3.1 Densification and Physical properties
Densification is affected by temperature (which enhance the atomic diffusion) and pressure (responsible for the plastic deformation) conditions [15]. When these two phenomena occurred, the polycrystalline aggregates of the powders are deformed, occurring static recrystallization and grain growth.
In order to analyses the densification of the Inconel 718 specimens, the density was calculated by using the conventional equation (\({\rho }=\text{m}/\text{V})\). The weight and volume of each specimen were measured values and the respective calculated density and densification are presented at Table 3.. The Inconel 718 theoretical density reported in literature is 8.20 g/cm3 [16] so, it is possible to conclude that the density results of the produced specimens were close to the desired values (94.59–99.77 % of ensification).
Table 3
Calculated density and densification of the Inconel 718 specimens produced by Hot Pressing.
Designation
|
Calculated density (g/cm3)
|
Densification (%)
|
T1000P50
|
7.756
|
94.59
|
T1000P60
|
7.998
|
97.54
|
T1068P50
|
7.917
|
96.55
|
T1068P60
|
7.826
|
95.44
|
T1150P50
|
8.036
|
98.00
|
T1150P60
|
8.181
|
99.77
|
Theoretical values
|
8.200 [16]
|
100
|
Regarding the specimens produced by applying a pressure of 50 MPa, there was verified a decrease in the porosity with increasing temperature (5.41% for 1000 ºC, 3.45% for 1068 ºC and 2.00% for 1150 ºC). These results allowedus to conclude that the sintering temperature has influence on the specimen’s densification. On the other hand, it has been verified a very light increase on the densification with the pressure sintering (except for 1068ºC), so it is not enough to conclude about its influence on the densification.
The analysis of the grain morphology is extremely relevant once it allows to assess with specimen’s densification and mechanical behavior. In this sense, the influence of temperature and pressure on the grain features can be observed at Fig. 4. The grain behavior during the solidification is based on two main stages: the nucleation, in which stable nuclei are formed, and the growth of nuclei that give rise to crystals and form a grain structure. At lower sintering temperature (1000 ºC), small grains are dispersed and mixed with large grains, indicating that they only nucleated (but did not have the necessary temperature and time to grow). The atomic diffusion is usually driven by high temperatures resulting in a reduction in grain surface area and local curvature of free surface. Grains poorly bonded specific contact points (necks) were observed and was originated by a decrease of free energy surface. The low free energy surface inhibits the powder particles interaction by atomic diffusion mechanisms, creating voids. At 1068ºC, the grains presented a bi-modal random structure, with intermixed small and large grains which the majority of grown grains but the total recrystallization is still not complete. The surface contact stress between the powder particles is gradually reduced and the metal powder begins to recover and recrystallize. The intimately contacted powders stick to each other and the abovementioned voids are filled. Inconel 718 specimens produced at 1150 ºC revealed a greater grain size homogeneity there was not found grains that just nucleated.
Figure 4 showed the grain size of Inconel 718 specimens processed at different temperatures (1000, 1068 and 1150 ºC), considering sintering pressures of P50 and P60. In order to evaluate the influence of the temperature on grain growth, the grain measurements only considered the representative part of each of the specimens. Thus, for example, for the sample sintered at 1000 ºC, the grains that grew little were not considered, since they are not representative of what is verified. The global analysis allowed to conclude that the grain size increase with the sintering temperature because it leads to increase the crystallinity of the material and hence increases the number of crystallites.
The diffraction patterns of Inconel 718 powder and specimens produced by hot pressing is shown in Fig. 5. The spectrum is coincident with solid solution of austenite (\(\gamma )\) which is a single phased structure made of the main structure elements (Ni, Cr, Fe). The main strain strengthening mechanisms of Inconel 718 are the γʺ phase and order strengthening from the γ’ phase as well grain boundary strengthening [17]. The absence of this strengthening phase in these works is due to all HIPed specimens have been sintered above the solvus of the γʺ phase (915ºC) [18]. However, all the XRD spectrums revealed the stable strengthening \({\gamma }^{\text{'}}{Ni}_{3}(Al, Ti)\) that came from the Inconel 718 powder, which is also important improve strengthening of Inconel 718 parts whose usually require high strength and good corrosion resistance in wide temperature range. The rapid precipitation of \({\gamma }^{\text{'}}{Ni}_{3}(Al, Ti)\) phase is relevant for improving the strength but the low growth rate, when compared with \({\gamma }^{{\prime }{\prime }}\left({Ni}_{3}Nb\right)\), makes it a secondary strengthening phase.
By increasing the sintering temperature, some elements such as Cr precipitated and formed chromium carbides such as \({Cr}_{23}{C}_{6}\). Chromium carbides (\({Cr}_{23}{C}_{6}\) ) were coincident with \(\gamma\) at 44º for sintering temperatures between 1068 and 1150 ºC. The amount of \({Cr}_{23}{C}_{6}\) tend to decrease with the increase of temperature once this phase due its dissolution into the matrix [19].
Niobium (Nb) is an element of Inconel 718 alloy which is highly predisposed to segregation, and consequently form precipitates. The low solidification rate an high temperature during Hot Pressing is probably to enhance macrosegregation of the Nb, leading to the formation of undesirable phases \(\left(Nb, Ti\right)C\) for improving tensile ductility, fatigue and creep rapture properties. On the other hand, these Niobium Carbides are known as beneficial for improving hardness. XRD spectrums revealed different types of Niobium carbides such as \({Nb}_{4}{C}_{3} and\left(Nb, Ti\right)C\), at 35 and 40º in spectra for sintering conditions from 1000 to 1150 ºC. Niobium carbides were not revealed in XRD Inconel 718 powder spectrum because they were formed during solidification. This phase is considered a stable phase because its melting point is much higher (3600 ºC) compared with Inconel 718 alloy (1336 ºC).
3.3 Mechanical properties
The SEM images of the fracture surfaces (Fig. 6) allow analyzing the fracture mode and effectiveness of sintering conditions. These images showed a ductile fracture mode once it is possible to observe an extensive plastic deformation. The ductile fracture includes two distinct modes: intergranular and intragranular. Some specimens seem to have one single fracture mode and the remaining specimens a mix between different ductile fracture modes (intragranular mode with a marked shift towards intergranularity).
As already concluded, it was not verified an effective densification likely due to insufficient presence of the liquid phase. The fracture analysis of the T1000P50 specimen showed a completely split up of the powders which probably lead to the observed pure intergranular fracture mode. As the grains were poorly bonded, the crack propagation tended to be along the grain boundaries. However, the increase of pressure from 50 to 60 MPa seemed to strongly contribute for improving the compaction of the powders once the contact area between powders was increased and so the local plasticity was enhanced. In this sense, fracture mode of T1000P60 specimens seems to be intragranular despite having some marks of intergranular mode.
The increasing of temperature to 1068ºC revealed a strong influence on the powder’s consolidation, resulting in a better metallurgical bonding when compared with the specimens sintered at 1000 ºC. The fracture surface of both T1068P50 and T068P60 specimens showed intragranular fracture mode with particle-boundary decohesion along dimples. These dimples may result from an increase on atomic diffusion and extensive plastic deformation. When increasing pressure from 50 to 60 MPa, the local plasticity is more evident at the fracture surface. The microvoids found on the T1068P60 fracture’s surface can have been originated by pure mechanical loads or a from oxidation of grain boundary precipitates.
For the higher sintering temperature (1150 ºC), there was verified a ductile intragranular fracture mode for both sintering pressures, with a good metallurgical bonding and powders consolidation. The specimens showed an almost completely dimple fracture which could be attributed to high atomic diffusion and plastic deformation [20].
Figure 7 presents the hardness results of Inconel 718 specimens grouped by temperatures of 1000, 1068 and 1150ºC in order to assess the hardness evolution with increase of sintering temperature.
When considering a temperature of 1000 ºC, the hardness results obtained were considerably lower than the remaining conditions (241.35 ± 33.93 and 219.98 ± 13.85 HV). Although the XRD spectrum have shown the presence of hardening phases such carbides (NbC and \({Nb}_{4}C\)), the poorly bonded powders (Fig. 6) are responsible for the global low hardness values. Although the smaller GS induces high hardness results due to the high amount of dislocations, the lack of powders consolidation was harmful to the hardness results. The specimens produced at 1068ºC and 1150ºC sintering temperatures revealed similar hardness results. The increase of temperature is not only essential for a strong metallurgical bonding (once enhance the atomic diffusion between the grains) but it can also promote the precipitation and solubilization of the elements. XRD spectrum revealed the precipitation of chromium carbides (\({Cr}_{23}{C}_{6})\) at 1068ºC sintering temperature but its dissolution at T1150P60 condition. Knowing that carbides enhance the hardness results, the decrease of the amount of \({Cr}_{23}{C}_{6}\) with temperature may are responsible for the decrease on hardness from 292.00 ± 0.00 to 283.90 ± 9.10 HV when considering 60 MPa as pressure sintering [19].
This study presented higher hardness values than those found in literature regarding Inconel 718 specimens produced by other sintering technologies (Table 4).
Table 4
– Hardness values (HV -Vickers) of Inconel 718 specimens produced by several manufacturing techniques (B – Bottom; T -Top; V-Vertical).
Process
|
Hardness (HV)
|
REF
|
This study
|
292.0 (100 g load)
|
-
|
As-SLMed
|
B (211.3) T (204.9) V (236.9) (50g load)
|
[21]
|
As-SLMed + Homogenization
|
B (289.1) T (260.0) V (282.6) (50g load)
|
As-SLMed + HIP
|
B (181.1) T (175.7) V (180.5) (50g load)
|
Laser Deposition
|
275–350 (300 g)
|
[22]
|
Field assisted hot pressing (FAHP)
|
256.3 (1000 g)
|
[23]
|
Microwave sintering
|
191.2 (1000 g)
|
Metal injection Moudling
|
211.3 (1000 g)
|
Seede et al. [21] compared the microhardness in the bottom, top and vertical surface of the as-printed, homogenized (1100ºC during 1 hour) and hot isostatic pressed (1160ºC under 100 MPa of pressure for 4 hours) SLMed specimens. Microhardness in the as fabricated (236.9 HV) is 19.3% lower than the homogenized specimens (282.6 HV) due to more even distribution of secondary precipitates and the nucleation of smaller grains. In the opposite, the specimens with less amount of nucleation of smaller grains revealed lower hardness results (T1000P50 and T1000P60). However, these specimens also revealed poor densification with lack of diffusion and voids which lower the hardness results. The homogenized specimens presented the higher hardness results and closer to values to those obtained at this study (292.0 HV). This was attributed to the large dispersion of \({\gamma }^{{\prime }{\prime }}\left({N}_{3}Nb\right)\) precipitates, revealing the presence of 100% of \(\gamma \left(CrNi\right)\) on XRD analysis. Parallelly, the specimens produced by Hot Pressing also revealed the \(\gamma\) phase at a similar temperature (1068ºC) and hardening Chromium Carbides such as \(NbC\) and \({Nb}_{4}{C}_{3}\) originated by the large macrosegregation of Nb during solidification. On the other hand, the specimens subjected to HIP process showed a decrease on the hardness when compared with both as-SLMed and homogenized specimens. The high temperatures, pressure and time employed leaded to the high dissolution of \({\gamma }^{{\prime }{\prime }}\)phase leading to a significantly reduction of hardness results. Parallelly, Zhang et al [22] evaluated the microhardness of fibre laser deposited Inconel 718 using filler wire and also suggested that the dissolution of the principal strengthening phase \({\gamma }^{{\prime }{\prime }}\) during the laser cladding leaded to a lower hardness results than other heat treated and aged conditions.
Dugauguez et al [23] compared hardness results between three distinct processing techniques. These results should not be compared with the microhardness values due the influence of the indentation load/size effect (ISE) [24]. The specimens produced by FAHP presented higher hardness result (256.3 HV) than when compared with the remaining processes studied. The specimens produced by microwave sintering process revealed the lower hardness result (191.2 HV) which was attributed to the presence of cracks on the surface due to the violent debinding and the occurrence of hot spots inside the specimen during sintering. The hardness of HIP treated specimens revealed proportional to the temperature because the increase in temperature enhances the solubilization and precipitation of the elements. In the present study, the low hardness results also were attributed of the presence of some defects such as voids (specimens produced at 1000ºC). The increase on hardness with temperature were also attributed the precipitation of some elements but also the increase on atomic diffusion and consequently powders consolidation.