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Effect of weldability of superalloys
Release time:
2022-07-28
The chemical composition of high-temperature alloys becomes more and more complex with the increase of temperature, so it is more and more difficult to weld. The four major factors affecting welding performance are material factors, design factors, process factors and service environment. The weldability of high-temperature alloys refers to the comprehensive evaluation of the sensitivity to cracks in the alloy, the uniformity of the tissue of the welded joints, the mechanical properties of the welded joints and the feasibility of taking process measures under a certain welding process condition.
The weldability of high-temperature alloys is mainly affected by the following factors:
1) Welding crack susceptibility of superalloys
In the welding process of superalloy, the welding cracks usually include hot crack and reheat crack, in which the hot crack is divided into crystallization crack and liquefaction crack, and the reheat crack mainly refers to the strain aging crack. The formation mechanism of liquefaction crack and crystallization crack is the same, both of which are due to the existence of fragile low-melting phase or eutectic between crystals, which can not withstand the action of force under the high temperature conditions of welding. The difference between the two is that the crystallization crack is formed during the solidification of the liquid weld metal, while the liquefaction crack is formed by the re-melting of the intergranular layer under the effect of the peak temperature of the solid base metal in the thermal cycle. Strain age cracking is generally produced when aging treatment is carried out after welding of precipitation strengthened high temperature alloy or when using at high temperature after welding. Due to the precipitation strengthening of high-temperature alloy crystal due to a large number of g' precipitation to strengthen, and the grain boundary strength in high temperature environment is generally lower than the intragranular strength, coupled with the adverse effects of impurity element segregation, the grain boundary is further weakened, resulting in plastic deformation in the grain boundary, increasing the tendency of strain aging crack, when the actual deformation of the grain boundary exceeds its plastic deformation capacity will produce strain aging crack.
2) inhomogeneity of welded joint structure
The microstructure of high-temperature alloy welded joints shows obvious inhomogeneity, and is obviously different due to the chemical composition and welding process. The organization of solid solution strengthened high temperature alloy is relatively simple, and after the welding of this kind of alloy, the weld metal changes from deformed tissue to casting tissue. Due to the fast cooling speed of the welding melt pool, the weld metal will form a layered tissue due to intra-crystal segregation, and the segregation will form a cocrystalline tissue between the branches. The heat-affected zone of welded joints produces local melting and grain growth along grain boundaries, such as solid solution-strengthened high-temperature alloys GH1015, GH1016 and GH1140, which have better weldability and fine weld tissue. In contrast, the organization of precipitation-strengthened high-temperature alloys and cast high-temperature alloys is more complex, and the organizational components of welds and heat-affected zones are more complex. The weld metal undergoes a melting and solidification process during the welding process, and the original g′ phase and carbide are all dissolved into the matrix to form a single g solid solution. The weld metal cools quickly and is prone to form dendrites with short transverse dendrites and long principal axes. Large composition segregation occurs between the dendrite and the main axis, resulting in the eutectic composition in the weld. In the heat-affected zone with a large periodicity of the thermal cycle, it will cause the dissolution of the strengthening phase g' and the transformation of carbides, so that the organization of the heat-affected zone becomes very complex and affects the performance of high-temperature alloys. Such as GH4169 base metal grain is small, mostly equiaxed crystal, belongs to the deformation alloy organization. The microstructure of the weld is completely different from that of the base metal, the branch structure is obvious, and the dendrite axis is perpendicular to the weld. The microstructure of this welded joint has little effect on the tensile properties, but it can significantly reduce the durability and fatigue properties.
3) Equal strength of weld joint.
The service environment of high-temperature alloys generally has to withstand high temperature and stress at the same time, so high-temperature alloy welded joints should have good high-temperature strength, plasticity, low-cycle fatigue performance and good oxidation and corrosion resistance. At the same time, it is hoped that the strength of the welded joint is the same as that of the base metal, that is, the strength of the welded joint. Usually, the main problem encountered in welding of high-temperature alloys is the reduction of mechanical properties in addition to cracks and microcracks during or after welding. Welding generally causes a significant reduction in tensile strength and yield strength, while reducing plasticity. In addition, weld melt solidification will cause element segregation, reduce oxidation and corrosion resistance, and deteriorate performance. Therefore, the use of reasonable welding process and excellent welding materials is essential to improve the strength of high temperature weld joints. If the high temperature alloy is welded by friction welding, the strength coefficient of the welded joint is almost 100%. If the heterogeneous welding wire is used, the strength of the joint is reduced more. The strength coefficient of the welded joint is caused by the inhomogeneity of the weld tissue, the grain tissue in the heat affected zone grows up, the dissolution of the strengthening phase g′ phase is easy to form a weakened zone, so the plastic deformation will first appear in the weakened zone, and eventually lead to fracture failure. Therefore, the strength and plasticity of high-temperature alloy welded joints are significantly reduced. Therefore, the process parameters should be optimized from the aspects of welding process, welding materials, welding methods and heat treatment to ensure that the coefficient of welded joints K σ is close to 100%.
The hot crack sensitivity of high-temperature alloy welding, the inhomogeneity of the joint structure and the equal strength of the welded joint are the key factors that determine the weldability of high-temperature alloys. In addition, the selection of a reasonable welding process is also an important basis for evaluating the weldability of high-temperature alloys. Therefore, before the use of high-temperature alloys, the weldability of high-temperature alloys must be analyzed and studied. Only by mastering the weldability of high-temperature alloy and its influencing factors can the production of high-temperature alloy welded components be successfully completed and the safe use of high-temperature alloy welded components can be guaranteed.
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