Studies on the grinding characteristics of directionally solidified nickel-based superalloy

Studies on the grinding characteristics of directionallysolidi®ed nickel-based superalloyChen Ming *,Li Xiaotian,Sun Fanghong,Xiang Yucheng,Xue BingyuanSchool of Mechanical Engineering,Shanghai Jiao Tong University,1954Hua Shan Road,200030Shanghai,PR ChinaReceived 18November 1999AbstractDirectionally solidi®ed nickel-based superalloy DZ4is a kind of very important structural material nowadays produced domestically.Grinding burn usually occurs in the grinding process,thus being an obstacle to further promoting grinding ef®ciency and grinding precision.This paper has discovered the grinding mechanism of this new type superalloy on the basis of systematic analyses of the variable characteristics:the grinding force,grinding temperature,topography of the ground surface,hardness distribution of the surface layer,as well as morphology of the surface layer from a metallographical viewpoint.The results will be bene®cial in the grinding of this new type superalloy with high ef®ciency and high precision.#2001Elsevier Science B.V .All rights reserved.Keywords:Grinding burn;Nickel-based superalloy;Grinding mechanism1.IntroductionNickel-based superalloy is mostly used as the material of aeroturbine blades for its high strength and high heat-resistance resulting from the regular dispersion of g H -phase.The operating temperature of aeroturbine blades need to be promoted high enough for the engine to create great pro-pulsive force and attain high combustion ef®ciency.The main method to secure a high operating temperature is to increase the volume ratio of the g H -phase and its solid solution temperature,but it is impossible to produce such alloy by hot-forming technology because of its narrow interval between the g H -solution temperature and melting point.Precision cast technology has proved feasible but,unfortunately,this technology may cause the decrease of the extensibility of the alloy.It is found that almost all the damage of equiaxed crystals obtained in the process of traditional cast technology such as high temperature creep and heat fatigue damage is related to the grain boundaries perpendicular to the axis of stress.By directional solidi®cation,a kind of newly developed cast technology,the blades can acquire columnar crystals with preferred orientation parallel to the stress axis,thus increasing the plasticity and ductility of the alloy by eliminating most of the grain boundaries perpendicular to the stress axis [1].DZ4is the directionally solidi®cated alloy nowadays developed domestically,which has been used on aeroturbineblades of a new model ®ghter plane.It is a kind of typical dif®cult-to-cut material with the smallest relative cutting coef®cient.In the process of manufacture of the cogging joint of aeroturbine blades,the demands on shape and dimen-sion precision as well as surface roughness of the cogging part of the blades are so high that creep feed grinding techno-logy is commonly used.In the process of grinding the cogging joint of a DZ4blade,grinding burn of the workpiece is very likely to take place on account of its high temperature strength,easy hardening and low heat conductivity.In this paper,experimental research on the grinding characteristics of DZ4was conducted systematically,espe-cially on grinding burn,enabling the authors to provide a theoretical and experimental basis for the optimization of the grinding process.2.Experimental conditions and measuring system The grinding force is measured by a Kistler force measur-ing system.The system consists of a 9272piezoelectric quartz force sensor,a 5019B charge ampli®er and a 12bits A/D converter/sampler CIO-DAS1602/12.The force trans-ducer can measure three pair-wise orthogonal forces F x ,F y ,F z and a torque M z .The charge ampli®er can deal with the four measured signals simultaneously with adjustable magni®ca-tion.The A/D converter/sampler is plugged into an ISA slot of a PC and the signals can be used with DynoWare,which is a special data processing software developed by theKistler.Journal of Materials Processing Technology 116(2001)165±169*Corresponding author.0924-0136/01/$±see front matter #2001Elsevier Science B.V .All rights reserved.PII:S 0924-0136(01)01024-XThe piezoelectric quartz transducer and charge ampli®er have a high measuring precision,sensitivity and dynamic response rate.The sensitivity is 7.65pC/N for tangential force and 3.55pC/N for normal force.The linearity distor-tion is less than Æ1%and the cross-talk between the tangential and normal force is less than Æ2%.The tangential natural frequency is 4000Hz and the normal natural fre-quency is 7000Hz,and so the force measuring system is suitable for dynamic or quasi-static force measuring.The grinding temperature is measured by a quasi-arti®cial constantan±DZ4electric thermo-couple,the cross-section of thermo-couple being 0:5mm Â0:1mm.The thermo-couple is calibrated by a high precision quick calibrator designed purposely by the authors.The result of calibration is as follows:T 13:74Uerror <5%where U is the electric potential produced by the thermo-couple (mV),and T the measured temperature (8C).The temperature signals were recorded using a Hp35665A dynamic signal analyzer which has high sensitivity and interference rejection.It can pick up weak signals of no more than millivolts,so a preampli®er is unnecessary.The analyzer can dynamically display the waveform of the signal and the signal can also be saved as a data ®le for further analysis by the PC.A diagram of the measuring system is shown in Fig.1.The surfaces of the ground test-pieces were polished by mechanical and electrolytical methods and acid-etched in sequence,the EDAX SEM (with energy spectrum analyzer)being used to scan the surface topography and the metallo-graphy of surface layer.The hardness distribution of the surface layer was measured using an HX-1000microhard-ness tester and the residual stress of the surface layer was measured by an x-350A X-ray stress analyzer.3.Experimental results 3.1.Grinding temperatureThe grinding temperature distribution in the grinding contact zone usually presents a curve consisting of a randomdistribution of ``peak point temperatures''on the base of an ``envelope temperature''.The envelope temperature is the average result of all effective abrasive particles in the contact zone,whereas the ``peak temperature''is caused by the process of individual abrasive grains cutting over the thermo-couple joint.The grinding temperature mentioned in this paper is the maximum value of the envelope tem-perature,which is the direct reason for grinding burn [2].Fig.2shows the grinding temperature signal curve for different feed rates.With high feed rate,as shown in Fig.2(a),the rapid rise of grinding temperature has apparent points of start and end,from which the length of the grinding contacting arc can be estimated.When the feed rate is relatively low,as in Fig.2(b),the grinding temperature shows the typical form of envelope temperature and peak temperature.The number and average spacing interval of effective grains can be evaluated by the time intervals of peak temperature points.3.2.Grinding forceThere are obvious variations of both tangential and nor-mal grinding forces when grinding burn appears,as shown in Fig.3.As shown in Fig.3,when grinding burn takes place,there are apparent jumps of both normal and tangential grinding force.The grinding force ratio F n /F t keeps approximately constant before grinding burn,but jumps from 3.5±3.7toFig.1.Diagram of the measuringsystem.Fig.2.Grinding temperature signals under different feed rates:(a)fast feed V w 17:72mm =s ;(b)slow feed V w 1:453mm =s .166 C.Ming et al./Journal of Materials Processing Technology 116(2001)165±1694.2±4.5when grinding burn occurs,as shown in Fig.4.The reason for the phenomenon is that when grinding burn appears,DZ4alloy is softened by high temperature and its adherence to grinding wheel is severe,and so the surface layer of the test-piece is strongly extruded,which causes an increase of normal grinding force.Because the force ratio F n /F t is a non-dimensional parameter,it can eliminate the variations caused by the change of grinding parameters,such as feed rate and grinding depth.It is a valuable characteristic parameter for monitoring grinding burn in real time [3].3.3.Surface topography of the test-pieceThe surface of the test-piece presents the color of light yellow when light grinding burn takes place.With the degree of burn increasing,the color of the surface turns from light yellow to a snuff color,even to a yellow-black color on some locations.Fig.5(a),(b)and (c)are SEM photographs of the surface topography of test-pieces in the normal state,the light burn state and the severe burn state,respectively.As Fig.5(a)shows,in the normal state,veining on the surface is clear,with straight edges,whereas in the light burn state,the veining turns rough and the edges become curved,as shown in Fig.5(b).This indicates that the plastic ¯ow of the DZ4alloy around the abrasive grains is enhanced and the alloy is extruded to the side of the abrasive grains.When severe burn appears,DZ4alloy is softened by high temperature,the alloy under the abrasive grains undergoing plastic ¯ow together with the alloy around the abrasive grains.The plastic ¯ow of the alloy under the abrasive grains leads to many scale-shape areas on the surface of the test-piece,as shown in Fig.5(c).There are some microcracks at the root segments of these scale-shape areas.At the edges of the scale-shape areas,tiny metal balls can be found which were formed by a small quantity of melted DZ4alloy generated in the grinding process.This indicates that the grinding tempera-tures at some points of the test-piece surface may exceed the melting point of DZ4alloy.3.4.Residual stress of the surface layerWhen grinding burn occurs in the grinding of a DZ4test-piece,the surface layer experiences severe plastic tensile strain,and the material will expand under high temperature.These factors can result in high residual tensile stress on the test-piece surface.Table 1lists the X-ray measurement results of the residual stress of the test-piece surface before and after grinding burn.Before grinding burn,the residual tensile stress 10m m below the surface of the test-piece is only 13MPa,butFig.3.Variation of grinding force when grinding burnoccurs.Fig.4.Variation of grinding force ratio when grinding burnoccurs.Fig.5.Surface topography of test pieces (2000Â):(a)normal;(b)light burn;(c)severe burn.C.Ming et al./Journal of Materials Processing Technology 116(2001)165±169167amounts to 290MPa for the severe grinding burn state at the same position.When grinding burn appears,the squeezing action of the abrasive grains is enhanced,and large friction between the grains and the test-piece due to the high adhesiveness of DZ4leads to severe tensile strain and tensile stress of the surface layer.The surface layer also expands under the high grinding temperature.When the grinding temperature drops rapidly,the expanded materials shrinks,so that residual tensile stress is produced when the shrinkage of the outer materials is hindered by the inner materials.Generally,high grinding temperature,large temperature gradient and high yield limit of the material can lead to high residual stress.In grinding DZ4alloy,the grinding tempera-ture and temperature gradient is high for its high temperaturestrength and low heat conductivity.DZ4alloy also has high yield limits,and all these factors lead to high residual tensile stress of DZ4alloy when grinding burn appears.Fig.6(a)is a microscopic metallograph of normal DZ4alloy,from which can be seen the directional arborescent structures of DZ4.Fig.6(b)shows the interdendritic g H -phases of DZ4,the white objects in the ®gure being g ±g H -eutectics.Interdendritic g H -phases are relatively coarse and present quadratic form and irregular arrangement,whereas dendritic g H -phases are relatively ®ne and regularly arranged.Fig.7is a microscopic metallograph of a burned surface layer.The black areas in Fig.7(a)are dendritic crystals,surrounded by interdendritic structures that appear gray in the photograph.The crystals are larger than normal ones and have larger separations from each other.This indicates that the grinding temperature has approached or even exceeded the melting point of DZ4at some locations,and a small quantity of melted materials has been generated on the test-piece surface.The borides in DZ4(0.012±0.025wt.%)are generally primary M 3B 2,which is a eutectic with low melting point.The melted M 3B 2can promote the dissolution of eutecticTable 1X-ray measurement results of residual stress State Stress,s (MPa)Severe burn 291Normal13Fig.6.The metallic phase of DZ4in the normalstate.Fig.7.The metallic phase of DZ4when grinding burn occurs.168 C.Ming et al./Journal of Materials Processing Technology 116(2001)165±169g H -phase in the DZ4[4].As eutectic g H and M 3B 2melt,the melting point of the DZ4matrix of the g -phase rge dendritic crystals are produced when the melted metals are re-solidi®ed.The melting of carbide and boride on DZ4grain boundaries are shown in Fig.7(b).Large dendritic crystals in the surface layer severely affect the uniformity of performance of DZ4.The high grinding temperature,which approaches DZ4s melting point,causes the solution of the g H -phase,which is the major strengthening phase in DZ4.After the grinding process,the temperature of a test-piece surface with burn color falls quickly below 7008C,which is far lower than the ageing temperature of DZ4,so that the soluted g H -phase cannot be fully precipi-tated.The strength of the DZ4will be reduced due to the decrease of g H -phase.The hardness distribution of burned surface layer was also measured,the measurement results being shown in Fig.8,which indicates clearly the weakening of the grinding burn layer.4.Conclusions1.In grinding DZ4alloy,there are definite correspon-dences among the grinding temperature,the topographyand the color of the test-piece surface.When the grinding temperature exceeds 11008C,a yellow-black color appears on the test-piece surface,and the topography and mechanical properties of the surface layer deteriorate rapidly.2.When grinding burn appears,the residual tensile stress of the surface layer is relatively high.3.The grinding force ratio F n /F t changes remarkably as soon as grinding burn takes place and can be used as the characteristic parameter for real time grinding burn monitoring.4.When grinding burn appears,the metallic phase of the surface layer changes.The main strengthening g H -phases are soluted into the g -phase,and large arborescent structures are produced,which cause deterioration of the strength and hardness of the DZ4alloy,the depth of the affected layer exceeding 0.5mm.AcknowledgementsFinancial support by the National Natural Science Foundation of PRChina is greatly appreciated.References[1]R.Trivedi,W.Kurz,Dendritic growth,Int.Mater.Rev.39(1994)49±74.[2]C.Ming,W.Yanchun,R.Xiaojian,Experimental study on surface andsub-surface quality during grinding of nickel-based superalloy,in:Proceedings of the First International Conference on Quality and Reliability,1995,pp.31±34.[3]P.Xuefeng,C.Ming,L.Wei,Studies on mechanisms and improve-ment of workpiece burn in grinding of difficult to grind metals,in:Proceedings of the Sixth International Manufacturing Conference,China,V ol.1,1993,pp.31±35.[4]L.Sun,Q.Wu,J.Zhang,Z.Hu,Mc morphology in a direc-tionally solidified nickel-based superalloy,Mater.Lett.17(1993)360±363.Fig.8.Hardness distribution of the burnt layer.C.Ming et al./Journal of Materials Processing Technology 116(2001)165±169169。