The chemical composition of high-temperature alloys becomes more and more complex as the service temperature increases, making welding more and more difficult. The criterion for measuring the ease of welding is weldability. Weldability refers to the ability of the same material or different materials to be welded to form a complete joint and meet the expected use requirements under manufacturing process conditions. Weldability is the adaptability of materials to welding processing. It includes not only the bonding performance, but also the use performance after bonding. The four major factors that affect weldability are material factors, design factors, process factors and service environment. The weldability of high-temperature alloys refers to a comprehensive evaluation of the susceptibility to cracks in the alloy, the uniformity of the joint structure, the iso-strength of the mechanical properties of the joints, and the complexity of process measures under certain welding process conditions. Weldability is an important characteristic of high-temperature alloys. It is one of the scientific basis for selecting high-temperature alloy materials. It is also an important basis for the design of welding parts and the formulation of welding processes. The following will be combined with the author’s own work to introduce the crack sensitivity of high-temperature alloys, the inhomogeneity of the joint structure and the iso-strength of the welded joints.
39.1 Welding crack susceptibility of high temperature alloys
There are usually two types of welding cracks in high-temperature alloys: hot cracks and reheat cracks. The former is divided into crystal cracks and liquefaction cracks, while the latter mainly refers to strain aging cracks.
39.1.1 Characteristics of hot cracks
During the welding process of high-temperature alloys, the welding cracks produced when the metal in the weld and the heat-affected zone cool to the high-temperature zone near the solidus line are called hot cracks. There are usually two types of crystal cracks (or solidification cracks) and liquefaction cracks.
1. Crystal cracks
Crystallization cracks only occur in the weld. They are mostly distributed longitudinally in the center of the weld, and some are distributed in an arc on both sides of the center line of the weld, and are perpendicular to the welding wave. The common characteristic of crystallization cracks is that they are distributed along the grain boundaries of primary crystallization, especially along the grain boundaries of columnar crystals [7].
ANTON METAL studied the weldability of Ni3Al alloy, which is the main strengthening phase of high-temperature alloys. The welding method uses electron beam welding, which is fusion welding. The material is Ni3Al alloy with three components, see Table 39-1. After vacuum induction melting, it is cast into cylindrical ingots, homogenized and annealed, and cold-rolled through multiple passes into thin plates with a thickness of approximately 1.5mm. The intermediate heat treatment temperature during rolling is 1050°C, and the holding time is 30 minutes.
The welding test equipment is a domestic ZSH-150 high-voltage vacuum electron beam welding machine, with an acceleration voltage of 100kV and a vacuum degree of 0.132Pa. The rolled thin plates of three alloys were autogenously welded at different welding speeds. The thin plates were mechanically cleaned before welding. During the welding process, the thin plates were not clamped. The welding direction was perpendicular to the rolling direction. The beam current and welding speed were related to each other. Matching is based on exact penetration.
After electron beam welding of rolled thin plates of three alloys, it was found that during high-speed welding, varying numbers of intergranular cracks appeared in the weld metal of the three alloys, but the macro and micro morphology of the cracks were similar. The morphology is a typical hot crack concentrated in the center of the weld. The scanned image of the crack fracture surface shows typical potato-shaped intergranular cracking characteristics. The grain surface is smooth, indicating that the grain boundary is still in a liquefied state when the crack occurs. Therefore, the nature of the crack is a crystalline crack.
2. Liquefaction cracks
Welding cracks formed in the near-seam area of the high-temperature alloy base material or on the grain boundary that is liquefied due to heating before multi-layer welding are called liquefaction cracks. Most of the liquefaction cracks are micro-cracks, generally less than 0.5mm, and some are as long as 1mm. Most high-temperature alloys have a tendency of liquefaction cracks.
Post-weld inspection found that cracks appeared in both the heat-affected zone of the welded joint and the weld metal, especially in the heat-affected zone, where the cracks were more obvious. The latter is a crystal crack, while the former is a liquefaction crack. Its characteristics are: the crack has strong directionality, the crack surface is parallel to the surface of the thin plate; the crack has the nature of intergranular cracking, and the crack expands along the recrystallization grain boundary of the base material. In the scanning energy spectrum analysis, it was found that there is a Zr-rich phase in the bridging part of the crack; the crack is located in the middle of the plate thickness direction, and this phenomenon was not found during the metallographic examination of the weld surface; the crack affects the welding speed. There is a certain sensitivity, and cracks tend to be obvious during high-speed welding.
39.1.2 Formation mechanism of hot cracks
1. crystal cracks
When the high-temperature alloy weld solidifies, it proceeds in the form of dendritic crystallization. The melting points of the elements on the dendrite axis are higher and the impurity elements are less. The low melting point elements between the dendrites are enriched and the impurity element content is high. This solidification segregation produces This phenomenon becomes more serious as the content of impurity elements increases. Usually as the columnar crystals grow, impurity elements are continuously repelled to the junction of the parallel-growing columnar crystals or the center line of the weld. Impurity elements segregate at the grain boundaries and reduce the grain boundary bonding force; form low melting point compounds and promote the precipitation of harmful phases at the grain boundaries; increase solidification segregation, affect the solidification process, and promote element segregation and harmful phase precipitation. For example, P and Si are enriched in the final solidification zone of K438 alloy, which significantly reduces the initial melting temperature and eutectic temperature. When the P content is 0.0005%, the initial melting temperature is 1280°C and the eutectic content is 0; when the P content is 0.015%, the initial melting temperature is 1120°C and the eutectic content increases to 1.5%. In the late stage of solidification and crystallization, the low melting point phase or eutectic structure remaining at the grain boundary is distributed on the surface of the grains in a liquid film state. Under the action of the tensile stress caused by cooling shrinkage, these liquid films, which are far more fragile than the grains, cause crystallization. The boundaries are separated and crystal cracks are formed.
ANTON METAL’s scanning electron microscope observations of the weld structures of three Ni3Al alloys containing Zr show that there is a second phase between primary crystals. The analysis of grain boundary phase, grain boundary and intragranular energy spectrum shows that the grain boundary phase is rich in Zr. It can be seen that Zr is a strongly positive segregation element with a segregation coefficient as high as 34, while the content of grain boundary impurity elements S and P is very low, so the severe segregation of Zr at grain boundaries is the main reason for the production of low melting point phases. Transmission electron microscopy selected electron diffraction analysis shows that the Zr-rich phase at the grain boundary has a face-centered cubic structure with a lattice constant of 0.68nm, while the Ni5Zr phase with a face-centered cubic structure has a lattice constant of 0.668nm, so it can be concluded that the grain boundary is Zr-rich. The phase is Ni5Zr phase.
2. liquefaction crack
The formation mechanism of liquefaction cracks is essentially the same as that of crystallization cracks. They are both due to the fragile low-melting phase or eutectic between the crystals, which cannot withstand the force at high temperatures and cracks. The only difference is that crystallization cracks are formed during the solidification (or crystallization) process of the liquid weld metal, while liquefaction cracks are formed after the solid base metal remelts the intergranular layer under the action of the peak temperature of the thermal cycle [7] .
Due to the presence of low-melting-point phases or eutectics in the grain boundaries of high-temperature alloys, the area near the seam of the base material or the previous pass of multi-layer welding, and the area close to the welding pool, is rapidly heated to the solid-liquid phase temperature during welding. The phase does not have time to undergo equilibrium transformation, and a liquid film forms on the original phase interface, which cannot withstand the action of the beam stress and is pulled into liquefaction cracks.
39.1.3 Influencing factors and control measures of hot cracks
Factors affecting hot cracking can be divided into metallurgical factors and mechanical factors.
1. Influence of metallurgical factors
(1) Influence of alloy elements
Alloying elements have the most obvious impact on hot cracking in high-temperature alloy welding, and the superposition of the effects of multiple elements is more serious. Most Ni-based and Fe-based high-temperature alloys are precipitation strengthened with the ¢ phase, and Al and Ti are the main strengthening elements that form the ¢ phase. However, Al and Ti have very adverse effects on welding hot cracking of high-temperature alloys.
Different high temperature alloys have varying degrees of susceptibility to crystallization cracks. Crystal crack sensitivity is often evaluated using the variable beam cross crack sensitivity test method.
Tianjin Anton Metal Manufacture Co., Ltd. is a company specializing in the production of various nickel-based alloys, Hastelloy alloys and high-temperature alloy materials. The company was established in 1989 with a registered capital of 10.0 million, specializing in the production and sales of alloy materials. Anton Metal’s products are widely used in aerospace, chemical industry, electric power, automobile, nuclear energy and other fields, and can also provide customized alloy material solutions according to customer needs. If you need to know the price consultation of alloy materials or provide customized alloy material solutions, please feel free to contact the sales staff.
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Post time: Sep-23-2023
