Lecture 12 : Basics of Materials Science-XVIII - Part 2

[Music] welcome back to the part two of lecture number 12 so we have been talking about processing of materials and we we started discussing about solidification processing how can you shap or get a components from the liquid state right and this is the last slide I shown you you can see here the fundamental factors right so we talked about factors like flood flow heat transfer are critical aspects critical factors for determining the Sol Bas and processing let's talk about temperature or the heat extractions and if I take a pure metal let's talk about pure copper and I have a liquid of copper start cooling it down how how this cooling Curve will look like that means as a function of Time how temperature will change right that's what is called a cooling curve you have to understand that as a function of Time how this cooling Curve will look like that's what we have to take so this is the y- AIS is temperature x-axis is time and it start cooling from liquid state you can clearly see it cools Down Still reaches point B and point B is C point to melting temperature of copper that is CC temperature of copper is how much any body remembers is 1084 de C so but you see it is at the temperature still nothing is going to happen temperature still is going down it reaches the point C then again temperature will increase further increase to point B and it remain constant for large length of time finally again it decreases so I marked this part is L this part is s you can clearly see that S and L right so now why this dip of temperature happens and again it rises that's something new unique right that's something new well at the meling temperature there is no diving Force for the system to go from liquid to solid state at the M temperature diving force is zero as you know you need to have certain kind of energy or diving Force for the system to go from liquid to solid state that's why liquid needs to be cooled below the melee temperature and that is what is known as under cooling you see BC corresponding to under cooling right and ab is above the mill temperature that is what is called superheating so now once you under cool you provide the system some energy and with that energy system will get some diving Force to transform from liquid to solid so this is from layo points of view so nucleation as you know it's not a very easy phenomena a easy phenomenon to understand is basically nucleation means temperature going down and your under cooling giving the system certain amount of energy to form the nucleus or nuclei after it has the soid has nucleated in the liquid okay like you can easily consider even water and ice system you take water start cooling down at 0° cus temperature water will never transform to ice you have to cool down below below 0° cius temperature to form this ice crystals inside the water once it is formed then they will keep growing and that's what is shown as the red region where the temperature remains constant that is what is called the solidification or the for growth of this nuclear in the m and after everything is over temperature will go further so this is a very simple cooling carve right very simple looking important aspect is here so you have a nucleation of phase taking place nucleation of solid from liquid taking place right and that's why you need some under cooling I'll not go into the energetics of of the systems that is available in standard textbooks also so now I show you three pictures here okay first one left side when there is no under cooling this is an ideal situation which will never arise in real life okay if you want to process any material the first curve I okay first one is never going to happen because you need to provide energ system to transform from liquid to Sol without that it cannot correct now the second one is more realistic that's what I discussed in the last slide right now what is the third one this is the third one what is the third one third one is for Alloys but is not for Pure metal so how it will happen let's see the next picture let's talk about this kind of simple phase diagram we talked about some phase diagram in the last few lectures start a simple is like this is for copper maybe nickel and copper yes nickel and copper nickel has M temperature of, 1452 and copper is 1084 so this they form this kind of phe diagrams now if I have to plot the cooling cup temperature versus time plot you can cely see there is no such thing like a dip temperature at the melee temperature because Mele temperature correspond to the liquid temperature here solid correspond the solidest temperature here marked here between b b and c t b is corespond to TL liquidus and CP to solidus TS so it continuously drops so that mean nucleation doesn't happen well it happens as you can see in the Liquid Plus solid region the temperature is changing as the temperature changes composition changes that's why the curve looks like this so nucleation can be observed nucleation deep can be obsorb when you take a pure metal but it can obsorb when you solidifying a alloy that's what we're trying to say so this is a feature which is very important now since we talked about nucleation let's talk about what are the different stages of solidification of any crystalline material well first thing is formation of nucleus what is nucleus nucleus is nothing but some cluster of atoms which can come together and form a regular array of arrangement of atoms do you remember crystalling structures right they atoms are regular manner very regularly ordered manner so in the liquid everything is hent because there is no crystalline structure in the liquid right but there nucleation nucleation is a fluctuation process or stochastic process in some part of the this whole liquid this solid atoms will align a regular water manner that is what is called nucleus so once this nucleus will form and they can form anywhere in the liquid or they can form some specific location also that we'll talk about a little later so then this nuclei in the Second Stage B this nuclear will grow the growth is means what attachment of atoms the nucleus and then once they grow fully grown this all this nuclear which are growing they don't know each other right at the beginning they'll heat on each other and form boundaries and these boundaries are called grain boundaries you can see here so this is an our shell what will happen in solidification solidification these kind of crystals will start forming as nucleus a nuclei L nuclear then they will grow and form bounds and then you see this gain structure in actual under microscope that's what is in a n shell is something which you should know so you understood there are two important basic aspects happens during solidification one is nucleation other one the growth nucleation is nothing but form this tiny stable soid particles from liquid and you see it is written even in this thing is the the number of atoms may be 50 to 60 or 100 atoms maximum so size of this nuclear is very small couple of nanometers right in diameter and as we talked about it about this you know cooling curbs under cooling is Mally required to create energy between solid and liquid interfaces you see see here this nucleus is solid and outside there is a liquid therefore there's an interface between solid and liquid right liquid atoms are are random solid atoms are orderly in orderly manner so you need to provide the energetics between the solid and liquid where from the energy will come energy is coming from undercooling of the liquid that's why you need to undercool the liquid to provide the energy for creation of the new soil liquid interfaces and larger the extend of under cooling G will be number of nuclear will form fin Crystal size will be forming if you want to have finer G size you to provide more diving Force now nucleation can be two types one is hetrogeneous other is homogeneous right hetrogeneous what is absorbed in real life hetrogeneous means what nuclear can form on a solid surface like on the walls of the mold some particulate on in the liquid solid particles can repres the liquid or even on the surface so so that means what nuclei form basically only perform certain sides whether it's a mold wall particulates or the surf or outside or the top surface doesn't matter okay it's in a liquid is in a container so therefore there's called hetrogeneous that means they are not same everywhere compared to that homogeneous nucleation means nuclear can form anywhere on their own now you know hogen nucleation is easier to occur that's why we find them more if you want to from Ice from water in the refrigerator what you do you take this water in a small container and then cool it down right in the fridge and then you'll see the ice crystals will start going from the container wall that's called hogen nucleation now in a very is n cell I thought of providing you this picture homogeneous means anywhere anywhere it can form and formation probability of nuclei is same everywhere in this volume but nucleation is not the same it can happen mostly on the walls of the container or the top surface or some particulate material that's the difference between hetrogeneous homogeneous well knowing this aspects so you know cooling CS you know some of nucleation nucleation important why because you have to create you know different grain sizes after casting how do you do that so let's talk about how metal casting processes what are the uh you know processes used to produce the casting remember there are many processor used I I suggest you to read this book if you have time s kalpam and SR Smith manufacturing engineering and Technology by ppen and this a very good book if you want to really go through but I know you not have much time to go the books but there will be videos available on PTL or some qu available you can go through that also so there are two important uh metal casting process one is called expander lble mold other one is called permanent mold you must have understood mold means the cavity where you want to create the threedimensional shape right so expandable means you can use it at one time and after that you simply destroy it and get out the casting from the mold that's basically typical sand mold casting you can make use the sand to create a cavity and then cast it permanent mold means mold is permanent you can break it down obviously par moles are made of metals right they must be and why they made of metals not Ceramics because Metals my thermal conductivity they can take out the heat faster as you understood you are you know solidifying liquid a liquid cify solidification requires to extract the heat then only you can make it solid so that's the permanent moles are widely used now green sand mold that is what is one of the expandable molding operations is what is green sand means add along with sand you add some water some other mola some other agents okay bentonite clay to make it you know nice okay so that surface fin can be retained and you can see here we have a lle you put the metal inside a cavity that is called sprew and then you have a RAR you can see this cavity will look like this this are to control the fluid flow and then it will go through the gret and come inside this mold cavity feel the mold cavity right but you know most of the metals shrink during solidification so therefore you have to provide extra liquid that's why you add a riser and this is will contain extra liquid once dur during solidification process it sings liar will provide Extra metal okay so this process is very commonly uh used in various kinds of you know engineering and Technologies so you can you have probably seen also that or many of you might have made this kind of moles and seen The solidification Happening not seen I made the casting out of it right also well different parts of this molding operation is this just to show you this is this is called a down sprew then you have a you know this is the Riser here you see here and this is what is it if you want to create a hole like a pipe inside a casting you can use a core actually cor is nothing but extra piece which are attached by this kind of uh Chets basically they are kind of pins and this to fix it so that it doesn't move okay during the fluid flow and then liquid will be surrounded by this and they already FY then you can create a pipe a hollow product you can see this a hollow product is creating by this way well as I already told about it talked about it the engine blocks are many holes right they prepared this way only you can see here this is one such engine box where so many holes are there you can see these are holes so many holes are there how they will prepar they'll prepared by this only there are many compressive Parts also prepared by castings okay nowadays even the connecting rod can be prepared by casting also many many compressor parts are prepared that well other than that there are many kinds of molding operation I'll talk about few of them what the time available to me one is called shell molding what is shell molding this is nothing but you have pattern pattern can made of metallic okay pattern means particular shape basically and then you have a sign rid binders and first you hit the pattern and then put it upside down whole thing is upside down you can clearly see in this picture too sand will be in contact with the with the mold and then sand will then form a shell around it because it has resin resin must have melted and then liquid resine along with the sand will form this kind of shell then you can remove the shell and create different kinds of pattern you can see here this is this is very simple technique which is used in the automobile Industries okay when in fact if you if you visit any automobile uh industry which are making castings they they use this kind of processes very widely only problem is in this process is to control the heating and also control the cell thickness well there are other expandable PO process like expand poing process what is it basically this is a mold you can clearly see this inside a mold you inject polyer pits easily available the market po bits with a heated aluminium Dy this is made of aluminium D which is heated right and then what will happen then this this is pressed okay this D two pieces of the D are pressed first you put inside is pressed right once you press this the poly beads will melt and then you cool it down they stify and form this kind of molds you can cely see this molds easily prepared then you can integrate this mold using all the ranas pro everything I told you and then pull liquid and form castings very very simple process right then after that you to break out the mold and take the casting out that's why it is called expandable molding process expandable poly process these are nothing but different ways of using different kind of material to prepare this kind of you know castings I'll talk about another important aspect which is used for Ceramics which is called slip casting slep casting is what first you have to produce a slip sleep is nothing but a slurry okay this a slurry nothing but a slurry and then sleep is pass pour into a absorbent mold you can see this is absorbent mold and if sleep is po inside this correct this looks like a water container right and then once it is poured once it is you know you know soaked a layer of clay forms as the mold surface absorb water okay this is what you see layer of clay has formed here and then when the shell is of suitable thickness excess slip is poured away basically it's upside down so whatever excess is there that's thrown out correct so to allow this this slurry to form what form some kind of a shell of suitable thickness that's what is important and then once it is form you take it out heat it up make it harder okay simple process and then you get the mold you can put the liquid inside it and get the casting very simple right this is called a sleep casting process because this SL is called slip this is shown here more detail manner you can cely see this was the IAL you know mold you put the SL here SL will form a thin you can see that what is form this thickness very thin layer and then this is your this is your your moldes this is widely used and many of the casting processes slip casting is used to prepare this kind of Cups this is a white clay you can see here and the face Fly cast cup is produced then once you fire this cups you'll get a solid piece which can be sold in the market that's what it is another important process which you will discuss is investment casting that's why the statutes are built okay statutes are built by this because invest and casting allows you to prepare integrate steps statues can be built up that and I'm I'm showing you a Sate or snapshot of that you can clearly see that you know these are the weight this are the wax pattern this is the wax pattern wax made pattern made of wax cor and this wax patters are joined like this this is the whole pipe and this is a sprew this is a pipe actually and they are joined now this wax pattern is deep inside a refractory SL and they dipped and then you know after dipping they taken out and then put sand okay they showed with refractory powders okay J2 l23 aluminum oxides and so that these powders will stuck with this SL and this process will be happening several times sometime five six times dip it in slurry take it out shower this you know refractory powder and allow it to get it cool down or get it covered by this Factory slur then again dip it into the slur so once this this will lead to formation of a thick cell okay around this wax patterns and then this whole thing is taken inside a furnace heat it up walks to M back and come out and the shell will from the mold that is what is called invest okay this this shell and then you can pull liquid once the liquid is solidified in the mold then remove this shell and get the casting out of it very simple process but very genuine process so remember I showed you the statue of dancing G from 5,000 years ago in indas B civilization this must have been prepared same route like this because the the amount of integrate pictures you can go back now your video amount of integrate is there in the casting is so nice and surface so good that can be obtained by sand casting so that's how investment casting is made I show you two pictures one is this nraa statue other one is the Oscar statue you can scarely see that how integrate these statues are and they are prepared by investment casting routs then they have what is know as a pressure casting you can have low pressure die casting or hyp do casting what is it very simple okay you can see this is a container or lle which is liquid and then this liquid is you know pass through this refractory tube reaches this mold and this mold is retractable that mean two pieces can be separated easily and by air pressure they allow the liquid to pass through that's why it's called Low Pressure dting and this this mold which is permanent mold basically it's made of metal it's a permanent mold casting and these two molds are joined together to make a cavity and allow the liquid metal to go ins the cavity once it is filled the whole cavity allow it to solidify and take it out a important version of this called pressure diecasting that a low pressure diecasting is called P diecasting what is done here you pull liquid metal inside this cavity and then using this injection you know piston you can pose this liquid metal to go inside this mold cavity and then allow it to solidify you can do many way also you can simply PL use a plunger to force the liquid go into the mole cavity right okay that also you can can form this kind of cast this is a casting you can see very easily you can see that casting so this is widely used P casting is widely used in case of aut components which are very thin shells are very good surface finish another important aspect is called Swit casting squee casting is what well this is also similar what you do is basically P the liquid from the leral inside this Dy and then you a punch to squish so what will happen is that in between the gaps between this D and the punch the liquid will take the shape and that is your casting because since you're squeezing using a punch it is called Squeeze casting so there are many many such process available in the engineering community so we'll not talk about it much details but we'll talk about some very specific things like CLE casting continous casting as well as single Crystal blade casting the next lecture and we'll talk about you know other methods of manufacturing because I want to complete these things in a couple of lectures so that I can go into the basic aspects of machine learning as quickly as possible so with that let us stop this lecture thank you hope you're enjoying the lectures and please go to my references uh read whatever associate uh literature you need to understand thanks [Music] [Music] [Music]