TC4 titanium alloy is widely used in aerospace, chemical, biomedical and other important fields because of its corrosion resistance, high specific strength, good toughness and excellent biocompatibility. However, the processing of TC4 titanium alloy under the traditional process has the shortcomings of low material utilization, high manufacturing cost and difficult deformation, which seriously limits the popularization and application of TC4 titanium alloy, and the emergence of 3D printing technology will improve this situation.
3D printing, whose scientific name is additive manufacturing, originated in the 1990s rapid prototyping technology. Distinct from the subtractive manufacturing, it uses the discrete/stacking principle, the use of computer technology to process parts of the 3D solid model cut into a series of thin slices with a certain thickness, 3D printing equipment for processing data analysis and processing of continuous processing of each thin slice and stacking, followed by the formation of dense solid parts. 3D printing technology is suitable for processing any shape of the parts and has a high rate of utilization of the material, low cost, 3D printing technology is suitable for processing parts of any shape, and has the advantages of high material utilization, low cost, high flexibility and high integration, especially suitable for the molding of TC4 titanium alloy. 3D printing technology mainly includes selective laser melting and molding technology (selective lasermelting (SLM), laser engineered net shaping technology (laser engineered net shaping (LENS)) and electron beam melting and molding technology (electron beam melting (EBM)), which is the most advanced technology in the industry. electron beam melting (EBM). Among them, EBM molding has many advantages compared to the other two molding technologies: (1) EBM molding uses electron beam as the energy source, no reflection in the manufacturing process, high energy utilization; (2) EBM molding is carried out in a vacuum environment, which can effectively avoid the contamination of other elements in the air; (3) EBM molding is more efficient than the other molding technologies due to the high energy input and high scanning speed; (4) EBM molding is more efficient than the other molding technologies due to the high energy input and high scanning speed; (5) EBM molding is more efficient than the other molding technologies. (4) EBM molding parts have less residual stress and do not require subsequent heat treatment, saving energy.
This paper starts from the principle of EBM technology, summarizes the relevant research results at home and abroad, describes the microstructure, defects and mechanical properties of TC4 titanium alloy parts molded by EBM under different process parameters, and finally looks forward to its application prospects.
1.Principle and main parameters of EBM
First of all, the Magicsl9.0 software slices and layers the 3D model of the part according to a certain thickness, so as to obtain the overall two-dimensional information of the part. Then, the EBM system uniformly spreads the alloy powder onto the substrate according to a certain thickness, and scans and melts the alloy powder on the substrate with the electron beam formed by the current passing through the tungsten filament as the heat source under the action of the focusing coil and electromagnetic deflection coil. Each scanning melting layer of the electron beam, the table will drop a layer height, and then re-lay the powder, the electron beam to repeat the scanning melting process, and each processed layer each other condensed into a whole. The entire manufacturing process is carried out in a vacuum environment, thus effectively avoiding the possibility of oxidization of the titanium alloy during processing. After fabrication, the EBM system removes the part from the build chamber and places it in a powder recovery system (PRS), which uses high-pressure air to remove the powder adhering to the part surface, resulting in a molded part with a smooth surface.
The main parameters of EBM technology are electron beam current, accelerating voltage, scanning speed, layer thickness, scanning line spacing, and focal point compensation, etc. Different energy densities can be obtained by adjusting these parameters, e.g., by increasing the electron beam current or decreasing the scanning speed, higher energy densities can be obtained. The size of the energy density greatly affects the microstructure, defects and mechanical properties of the molded parts, and the appropriate energy density will make the alloy have better mechanical properties. Due to the unique molding process of EBM technology, the microstructure and mechanical properties of EBM molded TC4 titanium alloy molded parts are different from those of conventionally manufactured (e.g. forged) TC4 titanium alloy molded parts.
2.EBM forming TC4 titanium alloy microstructure and defects
2.1 EBM forming TC4 titanium alloy microstructure and influencing factors
The temperature change of EBM forming TC4 titanium alloy in the forming process affects its microstructure. First of all, the powder melts under the action of the electron beam, and the liquid alloy temperature reaches about 1700 ℃, which is much higher than the β-phase transition temperature of TC4 titanium alloy (995 ℃), at this time, the liquid alloy consists of the original β grains; and then, with the electron beam away, the liquid alloy is rapidly cooled to the build temperature (generally 650-700 ℃) to remain stable, and becomes a solid state, at this time the alloy undergoes an α → α + β, and the precipitation of acicular α phase. Precipitation of acicular α phase and columnar β phase. A1-Bermani et al. believe that at this stage of the cooling rate greater than 410 ℃ / s, will precipitate sub-stable α 'martensite, a long time in the high temperature environment and the decomposition of α + β layered structure, and most of the fine acicular α laths and a small portion of the β phase. After the molding of TC4 titanium alloy by building temperature slowly cooled to room temperature, the alloy microstructure did not change significantly, still composed of α + β phase. EBM molding TC4 titanium alloy and forging molding of TC4 titanium alloy microstructure as shown in Figure 2.
Scholars at home and abroad for EBM molding TC4 titanium alloy microstructure has done a lot of research, found that the molding process parameters, the location of molding parts, molding part size and other factors will affect the cooling rate of the alloy in the molding process, which in turn affects the size of its grain size. Hrabe et al. found that, in the conditions of ensuring that the energy input can be made to make the TC4 titanium alloy powder completely melted to form a dense part, appropriately increase the e-beam The scanning speed causes the melt pool size to decrease and the cooling rate to increase, resulting in the precipitation of finer α-slats and β-phases.Murr et al. and Wang et al. found that the microstructures of EBM-formed TC4 titanium alloys were different at different locations. As shown in Figure 3, the position of lower deposition height, due to closer to the molding substrate and thus has a higher cooling rate, is an unstable growth zone, easy to precipitate fine needle-like α phase; the higher the deposition height of the position of the α lath is thicker, the larger the β grains; deposition of a certain height, the α lath is in a stable growth zone, as well as the β grain size tends to be stabilized. Wang et al. also investigated the effect of the size of molded parts on the microstructure of EBM-formed TC4 titanium alloy, and found that in the layer-by-layer melting and solidification process, the cooling rate of the smaller samples was larger, and therefore precipitated finer α-phase.Galarraga et al. further investigated and found that the change of microstructure of EBM-formed TC4 titanium alloy was related to its residence time in the building chamber, and that the residence time was too long, which would result in the deposition of lower height and higher microstructure in the bottom of the depositing height. height is lower, the microstructure is coarser results.
2.2 Defects in EBM molding of TC4 titanium alloy
Due to improper selection of process parameters or process interference, EBM molding of TC4 titanium alloy parts may produce a variety of defects. Zhai et al. found that there are two typical defects in the microstructure of EBM molding of TC4 titanium alloy: one is the porosity caused by argon gas entrapped in the defective powder; the other is the porosity due to poor melting of the alloy powder.
Gong et al. classified the TC4 titanium alloy defects into two main categories according to the level of input electron beam energy density. When the energy density is too low, it is not enough to completely connect the melt pool with the melt pool and the layers with the layers, forming irregular melting defects accompanied by a certain amount of porosity. When the energy density is too high, resulting in a rapid rise in local heat, the powder melting under the action of surface tension ballooning (the powder's low thermal conductivity), and thus the formation of the pores. kahnert et al. found that the energy input is too high, not only will the surface quality of the molded parts will be worse, and in serious cases, it will lead to the targeting machine of the powder coating system to stop working, making it necessary to abort the manufacturing process itself. In addition, when the electron beam current exceeds a certain threshold, the alloy powder will be blown away, leaving irregular pores in the layer, which in severe cases will make the entire powder bed collapse, as shown in Figure 5; preheating the powder bed to improve its adhesion and overcome the thrust of the electron beam on the alloy powder can avoid the occurrence of powder collapse. Defects will have a negative impact on the mechanical properties of C4 titanium alloy, and the EBM process parameters must be optimized, such as controlling the scanning speed, adjusting the scanning line spacing, and optimizing the electron beam current to reduce the generation of defects.
3.Mechanical properties of EBM molded TC4 titanium alloy
3.1 Tensile properties of EBM molded TC4 titanium alloy
Bruno et al. studied the tensile properties of EBM-formed and forging-formed TC4 titanium alloys, due to the fact that EBM-formed TC4 titanium alloys are prone to pore defects during the molding process and their microstructure distribution is not homogeneous, resulting in the highest tensile and yield strengths of 996 MPa and 919 MPa, respectively, which are slightly lower than those of the forging-formed TC4 titanium alloys (the tensile and yield strengths of 1034 MPa and 991 MPa:); Wang et al. yield strength of 1034MPa, 991MPa respectively:); Wang et al. also studied the tensile properties of EBM molding TC4 titanium alloy, and found that its tensile strength was 1002MPa, yield strength was 932MPa, and elongation was 14.4%, all of which were higher than those of the TC4 titanium alloy forgings after annealing and aging treatment.
Significant anisotropy exists in the mechanical properties of EBM molded TC4 titanium alloy.Bruno et al. and Hrabe et al. found that the tensile strength of EBM molded samples in the horizontal direction is stronger than that in the vertical direction, and the elongation of molded samples in the horizontal direction is smaller than that in the vertical direction. This is due to the B grain inhomogeneity within the alloy: the molded sample grows mainly in the vertical direction; the formation of smaller primary β grains in the horizontal direction reduces the stress buildup at the grain boundaries, which delays crack initiation and results in a slightly larger elongation.
Hrabe et al. found that an increase in the electron beam scanning speed (which is negatively correlated with the energy density) resulted in a slight decrease in the thickness of the α slats (1.16 μm → 0.95un), which in turn increased the tensile strength, yield strength, and microhardness by 2%, 3%, and 2%, respectively.
Formanoir et al. EBM molding TC4 titanium alloy were held at 950 ℃ for 60 min and 1040 ℃ for 30 min, both using water cooling and air cooling two cooling methods, the alloy's tensile strength and yield strength slightly reduced, the elongation did not get a significant increase, indicating that only the control of the key parameters of the EBM molding is an effective method of improving the performance of the alloy.
3.2 Fatigue properties of EBM molded TC4 titanium alloy
Chan et al. tested the fatigue life (number of cycles) of EBM-formed TC4 titanium alloy and rolled TC4 titanium alloy under alternating bending stress of 600 MPa (±10%). The results show that the fatigue life of EBM formed TC4 titanium alloy is only 17% of the fatigue life of rolled alloy; the fracture of EBM formed TC4 titanium alloy is distributed with different shapes of pores due to poorly melted regions and its surface roughness is much higher than that of rolled TC4 titanium alloy, which is an important reason for its low fatigue life.
Tammas-Williams et al. found that hot isostatic pressure treatment can effectively eliminate most of the pores in EBM-formed TC4 titanium alloy, but if some tunneling holes exist in the samples and are connected to the surface, the high-pressure argon gas under HIP treatment will penetrate into the tunneling holes, causing these tunneling defects to expand slightly, resulting in the failure of the HIP treatment; the addition of coatings to the samples prior to the HIP can remove the tunneling defects. Shui et al. found that after the HIP treatment of EBM-formed TC4 titanium alloy, although the slat becomes thicker, the dislocation density decreases, and the tensile strength as well as the yield strength decreases from 870MPa and 788MPa to 819MPa and 711MPa, respectively, the HIP treatment makes the organization become more homogeneous, and the alloy's relative density rises from 99.3% to 99.8%, which decreases the crack sprouting source, which in turn increases the fatigue strength from 460 boa to 580 MPa.
4.Conclusion
In summary, the results of domestic and foreign research on EBM forming TC4 titanium alloy show that: EBM forming TC4 titanium alloy macroscopic organization for the growth of columnar crystals along the building direction, microstructure for α + β layered structure, the faster the cooling rate, the easier it is to get a finer microstructure. Optimization of process parameters makes EBM have the best energy density, which can effectively avoid the generation of a large number of defects. The subsequent HOP treatment is also able to remove pores and homogenize the microstructure, which significantly improves the fatigue properties although it leads to grain coarsening, lower dislocation density, and a slight decrease in alloy strength. Optimization of the parameters of the EBM molding process, supplemented by appropriate follow-up treatment, can be obtained with conventional casting and forging of TC4 titanium alloys with comparable performance.EBM saves raw materials, fast, efficient, easy to form complex shapes of workpiece molding, and will gradually replace the current subtractive manufacturing methods used in aerospace, chemical and medical fields.
