TC4 titanium alloy is prone to β-phase and ordering transformation near the phase transition temperature due to its dual-phase composition characteristics. When the laser cladding is used to manufacture TC4 titanium alloy moldings, the high-temperature gradient and rapid solidification during the cladding process occur. The accumulation of thermal stress, an easy occurrence, will cause cracks to develop inside the molded part. Additionally, the structure of TC4 titanium alloy is unstable under high-temperature environments, and the rapid cooling and heating during the laser cladding process will make the base metal grains at the bottom of the molten pool epitaxial growth leading to the arrangement of the original β columnar grains along the deposition direction to form a typical solidified texture. The resulting columnar grain structure and nonequilibrium phase will further reduce the mechanical strength of the material. Furthermore, in the preparation of multilayer and multichannel samples, the change in the interlayer temperature will cause a change in the cooling rate, which will affect the transformation of the metastable β phase, and the different transformation products will affect the properties of the material. Therefore, given the uneven mechanical properties of the TC4 titanium alloy laser cladding structure during the reheating thermal cycle, the thermal simulator is used to reproduce the effect of the reheating thermal cycle during the multipass laser cladding process. The laser cladding thermal cycle and the effects of its characteristics on the microstructure and properties of TC4 titanium alloy are also investigated.

In this paper, the thermal simulation testing machine is used to conduct thermal cycle tests on TC4 titanium alloys to explore the effects of thermal cycle peak temperature and cooling rate on its microstructure transformation and mechanical properties. The coaxial powder feeding laser additive manufacturing (LAM) technology is used to prepare the TC4 titanium alloy cladding material on the surface of the titanium alloy substrate. Subsequently, the thermal simulator is used to prepare the samples based on the TC4 titanium alloy cladding material for thermal cycle simulation experiments at varying peak temperatures and cooling rates. The metallographic structure analysis, scanning electron microscope observation, X-ray diffraction composition analysis, and microhardness and microshear tests are conducted to determine the effect of thermal cycle on the microstructure and properties of the formed parts.

After thermal cycle simulations at different temperatures, as shown in Fig. 6, the β phase inside the sample decomposes, forming a microscopic region with alternating rich and poor solute atoms, and the microstructure exhibits bright and dark partitions. As the peak temperature increases, the proportion of dark parts gradually decreases, indicating that the β-segregation phenomenon has improved, especially when the peak temperature reaches above the phase transition temperature of TC4 titanium alloy (995 ℃), the matrix transformation effect dominates. The orientation of α-clusters is diverse, the number of primary phases α_p generated by β-phase transformation increases, the size becomes thicker, the aspect ratio decreases, and a small amount of flaky α is separated by the remaining β phases, forming the lamellar structure with alternating α/β distribution. As the cooling rate increases, the type of material phase transition changes. When the cooling rate is 21 ℃/s, the unstable β phase undergoes a diffusive phase transition and an orderly transition, as shown in Figs. 7(c), (g), and (k); in addition to the primary phase α_p, a small number of small particles dispersed in the interlamellar space also appear. When the cooling rate is 30 ℃/s, the α′ phase is formed. The results in Fig. 9 show that as the peak temperature increases, the original α-phase and α_p are more fully transformed into β-phase due to the effect of diffusion, so the content of α-Ti in the material decreases. The results in Fig. 10 show that the distribution of β phase is not uniform and the hardness change is not obvious. When the peak temperature of the thermal cycle is lower than 960 ℃, with the increase in cooling rate, the hardness of the postweld material is generally higher than that of the cladding layer, showing a trapezoidal change, rising sharply at first, then smoothly, and then slightly decreasing. There is no phase change in the sample, and the grain growth time is shortened with the acceleration of the cooling rate, thereby increasing the hardness. When the peak temperature of thermal cycle is higher than 960 ℃, the dissolution of the α phase and the transformation of the β phase occur during the heating and cooling of the material. With the increase in cooling rate, the hardness of the material first decreases and then stabilizes. According to the microshear test results (Fig. 11) and fracture morphology analysis (Figs. 12 and 13), the fracture mechanism of the material is micropore aggregation and fracture at the peak temperature below 960 ℃ has good toughness. The fracture mechanism changes from ductile fracture to brittle fracture with increased peak temperatures.

The results show that microstructure coarsening occurs inside the material when the peak temperature is lower than 960 ℃. The aspect ratio of primary phase α_p grows with the increase of temperature. The allotropic transformation and ordering transformation of the metastable β phase at the grain boundary occur as the peak temperature reaches 1100 ℃, resulting in the formation of Ti₃Al ordered phase, the reduction of the aspect ratio of the primary phase α_p, the refinement of the structure, and the significant increase of the hardness and the crack sensitivity. When the cooling rate increases, the phase transformation type changes from metastable β-phase allotropic transformation to martensitic transformation, the primary phase α_p morphology changes from lamellar to needle-like, and the material hardness gradually tends to be stable, but when the cooling rate is higher than 12 ℃/s, the hardness of the material increases slightly due to the formation of Ti₃Al.