Microstructure Evolution Of AM3 Directionally Solidified Superalloy Under Different Withdraw Conditions

Nickel base directionally solidified superalloy has excellent comprehensive mechanical properties in high temperature environment,so it has a wide and important application in the fields of aviation,aerospace,land transportation and ships.It is well known that in the directional solidification process,the drawing rate is the key process parameter to adjust the solidification rate,control the as cast structure and even affect the final performance of the casting.In this paper,liquid metal cooling(LMC)directional solidification process was used to study the effects of different withdraw conditions on dendrite,y' phase,carbide,eutectic and segregation of the first generation nickel base superalloy as cast and heat treated AM3 as the research object by using six different withdraw rates and two variable speed drawing methods.The main conclusions are as follows:(1)With the increase of drawing rate,the solidification interface morphology has experienced the evolution rule from flat to cellular to coarse to fine.The dendrite structure gradually becomes smaller,the primary dendrite spacing and secondary dendrite spacing also become smaller,and the relationship ?1=288V-0.16 and ?2=88 V-0.25 are obtained respectively.With the increase of drawing rate,the size of ?' phase decreases and the cubic degree increases.The size of ?' phase between dendrites is more irregular and coarser than that of dendrites,but with the increase of drawing rate,the difference between them increases first and then decreases.(2)With the increase of drawing rate,the segregation of elements increases first and then decreases.The maximum segregation peak value is achieved at the pull rate of 100?m/s.With the increase of drawing rate,the size and volume fraction of carbide and eutectic become smaller and smaller.At low speed(<100?m/s),the volume fraction of carbides increases with the drawing rate,but at high speed,the volume fraction does not change with the drawing rate.The volume fraction of eutectic has a linear positive correlation with the drawing rate.(3)The dendrites of alloys drawn at variable speed are smaller and more compact than those drawn at constant speed.The results show that the relationship between the primary dendrite spacing and the instantaneous drawing rate is ?1 104.58V-0.045 and ?1 104.45V-0.042,respectively.The size of ?' phase in the variable speed drawing alloy is smaller than that in the uniform speed drawing alloy,and the size of ?' phase decreases with the increase of instantaneous drawing rate.(4)It is easier to form eutectic under the condition of variable speed drawing,and the volume fraction of eutectic is positively related to the drawing rate.The volume fraction of eutectic in the alloy with variable speed drawing after a history of variable speed is larger than that of the alloy with uniform speed changing.The eutectic volume fraction of the alloy drawn by accelerating first and then decelerating is larger than that of the alloy drawn by decelerating first and then accelerating.In both cases,the degree of segregation is decreased.(5)Heat treatment A(1305?/3 h/A.C.+1080?/6h/A.C.+870?/20 h/A.C.)should be more suitable for the alloy drawn at a constant speed.Heat treatment B(1305?/3 h/A.C.+1080?/6 h/A.C.+870?/20 h/A.C.)should be more suitable for the alloy produced by variable speed drawing.The average size of the ?' phase of the heat-treated alloy obtained by a and B heat treatment systems decreases first and then increases with the increase of the drawing rate,and reaches the lowest value at 100?m/s.After heat treatment,the size uniformity of ?' phase reaches the highest value at 100?m/s.(6)With the increase of the pulling rate,the width of the ? channel becomes larger and larger,but the variation range is first large and then small,and the inflection point is 100?m/s.For the case of variable speed drawing,the width of ? channel increases with the increase of instantaneous drawing rate,and there is no obvious inflection point.Moreover,the width of ? channel of alloy with slow drawing is larger than that of alloy with accelerated drawing.With the increase of drawing rate,the integral number of ?' phase first decreases and then increases,reaching the lowest value at 100 ?m/s.The volume fraction of ?' phase decreases with the increase of drawing rate,and the size of residual eutectic decreases with the increase of drawing rate.