Stainless steel " target=_blank> stainless steel dissimilar material electron beam fusion brazing
At present, a large number of new-aerospace engines use a new material and a heterogeneous material connection structure to fully utilize the respective performance advantages of the materials and the special use of the structure to ensure the overall performance of the engine. Duplex stainless steel "target=_blank> stainless steel has excellent mechanical properties and full corrosion resistance, especially with good resistance to stress corrosion, so it has been widely used in petroleum, chemical, atomic energy engineering and aerospace engine manufacturing [1 Chromium bronze is a material with good corrosion resistance and high thermal conductivity. The addition of trace element Cr plays a dual role of refining crystal grains and further improving strength [2].
The effective combination of chrome bronze and duplex stainless steel "target=_blank> stainless steel dissimilar materials meets the requirements of engine thrust chamber cooling and high strength, which involves the welding of copper-steel dissimilar materials. It has high energy density and heating rate for electron beam welding. Fast, welding heat affected zone and small deformation, stable parameter reproducibility, easy control and suitable for welding refractory and dissimilar metals [3-5], electron beam welding of QCr0.8 and 1Cr21Ni5Ti The experimental study and the microstructure and mechanical properties of the joints formed by beam welding under different copper distances are analyzed. The results can be used to rationally develop the welding process of QCr0.8 and 1Cr21Ni5Ti to obtain the theory of high quality connection. And experimental basis.
1 Test materials and methods
The chemical composition and mechanical properties of the test chrome bronze and duplex stainless steel "target=_blank> stainless steel are shown in Table 1.
The welding equipment used in the test was a MEDARD43 vacuum electron beam welding machine manufactured by TECHMETA, France. The maximum accelerating voltage of the welding machine was 60KV and the maximum power was 6kw. The cathode diameter used in this experiment was Ф2.0. As shown in Figure 1, the cleaned chrome bronze is flush with the bottom surface of the duplex stainless steel "target=_blank> stainless steel test piece along the long side and placed in the self-made fixture on the welding machine vacuum chamber table. The gap between the seams must not exceed 0.25mm at most. Then the vacuum is 5.4×10-4mbr, the acceleration voltage is HV=60KV, the beam current Ib=45mA, the welding speed is v=1m/min, and the surface is in focus. The welding electron beam is changed relative to the offset value of the center line of the butt joint to the chrome bronze side.
Microstructure analysis of the joint area of ​​the welded sample was carried out using a PMG3 OLYMPOS optical microscope imported from Japan. The joint tensile test was carried out on an INSTRON MODEL 1186 electronic universal testing machine.
Test results and analysis
2 head state
There are significant differences in thermal physical properties such as melting point and thermal conductivity between QCr0.8 and 1Cr21Ni5Ti. Generally, the thermal conductivity of pure Cu is 6 to 10 times larger than that of pure Fe, so the heat transfer on the Cu side is much faster than that of Fe. Thus, when the copper partial value is 0 mm (ie, the centering welding), the distribution of heat on the sides of the butt joint of the QCr0.8 and 1Cr21Ni5Ti butt joints is extremely uneven, and the formation of the asymmetric temperature field in the midline of the butt joint is extremely uneven. Will cause the base metal on both sides to melt unevenly, and the melting amount of 1Cr21Ni5Ti is greater than QCr0.8, which is unfavorable for forming a reliable fusion welded joint. At the same time, it is considered that the burning of Cu element on the QCr0.8 side is severe under the action of high energy electron beam. To this end, we use the joint form of the unequal thickness of the copper side as shown in Figure 1 to balance the heat input of the base material on both sides of the weld, and at the same time make up for the shape of the collapsed weld caused by Cu burning. . It can be seen from Fig. 2(a) that the base metal on both sides of the joint has a partial copper value of 0 mm, and the weld microstructure is extremely uneven. The light-colored tissue area in the upper left part and the dark tissue area in the middle part and the upper right part are obvious. The boundary line, combined with the Fe-Cu binary phase diagram, we conclude that the dark structure of the middle and near stainless steel "target=_blank> stainless steel side of the weld is α+ε phase as-cast mixed structure; the upper left part of the weld is near QCr0.8 side The light-colored structure is the solid solution Cu (ss.Fe) of Fe in Cu, which contains a small amount of discretely distributed α+ε phase.
The volume content of α+ε two-phase structure in the weld is larger than that of Cu(ss.Fe) phase, which indicates that an asymmetrically distributed temperature field is formed on both sides of the weld head midline in the electron beam pair, which causes the base metal to melt without melting. all. The formation of such macrostructures and uneven welds is due to the difference in physicochemical properties of the dissimilar materials and the high energy density of the electron beam, and the welding characteristics of efficient and rapid seam formation. In the centering welding, both base metals are melted to participate in the formation of the molten pool, but due to the difference in melting point, density, atomic activity and high temperature fluidity of the two sides, the molten base material is melted on both sides under the action of fast moving electron beam deep penetration. The metal has not yet melted in the liquid state, that is, it begins to crystallize and solidify, thereby forming unevenness of the macroscopic structure of the weld.
When the copper value is 0.3mm, as shown in Fig. 2(b), the melting amount of the chrome bronze side is obviously increased, the degree of uniformity of the weld bead is improved, and the weld is Cu(ss.Fe) mixed with α+ε. The organization, in which the α+ε phase is no longer aggregated into a large area of ​​the tissue, but is discretely distributed in the weld by the block, and the obvious melting trace of 1Cr21Ni5Ti is still visible near the steel side fusion line. As shown in Fig. 2(c), with the further increase of the partial copper value, the partial copper value is about 0.8 mm, and the weld microstructure is basically the whole Cu (ss.Fe) phase, and the macroscopic structure of the joint region is uneven. Completely disappear. a) copper partial 0mm (centering welding) b) partial copper 0.3mm c) partial copper 0.8mm d) partial copper 2.0mm different partial copper value joint upper part microstructure microstructure of the joint steel side fusion line Further observation of the weld and heat affected zone (see Figure 3), we can see that in the short length of the steel side fusion line close to the upper surface of the test piece, a fusion transition zone appears, combined with its microstructure and Fe -Cu binary phase diagram, we analyzed that the microstructure is α+ε phase with higher Fe content; in the lower part of the weld steel side fusion line, the 1Cr21Ni5Ti base material is not melted, but is formed thin with the weld zone. Diffusion transition layer. Further, the offset value of the electron beam is further increased. As shown in Fig. 3(d), only the copper side base material is melted, and the steel side base material is not fused, and a clear unwelded butt joint surface can be seen from the figure. a) on the side of the steel side fusion line b) on the lower part of the steel side fusion line, as the electron beam distance increases the offset of the copper side of the center line, the distribution of the welding temperature field formed by the electron beam on both sides of the base material also The change in the weld structure is gradually uniformized. In the range of the partial copper value of 0.8-1.0 mm, the joint is characterized by a fusion weld joint. At this time, the chrome bronze base material is melted, and the steel side base material hardly melts, and the molten chrome bronze base material is used as a brazing material to be bonded to the steel side base material. If the amount of partial copper exceeds 2.0 mm, the joint cannot be fused.
2.2 Mechanical properties of the joint
In order to evaluate the connection performance of different copper-to-but joints, we performed a joint tensile test. It can be seen from Fig. 4 that the intensity of the QCr0.8/1Cr21Ni5Ti electron beam welded joint is nearly parabolic with the increase of the offset value of the copper side of the midline of the electron beam. When the offset value is 0 mm (ie, centering welding), the tensile strength of the joint is very low. It can be seen from the above-mentioned structural analysis that this is mainly caused by the macroscopic uneven distribution of the weld bead structure and composition of the welded joint. As the offset value increases, the joint structure and composition gradually become uniform until the offset value reaches 0.8-1.0 mm, and the joint strength peaks, forming a fusion joint in which the weld tissue composition is uniform. At this time, the joints are well connected, the strength is up to about 330Mpa, and the minimum joint strength of the joints is more than 90%. The partial copper value is further increased due to the large shift of the electron beam spot and the sharp heat loss of the copper side base material, so that the thermal effect of the electron beam temperature field on the butt joint side of the joint steel is lowered, resulting in atomic diffusion at the brazing interface. As the ability and degree decrease, the joint performance also decreases. When the amount of partial copper exceeds 2.0 mm, since the electron beam only heats the copper-side base material, an effective fusion joint cannot be formed, and the joint is not welded.
in conclusion
1) The increase of the offset value of the copper side of the mid-line of the electron beam-to-pair will lead to the homogenization of the weld microstructure and composition of the butt joint of QCr0.8/1Cr21Ni5Ti, and improve the welding state of the joint.
2) When the shift value reaches 0.8-1.0mm, the fusion joint forming the weld tissue composition is uniform, and the tensile strength can reach about 330Mpa, which can meet the practical use requirements.
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