Compared with other welding technologies, laser welding with a fast cooling speed, small overall deformation after welding, and easy control of the laser beam can yield high-quality welds of complex structural parts. The welding and assembly accuracies can also be improved. However, welded joints often have defects such as pores, cracks, inclusions, and incomplete penetration owing to improper selection of welding parameters, which significantly affect the service life and safety of the aircraft. The microstructure of the welded joint determines its mechanical properties, and the microstructural evolution in the welded joint is closely related to the welding process parameters. In the past, the microstructural evolution of welded joints of titanium alloys was mostly studied by traditional metallography and scanning electron microscope, and the elaborate analysis and understanding of the microstructural evolution of welded joints were limited. In this study, TA15-welded joints are prepared at different welding speeds using laser-welding technology. An electron backscatter diffractometer (EBSD) is used to analyze the microstructure characteristics in each area of the joint, and the relationship between the microstructure and mechanical properties of the joint is investigated in detail, providing a theoretical basis for the rapid application of titanium alloys in laser welding.

As shown in Figs. 7 and 8, as the welding speed increases from 1.25 m/min to 2.45 m/min, the hardness and tensile strength of the welded joint initially increase and then decrease; the tensile strength initially increases from 1090.9 MPa to 1140.1 MPa and then decreases to 1093.9 MPa. The increases in the hardness and strength of the welded joint are attributed to the continuous refinement of the grain size; however, the decrease in hardness and strength caused by the increase in the welding speed is attributed to the increase in the β-phase content in the joint. With an increase in welding speed, the elongation of the welded joint decreases from 4.0% to 3.7% and then increases again to 4.0%. The tensile fracture positions of all the welded joints are located in the heat-affected zone.

Titanium and its alloys have excellent properties, such as low density, good corrosion resistance, high-temperature performance, high specific strength and stiffness, and good fatigue and creep resistance. Therefore, titanium alloys are used in several structural parts of the aircrafts to lessen their weight and improve their service life. However, an advanced and efficient processing method for these materials has a significant impact on the widespread use of each material.

The effect of welding speed (1.25, 1.55, 1.85, 2.15, and 2.45 m/min) on the mechanical microstructure and properties of a TA15 titanium alloy laser-welded joint is studied. The microstructure of the TA15 titanium alloy laser-welded joint is analyzed using a scanning electron microscope and an EBSD probe. The samples for the EBSD tests are prepared through electrolytic polishing. The polishing voltage and current are set to 30 V and 0.65?0.90 A, respectively, and the calibration step is 0.15 μm. The microhardness values of the joints are measured using an automatic microhardness tester. The applied load is 200 g and the loading time is 10 s. Tensile tests are conducted on the welded joints using a microcomputer-controlled electronic universal testing machine. The tensile rate is 1 mm/min. The sizes of the tensile samples are shown in Fig. 1.

Large β columnar crystals shown in Fig. 2 are distributed in the weld zone of TA15 titanium alloy laser-welded joints at different welding speeds. With an increase in welding speed, the widths of the weld and heat-affected zones of the TA15 titanium alloy laser-welded joint decrease. As the welding speed increases from 1.25 m/min to 2.45 m/min, as shown in Figs. 4 and 5, the grain size of the weld zone decreases from 3.09 μm to 2.66 μm, and the volume fraction of high-angle grain boundary increases from 91.6% to 95.8%. The grain size of the heat-affected zone decreases from 1.16 μm to 0.94 μm, and the volume fraction of high-angle grain boundary initially decreases from 91.1% to 89.7% and then increases to 94.2%. The β-phase volume fraction of the heat-affected zone initially decreases from 1.09% to 0.64% and then increases to 2.34% as the welding speed increases. The β-phase volume fraction in the weld zone decreases from 0.13% to 0.03% as the welding speed increases.

An increase in welding speed results in a decrease in the welding heat input, resulting in a decrease in the grain size of the weld and heat-affected zones. The contents of large angle grain boundaries and β phase of the weld zone continuously increase and decrease, respectively, as the welding speed increases. However, the contents of the high-angle grain boundaries and β phase in the heat affected zone initially decrease and then increase as the welding speed increases. The tensile strength of the TA15-welded joints first increases and then decreases with increasing welding speed; however, the joint elongation first decreases and then increases with increasing welding speed. The tensile fracture locations of the TA15 titanium alloy laser-welded joints at different welding speeds appear in the heat affected zone. The contents of high-angle grain boundaries and β phase in the heat affected zone initially increase and then decrease, causing the joint elongation to initially decrease and then increase.