Lecture 55 : Laser Surface Engineering : Hardening and Melting

Welcome to the 55th lecture of Surface Engineering,we have beenin the last couple of lectures we discussed the various fundamental aspects of laser material processing and if you recall we actually discussed that the overall scopecan address four different possibilities namely the forming ,machining , joining and surface engineering So, these are the four majorscopes of application engineering applications of using laser as a pure source of heating, non contact source of heatingwe are not talking about spectroscopy or metrology or any other applications those also could be engineering applications, but essentially manufacturing related application So, where laser is purely a source of heating and a laser is useful because it can deliver a certain quantum of a very very precise quantum of energy at the desired spot and we haveextreme high level ofmany variabilityin terms ofdelivering the right quantum of energy at the desired spot and so on . Now,today onwards we are going to discuss pure surface engineering, we are not talking aboutadditive manufacturing or various other processes not mentioning or joining and so on Now, while we discuss surface engineering using laser as a source of heating I would like to divide the whole approach into two possible modes ofstrategies. Number one like here we are going to talk about only on the micro structural changes, so we are not changing the composition. So, the phase aggregate that we have on to the surface,we can subject the component surface torapid cycle of heating and cooling So, essentially ifyou consider a component and subjected to in terms of either temperature as a function of time or temperature as a function of depth . In either case you actually will see a rapid heating maybe a little bit of residue time at theof the surface during which the heat can propagate further and then we cool and this cooling is essentially a process of self quenching So, this is in terms of the temporal distribution, the time distribution similarly as a function of depth we expect thetemperature. So,at the surface the temperature will be the maximum and as a function of time or as a function of depth we will expect the temperature to gradually decreaseas we go below the surface So, using this kind of aheating cooling cycle and a distribution of temperature as a function of depth which will gradually decrease and of course, this as a function of time this will increase with time this will gradually increase. So, using this kind of a cycle we essentially would like to change the microstructure and please remember by micro structural change we mean the identity the shape size distribution morphology of the phase aggregate that we are dealing with So, when we do that we also would like to just make you recall that there are fourspecific reasons why we use laser because of coherency, because of monochomatic, because of very low divergence or very precise path ofmovement and then finally, very high power density at the desired spot. So, this is how laser is different than ordinary light and using thisdirected source of heating,we actually allow the solid to get heated up and undergo certain amount of micro structural changes So, the conversion of incident photon the packet of energy light energy to lattice heat, so photon to light heatlattice heat is an instantaneous process less than nanosecondthroughelectronic excitation and carrier relaxation. So, a very fast excitation of electrons to higher engine state followed by rapid falling back onto the ground state and in the energy is convertedthe incident energy is converted into lattice heat Thealso please remember that we are talking about an energy energy deposition profile let us say the intensity of heating or the lattice heat that we generate as a function of depth from the surface willdecay exponentially

So, this exponential decay this factor which involves a absorption coefficient times depth So, as a function of depth is dependent on the initial quantum of energy that is incident, but in course of depth or time it decreases because of this absorption coefficient and also because of the fact that much of the incident energy is reflected back So, yesterday we discuss as to how we can reduce thisreflectivity or improve the absorptivity of laser light, going to lower wavelength or creating certain absorbent coating on to the surface or certain texturing onto the surface and so on So, this huge source of heat is localized, confined to the surface which can go all the waywhich can vary over a wide range and because of which we can actually go to a situation where we can rapidly melt vaporize or simply on the other extreme we can say only heat and not even melt Now,why we actually are interested inoverall surfacing this is the same view graph you would have seen inthe very beginning at the introductory lectures and please I just want you to remember thatin all these components be it a gear or a or turbine engine component or this highly rotatingbearing assembly or these ceramic or metallic balls that are part of the ball bearing assembly, an engine block or a sprocket or a nozzle or the camshafts or these automatic implants. In all these cases they are very different compositionally, they may metallic, they may be ceramic, they may besteel, there may be aluminium or they may besilicon carbide or alumina titanium various kinds of metallic materials ceramic materials even polymeric materials And shape size or the interaction in terms of the forces acting on them or the environment in terms of temperature oxidative atmosphere or reciprocating wearing condition, fatigue conditions, high RPM rotations , interaction with body fluids So, all thesecombinations of various conditions they are very; very different component wise Yet there is one thing in common that all these components when they undergo failure the failure usuallyinitiates from the surface And that is purely because of the fact that the intensity of stress interaction with the surface is maximum and also the invented effect intensity at the surface is again higher than any other car parts So, the core does not experience as much stress intensity or environmentalattack intensity as the surface experiences. So, surface always undergoes failure or is prone to undergo failure beforeother components actually or the core of the material undergoes. So, we need to take care of the surfaces, we need to protect the surfaces against wear, friction, corrosion, oxidation various other kinds of surface damages So,this again we would have discussed that we said there are various properties surface dependent properties physical , chemical mechanical and I have just listed a few representative properties under them. So, when we want to improve uponany of these physical, chemical or mechanical properties we adopt the surface engineering approach and we have already discussed we do it by material removal by modification or by addition Now, we in in today’s lecture we are going to concentrate on this part , where we actually are onlymodifying the surface without changing dimension and that to a subset of that which will essentially be hardening or annealingonly requiring change in microstructure and no change in composition So, the overall scope of surface engineering is this is what we have discussed beat nanometer or few millimeter from from the surface we call it near surface region and we modify the microstructure and composition or both in today’s application using laser will be whirring. So, we will only change the surface microstructure and we will see how it can improve thesurface dependent mechanical properties So, we are not discussing the conventional process we have already done it at length in the past, we are not even considering ion beam or electron beam we are considering only laser beam which is a direct energy beam as a clean source of non contact heating So, the overall scopes can be only heating,

can be only melting. So, this is heating without any change of state this is heating with change of state, this is also heating with change of state and this is in solid state yetthe rate of heating is so large that will createa very large amount of stress on to the surface So, this is kind of a shock peening orshock hardening kind of applications But these are the more common types of applications, but please remember we can do many of these processes like annealing, solid state hardening,grain refinement texturing , even maybe marking or scribing all these processes without shock; shock peening, shock hardening all these processes only through change in microstructure and no change in composition We have already discussed this, so we can go and heat remain within below the melting temperature or we can cross the melting temperature, but not across the boiling temperature So, in the process we can doprocesses like bending which is a forming process we are not discussing that today , we are going to discuss transformation hardening maybe a bit of coloring or surface amortization not blazing , now all these processes actually do not require any change in competition right Now, when wearetalking about that we have to again recall the important parameter important point thatwe are only talking in terms of power density and interaction time and combination of this say for example, when you want transformation hardening you actually do not need to apply very high power density So, relatively lower amount of power density , but the interaction time would be sort of moderate . On the other hand if you want shock hardening or shockpeening, then you are talking about extremely highpower density, but for a very short period of time So, then the domain is only this much whereas, for hardening we are talking about a domain like this. So, just by controlling power density in an interaction time we can achieve either transformation hardening or shock hardening or even surface immortalization or various applications like that. So, in principle we must recall thattwo parameters independently can be varied the power density and interaction time and that is exactly what we will do in order to change the microstructure So, let us take up some examples ,see in case of a steel we are aware we just need to recall the basic phase transformationprinciples of steel weall we need to do is to if you recall the steel part of thephase diagram. So, we , so this is the ; so this is the part of ironcementite system and if we take a composition somewhere in themedium carbon range say about 0.4 weight percent carbon steel So, using if this is the component we are talking about . So, this component may have a certain depth or or thickness , but we use a laser beam only from the surface and this beam can move all the component can move So, that we heat here; we heat here and gradually the the heated zonemoves along y x direction and each time during the irradiation the temperatureof this region will undergo a rapid heating and then maybe a little bit of residual time and again will cool down So, this heating and cooling process is entirelyconfined to this region for the period of time of interaction and this time of interaction can be very very low anything like a millisecond to even nanosecond So, in this short period of time there is rapid heating and rapid cooling . So, in the process we please remember this is the composition, so, this is basically the carbon amount and this is the temperature So, in this rap in thisshort period of time we are dealing with the this alloy and the alloy from room temperature will go to this temperature or maybe this temperature. And in the process we will undergo a transformation from ferrite to austenite and subsequently this austenite will transform by rapid quenching by rapid it’s self quenching into martensite So, so when we cool it, when we move the beam away; when when we move the beam away to the next position . So, during this period the

cooling cycle starts and this cooling is a self quenching cooling and because of which this series this kind of a sequence happens ferrite transforms to austenite and on austenite on fast quenching undergoes martensitic transformation We must have adequate carbon, we otherwise we do not get say a sufficient hardening effect on to the portionwe haveactually irradiated So, let us say this is ashaft and this is the turbocharger . So, we want these corners and the skin to be hardened, also the shaft portion to be hardened such a way that from the surface until the until the I mean towards the core certain depth should transform to martensite. And when transforms to martensite not only it hardens it also creates residual compressive stress onto the surface So, in the process we actually ensure better wear resistance, better fatigue resistance and all these normal circumstances you would do conventionally, you charge the whole thing , so the whole component you charge inside the furnace. Now, you unnecessarily heat the entire volume you do not need to heat and then when you subsequently quench the whole component there will be scopes of lot of dust distortion or bending or a undue oxidation discoloration and so on But when you use a laser beam, so typically if you just consider this pointer. So, this pointer it moves like this and this is how exactly the laser beam also will move . So, this component will rotate, the shaft will rotate; the shaft will rotate and then the shaft rotates the stationary beam the beam remains stationary, but as the as the shaft rotates the beam also moves along the along the surface. And in the process point to point you actually continue to heat and the heating zone transfers So, this is how you actually do it very fast and at the same time you canallow the system to undergo rapid change of microstructure from one crystal structure to another , in in case of steel we just this change of phases from oneaggregate one type of an aggregate at room temperature to higher temperature and then subsequent cooling during cooling So, another transformation leads to significant improvement in various trends hardness and other things So so if, so this could be a circular section this could be a rectangular section or shape or with complex shapes like this with a geometry which is curved geometry and so on. The advantage of using laser is that if this is the surface we are talking about and if I know this is my beam diameter, I can make sure that the beam diameter at the surface is always is in focus and if it requires instead of flat if we are dealing with a curved surface So, I will have to basically trace exactly this contour of the surface using a computer aided mechanism I mean controlling the beam So, using that cadtrained beam delivery system I even if the contour or the shape changes I can still make the beam move up and down exactly to make sure exactly to ensure that the focal focus point remains in contact or remains exactly at the same position with respect to the surface So, in the process the heating cycle will be exactly the same, otherwise if the beam remains exactly at this level and you have a curved surface, then; obviously, when the when this trough region is under the beam, then the heating level will be much lower compared to this crest position when you have when the when the distance between the beam and thepeak position is very close by So, this is very important for us to make sure that weactuallymaintain thismaintain the identical position of thefocus with respect to this of substrate surface . So, in principle this is certainly possible with steel we can certainly do with many otherelements many other systems In fact, we do not necessarily have to always undergo ferrite austenite kind of phase transformation, even we will show some examples where we are able to take it to very fast cooling to to take it to surface meltinglimited melting

on to the surface ,followed byresolidification of this molten layer and that also can producesignificant amount of hardening, but that we will discuss later We can also do semiconductor annealing or basically annealing ofeven metals or nonmetallic systems and in the process we actually can change the color to acertain extent by way of allowing certain limited amount of oxidation,maintaining the thickness of this oxide layer and the composition of the oxide layer we can bring in various kinds of colors through an oxidative process. But otherwise we actually can also make micro structural changes like for example, this silicon wafer initially if it is inamorphous condition by way of laser beam heating we can convert this into a poly crystalline aggregate like this So, by changingfrom amorphous to poly crystalline aggregate we can make lot of changes in terms of not only appearance, but also a transport properties andcolor andelectrical properties Now, another very important process is a shock peening. So, I i told you that in terms ofin terms of power density and interaction time , if you choose a region which is very high power density, but verysmall interaction time So, of the order of few tens of nanoseconds or so, but your power density is a mega watt per centimeter square or even higher So, in that short period of time you actually can create a shock wave on to the surface So, in order to create that shock wave first you would create a layer you will cover it up with some. So, it is a polymeric membrane with certainchemicals or certain substances which can undergo very quick vaporization So, we use a pulsed laser and we allow thevery high fluence in terms of, so this in terms of energy or it can be in terms of power is watt per centimeter square for very small period of time So, weactuallyallow certain bursts oflaser irradiation at very high frequency, very high reputation rate and that creates that immediately vaporizesthe content inside between the membrane and the surface the solid surface. So, with vaporization this portion expands and if it expands then if it is prevented to go beyond a certain point on to the right, then; obviously, it creates a huge pressure inside and this pressure creates a shockwave So, you create a plasma which tries to expand and you confine the plasma within certain volume and that creates a shock wave and this shock wave is a very transient one . And, but nevertheless can createcan actually cross immediately very high stress level which can cause certainplastic deformation create certain amount of dislocation density and in the process canmake the surface harder and most importantly make the surface develop certain residual compressive stress So, the the the reason why we actually createresidual compressive stress is because of creation of this high dislocation density onto the surface . But the beauty of the whole process is it’s not like a mechanical deformation process by rolling or by short peening or something like that, the beauty of laser assisted shock peeningplease remember this we are talking about shock peening and not short pinning So, the shock peening actuallyallows very thin layerwe are talking about a very thin layer which is typically few tens of a micrometer or even less So, the dimension there is no dimensional change there is no compositional change, there is no distortion yet the surface develops fairly high few hundred mega Pascal’s ofresidual compressive stress. So,and just to remind you the utility of such residual compressive stress is that if you if this is the surface and ifyou create deformation onto the surface, thenessentially you createresidual stresses which are sort of acting towards each other So, if there is acrack form for whatever by whatever reasonsmay be due to fatigue or wear or something, this crack first has to overcome thisresidual compressive stress and then convert this into residual tensile component and then only the crack can propagate. So, under normal circumstances when you create such high residualcompressive stress on to the surface near surface region they act in a way to prevent crack formation or crack propagation and that helps the component

to acquire higher resistance to wear, fatigue or any amount of mechanical damage to the surface And this is done by laser in a very precise manner, so if you have for example, a component a short teeth of a gear component or sharpedges of the of a cutting device or in a semiconductor you want a small region to be heated and leaving rest of the regions unaffected So, very precisely you can heat treat only this portion the dimension of which could be a few micrometers and remaining 90 percent 99 percent of the surface can be left unaffected So, here also you can affect only this very precise corner of the of the cutting device or the shear component or or somesharp edges or maybethe cup the curved portion of the acetabular cup can be given this kind of a treatment to create residual compressive stress And you can do that very precisely exactly at where you want and avoid heating other zones where you do not want So, this is the advantage of a such kind of shock peening and this is the kind of machine and this is what you are seeing here is on aluminum alloy and these are aluminum alloy and this is how the shock wave propagates because of the creation of this temporary plasma pressure that we create by way of this kind oflaser irradiation So,so this is typically the load displacement curve, so when you actually apply such shockwaves, then you temporarily cross theyield strength of the material locally very; very confined region and that is how you create certain dislocation density, but this deformation is so small volume that there is no overall distortion or change of shape of the material So, it is important that we actually choose the right kind of paint and then have an impervious membrane through which we can actually use the laser beam to irradiate We can also do texturing which means we actually this texturing is different than crystallographic texturing . So, we are not orienting the crystallites in certain crystallographic directions , here we are basically creating certain pattern on to the surface. So, certaintiny little holes orsmall markings at regular intervals or certain grooves at regular intervals . Now, all thesecuts or grooves ordents actually creates a very nice geometric pattern onto the surface, which improves the lubrication allows higher speed of rotation, power also produces fuel consumption So, for any rotating parts particularly let us say the engine blocks and so on Now, the pistonsthe exhaust levels are reduced and most importantly the wear and tear or the mechanical losses on the surfaces is reduced and this is done. So, actually when you do this with a laser you actuallycreatevery localizedevaporation from the surface or deformation from the surface Now, for in case of mechanical system this deformation is spread wider in case of laser it is confined to very very small level So, you can do for example, using certain electrochemical process or chemical process by using certain acids and so on, but with laser you actually canreach a very higher level of absorbance and hence theeffect is much higher and is can be done over a very widerange of a wavelength incident wavelength So, in order to summarize we will say that we can do various kinds of surface engineering using laser; laser surface engineering where either we change the composition and microstructure both or we change only themicrostructure and not the composition. And that is possiblevery easilybecause we can control the heating rate the heating process and so on the main process parameters are as I said the power density and interaction time and laser is a laser fastening is more suitable than electron beam or ion being purely because of this kind ofexponential decay of the deposition profile compared to Gaussian profile of electron or ion beam the the shock peening creates a compressive stress because you actually deform locally very; very thin there from the surface increase

the dislocation density ah, but thiseffect is different than what is what happens in case of surface melting because in case of surface melting you actually create residual tensile stressbecause of the tendency for contraction duringsolidification, we will discuss that again in the next lecture. In case of steel we actually adopt the strategy of austenitization followed by martensitic transformation which is absent in non ferrous metal So; obviously, the strategies forhardening surface hardening using laser is going to be different in non ferrous metals we generally adoptrapid melting followed byquenching. In case of semiconductor we actually allow very transient heating in the process because of which there could be a annealing effect,annealing of the point effectdensity onto the surface or we can change color or instantly by way of heating we can convert an amorphous aggregate into polycrystalline aggregate And texturing involves creating certain surface patterns by way of creating grooves ordents or marksand so on onto the surface and this is different than changing the crystallographic distribution onto the surface in terms of crystallographic directions andplanes orbringing in certain pattern in that this is more ofthe topological texturing than a crystallographic texturing So,we will stop here and then we will move on to the next lecture where we will discuss specific examples Thank you very much