With its high temperature resistance, corrosion resistance, and complex stress resistance, nickel-based superalloys have wide applicability in the manufacture of high-temperature parts such as turbine engine working blades, guide blades, aircraft engines, and industrial gas turbines. "The heart of the engine." However, when these parts work under high temperature and complex stress, especially in complex environments such as seawater, they are prone to cracks, abrasion, fracture and corrosion, resulting in a large number of scrapped parts. The use of additive remanufacturing technology to "remanufacture" used parts can maximize their value and generate huge economic benefits.
Additive remanufacturing technology is the use of additive manufacturing technology to remanufactured and used parts:
First, use some principles of digital processing to scan the 3D digital model of the component;
Then, the digital model is post-processed to obtain a 3-dimensional digital model of the defect part;
Finally, the defects are directly and quickly processed by layer-by-layer superimposition.
It is built on the existing mature technologies such as CNC CAD / CAE / CAM, welding, new materials, etc. The core concept is "layer-by-layer overlay, layered forming". Since the 20th century, the United States has carried out additive remanufacturing on military equipment such as B-52 bombers and M1 tanks, and has listed the upgrading and remanufacturing of weapon systems as key research areas for national defense science and technology. China has also successfully applied additive remanufacturing technology to various military equipment, which has generated huge economic benefits. Because the technology of additive remanufacturing is not mature enough, the current research is still in its infancy, so there are many problems to be solved urgently. For this reason, the current research status of welding-based additive remanufacturing technology is briefly introduced, and the future research hotspots are proposed by comparing different welding processes.
1 welding process
Tungsten arc welding
Gas Tungsten Arc Welding (GTAW) is a gas shielded arc welding with tungsten rods as the pole of the arc. Its application is very flexible, especially compared with laser cladding, it can more easily process additive remanufacturing of non-ferrous metals such as copper, aluminum, and magnesium. In addition, its arc length and arc stability are good, and the lower limit of the welding current is not restricted by factors such as wire droplet transfer. The minimum welding current can be 2A. However, it still has some shortcomings: on the one hand, the tungsten electrode has a limited carrying capacity, and excessive current can easily burn the tungsten electrode, which limits the penetration depth; on the other hand, as the current increases, the tungsten electrode arc diverges It becomes serious, causing the molten pool to collapse after forming, which seriously affects the forming quality.
The factors that affect the GTAW process are welding current, tungsten electrode diameter, arc length, arc voltage, and welding speed. Among them: welding current is the key parameter that determines the formation of GTAW welds. When other conditions are unchanged, the increase of welding current can lead to the increase of arc pressure, heat input and arc pillar diameter, which increases the weld penetration and weld width. The length range is usually 0.5 ～ 3.0mm. When the deformation of the forming part is small, the lower limit of the arc length is taken, otherwise the upper limit is taken. The welding speed is an important parameter for adjusting the heat input of the GTAW and the shape of the bead. After the welding current is determined, the welding speed has The corresponding value range exceeds the upper limit of this range, which is prone to defects such as cracks and undercuts.
Plasma arc welding
Additive remanufacturing method using plasma arc as welding heat source is called Plasma Additive Remanufactu-ring (PAR). Among them, the plasma arc is a compressed tungsten argon arc. The maximum temperature of the tungsten arc is 10000 ～ 24000K and the energy density is less than 104W / cm2, while the temperature of the plasma arc is as high as 24000 ～ 50,00000K and the energy density can reach 106 ～ 108W / cm2. Relying on the mechanical compression of the nozzle, accompanied by the thermal compression generated by the principle of minimum voltage and the electromagnetic compression of the arc column itself, the energy density of the plasma arc far exceeds that of the tungsten argon arc, and even the energy density of the laser can be reached. In comparison of the arc shapes of the two, the diffusion angle of the free arc is about 45 °, while the plasma arc is only 5 °.
Compared with the use of laser welding power, PA has an absolute cost advantage. According to data, the price of a common laser welding power source is generally around $ 500,000, while the price of a plasma arc welding power source is only $ 7,000, less than 2% of the price of a laser welding power source. Compared with GTAW, the process adjustment of BAR is more complicated, which mainly includes nozzle structure, electrode shrinkage, ion gas flow, welding current, welding speed and nozzle height. Among them: the nozzle structure and electrode shrinkage are the prerequisites for the selection of other process parameters, which are usually determined according to the type of material and forming conditions; the flow rate of the ion gas determines the penetration force of the plasma arc. Strong.
Laser cladding
Additive remanufacturing technology with laser as the heat source is often called Laser Deposition Forming (LDF), and it is one of the most widely developed additive manufacturing technologies. The factors that control the forming quality of LDF are mainly laser power, scanning speed, powder particle size, powder feeding amount and carrier gas flow. Compared with GTAW and PA, the LDF forming process needs to consider the laser absorption of the powder. When the amount of powder is constant, the required laser energy can be obtained by adjusting the laser power and scanning speed. The outstanding characteristics of LDF are high energy density, concentrated arc heat, small welding heat affected zone, large temperature gradient, and high forming efficiency, but there is a high residual stress after welding, so the pulse method is often used to adjust the heat input of the laser. The current research results show that: using pulsed laser cladding forming can obtain a slightly lower heat input and can better control the welding forming.
In contrast, GTAW and PA, while providing high heat input, will increase the post-weld heat-affected area and deteriorate the microstructure and properties of the workpiece after forming. With pulse technology, the pulse peak current can be used to melt the substrate and the base current dimension arc. Through the alternating changes of the peak current and the base current, the heat accumulation in the welding process can be effectively dispersed, thereby reducing the welding heat affected zone. Balachandar and other studies have shown that: using suitable pulse process parameters can effectively reduce the welding heat affected zone of GTAW, so as to improve the mechanical properties of the welded joints, it also improves and stabilizes the hardness of the welded joints, and even the mechanical properties are better than the welding heat treatment After mechanical properties. Chen et al. Used a pulse process to comparatively analyze the small-hole par and GTAW, and found that: pulsed plasma arc welding can effectively reduce the width of the heat-affected zone and make the metal structure in the fusion zone denser.
In order to compare the comprehensive effectiveness of rapid forming under different welding processes, Martina et al. Used direct forming width and interlayer height for modeling, and the results showed that: PA has significant advantages over GTAW and LDF direct forming. The comparison of the formability of different welding processes is shown in Figure 2. In short, the additive remanufacturing technologies based on different welding processes have their own characteristics: GTAW has high efficiency and low equipment costs, but its input heat is large and the part forming accuracy is not high; the pulse LDF heat input is small, the welding heat affected zone is small, The forming effect is excellent, but its equipment is expensive; the technology of the BAR technology has a significant advantage over LDF in terms of equipment cost, its deposition efficiency is about 98%, the maximum deposition rate can reach 1.8kg / h, the effective width and deposition rate of the formed part Higher than GTAW and LDF.
2 organization and performance
Welding-based additive remanufacturing technology is a complex welding process that is affected by multiple parameters: heat source energy input, CAD model scan data, and welding path planning all affect the microstructure shape, grain growth mode, grain boundary inclusions, and segregation. It will affect the overall performance of nickel-based superalloys. Scholars at home and abroad have done a lot of in-depth comparative research on this.
Organizational characteristics of forming parts
He Shaohua used Inconel 718 alloy to obtain shaped parts through LDF. An in-depth analysis of the deposited structure shows that the structure of the cladding layer is composed of columnar dendrites with directional growth. There is segregation of elements such as Mo and Nb and a small amount of carbides, which adversely affects the tensile strength of the matrix. The tensile strength of the deposited sample at room temperature is less than 50% of the deformed alloy. After the process, the grains are refined, and partial dendrite segregation is eliminated, and the yield strength and tensile strength of the test piece are improved. This is similar to the results of the study by Dinda et al. In the United States, who found that the deposited columnar crystals can be oriented to grow upward along the height of the deposition trajectory. The different cooling rate of the molten pool is the cause of the uneven structure of the formed part from bottom to top, as shown in FIG. 3. At the same time, Dinda et al. Found that during heat treatment, columnar dendrites can be transformed into equiaxed crystals at a temperature of 1200 ° C, and the precipitation of γ 'and γ ″ phases at 700 ° C increases the microhardness of the sample.
Xu Fujia used PA to form Inconel 625 thin-walled parts, as shown in Figure 4, the results show that the tissue from the bottom to the top presents different morphological characteristics:
1) The bottom structure presents small cell-like crystals without developed secondary dendrites;
2) The central tissue has obvious cell dendritic morphology, and the dendrite spacing increases;
3) A more developed secondary horizontal branch appeared in the upper part of the sample, and the dendrite spacing increased significantly;
4) At the top of the sample, a transition zone from columnar crystals to equiaxed crystals appeared.
The author's research found that in the process of additive remanufacturing, increasing the temperature gradient, increasing the cooling rate, and decreasing the heat input can increase the nucleation rate of the structure, thereby making the grains very fine and the overall structure. It is more compact, and it is tested in this case that the tensile mechanical properties of the formed part are improved. The above research results show that the change of cooling rate and heat input is the reason for the formation of columnar dendrites in the deposited state, and most of them are qualitative descriptions. The quantitative relationship between the size, distribution and spacing of dendrites and the cooling rate and heat input Less research.
Effect of process parameters on tissue performance
Scan path
Wuri Kaixi Aiyi used the PA technology to study the effect of different scanning paths on the mechanical properties of the formed part. The results showed that the tensile strength of the specimen along the parallel direction of the scanning path was higher than other directions, and the plasticity was optimal, which showed that The shaped part is anisotropic in the macro. The samples obtained by Xi Mingzhe and other multi-directional combinations (alternative cladding in different directions) showed isotropy. The tensile strength of the test piece was better than that of the welding wire, while the former was less plastic than the welding wire. Liu et al. Systematically studied the structure and properties of Inconel718 alloy LDF based on the changes of different deposition paths, and found that the tensile strength of the samples obtained by the single deposition path and the changed deposition path was equivalent, but the elongation of the former was significantly lower than the latter. Under specific path conditions, the formed parts produced by additive remanufacture show anisotropy in performance, so the study of the mechanical properties of the joint between the additive part and the substrate perpendicular to the forming direction is particularly important. There is less research.
Heat input
Fei Qunxing et al. Studied the influence of different process parameters of LDF on the structure and properties of the test specimens, and found that:
1) The cross section of the remelting zone along the deposition direction is flaky, mostly columnar crystals, and the crystal grains grow radially upward;
2) When the laser power is increased and the heat input is increased, the phenomenon of cross-layer growth of grains can be observed, and the thickness of the remelted zone is significantly increased;
3) Excessive power will increase the heat accumulation, which will cause the sample to be textured, and the external interface of the columnar crystals is prone to thermal cracks.
Ganesh et al. Studied the influence of process parameters on forming performance and found that process parameters can affect the deposition efficiency, promote the obvious change of the microstructure, and form a mixed morphology of columnar dendrites and cell crystals. Xu Fujia et al. Studied the effects of peak current, pulse frequency, welding speed, and wire feeding speed on the microstructure and performance of the formed parts of PA. They found that low peak current and high welding speed can obtain fine and dense dendrite structure, precipitated Laves phase and metal carbides. It has a diffuse distribution characteristic; increasing the pulse frequency or decreasing the wire feeding speed will make the tissue coarse, the Laves phase and metal carbides will increase, and it will have a continuous distribution characteristic. The above research results reflect the influence of the heat accumulation effect of cyclic heat input on the microstructure and properties of the formed parts during the remanufacturing process of additive, but they are all described qualitatively, and there is a lack of quantitative research on the heat accumulation effect.
Cooling rate
Due to the small heat input and low energy density of the GTAW, the cooling rate of the formed part during heating is lower than that of the BAR and LDF. Wang Wei et al. Studied the effect of different cooling rates on the precipitation of carbides and Laves phases in Inconel718 alloy, as shown in Figure 5. The above results show that:
1) When the cooling rate is low, the carbides are distributed in chains between dendrites and connected in large blocks;
2) As the cooling rate is accelerated, the carbides gradually change to small blocks, and the size also decreases;
3) The aggregation state is similar to carbides, but with the increase of the cooling rate, it is dispersed and the size gradually decreases.
Yin et al. Proposed that the precipitation amount and morphology of carbides both have an important effect on the mechanical properties of the alloy, and the morphology of the carbides with a smaller distribution and smaller size is better. When the Laves phase size is reduced by 1 μm, the cross-sectional shrinkage at room temperature can be increased by 2.5%. At present, there are no related reports that it is feasible to completely eliminate Laves phase. Therefore, it is of great significance to explore the quantitative influence of process parameters on the size and number of Laves phases.
3 Development and Outlook
Aiming at the characteristics of additive remanufacturing technology process and organizational performance, future research hotspots will focus on the following aspects:
1) Improve the accuracy of formed parts and reduce the heat affected zone. The pulse process is introduced, and the process parameters such as peak current, base current, pulse frequency, and duty cycle are adjusted to accurately control the heat input and cooling rate of additive remanufacturing, so as to better control the size of the molten pool and improve the forming accuracy.
2) Optimize the structure of forming parts.
The first is to study the quantitative relationship between the size, distribution and spacing of dendrites and the cooling rate and heat input;
The second is to study the mechanical properties of the joint between the additive part and the substrate in the direction perpendicular to the forming direction to avoid the adverse effects of anisotropy;
The third is to study the influence of the heat accumulation effect of cyclic heat input on the structure and performance of the formed parts during the remanufacturing process, reduce the precipitation of harmful Laves phases, and improve the mechanical properties of the formed parts.