| As the thrust-to-weight ratio of aero-engine continuous increase,the service temperature of critical components in aero-engine hot-end section will also increase significantly.Thus the temperature-bearing capacity of the selected material for the integral casted complex thin-walled structure,skew plate frame,which is an important load-bearing component in the aero-engine hot-end section,needs to be greatly increased.Nowadays,small amount of defects such as shrinkages and inclusions are inevitably occurred during fabrication of skew plate frame.Thus welding repair of these defects is necessary.In addition,the skew plate frame needs to be welded with GH536 alloy to fabricate a cavity structure for actual use.At present,the temperature-bearing capacity of the skew plate frame is insufficient because of the use of K4169 alloy whose temperature-bearing capacity is only 650?.Thus an alloy,named K4750,with the temperature-bearing capacity of 750? was developed for the skew plate frame.At the same time,another alloy,temporarily named 850 alloy,with the temperature-bearing capacity over 800? was also being developed for future advanced aero-engine skew plate frame.Simple repair welding test of K4750 indicated good repair weldability.However,as a newly designed alloy,the study of microstructures,phase precipitation and mechanical properties of the K4750 similar and K4750/GH536 dissimilar alloy joints are not conducted.In addition,repair welding joints of 850 alloy exhibited high susceptibility of liquation cracking and SAC(strain ageing cracking,SAC)in the HAZ(heat affected zone,HAZ).However,the cracking formation mechanism of 850 alloy joint is unclear and there is no effective method to suppress the cracking.Thus,comprehensive study of the microstructures and mechanical properties of the K4750 similar and dissimilar alloy joints,the defects formation mechanism and the control method of 850 alloy joints is very important.The microstructures,fusion zone phase precipitation,and the mechanical properties of the similar alloy joints of K4750 were studied.And the results show that the solidification pattern of the K4750 joint fusion zone is as follows:L?L+??L+?+(Ti,Nb)C??+(Ti,Nb)C.The result of elemental segregation indicates that Fe,Al and W tend to segregate to dendrite cores,while Nb,Ti,Si,and Mo tend to segregate to dendrite boundaries.The liquated MC carbides can be detected at the joint HAZ adjacent to the fusion boundary and cannot be removed by post-weld heat treatment.However,crack-free joints can be obtained in as-welded and post-weld heat treated conditions.The micro-hardness and tensile properties of the joints can be significantly improved by both Tl(1120 ?/4h+800 ?/20h)and T2(1050 ?/2h+800 ?/20h+700?/20h)post-weld heat treatment regimes,however,the ductility of the joints is decreased.The average micro-hardness of the as-welded joint fusion zone is about 250Hv,and that in the base metal and HAZ are about 300Hv.However,after Tl and T2 heat treatment,the micro-hardness profile across the joint become harmony with the average values of 380Hv and 400Hv,respectively.Meanwhile,the tensile strength coefficient of the joint in as-welded condition is 76%whereas in post-welded heat treated condition is 100%.The microstructures,interfacial structures and mechanical properties of the dissimilar alloys joints between K4750 and GH536 alloys were investigated on the bases of the K4750 similar alloy joint.The results show that both the K4750 filler material and the GH536 filler material can be applied to join GH536 and K4750 alloys successfully.The carbides formed in both filler materials fusion zones are MC type carbides.The elemental segregation in joint fusion zone with GH536 filler material is more severe than that with K4750 filler material.At the same time,element A1 segregated to dendrite boundary at GH536 filler material fusion zone while to dendrite core at K4750 filler material fusion zone.The structural transition areas are occurred at interfaces between the K4750 base metal and the GH536 filler material,and between the K4750 filler material and the GH536 base metal,respectively.And the transition area at the interface with GH536 filler material is wider than that with K4750 filler material.The deformed grains and the residual strain are mainly concentrated at the transition area.Both eutectic transformation of M6C carbide and grain growth were taken place at GH536 alloy side HAZ during welding.The GH536 alloy grain growth after T1 heat treatment is more serious than that after T2 heat treatment.Micro-hardness and mechanical properties of the dissimilar joints can be significantly improved by T1 and T2 post-weld heat treatment regimes,however,ductility of the dissimilar joints is decreased.Compared with GH536 alloy,the minimum tensile strength coeff-icient of the dissimilar joints is 98%,while the maximum is 108%.Both post-weld heat treatments are detriment to the tensile ductility of the dissimilar joints.However,elongations of the dissimilar joints are larger than 10%.The joints with K4750 filler material exhibit optimum comprehensive mechanical properties after T2 heat treatment.The microstructure,phase precipitation and cracking susceptibility in the joint of 850 alloy were characterized.Liquation cracking and SAC will occur at the 850 alloy repaired joint HAZ,and the size of the SAC is larger than that of the liquation cracking.Post-weld HIP treated can heal the liquation cracking and the SAC successfully.However,the SAC healed trace which is concentrated with B,Ti and Zr can be detected.The HAZ cracking susceptibility of the 850 alloy can be successfully evaluated by STF test.The fracture ductility and the rupture time of the pre-solutionized 850 alloy simulation HAZ are smaller than that of the pre-overaged 850 alloy.The minimum fracture ductility and the rupture time of 850 alloy simulation HAZ are at the temperature between 850? to 950?.Meanwhile,as the thermal cycle peak temperature increased,the fracture ductility and the rupture time of the pre-overaged 850 alloy simulation HAZ are gradually decreased.Welding process optimization experiment of 850 alloy was conducted.And the results show that liquation crack-free joint can be achieved when the welding current is 70A.When the welding current is over 90A,the amount of liquation cracking is increased with the increasing of welding current.The welding residual stress increase not only with the increasing of welding current but also the content of the strengthen precipitate in filler material.At the same time,the residual stress in the joints with pre-solutionized alloy is bigger than that in the joints with pre-overaged alloy.The SAC can be formed during heating and cooling procedure in the post weld heat treatment.The increase of the residual stress and the difference of thermal expansion coefficient(TEC)between the filler material and the base metal contribute to the increase of the joint SAC susceptibility.The SAC susceptibility can be reduced by decreasing welding current and post-weld heat treatment cooling rate. |
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