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This article shares a summary for you: the application of synchronous X-ray imaging and diffraction in additive manufacturing.
1.1 Mechanical properties
Compared with cast parts, the mechanical properties of additively manufactured parts are mainly affected by the internal microstructure, complex growth direction and defects of the parts. Additive manufacturing components have high tensile strength, low plasticity and strong anisotropy associated with construction direction [56,57]. XCT can also be used to characterize tensile fractures and internal pores of reinforcements to better understand the relationship between defect characteristics and the performance of the printed member.
In a 2016 study of Ti6Al4V manufactured by SLM, Krakhmalev et al. [58] found through XCT that the average porosity is less than 0.0022% (density 99.9%). The results show that in the fractures of typical cup cone fracture morphology, pore mergers are the main crack formation mechanism, indicating that XCT is an effective method to study the impact of defects on mechanical properties. Carlton et al. [59] performed in situ tensile tests on the AM SS316L using synchronous radiation X-ray imaging to track damage evolution inside the material. While applying tensile load, the volume, distribution and morphology of 3D pores at the micron scale in SS were measured, as shown in Figure 16. Although the high porosity samples were improved by annealing, they still showed poor mechanical properties. They also found that porosity distribution plays a more important role in influencing the fracture mechanism than measuring volume density.
Figure 16 Results of high porosity SS316L specimens. a3D rendering of the segmented pore distribution of high-porosity AM SS specimens before mechanical test (left) and before sudden failure (right); b Tomography images under different loads and displacements during tensile loading
Zekavat et al. [60] used XCT to study the effect of preparation temperature on the mechanical properties of melt deposition molding (FDM) polylactic acid (PLA) filament parts. They found that the samples prepared at lower temperatures have a larger fracture strain, but their tensile strength is relatively low. However, the samples prepared in a higher temperature range have a higher tensile strength due to the good bond between the extruded fibers. The results show that the different mechanical responses are highly correlated with the internal geometry of the sample, but independent of porosity. Prove that CT as a lossless tool for the development of FDM method shows great potential.
Similarly, Stef et al. [61] proposed a detection method for printing Ti6Al4V based on 2D fracture and 3D XCT analysis. They associate the 3D spatial distribution, morphology, and orientation of the hollow with the scanning strategy pattern. Their results show that pores are mainly concentrated in the cover area and support the energy deficiency caused by lower energy periphery of the laser spot is the main mechanism for pore formation. They found that tensile properties and crack paths were affected by the 3D distribution of the voids, and the crack paths followed the alignment direction of the voids, as shown in Figure 17, where the crack paths were parallel to the construction direction. This further confirms the study of Krakhmalev et al. [58].
Figure 17 A XCT view of the tensile sample breaking along the x-axis; b Double view of the top fracture surface and bottom 3D void distribution of the fracture sample. c. d. Two different views of the projection of the two slices of the fracture surface and the gap on the (XY) plane
For AM parts in the fields of aviation, aerospace, electricity, energy, etc., many parts bear varying degrees of alternating stress during use, resulting in fatigue failure. It is confirmed that the air hole defect has an important impact on the fatigue performance of AM parts during use. Siddique et al. [62] used CT to evaluate the effect of porosity-induced stress concentration on fatigue dispersion of AlSi12 samples. They found that pores are potential places for crack invasion, while samples without pores only had surface crack invasion. When pores are found near the surface, the pores are more pronounced in the initiation of cracks.
Based on this, a suggestion is made to set the area of the contour to 200 μm in the SLM process for fatigue key components. They also developed a model that describes the stress concentration factor (kt) as a function of pore characteristics (pore size and distance from the surface) based on the results of the CT scan.In the study of Sandgren et al. [63], the fatigue crack propagation (FCG) behavior of Ti6Al4V prepared by laser near-net shaping (LENS) was observed in situ based on high-energy synchronous X-ray imaging technology. As shown in Figure 18, cracks in Ti6Al4V martensite mainly grow in the tensile axis, emphasizing the importance of local 3D observation and characterization of FCG. The research results further confirm that the use of synchronous radiation X-ray imaging can more realistically understand and characterize the FCG behavior in Ti6Al4V prepared by LENS.
Figure 18 Snapshot of 3D microscopic tomography reconstruction: A-g crack propagation corresponding to seven tomography scans; crack height at different locations
Larrosa et al. [64] used related XCT and other traditional characterization methods ( optical microscope , electron backscattering diffraction, SEM and TEM) to analyze the effects of AM generation orientation on the porosity and related mechanical behavior of AlSi10Mg samples for preparation of SLM. They found that fatigue life was dominated by the presence of pancake-like defects perpendicular to the loading direction, and building samples transverse to the highest fatigue load may help enhance fatigue performance. This study reveals to a certain extent the effect of defects in SLM parts on experimental fatigue behavior. Based on the self-developed in-situ fatigue test bench that is fully compatible with the Shanghai Synchronous Radiation Device (SSRF) BL13W1, Feret diameter and extreme value statistics were used to characterize the size, morphology, quantity, location and its impact on fatigue life of the defects (Figure 19). The results show that fatigue cracks mostly originate from the sample surface or near the surface, showing typical semi-elliptical cracks, and the unfusion defect has a relatively large impact on fatigue life. Compared with Sandgren et al. [63]'s study on Ti6Al4V fatigue behavior, they explored the types and growth characteristics of cracks more comprehensively. In addition, defects with a spherical degree of less than 50 μm and a spherical degree of 0.4~0.65 are the main factors in the fatigue behavior of SLM Ti6Al4V. It was also found that the larger the feature size of the defect, the lower the fatigue life. The above conclusions obtained in the in-situ fatigue test of synchronous radiation X-ray imaging beamline can provide theoretical basis and support for predicting the fatigue performance of SLM Ti6Al4V.
Figure 19 3D reconstruction results of cracks and fracture morphology of in-situ fatigue specimens. a 3D XCT of fracture diagram; b 3D rendering of defects and crack propagation after 1850 cycles; c morphology of fatigue fracture of the sample at maximum stress 1175MPa; d 3D drawing results are projected along the direction of the main stress, yellow represents cracks, blue represents defects, and red represents crack surface defects
In short, XCT combined with in-situ loading provides the comprehensive characteristics of pore defects (volume, size, distribution, morphology, topology , etc.) and its impact on the mechanical properties of additive manufacturing parts, as well as the potential influence mechanism. These studies help us better understand the relationship between processing parameters, pore defects and mechanical properties.
2. Online characterization by Synchrotron X-ray imaging AM
As we all know, traditional characterization of AM components is usually performed by post hoc detection. However, this non-in-situ detection method would hinder us from studying microstructure changes and defect formation mechanisms in samples during AM. In recent years, synchronous radiation X-rays in in-situ characterization techniques have proven to be one of the most effective ways to track defect formation or crack evolution during AM. The third generation synchronous radiation light source with strong penetration, high spatiotemporal resolution and high throughput can quickly image parts (milliseconds to microseconds). It has natural advantages for studying the evolution of size and shape of melt pool , defect formation mechanism and unbalanced solidification behavior.
AM parts are produced by repeated processing layer by layer. When laser light is irradiated on the powder, it will inevitably interact with the powder particles, the molten pool and the metal vapor. This process involves multiple heat transfer methods, as well as the fluid mechanics behavior inside the molten pool. Some scholars have better observed the dynamic behavior of the melt pool through synchronous radiation X-ray imaging and explored the consolidation mechanism of the powder. Leung et al. [68] used high-speed synchronous radiation X-ray imaging technology to reveal the physical phenomena during the deposition of the first and second layers of melt trajectories during the SLM process.
They found that there are two main mechanisms for forming the melt pool: one is that the newly formed melt pool promotes the growth of the melt pool by wetting with the melt beads, because the laser beam reduces its surface tension while heating the melt pool; the other is that the laser-induced gas or steam jet pulls the powder particles into the melt track, resulting in the growth of the melt track (Figure 20). The former is the main mechanism. In addition, the time-resolved quantification of pores and splash motions also provides critical information about their speed and direction, which is a unique advantage of ultrafast synchronous radiation X-ray imaging. This is an important step in revealing the causes of pore formation and exploring the interaction between laser and matter. Later, Chen et al. [69] further captured the morphological evolution of each melt trajectory in the five-layer melt trajectory by ultrafast synchronous X-ray imaging, with metal vapor jets vertically ejecting droplets and powder splashing from the erosion zone and forming small holes in the deposition layer. Keyhole-related phenomena in multilayer structures are similar in all layers. The authors' findings elucidate the mechanism of molten pool growth, laying the foundation for subsequent combination with modeling to improve the quality of LPBF parts.
Figure 20 Time series radiographs obtained during Invar 36 single-layer melt track additive manufacturing under conditions of P=209W, V=13mms-1 and LED=16.1 Jmm-1
Gong et al. [70-72] summarized three melt pool formation modes according to the different laser energy density. One is the unfusion mode formed when the energy density is low, the second is the conduction mode when the energy density is moderate, and the third is the keyhole mode when the energy density exceeds a certain critical value. In actual production process, the small-pore mode more effectively transfers the laser energy to the powder layer, but if the process parameter is not controlled properly, the quality of the forming part will decrease due to too many pores, thereby reducing its mechanical properties. Therefore, it is particularly important to effectively control the generation and formation mechanism of air holes in the keyhole mode.
Zhao et al. [73] monitored the LPBF process of Ti6Al4V in situ for the first time based on high-speed synchronous radiation X-ray imaging, as shown in Figure 21. They not only show many phenomena of scientific and technological significance, including molten pool dynamics, powder injection, rapid solidification and phase transition, but also reveal the entire process of keyhole formation in Ti6Al4V. The results show that the closing time of the pores is less than 50μs. The reason for the formation of small holes is explained in detail: the absorption of laser energy causes strong evaporation of surrounding metals, and the rapid movement of metal vapors produces a backlash pressure gradient, causing the melt to eject. When the laser is turned off, the local negative pressure environment causes the liquid metal to flow to the center of the molten pool.
Due to the large depth of the cavity, the top liquid metal maintains a high horizontal fluidity, while the bottom horizontal movement is too slow, resulting in the formation of keyholes. Based on previous research, Cunningham et al. [74] also determined the threshold value from the conduction mode to the keyhole mode in 2019, and finally derived four transformation steps of the keyhole: vaporization, liquid level depression, instability, and then to the formation of deep keyholes. They then observed the holes and found that the boundary of the stomatal area was sharp and smooth, and the critical holes were instable to produce sound waves in the melt pool, providing additional important driving force for the stomatal near the tip of the hole to stay away from it, becoming a defect. In addition, the relationship between the hole depth, front wall angle and laser power density is also discussed.
Figure 21 Dynamic X-ray image of Ti6Al4V laser powder bed fusion process. The laser power is 520W. The laser beam size is approximately 220 µm (1/e2). The powder particle size is in the range of 5-45 µm, and the powder layer thickness is about 100 µm. Numbers represent time nodes. The laser is turned on at t=0 and continues to heat the sample until t=1000µs. The raw data is obtained at at frame rate of 50kHz. The exposure time per image is 350ns. All scale bar is 200 µm
Later, Guo et al. [76] also pointed out that under constant line energy density (IDE), the melting zone can be transformed from the non-melting pool zone to the conductive zone, the transition zone and the pore zone in turn. The three-dimensional size of the molten pool has increased. Importantly, the authors found that at a constant IDE level, energy absorption tends to increase when the laser power and laser scanning speed simultaneously increase, which is an important factor leading to the melt pool change.
Similarly, Martin et al. [77, 78] observed the vapor suppression and surface instability of Al6061 and Ti6Al4V in the LPBF process through high-speed transmission X-ray imaging, describing the effect of laser reflection and ablation of materials on instability, as well as the formation mechanism of surface driving and small hole driving. This paved the way for their later research on multiphysics simulations. To eliminate pores, the authors believe that changing the laser power at the pore formation threshold can reduce pore formation. They then designed a power distribution strategy, successfully eliminating the small holes when the molten pool track is formed. Hojjatzadeh et al. [79] also found that the high temperature capillary force generated by the high temperature gradient of the laser action zone can quickly eliminate pores in the molten pool during the LPBF process, thereby achieving additive manufacturing of poreless metals.
In addition to the small holes formed at the bottom of the molten pool due to the recoil pressure, the pores in other locations cannot be ignored. Bobel et al. [80] used ultrafast synchronous radiation X-ray imaging technology to directly observe the fusion process of the AISI 4140 laser powder bed. They believed that the pores in the stacking layer mainly come from the retention gas in the original powder and are independent of the setting of process parameters (Figure 22). Through the study of the powder-oriented energy deposition process, Wolff et al. [81] found that the flow velocity and distribution of individual particles lead to different mechanisms of powder melting, melt pool merger and solidification, as well as different pore formation and movement.
Figure 22 X-ray image of the melt pool, keyhole and pores in the base plate. aFill; bHigh energy (HE); keyhole
During laser processing and manufacturing, splashing will inevitably occur. This splash refers to the area where molten material is sprayed out of the molten pool and deposited in the vicinity of the molten pool, which not only affects the surface roughness of the AM part, but also reduces its mass. In order to reduce the splashing phenomenon of powder beds, some scholars have made a lot of efforts. Khairallah et al. [82] attempted to explain the causes of splashes. They believe that surface tension opposite to the compression effect of recoil force causes depression and material splash.
In order to better explain the mechanism of splash formation, Leung et al. [83] studied the laser-matter interaction of SS316L and 13-93 bioactive glass in the AM process in 2018 based on ultrafast synchronous radiation X-ray imaging. They found that droplet splashes are often formed by Marangoni-driven flow during the AM process, increasing the transmission of pores, which further confirms the results revealed by Khairallah et al. [82]. In addition, they also claim that low viscosity melts such as SS316L are more likely to splash and seep into the melt pool, forming melting trajectories. On the other hand, the high viscosity melt will hinder the formation of splash by inhibiting the drive of Marangoni.
is different from the reasons for the splash found by the above authors. Guo et al. [84] found that the powder splash around the laser beam is driven by a steam jet. If the particles in the area behind the laser beam are wrapped by the argon gas stream, the particles in front of the laser beam are also affected by the argon gas stream (Fig. 23). Similarly, Matthews et al. [85] used high-speed imaging to study the erosion of metal powders observed near the laser scanning path. They found that the depletion of metal powder particles observed in the region close to the curing trajectory is due to competition between the outward metal steam flux away from the laser point and the powder particle sweep in the shear airflow driven by the metal steam jet on the melt track.
This is an important step in predicting and reducing void defects of AM metal components. Anwar observed the transient dynamics of powder splashing during LPBF through in situ high-speed and high-energy X-ray imaging, and quantified the velocity, acceleration and driving force of metal steam injection and argon flow induced powder movement.
They found that the sputtered particles are mainly observed along the scanning direction and have a smaller orthogonal distribution. As the gas flow rate increases, heavier and larger splashes are added, resulting in more even distribution of gas flow downstream. This laid a good foundation for them to use discrete phase model (DPM) to study the motion trajectory of splash particles. Subsequently, Zhao et al. [88] used MHz single-pulse synchronous radiation X-ray imaging to study the sputtering behavior of Ti6Al4V's micro-space resolution and sub-nanosecond time resolution.Finally, their research reveals a new laser sputtering mechanism: the large explosion of tongue-like protrusions on the front keyhole wall causes molten metal ligamentation at the edge of the keyhole and subsequent sputtering. Their research opens a door to create splash and defect-free metal parts by precisely controlling keyhole dynamics.
Figure 23 Dynamic X-ray images show the movement of powder at different moments and under different environmental pressures
Other scholars have also contributed to other aspects of the formation of the melt pool. Escano et al. [89] reported the powder diffusion kinetics at the particle scale during additive manufacturing based on powder beds and quantified the evolution of slope velocity, slope roughness and coagulation kinetics at the powder front. The influence of powder particle size on powder flow kinetics is revealed. This result is very important for a deep understanding of the powder diffusion behavior of powder beds during the AM process. Parab et al. [90] used the fastest X-ray imaging speed of 6.5 MHz to quantify the steam suppression and rapid oscillation of high-speed rotating powder particles for the first time.
This helps to verify the numerical model, determine processing conditions, study functional gradients and preparation of multi-material products. Calta et al. [91] used in situ X-ray imaging technology to directly detect the changes in the liquid-gas interface morphology with ambient pressure and oxygen partial pressure during laser melting of SS316L, Ti6Al4V, Al6061 and Ni400 under LPBF conditions. It was found that changes in the surface tension of liquid metals related to temperature and composition influenced the formation and final morphology of partial defects of LPBF. Recently, Chen et al. [92] correlated the manufacturing quality and processing parameters such as powder feeding speed, laser power and lateral velocity of the powder, and determined the process conditions required to suppress surface disturbances resulting in roughness.
With the combined application of crystal imaging and 3D finite element simulation technology based on high-energy synchronous radiation X-ray source, and the development of high-throughput material preparation technology based on AM technology, it has become possible to observe and quantitatively characterize additive manufacturing materials in full cycle in situ. Synchronous X-ray imaging will play an important role in process parameter optimization and defect tolerance evaluation, promoting our deeper understanding of AM.
3. Measurement of residual stress of AM parts by X-ray diffraction
3.1 Source and type of residual stress
SLM During the shaping process, under the irradiation of the Gaussian laser energy distribution heat flow, a series of complex unequalized physical and chemical metallurgy processes occur, thereby generating a complex thermal phase change field. Mercelis et al. [93] proposed a critical temperature gradient mechanism (TGM) and cooling stage model in 2006, explaining the source of residual stress of the part from a macroscopic perspective [94]. In the TGM model, after the laser is turned on, the heated metal particles melt rapidly, and the solidified phase tends to expand. As shown in FIG. 24 , as the laser beam moves, the previous area begins to cool down and shrink, and residual tensile stress is generated in this area. As the parts are manufactured layer by layer, the compressive stress and tensile stress are constantly balanced [95].
Residual stress can be divided into three types: grain size, microscale and nanoscale. It is difficult to calculate residual stress on the nanoscale using modern measurement methods. Most of the residual stresses detected are at the grain scale, and their macroscopic characteristics affect the physical performance of the part.
Figure 24 Residual stress formation model: a heating stage; b cooling stage
3.2 Measurement of residual stress in AM by X-ray diffraction
Residual stress measurement is increasingly valued by researchers. XRD has been widely considered as an effective residual stress measurement method. Simson et al. [96] studied the residual stress of AISI 316L additive manufacturing parts in 2017 with XRD and tested the residual stresses at different depths and both outer surfaces. They found that on the top surface of the part, the residual stress along the scanning direction was higher than the residual stress in the vertical direction. On the contrary, on the side surface, the maximum principal stress is perpendicular to the scanning direction and parallel to the construction direction. This is consistent with two mechanisms of residual stress generation, namely the TGM and the cooling stage.
In addition, the research results also show that the stress value is related to structural density, unfused and adhesion of partially fused powder particles.Later, Marola et al. [97] used XRD to study the Al stress levels on the surface and inside of LPBF AlSi10Mg samples. It can be concluded that the stress level perpendicular to the construction direction is higher, which is due to the small number of Al grain boundaries along the construction direction. This result is different from the stress maximum position studied by Simson [96]. In addition, it was found that the stress value slightly decreased as the slice depth increased. Their work provides a clear distinction for the stresses occurring in the LPBF sample of AlSi10Mg and contributes to subsequent stress research.
Residual stress of additive manufacturing parts is affected by a variety of process parameters, such as scanning strategy, scanning speed, insulation time, etc. The residual strain of Ti6Al4V under different process parameters was measured using X-ray diffraction (XRD) technology. They found that when the scanning lengths of the cube samples were 5 and 1 mm, respectively, the residual stress decreased from 185 MPa to 90 MPa.
results show that using a shorter scanning vector can increase the surface temperature, reduce the temperature gradient at the solidification front, and thus reduce residual stress. This may reduce the need for post-processing and reduce the chances of parts failure during production. In addition, Levkulich et al. [99] performed XRD tests on the surface of Ti6Al4V samples made by LPBF. They observed that LPBF process parameters (scanning speed, laser power, construction height, construction plane area, substrate conditions) have a great influence on the development of residual stress and deformation in the substrate (Figure 25). By increasing the laser power, reducing the scanning speed, and reducing the forming plan area, the residual stress on the top surface of the LPBF sample can be reduced. In addition, their findings provide a valuable basis for modeling and simulation of residual stress and distortion evolution in the future.
A large number of analysis shows that the change in temperature over time has a very important impact on residual stress. Synchronous X-ray diffraction (SXRD) is a detection method that can monitor the thermal working state of parts in real time. In recent years, it is not uncommon to use SXRD to explore residual stress caused by thermal changes in AM. Oliveira et al. [100] used SXRD to measure the local transition temperature of the thermally affected zone of the laser-processed NiTi thin plate, as shown in Figure 26. The phase transition temperature gradient was observed to be related to local chemical composition changes caused by Ni depletion and residual stress. The new results provide a more basic understanding of the observed microstructure in AM components and its relationship with phase transition characteristics. Schmeiser et al. [101] studied strain and stress formation in the manufacturing process of SLM IN625 multi-layer thin-walled parts. They found that the correlation between temperature and yield strength resulted in the maximum stress generated from 300 μm of the top layer of the IN625 sample. This study demonstrates the potential of high-energy SXRD in in situ SLM studies.
At present, the research on residual stress of additive manufacturing parts is in a stage of rapid development. The stress field is not only affected by microstructure, but also by macroscopic parameters. The above results prove that XRD can better measure residual stress during AM. In the future, a variety of measurement methods can be used to fully understand the residual stress state of additive manufacturing parts, and combine quantitative theoretical models to effectively improve stress distribution and part quality.
Figure 25 XRD and drilling main stress measurements are performed on the top surface of sediments with different construction heights
Figure 26 a Schematic diagram of the detection area in the in-situ XRD experiment; b Room temperature X-ray diffraction pattern; c Diffraction pattern of the heat-affected zone at 150°C
4. Summary and future prospect
This article summarizes the application research of synchronous radiation X-ray imaging and diffraction in AM artifacts. Since scientific and technological problems affect the quality and cost of AM products, they include typical material defects (such as Ti6Al4V and AlSi10Mg), surface roughness, residual stress, etc. [102,103]. Using synchronous radiation X-ray characterization method, not only can high-precision analysis of parts with complex geometric shapes be performed, but the main defect information can be detected through rapid scanning or imaging. The defect formation mechanism and the impact of defects on mechanical properties are better understood, thereby improving quality and better developing post-treatment technologies.
Although synchronous radiation X-rays have been widely used to detect the microstructure, defect formation and evolution of AM, which has opened the door to a deeper understanding of AM, it still needs to further develop to broaden the application of X-rays in the AM process.The future development direction is as follows. Online monitoring remains a challenge. Important factors affecting the quality of parts, such as temperature field, velocity field, cooling speed, solidification parameters, etc., must be combined with real-time models to reduce defects and improve quality. In addition, combining ultrafast synchronous X-ray imaging, high-speed photophotography and infrared thermometer , it is expected to conduct online detection of the real laser printing process of multi-layer structures to reveal the microstructure, fluid convection and temperature field during the AM process. At the same time, it is of great significance to use synchronous radiation X-ray imaging or diffraction results to verify and correct the numerical model of AM, so as to accurately simulate the unequalization phenomenon in the melt pool during the AM process.
This article is over, and the Yangtze River Delta G60 Laser Alliance warmly welcomes your continued attention!
Article source: An, N., Shuai, S., Hu, T. et al. Application of Synchrotron X-Ray Imaging and Diffraction in Additive Manufacturing: A Review. Acta Metall. Sin. (Engl. Lett.) 5, 25–48 (2022). https://doi.org/10.1007/s40195-021-01326-x
Yangtze River Delta G60 Laser Alliance Chen Changjun reprinted
