Assessing the thermal stability of laser powder bed fused AlSi10Mg by short-period thermal exposure

ABSTRACT Laser powder bed fused (LPBFed) AlSi10Mg is recognised for its superior mechanical properties. However, its thermal stability has never been justified. Herein, we exposed as-built AlSi10Mg to different temperatures (200–500°C) for only 3 min to evaluate its thermal stability. Results showed that LPBFed AlSi10Mg had relatively low thermal stability. Only 3 min of thermal exposure at 200°C would deteriorate its tensile strength dramatically. Microstructural analysis revealed that with increasing thermal input, as-built AlSi10Mg exhibited a microstructural evolution similar to annealing of cold-worked metals, namely recovery, recrystallisation followed by grain-growth. The excessive energy stored in as-built microstructure due to fast cooling during LPBF was deduced as the driving force for this phenomenon. Therefore, such microstructural change was at the expense of dislocations stored in the as-built material, which in turn caused deterioration in tensile strength. The present findings may provide guidance for the application of LPBFed AlSi10Mg.


Introduction
The origin of laser powder bed fusion (LPBF), also known as selective laser melting (SLM), dates back to 1995, when a patent (DE19649865) was filed for LPBF by Fraunhofer Institute for Laser Technology (ILT) (Wilhelm, Konrad, and Andres 1998).Later, Dr. Matthias Fockele and Dr. Dieter Schwarze developed the first commercial LPBF machine (Gunasekaran et al. 2022).As the LPBF technique becomes more and more powerful and sophisticated, it has attracted significant attention in materials world (Herzog et al. 2016;Gu et al. 2021;Li et al. 2021).One of the most fascinating advantages of LPBF is its near-net-shape forming characteristic, which has enabled unlimited possibilities in producing complicated and customised components to be used as prototypes or even applied in real world.Up to date, it has been used to process many metallic alloys such as steels (Sun et al. 2018;Li, Qu, and Bai 2021), Ni alloys (Li, Thomas, and Hutchinson 2022;Wu et al. 2020), Ti alloys (Zhang, Qiu, et al. 2019;Kong et al. 2022), Mg alloys (Liang et al. 2022;Wang, Liu, et al. 2022), Zn alloys (Zhao et al. 2022), Al alloys (Schimbäck et al. 2022;Tan et al. 2022), high entropy alloys (Han et al. 2020) as well as their composites (Yu et al. 2019;Wang et al. 2020).Within all those metallic alloys, Al alloy is more challenging to be fabricated by LPBF due to its inherent low density, low fluidity, high thermal conductivity and low laser absorptivity (Aboulkhair et al. 2014(Aboulkhair et al. , 2019)).
The typical tensile strength of LPBFed AlSi10Mg is ∼450 MPa (Chen et al. 2018;Zhang et al. 2021;Wu et al. 2021), which is about 43% higher than conventional casting AlSi10Mg (∼315 MPa) (Tang and Chris Pistorius 2017).This is not only much superior to LPBFed conventional AA2024 (∼20-200 MPa) (Wang, Lin, et al. 2022), but also comparable to those LPBFed AA2024 modified by various inoculants (∼280-450 MPa) (Chen et al. 2022;Tan, Zhang, Sun, et al. 2020).Such outstanding mechanical property endues LPBFed AlSi10Mg with promising potential in various applications, which could trigger unlimited possibilities for relative fields.However, thermal stability is an important property for structural applications.By definition, thermal stability represents the material's ability of retaining its microstructure and mechanical properties at elevated temperatures (Czerwinski 2020).It is vital for Al alloys to maintain a stable mechanical property at particular temperatures required for different applications.For example, Al alloys used for commercial aeroplane need to have a good thermally stability at room and slightly elevated temperatures (Boeing).Packaging and transportation of radioactive materials require Al alloys to have thermal stability at temperatures of 100-200°C for over 60 years (Ishiko et al. 2015).Cast Al alloys used for automotive combustion engines need to have high-thermal stability at temperatures over 200°C (Czerwinski et al. 2017).As an upstart structural material, the thermal stability of LPBFed AlSi10Mg has never been justified.Therefore, in order to facilitate the application of LPBFed AlSi10Mg, it is essential to assess its thermal stability.
It should be noted that due to the continuous thermal input during LPBF, the temperature of in-processing materials is ∼100°C.Therefore, it is safe to deem that the thermal stability of LPBFed AlSi10Mg is acceptable at 100°C.In the present study, to assess its thermal stability at higher temperatures, we exposed as-built samples at temperatures of 200-500°C for only 3 min.Comprehensive microstructural and mechanical property analyses were conducted.The change of mechanical property was correlated with the microstructural evolution.Eventually, the thermal stability of LPBFed AlSi10Mg was ascertained.

LPBF process
Gas atomised AlSi10Mg powder (15-53 μm in diameter) was processed to fabricate bulk AlSi10Mg samples using an LPBF machine (SLM 250HL, SLM Solutions GmbH, Germany), which was operated under argon gas atmosphere to prevent oxidisation during the process.The substrate was made up of AlSi10Mg and pre-heated to 100°C before deposition.The laser power, scan speed, layer thickness and hatch spacing were 350 W, 1170 mm/s, 50 μm and 0.24 mm, respectively.A stripe scanning strategy with 90°of rotation after each layer was adopted.The as-built sample was 50 mm in length, 50 mm in width and 20 mm in height.

Thermal exposure process
The as-built material was sectioned using electrical discharge machining wire cutting for thermal exposure.The exposure duration was set to be 3 min and the exposure temperatures were 200°C, 300°C, 350°C, 400°C and 500°C.The whole process was performed under argon gas atmosphere and cooled in air.Based on the thermal exposure temperatures, the specimens were denoted as TE200, TE300, TE350, TE400 and TE500, respectively.

Microstructural characterisation
The microstructural characterisation was conducted using optical microscopy (OM) and scanning electron microscopy (SEM, JEM-6500F, JEOL, Japan) equipped with an electron backscattered diffraction (EBSD) detector.Transmission electron microscopy (TEM, JEM-2010, JEOL) was also utilised to investigate the detailed microstructure under different modes including bright filed (BF), dark field (DF), energy dispersive spectroscopy (EDS), high resolution transmission electron microscopy (HRTEM) and selected area diffraction (SAD).In-situ hightemperature EBSD analysis was performed on the asbuilt sample while being simultaneously heated to different temperatures up to 450°C to directly observe the microstructure evolution.Phase identification was realised using X-ray diffraction (XRD, XRD-600, Shimadzu, Japan, Cu Kα, 2°⋅min −1 ).Samples for OM, SEM and XRD analyses were mechanically polished and then chemically etched using an acid solution consisting of 25 vol.%HNO 3 , 15 vol.%HCl, 10 vol.%HF and 50 vol.%H 2 O. EBSD samples were prepared by mechanical polishing followed by vibratory polishing.TEM samples were prepared using a focused ion beam (FIB) system (FB-2000S, HITACHI, Japan).

Mechanical property test
Tensile tests were performed with the tensile axis perpendicular to the building direction using a universal mechanical testing machine (AUTOGRAPH AG-I 50 KN, Shimadzu, Japan).The gauge dimensions of the tensile specimens were 10 mm in length, 2 mm in width and 1 mm in thickness.Three tensile tests were conducted for each condition at a constant strain rate of 5 × 10 −4 s −1 .

As-built microstructure
Figure 1 showed the as-built microstructure for both horizontal plane (HP) and vertical plane (VP).As shown in Figure 1(a, c), except few micro-voids, there were no observable defects in the as-built microstructure, which is consistent with its measured high relative density of ∼99.4%, indicating that the LPBF parameters selected in this study were effective in fabricating AlSi10Mg samples with nearly full density.Similar to previous studies (Thijs et al. 2013;Liu, Zhao, et al. 2019), the HP exhibited equiaxed grains with an average grain size of approximately 9.3 μm (Figure 1(b)), while VP demonstrated columnar grains with an average grain size of approximately 14.6 μm (Figure 1(d)).Further SEM analysis on HP revealed that each grain consisted of many smaller cells (Figure 1(e-f)).This microstructural characteristic is attributed to the unique border-to-centre solidification pattern in the melting pools and is common to see in many FCC materials fabricated via LPBF (Liu, Zhou, et al. 2019;Thijs et al. 2013).As shown in Figure 1(g), the cell size was around 500 nm in diameter and the thickness of the cell boundaries were ∼50 nm.SEM-EDS analysis suggested that those cell boundaries were rich in Si.TEM analysis further revealed that the cell boundaries were made up of alternate Al-rich phase and Sirich phase (Figure 1(j-n)).Based on the Al-Si phase diagram (Rosenthal, Stern, and Frage 2014), as a hypoeutectic composition, the as-built microstructure of AlSi10Mg can be interpreted as primary Al (Al P ) cells plus eutectic networks (Al E + Si E ).It should be noted that there were minor micro-voids locating at the interface between primary Al cells and eutectic networks (Figure 1(j)), which may be attributed to the high residual stress at the phase boundary.

Effect of thermal exposure on microstructure
Figure 2 showed the XRD spectra of each material, where the numbers indicated the intensity ratio between the primary Si peak at 28.44°and the primary Al peak at 44.74°.It can be seen that only peaks corresponding to Al (( 111), ( 200), ( 220), ( 311) and ( 222) at 38.47°, 44.74°, 65.13°, 78.23°and 82.43°, respectively, PDF card no.04-0787) and Si (( 111), ( 220), ( 311), ( 400), ( 331) and (422) at 28. 44°, 47.3°, 56.12°, 69.13°, 76.38°and 88.03°, respectively, PDF card no.27-1402) were detected for all materials.The as-built sample had a similar phase constitution with the as-received powder.After 3 min of thermal exposure at 200°C (TE200), the phase constitution seemed to be the same as the as-built condition.However, at a higher thermal exposure temperature of 300°C, the intensity of each Si peak increased, which kept increasing with further increase in thermal exposure temperature.
Figures 3 and 4 showed the microstructure on HP for both as-built sample and thermally exposed samples at magnifications of 5000x and 20000x, respectively.It can be clearly seen that the short-period thermal exposure had no distinct effect on the micro-voids until the temperature was raised to 500°C, where the micro-voids appeared to be bigger.This is in good agreement with the measured relative densities (∼99.4% for TE200-TE400 and ∼98.5% for TE500).Furthermore, as mentioned in Section 3.1, the as-built sample demonstrated a cellular microstructure (Figures 3(a) and 4(a)), where the cells were made up of primary Al and the cell boundaries were made up of eutectic Al and eutectic Si.After 3 min of thermal exposure at 200°C and 300°C, some of the cell boundaries vanished, resulting in coalescence of neighbouring cells.Consequently, the cell size appeared to be bigger in TE200 (Figures 3(b) and 4(b)) and TE300 (Figures 3(c) and 4(c)).As the thermal exposure temperature increased to 350°C (Figures 3(d) and 4(d)), the cell boundaries vanished completely and the cellular microstructure was replaced by a more uniform microstructure.According to Albu et al. (2020;Takata et al. 2020), the breakdown of those eutectic cells was resulted from the spheroidisation and coarsening of Si nanoparticles, which was governed by surface self-diffusion and Al-Si interdiffusion.As the thermal exposure temperature further increased to 400°C (Figures 3(e) and 4(e)), many small particles precipitated out from the matrix, which were averaged to be approximately 100 nm.Finally, in TE500 (Figures 3(f) and 4(f)), those particles coarsened significantly to around 1470 nm.According to the XRD analysis results shown in Figure 2, the only change in phase constitution is the increase in the intensity of Si peaks.Therefore, the particles in TE400 and TE500 can be speculated as Si precipitates.
To identify the composition of the particles precipitated during thermal exposure, a combination of SEM and EDS was performed for sample TE500.As shown in Figure 5(a-c), the particles were detected to be rich in Si and lean in Al, which suggested that they were Si precipitates.This is in good agreement with the XRD analysis results (Figure 2).Furthermore, there were some areas rich in Mg (Figure 5(d)), which should be a result of Mg or Mg 2 Si phases.Compared with the Si precipitates, the content of Mg-rich phase was relatively low.Therefore, they were not detected by XRD (Figure 2).

Effect of thermal exposure on Si size
In as-built microstructure, the Si-rich regions (Si E in Figures 1(g) and 6(a)) making up the cell boundaries were occasionally misinterpreted as Si particles (Prashanth et al. 2014;Wang, Sun, et al. 2018).Herein, detailed TEM analysis was conducted to examine the Si precipitate in the as-built sample (Figure 6(a-c)).Figure 6(a and b) displayed as-built microstructure for the same region under BF-TEM mode and DF-TEM mode, respectively.It is obvious that the BF-TEM image can be misleading with respect to the Si precipitate.However, the DF-TEM mode highlighted the Si crystals, which appeared as tiny bright particles within the eutectic networks.Further HRTEM image suggested that those Si particles had an average size of ∼8 nm (Figure 6(c)).
Similar results were acquired by Casati et al. via XRD analysis (Casati et al. 2018).After subjected to 3 min of thermal exposure at 350°C, coarsening of those Si particles occurred and their particle size was averaged to be ∼20 nm (Figure 6(d)).Meanwhile, besides the spherical Si particles, few Si nanorods appeared (Figure 6(e)).It should be noted that those Si particles in TE350 were too small to be resolved by SEM (Figures 3(d) and 4(d)).With increasing the thermal exposure temperature, the Si particles further coarsened, which reached ∼100 nm in TE400.This is in good agreement with the SEM analysis results (Figures 3(e) and 4(e)).Similar to previous studies on LPBFed AlSi10Mg, coarsening of Si particles during thermal exposure was at the expense of solutionised Si, which can lead to strength deterioration (Cao et al. 2021).

Effect of thermal exposure on grain size
Figure 7 showed the EBSD analysis results on HP.As mentioned in Section 3.1, the as-built microstructure had an average grain size of ∼9.3 μm (Figures 1(b) and 7(a)).After 3 min of thermal exposure at 300°C, the grain size appeared to be similar to as-built specimen (Figure 7(b)).However, when the thermal exposure temperature increased to 350°C, the microstructure became much finer (Figure 7(c)).The grain size decreased dramatically to ∼0.67 μm, which reflected approximately 93% of decrement compared with asbuilt condition.As the thermal exposure temperature

In-situ high-temperature EBSD analysis
In order to investigate the grain refinement mechanism shown in Figure 7, in-situ high-temperature EBSD  analysis was performed on HP.As shown in Figure 8(a), besides ambient temperature, the in-situ EBSD analysis was conducted at temperatures from 200°C to 450°C with an interval of 50°C.It can be observed that there was no obvious microstructural change at temperatures no more than 350°C (Figure 8(b-d)).However, when the testing temperature increased to 400°C, few tiny grains formed around the original grain boundaries, as indicated by arrows in Figure 8(e).As the testing temperature increased to 450°C, much more tiny grains formed and they appeared not only around the original grain boundaries but also within the original grains.It is worth noting that the newly developed grains had an average grain size of ∼500 nm, which was very close to the grain size  achieved in TE400 (Figure 7(d)) as well as the cell size in as-built microstructure (Figures 1(f, g), 3(a) and 4(a)).

Misorientation angles between neighbouring ultra-fine grains in TE400
TEM analysis was performed to further reveal the crystallographic information of the ultra-fine microstructure in TE400 (Figure 7(d)).Figure 9(a-c) was three representative BF-TEM images for TE400, where the circles indicated ultra-fine grains in each field selected for SAD analysis.All SAD patterns (Figure 9(a1-a2, b1-b2, c1-c2)) were indexed to determine the zone axes of those grains, which were then used to calculate the misorientation angles between two neighbouring grains in each field.As shown in Figure 9(a-c), the misorientation angles of a1/a2, b1/b2 and c1/c2 were 25°, 19°and 87°, respectively.In general, grain boundaries with misorientation angles greater than 15°are considered as HAGBs (Li, Wang, and Zhao 2022).Therefore, it can be deduced that the boundaries between ultra-fine grains developed in TE400 were HAGBs, which is in good agreement with the continuous increase of fraction of HAGBs in the EBSD analysis results (Figure 7(f)), proving the occurrence of grain refinement phenomenon during thermal exposure of LPBFed AlSi10Mg.

Effect of thermal exposure on mechanical properties
Tensile tests were conducted to reveal the effect of thermal exposure on mechanical properties.Figure 10 showed the representative engineering tensile stressstrain curves for each condition and the detailed tensile property including elongation (El.), yield strength (YS) and ultimate tensile strength (UTS) as well as their standard deviations have been summarised in Table 1.Overall, with increasing the thermal exposure temperature, the YS and UTS exhibited a similar decreasing trend while the El.increased.As shown in Figure 10(a), after 3 min of thermal exposure at 200°C, both tensile strength and elongation decreased compared with asbuilt condition.When the thermal exposure temperature increased to 300°C, the tensile strength decreased but the elongation increased.As the thermal exposure temperature further increased to 350°C, 400°C and 500°C, the tensile strength decreased continuously accompanied with continuous increase in elongation.Kernel average misorientation (KAM) maps (Figure 10 (b-e)) were obtained for the deformed microstructure near the fracture surface of the post-tensile specimens, which quantifies the orientation deviation from pixel to pixel inside each individual grain, reflecting microdeformation and residual stress across the microstructure (Tan, Zhang, Mo, et al. 2020;Wang, Voisin, et   2018; Zhou et al. 2019).It can be observed that with increasing the thermal exposure temperature, the micro-deformation across the whole filed became more and more uniform.It suggested that a higher thermal input could alleviate local deformation and promote the plasticity of LPBFed AlSi10Mg.

Rationale for grain refinement phenomenon
Grain orientation spread (GOS) reflects the misorientation level for each individual grain by averaging the misorientation of each point within that grain (Field et al. 2007).It is widely used to differentiate recrystallised grains in deformed microstructure (Prithiv et al. 2018;Barrett et al. 2017;Ma et al. 2021).Figure 11 showed the GOS maps overlayed with HAGBs for in-situ high-temperature EBSD test (Figure 8(b, f)) and thermal exposure analysis (Figure 7(c, d)), where the recrystallised grains were coloured in green by using 2°as the critical GOS value.Note that the temperatures monitored from in-situ EBSD analysis were not accurate and should not be compared with those in thermal exposure analysis.As shown in Figure 11(a), most of the original grains had a GOS value higher than 2°even though not subjected to any mechanical deformation.However, as the material was heated to 450°C during the in-situ EBSD test, many tiny grains appeared (Figure 8(f)), which were characterised as recrystallised grains.The GOS maps in Figure 11(c  and d) suggested that TE350 was partially recrystallised, whereas TE400 was almost fully recrystallised.It is obvious that as the recrystallisation process continued, the microstructure became finer.An ultra-fine microstructure with an average grain size of ∼0.54 μm was achieved for TE400 (Figure 7(d)).What is more, as indicated by the unit cells in Figure 11(a and b), the new grain exhibited completely different crystal orientation compared with the parent grain in as-built microstructure.This is consistent with the TEM analysis results for misorientation angles between neighbouring ultra-fine grains in Section 3.6.Taking the above-mentioned into consideration, one can conclude that the ultra-fine grains formed in this study was a result of the recrystallisation process.

Rationale for the deformation-free recrystallisation
By definition, 'recrystallisation' is the formation of new grains in a deformed microstructure by the formation and migration of HAGBs (Doherty et al. 1997;Shen et al. 2022;Ayad et al. 2021).However, the present study manifested a grain refinement phenomenon (Figure 7) induced by DFRX.It should be noted that deformation enables recrystallisation by storing excessive energy in the material, which in turn acts as a driving force for recrystallisation.Therefore, deformation is not essential for recrystallisation only if enough excessive energy can be introduced and stored in the material by other approaches.In contrast with conventional manufacturing processes, the cooling rate of LPBF is far higher (∼1 vs. ∼10 3 -10 6 °C/s) (Lu, Nogita, and Dahle 2005;Wan, Qing, and Xu 2019;Lin et al. 2019;Tan et al. 2022;Tang et al. 2016).Therefore, the residual stress in LPBFed materials  is extremely high (Sanaei and Fatemi 2021;Jia et al. 2021;Wu, Wang, and An 2017).According to the XRD analysis done by Zhuo et al. (2019), the residual stress in as-built AlSi10Mg is about −111 MPa.With this being clarified, the DFRX phenomenon in the present study can be depicted in Figure 12.Firstly, a great deal of the residual stress was stored as dislocations in the as-built microstructure, especially at the phase boundary between primary Al cells and eutectic networks (Figure 12(c)) (Wei et al. 2021;Zhang et al. 2021).Then, those stored  dislocations were activated by thermal input (Figure 12 (d)).As the thermal input increased, the dislocations started to interact to release the excessive energy stored during LPBF, which led to recrystallisation along the interface between primary Al cells and eutectic networks (Figure 12(e)) (Lu, Lu, and Suresh 2009).As the thermal input kept increasing, TE400 for this study, the growth of recrystallised grains consumed all parent grains and a grain size as fine as the primary Al cells were achieved (Figure 12(f)).It should be noted that the refined grains would grow towards each other and coarsen eventually if the thermal input went to an even higher level, as shown in Figure 7(e).A similar phenomenon was also reported in a heat treatment study of LPBFed Ti-5Al-2.5Sn(Wei, Wang, and Zeng 2018).In general, the excessive energy induced by fast cooling characteristic of LPBF acted as the driving force for recrystallisation of as-built microstructure, which enabled the DFRX process in the LPBFed materials.

Thermal stability of LPBFed AlSi10Mg
Figure 13 illustrated the change of microstructure and mechanical properties along with the thermal exposure temperature, which appeared to be similar to the textbook recovery, recrystallisation and grain-growth diagram for annealing of cold-worked materials (Semiatin 2005).As shaded by different colours, the whole diagram can be roughly divided into three zones.
There is no doubt that the starting material contained vast amounts of dislocations (Cao et al. 2021;Wei et al. 2021).Since the as-built material suffered an inprocess temperature of ∼100°C for ∼8 h, it can be regarded as a thermally stabilised microstructure at 100°C.In Zone I (100-300°C), the microstructure was similar to as-built one but the UTS decreased significantly in accompany with El. increase.Therefore, it can be deduced that recovery involving partial defect annihilation and rearrangements occurred in this zone.In Zone II (300-400°C), the DFRX occurred, which induced grain refinement phenomenon.This was at the expense of dislocations stored in the asbuilt material.Therefore, the UTS further decreased.Finally in Zone III (400-500°C), growth of recrystallised grains occurred, which led to a further decrease in UTS.It should be noted that the Si particle size increased significantly to 1470 nm in this zone, which also contributed to the UTS decrease.In general, despite its superior mechanical property, LPBFed AlSi10Mg showed relatively low thermal stability.Only three minutes of thermal exposure at 200°C would degrade its tensile strength dramatically.The ultra-fast cooling characteristic of LPBF induced exceptional strength to the material via solid solution strengthening from super-saturated Si (Cao et al. 2021), precipitation strengthening from ultra-fine Si precipitates (Casati et al. 2018) and dislocation strengthening (Wu et al. 2016).However, every coin has two sides.Those strengthening components were thermal unstable, which induced thermal instability to the material.Therefore, extra caution should be taken for the utilisation of LPBFed AlSi10Mg in applications.

Conclusion
The thermal stability of LPBFed AlSi10Mg was evaluated systematically by exposing as-built AlSi10Mg at temperatures ranging from 200°C to 500°C for relatively short period (3 min).Tensile tests for the thermally exposed samples showed that LPBFed AlSi10Mg had relatively low-thermal stability.Only three minutes of thermal exposure at 200°C would degrade its tensile strength dramatically.By correlating the microstructure change with mechanical property change, it was found that with increasing the temperature of thermal exposure, as-built AlSi10Mg exhibited a microstructural evolution similar to annealing of cold-worked metals, namely recovery, recrystallisation followed by grain-growth.The excessive energy stored in the as-built microstructure resulted from fast cooling during LPBF was deduced as the driving force for this phenomenon.In other words, such microstructural change was at the expense of dislocations stored in the as-built material, which in turn led to deterioration in tensile strength.It is recommended that extra caution should be taken for the utilisation of LPBFed AlSi10Mg in applications.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Figure 2 .
Figure2.Comparison of XRD patterns among as-received powder, as-built sample and thermally exposed samples.The numbers indicated the intensity ratio between the primary Si peak at ∼28.5°and the primary Al peak at ∼44.7°.

Figure 3 .
Figure 3. SEM images showing the microstructure on HP for both as-built sample and thermally exposed samples at a magnification of 5000x.Note that the bright particles were polishing artefacts.

Figure 4 .
Figure 4. SEM images showing the microstructure on HP for both as-built sample and thermally exposed samples at a magnification of 20000x.Note that the bright particles were polishing artefacts.
Figure 7(f) summarised the average grain size and high angle grain boundaries (HAGBs) for each condition.It is worth noting that the finest grain size was ∼0.54 μm (Figure 7(d)), which was very close to the cell size (∼500 nm) in as-built microstructure (Figures 1(f, g), 3 (a) and 4(a)).

Figure 5 .
Figure 5. Microstructure on HP for TE500.(a) An SEM image.(b-d) SEM-EDS elemental mapping on (a) for Al, Si and Mg, respectively.

Figure 7 .
Figure7.EBSD analysis results on HP. (a-e) Orientation maps for the as-built sample, TE300, TE350, TE400 and TE500, respectively.(f) Grain size and HAGBs evolution.Note that ultra-fine grains were developed in TE350 and TE400.

Figure 8 .
Figure 8. In-situ high-temperature EBSD analysis results on HP.(a) Temperature profile.(b-f) Orientation maps.Arrows indicated the grains newly formed during in-situ EBSD analysis.Note that the temperatures monitored from in-situ EBSD analysis was not accurate and should not be compared with those in thermal exposure analysis. al.

Figure 10 .
Figure 10.Mechanical property analysis results.(a) Nominal tensile stress-strain curves.(b-e) KAM images for deformed microstructure near the fracture surface of the post-tensile specimens.

Figure 11 .
Figure 11.GOS maps overlayed with HAGBs, where the recrystallised grains being coloured in green.(a and b) As-built and 450°C conditions from in-situ high-temperature EBSD test, where the unit cells indicated the crystal orientation of circled areas.(c and d) TE350 and TE400 from thermal exposure analysis.

Figure 12 .
Figure 12.A schematic showing the grain refinement phenomenon (a and b) induced by the DFRX mechanism (c-f) in LPBFed AlSi10Mg.

Figure 13 .
Figure 13.A picture showing the effect of 3 min of thermal exposure on grain size, Si particle size and tensile properties.Note that the picture was roughly divided into three zones based on the change of microstructure and mechanical properties.

Table 1 .
Summary of the tensile properties of LPBFed AlSi10Mg in both as-built and thermally exposed conditions.