The present study aims to reveal the mechanism of element vaporization of Ti-6Al-4V alloy during selective laser melting (SLM). The equations of Redlich–Kister and the thermodynamics principles were employed to calculate the vaporization thermodynamics, which contributes to the obtaining the vaporization kinetic based on the Chapman-Enskog theory and the diffusion model. According to the achieved vaporization model, the elements with the most prominent tendency and flux to vaporize were distinguished. Moreover, the effect of the process parameters on the vaporization of Al and Ti is experimentally investigated using inductively coupled plasma optical emission spectrometer (ICP) technology. The analyzed results of the chemical composition of the powders and builds show a great agreement with the kinetic results calculated by the vaporization model. Notably, the element vaporization can be curbed by regulating the laser energy input.

Additive manufacturing (AM) of titanium alloy entails severe microstructural heterogeneity and layer bands due to diverse thermal histories. While the thermal-microstructure relationship in AM has been reported, the details on how complex thermal histories influence the microstructural evolution have not been so addressed, and the formation of layer bands in multi-layer multi-pass builds is still unclear. To undertake such investigation, a thermal model is firstly calibrated using two part-scale blocks fabricated on differently sized substrates, and then used to study the relationship between key microstructural characteristics and the thermal cycling involved. Results show that the different evolutions of the temperature ranges just underneath the β-transus temperature (T_β) controlled by the printing path are responsible for the different band distributions at the centre and corner of the blocks. Also, the α sizes in the normal region are closely linked to the integral area obtained from the thermal curve as temperature fluctuates between T_β and α dissolution temperature, which helps linking AM variables to metallurgy. This further demonstrates that the α coarsening during thermal cycles is primarily driven by multi dissolution and precipation transformations instead of Ostwald ripening. Finally, the quantitative thermal-microstructure-microhardness relationship is established, helpful for the microstructural design.

TA15/Ti₂AlNb multiple-layer samples and a dual-alloy sample were fabricated by laser solid forming (LSF) in this study. The formation mechanism of the dilute region and microstructure evolution of the dual alloy were analyzed. The results confirmed a “step” distribution of the composition among several initial layers in the multiple-layer samples, which can be explained by calculating the ratio of the remelted zone to the deposited Ti₂AlNb zone in each deposited layer. However, the “step” compositional distribution disappears, and the compositional variation tends to be more continuous and smooth in the TA15/Ti₂AlNb dual-alloy sample, which is attributed to alloy elements’ diffusion at the subsequent multiple re-melting and the longer thermal cycle. The macrostructure of the TA15/Ti₂AlNb dual-alloy sample consists of epitaxially grown columnar prior β grains at the TA15 side and equiaxed grains at the Ti₂AlNb side, while the microstructure shows a transition of α+β→α+α₂+β/B2→α₂+β/B2→α₂+B2+O with increasing amounts of Ti₂AlNb, leading to the microhardness also changing significantly.

A near β titanium alloy, Ti5Al2Sn2Zr4Mo4Cr, was fabricated by directed energy deposition (DED) with different powder feed rates to investigate the formation of fully equiaxed β grains. A two-dimensional numerical model was developed to investigate the thermal conditions of the molten pool. Experimental results showed that the formation of an epitaxial cellular structure at the bottom of the molten pool is almost unavoidable. An increase in the powder feed rate produces a moderate thermal condition and promotes the formation of equiaxed grains in a single cladding layer. However, it could not guarantee the formation of a fully equiaxed microstructure in a block sample. From a low to high powder feed rate, fully columnar, mixed equiaxed–columnar, and fully equiaxed microstructures were obtained. Grain morphology was also affected by the remolten process. Increasing the powder feed rate reduced the remolten depth and broke the continuity of the epitaxial cellular structure, leading to different grain morphologies.