Dynamic Tensile Deformation and Fracture Mode of Superalloy Welded Joints

0 Introduction In the application of superalloys, welding is an important manufacturing process, and welding quality plays a decisive role in the entire welding structure. The high-temperature alloy welded parts that need to withstand alternating loads during use must also undergo fatigue check. In-situ stretching can perform real-time dynamic observation of the microscopic failure process of the material, and use the specimens taken from the same welded joint fatigue test piece to perform in-situ stretching to understand the dynamic fracture behavior of the weld and compare it with the results of the fatigue test , It is helpful to improve the understanding of the fatigue failure process of welded joints, so as to seek effective ways to improve the welding quality. 1 Test materials and test methods The test materials are domestic GH3030 and GH150 high-temperature alloys. Welding sample preparation: Take the above alloy plate with a thickness of 2 mm, and use automatic butt argon arc welding. The welding wire is GH3128, the diameter is 1.6~2.5mm, the diameter of the tungsten electrode is 2.0~2.5mm, the welding current is 70~90 A, The voltage is 12 V, the argon gas flow rate is 12~15 L/min, and it is inspected by X-ray inspection after welding. Cut the in-situ tensile specimens on the welded specimens that have not been fatigue-tested, and cut the in-situ tensile specimens with wire cutting in the direction perpendicular to the weld. The weld is in the middle of the specimen, as shown in Figure 1. The sample is ground, polished on one side and slightly corroded in order to observe the tensile deformation process of the structure. The dynamic stretching is performed on the special stretching table SM-TS40, which is attached to the JXA-840 scanning electron microscope. The maximum tensile load is 2kN, the afterburner is calibrated to 150mV, and the voltage and load have a linear relationship with a slope of 1. The maximum stretching distance is 20mm, and the stretching speed is adjustable up to 50mm/min. In dynamic stretching, an X-ray energy spectrometer (EDS) can be used to analyze the elements of the observation point at the same time. The fatigue test of the welded joint specimen was carried out on the hydraulic servo fatigue testing machine EHF-EG250KN-40L. The fatigue load was controlled by the axial tension-tension stress, sine wave pulsation loading, f=4.5Hz, R=0.1. Record the number of cycles and check where the fatigue fracture occurs. 2 Analysis of the test results 2.1 The macro morphology of the welded joint in situ tension Figure 2 shows the macro morphology of the welded joint at the later stage of in situ tension. The weld seam is a solidified as-cast structure in a non-equilibrium state. The structure is very complicated with the proportion of matching welding materials, the chemical composition after mixing with the base metal and the cooling conditions. As shown in Figure 2, during the in-situ stretching process, when necking occurs, There is obvious unevenness of deformation at the weld. This is caused by the inhomogeneity and complexity of the microstructure of the welded joint. It shows that after the sample is loaded, the microstructure of the weld is deformed. The grain structure of different orientations and its internal dislocation movement cause the corresponding part of the sample to continue to deform. The local stress to be applied is not consistent, so the welded joint The phenomenon that the center of the weld is enlarged, forming a two-stage necking, conforms to the principle of the minimum strain energy path followed by the necking of the plate sample. The metallographic examination found that the welds of the test pieces were mainly dendritic structure. Studies have shown that alloys with fine-dendrite solidification interfaces at the crystallization front have the best durability. This structure promotes work hardening and delays the necking time. The base material is a fine equiaxed crystal structure. The structure of the fusion zone is more complex. The junction with GH150 is dominated by equiaxed crystals of different sizes, and the junction with GH3030 is dominated by columnar crystals. The heat-affected zone has local grain coarsening, because the superalloy has no allotropic transformation. During the welding process, except for the solidification structure of the weld, the transition zone with the base metal grew irregularly due to overheating. In this way, the overall microstructure of the welded joint has three different orientations. Therefore, there are serious microstructure inhomogeneities in the joints, which will inevitably affect the deformation behavior of the joints. The interaction of the crystal grains of the polycrystal is the main form of resistance to plastic deformation, otherwise it is very easy to crack locally. The above-mentioned structural inhomogeneity is easy to cause the internal micro stress concentration and the difference of the strain rate of each grain during the fatigue performance test of the welded joint, and eventually fatigue cracks will be induced in the weaker part. 2.2 The microscopic deformation process of the sample during in-situ stretching Figure 3 shows the microscopic deformation of the sample surface during the stretching process. As the stretching progresses, a large number of dislocations, stacking faults, etc. in the stressed crystal grains will be activated under the action of stress, forming mutually parallel slip lines, as shown in Figure 3a. Plastic deformation must consider the loading speed. Due to the low in-situ tensile deformation rate, the grain deformation is relatively uniform, the slip zone is straight and the spacing is basically the same. However, the figure only shows the slip band in one direction, which means that in the uniform deformation stage, only one direction slip system dominates, and the alloy will undergo initial hardening at this stage. As the stretching progresses, slip bands in different directions can be seen in other crystal grains, forming spikes or triangles. According to crystallographic theory, the <111> crystal plane is the slip plane of austenite and the plane where stacking faults are formed. The crystal plane has 4 basic orientations, and the angle between each crystal plane is theoretically calculated as 70°32′. If a grain cross section is parallel or approximately parallel to <111>, Since the angle between the reference vectors of the three slip directions on the <111> plane is 60°, which is a regular triangle, the characteristics of the slip line in the austenite during stretching conform to the crystallographic orientation relationship. As the amount of deformation increases, the slip zone will gradually become wider and bend. Because the orientation of each crystal grain is different, not the direction of the slip system of each crystal grain is consistent with the direction that is most favorable for deformation, which results in a difference in the degree of deformation of each crystal grain. As the amount of strain increases, the grains begin to obey the coordinated torsion under the stress conditions, the deformation intensifies, and the relief appears. From the initial twisting of individual crystal grains in only a few areas, to the end almost every crystal grain has been twisted, and the surface relief becomes more obvious.As shown in Figure 3b, under the action of tensile stress, the weld fusion zone The crystal grains are obviously elongated, and the preferred orientation appears. However, the grains in the heat-affected zone above it have coarsened and bent slip lines. This microscopic morphology change can explain the unevenness of the macroscopic deformation to a certain extent. At the same time, it was found that under the action of tensile stress, the intragranular brittle second phase particles (the particles in the figure are titanium nitride) separated from the matrix and fragmented, as shown in Figure 3a. This kind of second phase particles has a small slip system and is easy to brittle or fracture along the boundary to become the source of cracks under external stress, but sometimes it is also helpful to improve the deformation strengthening ability of the material. Inhomogeneous structures such as grain boundaries, inclusions, etc., will also affect the fracture of welded joints. Figure 4 shows the influence of grain boundaries and inclusions on the microscopic fracture mechanism under tensile stress. It can be seen from Figure 4a that the weld grain boundary cracks before the inclusions at a small stress level, but the influence of the grain boundary precipitates cannot be judged yet. Figure 4b shows the source of cracks caused by inclusions. X-ray energy spectrum analysis shows that such inclusions are mainly compounds of aluminum, titanium or niobium. In-situ stretching shows inclusions falling off the grain boundary or matrix. Due to the stress concentration caused by inclusions, microcracks perpendicular to the tensile direction will be formed around the holes. It can be seen that under the action of tensile stress, inclusions and surrounding tissues are the weak link of the weld. 2.3 Crack initiation The high-temperature alloy weld has good strength and toughness. When the sample necks down, because the plate-like thin sample cannot cause deformation constraints in the thickness direction, it cannot form a plane strain state, and the initial crack can be The polished surface of the specimen was directly observed. Once a crack is formed on the surface, the deformation of the area near the front end of the crack is basically unconstrained and can be deformed freely, showing a fully yielded state. Under the action of external force, the front end of the crack is torn in an open shape along the thickness of the sample and on both sides due to plastic deformation. It is consistent with the macroscopic fracture behavior of the thin plate tensile specimen under the plane stress condition. Figure 5 shows the microscopic morphology and mechanism of fracture in different parts of the sample during in-situ stretching. As shown in Figure 5a, in the central zone of the plate-shaped sample, the slip line is thickened and bent; the crystal grains are tilted to the side and the surface is embossed; because it is in a plane stress state, the crack is of a slip-open type. However, the stress state at the sharp edges and corners of the sample is complicated, which is easy to cause stress concentration. A small amount of plastic deformation will cause deformation strengthening, which will hinder further plastic deformation. As a result, obvious brittle transgranular fractures are formed at the edge of the sample, and the elongated grains are formed. And the surface slip line shows that it has experienced plastic deformation in the early stage. 2.4 In-situ observation of the main crack growth Figure 6a~f shows the in-situ observation results of the main crack initiation and growth. The deformability and deformation resistance of grains with different orientations are different. Under tensile load, the grains with the most favorable orientation for slippage and the most easy concentration of microscopic internal stress must start first, and the more uneven the structure, this kind of initiation The selectivity of plastic deformation becomes more prominent. When the deformation of the first deformed grain exceeds a certain value, an initial crack will be initiated. As shown in Figure 6a, the tip of a sliding triangle and the twisted bottom of the twin grain boundary, due to the deformation strengthening, the material becomes brittle, and a cavity is generated to form the main crack source; the generation of the cavity softens the stress state as shown in Figure 6b. It shows that the cavity is elongated in the force direction and torn to the left along the grain boundary. After passing through the twin grain boundary, another crack source is generated; Figure 6c shows that the twin crystal is pulled off, and the crack is perpendicular to the tensile direction. Part of it is easier to slide apart, and a crack is generated inside the main crack; Figure 6d shows that new cracks continue to appear, and the main crack has been connected; Figure 6e shows that a second phase particle is brought out from the original observation point; dimples are formed locally on the main crack surface At the same time, a new cavity appeared. The left front edge of the main crack was quickly torn to the left and entered a full collapse; Figure 6f shows the crack morphology before the final fracture of the sample. The whole process is: the crack source is formed and connected to each other to form the main crack; the main crack slides and opens diagonally upwards, and the snake slip pattern and small dimples are visible on the tear surface; as the crack opens, the second phase zone of different properties The cracks spread laterally and finally fractured. The fracture surface is full-sheared and has no radiation zone. It is a cut-off fracture caused by a typical plane stress condition. 2.5 Fatigue test results Table 1 shows the fatigue test results of two groups of welded joints, each group contains 6 data, S is the standard deviation of the sub-sample. The test results show that the fatigue performance of superalloys welded by automatic butt argon arc welding is good, and the data uniformity under low stress levels is better. Fatigue fracture inspection found that the source of fatigue cracks started at the weld toe or weld root. At the same time, it is found that the fatigue source is firstly related to the micro-quality of the weld surface, and secondly is the microstructure and structure of the weld. When testing the fatigue performance of superalloy butt argon arc welding joints, it is found that the fatigue life is much lower than that of the smooth specimens of the corresponding alloys, and all fatigue sources are generated in the fusion zone of the welded joints, and the fatigue crack propagation leads to the fatigue failure of the joints. The in-situ tensile observation shows that the complexity of the microstructure of the welded joint and the distribution characteristics of the structure in each area of ​​the joint indicate that the fatigue failure process is different from that of static tension. The uniformity will be greater. Studies have shown that even the same metal structure, on the basis of different plastic deformations, will lead to different degrees of deformation strengthening. The obvious structural inhomogeneity in the fusion zone of superalloy welded joints will aggravate the initiation of fatigue cracks. In this way, this area will become the weaker part of the fatigue performance of this type of joint. In the application, it is found that the destruction of high-temperature alloy welded structures is often dominated by low-cycle fatigue. At present, it is generally reported that cracks often occur at the weld toe of welded joints, and the main reason is the effect of macroscopic residual tensile stress and stress concentration of welded joints. However, in the in-situ tensile test where the weld toe is removed, it is found that the sample is often broken in the fusion zone of the weaker alloy in the weld. In the inspection of fatigue fractures under different welding conditions, it was found that the sources of fatigue are common in slag inclusions, surface defects and early deformation and hardening of the fusion zone, which are consistent with the results observed in the in-situ tensile of welded joints. In the process of high-temperature alloy welding, under the general conditions that the multi-element metal is not sufficiently diffused, the brittle phase of intermetallic compounds is easily formed in the weld fusion zone, which leads to the weakening of the weld. Therefore, it is very important to improve the welding quality, especially the surface quality, which helps to reduce the adverse effects of the weld toe. 3 Conclusions (1) The in-situ tensile of the plate-shaped sample shows that the superalloy It will also lead to varying degrees of deformation strengthening. The obvious structural inhomogeneity in the fusion zone of superalloy welded joints will aggravate the initiation of fatigue cracks. In this way, this area will become the weaker part of the fatigue performance of this type of joint. In the application, it is found that the destruction of high-temperature alloy welded structures is often dominated by low-cycle fatigue. At present, it is generally reported that cracks often occur at the weld toe of welded joints, and the main reason is the effect of macroscopic residual tensile stress and stress concentration of welded joints. However, in the in-situ tensile test where the weld toe is removed, it is found that the sample is often broken in the fusion zone of the weaker alloy in the weld. In the inspection of fatigue fractures under different welding conditions, it was found that the sources of fatigue are common in slag inclusions, surface defects and early deformation and hardening of the fusion zone, which are consistent with the results observed in the in-situ tensile of welded joints. In the process of high-temperature alloy welding, under the general conditions that the multi-element metal is not sufficiently diffused, the brittle phase of intermetallic compounds is easily formed in the weld fusion zone, which leads to the weakening of the weld. Therefore, it is very important to improve the welding quality, especially the surface quality, which helps to reduce the adverse effects of the weld toe. 3 Conclusions (1) The in-situ tensile of the plate-shaped sample shows that the superalloy It will also lead to varying degrees of deformation strengthening. The obvious structural inhomogeneity in the fusion zone of superalloy welded joints will aggravate the initiation of fatigue cracks. In this way, this area will become the weaker part of the fatigue performance of this type of joint. In the application, it is found that the destruction of high-temperature alloy welded structures is often dominated by low-cycle fatigue. At present, it is generally reported that cracks often occur at the weld toe of welded joints, and the main reason is the effect of macroscopic residual tensile stress and stress concentration of welded joints. However, in the in-situ tensile test where the weld toe is removed, it is found that the sample is often broken in the fusion zone of the weaker alloy in the weld. In the inspection of fatigue fractures under different welding conditions, it was found that the sources of fatigue are common in slag inclusions, surface defects and early deformation and hardening of the fusion zone, which are consistent with the results observed in the in-situ tensile of welded joints. In the process of high-temperature alloy welding, under the general conditions that the multi-element metal is not sufficiently diffused, the brittle phase of intermetallic compounds is easily formed in the weld fusion zone, which leads to the weakening of the weld. Therefore, it is very important to improve the welding quality, especially the surface quality, which helps to reduce the adverse effects of the weld toe. 3 Conclusions (1) The in-situ tensile of the plate-shaped sample shows that the superalloy

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