α-variant selection. The combination of appropriate elemental alloying and heat treatments could provide some guidance for LDED Ti alloys of good mechanical performance with potentials for wider applications.


Introduction
Ti6Al4V alloy shows high strength to weight ratio, good creep resistance and excellent biocompatibility, considered as the ideal structural material for the aerospace and biomedical applications [1,2].Ti6Al4V alloy is traditionally manufactured by forging and machining techniques, often suffering material underutilization and long lead time, which hinders their broader applications [3].As a new manufacturing method, laser directed energy deposition (LDED) can achieve near-net-shaped fast fabricating and high material utilization [4,5]; however, the rapid heating and cooling rates during the LDED process can introduce high levels of thermal stresses and undesired microstructures, making it difficult to have high strength while maintaining good ductility [6,7].
Elemental alloying is often used to improve mechanical performance of LDEDed Ti6Al4V alloy.As a typical α + β two-phase alloy, microstructure refinement of Ti6Al4V alloy is usually achieved by adding β-stabilizing elements, as the β-isomorphous group (Ta, Mo and Nb) can optimize the microstructure and enhance mechanical properties [8][9][10][11].Among these β stabilizers, Ta element has the advantages of high temperature resistance, high corrosion resistance and non-toxicity, yet to be explored systematically.In addition to elemental alloying, heat treatment is also important method to balance the strength and ductility by tailoring the microstructure.Recently, Chen et al acquired the balance of strength and ductility of a novel Ti alloy with multiscale microstructure containing nano-scale acicular secondary α (α s ) and equiaxed primary α (α p ), by setting the solution temperature at 900 °C for 1 h then aging at 550 °C for 6 h [12].Zhao et al used the solid solution treatment and aging (STA) approach in LDEDed Ti6Al4V alloy, and obtained multi-scale microstructures consisting of equiaxed α p and ultrafine α s lamellar, which largely enhanced the overall ductility with good tensile strength [13].These studies indicate that STA approach is very effective in balancing the strength and ductility of LDEDed Ti alloys by optimizing the multi-scale microstructure.
This work aims to research the effect of different aging temperatures on the microstructure and mechanical performance of LDEDed Ti6Al4V with the addition of Ta (4 wt%).The content (4 wt%) of Ta was ascertained from Ti6Al4V4Ta alloy exhibiting the best mechanical tensile results among different weight percentages (2, 4 and 6 wt%) [14].Multiscale-α microstructure was achieved by choosing appropriate STA approach in the Ti6Al4V4Ta alloy, showing strength reached almost 1000 MPa with total ductility over 15%.

Materials and methods
The Ti6Al4V and Ta powders with the particle size of 45-120 μm were used to fabricate Ti6Al4V4Ta Ti alloy samples by a LDED system, including a 6 kW fiber laser cladding head equipped with the coaxial powder feeding, and the schematic representation of the LDED process is shown in figure 1.The forged Ti6Al4V plates with the three-dimensional size of 100 × 100 × 15 mm were adopted as substrates in the LDED experiment.
Based on our previous research [14], the optimal LDED processing parameters for depositing Ti6Al4V4Ta alloy are given in table 1.In the design of STA process parameters, the β transus temperature (T β ) of Ti6Al4V4Ta alloy was calculated by the Thermo-Calc software to be 974 °C, as presented in figure 2(a).In order to inhibit the growth of grains, the solution temperature is generally selected at 30-50 °C below T β [6,13].Therefore, the solution treatment for deposited Ti6Al4V4Ta samples was carried out at 930 °C for 1 h, then cooled in the air.Based on STA approaches of LDEDed Ti6Al4V alloy [13], subsequent aging treatment was set at 500 °C, 550 °C, and 600 °C for 4 h, then cooled in air as illustrated in figure 2(b).These specimens were labeled as HT500, HT550 and HT600, respectively, according to their aging temperatures.
The phase composition of the as-deposited and heat-treated Ti6Al4V4Ta alloy was identified by x-ray diffraction (XRD, Empyrean).Microstructural characterization in prior-β grains was performed on scanning electron microscopy (SEM, Zeiss SUPRA 55).The preferred selection of α-variants was characterized by  electron back-scattered diffraction (EBSD).Tensile tests were performed using SANS CMT5305 at a constant speed of 0.5 mm min −1 at room temperature.

Results and discussions
The XRD patterns of the as-deposited and heat-treated samples are presented in figure 3, composed of only α and β phases despite of the different STA approaches.However, a diffraction peak of the β phase appears at 39°a fter the STA process, suggesting the increase of β phase content.This is also confirmed by the EBSD results, as the β area fractions of as-deposited, HT500, HT550 and HT600 samples are found to be 5.6%, 15.7%, 16.9% and 14.9%, respectively.During the STA process of the Ti6Al4V alloy, some α phase can be retransformed to β phase, resulting in a supersaturated β matrix under the AC condition [13].Therefore, during the solution treatment process at 930 °C for 1 h followed by AC in Ti6Al4V4Ta alloy, some α-laths transformed into β phase, which resulted in the increase of β area fraction.
Figures 4(a)-(d) presents the microstructure evolution of the as-deposited and heat-treated samples containing α and β grains.These grains were measured by Image-Pro Plus (IPP) image processing software for quantitative analysis.The as-deposited Ti6Al4V4Ta alloy exhibits the basket-weave microstructure that consisted mainly of acicular α-laths with the width of ∼1.5 μm and the length of ∼10 μm, as shown in figure 4(a).After STA, the basket-weave microstructure is replaced by a multiscale-α microstructure in HT500, HT550 and HT600 samples, which is comprised of coarse α p with the width of ∼2 μm and the length from ∼5 μm to ∼20 μm, small equiaxed-α (α e ) with the grain size of ∼3 μm and ultrafine nano-scale α s , as exhibited in figures 4(b)-(d).The dimensions of coarse α p and α e are similar in HT500, HT550 and HT600 samples.With the aging temperature increasing, the morphology and content of α p -laths and α e are not changed significantly,  indicating that α p and α e are insensitive to aging temperature.However, it is obvious that most ultrafine α s -laths are formed uniformly when the aging treatment temperature set at 550 °C.During solution treatment process at 930 °C for 1 h, the acicular α-laths grow up to the coarse α p .This process of α-laths coarsening could be attributed to the effect of Ostwald ripening during the holding solution process [15].The following AC process suppresses the further growth of the α-laths, which results in the formation of small equiaxed α p .At the aging heat treatment of at 500 °C-600 °C for 4h, substantial dispersed ultrafine α precipitates (nano-scale α s ) in the β phase [6].Finally, the multiscale-α microstructure with coarse α p , equiaxed α p and ultrafine nano-scale α s is obtained through the STA approach.
To analyze the elemental evolution in α and β phases, the SEM-EDS point analysis was performed by selecting four different points in as-deposited and HT550 samples, and the corresponding results are shown in table 2. In the as-deposited Ti6Al4V4Ta alloy, the elemental distribution inside α and β phases suggests that Al element (α phase stabilizer) is enriched in the α phase while V and Ta (both are β phase stabilizers) are enriched    [9,16] as the EDS results of LDEDed Ti6Al4V show the high content of Al in α phase and that of V in β phase.By comparative analysis, the composition difference of the as-deposited and HT550 samples is small in α phase and β phase, respectively, indicating the STA process has little effect on the elemental distribution.
Based on the crystal symmetry between α and β, there are twelve different α-variants in prior-β grain [17].Figures 5(a)-(h) shows IPF maps and the areal fraction of the α-variants of the as-deposited and heat-treated Ti6Al4V4Ta alloys, which are labeled α1 -α12.As seen in figure 5(f), all 12 α-variants appeared in HT550 samples, and eight variants are stable at 10% and four variants are stable at 4%.However, although eight variants are stable at 7.5% in HT600 samples (figure 5(h)), α12 variant completely disappears and α7 variant takes the highest areal fraction, accounting for ∼18% of total area.Comparing and analyzing statistics of different αvariants in these samples, it can be seen that the area fractions of most α-variants are basically stable at ∼10% in the HT550 specimen, showing the least variant selection, which is beneficial to enhance the ductility [17].
Figure 6(a) exhibits the engineering tensile curves of as-deposited and heat-treated Ti6Al4V4Ta alloys.Compared with as-deposited alloy, heat-treated alloys obtained the higher elongation, but the tensile strength was slightly lower than that of the as-deposited alloy.As reported by Zhang et al [18], although the coarsening of α p -laths can decrease the strength after STA, a large amount of ultrafine α s can enhance the strength by preventing the dislocation movement.Therefore, more ultrafine α s are formed through 930 °C solution treatment and 550 °C aging, resulting in the maintenance of a comparable ultimate tensile strength (UTS, ∼990 MPa), which is 10% higher than that of the forged Ti6Al4V standard (∼895 MPa) in ISO 7209 [19].Excitingly, the elongation is improved to 16%, showing 60% enhancement in comparison with that of the forged alloy.The desirable elongation is attributed to a small amount of equiaxed α p with good deformation ability [13], the increase of β phase content and the weakened α-variant selection.Base on this, compared with the forged Ti6Al4V alloy (figure 6(b)), Ti6Al4V4Ta alloy obtained an excellent balance of strength and ductility (UTS of ∼990 MPa and elongation of ∼16%) by 930 °C solution treatment and 550 °C aging.
Table 3 shows a clear comparison of the UTS and total elongation to failure for the element alloyed Ti6Al4V alloy made by LDED, cold metal transfer additive manufacturing and vacuum non-consumable electrode arc melting under heat treatment or not [8,16,20,21].It can be seen that the ductility of Ti6Al4V4Ta alloy (HT550 sample) well exceeds that of the other elements (Mo, Nb and Y) alloyed Ti6Al4V or corresponding heat-treated samples.Additionally, although the UTS and total elongation of both as-LDEDed Ti6Al4V [20] and HT550 sample are exceeding the forged Ti6Al4V standard (ISO 7209) [19], HT550 sample performs better total elongation, indicating that the Ti6Al4V4Ta alloy can obtain an excellent balance of strength and ductility by the STA process.
The fracture morphologies of as-deposited and heat-treated Ti6Al4V4Ta alloys are presented in figures 7(a)-(d).Deep and large dimples appeared on the fracture surface of the heat-treated samples, showing a typical ductile failure characteristic.The formation of deep and large dimples indicates that heat-treated samples possess an excellent ductility, which corresponds to their engineering tensile curves.
The Ti6Al4V4Ta alloy prepared in this study using LDED shows excellent strength and ductility, well exceeding the forging standard requirements of Ti6Al4V alloy (ISO 7209).This is benefited from adding Ta as the key β-stabilizing element, in combination with suitable heat treatment process.Our results confirm that the optimized microstructure should consist of multiscale-α microstructure, reasonable β phase size and distribution.In addition, adding the Ta element can also promote the equal growth of more α-variants.These results could provide some basic guidelines for fabricating high-performance LDEDed Ti alloys using elemental alloying and STA approach.

Conclusions
In this work, we studied the effect of aging temperature on the microstructure and tensile property of the new Ti6Al4V4Ta alloy.The multi-scale microstructure consisting of coarse α p , small α e , ultrafine nano-scale α s and retained β is achieved by choosing appropriate STA approaches.Ti6Al4V4Ta alloy obtained an excellent balance of strength and ductility (UTS of ∼990 MPa and elongation of ∼16%) through solution treatment at 930 °C and aging at 550 °C.The tensile strength mainly derives from the ultrafine nanoscale α s while the good ductility results from equiaxed α p , the increased β phase and the weakened α-variant selection.Therefore, this work  clearly demonstrated that the combination of elemental alloying and appropriate heat treatment can be used to improve the mechanical performance of additive manufactured Ti alloys.

Figure 1 .
Figure 1.The schematic representation of the LDED process.

Figure 2 .
Figure 2. (a) Calculated Ti6Al4V4Ta phase diagram through Thermo-Calc software, (b) schematic illustration of the solid solution treatment and aging.

Figure 3 .
Figure 3.The XRD patterns of the as-deposited and heat-treated samples.

Table 2 .
The SEM-EDS result of the selective spots from the asdeposited and HT550 samples in figures 4(a), (c) (wt%).
in β phase, showing very different solubility.The distribution of Al and V elements is consistent with the previous studies

Table 3 .
Tensile properties compared with the element alloyed Ti6Al4V alloy under heat treatment or not.The representative value chosen is the maximum in the literature.