Research On Microstructure And Defects Of Two-phase Titanium Alloy With 3D Printing

In this paper, testing methods such as stereo microscope, optical microscope, scanning electron microscope, microhardness tester, etc. have been applied to do researches on the features of three-dimensional tissue, defects and fatigue fracture behavior of 3D printing TC4, TC11 and TC18, two-phase titanium alloy. And use three non-destructive testing methods such as ultrasonic C-scan, fluorescence and industrial CT to detect different types of defects in order to provide a theoretical basis for the manufacturing, service and safety evaluation of 3D printing two-phase titanium alloy.Metallographic analysis shows that three-dimensional tissue characteristics of 3D printing two-phase titanium alloy in different forming directions are various. Microstructure features from top to bottom are as follows: a thin layer of equiaxed crystals, columnar crystals, equiaxed crystals. Microstructure before and after the heat treatment will shift from Widmanstaten consist of needle phase to basketweave microstructure consist of rod-likephase; from the top to the bottom, phase turns to be more coarse. In addition, the microstructure of layer bands is more coarse than that out of layer bands, the microstructure of overlapping region is more coarse than that of non-overlapping zone.There are five types of defects in two-phase titanium alloy: pores, high density inclusions, lack of fusion, cold shut defects and unmelted powder particles by fracture analysis and microstructure analysis. In addition to the pores, the morphologies of defects are significantly different from cast and wrought alloys. Pores and high density inclusions have no significant effect on the surrounding tissue and hardness as point defects, unmelted powder particles and lack of fusion occurred mainly between the layers or tracks as area defects, which hardly affect the tissue and hardness surrounding. However, there is also a kind of special lack of fusion which makes the microstructure and hardness changed. Cold shut mainly in the confluence of the flow between cladding layers produces a certain influence on the tissue of surrounding as volume defects.Compared with traditional wrought titanium alloys, there are both similarities and differences in fracture behavior of the fatigue fractures of 3D printing two-phase titanium alloys with different directions loading. Characteristics of fracture dimple in short break area and fatigue striation of the source and fatigue region are similar. However, the rapid expansion zone of fatigue fracture of 3D printing two-phase titanium alloy is significantly different from that of wrought alloy. The fatigue life of 3D printing two-phase titanium alloy is different with different loading direction. Fatigue performance with the loading vertical to the deposition direction is more excellent. Besides, pores impact the fatigue life of 3D printing two-phase titanium alloy.The internal and external defects of 3D printing two-phase titanium alloy can be detected by conventional non-destructive testing methods. Fluorescent penetrant testing can effectively detect surface breaking defects while ultrasonic C-scan can effectively detected internal pores, lack of fusion and inclusions. Industrial CT can also effectively detect internal pores, inclusions and lack of fusion defects. During the ultrasonic testing, there may be a certain error of defect size measurement when using forged alloy as calibration test blocks, because that the defects morphologies in forged alloy are different from that in 3D printing titanium alloy.