Z ‐Direction Weld Zone: Microstructural and Mechanical Inhomogeneity in 140‐mm Ultra‐Thick Ti Alloy Double‐Sided Electron Beam Welded Joints

ABSTRACT TC4 titanium alloy is widely used in aerospace, biomedical, chemical, and marine engineering due to its high specific strength, corrosion resistance, and biocompatibility. Existing studies on thick titanium alloy EBW joints mainly focus on welded joints, while through‐thickness fatigue behavior of double‐sided welds remains underexplored. This work systematically studies the distribution of microstructure and mechanical properties along the thickness direction in double‐sided electron beam welded joints. Experimental results indicate that the weld zone of ultra‐thick Ti alloy joints shows significant heterogeneity in microstructure and mechanical properties through thickness. The average ultimate tensile strengths of the welds without and with the overlapping zone are 955 and 1007 MPa, respectively. The high‐cycle fatigue properties of welds with and without overlapping zones are compared, with fatigue strengths measured as 544 and 498 MPa, respectively. All welds contain abundant acicular martensite, and the overlapping region exhibits finer microstructure and higher microhardness. These results provide experimental support for the safe application of ultra‐thick titanium alloy welded structures.