Xiangjun Tian

Microstructure of TA2/TA15 graded structural material by laser additive manufacturing process

Abstract TA2/TA15 graded structural material (GSM) was fabricated by the laser additive manufacturing (LAM) process. The chemical composition, microstructure and micro-hardness of the as-deposited GSM were investigated. The results show that the TA2 part of exhibiting near-equiaxed grains was Widmanstatten α-laths microstructure. The TA15 part containing large columnar grains was fine basket-weave microstructure. The graded zone was divided into four deposited layers with 3000 μm in thickness. As the distance from the TA2 part increases, the alloy element contents and the β phase volume fraction increase, the α phase volume fraction decreases and the microstructure shows the evolution from Widmanstatten α-laths to basket-weave α-laths gradually. The micro-hardness increases from the TA2 part to the TA15 part due to the solid solution strengthening and grain boundary strengthening.

Microstructural evolution and mechanical properties of laser melting deposited Ti–6.5Al–3.5Mo–1.5Zr–0.3Si titanium alloy

A rectangular plate of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si titanium alloy was fabricated by laser melting deposition (LMD) technology. Macrostructure and microstructure were characterized by optical microscope (OM) and scanning electron microscope (SEM). Room temperature tensile properties were evaluated. Results indicate that the macro-morphology is dominated by large columnar grains traversing multiple deposited layers. Two kinds of bands, named the wide bands and the narrow bands, are observed. The wide band consists of crab-like α lath and Widmanstatten α colony. The narrow band consists of α lath and transformed β. The formation mechanism of the two bands was explored. The influence of heat effect caused by subsequent deposition layers on microstructural evolution during deposition process was discussed. The room temperature tensile test demonstrates that the strength of laser deposited Ti–6.5Al–3.5Mo–1.5Zr–0.3Si is comparable to that of wrought bars.

Low cycle fatigue behavior of laser melting deposited TC18 titanium alloy

Abstract Low cycle fatigue (LCF) behavior of laser melting deposited (LMD) TC18 titanium alloy was studied at room temperature. Microstructure consisting of fine lamella-like primary α phase and transformed β matrix was obtained by double annealed treatment, and inhomogeneous grain boundary α phase was detected. Fatigue fracture surfaces and longitudinal sections of LCF specimens were examined by optical microscopy and scanning electron microscopy. Results indicate that more than one crack initiation site can be detected on the LCF fracture surface. The fracture morphology of the secondary crack initiation site is different from that of the primary crack initiation site. When the crack grows along the grain boundary α phase, continuous grain boundary α phase leads to a straight propagating manner while discontinuous grain boundary α phase gives rise to flexural propagating mode.