Objective In recent years, many advances have been made on TC4 alloys, which have received significant attention in the research field of laser additive manufacturing (LAM) owing to their high specific strength, excellent corrosion resistance, and good biocompatibility. However, these alloys have been fabricated without considering the specific characteristics of LAM technology. Consequently, several metallurgical defects, such as pores, inadequate fusion, and cracks, can easily be produced during the LAM process owing to their high metallurgical defect sensitivity. Moreover, metastable phases are often formed locally because of a combination of an alloy’s low structural stability and the layer-by-layer nature of the fabrication process. This, in turn, affects the uniformity of mechanical properties, which cannot be completely achieved by optimizing the LAM processing parameters. The main characteristic of the LAM process is that the alloy undergoes the entire process from melting to solidification within a very short time. To effectively control this process, the LAM alloys must possess not only high mechanical, chemical, and tribological properties but also good formability because good formability is associated with characteristics such as high liquid-state fluidity, weak defect sensitivity, and low microsegregation. Therefore, regulating the structure and properties of TC4 through alloying is of great significance. In this work, new alloys were designed based on a basic TC4 composition alloyed with different Al additions. Then, these new alloys were prepared by applying LAM on a TC4 substrate. In addition, a systematic study of the microstructure and properties of these alloys system was performed with the aim to comprehensively improve the mechanical, chemical, tribological, and forming properties of TC4 alloys through composition optimization.
Methods Alloy design in this work was based on the compositional analysis of a TC4 alloy and the specific characteristics of LAM technology. Then, LAM was employed to fabricate the alloys on a TC4 alloy substrate. The microstructure of as-deposited alloys was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The hardness of these alloys was measured using a Vickers hardness tester. Compressive tests were performed using a universal testing machine. Their tribological properties were evaluated using a wear tester. Further, their electrochemical behavior in an HCl solution was investigated using an electrochemical workstation and their surface roughness was examined using a laser scanning confocal microscope.
Results and Discussions a) The microstructures of all as-deposited alloys exhibited the morphological characteristics of a basket weave, where a multitude of individual α-Ti laths were separated by a thin layer of retained β-Ti (Fig. 2). In these structures, the α-Ti content gradually increased with an increase in Al addition, whereas their size initially decreased with the increase in Al addition (mass fraction) to 1.5% and then increased. This is due to the stabilizing effect of Al on the α-Ti phase and the variation of solidification interval. Moreover, the c/a axial ratio of α-Ti phase gradually increased with the increase in Al addition, indicating that the Al substitution of the Ti atoms exhibits a preference to occur at the {1100} α-crystal plane. b) The hardness (Fig. 5) and strength (Table 2) increased with the increase in Al addition owing to the enhanced solution strengthening and increased α-Ti phase, while the ductility (Table 2) reached its highest value at 1.5% Al addition (mass fraction), resulting from the formation of the first α-Ti phase. c) Under dry sliding friction and wear conditions, the wear mechanism of the as-deposited alloys was abrasive wear (Fig. 9). By increasing the Al addition, both the antifriction property and wear resistance gradually increased because the increased hardness effectively enhanced the resistance to abrasive wear (Fig. 8). d) The corrosion surfaces of all the as-deposited alloys exhibited etched-structure characteristics owing to electrode corrosion between the α- and β-Ti phases (Fig. 11). The corrosion resistance initially increased with the Al addition up to 1.5% and then decreased. This is mainly attributed to the changes in the content and size of the α-Ti phase. e) The surface roughness initially decreased and then increased with the increase in Al addition. Its lowest value was obtained at 1.5% Al addition (Fig. 12). This is mainly due to the fluidity and wettability variation of the melted alloy.
Conclusions The microstructures of as-deposited alloys with different Al additions exhibit a basket-weave morphology, where a multitude of individual α-Ti laths are separated by a thin layer of retained β-Ti. In these structures, the content of the α-Ti phase gradually increases with the increase in Al addition, whereas its size initially decreases and then increases. Its lowest value is obtained at 1.5% Al addition. Thus, the hardness, yield strength, and tribological properties of as-deposited alloys increase, while the ductility, corrosion resistance, and surface roughness reach their optimal values at 1.5% Al addition. These results suggest that as-deposited alloys with a 1.5% Al addition exhibit the best combination of mechanical, tribological, chemical, and forming properties, which are superior to those obtained from as-deposited TC4 alloys.
