Research On Microstructure And Process Optimization Of Directional Solidified Turbine Blade From DZ125-X Nickel-based Superalloy

As a key component of aircraft engines,turbine blades work in high service temperature and under complex stresses,which makes them the worst hot-end components.With continuous increase of thrust-weight ratio in aircraft engines,service temperature and working load of turbine blades are continuously rising.The manufacture of high performance directional turbine blades has become the pivotal technology of aircraft engines development.Therefore,research of the preparation and structure optimization process of directional turbine blades has great practical significance.In this paper,the turbine blades from a new type of nickel-based directional solidification superalloy(DZ125-X)were prepared with high rate solidification process(HRS).The influences of main process parameters on the blade structure were studied;Furthermore,the organization evolution of the blade at different stages during solidification process were observed and analyzed;Finally,the dendrite segregation and shrinkage defects in the as-cast microstructure of the blade were observed and analyzed;In additon,the heat treatment processes of the blades and their influence on the mechanical properties of the blade body were also studied.The research results are as follows:For the new directional superalloy and the selected blade structure,the growth direction of crystals in the directional solidified blade deviates from the stress axis,if the withdrawal rate is not suitable.Too slow withdrawal rate causes crystal breakage probably.If the withdrawal rate is too fast,stray grains occur easily.With the increase of withdrawal rate,the primary and the secondary dendrite arm spacings become smaller,the eutectic content decreases first and then increases,and the size of ?' strengthening phase decreases.As the holding temperature of mould increases,the crystal number of blade body is increased,the primary dendrite spacing and the size of ?' strengthening phase are decreased and the eutectic content is reduced.It is showed that the lead crystal section advances with the solid-liquid interface,the number of crystals decreases,the orientation is optimized,and the spacing of primary dendrite increases gradually.From bottom to top,the crystals in the roots of the blade are introverted to grow,and some crystals on the blade body are scattered.With the increase of solidification height and wall thickness,the primary dendrite arm spacing becomes larger and the eutectic content increases.The ?' strengthening phase size increases with the increase of wall thickness.Studies on dendrite segregation and shrinkage in directional solidified turbine blades showed that Ti,Al,Ta and Mo are positive segregation elements,and Cr,Co and W are negative segregation elements.The degree of dendritic segregation increases with the increase of solidification height of the blade body.Macroscopic shrinkages usually occur between the dendrites along the direction of gravity,sometimes forming a “grooved” tunnel shrinkage on the blade body,and microscopic shrinkages occur in the vicinity of the interdendritic eutectic structure,with irregular strips shape,circle and triangle,and with the increase of the solidification height of the blade body,the overall level of microscopic shrinkages increases,and the single shrinkage size first increases and then decreases.The as-cast blade after 1285°C/2h+1305°C/2h+1325°C/12 h,AC,solution treatment,the size of ?' phase is about 0.30?m,eliminates the eutectic structure basically and reduces element dendritic segregation.If the temperature for high temperature aging is too high and the aging time is too long,it causes the ?' phase edge passivation,the cube decreases,and the ?-phase is precipitated three times ?' phase.Low temperature aging treatment at 870°C/24 h can effectively adjust the cube and volume fraction of the ? ' phase.The microhardness distribution of the blade body sample after heat treatment was uniform to 500 HV.The tensile strength of the as cast and heat-treated state at 980 °C was 486.5MPa and 558.6MPa respectively,and the elongation was 8.7% and 16.1% respectively.At 1000°C the longevity life of as cast and heat-treated samples under 140 Mpa is 51.2h and 108.7h respectively,and the elongation is 12.8% and 19.2% respectively.After heat treatment,the mechanical properties of the blade are significantly improved.