Progress Of EBM Forming TC4 Titanium Alloy

First, the 3D model of the part is sliced and layered according to a certain thickness through Magicsl9.0 software to obtain the overall 2D information of the part. Then, the EBM system evenly spreads the alloy powder to a certain thickness on the substrate, and uses the electron beam formed by current passing through the tungsten wire as the heat source. Under the action of the focusing coil and the electromagnetic deflection coil, the alloy powder on the substrate is processed. Scan melt. Each time the electron beam scans and melts a layer, the workbench drops by one layer height, and then the powder is spread again. The electron beam scans and melts the process repeatedly, and each processed layer condenses into a whole. The entire manufacturing process is carried out in a vacuum environment, effectively avoiding the possibility of titanium alloy being oxidized during processing. After the manufacturing is completed, the EBM system takes the parts out of the build chamber and places them in the powder recovery system. High-pressure air is used in the PRS to remove the powder adhering to the surface of the parts, and finally obtains molded parts with a smooth surface.

The main parameters of EBM technology include electron beam current, acceleration voltage, scanning speed, layer thickness, scanning line spacing and focus compensation. By adjusting these parameters, different energy densities can be obtained, such as increasing the electron beam current or reducing the scanning speed. Higher energy density can be obtained. The amount of energy density greatly affects the microstructure, defects and mechanical properties of molded parts. Appropriate energy density will make the alloy have better mechanical properties. Due to the unique forming process of EBM technology, the microstructure and mechanical properties of EBM formed TC4 titanium alloy formed parts are different from those of conventionally manufactured (such as forging) TC4 titanium alloy formed parts.

2.1 Microstructure and influencing factors of EBM formed TC4 titanium alloy

The temperature change of EBM formed TC4 titanium alloy during the forming process affects its microstructure. First, the powder is melted under the action of the electron beam, and the liquid alloy temperature reaches about 1700°C, which is much higher than the β phase transition temperature of TC4 titanium alloy (995°C). At this time, the liquid alloy is composed of original β grains; then, as As the electron beam moves away, the liquid alloy rapidly cools to the construction temperature (generally 650-700°C) to remain stable and become solid. At this time, the alloy undergoes α→α+β, and the needle-like α phase and columnar β phase precipitate. A1-Bermani et al. believe that when the cooling rate is greater than 410°C/s at this stage, metastable α' martensite will precipitate, which will decompose into an α+β layered structure after being exposed to a high temperature environment for a long time, and most of it will be Fine needle-like α laths, with a small portion of β phase. Then the formed TC4 titanium alloy is slowly cooled from the construction temperature to room temperature, and the alloy microstructure does not change significantly and is still composed of α+β phases. The microstructure of EBM formed TC4 titanium alloy and forging formed TC4 titanium alloy are shown in Figure 2.

Domestic and foreign scholars have done a lot of research on the microstructure of EBM-formed TC4 titanium alloys and found that factors such as molding process parameters, the position of the molded parts, and the size of the molded parts will affect the cooling rate of the alloy during the molding process, thereby affecting its grain size. Hrabe et al. found that, under the conditions of ensuring that the energy input can completely melt the TC4 titanium alloy powder to form dense parts, appropriately increasing the electron beam scanning speed will cause the size of the molten pool to decrease, the cooling rate to increase, and thus finer α particles will be precipitated. lath and beta phase. Murr et al. and Wang et al. found that the microstructure of EBM-formed TC4 titanium alloy is different at different locations. As shown in Figure 3, the position with a lower deposition height has a higher cooling rate because it is closer to the molding substrate. It is an unstable growth zone and is prone to precipitate fine needle-like α phase; the position with a higher deposition height has a higher cooling rate. The thicker the lath, the larger the β grains; after being deposited to a certain height, it is in a stable growth zone, and the size of the α lath and β grains tends to be stable. Wang et al. also studied the effect of molded part size on the microstructure of EBM-formed TC4 titanium alloy and found that during the layer-by-layer melting and solidification process, smaller samples had a larger cooling rate, thus precipitating finer α phases. Galarraga et al. further studied and found that the changes in the microstructure of EBM molded TC4 titanium alloy are related to the residence time in the build chamber. If the residence time is too long, it will cause the deposition height at the bottom of the deposition height to be lower and the microstructure to be coarser. result. ​

2.2 Defects of EBM molded TC4 titanium alloy

Due to improper selection of process parameters or process interference, EBM formed TC4 titanium alloy parts may produce various defects. Zhai et al. found that there are two typical defects in the microstructure of EBM-molded TC4 titanium alloy: one is the pore caused by the argon gas entrained in the defective powder; the other is the pore caused by poor melting of the alloy powder.

Gong et al. classified TC4 titanium alloy defects into two major categories based on the energy density of the input electron beam. When the energy density is too low, it is not enough to completely connect the molten pools to the molten pools and between layers, forming irregular melting defects accompanied by a certain amount of pores. When the energy density is too high, local heat rises rapidly. When the powder melts, it spheroidizes under the action of surface tension (the thermal conductivity of the powder is low), thereby forming pores. Kahnert et al. found that if the energy input is too high, not only will the surface quality of the molded parts deteriorate, but in severe cases, the target machine of the powder coating system will stop working, so that the manufacturing process itself must be stopped. In addition, when the electron beam current exceeds a certain threshold, the alloy powder will be blown away, leaving irregular pores in the layer. In severe cases, the entire powder bed will collapse, as shown in Figure 5; Preparation of the powder bed Heat is used to improve its adhesion, overcome the thrust of the electron beam on the alloy powder, and avoid powder collapse. Defects will have an adverse impact on the mechanical properties of C4 titanium alloy. EBM process parameters must be optimized, such as controlling scanning speed, adjusting scan line spacing, and optimizing electron beam current, to reduce the occurrence of defects.

3.1 Tensile properties of EBM formed TC4 titanium alloy

Bruno et al. studied the tensile properties of TC4 titanium alloy formed by EBM forming and forging. Since EBM formed TC4 titanium alloy is prone to pore defects during the forming process, and its microstructure is unevenly distributed, resulting in its tensile strength, The highest yield strengths are 996MPa and 919MPa respectively, which are slightly lower than the strength of forged TC4 titanium alloy (tensile strength and yield strength are 1034MPa and 991MPa respectively); Wang et al. also studied the tensile properties of EBM formed TC4 titanium alloy. It was found that its tensile strength is 1002MPa, yield strength is 932MPa, and elongation is 14.4%. All performance indicators are higher than those of TC4 titanium alloy forgings after annealing and aging treatment.

There is significant anisotropy in the mechanical properties of EBM formed TC4 titanium alloy. Bruno et al. and Hrabe et al. found that the tensile strength of EBM molded samples in the horizontal direction was stronger than that in the vertical direction, while the elongation in the horizontal direction of the molded samples was smaller than the elongation in the vertical direction. This is caused by the uneven B grains inside the alloy: the molded sample mainly grows in the vertical direction; the formation of smaller primary β grains in the horizontal direction reduces the stress accumulation at the grain boundaries, thereby delaying the initiation of cracks and making it Slightly greater elongation.

Hrabe et al. found that increasing the electron beam scanning speed (negatively related to the energy density) will slightly reduce the thickness of the α plate (1.16μm → 0.95un), thereby increasing the tensile strength, yield strength and microhardness by 2% respectively. , 3% and 2%.

Formanoir et al. maintained the EBM-formed TC4 titanium alloy at 950°C for 60 minutes and 1040°C for 30 minutes respectively, using two cooling methods: water cooling and air cooling. The tensile strength and yield strength of the alloy were slightly reduced, and the elongation was not significantly improved. It shows that only controlling the key parameters of EBM forming is an effective way to improve the properties of the alloy.

3.2 Fatigue properties of EBM formed 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 the action of alternating bending stress of 600MPa (±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 pores of different shapes due to poor melting areas, and its surface roughness is also far away. Higher than rolled TC4 titanium alloy, which is an important reason for its low fatigue life.

Tammas-Williams et al. found that hot isostatic pressing treatment can effectively eliminate most pores in EBM-formed TC4 titanium alloy, but if there are some tunnel holes in the sample and are connected to the surface, the high-pressure argon gas under HIP treatment will penetrate into the tunnels. In the pores, these tunnel defects expand slightly, causing the HIP treatment to fail; adding a coating to the sample before HIP can remove the tunnel defects. Shui et al. found that after HIP treatment of EBM-formed TC4 titanium alloy, although the laths became thicker, the dislocation density decreased, and the tensile strength and yield strength decreased from 870MPa and 788MPa to 819MPa and 711MPa respectively, HIP treatment made the structure more Uniform, the relative density of the alloy increased from 99.3% to 99.8%, reducing the sources of crack initiation, thereby increasing the fatigue strength from 460boa to 580MPa.