3.1 Microstructures
Optical microstructures of Ti-6Al-4V round specimens with typical sintering and HIP conditions are shown in Fig. 3. After sintering, there are several tiny isolated pores with preferentially rounded shape. At the sintering temperature of 1000°C, gaps between adjacent particles gradually spheroidized due to the diffusion of atoms, and the crystal structure of the raw material powders was partly retained due to incomplete sintering [15]. The higher sintering temperature of 1150°C could promote the migration of material and the discharge of pores, resulting in the reduction of pore number. When the sintering temperature was too high (1300°C), closed pores appeared early and gas in the pores could not be discharged smoothly, which resulted in excessive grain growth and increased pore size at grain boundaries.
Compared with argon atmosphere [16], vacuum sintering in this study could prevent the accumulation of gas in the pores, and thus decreasing the porosity. The vacuum atmosphere could also lower the melting point of Ti-6Al-4V, thereby decreasing the sintering temperature. The use of fine powder (D50 = 14.1µm) is another reason for lowering sintering temperatures compared with previous studies [26, 27]. Fine powder allows the formation of fully homogeneous and fine-grained material at relatively low temperature. If coarse powder had been used under the same sintering condition, a large number of residual pores would appear. In the latter case, higher sintering temperatures and/or longer holding times were required to produce homogeneous and high-density materials, which inevitably led to undesirable grain growth [28]. The pore number decreased for all specimens after HIP. The lower the sintering temperature, the more obvious improvement of HIP on porosity. The mechanism of pore healing is its diffusion along the grain boundary from the core of the material to the surface [29].
The thickness of round specimen after sintering is about 2.3 mm. With LAMOS APorosity software, the thickness is divided into several rectangular zones. The width of each sub-region is 50 µm, as shown in Fig. 4a. The relationship between the porosity of each sub-region and the corresponding thickness is shown in Fig. 4b. The surface porosity of the specimen is relatively low and increases towards the core of the material. Sub-regions with relatively low porosity (≤ 0.15%) near the surface can be defined as “dense layer”. The thickness of dense layer can be obtained through the porosity - thickness diagram in Fig. 4b, which is represented by the red dotted line and arrow. The phenomenon of the dense layer is very similar to that of bread (as shown in Fig. 4c) and can also be called a “bread model”. Dense layers under different conditions are marked with red dotted lines and boxes in Fig. 3. After debinding process, the binder was burnt out and the brown part had porous microstructure from surface to core. During sintering process, heat was generated by heating elements and transferred to specimens via radiation (shown in Fig. 4d). The surface of specimen received heat preferentially, and thus had a higher temperature and densified first. Some pores were entrapped in the core due to the elimination of discharge channel. The thickness of dense layer for specimens under different sintering and HIP condition are shown in Fig. 4e. For specimens without HIP, the thickness of dense layer increases first and then decreases with the increase of temperature. However, specimens with lower sintering temperature have thicker dense layer after HIP. The reason will be analyzed later.
XRD patterns of Ti-6Al-4V are shown in Fig. 5. The alloy under different conditions are all composed of α phase and a small amount of β phase. Alpha-case often appears in the casting and forging process of titanium alloys, and TiO2 and intermetallics are necessary components of it [30]. No related phases were detected in the XRD pattern, which also shows that the dense layer of injection molded Ti-6Al-4V formed after sintering and HIP is essentially different from the alpha-case. Actually, high vacuum in sintering process and high purity argon atmosphere in HIP process avoid the formation of alpha-case.
EBSD maps of Ti-6Al-4V without and with HIP are shown in Fig. 6 and Fig. 7, respectively. The red area represents α phase and yellow area is β phase. Inverse pole figure (IPF) maps for different conditions are shown in Fig. 6d, e, f and Fig. 7d, e, f. The microstructure of specimen sintered at 1000°C (Fig. 6a) shows typical equiaxed structure, which composed of equiaxed α phase and a small amount of intergranular β phase.
The reason is that the sintering temperature of 1000℃ is close to the phase transition temperature of Ti-6Al-4V, and the phase transition is not sufficient, which basically keeps the phase state of the powder. With the increase of sintering temperature, phase transformation becomes more complete. Densification is achieved by interactions among the powders, while nucleation and grain growth are mainly accomplished by the interactions of adjacent phases [15]. After HIP, the pores reduced and there was no trace left during processing.
Figure 8 shows EBSD grain size distribution of Ti-6Al-4V for different sintering temperature without and with HIP. The grain size increases with the increase of sintering temperature not matter for α or β phase. HIP also make the grain size slightly larger compared with the corresponding sintered state. Larger grain sizes produce larger pores at grain boundaries. Therefore, the higher the sintering temperature, the weaker the improvement effect of HIP on the dense layer. The improvement of densification by HIP also gradually weaken from surface to core.
The grain boundary angle of α phase was calculated and calibrated according to the misorientations between adjacent grains, as shown in Table 2. An orientation difference of < 10° was defined as low-angle grain boundaries (LAGBs) and > 10° was defined as high-angle grain boundaries (HAGBs) [31]. Specimens sintered at 1000°C have equiaxed grain, and thus HAGBs occupy the majority for α phase. As the sintering temperature increased, the grains gradually changed from equiaxed to lamellar, and the grain orientation tended to be consistent, and thus decreasing the proportion of HAGBs. In addition, HIP increased the ratio of HAGBs for α phase.
Table 2
EBSD misorientation angle of α phase of Ti-6Al-4V under different conditions
Process | LAGBs (%) | HAGBs (%) |
Sintering at 1000℃ without HIP | 21.5 | 78.5 |
Sintering at 1150℃ without HIP | 37.4 | 62.6 |
Sintering at 1300℃ without HIP | 81.1 | 18.9 |
Sintering at 1000℃ with HIP | 11.1 | 88.9 |
Sintering at 1150℃ with HIP | 29.1 | 70.9 |
Sintering at 1300℃ with HIP | 51.2 | 48.8 |
3.2 Mechanical properties
Figure 9 shows the mechanical properties of Ti-6Al-4V without and with HIP. For specimens from 1000°C to 1150°C without HIP, the elongation increases significantly, while the tensile strength and yield strength increase slightly. Strength and elongation decrease gradually with the further increase of sintering temperature. All of the increases in tensile strength, yield strength and elongation were achieved by applying an additional HIP process after sintering. Specimens sintered at 1000°C without HIP have lower elongation compared with that of 1050–1150°C, the reason should be the positive effect of fine grains on elongation is masked by higher porosity [28, 32]. Lower porosity and fine grain size were achieved for specimens sintered at 1000°C after HIP, elongation have also been significantly improved. In tensile test, porosity reduced the effective load bearing cross-section area and acted as a stress concentration site for strain localization and damage, decreasing both strength and elongation. This explains why the strength of the as-sintered specimens is lower than that of as-HIPed specimens [33–36]. In addition, the oxygen equivalents before and after HIP were both below 0.4 wt.%, and thus the content of interstitial elements had no significant effect on the mechanical properties [20].
The strength of metallic materials is improved with decreasing grain size. The relationship between yield strength and grain size of metallic materials that have equiaxed grain, is explained by mechanism of fine grain strengthening and Hall–Petch relationship [37]. For specimens with fine equiaxed grains, there are more grain boundaries per unit volume. When the adjacent grains with different orientations were subjected to deformation, the dislocation sources in some grains were activated first, and then slipped and multiplied along a certain crystal plane. The dislocations that slip before the grain boundaries were blocked by grain boundaries, thereby increasing the strength. Equiaxed grain reduced the stress concentration and tended to be isotropic, and thus benefiting the elongation of materials. The coarse martensite morphology in higher sintering temperature directly led to decreases in the strength and elongation of material.
The typical process and corresponding mechanical properties in this study are compared with the standard and previous studies, as shown in Table 3. Compared with previous research [38–41], finer powder was used in this study to increase the sintering activity, and thus decreasing the sintering temperature and energy consumption. Relatively fine powder and low sintering temperature promote grain refinement and grain boundaries increase, which is conducive to high tensile strength and elongation. In this study, the comprehensive properties of elongation and tensile strength for specimens sintered at 1150℃ without HIP and 1000℃ with HIP not only meet the requirements of ASTM F2885-17 and ISO 22068 − 2012, but also are superior to that of corresponding literature.
Table 3
Comparison with standard and previous study
Alloy | Median powder diameter (D50, µm) | Sintering temperature (℃) | HIP | Tensile strength (MPa) | Elongation (%) | Ref. |
Ti-6Al-4V | N/A | N/A | without | 780 | 10 | ASTM F2885-17 |
Ti-6Al-4V | N/A | N/A | with | 900 | 10 | ASTM F2885-17 |
Ti-6Al-4V | N/A | N/A | without | 800 | 3 | ISO 22068 − 2012 |
Ti-6Al-4V | 18 | 1200 | without | 934.3 | 12.14 | [38] |
Ti-6Al-4V | 24.5 | 1250 | without | 902 | 13 | [39] |
Ti-6Al-4V | 24.5 | 1250 | with | 973 | 16 | [39] |
Ti-6Al-4V | 28.41 | 1300 | without | 923 | 5.9 | [40] |
Ti-6Al-4V | 28.41 | 1300 | with | 978 | 6.3 | [40] |
Ti-6Al-4V | 28.8 | 1350 | without | 837 | 15.1 | [41] |
Ti-6Al-4V | 14.1 | 1150 | without | 1015 | 14 | This work |
Ti-6Al-4V | 14.1 | 1000 | with | 1067 | 17.5 | This work |
Relationships between mechanical properties and dense layer of Ti-6Al-4V are shown in Fig. 10. The mechanical properties (tensile strength, yield strength and elongation) of the sintered Ti-6Al-4V without HIP have little correlation with the thickness of the dense layer. In contrast, the mechanical properties (tensile strength, yield strength and elongation) of Ti-6Al-4V with HIP have an exponential relationship with the thickness of dense layer, but the growth rate of mechanical properties gradually slows down with the increase of dense layer thickness. The proportion of dense layer thickness in the whole thickness direction determines its influence on mechanical properties. After sintering, the thickness of dense layer is small, and the proportion in the whole thickness direction is also small, thus it has little influence on the mechanical properties. The thickness of dense layer is significantly increased after HIP, and its proportion in the whole thickness direction and influence on mechanical properties are also increased. However, with the further increase of dense layer thickness after HIP, the porosity of both core and surface reach a very low level, its influence on the mechanical properties tends to be fixed. |
In summary, mechanical properties are the combined effects of porosity, dense layer, grain size, grain morphology and grain orientation. In addition, secondary processes such as finish machining and polishing are always needed for some MIM products with relatively high dimensional accuracy or cosmetic requirement. The thick dense layer can ensure the residue after machining or polishing, thereby avoiding the adverse effect of pores on the appearance of products, especially mirror-polished products.