Solidification Processing凝固处理.ppt

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,Solidification Processing,Liao,Hengcheng,Southeast University,School of Materials Science and Engineering,10-11 Autumn,Tel:[phone],Room:430,Email:,hengchengliao,Knowledge,Product,Performance Requirement,Components,Property and microstructure Design,Processing,Ingot or casting,Microstructure Design,Alloy,Composition Design,Solidifying,Integrating,Casting,Machining,Forging,Rolling,Extruding,Drawing,Ingot,Al-11.6%Si alloy,Chapter 1,Hot Flow in Solidification,1.1 Growth of Single Crystals,Three categories to produce single crystals from melts,Normal Freezing,The entire charge is melted and solidified from one end.,Horizontal boat,Bridgeman,method,Figure 1.1a,Zone melting-Zone freezing,Only a small zone of crystal is melted at any time.,To melt initially only a portion of the charge and move this molten zone slowly through the charge.,Floating zone,Figure 1.1c,3 Crystal pulling,Czochralski,method,A large of charge is melted and a small crystal withdrawn slowly from it.The crystal is rotated slowly as it is pulled.,Figure 1.1b,The basic heat flow objectives are:,1 to obtain a thermal gradient across a liquid-solid interface which can be held at equilibrium;,2 subsequently to alter or move this gradient in such a way that the liquid-solid interface moves at a controlled rate.,A heat balance at a planar liquid-solid interface:,Thermal conductivity of solid metal,Thermal conductivity of liquid metal,Temperature gradient in solid at the liquid-solid interface,Temperature gradient in liquid at the liquid-solid interface,density of solid metal,Heat of fusion,Growth velocity,G,L,G,S,1,R,is dependent,not on absolute thermal gradient,but on the difference between,K,S,G,S,and,K,L,G,L,.Hence,thermal gradients can be controlled independently of,R,.,2,R,would be at maximum when,G,L,becomes negative;However,good crystals cannot be grown in,undercooled,liquids,and so the practical maximum,R,occurs when,G,L,0.,Evaluate,G,S,Supposed:,1 crystal is circular cross section;,2 heat transfer from the crystal to surroundings is by convection;,3 growth is steady state;,4 temperature gradients within the crystal transverse to the growth direction are low.,Consider a cylindrical element in the solid crystal,dx,in the thickness,moving at R of the liquid-solid interface.,Example:floating zone(,crucibleless,)crystal growth,Solid,Liquid,dx,0,x,Net heat change from conduction,Net heat change from loss to surrounding,Net heat change from moving boundary,Thermal diffusivity of the solid crystal,S,=,K,S,/,S,c,S,Specific heat of solid metal,Ambient temperature,Temperature at,x,Heat transfer coefficient for heat loss to surrounding,Radius of crystalDistance from liquid-solid interface,Solid,Liquid,dx,0,x,Boundary conditions:,Solid,Liquid,dx,0,x,1.2 Solidification of Castings and Ingots,Heat flow is not at steady state.,Hot liquid is poured into a cold mold;Specific heat and heat of fusion of the solidifying metal pass through a series of thermal resistances to the cold mold.,Thermal resistances which,in general,must be considered are those across the liquid,the solidifying metal,and the metal-mold interface and those in the mold itself.,Distance,Temperature,T,M,T,0,air,mold,solid,liquid,1.3 Casting Processes Employing Insulation Molds,Sand casting,Investment casting,The important characteristic of solidification of a metal:,The metal is a much better conductor of heat than the mold.,The solidification rate depends primarily on the thermal properties of the mold;,The thermal conductivity of the metal has practically no influence on the heat-flow.,The mold can be considered to be,semi-infinite,in extent,i.e.,the outside of the mold does not heat up during solidification.,Assume:the metal is poured with,no superheat,that is,exactly at its melting point,T,M,Distance,x,T,M,T,0,mold,solid,liquid,S,Temperature,0,Distance,x,T,M,T,0,mold,solid,liquid,S,Temperature,0,A transient one-dimensional heat-flow problem,Temperature of the mold at,x,Distance from mold wall(negative into the mold),Time,Thermal diffusivity of mold,m,=,K,m,/,m,c,m,Boundry,condition:,x,=0,T=T,M,;,x,=-,T=T,0,Initial condition:,t=0,T=T,0,The rate of heat flow into the mold at mold-metal interface:,Rate of heat flow,Area of the mold-metal interface,Note:the heat entering the mold comes only from heat of fusion of the solidifying metal since both the solid and liquid metal are exactly T,M,metal,mold,Rate of absorbing heat of the mold,thermal diffusivity,S,K,S,/,S,c,S,heat diffusivity,Applicable case:,a metal cast into a relatively insulating mold,Veracity:,more accurate for sand castings of high conductivity such as nonferrous metals(Cu-,Mg-and Al-base alloys)than for iron and steel,Deduction:,1 high melting temperature and low heat fusion favor rapid solidification;,2 the solidification rate is initially very rapid and decreases as the mold becomes heated.,Figure 1.7,For semi-infinite plane mold,T in the mold:,Distance,x,T,M,T,0,mold,solid,liquid,S,Temperature,0,Employing Insulation Molds,Complex shapes:,The contour of the mold wall has some influence on its ability to absorb heat.,Heat flow into the concave surface will be divergent and therefore slight more rapid,and into the convex surface less rapid than into a plane wall.,Plane wall;concave surface;convex surface,For a simple shapes,the differences will not be large.,A useful approximation,Assume,:a given square centimeter of mold surface has a fixed ability to absorb heat regardless of its contour or location on the casting.,Volume solidified of solid metal at time,t,Area of the mold-metal interface,The total solidification time of a casting of volume V,Chvorinovs,rule,A constant for a given metal-mold material and mold temperature,Experimental confirmation:,Figure 1.8,For simple shape castings:spheres or cylinders,Relation of,t,f,V,/,A,without retaining the assumption of non-divergency of heat flow,Controlling equation:,r=casting radius;,n=1for cylinder,2 for sphere,More exact expression,Approximate expression,Validity?,Validity?,1 the simple approximation becomes increasingly valid as,K,m,decreases;,2 it is also more nearly valid for cylinder than for sphere.,Deduction:For a given volume-to-surface area ratio,a sphere freezes more rapidly than a cylinder and a cylinder more rapidly than a plate.,1.4 Casting Processes in Which Interface Resistance is Dominant,Permanent mold casting,Die casting,Splating,casting,The mold-metal interface resistance is of overriding importance,controlling the heat flow to significant extent.,Temperature,Distance,x,T,M,T,0,mold,solid,liquid,S,0,All temperature drop is across the interface,Rate of heat flow across this interface for metal poured at its melting point,T,M,Heat transfer coefficient of the mold-metal interface,Generalizing for simple shaped castings,Shape in no way alters the heat transfer across the interface,The mold,being assumed infinite in extent,remains at its original T,0,Validity?,Conditions for validity:,The resistance to heat flow across the mold-metal interface is large compared with other resistance in the metal and mold.,Except when the mold is relatively insulating:,when the mold is relatively insulating,the added necessary condition:,Die casting,Permanent mold casting,Heat transfer coefficient are in the range from 0.04 cal,cm,-2,C,-1,s,-1,(for simple thin graphite-base washes)to much lower values for insulating washes,Splat cooling,Heat flow is generally interface-limited.,1.5 Analytic Solutions for Ingot Casting,For many ingot-casting,Thermal resistance of the metal,mold-metal interface,mold,and mold surroundings must be considered for complete solution.,Limiting cases:,1 heat flow is one-dimensional;,2 mold-metal interface resistance is negligible;,3 the mold either is held at constant temperature(as by water cooling)or is very thick.,Distance,Temperature,T,M,T,0,air,mold,solid,liquid,Temperature,Distance,x,T,M,T,0,mold,solid,liquid,S,0,Distance,x,T,M,T,0,mold,solid,liquid,S,0,T,S,Resistance of solidifying metal is controlling heat flow,Combined resistances of solidifying metal and mold are controlling,Water-cooling chill,Non-water-cooling chill,Considering the water-cooled chill mold,Controlling equation,Integration constant,Carslaw,and Jaegers solution,Temperature in the solidifying metal,Boundary condition:,Temperature,Distance,x,T,M,T,0,mold,solid,liquid,S,0Considering the non-water-cooled chill mold,Controlling equation,Integration constant,Temperature in the solidifying metal,Temperature of mold-metal interface,Boundary condition:,Distance,x,T,M,T,0,mold,solid,liquid,S,0,T,S,insulating mold,Temperature in the mold,Water-cooled chill metal mold,Temperature in the solidifying metal,Non-water-cooled chill mold,Temperature in the solidifying metal,With zero superheat and zero heat resistance of metal-mold interface;,Semi-infinite plate mold wall,Calculated results,experimental results,The calculated results agree well with experimental measurements made of unidirectional solidification against metal chill walls except that the experimental curves obtained are generally displaced slightly on the time axis.,Water-cooled chill metal mold,Non-water-cooled chill mold,The apparent delay in beginning of solidification?,1 Convection during and just after pouring results in rapid removal of superheat from the liquid,thus slowing the start of solidification and its initial rate;,2 The second effect is finite mold-metal resistance to heat transfer.,Solidification rate is actually finite at zero time but is interface-controlled and so proceeds at a slower rate than in absence of this resistance.Initially,the amount solidified increases linearly with time.Later,as resistance of the solidifying metal becomes large compared with mold-metal resistance,the amount solidified increases linearly with the square root of time.,1.6 Solidification of Alloys,Pure metals,:solidifying at a discrete melting point;,Alloys,:solidifying over a range of temperature rather than at,Carslaw,and Jaegers analytic solution,Unidirectional solidification,Non-water-cooled chill,Assume:semi-infinite metal and mold,no interface resistance,constant thermal propertiesand heat of fusion distributed evenly over the solidification range.,Other methods:,Approximate analytic techniques;,Integral profile methods;,Numerical solution techniques,A fully solid skin forms immediatel1y,and both the,liquidus,and,solidus,isotherms then move linearly with the square root of time.The velocity of the,liquidus,isotherm is not affected by approaching the upper end of,the ingot,but the velocity of the,solidus,isotherm is significantly altered by this end effect.The velocity begins to increase shortly after the,liquidus,isotherm reaches the end.,Campagnas,numerical solution:Al-4.5%Cu alloy unidirectional solidification,No interface resistance and superheat,water-cooled chill,h=0.04 cal,cm,-2,C,-1,s,-1,The presence of interface resistance alters behavior of the isotherms only in the vicinity of the chill face.Here,initial rate of isotherm movement is slowed,and there is a finite delay time before movement of the,solidus,isotherm begins at all.,No interface resistance,Mushy zone,Region of solid and liquid coexisting,Casting characteristics such as feeding,hot tearing,and,marcosegregation,are strongly influenced by the width of the mushy zone,t,f,Local solidification time,Local solidification time is inversely proportional to average cooling rate at a given location during solidification.,Important aspects of the solidification structure(including dendrite arm spacing and inclusion size)depend strongly on this local solidification time.,In the case of finite,h,coefficient,liquid remains in contact with the mold surface a finite time during solidification.This is a necessary condition for formation of certain types,macrosegregation,including inverse segregation and exudation.,