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Reduction of residual stress in porous Ti6Al4V by in situ double scanning during laser additive manufacturing

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  • Corresponding author:

    Shu-qiang Jiao    E-mail: sjiao@ustb.edu.cn

  • Received: 28 May 2020Revised: 15 October 2020Accepted: 19 October 2020Available online: 20 October 2020
  • Selective laser melting (SLM) technology plays an important role in the preparation of porous titanium (Ti) implants with complex structures and precise sizes. Unfortunately, the processing characteristics of this technology, which include rapid melting and solidification, lead to products with high residual stress. Herein, an in situ method was developed to restrain the residual stress and improve the mechanical strength of porous Ti alloys during laser additive manufacturing. In brief, porous Ti6Al4V was prepared by an SLM three-dimensional (3D) printer equipped with a double laser system that could rescan each layer immediately after solidification of the molten powder, thus reducing the temperature gradient and avoiding rapid melting and cooling. Results indicated that double scanning can provide stronger bonding conditions for the honeycomb structure and improve the yield strength and elastic modulus of the alloy. Rescanning with an energy density of 75% resulted in 33.5%–38.0% reductions in residual stress. The porosities of double-scanned specimens were 2%–4% lower than those of single-scanned specimens, and the differences noted increased with increasing sheet thickness. The rescanning laser power should be reduced during the preparation of porous Ti with thick cell walls to ensure dimensional accuracy.
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Reduction of residual stress in porous Ti6Al4V by in situ double scanning during laser additive manufacturing

  • Corresponding author:

    Shu-qiang Jiao    E-mail: sjiao@ustb.edu.cn

  • 1. State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China
  • 2. Department of Orthopedics, Peking University Third Hospital, Beijing 100191, China

Abstract: Selective laser melting (SLM) technology plays an important role in the preparation of porous titanium (Ti) implants with complex structures and precise sizes. Unfortunately, the processing characteristics of this technology, which include rapid melting and solidification, lead to products with high residual stress. Herein, an in situ method was developed to restrain the residual stress and improve the mechanical strength of porous Ti alloys during laser additive manufacturing. In brief, porous Ti6Al4V was prepared by an SLM three-dimensional (3D) printer equipped with a double laser system that could rescan each layer immediately after solidification of the molten powder, thus reducing the temperature gradient and avoiding rapid melting and cooling. Results indicated that double scanning can provide stronger bonding conditions for the honeycomb structure and improve the yield strength and elastic modulus of the alloy. Rescanning with an energy density of 75% resulted in 33.5%–38.0% reductions in residual stress. The porosities of double-scanned specimens were 2%–4% lower than those of single-scanned specimens, and the differences noted increased with increasing sheet thickness. The rescanning laser power should be reduced during the preparation of porous Ti with thick cell walls to ensure dimensional accuracy.

    • Titanium (Ti) and Ti-based alloys have been extensively applied to the fields of aerospace, petrochemicals, engineering, and medical devices on account of their excellent mechanical properties, outstanding corrosion resistance, and biological performance [14]. Ti6Al4V (TC4 in China), a typical Ti alloy showing high specific strength and bioinert properties, is widely used in the biomedical field for implant fabrication [5]. Previous research demonstrated that mismatches in the stiffness of the bone tissue and Ti implants lead to the stress shielding effect, which results in bone resorption and implant loosening [68]. Fabrication of porous structures has been recognized as an effective solution to reduce the Young’s modulus of Ti implants and match it with those of natural bones. Porous Ti6Al4V, which combines the properties of Ti-based alloys and foam metal, possesses unique characteristics, such as low density (ρ), large specific surface area, and excellent toughness and stiffness [9]. Traditional methods to produce Ti6Al4V alloys with porous structures, such as direct metal foaming, vapor deposition, and powder metallurgy, generally give rise to poor connectivity, uncontrollable pore distributions, and inhomogeneous porosities. These defects lead to limitations in the porosity control, porous characteristics, and mechanical properties of the implants [1012]. The requirement of porous structures with highly controllable properties has promoted the development of additive manufacturing (AM) techniques, such as selective laser sintering [13], selective laser melting (SLM) [14], and electron beam melting [15]. The SLM technique integrates advanced laser technology, computer aided design (CAD), and powder metallurgy technology. Compared with traditional methods, SLM yields microporous structures with global morphological properties that are precisely controlled by a computer, which presents great advantages in the preparation of implants with complex structures and precise sizes. Moreover, because the Ti6Al4V powder for SLM is prepared by atomization, the powder particles exhibit a low degree of submicron-scale roughness, which increases the surface roughness of the implant and reduces the nucleation free energy [16]. These features are beneficial to the bioactivity of an implant.

      Unfortunately, implants processed by lasers as a heat source generally feature high levels of residual stress due to the rapid heating and cooling of the implant material [17]. Residual stress leads to deformation, distortion, and in some extreme cases, even cracking [18]. Conventional stress relief by annealing can eliminate the residual stress of implants; however, if the material is deformed or cracked, recovery by subsequent heat treatment is improbable. This problem can be solved by the rescanning method, which reduces the residual stress of the material in situ during SLM. However, most of the related research focuses on solid alloys, and studies on porous components are rare [1920]. Porous Ti is characterized with low ρ; it is often used for medical purposes, such as implants and cable systems, because it presents structural functions for bearing loads as well as biological functions for tissue growth, synostosis, and fluid transport [21]. That means the strength and stiffness of lightweight structure, porosity, and inner surface of the pores should reach a high level at the same time. The evolution of mechanical properties after the rescanning of porous components remarkably differs from that of solid alloys. Therefore, mitigating the residual stress and its influence to improve the mechanical properties of porous implants is a crucial endeavor.

      Previous studies focused on the application of heat treatment, such as aging or annealing, after laser AM to eliminate the residual stress of porous Ti [2224]. However, heat treatment can result in additional processing steps and modify the as-built microstructures. The aim of the current study is to reduce the residual stress and improve the mechanical strength of porous Ti during three-dimensional (3D) printing while minimizing the necessary production steps and supporting the rapid prototyping of products. Methods to eliminate residual stress during AM include heating of the build plate [25] and alteration of the scanning strategy [26]. The first method leads to decreased efficiency as the height of the component increases, while the second method can reduce the residual stress to some extent but requires further optimization for practical applications.

      In this work, the residual stress of porous Ti6Al4V was adjusted by SLM with a dual laser system. The novel strategy of rescanning of the solidified cross-section of the alloy under a low power and scan rate immediately after completion of the first laser melting reduced the temperature gradient and prevented the rapid melting/cooling of the alloy. The theoretical relationship between the mechanical properties and deformation behaviors of porous Ti6Al4V, as well as the pore structural parameters, obtained from the new scanning strategy was subsequently obtained using experimental and statistical methods. The results of this work are expected to promote the development of biomaterials featuring reasonable porosity and sufficient mechanical strength.

    2.   Experimental
    • The starting Ti6Al4V powders were obtained by electrowinning according to the patent No. US10081874B2. High-purity Ti powders electrolyzed from a Ti-containing soluble anode were mixed with a certain proportion of Al and Va powders, as listed in Table 1. The powders had an oxygen content of 0.15wt%, which is below the minimum requirement of biomedical materials (i.e., 0.20wt%) [27]. Fig. 1 shows the size distribution and scanning electron micrographs of the Ti6Al4V powders used in the present study; the powders had diameters in the range of 10–53 μm, as determined via measurements taken with a laser diffraction particle size analyzer (CLF-2, Malvern Panalytical, UK). The sphericity and particle size of the powders also met the requirements of SLM 3D printing.

      TiAlVFeONCH
      Bal.5.5–6.53.5–4.50.1<0.15<0.01<0.03<0.01

      Table 1.  Chemical composition of the Ti6Al4V powders wt%

      Figure 1.  Micrograph and particle size distribution of the Ti6Al4V powders.

    • The unit cell representing the microarchitecture of the porous Ti alloy featured a honeycomb structure, the porosity and pore size of which could be controlled by adjusting the CAD model parameters D and d (Fig. 2(a)). This specific unit cell was chosen for the convenience it offers during evaluation of the accuracy among porous structures of different print thicknesses and CAD models. As the minimum size the SLM equipment can produce is 100 μm, five porous structures with identical pore sizes (D = 600 μm) and different thicknesses (d = 100, 200, 300, 400, and 500 μm) were designed and manufactured through SLM. Cylindrical specimens with a height of 15 mm and diameter of 10 mm, cubic specimens with dimensions of 10 mm × 10 mm× 10 mm, and tensile test specimens were produced with their longitudinal axis perpendicular to the build plate (Figs. 2(b) and 2(c)).

      Porous Ti6Al4V was manufactured from the Ti6Al4V powders using SLM technology (EOS, M300, German) equipped with a dual laser system. The build chamber was first vacuumed and then filled with Ar gas to maintain an oxygen content of less than100 ppm. The processing parameters used in this experiment were as follows: power, 240 W; scanning speed, 0.8 m·s−1; layer thickness, 30 μm; and scanning interval, 50 μm. Specimens prepared under these process parameters were referred to as C-AM. Another series of specimens were scanned by the double laser system immediately after each layer demonstrated laser melting. The laser power and scanning speed of the second scan in this experiment were set to 180 W and 0.5 m·s−1, respectively; these parameters provided 75% of the energy density of the first scan.

      Figure 2.  Schematic diagram and photograph of porous Ti6Al4V structure: (a) unit cell; (b) computer aided design (CAD) model of the test sample; (c) specimens fabricated by SLM.

    • The cell wall and fracture morphology of the porous Ti6Al4V alloys were characterized through metalloscopy (Leica, Leica-DM4M, German) and field emission scanning electron microscopy (Carl Zeiss, Zeiss-EVO18, German). Metallographic photographs of the specimens were analyzed using Image-Pro Plus software to measure pore sizes. Porosity (P) was calculated using the formula [28]:

      where the density of bulk Ti6Al4V (${\rho _{\rm{b}}}$) is equal to 4.37 g·cm−3 and the density of porous Ti6Al4V ($\rho $) is obtained from density measurements (Quantachrome, Ultra PYC 1200e, USA)

      Static uniaxial compression and tensile tests were carried out using a universal testing machine (XinSanSi, CMT4305, China) at room temperature following ASTM E9–09 and ASTM E8M–11. Average values were derived from the mean of five specimens for each Ti6Al4V alloy with a particular sheet thickness. Cylindrical specimens with a diameter of 10 mm and height of 15 mm were used for the axial compression test, during which the load was applied to the top of the cylinder. Cubic specimens with dimensions of 10 mm × 10 mm × 10 mm were used for the radial compression tests, during which the load was applied to the cell walls. An electronic universal material testing machine (Kexin, WDW3020, China) was used, and a loading speed of 1 mm/min was applied to all samples. Load displacements under tension and compression were recorded to plot stress–strain curves.

    • Virtual testing by finite element analysis (FEA) was conducted to understand the mechanical properties of the designed porous structures. The relevant CAD model was imported into FEA software and assumed to be linearly elastic and homogeneous. Two constraints were applied to the top and bottom of the porous structures to simulate their compression process. The bottom of the model was fixed, and the top of the model was loaded with some pressure at a constant rate. A constant-velocity tension was applied to both ends of the porous structure to simulate the tension process. The elastic modulus (E) and yield strength (σps) of each model were determined from the experimental results of the mechanical properties, and the Poisson’s ratio was assumed to be 0.31. The designed models were meshed and calculated under these parameter settings to analyze the deformation and stress of porous Ti6Al4V alloys with different sheet thickness under a tensile or pressure load.

    • A nano-indenter (MNT, N2100, USA) was employed to measure the residual stress of Ti alloys with different sheet thickness. Lamellar specimens with thicknesses of 100, 200, 300, 400, and 500 μm were prepared separately for use in the residual stress tests to guarantee the accuracy of the measured data. The specimens were polished and measured under a fixed load (1 mN). The measurement range of each sample was a 3 × 3 lattice with a spacing of 10 μm between each indentation. The residual stress of each sample was then calculated by the Suresh model [29].

    3.   Results and discussion
    • Figs. 3(a)3(e) shows the metallographs of double scanned porous Ti6Al4V alloys (D-AM100–D-AM500) with different porosities fabricated by SLM. No noticeable crack or defect can be found in any of the specimens. The pore size of the CAD models is 600 μm. The actual pore sizes of the SLM-manufactured specimens are generally smaller than that of the CAD models (Fig. 3(f)), and the difference noted increases with increasing cell thickness. For example, the difference in pore size between the CAD model and an actual sample is 32 μm when the cell thickness is 100 μm but 102 μm when the cell thickness is 500 μm. This phenomenon could be explained by the inherent limitations of the SLM technique. The CAD models are constructed with porous structures but smooth surfaces, whereas the surfaces of the SLM-manufactural specimens are covered with a powder layer because of heat radiation around the laser spot. The energy of the laser beam presents a typical Gaussian intensity distribution; thus, it shows strong intensity at the center and low intensity at the edge. Larger laser spot diameters provide greater heat radiation effects that promote the melting and attachment of the Ti6Al4V powders to the pore edges. The spot diameter of the SLM scanning component, 70–100 μm, is largest when the laser is located at the center of the build plate and decreases in size as the edges of the plate are approached. Consequently, under the same parameters, the dimensional error of the specimen located at the center of the build plate (i.e., D-AM500 in this experiment) is much more evident than that of a specimen located at the edge of the plate. Narrow laser spots benefit the manufacture of precise porous structures and morphologies.

      Figure 3.  Metallographs and porosities of the porous Ti6Al4V structures produced by SLM: (a) D-AM100, (b) D-AM200, (c) D-AM300, (d) D-AM400, (e) D-AM500; (f) comparison of the pore sizes of the SLM specimens and CAD models.

      Fig. 4 compares the porosities of the CAD models and C-AM and D-AM specimens. According to the CAD data, the porosity of the specimens is in the range of 40.4%–78.6%. As discussed earlier, the actual porosities of the C-AM and D-AM samples are smaller than that of the CAD models and the error of the D-AM specimens is 2%–4% larger than the error of the C-AM specimens. The microstructures of the cell walls of C-AM and D-AM are shown in Figs. 5(a) and 5(b). Powder adhesion occurs more extensively in the D-AM specimens than in the C-AM specimens. The adhesion of powders to porous Ti6Al4V is mainly related to the parameters of SLM and the physical properties of the metals. The temperature of molten Ti6Al4V under the action of a laser source is higher than the liquidus temperature [30]. D-AM specimens scanned twice per layer are subject to significantly higher laser energies, leading to increased temperatures and liquid formation and, in turn, the adhesion of more unmelted powders on the surface of the specimens. Accordingly, the Ti6Al4V powder in the powder storage receives twice thermal radiation and heat conduction, which will also affect the viscosity of recycled powder. Previous studies [3133] demonstrated that high laser energy densities result in over-melting and aggravate the phenomenon of powder adhesion, which leads to high surface roughness. By comparison, the viscosity of powders irradiated by a laser iteratively and recycled repeatedly decreases, thus significantly reducing the dimensional accuracy of the specimens. Hence, the laser power of the second scan should be reduced suitably to avoid the influence of adhesive powders on the dimensional accuracy and surface quality of porous Ti6Al4V.

      Figure 4.  Porosities of the CAD models and C-AM and D-AM specimens.

      Figure 5.  SEM microtopography of the specimens fabricated by different processes: (a) surface of C-AM200; (b) surface of D-AM200; (c) microstructure of C-AM200; (d) microstructure of D-AM200.

      Microscopic images of the cell walls prepared by conventional and double scanning are shown in Figs. 5(c) and 5(d). The microstructure of the Ti6Al4V alloy is mainly composed of α acicular martensite because of rapid cooling. The grain boundaries are clearly visible in the absence of double scanning. By contrast, the grain boundaries of specimens subjected to rescanning are obscured by the redistribution of chemical substances at high temperatures. The grains melt once more when the powder layer is rescanned by the laser, which leads to the epitaxial growth of strengthened grains and martensite. The growth of martensite improves the strength of the Ti6Al4V alloys. The main defects of Ti6Al4V alloys melted using conventional technology mainly include irregular pores caused by unfused powder; these defects are obviously reduced after rescanning.

    • The porosity of Ti6Al4V alloys exerts a significant influence on their mechanical and biological properties. As the thickness of the cell wall increases, the strength of an alloy increases but its porosity decreases, which is detrimental to the biological activity of implants. Increases in porosity result in corresponding decreases in alloy strength. Hence, the porosity of a biomaterial should be increased as long as the desired mechanical properties are satisfied to achieve a suitable balance between mechanics and biology. Static stretching and uniaxial compression tests in the radial (RD) and axial (AD) directions are conducted on the porous Ti6Al4V alloys. Table 2 illustrates the details of the mechanical performance of the single- and double-scanned samples. Because of the presence of pores, the σps and E of porous Ti6Al4V are lower than those of the bulk alloy and decrease with increasing porosity. By contrast, the double-scanned structures reveal higher E and better mechanical strength than the conventional porous structures because double scanning maintains a more temperature and improves solute segregation in the cell walls. The E of porous Ti6Al4V prepared by the double-scanning process proposed in this work is in the range of 4.59–23.96 GPa. Previous studies demonstrated that the E of cortical bone is approximately 10–30 GPa; the E of bone trabecula is even lower than this value [34]. The experimental data collected above clearly overlap with the apparent E reported for human bones. The σps values of samples D-AM200 and D-AM300 are 60.65 and 111.49 MPa, respectively; these values are well within the expected yield strength of human bone tissues, which ranges from 55.3 ± 8.6 MPa to 122.3 MPa [3537]. However, the E of D-AM200 is lower than that of cortical bones. When the sheet thickness is 400 or 500 μm, the E of the porous structure is suitable but its σps does not match those of human bones.

      Sheet thickness /
      μm
      Yield strength / MPaElastic modulus / GPaRadial compressive strength / MPaAxial compressive strength / MPa
      C-AMD-AM C-AMD-AM C-AMD-AM C-AMD-AM
      100 33.58 43.19 2.05 4.59 24.85 51.73171.81259.69
      200 40.06 60.65 3.54 6.05 85.36143.47193.29289.61
      300 97.03111.49 6.2210.34169.48316.43202.31350.28
      400147.84184.57 9.1416.68211.54384.26263.16405.65
      500206.41275.3811.3323.96304.39447.58335.97490.12

      Table 2.  Comparison of the mechanical properties of different types of porous Ti6Al4V

      The quasi-static stress–strain relationships of specimens C-AM and D-AM with various porosities are shown in Figs. 6(a) and 6(b). The relationships among σps, E, and relative ρ can be approximated by the Gibson and Ashby model [38]:

      Figure 6.  Tensile stress–strain curves of the porous Ti6Al4V specimens: (a) C-AM and (b) D-AM; (c) Changes in the elastic modulus of the specimens as a function of their relative density; (d) Changes in the yield stress of the specimens as a function of their relative density.

      the elastic modulus (Eb), density (ρb), and yield strength (σys) for bulk Ti6Al4V alloys are 110 GPa, 4.37 g·cm−3, and 827 MPa, respectively [39]. C1, C2, n1, and n2 are constants related to the structure and experimental parameters of the porous Ti6Al4V alloys and could be obtained through nonlinear fitting of the experimental data.

      As shown in Figs. 6(c) and 6(d), the C1, C2, n1, and n2 of the C-AM structures are equal to 0.28 ± 0.018, 0.87 ± 0.063, 2.06 ± 0.128, and 2.20 ± 0.149, respectively; the corresponding C1, C2, n1, and n2 of the D-AM structures are 0.95 ± 0.046, 0.91 ± 0.087, 2.57 ± 0.171, and 2.26 ± 0.020, respectively. In the Gibson and Ashby model, the coefficients C1 and C2 range from 0.1 to 1 for open-cell structures [38], and the specific values of these coefficients depend on the bonding strength of the cell walls. In the present experiment, C1 in the equation of C-AM is smaller than that of D-AM, which suggests that the bonding condition of the latter is stronger than that of the former. The density index (n) is one of the most important structural characteristics of a porous material. Earlier studies revealed that n is determined by the deformation of the pore walls, i.e., bending, buckling and yielding, and approximately distributed in the range of 1–6.3 [40]. Yamada et al. [41] also reported the relationship between n and the deformation mode of cellular alloys; specifically, n2 = 1 when the deformation of porous structure is dominated by the yielding of the cell walls, n2 = 1.5 when the deformation is dominated by the bending mode, and n2 = 2 when the deformation is dominated by the buckling of the cell walls. The calculations in this work indicate that the deformation mechanism of specimens C-AM and D-AM involves the buckling of cell walls, which implies that porous Ti loses its bearing capacity because of the instability or collapse of its cell walls before yielding.

      Fig. 7 depicts the RD and AD quasi-static stress–strain relationships of the specimens during compression. The compressive stress–strain curve of porous Ti usually consists of three stages: the linear elastic stage, the platform stage, and the densification stage [42]. Fig. 7 reveals the following characteristics:

      Figure 7.  Compressive stress–strain curves of the porous Ti6Al4V specimens: (a) radial direction; (b) axial direction.

      (1)In the first stage of the stress–strain curve, the deformation of Ti6Al4V with different porosities presents a linear elastic behavior in the RD and AD. E decreases with decreasing sheet thickness.

      (2)The transition from the first stage to the second stage is smooth, and no obvious yield point can be observed in the stress–strain curve. This finding suggests that the stress increases in a nonlinear manner with increasing compression stress.

      (3)In the second stage, the porous Ti6Al4V shows obvious variations in plateau stress in the RD and a fairly stable plateau stress in the AD. Moreover, specimens D-AM400 and D-AM500 are fractured directly under radial stress after yielding without plastic deformation; these phenomena are attributed to a reduction in the plasticity of these samples due to the thickening of their cell walls.

      (4)In the third stage, specimens with low porosities (e.g., D-AM300, D-AM400, D-AM500) are fractured when the compressive stress reaches a peak value. By contrast, porous Ti6Al4V with high porosities (e.g., D-AM100, D-AM200) enter the densification stage without obvious fractures.

      Thus, porous Ti alloys with different sheet thicknesses show identical elastic deformation stages but different plastic deformation stages.

      Fig. 8 shows the fractures of the samples. Specimens with different sheet thicknesses present different failure modes in the RD and AD. Under radial quasi-static loading, the sheets of the unit cells of all specimens are generally crushed at a 45° angle (Figs. 8(a) and 8(b)). Cracks are initiated at the edge or tip of the specimens and propagate toward the center of the sheets. Under axial quasi-static loading, specimens with low sheet thicknesses show excellent toughness and do not crack. In this case, the waist of the specimen bulges and the cell walls compress onto each other (Fig. 8(c)). By contrast, specimens with higher sheet thicknesses break after densification (Fig. 8(d)), thus revealing that the deformation mode of the cell walls is closely related to the characteristics of their pore structure.

      Figure 8.  Failure modes of the specimens during the static compression test: (a) shear line of D-AM500; (b) diagonal shear fracture and its microstructure of D-AM100; (c) plastic deformation in D-AM100 without cracks; (d) shear line and dense pores of D-AM500.

    • The relationship between the stress and strain fields of Ti6Al4V samples with different porosities could be obtained by FEA. The FEA results of the von Mises stress field shown in Fig. 9 could help evaluate the deformation levels of the specimens. The input parameters of each model, including E and σps, are derived from the experimental data (Table 2). As shown in Fig. 9, porous structures present different stress concentration fields in the out-of-plane compression and in-plane tensile tests. Under a load pressure of 100 MPa, the von Mises stress is concentrated in the diagonal direction of the cell walls forming an angle of 30° relative to the build plate. Hence, the angle between the stress concentration field and build plate increases with increasing sheet thickness (Fig. 9(a)). This FEA result is consistent with the experimental results illustrated in Fig. 9(b). As the load increases, buckling of the cell walls dominates the deformation mechanism of porous Ti and results in the local deformation of the specimens. Continuous increases in the load cause the porous structure in the deformation area to collapse and form a deformation belt. Calculations of n2 indicate that the deformation mechanism of the cell walls during compression is buckling rather than bending or yielding. The tensile von Mises stress is concentrated between the upper and lower vertices of two adjacent layers of pores (Fig. 9(b)). Thus, compared with cell walls perpendicular to the build plate, inclined cell walls appear to bear more stress under tension and compression.

      Figure 9.  FEA results of the deformation of porous Ti6Al4V before failure: (a) radial compression; (b) tensile test.

      Fig. 10 shows the relation between residual stress and sheet thickness. The residual stresses of the rescanned specimens are 33.5%–38.0% lower than those of no rescanned specimens. The observed decrease in residual stress with increasing sheet thickness is consistent with the findings of Wang et al. [43]. Ali et al. [44] reported that rescanning with 150% energy density results in a 33.6% reduction in residual stress. However, the mechanical properties of the samples deteriorated, and the samples failed prematurely. In the present study, the σps and E of porous Ti6Al4V are improved by double scanning with a low power and slow speed.

      Figure 10.  Effect of rescanning on the residual stress of porous Ti6Al4V.

    4.   Conclusions
    • In this work, 3D-printed porous Ti6Al4V alloys with different cell wall thicknesses were produced by double scanning with 75% energy density. The porosities of double-scanned specimens are 2%–4% lower than those of single-scanned specimens. Moreover, double-scanned structures reveal higher E and better mechanical strength than conventional porous structures. The double-scanning process may thus be suggested to maintain more uniform temperatures and promote solute segregation in cell walls. The E of the porous Ti6Al4V prepared by the process proposed in this work is in the range of 4.59–23.96 GPa, and its n is fairly large, thus revealing that the bonding conditions of this sample are stronger than those of single-scanned specimens. The deformation mechanism of the double-scanned specimens involves the buckling of the cell walls. Rescanning with an energy density of 75% resulted in 33.5%–38.0% reductions in residual stress. The residual stress of porous Ti6Al4V decreased with increasing sheet thickness. The proposed method could prove useful for decreasing residual stress in in situ laser AM while ensuring the dimensional accuracy of thin-walled structures.

    Acknowledgements
    • This work was financially supported by the National Natural Science Foundation of China (Nos. 52004026 and 51725401) and the Fundamental Research Funds for the Central Universities, China (No. FRF-TP-18-003C2). The helpful comments, suggestions, and encouragement from the editors and anonymous reviewers are gratefully acknowledged.

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