| Author: | Jiang, Qinghong |
| Title: | Material removal mechanism and surface integrity of wrought and SLM-ed Ti6Al4V alloys in ultra-high-speed machining |
| Advisors: | Fu, Mingwang (ME) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Titanium alloys Machining High-speed machining Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xiv, 191 pages : color illustrations |
| Language: | English |
| Abstract: | Titanium alloy (Ti-alloy) is a preeminent structural material with remarkable mechanical properties such as high specific strength, excellent corrosion resistance, and ideal biocompatibility. It has been extensively applied in various industries including aviation and aerospace, navigation, automotive, and biomedical industries. The conventional manufacturing and processing of high-performance Ti-alloy parts, however, encounter many technological bottlenecks and limitations, such as low productivity, difficulty in fabricating complex structures, high material wastage, and high production cost. Recently, selective laser melting (SLM) has emerged as an advanced additive manufacturing technique for rapid, integrated, and lightweight manufacturing of Ti-alloy parts. Nevertheless, the SLM-manufactured (SLM-ed) Ti-alloy parts suffer from the trade-off dilemma between strength and ductility due to the internal defects and the non-equilibrium microstructure induced by high cooling rate and large temperature gradient during the solidification process. Moreover, the inherent inferior surface quality of SLM-ed parts limits their applications in critical areas. Therefore, surface machining is imperative for achieving high surface integrity of SLM-ed Ti-alloy parts. However, Ti-alloy is known as a typical difficult-to-machine material, characterized by high specific strength, low thermal conductivity, high chemical reactivity, and low elastic modulus. Consequently, high machining force, elevated machining temperature, rapid tool wear, and compromised surface integrity tend to arise in the conventional machining of Ti-alloy. Furthermore, SLM-ed Ti-alloy exhibits completely different microstructures and properties compared to its wrought counterpart. These alterations not only affect machinability but also introduce additional challenges in the machining process. Ultra-high-speed machining (UHSM) is a potential technique to address the machining challenges associated with Ti-alloy, given its capacity to enhance both surface integrity and machining efficiency. However, the dynamic responses and deformation behaviors of materials at ultra-high strain rates differ significantly from those at low strain rates. This leads to distinct material removal mechanisms in UHSM, which remains inadequately explored. Furthermore, there is limited research on the variation of microstructure evolution and surface integrity with machining speed in the machining of Ti-alloy. Additionally, the influence of different microstructures in SLM-ed and wrought Ti-alloys on their machinability also requires thorough study and analysis. In tandem with these, this study starts with the SLM manufacturing of Ti6Al4V to fabricate strong and ductile Ti6Al4V by tailoring process parameters and microstructures. Subsequently, single-point-scratching (SPS) experiments and finite element simulations are conducted together to reveal the material removal and deformation mechanisms of both wrought and SLM-ed Ti6Al4V alloys, covering the spectrum from conventional speed machining (CSM) to ultra-high-speed machining (UHSM). Furthermore, the surface integrity of Ti6Al4V alloys in ultra-high-speed grinding (UHSG) is systematically investigated. The first part of this thesis reports the investigations on the densification behaviors, defect formation mechanisms, microstructure evolution, and mechanical properties of SLM-ed Ti6Al4V at different energy densities. The densification map and process map of SLM-ed Ti6Al4V were developed to support the fabrication of near fully dense parts. The microstructure analysis revealed a progressive transformation with the decrease in energy density: from coarse α+β lamellar, ultrafine α+β lamellar, and to fully α' microstructure. The remarkable tensile strength combining high strength and ductility (Tensile strength: 1,390 MPa; elongation: 9.66%) was achieved at energy density of 76 J/mm3 due to the high densification level and ultrafine microstructures. This study further revealed the fracture mechanisms and established the process-structure-property relationship of SLM-ed Ti6Al4V. These findings provide guidance for realizing the fabrication of strong and ductile Ti6Al4V by SLM. The second part focuses on the material removal mechanisms of wrought coarse-grained Ti6Al4V under low-speed to ultra-high-speed conditions based on a developed SPS system. The material removal mechanisms were investigated in terms of surface creation, subsurface deformation, and chip formation. Multiscale characterization combining TKD, FIB, and STEM techniques was employed to investigate the microstructure evolution at the speed ranging from 20 to 220 m/s. The results indicated that material pile-up was suppressed at higher machining speeds due to the inhibition of plastic deformation. A deep machining-deformed zone (MDZ), consisting of a dynamic recrystallization zone (DRXZ) and a plastic deformation zone (PDZ), was induced at 20 m/s. The depth of PDZ was considerably reduced at higher machining speed and was absent at 220 m/s. Moreover, under ultra-high strain rate deformation, dislocation slip was inhibited, resulting in a transition of deformation mechanism from dislocation-mediated deformation (DMD) to twinning-mediated deformation (TMD). Consequently, a deformation-induced twin zone (DITZ) was generated in the topmost layer, in which a distinct microstructure characterized by ultrafine grain embedding nanotwins (UGENTs) was induced. Additionally, the segmented chips transitioned to the fragmented chips with the increasing machining speed. This study enhances the understanding of material removal and deformation mechanisms at ultra-high strain rates. The third part delves into the material removal mechanisms of SLM-ed Ti6Al4V based on part 2. SLM-ed Ti6Al4V exhibited a decline pile-up ratio as the machining speed increased. However, the higher brittleness of SLM-ed Ti6Al4V resulted in less material accumulation on the edges of scratches. Additionally, the MDZ exhibited “skin effect” with an increase in machining speed, but it was shallower in SLM-ed Ti6Al4V compared with that in wrought Ti6Al4V under same speed conditions. As the strain rate increased, the deformation mechanism of SLM-ed Ti6Al4V also transitioned from DMD to TMD to coordinate the deformation. The multiple-fold twins were induced in the UGENTs, and the formation mechanism was revealed by multiscale characterizations and analyses. Regarding the chip formation, wrought Ti6Al4V exhibited a higher sensitivity to strain rate compared to SLM-ed Ti6Al4V. This can be attributed to different chip formation mechanisms. The segmented chips were formed as the phase transformation activated the adiabatic shear bands (ASBs) in wrought Ti6Al4V, while the relative slip along the lath boundaries triggered the segmented chips in SLM-ed Ti6Al4V. In the last part, a UHSG system was developed to achieve high-efficiency and high-quality machining. A series of grinding experiments of both wrought and SLM-ed Ti6Al4V alloys with the linear grinding speed ranging from 60 to 250 m/s were conducted. The surface integrity of the machined samples was systematically analyzed by considering both surface and subsurface characteristics. The results verified the material removal mechanisms elucidated in previous chapters and demonstrated the potential of UHSG in improving surface integrity. Meanwhile, the grinding forces of both materials at different grinding speeds were measured to evaluate their machinability. The effects of strain rate and microstructures on the deformation mechanism and machinability were elucidated based on the systematic investigations. This thesis presents an original study on the material removal/deformation mechanisms and surface integrity of wrought and SLM-ed Ti6Al4V alloys in UHSM by applying multiscale characterizations. These findings not only provide a scientific and theoretical basis but also offer instructive insights into the manufacturing and processing of high-performance Ti-alloy parts. |
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| Access: | open access |
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