Lecture 14

[Music] [Music] welcome to the second part of solidification of parts in metal additive manufacturing series so in this lecture we will try to understand more on equilibrium and non equilibrium phases for solidification then theory and mechanism for solidification last time we saw very trivially the solidification zone freezing zone and the melting zone so we saw melting first freezing next and solidification so at each phase what happens so now we will go little bit in understanding the theory and mechanism for solidification so in equilibrium phase and in non equilibrium phase the last part of the lecture we will try to see what are raw metal additive manufacturing printed parts look like so when we talk about solidification of metal and we talk about equilibrium here after melting in conventional casting or laser slash electron beam additive manufacturing the liquid metal solidifies through cooling so cooling is one of the process through which the solidification starts happening and there is a heat transfer happening thermodynamically solid and liquid have the same energy at their melting point so both are steady so this is very important because when you try to do modeling of additive manufacturing part in a metal melting pool or in a molten pool then this point is very important thermodynamically solid and liquid have the same energy at the melting point now after the melting is done and cooling starts there is a thermodynamical instability as the liquid metal cools it loses latent heat low liquid temperature reduces the average inter atomic space between movable and disordered atoms very important low liquid temperature reduces the average inter atomic spacing so now slowly slowly slowly the atoms are going closer between the movable and disordered atoms as atom cools their interaction forces stops them from further moving apart and the liquid turns solid so atoms goes here and there randomly and then slowly slowly slowly now it is put in a constrained fashion like a small kid left in a ground it goes havac here and there with lot of energy but then moment it sees a ringing of a bell happening under school teacher coming in front slowly he gets confined and then he gets into his classroom so almost like that so much of free energy right so then as the atom cools their interaction forces stop them from moving apart and then liquid turns solid a large ingot of metal alloy is cast into a single crystal while a controlled solidification protocol is done so when you do that you can also have this grain orientation happening in this grain orientation happening you can try to have single crystal all throughout the in got which is used to be made for titanium so titanium ingots are made for turbine blade applications to have better mechanical and service condition response in practice it is difficult to transform a homogeneous liquid into a perfect homogeneous solid this requires knowledge at the armstrong micron miso and macro level that is why today additive manufacturing solidification part is done using molecular dynamic simulation tools these tools will try to give more understanding at micro scale meso scale and macro level macro and meso you can do it through any of the existing softwares like console other things can be used you can make when it goes to micro we always try to go towards molecular dynamic simulation and armstrong's is exactly only molecular dynamic simulation softwares the amstrom level atomic movement and conformation of the solid liquid interface are important whether the crystal is growing or not if it is not growing then you can use that time period for diffusing or doing something else if it has started solidifying then start properly using it so the heat is extracted the grains are formed and the solid is formed it is something like your quenching cycle you heat the liquid ah you heat the solid to a particular temperature try to deform it whatever you want and instantly quench it in water so then when you quench it in water you freeze right in the same way you start playing with this amstrom level atomic movement you start playing it to get the output here friends since metal is in a powder form the surface area is large volume is too low the laser energies are very high the melts controlled solidification can be done very easily whether the crystal is growing or not also crystal growth and nucleus formation should be studied this depends on the thermodynamics free energy available and the stability of the nucleus it is important to study the micro level redistribution of solvent if solid crystal structure have different chemical composition than the liquid so once the different phases are getting formed then there is a solidification difference which is happening so the material gets redistributed and then it is trying to get formed this can lead to undesirable segregation or extreme refinement of the solid so now you see this undesirable segregation or extreme refinement of the solid leads to better mechanical properties which must consider the situation where the solid liquid interface becomes unstable and form cellular or dendritic structure or crystal imperfections very important if you understand this then you try to play with the laser matter interaction what is the laser matter interaction laser power with respect to time whether to stop for micro second millisecond nano second or femtosecond at that spot and try to have multiple frequency hits to generate liquid and then you move in a feed rate such that this liquid is converted into a solid during this conversion it goes from liquid solid liquid interface and then it goes to solid so here when it goes into this transition again time and temperature comes into existence this time and temperature depending upon the composition tries to push people or the particles here and there forms precipitates this is what happens if you want to have equiaxed grains or if you want to have dendritic grains then you again play with the time so all these things are getting made in a fraction of a second in a given spot macro level solidification knowledge is applied to heat and fluid flow in liquid metal during cooling so whatever was a knowledge which we get from solidification point of view in casting the same thing can be brought in here the only difference is going to be the interaction time and the interaction area so non equilibrium let us see since additive manufacturing involves rapid melting of metal solidification theory is important for predicting and monitoring the process see while doing the additive manufacturing we studied about closed loop system you can put sensors and find out whether the fumes are created where the temperature is going high or where there is an explosion of the powder we saw that what is the reason for exploding the powder while additive manufacturing is done through a on a powder bed fashion so we saw that in the past lectures so predicting and monitoring the process so for predicting and monitoring apart from all these things you also use solidification theory laser or electron beam based additive manufacturing uses a moving heat source this moving heat source is a small diameter feed rate given when it is moving how is the heat getting conducted through the manufactured part with us with a short interaction time the laser powder bed fusion uses 4 meters per second laser with a spot diameter of 100 microns size so that you can try to do all this rapid melting and solidification in a small spot and 100 micron what we talk about is the spot size and our powder size varies from 15 micron to 100 micron so in a very very crude fashion you can see the spot diameter and the powder diameter is the same but generally you will not melt one powder because these powders will be oriented in different directions so you will try to melt three four powders and these three four powders when you melt it can go if you go along a straight line it also can melt in this direction in this direction so this makes an area and this area is a two d so there is also the laser waste which is there in the focal plane this is laser waste right so if you this heat is there so it you are trying to move along a line so the heat gets disseminated across laterally then it tries to have an area of melt this area of melt is 2d so when you try to put it in the powder bed fusion method it will have a volume so this is a 3d instant so this is called as the melt spool ok so this means 100 micron process zone is heated with its maximum temperature can be reached in less than 25 micro seconds within a second you can try to melt and if the laser is moving fast it starts solidifying so before that whatever we were thinking about rattling moving inter atomic distance freezing everything happens in 25 below 25 micro seconds so thermal conductivity is a very important parameter which tries to dictate the solidification suppose if the thermal solid conductivity is very low all the heat gets accumulated at that spot it does not allow the heat to go so then the melt spool will be existing for a longer time so longer time means then it will try to have a different non equilibrium conditions to do it for example in titanium what happens when you start measuring titanium entire accumulation of the heat happens at the mission zone itself it does not disseminate very fast this try to distort the tool so in the same way when we try to do copper powder the thunder conduct the thermal conductivity is extremely large heat applied it gets disseminated so you have a larger melt pool area so when you have a larger melt pool area the the grain the grain boundary the grain dynamics completely changes depending upon the thermal conductivity property of the powder the equilibrium and non equilibrium states gets achieved very fast due to thermal conductivity solidification and cooling can take twice as long as heating so this is a very very important property when many of us do modeling with for an additive manufacturing process we try to take the thermal conductivity of a solid and then we try to fit into the program but you should understand here you are trying to work with powder and this powder is not continuous it is discrete see when you try to take metal block it is continuous and here the thermal conductivity is different now here when we try to take powder it is completely different and the response for a slab or a powder is different the conductivity is different and here there is a discontinuity coming so here it is continuous so the modeling you will always try to have a error in the final prediction right so due to the substrate material the surrounding heat conduction convection regimes causes high solidification and cooling rate what are we talking about convection is something called as marangoni convection what is marangoni convection you have a melt zone in the melt zone this is heat or hot zone and this is cold zone so now there is a difference in temperature this difference in temperature tries to bring in some small circulation of heat so this heat circulation creates a vortex so this vortex is called as marangoni convection because of heat and cold zones you try to do this so that is what is the convection which is there regime causes high solidification and cooling rates the point heat source produce a highly localized heat flux in the melt pool zone and a massive temperature gradient in the deposition area so now it is very interesting you are trying to move along a line so this is a straight line so now if you try to have hatch patterns right if you have hatch patterns like this then it is a different story if you have something like out to in or into out hatch patterns then completely different different phenomenas are happening when the point heat source and point heat source you try to move to a moving heat source it is different so the point heat source produces highly localized heat flux point melting a powder or a set of powders in the milk pool zone and a massive temperature gradient true because there are four powders which are melt rest of the powders were about to be melt and the rest of the powders is perfectly powder at room temperature or at the bed temperature what we said in the deposition layer the substrate bottom has a higher temperature gradient than the deposition top so here at the bottom portion so of the layer when it is recently formed this will be of a higher temperature as compared to that of a new layer which is brought in in the powder bed fusion method for the laser to move on top of it so that is what the substrate bottom has a higher temperature gradient from here it goes down than the depositor top so when we are trying to work on alloy partition during solidification the alloy participation coefficient drops rejecting solute atoms at the solid liquid boundaries the solute concentration rises until the solution is stable the solidification temperature at the solid liquid boundary is affected by liquid composition process speed boundary angle relative to the heat source center line very important you see here solidification temperature at the solid liquid boundary is affected by liquid composition because alloys are mixed processing speed and boundary angle in am metallurgy is controlled by the chemistry of raw material and the thermal history of the material during manufacturing this is very important this makes the life of simulation experts difficult to bring in reality what happens during additive manufacturing still a long way to go in the research front also as far as powder bed fusion itself laser powder bed fusion and laser directed energy deposition may have different heat transfer mechanism so laser heating at the powder bed powder bed fusion and laser trying to have powder fed in either coaxially or laterally this is laser this is nozzle so laterally or actually the heat transfer mechanisms are different you cant take the same laser and electron beam is also different by the way its not the same heat transfer mechanisms ok but laser and electron beam have the same metallurgical principles for full melting selective laser sintering is different from selective laser melting cooling to ambient temperature after solidification controls the deposited layers early phase distribution and grain morphology so here when it cools from the freezing to the room temperature so here it tries to the deposited layers early phase distribution and grain morphology is very important the heat source speed power and size govern melt pool geometry which controls solidification kinematics important point heat source speed power size governs the melt pool geometry whether the milk pool also we assume it like a cylinder but in reality it will have a shape something like this so melt pool geometry is very important so this is dictated by the heat source speed power size governs the melt pools geometry which controls solidification kinematics i am not getting into details of solidification kinematics because that will be covered in few of the casting courses offered by moocs or nptel the thermal cycling and cooling control precipitation kinematics face growth and grain boundaries are different the thermal cycling and cooling controls thermal cycling is a single spot what happens when you try to have a laser powder second laser powder when it tries to melt it not only melts this top powder it also melts the second layer or which is already melt layer it tries to hit it and this portion almost tries to get to a semi solid state and there is a solidification happening so the thermal cycling and cooling controls precipitation kinematics face growth and grain growth when we try to compare the solidification of additive manufacturing with casting solidification in some cases additive manufacturing solidification differs from casting for additive manufacturing melt pool size ranges from hundreds of micrometer to millimeter regardless of the final components size and shape and is therefore different from casting with large melt pool area very different so here it is hundreds of micrometer to a millimeter small melt pool have less macro segregation than larger castings this is what i said you can dictate the product whatever you want and the quality and the mechanical properties of it in laser powder bed fusion the powder bed around the melt area has lower thermal conductivity than bulk alloy material due to voids between the particles laser and electron beam additive manufacturing processes can use a similar hypothesis am solidifies quickly metallic am products undergo a complex thermal cycle including directional heat conduction repeated heating and cooling and rapid solidification so in order to make our life simple we are trying to have this laser and electron beam because in electron beam it happens inside a vacuum you can still dictate the three of these processes but whereas in laser you have an ambience which is created but that is not as controllable as that of electron beam but in order to have a simplicity in life we try to take laser and electron beam have the same hypothesis these are the major complex thermal cycle phenomenas which include directional heat conduction repeated heating and cooling and rapid solidification there are mainly two reasons for rapid solidification enormous under cooling of the metal and fast moving temperature flux these two tries to give a rapid solidification what is enormous under cooling enormous under cooling is basically trying to go down rapidly cooling and then you come up we will see that in detail in the forthcoming slides rapid solidification is often accompanied by robust liquid flow such as marangoni convection which can reach 1 to 4 meters per second very high these factors does not support the assumption that microstructural development and properties are the same as conventional technique we try to understand casting very well we have over a period of time modeled casting process to a large extent very well now from that understanding we are trying to move to additive manufacturing where in which we see all these challenges enormous under cooling of the metal and fast moving temperature flux rapid solidification reduces elemental partitioning which extends solid solubility and causes metastable faces new new new faces which was not earlier possible in casting could be achieved by fast moving temperature flux due to heat conduction grain growth may be directional rapid and directional solidification and thermal induced phase transformation affects the microstructure of deposited parts rapid and directional solidification and thermal induced phase transformation affect the microstructure of the deposited part rapid solidification in the melt spool improves mechanical properties by allowing more structural latitude than in conventional manufacturing so rapid solidification is nothing but your something like your quenching its not equivalent but something like under cooling before nucleation and solidification growth causes unique structures so rapid solidification happens directional solidification happens you get this by doing thermal cycle so here the other important thing is the cooling rate we are only talking about heating heating but also the cooling rate it follows a fatigue cycle so heat cool heat cool heat cool so the cooling rate is vital to achieve the desired under cooling that eventually minimizes the increased temperature due to recollections the cooling rate is vital to achieve the desired undercooling that eventually minimizes the increased temperature due to recollections rapid solidification characteristics include microstructural refinement increased solubility limits reduced micro segregation and non equilibrium meta stable phase formation look at it you have something like a kitchen everything mixed rapid solidification characteristics include micro structural refinement increased solubility limits reduced micro segregation and non nuclear meta stable phase formation this meta stable phase sometimes can lead you to immiscible alloys also right so this metastable phase can also lead you to something like inter metallic faces forming compared to standard casting methods fast cooling reduces the formation of grains lamellae and secondary face particles dentitic segregation can be reduced to achieve compositional homogeneity in additive manufactured parts additive manufacturing solidification affects grain size shape distribution growth kinematics elemental segregation precipitation phase transformation and material properties all are dictated the key factor in the process are multi scale multi physics phenomena what is that fluid flow heat and mass transfer beam interaction with material causes additive manufacturing localized heating and cooling very important it is multi scale multi physics phenomena because solid gets converted into a liquid liquid will try to flow in the flow you have viscosity you have surface tension and when these two are there there is a temperature gradient coming then you have the heat and mass transfer happening then the beam keeps moving very fast beam interaction time so all these things make additive manufacturing process your robust and a good process to make sound products selectively in a flexible manner if you want to have on surface wear resistance property you just play with laser interaction time with material the laser can be used for cutting melting heat treatment it can also be used for polishing the laser interaction material with time and power you can do any of these things you can also do sintering all same with electron beam also you play with the energy densities you can get that so when we try to talk about laser so this is the laser coming this is a directed energy deposition method where there is a powder stream coming this is hitting the surface you have a substrate then you see the substrate as and when it is getting developed it is getting cooled so you have dendrites getting formed so this is the direction in which you are moving and this in order to protect the melt pool getting oxidized or explosive reactions happening so we try to put a shield around it so which is almost like a neutral gas which prevents it from getting interaction with the surface so you see here the powder particles again here powder particles you will have alloys when it melts you will have different different responses pure solidification time and alloy solidification time are different in alloys again segregation of the grain boundaries happen so you dictate the temperature whatever is getting formed so you not only melt the powder you also try to melt one more layer down and these two fellows get stitched because of the melt solidification process happening here so little bit on thermodynamics so thermodynamics can analyze solidification kinematics it evaluates alloy phase composition solidification method fundamental alloy properties and solidus liquidus line slope so all these things are part of thermodynamics and solidification kinematics pressure temperature composition affect materials free energy so here we use these two we do not use pressure when gibbs free energy is low equilibrium is reached under the assumption of thermodynamic equilibrium the equilibrium phase diagram can be used to estimate the solid liquid alloy composition by using the equilibrium phase diagram this equilibrium phase diagram is drawn for solid casting solid material or bulk material powder interacting with laser short time these equilibrium phase diagrams get tweaked little bit for additive manufacturing parts and especially with respect to metal additive manufacturing parts the materials structure a function of composition and temperature can be identified from an equilibrium phase diagram using a slow transformation rate or fast diffusion rate this is the typical cooling curve which we always discuss in casting process so you can see here a b is the cooling from liquid to freezing b and e so b e is the liquid solid happening here so you will have freezing going on and e to f will be solidification complete so you see here this is liquid this is solid so in between b and e there are so many phenomenas which happen and this so many phenomenas will try to dictate the output so you see here a b it is a melt pool you take an example a b is liquid then bc what will happen there is an under cooling which is necessary for initiating nucleation process then from c to d there is recalcins happen then d e is isothermal solidification all these things happen in casting as well as an additive manufacturing in the melt pool the time will be small so because of that rapid timing going on so you will have multiple new new faces coming into action so the cooling curve shows the phase transition temperature over time for a pure metal or alloy as they cool when you try to do it for a pure metal it is like this when you try to do for an alloy it is something like this so you will have this freezing will go like this so this is the freezing zone this is the solidification zone in the liquid and solidification zone there will be not much change except for the slope change but in the freezing zone there will be lot of change so it will not be isothermal solidification the pure liquid metal cools to its melting point before solidifying to understand the solidification of a pure metal super cooling curve is used where the liquid metal cools from a to b then from b to c must be under cooled since nucleation begins at the point c so till that time there will be a rapidly struggle of the fittest so nucleation will start it will try to expand the external temperature will kill so this will go back and forth so since nucleation begins at point c the liquid temperature rises due to latin heat of fusion so it rises so this is recall sense from the point c to d from here solidification temperature at d m until the point e the freezing zone the cooling rate indicates that the solid starts to cool and based on the atomic configuration solid and liquid metal having different thermal conductivity you have different different atomic configurations can happen different direction regions having different cooling curve slopes are seen before solidification begins the temperature falls below the equilibrium cooling called as supercooling to develop a thermodynamic driving force so when we try to talk about this is what we have been talking in plenty super cooling is also called under cooling so do not get confused supercooling is different undercooling both are the same this is what is undercooling where the liquid temperature is lowered below the melting point without solidifying pure metal cooling curve has zero slope constant temperature during solidification this is an ideal case but whereas when you do alloys this is what will happen for all engineering materials let it be bulk or let it be powder you will have an alloy so if you see it you will have melting then you will have a face change will happen start temperature phase change finish temperature and here is what a big paradigm shift happens from casting to additive manufacturing process and this henceforth here will be almost the same the alloy cools differently than the pure metal as shown here so here there is lot of in equilibrium thermodynamic stability there is thermodynamic kinematics is there to dictate what you want so when we look at the cooling curve and phase diagram you can see here this is liquid there is a solid and in between the liquid and the solid you have liquid as temperature line you would have a solid as temperature line and in between you have a mushy zone this mushy zone you try to dictate in additive manufacturing so if you try to take the cooling curve and the phase diagram the cooling curve from a two phase region the solid precipitates are delayed in liquid through a mushy zone so this is what is the liquid this is what is the solid so start of liquid to get into a solid so it is called as liquidus temperature the solid is called a solidus temperature so you can see here this is t l temperature of the liquid s line temperature of the solid s line temperature of liquid solid line also you can see here okay the metal alloy shows a mussy zone between solidus and liquidus temperature where solid and liquid phase occurs during solidification so here you can try to have various compositions and this various compositions with the heating cycle you can try to dictate your grain so if you see a cooling curve and a phase diagram alloy cooling curve at different compositions you can see this is what is the temperature melting zone this is called the freezing zone and then it is solidification zone fine so here if you see that in an alloy you will try to have this is solid as this is a liquidus line this is a solidus line and now here is a mushy zone so now if you try to draw plots you will try to see various compositions you can try to take for an alloy this top one talks about here it is hundred percent of a and zero percent of b and when it comes to here it is 20 and 80 you can try to have this is called as upper critical temperature lower critical temperature right t a and t b t b is a point when alloy b melting temperature is reached and t a is a point where alloying a melting temperature is reached so now with this composition you with this solidus line when you draw you can try to find out what are all the faces and what are all the segregations can happen this curve tries to talk about cooling curve to construct binary phase diagram but in reality you will have ternary phase diagram which makes the life little more complex cooling curve and phase diagram this region of cooling is non equilibrium or non linear so the region of cooling is non-linear due to the evolution of latin heat caused by the solidification process the temperature from which the slope first deviates corresponds to the formation of the first solid and it is called as liquidus temperature the alloy solidifies below the solidus temperature this is a typical cooling curve and a phase diagram for steel so this is iron ion carbide diagram which you would have gone through in your initial days in your school or in your engineering materials so this is how we try to get for a iron carbide diagram at various temperatures various compositions what are the different phases you get with this we can try to understand when iron and carbon are getting mixed what will be the response like this you will try to see for various alloys the cooling curve and phase diagram a phase diagram shows how faces form and transform during heating and cooling with different alloy composition different c content affects alloy phase transformation temperature optimizing additive manufacturing process parameter and microstructure can help during laser based additive manufacturing stainless steel production the laser input temperature must be optimized to control austenitic and ferritic formation so these are the cooling curve which were again taking it for steel because it is very much available but when you do additive manufacturing this is still a question lot of people are trying to understand will it follow the same or will there be a deviation if there is a deviation how is that so the cooling curve and the phase diagram says that this y axis is the temperature x as is with respect to time so you will have fast cooling you say you get martensite when you do slow or when you do moderate cooling you try to get bainite and when you do slow cooling you get ferrite and pearlites in the microstructure so when we were looking into the microstructure we were looking like a fingerprints impression i was saying these are it can be martensite or this can be austenite in between whatever it is so this will try to talk about the cooling curve and formation of different faces with respect to temperature for steel like this you have different different cooling curves and phase diagrams for different materials the continuous cooling transformation diagram cct diagram tries to talk about the phase that forms in alloys at different cooling rate so the same if you want to do it for additive manufacturing we might get the same response or we might not get the same response see if you look at it going back see the cooling rate in 10 seconds you are trying to move from 800 830 degrees you are trying to come to 200 degrees within 10 seconds but whereas in laser additive manufacturing you get it within a second peak transformation so here you might get completely martensite or in martensite also you might get some change in the microstructure to do that each deposition layer undergoes repeated thermal cycle and have different cooling rates in additive manufacturing at fast cooling rates a complete martensitic structure forms while bainite and ferrite and pearlite forms at slower cooling rates as compared to that of the faster one in additive manufacturing the c c t diagram can be used to estimate phase formation when the thermal profile of each deposition layer is known which is a challenge this is a challenge we do not know today because between one layer and the next layer what will be the temperature dissemination happening we are not able to measure even if you use an infrared pyrometer or thermocouple the layer size is too small for a thermocouple to measure when you use thermal imaging it averages out so all these things makes the life misa very difficult to draw a cct diagram for additive manufactured part so this is the cycle which it undergoes in temperature time profile this is called as a thermal cycle so you can see in the first layer when it is moving it will try to be in liquid as form when the second layer is moving on this on say first layer is on the top let us do it like this so this is the first layer this is the second layer in the third layer you see the temperature will be like this in the fourth layer you see it will be like this in the fifth layer it will be like this and in the sixth so first second third fourth fifth and sixth layer so you see there is lot of thermal cycle happening you aimed at only the first layer but second layer also gets the temperature diffused and the third layer also it is getting fourth layer so now each so if you try to see this is a slope which we try to get so it is pretty interesting and to get and validate the experiments to get with respect to timing cycle it is a real challenge so heating and cooling can cause suppressed phase changes or super saturated faces during additive manufacture solidification in additive manufacturing when every layer undergoes a repeated heating and cooling cycle the temperature may exceed the phase transformation temperature this causes multiple phase transformation then intricate microstructure and residual stresses for example typically thermal profiling of a single layer ti 6 alv 4 alloy manufactured in am for a single layer the time temperature profile fluctuates and reaches phase change temperature causes phase changes to happen knowledge on phase transformation during fast solidification and repeated thermal cycles is essential to control the microstructure in additive manufacturing additive manufacturing process optimization involves equilibrium non equilibrium phase diagram thermodynamics thermo physical quantities laser electron beam material interaction diffusion kinematics so this laser electron beam material interaction it is completely different when you are talking about laser we talk about the surface reflection and thermal conductivity when we talk about electron beam we talk about the electronic structure which is there during rapid solidification some phases that form under equilibrium condition may not form causing metastable phases the figure on the next slide compares ti6 alb for cct diagram for standard casting and additive manufacture which is published in one of the renowned journals in additive manufacturing the alloy cooling faster forms a meta stable martensitic structure so this is what it is so you can see the characteristics of meta stable phase depend on alloy chemical structure and cooling method during thermal behavior so this is nucleation of phase one then it is nucleation of phase two this is t two this is t1 this is for titanium so the time temperature diagram presenting the nucleation onset of two dissimilar theoretical phase with unlike cooling behavior so from the time temperature diagram at cooling rate t one the primary phases is nucleated as phase one whenever at faster cooling rate t two another phase phase t two is nucleated by detouring phase one so you do not even get into phase one you got into t2 so its repeated thermal cycles so you do not even get phase one which is called as trailer grain boundary engineering figure on the next slide shows the phase diagram with phase delta at equilibrium due to rapid cooling a dashed meta stable phase diagram may be created so you see here this is a comparative presentation of theoretical equilibrium and metastable non equilibrium phase diagram which is drawn it makes life very complex so you see here these are all dotted lines which are solid lines are theoretical ones solid lines and metas table you see here you completely new one you see meta stable this one right so it completely changes so some of the raw printed parts which is made through additive manufacturing you can see you can make a bowl multiple parts can be made in one shot high aspect ratio can be made in one direction zero direction high aspect ratio parts have been made in x y plane in x y plane high aspect ratios are also made additive manufactured components after cooling just after being taken out from the built chamber without any processing looks like this you can see there is lot of undulations on the surface which can be because of staircase effects or it can be due to shrinkage or improper solidification in order to avoid such things we try to decide the orientation of the part before printing and friends keep in mind that it is not a single part getting printed you can print multiple parts in one shot also so in this lecture what did we go through we tried to compare manufacturing of metallic material with rest of the conventional additive manufacturing we understood and we wrote as notes on solidification which we saw these two in the previous lecture in this lecture what we saw was we saw the difference between equilibrium and non equilibrium phase for solidification try to draw for titanium and copper this equilibrium and non equilibrium phase diagrams for solidification try to do it for by yourself as an assignment so that you get more insight then explain the mechanism of solidification in equilibrium and non equilibrium phases we saw for the same can you do it for titanium and copper then understanding and draw different cooling curves for pure metal and alloy we saw and finally we saw some of the raw printed parts so that you get an understanding different from that of your casting the assignment which is given here try to look at varying thermal cycling graphs for am process both laser and e beam separately to multiple part printing process through am what is their micro structural variation along the cross section for example you had a you had a this is the bed bed so you had a component something like this you have a component something like this and you had this component right look at some literature for it and then you try to take a cross section and see how what is the difference in the microstructure if somebody has published you can see this you can take it for again copper or titanium because copper is highly conducting titanium it is conducting but all the heat gets accumulated in one place so because of that i am trying to always give you try to look at these two steel is available in the open literature so let us look into these two things varying thermal cycle whatever we said is this this is the varying thermal cycle so you try to see what are the different phases you get again for copper thank you very much [Music] [Music] [Music] you