Special Issue on Advanced Metal Forming Technologies in Asia
Mechanism Study and Optimized Technology of Multi-Pass Drawing Process for Ultrafine-Grained Inconel 718 Micro-Tubes
2020 Volume 61 Issue 2 Pages 234-238
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2020 Volume 61 Issue 2 Pages 234-238
In this paper, Inconel 718 tube blanks are processed into ultrafine-grained Inconel 718 micro-tubes by multi-pass drawing. The mechanism of grain size variation and microstructure evolution has been discussed. Moreover, parameters of tubes have been compared and studied for each pass. Furthermore, optimized sequence on drawing process was determined. Optical microscope (OM) and surface roughness tester (SRT) were used for analyzing the evolution of tube size and surface roughness, separately. To further investigate the mechanical properties and microstructure of Inconel 718 tubes, tensile tests and microstructure analysis were conducted. Ultrafine-grained Inconel 718 micro-tubes with outer diameter of 0.9 mm have been produced successfully. The results show that grains were continuously refined during the drawing process and low angle grain boundaries were finally obtained. On the other hand, plasticity reduced while the strength increased obviously with the drawing process. Compared with the results of two manufacturing plans (5 passes and 11 passes), the manufacturing plan with 7 drawing passes has been accepted as the most topgallant option due to its comprehensive advantages. In general, mechanism study and optimized technology of multi-pass drawing process for ultrafine-grained Inconel 718 micro-tubes were discussed in detail, which provide a theoretical basis for high precision micro-tubes manufacturing process with hardly deformable materials.
In last decades, scientists have paid more and more attention to explore the aero-engine of hypersonic vehicles.1) As the key components of high effective pre-cooler of hypersonic aircraft engine, Inconel 718 micro-tubes have unique characteristics, such as small outer diameter, thin wall thickness and outstanding heat exchange ability. Commonly used micro-tubes are derived from low-strength, high-plastic materials such as Cu,2) Mg3) and stainless steel.4) However, Inconel 718 micro-tubes are more difficult to fabricate due to the greater work hardening effect and longer preparation process. Up to now, superalloy micro-tubes are mostly thick-walled tubes.5) Studies on the microstructure and properties of ultrafine-grained superalloy micro-tubes are relatively scarce.
In this paper, a complete study is proposed in order to study deformation mechanism of micro-tubes and optimize multi-pass drawing process. Optical microscope (OM) and surface roughness tester (SRT) were used for analyzing the evolution of tube size and surface roughness, separately. To further investigate the mechanical properties and microstructure of Inconel 718 tubes, tensile tests and microstructure analysis were conducted. Generally, this study fills the gap in the research field of thin-walled superalloy micro-tubes and provides a theoretical basis for high precision micro-tubes manufacturing process with hardly deformable materials.
The Inconel 718 bars was processed into tube blanks with outer diameter of 2.1 mm and wall thickness of 0.05 mm by combining piercing extrusion, rolling, floating plug drawing and annealing processes. Multi-pass hollow sinking was carried out to reduce the diameter of Inconel 718 tubes on electronic universal tester (MTS E44.204). Seven diamond dies were used in drawing process.
The dimensions of tubes are measured by an optical microscope (OM). Surface roughness tester (SRT) and scanning electron microscope (SEM) were used for analyzing the evolution of surface roughness. In each test, ten sets of data for every specimen were measured and then averaged.
Specimens for microstructure observation were cut from the longitudinal section and cross section. The grain size was observed by OM. Electron backscattered diffraction (EBSD) analysis was carried out on the cross section to measure the crystal orientation and texture of tube specimens. EBSD was performed on a JEOL-JSM-7001F field emission scanning electron microscope.
Uniaxial tensile tests were performed on the Inconel 718 tubes at room temperature. Tube specimens with an overall length of 150 mm and a gage length of 50 mm were used. Tensile tests were conducted on MTS at a speed of 2 mm/min. To ensure accuracy of the results, three specimens were measured for each group of tests and the intermediate value was then used in the results.
Figure 1 displays the image of Inconel 718 tubes for each drawing pass. The micro-tube specimens display bright white metallic luster and have smooth surface appearance without any shear cracking or tearing. Figure 2 shows the SEM images of the tubes surface morphology. Figure 3 show Surface roughness value curve during the drawing process. The roughness of the first three passes decreases obviously mainly due to the finishing effect of the drawing die, which removes extremely small scratches. However, the reduction of the last four passes becomes weak because the grain refinement plays a major role in the reduction of tubes roughness.6) Grain size decreases and grain boundary degree of freedom increases under large strain conditions, so that the micro cracks automatically heal. Finally, the surface roughness of Inconel 718 micro-tubes is as low as 0.076 µm.
Inconel 718 tubes (from left to right: Φ2.1, Φ1.6, Φ1.2, Φ0.9).
Surface morphologies of Inconel 718 tubes ((a) Φ2.1, (b) Φ1.6, (c) Φ1.2, (d) Φ0.9).
Surface roughness value curve during the drawing process.
Figure 4 presents the variation of theoretical and actual tubes dimensions. It can be seen that the actual value of tube diameters are less than the theoretical value, and the difference decreases with the drawing process. After the tube blank is pulled out from the working belt, the blank is not rigidized immediately. There are still radial shrinkage and reverse drawing direction flow in a larger area near the outlet of the working belt. Moreover, the drawn tube has to bear a certain drawing load. So a certain degree of necking occurs.7) Necking becomes weaker due to work hardening of tubes with the drawing process. Besides, the actual wall thickness is slightly larger than the theoretical value which can be calculated by the formula since the drawing pass is more than the theoretical value. The Inconel 718 micro-tube has an outer diameter of 0.899 mm and a wall thickness of 66.8 µm. The errors of outer diameter and wall thickness are 0.11% and 3.94% respectively.
Theoretical and actual value curve of tubes dimensions (out diameter and thickness).
During the drawing process, the grains of tubes have changed significantly. Figure 5 shows Optical micrographs of the cross-sectional microstructures of Inconel 718 tubes. For the initial tube blank, the equiaxed coarse grains were observed as shown in Fig. 5(a). By two passes of drawing, equiaxed coarse grains are elongated (Fig. 5(b)). When the outer diameter of the tube is drawn to 1.2 mm, the grains are obviously long strips (Fig. 5(c)). As shown in Fig. 5(d), ultrafine rectangular grains are formed in the finished Inconel 718 micro-tubes and the mean grain size is estimated to be about 4 µm.
Optical micrographs of the cross-sectional microstructures in Inconel 718 tubes: (a) Φ2.1 mm, (b) Φ1.6 mm, (c) Φ1.2 mm, (d) Φ0.9 mm.
Figure 6 shows EBSD inverse pole figures on the cross section of Inconel 718 tubes. The texture intensities of initial tube blanks are distributed homogeneously (Fig. 6(a)). As drawing proceeds, the orientation evidently began to concentrate with the ⟨101⟩ poles (Fig. 6(b) and (c)). It is seen that the Inconel 718 micro-tubes show a clearly texture component ⟨101⟩ with an intensity of 4.99 in the drawing direction (Fig. 6(d)). The results show that all grains rotate in the ⟨101⟩ direction during the drawing process, thus forming fiber texture. The texture has an additional influence on the evolution of residual stress.8)
EBSD inverse pole figures on the cross section of Inconel 718 tubes: (a) Φ2.1 mm, (b) Φ1.6 mm, (c) Φ1.2 mm, (d) Φ0.9 mm.
The boundary misorientation angle distributions of Inconel 718 tubes calculated from EBSD measurements are presented in Fig. 7. As shown in Fig. 7(a), the fraction of boundaries with misorientation angle >15° in the initial tube blank is 86.4%, so the predominant boundaries in the initial tube blank are high angle grain boundaries (HAGB). It can be seen from Fig. 7(b) and (c) that the fraction of low angle grain boundaries (LAGB) increases, and the fraction of HAGB decreases. The predominant boundaries of Inconel 718 micro-tube (Fig. 7(d)) become LAGB. The greater LAGB fraction indicates the higher dislocation density.9)
The boundary misorientation angle distributions of Inconel 718 tubes calculated from the EBSD measurements: (a) Φ2.1 mm, (b) Φ1.6 mm, (c) Φ1.2 mm, (d) Φ0.9 mm.
The results of inverse pole figures and boundary misorientation angle distributions in different Inconel 718 tubes indicate that no dynamic recrystallization occurs during cold drawing of Inconel 718 tubes, which is responsible for grain elongation and refinement. During cold drawing, the main mechanism of metal deformation is dislocation change in crystals. Previous studies10–12) have shown that when the strain is large enough, dislocations are mainly located on the cell wall with high dislocation density and then form cellular structures. The size of subcell decreases with the increase of strain, and the dislocation entanglement areas are rearranged into obvious subgrain boundaries. Macroscopically, the grain elongates gradually along the direction of force with the increase of deformation, and many subgrains are produced by grain fragmentation.
3.3 Mechanical behaviorFigure 8 shows mechanical properties of Inconel 718 tubes. The tensile true stress-strain curves of Inconel 718 tubes are displayed in Fig. 8(a). Changes of yield strength (abbr. as YS), ultimate tensile strength (abbr. as UTS) and elongation of the four sizes tubes are illustrated in Fig. 8(b). The YS of the initial tube blanks is 804.6 MPa, while that of the ultrafine-grained Inconel 718 micro-tubes increases to 1565.1 MPa. Besides, the elongation of tubes decreases from 16.4% to 2.4% with the drawing process. The YS, UTS and elongation all change greatly at the beginning, but basically remained stable afterwards.
Mechanical properties of Inconel 718 tubes: (a) typical tensile true stress-strain curves and (b) curves of strength and elongation changes during the drawing process.
Generally, the texture and the grain size13) would affect the stress-strain behavior of alloy materials. The increase of strength is consistent with the change of texture density. Moreover, the relationship between mechanical strength and grain size in polycrystalline alloys is usually preformed according to Hall–Petch equation:14,15)
| \begin{equation} \sigma_{y} = \sigma_{0} + K \cdot d^{-1/2}, \end{equation} | (1) |
Some studies16–18) have shown that the residual stress and uneven strain caused by multi-pass drawing is much larger than single pass drawing due to the accumulative effect. In addition, excessive deformation degree in a single pass will increase inner defect, causing instability. Therefore, optimizing drawing sequence is crucial to improve the comprehensive quality of Inconel 718 micro-tubes.
To optimize drawing sequence, 11 passes drawing process (Plan 2) and 5 passes drawing process (Plan 3) are added on the basis of 7 passes drawing (Plan 1). The dimensions, surface roughness and mechanical properties of Inconel 718 micro-tubes produced by three schemes are displayed in Table 1. Compared with Inconel 718 micro-tubes produced by Plan 2 and Plan 3, the micro-tubes fabricated by Plan 1 has a low degree of work hardening, which reduces the possibility of defects such as cracks. Figure 9 shows the cross-sectional microstructures of the Inconel 718 micro-tubes from three different drawing schemes. In conclusion, ultrafine-grained Inconel 718 micro-tubes manufactured by plan 1 have the best comprehensive performance.
Optical micrographs of the cross-sectional microstructures in Inconel 718 micro-tubes from (a) Plan 1, (b) Plan 2 and (c) Plan 3.
The ultrafine-grained Inconel 718 micro-tube was successfully produced by multi-pass hollow sinking. The results show that grains were continuously refined during the drawing process and low-angle grain boundaries were finally obtained due to the absence of dynamic recrystallization. The surface roughness is weakened obviously. Besides, the inverse pole figures show that all grains rotate in the ⟨101⟩ direction during the drawing process, thus forming fiber texture. Furthermore, plasticity reduced while the strength increased obviously with the drawing process because of the greater work hardening effect. Compared with the results of two manufacturing plans (5 passes and 11 passes), the manufacturing plan with 7 drawing passes has been accepted as the most topgallant option due to its comprehensive advantages. Consequently, this study provides a theoretical basis for high precision micro-tubes manufacturing process with hardly deformable materials.
The author would like to acknowledge the funding support from the project of “The manufacturing of the new type exchanger for hypersonic flight (23100002015104001)” from BEIHANG University, the project (614270202010317) from Key Laboratory, the project (JCKY2018601C207) from State Administration of STI for ND PRC and the project (51635005) from National Natural Science Foundation of China.
