Influence of scanning speed on microstructures and mechanical properties of SLM produced Hastelloy X: as-built and solution-treated

Hastelloy X (HX) alloys with ideal strength and ductility match can be obtained by selective laser melting (SLM) and a proper follow-up heat treatment. This work studies the influence of scanning speed on grain size, grain boundary distribution, recrystallization and mechanical properties of as-built HX. These influences are reevaluated after a solution treatment at 1175 °C for 4h. The results reveal that the average grain size decreases, while the aspect ratio, texture intensity and the proportion of high-angle grain boundaries (HAGBs) increases with the increase of scanning speed. A small amount of recrystallization has occurred in the as-built alloys due to the cyclic thermal effect of SLM scanning. The finer grains and larger aspect ratio imply the higher energy storage during SLM, which will increase the recrystallizing nucleation rate. Solution treatment eliminates the fiber texture of 〈100〉//BD, significantly increases the HAGBs fraction and recrystallization fraction, reduces the grain aspect ratio, and coarsens the grains. With the increase of scanning speed, the strength of the Hastelloy X increases and the elongation decreases. The decrease of grain size is the main reason for the increase of yield strength.


Introduction
Hastelloy X (HX) is a solid solution strengthened Ni-Cr-Fe-Mo based superalloy with good oxidation resistance, corrosion resistance, formability and weldability [1,2].Due to the medium durability and creep resistance strength at 900 °C, it has been widely used in aerospace industries for hot gas path components such as combustors, exhaust-end components and transition pieces [3].These components typically have complex geometrics.However, owing to the high work-hardening capacity, it is difficult to machine complex parts with Hastelloy X [4,5].Therefore, additive manufacturing of Hastelloy X has recently attracted much attention [6,7].As one of the most popular metal additive manufacturing techniques, selective laser melting (SLM) technology uses a focused laser to selectively melt powder along the certain route and quickly bond it to form a part layer by layer [8].Compared with other metal additive manufacturing technologies, SLM has the advantages of high forming accuracy and good surface finish [9,10].
The SLM processing involves the interaction of laser, powder and solidified part, the microstructure, consequently, the forming quality and mechanical properties are affected by the composition and characteristics of the powder material, processing parameters and scanning strategies [6,11,12].Therefore, the exploration of the processing parameters is very important for obtaining good mechanical properties and forming quality.Besides laser power, scanning speed is also a key parameter of SLM, which affects the input energy density, microstructure, mechanical properties and forming efficiency.Esmaeilizadeh et al [13] reported the effect of laser scanning speed on the quasi-static tensile response of HX, where the extremely high (>1300 mm s −1 ) and low (<550 mm s −1 ) scanning speeds resulted in lack of fusion and keyhole defects, respectively.While moderate speeds (850-1300 mm s −1 ) made parts with high density in which grain size and yield strength showed a good agreement with the Hall-Petch equation [13].It was revealed by Deirmina [14] that the increased volumetric energy density (VED) led to the stronger texture, the larger lattice micro-strain, the higher fraction of low angle boundaries, and the increased yield strength of HX alloy.
Owing to grain boundary strengthening and dislocations hardening [15], the SLM produced HX part usually exhibits high strength, low plasticity and significant anisotropy [8,16], thus the effects of subsequent heat treatments on microstructure and mechanical behaviors have attracted much attention [17,18].Huang et al [19] gave a comparative study on the effect of hot isostatic pressing (HIP) , solution treatment (ST), and HIP+ST on the mechanical properties of SLM produced HX alloy.It was reported that the linear carbides at grain boundaries result in the decreased elongation of the HIP specimen at room temperature.However, the carbide size in HIP specimen was appropriate to restrict the crack propagation within a single grain boundary, giving rise to the better elongation at high temperature [19].Chen et al [20] reported that a high-temperature static recovery, and a mass precipitation of M 23 C 6 carbides were observed when the original SLMed HX alloy went through a solution treatment at 1050 °C for 1h.While, a complete recrystallization occurred when the holding temperature rose to 1150 °C.With the increase of holding temperature, the strength of SLMed HX decreased but the elongation increased significantly.
Accordingly, the investigation of the influence of processing parameters on the microstructure and properties of SLMed HX alloy, along with a comparison to the results obtained after typical heat treatment, is instrumental in optimizing the process and comprehending the relationship between microstructure and properties.Our previous work reveals that the average grain size, the aspect ratio, the proportion of low-angle grain boundaries and texture intensity of SLMed HX increase with the increase of laser power [21].However, other work shows that as the energy density increases, the aspect ratio of the average grain shape decreases significantly [22].Therefore, from the perspective of energy density, it is worth exploring whether the effect of scanning speed on the microstructural characteristics of HX alloy is opposite to that of laser power.In this work, the influences of scanning speed on grain size, grain boundary distribution, recrystallization and mechanical properties of as-built HX are studied.These influences are reevaluated after a solution treatment at 1175 °C for 4h.

Experimental procedure 2.1. Materials and SLM processing
The raw material for SLM is Hastelloy X alloy powder which was produced by Ningbo Zhongyuan Advanced Materials Technologies Co., Ltd using argon atomization technics.The chemical composition of the powder detected by the National Iron and Steel Material Testing Center is given in table 1.The particle size is distributed in the range of 15 ∼ 53 μm with a D10, D50 and D90 of 19.8 μm, 34.7 μm, and 55.8 μm, respectively.The SLM processing was carried out on a BLT-S210 machine with the detailed parameters as listed in table 2. The energy density E can be calculated as, where P, v, h and t refers to laser power (W), scanning speed (mm/s), hatch spacing (mm) and layer thickness (mm), respectively.The energy density is 70.3, 56.3 and 46.9 J mm −3 corresponding to the scanning speed of 800, 1000 and 1200 mm s −1 , respectively.Cubes (15 × 15 × 15 mm 3 ) were built for microstructure observation, while barbell-like vertical specimens were built for tensile experiment, as shown in figure1(a).Part of the as-built specimens were subjected to a solution treatment at 1175 °C for 4 h, then cooled by air.

Microstructure characterization
The microstructure and crystal orientation of specimens were characterized on a Zeiss Sigma 500 scanning electron microscope (SEM) equipped with the Oxford NordlysMax 3 electron backscatter diffractometer (EBSD).The vertical side of specimen was ground, and then mechanically polished with OPS solution for the EBSD detection.The EBSD field of view was 1500 μm × 1120 μm, with the step size of 2 μm.The post-data processing was performed by the HKL-Channel 5 software.Misorientation angles of adjacent grains above 15°w ere classified as high-angle grain boundaries (HAGBs), and those between 2°and 15°were classified as lowangle grain boundaries (LAGBs).

Tensile test
The tensile specimens were machined according to ASTM-E8M standard, with the geometry and dimension shown in figure1(b).The room-temperature tensile properties were tested at a loading speed of 1 mm min −1 on a SENS CMT-5105 electronic universal testing machine.The tensile tests were repeated twice to examine the repeatability of the results.

Results and discussion
3.1.As-built microstructure Figure 2 displays inverse-pole-figure (IPF) maps, grain boundary (GB) maps, histogram of grain boundary misorientation distribution (GBMD) and grain diameter distribution (GDD) of Hastelloy X produced at different scanning speeds.The GBMD histogram was obtained by classifying and statistical analysis of grain boundaries according to the misorientation angle.Figure 2(a) shows the 'fish scale' like microstructure obtained at scanning speed of 800 mm s −1 .With the increase of scanning speed, the grains become narrower and longer along the building direction (BD), showing the columnar growth characteristics in figure 2(c).During the solidification process, the temperature gradient G in the liquid phase at the front of the solid/liquid interface and the growth rate V at the front of the solidification jointly determine the morphology and size of the grain and the internal substructure [23].The larger the G/V is, the easier it is to form columnar crystal structure.In a single molten pool, G is usually large while V is small at the bottom of the molten pool, making columnar grains easy to form.On the contrary, the equiaxed grains are easily formed at the top.SLM uses layer by layer deposition, when the solidified part is remelted, the columnar crystals at the bottom of the molten pool grow epitaxial along the temperature gradient (mainly in the vertical direction) [24].According to the above solidification theory, it can be understood that under current experimental conditions, the increased scanning speed increases the temperature gradient and promotes the epitaxial growth of columnar grains.For the as-built HX alloys, grain boundaries with misorientation angle between 2 ∼ 4°occupy a dominant position, of which the proportion decreases from 25.5% to 19.3% with the increase of scanning speed.Meanwhile, the HAGBs fraction increases from 61.1% to 68.6%.The LAGBs can be regarded as consisting of a series of dislocations, and the decrease of its proportion indicates the decrease of dislocation density [25].
The grain size is expressed by the equivalent circular grain diameter.Based on the grain statistics, the average grain diameter (AGD) is calculated to be 32.5 μm, 28.0 μm and 25.7 μm for v = 800, 1000 and 1200 mm s −1 , respectively.The decrease of energy input leads to the decrease of molten pool volume and the increase of cooling rate, as well as an increase in partially melted particles [13], which lead to the refinement of grains [26].While, the mean aspect ratio (MAR) is 3.31, 3.55 and 4.14, respectively.It indicates the grains become slender with the increase of scanning speed or the decrease of input energy density.
Figure 3 shows the pole figures of SLMed specimens at various scanning speeds, which clearly demonstrates that there is a 〈100〉//BD fiber texture with a maximum pole intensity of 2.80, 3.87 and 4.81, respectively.The specimen obtained at higher scanning speed have stronger 〈100〉 texture intensity, which is related to its larger aspect ratio of grains.When the scanning speed increases, the larger temperature gradient and cooling rate are generated in the molten pool, which promotes the growth of fine cells and dendrites, and leads to the formation of a strong texture along BD [23].Compared to our previous work [21], it is noticed that in the current work, SLMed HX alloys have relatively low texture intensities, smaller grain sizes and aspect ratios, which could also be related to composition differences between powder batches, in addition to being affected by processing parameters.In some literatures [14,27], as the energy density increases, the texture intensity increases.While in the current work, the texture intensity gradually increases with the increase of scanning speed.This indicates that there may not always a positive correlation between energy density and texture strength over a wider range of processes.Figures 4(a)-(c) shows the polygonal/equiaxed crystal microstructure that is quite different from the as-built state, indicating that the static recrystallization has occurred.The driving force of recrystallization is the release of stored energy produced by the SLM process which mainly from the high-density dislocation region.It can be observed in figures 4(d)-(f) that the LAGBs depicted in red are quite rare, which means that the high-density dislocation regions have been almost eliminated and the dislocation density has been significantly reduced.As shown in figure 4(g), the HAGBs fraction of 800 mm s −1 specimen reaches 96.9%, and it increases slightly to 98.0% with the increase of scanning speed.It is noticed that the difference in grain misorientation distribution induced by varying scanning speeds (figures 2(g)-(i)) has been wiped out by heat treatment.The 60°grain boundaries are mainly distributed on the twin boundaries.These twin boundaries with misorientation angle of 60°are denoted as Σ3 boundaries.The proportion of 60°-Σ3 twin boundaries ranges from 57% to 60%, which indicates the presence of a large number of recrystallized annealing twins.For superalloys with low stackingfault energy (Hastelloy X, 45 ∼ 55 mJ m −2 ) [28], twinned grains are always observed in the microstructure after cold deformation and recrystallization annealing [29], as well as in the SLM+ heat-treated microstructure [30].It is generally believed that annealing twins are formed during grain growth process.
Based on the grain statistics (figures 4(j)-(l)), the AGD is calculated to be 34.6 μm, 33.2 μm and 28.5 μm for v = 800, 1000 and 1200 mm s −1 , respectively.Meanwhile, the MAR is 2.97, 2.96 and 2.68, respectively.It is worth noting that the AGD increases by 6.5 ∼ 18.6% and the MAR decreases by 10.3 ∼ 35.3% after heat treatment.The smaller grain size and aspect ratio (v = 1200 mm s −1 ) imply that static recrystallization is more fully occurred, since the finer original grain provides more recrystallization nucleation sites [31].Accordingly, it should have lower maximum pole intensity and higher recrystallization degree.
Figure 5 gives the pole figures of solution-treated specimens.After heat treatment, the crystal orientation shows a random distribution where the texture characteristics can be no longer observed.The maximum pole intensity decreases with the increase of scanning speed, which is inferred to be related to the original grain size.Grain refinement increases the number and area of grain boundaries, improves the recrystallization nucleation rate, and makes the recrystallization grains finer and the orientation distribution more uniform.
Figure 6 gives the recrystallization maps of as-built and solution-treated HX, where recrystallized grains, subgrains and deformed grains are depicted in bule, yellow and red, respectively.There are a large number of LAGBs in the red deformed grains, which contain a high density of dislocations.The blue parts are the undistorted recrystallized grains with the misorientation angle between 0 and 1.5°.The yellow parts can be considered as the subgrains with the misorientation angle between 1.5°and 7.5°that have undergone hightemperature recovery, but have not yet recrystallized.The recovery and recrystallization behavior of the as-built specimen has a certain relationship with the thermal influence of the subsequent scanning passes, and the subsequent laser scanning is equivalent to the cyclic heat treatment of the solidified part in a certain range [32].It is speculated that some of the solidified microstructures have been recovered or recrystallized during this process [33].
It can be observed from figures 6(a)-(c) that there are a large number of recrystallized nuclei in as-built specimens, some of which have grown into recrystallized grains, and the recrystallized fraction ranges from 5.7 to 11.1%.The recrystallized fraction increases gradually with the increase of scanning speed.As the scanning speed increases, the grains become finer with larger aspect ratio, which is similar to the microstructure that has undergone a lot of cold deformation.It implies the higher energy storage during SLM and the greater driving force for recrystallization, which will increase the recrystallization nucleation rate.Accordingly, the dislocation density in the alloy should be lower, which is consistent with the higher proportion of HAGBs (figure 2(i)).From the above perspective, the recrystallization behavior can be reasonably explained.After the solution treatment for 4h, the recrystallized fraction is larger than 84.3%,only a small number of recovered subgrains still remain.
Figure 7 depicts the variation of AGD (a), MAR (b), HAGBs fraction (c) and recrystallized fraction (d) with scanning speed, respectively, where the influence of solution treatment can also be observed.On the one hand, comparing with the as-built (AB) state, solution treatment (ST) reduces the MAR of grains via recrystallization, and coarsens grains by 6.5 ∼ 18.6% in diameter.Solution treatment leads to the increase of HAGBs fraction from 61.1% ∼ 68.6% to 96.9% ∼ 98.0% and the increase of recrystallized fraction from 5.7% ∼ 11.1% to 84.3% ∼ 95.9%.On the other hand, as the scanning speed increases, AGD gradually decreases, the HAGBs fraction and recrystallized fraction slightly increase, the MAR of as-built specimen increases while that of solution-treated specimen decreases.

Tensile properties
The histograms of tensile properties including yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) of the vertical specimens are given in figures 8(a)-(c), respectively.As can be seen in figure 8(d), the engineering stress-strain curves of the two specimens with the same parameters almost coincide, and the performance deviation of the vertical specimens is very small.The results can reflect the differences in tensile properties among different parameters.The YS is 480.0, 498.5 and 527.2 MPa for SLMed HX at scanning speed of 800, 1000 and 1200 mm s −1 , respectively.After solution treatment, the corresponding YS decreases significantly to 301.8, 305.7 and 309.5 MPa, respectively.At the same time, the elongation of as-built HX increases significantly from 45.8% ∼ 38.5% to 65.4% ∼ 58.6%, that is, the plasticity of SLMed HX alloys is improved.Comparatively, the influence of solution treatment on UTS is not significant, with a reduction from 720.0 ∼ 757.2 MPa to 698.7 ∼ 718.8 MPa.As mentioned before, the significant decrease in YS and the improvement in plasticity are mainly due to the reduction of dislocations and the occurrence of static   recrystallization during heat treatment [19].As can be seen from figure 8(d), the strain hardening ability of the HX alloys is enhanced by heat treatment, and a longer uniform plastic deformation stage is presented.The UTS values are maintained at a high level and the elongations are greatly increased, which is related to the reduction of dislocation density and yield strength.
From another point of view, the effect of scanning speed on tensile properties is also regular.That is both of YS and UTS increase with scanning speed, while EL decreases with scanning speed.This regularity is more obvious in the as-built state.The variation in YS can be explained by the Hall-Petch relation [13], as can be seen in figure 7(a), for both as-built and solution-treated alloys, grain size gradually decreases with the increase of scanning speed.A greater number of grain boundaries increases the ability to impede dislocation movement, resulting in an increase in yield strength.Heat treatment weakens the strength difference among specimens SLMed at different scanning speeds, which is related to the almost elimination of LAGBs (figure 7(c)) and the high degree of recrystallization (figure 7(d)).The elongation decreases with the increase of scanning speed, which is same as reported in literatures [13,33,34].As depicted in figure 7(a), the grain size decreases with the increase of scanning speed.This refinement of the microstructure, which may also be indicated from figures 6(a)-(c), means that the total number of grain boundaries has increased with the increase of scanning speed, which can result in an increase of the dislocation resistance and then in a reduction of the elongation.

Conclusions
This work has studied the influence of scanning speed on grain size and shape, grain boundary distribution and mechanical properties of as-built HX.The effect of solution treatment on the microstructure and properties of the as-built alloys has also been revealed.The main conclusions are given as follows.(1) With the increase of scanning speed, grain size decreases, while the aspect ratio, the HAGBs fraction and recrystallized fraction increase.The specimen obtained at higher scanning speed have stronger 〈100〉//BD fiber texture intensity, which is related to its larger aspect ratio of grains.A small amount of recrystallization has occurred in the as-built alloys due to the cyclic thermal effect of SLM scanning.The finer grains and larger aspect ratio imply the higher energy storage during SLM, which will increase the recrystallizing nucleation rate.
(2) As compared to the as-built state, solution treatment leads to the elimination of fiber texture, the increase of HAGBs fraction from 61.1% ∼ 68.6% to 96.9% ∼ 98.0%, and the increase of recrystallized fraction from 5.7% ∼ 11.1% to 84.3% ∼ 95.9%.The proportion of 60°-Σ3 twin boundaries is 57% ∼ 60%, indicating that recrystallized grains mainly grow via twinning.Solution treatment reduces the mean aspect ratio of grains, and coarsens grains by 6.5% ∼ 18.6% in diameter.
(3) With the increase of scanning speed, the strength of the Hastelloy X increases and the elongation decreases.The decrease of grain size is the main reason for the increase of yield strength.Heat treatment weakens the strength difference among specimens SLMed at different scanning speeds due to the almost elimination of LAGBs and the high degree of recrystallization.The yield strengths of as-built specimens are obviously decreased while the elongations are increased by solution treatment, which is mainly due to the reduction of dislocations and the occurrence of static recrystallization during heat treatment.

Figure 1 .
Figure 1.Photographs for the vertical specimens (a) and engineering drawing of the specimen for tensile test (b), the dimensions given are in mm.

Figure 7 .
Figure 7. Point plots of average grain diameter (a), mean aspect ratio (b), HAGBs fraction (c) and recrystallized fraction (d) Versus scanning speed.AB and ST are abbreviations for as-built and solution-treated, respectively.

Figure 8 .
Figure 8. Histograms of yield strength (a), ultimate tensile strength (b) and elongation (c) of as-built and solution-treated vertical specimens.The engineering stress-strain curves for vertical specimens produced at scanning speed of 1000 mm s −1 are given in (d).S1 and S2 are abbreviations for specimen 1 and specimen 2, respectively.

Table 1 .
Chemical composition of hastelloy X powder (wt%) given by the national iron and steel material testing center.