StefanieMeidinger
StefanieMeidinger
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Original title: Effect of electric pulse on microstructure and microhardness of selective laser melting Ti6Al4V alloy Jiangsu Laser Alliance Guide: The effect of electric pulse on the microstructure and microhardness of selective laser melting Ti6Al4V alloy was investigated. The optimization of microstructure and mechanical property of Ti6Al4V selective laser melting (SLM) alloy is an important subject in the field of science and engineering. In this study, rapid hardening and softening of SLM-Ti6Al4V alloy were achieved during pulsed electropulsing treatment (EPT) at different discharge voltages due to the obvious microstructure evolution. The microhardness of the EPT-7. 5 kV sample is about 7% lower than that of the untreated sample, while the microhardness of the EPT-8 kV sample is about 10% higher than that of the untreated sample. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used to analyze the evolution of α, β grains and dislocation structures, which corresponded to the microhardness changes caused by transmission electron microscopy. The results show that EPT can reduce the thermomechanical barrier of αβ solid phase transformation in SLM-Ti6Al4V alloy and further refine the microstructure of the alloy. This study provides a new way to improve the microstructure and mechanical property of the metal components quickly and effectively. Abstract graphics 1. Introduction In recent years, additive manufacturing technology (AM) has achieved design freedom and energy saving by using high-intensity energy sources (laser beam, electron beam or arc, etc.) To melt metal powder or wire and manufacture parts on substrates according to CAD models. As a typical laser AM technology, selective laser melting (SLM) is widely used in the fields of titanium alloy, aluminum alloy, steel and high entropy alloy in the case of manufacturing complex parts and high material utilization. Ti6Al4V alloy is widely used in advanced manufacturing fields such as biomedical engineering, aerospace and so on because of its high strength-to-weight ratio, excellent corrosion resistance and biocompatibility. Expand the full text Advantages of SLM technology over traditional biomedical device manufacturing methods. SLM, selective laser melting. The key advantages of fabricating biomedical devices via SLM over conventional fabrication techniques are shown in the figure above. However, SLM-Ti6Al4V alloy usually forms α 'martensite phase, which has high residual stress and lower plasticity than traditional process. At present, SLM components use post-treatment heat treatment, such as conventional heat treatment and hot isostatic pressing (HIP) heat treatment, to eliminate residual stress and improve the ductility of components by reducing voids or defects. These processes can effectively eliminate the residual stress, reduce the number of defects, change the original acicular α ′ martensite into a lamellar mixture of α + β, and improve the plasticity, but reduce the strength. However, the first process will lead to abnormal grain growth, which is not conducive to the improvement of mechanical property, while the HIP process is relatively expensive and inconvenient, and there are problems of deformation and structural uniformity when applied to parts with complex geometries. In recent years, the non-equilibrium transient method based on the selective Joule heating effect and the electron wind velocity, the electric pulse treatment (EPT), has been applied to adjust the microstructure and mechanical property of conventional metals in a short time and with high efficiency. In 2205 duplex stainless steel, EPT can promote the formation of grain boundary, refine the microstructure and improve the mechanical property. Our recent research shows that EPT not only lowers the threshold of recrystallization and accelerates the nucleation of recrystallized cold-rolled 316 L austenitic stainless steel, but also realizes the healing of pores and cracks, effectively improving the fatigue performance of machinery and materials. Microsecond pulsed electric field (EPT) is expected to be the best process to repair the defects caused by SLM and suppress the rapid grain growth during post-treatment. However, little is known about the effects of electromagnetic waves on SLM materials. The effects of EPT on the microhardness and microstructure evolution of SLM-Ti6Al4V alloy were investigated. The potential regulation effect of the critical discharge voltage of pulsed electric discharge on the mechanical property was revealed.
2. Materials and methods 2. 1. SLM manufacturing SLM-ti6al4v samples were prepared in an argon atmosphere on a commercial SLM facility (ConceptLaser, M2 facility) with the preset optimum parameters shown in Table 1. An island sharpening scanning strategy with a zigzag path is adopted in the SLM process. The average diameter of the powder used for production is about 20.m u.m, as shown in fig. 1. The chemical composition of the powder is shown in Table 2. Make cylindrical specimens 100 mm in length and 17 mm in diameter, with the axis of the cylindrical specimen in the direction of the building. Cylinders are annealed at 800 ± 14 ° C for 2 hours ± 10 minutes and then furnace cooled. It should be clarified that the SLM-Ti6Al4V alloy used for the EPT is stress-relieved because it is easier to remove the printed sample from the substrate and is less likely to bend or crack than the as-built state. Table 1 SLM process parameters used for sample fabrication. Fig. 1 Morphology of Ti6Al4V powder used for fabrication. Table 2 Chemical composition of Ti6Al4V powder (wt%). 2. 2. EPT process As shown in Fig. 2a, the annealed cylinder was cut into a plate-like sample with a size of 50 × 6.5 × 2.5 mm3 by an electric discharge wire, and subjected to electric discharge field treatment. A self-made device was used to carry out the discharge experiment under pulsed electric field at ambient temperature (Fig. 2b), and the clamping device is shown in Fig. 2C. According to the experience of predecessors, we chose the electric field discharge voltage of 6 kV ~ 8.5 kV as the experimental parameters (Table 3), which effectively improved the mechanical property of steel. The EPT lasts about 400 ns and is then air-cooled. Hereafter, samples treated at 6 kV are referred to as EPT-6 samples, and so on. The original samples without EPT were labeled as EPT-0 samples accordingly. Fig. 2 is a schematic diagram of the experimental process and the electronic pulse processing equipment. Table 3 EPT process parameters of SLM-Ti6Al4V alloy. 2.3 Microhardness and microstructure characterization Vickers microhardness was tested by AMH43 microindentation hardness automatic testing system under 200 G load and 15 s dwell time. Phase detection was carried out on a Cu-Kα X-ray diffraction (XRD) apparatus with scanning angles ranging from 30 ° to 60 ° and a step size of 10 °/min. The OM observations were performed on a device at Olympus. Optical microscopically observed samples were mechanically polished and chemically etched in an etching solution of 10% hydrofluoric acid, 5% nitric acid, and 85% distilled water. Scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) analyses were performed by FEI Inspect F50 and LEO Supra 35, and the samples were electropolished in a mixed solution of 5% perchloric acid, 35% n-butanol, and 60% methanol at − 30 ° C. Transmission electron microscopy (TEM) was performed with FEI Tecnai Spirit TEM T12. TEM films were prepared by electropolishing in an electrolytic solution (perchloric acid: n-butanol: methanol = 5:35:60) at − 30 ° C using a double-jet with a voltage of 20 V. In order to perform EBSD experiments,ti6al4v, the sample must have a highly flat and well-polished surface, and the beam must strike the sample at a grazing angle, usually 20 degrees, so the table carrying the sample should be tilted 70 degrees (above). When the accelerating voltage is 10-30 kV and the incident current is 1-50 nA, the electron beam diffracts at the incident point on the sample. In the case of a stationary beam, the EBSD map emits spherically from this point. When the main beam interacts with the crystal lattice, backscattered electrons with a small amount of energy loss are guided and cross different paths, resulting in constructive and destructive interference. Placing a phosphor screen not far from the tilted sample, the diffraction pattern in the path of the diffracted electrons can be observed (Wells, 1999). The phosphor screen converts the diffracted electrons into light, which is detected by a charge-coupled device (CCD) type camera.
EBSD images detected by the CCD camera were recorded and analyzed. It is defined by the spatial orientation of the crystals in the sample, the wavelength of the incident electron beam (which depends on the acceleration of the electron beam), and the distance between the phosphor screen and the sample. 3. Result 3. 1. Rapid softening and hardening Fig. 3 shows the microhardness changes of SLM-Ti6Al4V alloy after electric field treatment at different discharge voltages. The average microhardness of the sample decreases first and then increases. The microhardness of EPT-7.5 sample is the lowest, 336HV, which is about 7% lower than that of EPT-0 sample (362 HV). Under the electric field of 8 kV and 8. 5 kV, the microhardness of the sample is 400 HV and 394 HV, respectively, which is about 10% higher than that of EPT-0 sample. Therefore,Titanium welding pipe, the application of different EPT parameters can rapidly change the microhardness of SLM-Ti6Al4V alloy. Fig. 3 Variation of microhardness of the sample under different electric fields. 3. 2. Phase composition of SLM-Ti6Al4V sampled The XRD pattern of the SLM-Ti6Al4V sample after EPT treatment is shown in Figure 4. It can be found that all samples are composed of α and β phases (fig. 4A). In addition, the full width at half maximum (FWHM) of the α-phase peak at 2θ ≈ 40.47 ° changes in the EPT-0 and EPTed samples (Figure 4 B). The FWHM value is closely related to the grain size and dislocation density of the material. It can be seen from fig. 4 that the phase composition of SLM-Ti6Al4V remains unchanged under the electromagnetic wave electric field, while the grain size (α lamellar width) and dislocation density are significantly affected. Specifically, the relationship between these two variations can be explained by the optimized Williamson-Hall equation: Fig. 4 (a) XRD spectra of samples EPT-0, EPT-6, EPT-7, EPT-7.5, and EPT-8; (B) FWHM value of (101) α peak at 2θ ≈ 40.47 °. Where G = 1 dhkl dhkl ∆ G = cos θ cos θ ∆ 2θλ, Δ2θ measurement application of the HKL peak, D is the size of the crystal, B is the size of the Burgess vector, M is the contrast factor HKL crystal plane for the average dislocation with constant and external correlation cutoff radius, dislocation density ρ, C. The FWHM of Ti6Al4V alloy [24] is linearly correlated with FWHM. The FWHM increases with the decrease of grain size and the increase of dislocation density. In this experiment, the TEM observation results show that the effect of dislocation density in EPTed samples is not obvious (Fig. 9), that is, the smaller the half-width value of EPT-7 samples is, the coarser the grains are under the action of EPT. The higher FWHM value of the EPT-7.5 sample may be related to the shorter α layer shown in Figure 7d. While in the EPT-8 and EPT-8.5 samples, the higher FWHM values indicate grain refinement at higher processing voltages. 3. 3. Microstructure evolution of SLM-Ti6Al4V Fig. 5 is an optical micrograph of an SLM-Ti6Al4V sample in different EPT States. It is important to note that the direction of construction of the sample is vertical. It can be seen that the epitaxial columnar β grains grow along the building direction driven by the temperature gradient. Interestingly, in the discharge voltage range from 6 kV to 7.5 kV, the β grains exhibit a columnar structure, and the width of the β grains remains basically unchanged. The relatively straight boundaries of the columnar grains gradually change to zigzag boundaries, as shown in figs. 5a-d. When the discharge voltage reaches 8 kV and 8.5 kV, the β grains are equiaxed, and the grain diameter increases from 108 µm to 302 µm. Table 4 lists the columnar width or equiaxed grain size of the previous β grains, which increased very slightly with increasing discharge voltage, except for the EPT-8.5 sample, which increased significantly.
Figure 5 Optical images of (a) EPT-0, (B) EPT-6, (C) EPT-7, (d) Ept-7.5, (e) Ept-8, and (f) Ept-8.5. Table 4 Size of β grains in non-EPTed and EPTed samples. Microstructure evolution of SLM-Ti6Al4V alloy before and after EPT treatment was investigated by scanning electron microscopy (SEM) and electron scanning electron microscopy (EBSD). It can be seen from fig. 6 that there are a large number of fine α lamellae with high aspect ratio inside the β columnar grains in the ep-0 sample, which is in good agreement with the XRD results of fig. 4. The α/β structure of the alloy is controlled by the Burgers relationship, which plays a key role in the strength and plasticity of the alloy. An interesting finding is that the α-layered structure of EPT-7.5, ti6al4v eli ,6al4v titanium bar, EPT-8, and EPT-8.5 samples is denser than that of EPT-0, EPT-6, and EPT-7, which means that the metallurgical bonding is better. In addition, a more pronounced Widmannstatten organization and a thinner lamellar α phase can be found in the EPT-8 and EPT-8.5 samples (Figure 6 e and f). Figure 6 Scanning electron microscope (SEM) images of (a) EPT-0, (B) EPT-6, (C) EPT-7, (d) Ept-7.5, (e) Ept-8, and (f) Ept-8.5. Further EBSD observations of the lamellar α phase are shown in Fig. 7, indicating that these samples are mainly composed of lamellar α structures of different widths, as shown in Fig. 8. In the EPT-0, EPT-6, EPT-7, and EPT-7.5 samples, the α sheet width increases from 1.0 µm to 2.29 µm with increasing discharge voltage (Figure 7 a-d). It is worth noting that when the electric field discharge voltage reaches 8 kV and 8.5 kV, respectively, the α lamellar structure is significantly refined to 0.73 μm and 0.88 μm (Fig. 8 e and f). Figure 7 IPF images of (a) EPT-0, (B) EPT-6, (C) EPT-7, (d) Ept-7.5, (e) Ept-8 and (f) Ept-8.5. Fig. 8 The distribution of α lamella width in different samples are (a) EPT-0, (B) EPT-6, (C) EPT-7, (d) Ept-7.5, (e) Ept-8 and (f) Ept-8.5. TEM experiments were carried out to verify the evolution of the dislocation structure in the sample. Fig. 9a is the microstructure of sample EPT-0. The inserted selected area electron diffraction (SAED) pattern of the area marked with "A" shows the HCP structure of the alpha phase. In the EPT-0 (fig. 9 a) and EPT-6 (fig. 9 B) samples, dislocations are visible inside the lamellar α phase, as indicated by the arrows. In contrast, the EPT-7.5 sample shows a clear decrease in dislocation density. When the electric field voltage reaches 8.5 kV, an α ultrafine lamellar structure with a width of ~ 300 nm appears (Fig. 9d). Fig. 9 Bright field (BF) images of (a) EPT-0, (B) EPT-6, (C) EPT-7.5, and (d) EPT-8.5. The corresponding SAED diagram of the α phase is inserted in (a). 4. Discussion 4. 1. Rapid phase transition under electric field The microhardness and the corresponding microstructure evolution are caused by the coupling action of the thermal and non-thermal effects induced by EPT. On the one hand, we discuss that the high cooling rate inherent to the SLM process induces the slm-ti6al4v alloy to have a basketweave microstructure with fine alpha laths (figs. 5A and 6a). In addition, the higher dislocation density inside the alloy (Figure 9 a) hinders the movement of dislocations, resulting in a relatively high microhardness (Figure 3). From the optical micrograph of fig. 5, it can be seen that the β phase transition occurs with the increase of the discharge voltage under the electric field.
The thermal effect caused by Joule heating is completed in nanoseconds in a very short time. Periodic violent impacts between electrons and atoms can provide non-thermal effects, which can also be explained by electron wind theory. The microstructure evolution of cast iron treated with Mg and properly inoculated with CE > 4.3 during casting is shown in the following figure. Describe the phase diagram, cooling curve and steps of the solidification process of ductile iron. At the lower discharge voltage of 6 ~ 7.5 kV, the initial β columnar grains and α lamellae do not change obviously. It is speculated that the change in the morphology of the original β columnar crystals may be related to the thermal stress, which is generated during rapid heating due to thermal expansion lagging behind the temperature, and can be expressed as σ = EαEα ∆ T, where E is the Young's modulus and α is the thermal expansion coefficient. At the same time, the current through the sample is constrained by the electrode during the electric pulse treatment, which will also produce transient thermal stress. Therefore, the β grain boundary changes from the original straight grain boundary to the zigzag grain boundary. However, according to previous studies, Joule heating has an effect on the size of α lamellae, which may be caused by grain boundary migration. The size of α lamellar structure directly affects the mechanical property of the alloy. The average width of the α layer increased from ~ 1 μm in the EPT ‐ 0 sample to 2.29 μm in the EPT ‐ 7.5 sample, and the microhardness decreased from 362 HV in the PT ‐ 0 sample to 336 HV in the PT ‐ 7.5 sample. The force of the electron wind can promote the movement of dislocations, which may be the reason for the decrease of dislocation density in the α lamellar structure shown in Fig. 9 a-c, which leads to the decrease of microhardness. To sum up, the increase of α lamellar width and the decrease of dislocation density are the main factors leading to the softening of the sample under the discharge voltage of 6 kV ~ 7.5 kV. The EPT discharge voltage is relatively high at 8 kV 8.5 kV, and the original β columnar particles become uniformly sized grains and grain scarlet as the voltage increases (Fig. 5e and f). Morphological changes in β grains can be attributed to the thermodynamics of solid-state phase transitions [36]. As mentioned before, the phase transition during the electromagnetic wave pulse can be attributed to the reduction of the phase transition barrier caused by the electric pulse. The energy change ∆ Gre and the total Gibbs free energy ∆ GrEPT due to the current during the phase transition can be expressed as: μ0 is the vacuum susceptibility, G is the positive geometric factor of the coarse-grained material, J is the current density, ∆ V is the volume of the core, and σα and σβ are the conductivities of the α and β phases. ξ (σα, σβ) is the conductivity σα and σβ between the differences caused by one factor, ξ (σα, σβ) = (σβ − σα)/ (σα + σβ). ∆ Gro is the change in free energy in a current-free system. (A) g-C3N4 , (B) Ag/g-C3N4 , (C) Pd/g-C3N4 The lamellar structure of g-C3N4 was studied by TEM. The upper image is a TEM image of a typical pure g-C3N4 nanosheet, in which the plate-like structure can be seen. In addition, when g-C3N4 is doped with a substance, TEM imaging can be used to determine the doping and the distribution of particles on the slab. As shown by Yang et al. Using TEM, Ag nanoparticles were successfully doped on g-C3N4. In another study, Raghu et al. found the correct doping of palladium nanoparticles on g-C3N4 using TEM technique. From the above analysis, we can see that EPT causes the αβ phase transition by reducing the thermodynamic barrier. At the same time, under an electric field of 8 kV, the α lamellae become thinner (fig. 8e) and the β grains become equiaxed (fig. 5e). A fully lamellar α + β microstructure with refined β grains can be obtained by rapid heat treatment of Ti6Al4V alloy due to epitaxial recrystallization formed by substructural transformation in the matrix martensite at the α/β interface. Meanwhile, since the α/α ′ lath is formed by the nucleation at the β grain boundary, the smaller the β grain boundary is, the smaller the size of the lamellar α structure is.
In addition, as the electric field voltage increases from 8 kV to 8.5 kV, Joule heating coarsens the α lamellae and equiaxed β grains, as shown in Figure 5 e and f, and decreases the microhardness from 400 HV to 394 HV. Samples EPT-8 and EPT-8.5 are mainly strengthened by refined α lamellae, which can be described by the Hall-Petch relation: Where H is the microhardness, H0 and kH are the material constants; D is the average thickness of the layered alpha structure. As shown in Fig. 10, the experimental data of all samples are in good agreement with the typical Hall-Petch equation, indicating that the softening and strengthening phenomenon of SLM-Ti6Al4V alloy under EPT is mainly caused by grain coarsening and refinement. Fig. 10 Relationship between microhardness and grain size of SLM-Ti6Al4V alloy. 4. 2. Mechanism of microstructure evolution induced by electromagnetic wave radiation To further describe the evolution of microstructure and microhardness of SLM-Ti6Al4V during EPT, it is shown in Fig. 11. The untreated specimen consisted of relatively straight β columnar grains at the grain boundaries and an α lamellar structure, as shown in Figure 11A. At a lower electric field voltage, the sample softens and the microhardness decreases. As shown in Fig. 11b, the straight β columnar grain boundary is transformed into a serrated grain boundary, and the α lamellar structure is obviously coarsened. Under higher electric field voltage, the sample is hardened and the microhardness is increased. It can be speculated that the sample may reach the β transformation point, and the β columnar grains are transformed into equiaxed grains, forming thinner α lamellae (Fig. 11c). When the electric field voltage is further increased, the microhardness remains basically unchanged, while the β equiaxed grains and α lamellae become thicker (Fig. 11d). Fig. 11 Microstructural evolution mechanism of ept-induced rapid softening and hardening in SLM-Ti6Al4V alloy. 5. Conclusion The effects of EPT on the microhardness and microstructure evolution of SLM-Ti6Al4V alloy were investigated. The main conclusions are as follows: (1) The microhardness of SLM-Ti6Al4V alloy can be quickly changed by applying appropriate electric field voltage. The microhardness of EPT-7. 5 with relatively low voltage decreased by about 7%, and the microhardness of EPT-8 with relatively high voltage increased by about 10%. (2) The combined effect of α lamellar martensite and dislocation is the main reason for the rapid softening and hardening. The solid phase transformation at EPT-8 is due to the reduction of the thermodynamic barrier of αβ phase transformation, which further refines the microstructure of SLM-Ti6Al4V alloy. (3) EPT is a potential processing method for adjusting the microstructure and mechanical property of additive materials.
Origin: Effects of electropulsing on the microstructure and microhardness ofa selective laser melted Ti6Al4V alloy, Journal of Alloysand Compounds,https://doi.org/10.1016/j.jallcom.2021.160044 Reference: S. Liu, Y. C. Shin, Additive manufacturing of Ti6Al4V alloy: a review, Mater. Des.,164 (2019), Article 107552; N.A. Rosli, M.R. Alkahari, M.F. Abdollah, M.F. Abdollah, S. Maidin,F.R. Ramli, S.G. Herawan, Review on effect of heat input for wire arc additive manufacturingprocess,J. Mater. Res. Technol., 11 (2021), pp. 2127-2145 Original works by Chen Changjun of Jiangsu Laser Alliance! Return to Sohu to see more Responsible Editor: (function() { function getBrandHtml() { var brands = [],Titanium 6Al4V wire, html = ''; for(var i = 0; i < brands.length; i++) { var brand = brands[i]; if(brands.length i+1) { html+= ''+brand.name+''; } else { html+= ''+brand.name+'、'; } } return html; }; if(document.getElementById('linkBtn')){ document.getElementById('linkBtn').onclick = function() { $('#brands').removeClass('brand');$ ( '# tipInfo').text ( 'Real name responded'); $ ('#linkBtn').remove();$ ('.real-response .content').css('line-height', '20px');$ ('.real-response .time').css('line-height', '20px'); }; document.getElementById('brands').innerHTML = getBrandHtml(); }; })();。 yunchtitanium.com

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