Investigation On Burst Behavior Of GH3625 Alloy During Hot Extrusion

GH3625 alloy is widely used in chemical industry,nuclear power,aerospace,national defense and other fields in the form of seamless tube due to its excellent comprehensive properties below 650?.However,due to its high degree of alloying,large deformation resistance,and narrow temperature range for hot working,problems such as cracks,stuffiness,and bursting during the hot extrusion forming of seamless tubes are prone to reduce production efficiency.This thesis takes GH3625 alloy hot-extrusion burst tubes as research object,analyzes the tube bursting behavior mechanism of GH3625 alloy seamless tube during hot extrusion forming,and reveals the influence rule and action mechanism of key extrusion parameters on GH3625alloy profile defects,which provides theoretical guidance for further optimizing the hot extrusion process parameters of GH3625 alloy seamless tube and improving the yield of tube.The influence of different billet structure states on hot extrusion forming of GH3625 alloy seamless tube was studied.The results show that the as-cast billet has burst and the homogenized billet has successfully extruded;this is due to the existence of a large number of Laves phases in the microstructure of as-cast GH3625alloy due to severe micro-segregation;the severe adiabatic heating during the pressing process caused the temperature in the middle position of the tube blank to exceed the melting point of the Laves phase and some carbides,causing the Laves phase and carbides to over-burn,they melted and diffused into the surrounding matrix to form a large number of micro-cracks and holes,under the action of high tensile stress at the die exit,these cracks and holes quickly expand and connect to each other,resulting in the bursting of the tube.The effects of different extrusion ratios on hot extruded GH3625 alloy seamless tubes were studied.The results show that the relatively large extruded billet has burst,and the relatively small extruded billet has been successfully extruded;this is because the used billet has undergone forging treatment,eliminating a large amount of Laves phase,however,a small amount of Laves phase still exists;during the hot extrusion process,the severe adiabatic heating of the billet melts and diffuses these remaining Laves phase into the surrounding matrix,thereby forming micro-cracks and holes;the extrusion force and the tensile stress at the die exit are not large enough to make these micro-cracks and holes expand,so the tube is successfully extruded;when the extrusion is relatively high,the extrusion force and the tensile force at the die exit is large,under the action of high tensile stress at the die exit,these cracks and holes rapidly expand and connect together along the grain boundary with less resistance,which causes the tube to burst.The mechanical properties and fracture mechanism of GH3625 alloy at high temperature were studied,which supplemented and verified the proposed tube burst mechanism.The results show that at a strain rate of 0.001 s^-1,the stress-strain curve of the GH3625 alloy shows a typical tensile curve of the metal;while at a strain rate of 1 s^-1,the stress-strain curve of the GH3625 alloy is completely absent the typical tensile curve of the metal which has a yield phenomenon;the strength of the material increases rapidly with increasing strain and breaks directly after reaching the tensile strength of the material;however,the mechanical properties of the alloy are greatly reduced at 1200?,which is due to the fact that 1200?exceeded the melting point of the Laves phase in the structure,causing the Laves phase to melt and diffuse into the surrounding matrix to form cracks and holes,and part of the Laves phase directly liquefied into droplet particles;under the action of an applied load during the stretching process,these cracks and holes continue to expand and connect with each other,resulting in liquid fracture of the alloy,which greatly reduces the mechanical properties of the alloy;this is fully consistent with the tube burst mechanism we analyzed,and further supporting the mechanism for the tube burst phenomenon during the hot extrusion process we have proposed that Laves phase is one of the reason for the tube burst phenomenon.