Ti6Al4V在不同成形工艺下的数值分析及组织性能

At present, there are many processing methods for Ti6Al4V alloy, and there are great differences between the respective processes. However, the change of transient temperature field, the solidification process of molten metal and the difference of cooling rate would have an impact on the micro-mechanism and mechanical properties. At the same time, traditional research methods could not deeply study the grain structure and the dynamic change process of forming mechanism. Therefore, in order to explore the differences of temperature field, microstructure and mechanical properties of Ti6Al4V alloy under selective laser melting (SLM) and investment casting, a combination of numerical analysis and experiment was used for research. Based on the different process forming principles of SLM and investment casting, the finite element numerical analysis software and the DLUX subroutine written in Fortran language were used to carry out the finite element simulation of the SLM forming process. The CAFÉ module was used to simulate the grain orientation and nucleation growth during the investment casting process. The dynamic process of cooling rate and temperature field of the samples under different forming methods was studied. In the experiment, SLM and investment casting were used to prepare different Ti6Al4V titanium alloy samples, and the internal relationship between the microstructure, grain structure and mechanical properties of the cooling process was explored. Through numerical analysis and experimental verification, it was found that: (1) The metallographic structure of the sample formed by SLM was evenly distributed with closely packed hexagonal acicular martensite α' phase. The shapes were different, the forming orientations are various, and the layers were stacked alternately to form a basket structure. At the same time, there were black burning areas and small holes (about 2 μm). The black burning area was mainly caused by the high energy of laser beam. The formation of fine holes was due to the rapid reaction of laser beam energy melting powder and short operation time during SLM forming. Under the high cooling rate, the impurity gas could not be eliminated in time when it merged into the liquid phase, and it was left in the metal to form holes under the rapid solidification. The metallographic structure of the investment casting sample was coarse flaky original β crystal grains, and long α phases were distributed in β crystal grains, and each α slice was separated by β phase. Compared with SLM process, β phase had little thermal deformation during casting process, while the casting cooling was slow and the cooling time was too long to form Widmanstatten structure, which had certain strength but low plasticity. (2) In order to characterize the different characteristics of elements under different processes, X-ray energy spectrum analysis was performed on SLM and investment casting samples. Under the two processes, the distribution of elements were mainly C, Al, Ti and V, and their contents were basically the same, among which the content of Ti was up to more than 80%. Al, Ti and V were bright spots and evenly distributed in the graph, without obvious element enrichment or element poverty, but the distribution of C was different. Compared with casting, the distribution of C element in SLM process was relatively uniform and dense, but the content of C element in SLM was smaller than that in investment casting. The main reason was the contamination of the sample surface caused by residual C impurities in graphite shell and smelting furnace. (3) The CAFÉ module based on PROCAST found that the grain size of investment casting gradually increased and the number of grains gradually decreased during solidification, resulting in preferential growth of grains. In the numerical simulation process, the temperature rose rapidly when the laser beam irradiated the powder, and the peak temperature reached about 2580 ℃, which immediately presented a dynamic non-equilibrium process. The average cooling rate of SLM was 8.88×10⁵ ℃∙s^-1, which was 1.02×10⁶ times higher than that of investment casting (0.87 ℃·s^-1). (4) In addition, the fitting curve of average cooling rate and grain size showed that the grain size decreased with the increase of cooling rate, which explained the mechanical reason why the grain size of SLM was smaller than that of investment casting. At high cooling rate, the fine grains produced by SLM samples produced fine grain strengthening characteristics, and the tensile strength and yield strength reached 1132.14 MPa and 1057.11 MPa, respectively. Compared with investment casting, it was increased by 20.76% and 25.30%. SLM samples had higher plasticity, and their comprehensive properties were superior to those of investment casting, which met the GB/T 2965-2007 standard and met the requirements of industrial standards. Through numerical analysis and experimental verification, the differences in temperature field, microstructure and grain growth between SLM and investment casting technology were found, which further affected the different changes of mechanical properties. Therefore, the dynamic change of temperature field was explored by simulation. Combined with experiments, we could explore the differences between processes and build the relationship between different processes, microstructure properties, and industrial needs.