Mod-3 Lec-13 Resistance Welding Process

Mod-3 Lec-13 Resistance Welding Process


welcome students this is the eighth lecture in welding and this lecture is based on the resistance welding processes we have seen in arc welding processes that the heat required for producing the joint is generated by the arc which is struck between an electrode and the work piece and in this process the molten metal is generated but here in resistance welding processes the required heat for producing the joint is generated by the flow of current through the area where joint is to be formed so electrical resistance heating is basically responsible in all those processes which come under this category so heat required for producing the joint is generated because of electrical resistance heating through the contact interface where joint is to be formed the need of this process was a felt when it was required to join really thin sheets at a very high scale so if we see the capabilities of this process this process can produce the joints at very large scale and that to very high at a very high production rate so here we can say for a large scale and high production rate of the sheet metal component it is possible to use the resistance welding processes because it forms the joint in very less time and whatever joint is formed that is largely free from and the distortion or the distortion is very less and that is required for producing a successful successful joint of thin sheets and by any joining process and that role is performed by the resistance welding process why it is called so because heat required for producing joint by using this process is generated by the flow of generated by the resistance to the flow of current through the contact area where joint is to be formed and therefore this process is called a resistance welding process that there are various types of the resistance welding processes which are used in different industrial applications but the eight are the common one and out of these aides and here the first three types of the resistance welding processes are the most common and others are used in very specific areas the first three resistance welding processes are a spot welding seam welding and projection welding the application of these three welding processes are very extensive in in in the industrial sectors while the flashed welding works on welding upset but welding high-frequency resistance welding and high frequency induction welding are used in very localized manner for example flash welding is mainly used for joining the rail rails in the railway industry and upset but welding is used for producing in the rods in butting position or when ends to end are joined in by the upset but welding high frequency resistance welding and high frequency induction welding processes are used for joining for the fabrication of the pipes and therefore we can say that the spot welding seam welding and projection welding we should understand more clearly as these are very extensively used in the industry while others are used in very localized manner the resistance welding processes are slightly different from the arc welding processes in respect of the many parameters and if you will see here that the resistance welding processes use presser while in arc welding process no pressure is used pressure is used in resistance welding processes mainly to consolidate the partially melted metal present at the interface so that sound nugget can be formed and in in terms of other other other factors in light of which the resistance welding process differ from the arc welding processes is like no flux is used in resistance welding processes and mostly no filler method is also used sometimes it can be used so these are the two main differences which you will see in resistance welding processes compared to the arc welding processes where fluxes are used to remove the impurities from the molten metal and filler metal is normally used in thick sheet welding to to fill the groove between the plates to be joined and most of the resistance welding processes are automatic while arc welding processes can be automatic or the manual one and because of this nature of the resistance welding processes here the process variables are identified and established first and those are set before proceeding for the welding and during the welding the entire process parameters are maintained while in the arc welding process those are identified and then as per needs parameters can be adjusted so here since the process is mostly automatic in means resistance welding processes are mostly automatic and that is why identification of the variables is very important and those parameters are to be maintained for producing the success joined continuously in production the basic principle of the resistance welding process is lies in the generation of heat by the electrical resistance heating in very localized manner heat is mainly generated in the interface zone where joint is to be formed and for that purpose heat is generated by the flow of current through the interface and heat generated must be enough to bring the materials near the interface in the molten state or partially molten state and here the parts are joined with the application of the pressure once the partial molten state is attained then they are joined by using the forging pressure to form some nugget joint at the interface and heat generated during the resistance welding is expressed by the equation like H is the heat generated is equal to I square RT where I is the current R is the resistance and T is the time during which current flows here we can see heat generated in Joule and I is the current in amperes R is the resistance in ohms of the area of the contact where well T is to be formed and D is the time in seconds for during which the flow of current through the interface takes place so here the role of current and the interface resistance and the time for which current flows is significant in generation of the heat for producing the sound weld joint the the resistance or at the interface depends upon the condition of the work surfaces which are to be joined impurities presence of impurities and rough surfaces caused higher electrical contact resistance compared to the finished and smooth surfaces and to generate the higher amount of heat if the current is set on the higher side it may lead to the complete melting also or incomplete heating can also take place if the current is not set properly so the selection of all these parameters like current and the time is very important for generating the heat which can bring the interface region or the contact surfaces and to the molten state and then forging pressure can be applied to produce the nugget for producing the joint between the seats to be welded so here this the role of the current and the time is very important in a generation of the heat when current flow when flow of current takes place through the interface that temperature varies across the section of the plate being welded the temperature at different positions is found different and it is mainly determined by the resistance to the flow of current in the different zones as heat is generated and direct heat generated is directly proportional to the resistance offered by any point in this section which is being welded and then accordingly the temperature of that particular zone is determined higher the resistance for a given flow of current greater is the temperature which is no test say these are the two plates which are to be welded by the electrical resistance welding these are the two electrodes which are there on both the sides of the plates to be welded and between these two electrodes the work pieces are placed and when the the electrodes are brought in contact with the work piece we find the three interfaces one is between this electrode and the upper surface of the one plate is this one the region one we can say and another interface is formed between the two plates of the workpiece that is the two plates of the work pieces like in the zone two is formed by the between the two plates which are to be joined and the third region or third interface is formed between the electrode and the lower surface of the work piece and at the contact surfaces one two and three and the contact resistance is found higher compared to the other areas and that is why these 0.1 means region 1 region 2 and region 3 are subjected to the higher temperature compared to the other areas or other regions in the electrode or in the workpiece or II or in the other sections of the electrodes so here if we see that how the contact resistance varies with the from one end to the another one means from one electrode site to the another electrode side through the work pieces which are to be joined and and accordingly only we get the variation in temperature at the different zones so here we can see when the plates are to be welded are placed between the electrodes and the flow of current takes place and a nugget is formed at the interface between the two plates and this nugget acts as a joint for joining the two plates here we can see here how the nugget is formed say this is one electrode this is another electrode and this is the tip of one electrode and the tip of another electrode one electrode remains stationary another becomes moveable and between the two plates between the two electrodes workpieces to be joined are placed and then moveable electrode and comes in firm contact with the plates to be welded and then flow of current is started and due to the electrical resistance heating here well nugget is formed at the interface between the work sub work pieces are the plates to be joined here if we see that Y joint is formed or weld nugget is formed only at the interface between the work pieces to be welded to understand that we have to see that how the contact resistance and varies from region one in one electrode to the region seven in another electrode through the plates which are to be welded here we can see the region one lies in an electrode a moveable electrode and the region seven lies in in the fixed electrode and here region three is is in the middle section of the workpiece middle section of the workpiece and region two is at the interface between the upper surface of the work piece and the electrode so this is region 2 region 3 is in the middle of somewhere in the middle of the upper plate and the region 4 is the interface between the 2 work pieces which are to be welded region 5 again is in the middle of the two plates are middle of the lower plate which is to be welded and region 6 is the interface between the lower electrode and the lower surface of the work piece that is this and the region 7 lies in the electrode and if we see the contact resistance at these different regions we will find the contact resistance in the electrode at the electrode moveable electrode and and the fixed electrode is same here that is the lowest one this contact resistance magnitude is shown here and this y-axis represents the distance from the region 1 to the region 7 here so across so the variation in the contact resistance from one side one electrode another electrode can be seen from this diagram here it shows that contact resistance at region 1 and region 7 is the minimum one because these are made of of high electrical conductive conducting materials like copper or tungsten or they are alloys that is why the contact resistance of means resistance at region 1 and region 7 is the minimum one if you see the contact resistance at region 2 and region 6 which are indicating the contact resistance between the electable electrode and upper surface of the work piece and the the contact region 6 which is that indicating the contact resistance between the lower surface of the plate and a fixed electrode and the contact resistance here is somewhat higher here corresponding to this value and corresponding to this value so here we can say the contact resistance is somewhat higher between the electrode and the work piece and that is why for a given flow of current more heat is generated and that is why accordingly more temperature higher temperature we find at the interface between the electrode and work piece here between the upper surface of the work piece and the movable electrode and the same way we also find the higher temperature between the lower surface of the work piece and the electrode that is corresponding to this one and if we see that the contact resistance decreases because we pass through contact resistance at at point at region 3 and the 5 is is somewhat lower than the contact resistance at Point 2 at region 2 and region 6 because these regions 3 and 5 lie in between the workpiece surface in in in the workpiece itself that is where metallic continuity exists and this.y resistance will be somewhat lower for the flow of current and that is why somewhat lesser temperature is generated in the middle of the plates to be welded but if we see in the region for the interface between the two plates which are to be joined are subjected to the maximum or they have the maximum contact resistance and that is why maximum temperature is generated and this is the reason behind why the the melting of the metal starts first near the interface of the workpiece or at the interface of the plates to be joined so this is how the contact resistance and the temperature variation from electrode one to another electrode takes place and this explains that the maximum temperature is generated at the interface because of the high contact resistance at the interface here we’ll see schematically and we can see here the maximum temperature is generated and near the interface these are the two plates which are to be joined and the temperature is generated in around the interface of the two plates which are to be joined in it when under these conditions a weld nugget is formed we find that weld nugget penetrates to both the sites here partial penetration we can see penetration of the weld nugget is here in the upper plate side and also in the lower plate side so the this is the cost structure which is formed and the region around it we can see the area where because of heat the material has been affected and that region is this entire zone is termed as the heat affected zone and this is the and the cast zone where melting has taken place and we can see here some sort of depression also has been formed because of the electrode pressure and when negative formed the the penetration has to be optimum one not excessive or not less penetration will required here we can see the penetration both the sides of the plate is is the optimum one here we can see access to penetration of the weld nugget has taken place on both the sides and very thin seed has left unmelted and here this may we can the joint and here very less penetration has taken place on both the sides and this planet extent of penetration will depend upon how much heat is being generated and during the flow of current through the interface so the setting of the welding current and the time for which current flows is very important for the required penetration to form and the sound weld joint by resistance welding process the important variables which are to be kept under control in resistance welding process for a successful welding it is necessary to see that its current value optimum current value is selected and it is allowed to flow for preset proper preset time and after when the melting has taken place the enough pressure is applied so that the molten metal generated at the interface can be consolidated effectively to form sound joint here the if we see the variables which are to be controlled current current normally selected in the range of few kilo amperes because these processes are known as low voltage and high current the how much current is to be said that depends upon the material to be welded and thickness of the seat which are to be processed by the resistance welding process the voltage can range from 2 to 12 volts and the time can vary from few microseconds to few seconds and the force to be applied for the consolidation purpose may vary to generate a pressure in range of the 3200 MPA and the amount of presa which is to be used will depend upon the thickness of the seat to be welded the welding current which is to be used and the material which is to be welded and the purpose of applying pressure is to have the good electrical contact between the surfaces of the workpiece and the good contact between the electrode and the work piece surface itself and also to force the weld metal and during during and after the welding or of men when molten metal is formed it has to be consolidated firmly by using pressure so that there is no expulsion of the molten metal and for that purpose to force the molten metal and form a sound nugget now the required pressure is to be applied which can range from 3200 mpa and that the welding current is an important parameter and it has to be selected after giving the due considerations related to the material which is to be welded and the other related factors out of the factors which significantly affect the selection of the welding current are electrical and thermal conductivity of the metal the electrical and the thermal conductivity of a given metal is high then will require higher welding current because whatever heat is generated at the interface that will be dissipated easily because of high thermal conductivity and due to the high electrical conductivity the heat generated at the interface will be less so if the material is having high thermal and electrical conductivity will have to set the higher welding current and the same is applicable here also thicker the plates to be welded higher will be the welding current required for producing the sound nugget and like for welding of the mild steel and the current can be selected in such a way that the current density that is ampere per mm square lies in the range of 190 to plus minus K into e raised to your minus T where K and E are constants and T is the thickness of the seat in mm to be very dead this helps to get some idea about the the current density which should be there for welding of the mildly steel sheets at hundred four ten Hertz weld time and the weld conductive welding of the high conductivity materials like copper can be done by sandwiching the thin sheets of the electrical resistivity at the interface to generate the required heat actually welding of the high electrical and thermal conducting materials like copper becomes difficult by using the conventional resistance welding process because the amount of heat generated becomes less and whatever heat is generated because of high electrical conductivity and that is dissipated rapidly from the interface and to the base metal region rapidly due to the high thermal conductivity and that is why and mult it becomes difficult to get the molten metal in the interface region to produce the weld nugget and that is why a the welding of the copper and other high conducting electrical and thermal conducting materials becomes difficult and to make the weld joint possible of these materials and normally a thin sheet of the high electrical resistivity material is placed between the plates to be welded and then the flow of current generates high heat which is required for producing the joint but in that case the such process is not termed as electrical resistance welding but it is termed as resistance brazing and if we see the relationship between the strength of the joint and the welding current then we notice that increase in welding current increases the shear strength rapidly and it goes on increasing up to a certain critical level thereafter reduction in strength and takes place with the further increase in welding current and this is the attributed to the increase in very negative if we increase the welding current then it increases the heat generated and it also increases the amount of the molten metal which is formed at the interface which in turn increases the size of the weld nugget and increase the size of weld nugget helps to increase the shear strength so increase in shear strength is attributed to the increase in the diameter of the very nugget but after reaching to a critical value due to the excessive heating and the expulsion of the metal from the interface region X starts and because of which a reduction in the shear strength of the metal takes place another important variable of the resistance welding process is the welding time welding time normally is very short in resistance welding processes it can range from to 200 Hertz for 50 Hertz main supply and the welding time affects the amount of heat generated at the interface and so the strength and soundness of the weld joint because heat generated is found proportional to the time for which current flows through the interface longer the time greater the heat generated and accordingly there will be increasing in strength of the joint and actually this very time is controlled through the various means like the mechanically electronically and in pneumatically and the how long time for how long time flow of current should be there it depends on thickness of the metal to be welded thicker the plates to be welded greater will be the requirement of heat and accordingly longer should be the time for which current should flow through the interface for producing the desired heat and the thermal conductivity and electric that the weld time or the time for which current should flow and increase in thermal and electrical conductivity also needs the longer time for the flow of current here if we see the relationship between the strength and the weld time then here also we can notice that increase in very time increases the shear strength of the metal and this increase in shear strength of the metal up to a certain level is attributed to the increase in weld nugget diameter and due to the increased heat input when a heat input is increased that increases the weld nugget diameter due to the increased melting of the metals at the interface but beyond the limit here also expulsion starts and the reduction in is a shear strength of the weld joint and takes place the pressure is the another variable which plays significant role in producing the successful very joint by resistance welding process and the pressure is used to perform certain functions like the pressure increases the electrical contact between the workpiece workpiece and electrode and between workpiece offices which are to be joined to avoid the arcing between these components and at the interfaces and then initial and it also lowers the initial contact resistance at the interface otherwise there won’t be any flow of current due to the very high due to the large gaps between the plates or the electrode to be joined the pressure also suppresses the expulsion of the molten metal which is formed due to the heat generator during the welding and when molten metal is formed it is consolidated under the pressure to form sound nugget which is free from the defects the amount of pressure which is to be exerted depends upon the mechanical properties of the material to be welded stronger the material greater will the pressure required for producing the sound joint mechanical properties of the metal to be welded affects the pressure which is to be used for producing the sound joint soft materials may get damaged under pressure and form the indentation for example mildly steel needs about 70 mph or for producing the joint and for high-strength steel this magnitude of pressure can be the number of times higher than that of earth required for mildly steel and the metal plate metal plate thickness or the thickness of the sheet to be welded also affects the pressure to be used thicker the sheets greater the pressure required and the same is for the current if the higher is the current is to be used then pressure requirement decreases but there is a limit up to which increase in pressure can be used to reduce the pressure requirements excessive pressure with the soft materials particularly can lead to form indentation at the surface of the workpiece and which can damage at the interface we can see here the depression formed or the indentation formed at the surface of the work piece due to the heavy press are transferred from the electrode to the workpiece surface this indentation can reduce the esteem value of the product or can damage to the surface of the work piece so the process parameter selection like welding current welding time and the pressure is important for are important for the selection of the weld metal for for the successful joint for producing the successful joint and the selection of these parameters to a great extent depends upon the material of the component to be welded in terms of the mechanical thermal and electrical characteristics of the plates to be joined thickness of the sheet to be welded and the type and size of the electrodes which are being used for producing the joint the steps which are used in spot welding for producing the joint starts with the with the from contact between the electrode and the work piece and the form contact between the workpiece interface so under the pressure the electrode surface means the pressure is applied through the electrode on the surface of the workpiece to have the firm contact between electrode and work piece and the form contact between the workpiece surfaces so the well cycle starts with the upper electrode moving and contacting the workpiece resting on the lower electrode which is stationary and the work pieces are held and repressor and then flow of current is started for a preset time and when flow of current starts he generates heat at the interface and the maximum heat is generated at the interface and which leads to the partial melting of the workpiece of surfaces at the interface and under pressure and the molten metal is squeezed and consolidated to form nugget and when nugget is formed well- formed it is allowed to cool under pressure and then pressure is released gradually these are the four steps in producing the joint by spot welding and the entire all these four steps are termed as the resistance spot welding cycle if we have to see then it includes the squeezing time with the time for which the initially pressure is applied and during which pressure increases from zero value to the maximum squeezing pressure and this is applied for form electrical contact between the electrode and work piece and initiate the application of the pressure so means when there is no pressure pressure application is started in squeezing time and until it reaches to the maximum level welding time is the time during which flow of the current takes place and it determines the amount of heat which will be generated at the interface hole time is the time during which pressure is maintained but current is switched off and during this period the consolidation of the partially melted metal takes place to form weld nugget and off time is the period during which the shifting of the workpiece from one station to the another station takes place before the weld cycle can be repeated schematically we can see these four steps like this represents to the off cycle period where shifting of the workpiece from one station to another station takes place and in this case there is no firm contact between the electrode and the work piece and at the location to here here squeezing starts and the direct electrode comes in contact with the work piece and the pressure is increased gradually and here in this period in the squeezing period squeezing time the electrical pressure is increased gradually so that the electrode and the plates come in from electrical contact with each other and then this maximum pressure is maintained here maximum pressure is maintained and the flow of current is started so the time this period during which flow of current takes place is termed as weld time and after that current is switched off this pressure is maintained for some more time and this pressure is known as force time during this period only consolidation of the molten metal takes place and then pressure is released gradually so here this way we can see again we reach to the same stage there is no contact between the electrode and work piece so this way four steps are repeated for repeated for producing the joints and continuously first squeezing then welding then holding and then off period during which and the electrodes are not in contact with the work piece and for producing a successful joint sound joint the surface must be free from the impurities and the dust and dot if the rust is present or dust and oil and grease is present these must be removed by using the suitable techniques if the rust is present at the surface we should follow this procedure like liquid pickling treatment is done first by dipping in dilute acidic bath and then washing in hot water bath washing in cold water bath and followed by drawing using the compressed air jet and if the surface is free from the rust then pickling is not required but only surface cleaning is done using the acetone to remove the oil and grease from the surface the different types of the joints which are produced by the resistance welding process is like a spot same and projection welding the most common one is the lap joint however but wind can also be produced in some cases in the resistance welding process like in case of reduction of the welded tubes by seam welding butt joint is used while in butt and flesh welding process but vent is produced for joining the dots where ends are butting and together and here the lap joint is formed in this manner when one plate overlaps another and at the interface joint is formed and in case of the bird joint and so are touching each other on they are aligned and the joint is formed between them in the spot welding process it is the first and most important resistance welding process which is most extensively used in this process two or more sheets of the metal are held between the electrodes through which welding current is supplied for a definite period and then force is exerted on the work pieces to produce the joint this is how power is supplied and the workpiece is to be welded on captive between the two electrodes and by flow of the current heat is generated for a limited period and then weld nugget is formed like this at the interface and the power supply for the resistance spot welding process can be of low AC value or the high DC value low AC current is supplied using a single phase step-down transformer and the high DC current is supplied using three phase rectifier to balance the load of three-phase supply and the welding current is to be controlled properly to avoid the defects and to form the sound weld joint like the rate of increase in current and the rate of decrease in current that is termed as upslope and downslope should be controlled to get the desired weld upslope should be controlled to avoid the overheating between the electrode and the work piece surface and also to avoid the expulsion and between the interface of the workpiece and interface between electrode and the work piece if the rate of rise is very high then it can lead to the excessive heating between the electrode and work piece surface and which can ultimately lead to the expulsion and between the electrode and work piece interface and the down slope is controlled to to have the slow cooling rates if that current is switched off rapidly and then the high cooling rate can lead to the cracking in the materials particularly which having high harden ability like high hardenable steels can show tendency to get cracks if the current is switched off rapidly and so that can be controlled means this cracking can be avoided by controlling the downslope by low and which in turn lowers the cooling rate and them and also lowers the solidification rate electrodes which are used in a spot welding can be of different shapes the pointed type or truncated pointed type or which are also in truncated a cones in shape having the angle from 120 to 140 degree are used for the welding of the ferrous metals but these shows these show tendency to we are out rapidly and that is why our to be reshaped regularly and don’t shape the electrodes having the dome radius from fifty to hundred mm used for the heavier loads and where severe heating is to be used for producing named and the weld joints of the non-ferrous metals and the flat tip electrodes are used where minimum distortion is required in the plates to be welded we can see three different shapes of the electrodes which are used in a spot welding truncated cone dome shaped or flat chipped electrodes materials of the electrode should have certain characteristics means the electrodes are made of certain materials which can offer very good thermal and electrical conductivity and having ability to withstand under the pressure because pressure is to be transferred through the electrode on to the workpiece so these electrode materials must be strong enough to sustain high pressure at elevated temperature and these are the common materials which are used for making and the spot welding electrodes these are basically copper or kangas turn-based and wise because the both these materials show good thermal conductivity and electrical conductivity but addition of the alloying elements in turn reduces the electrical conductivity but these additions are frequently used to increase the strength of the copper base l wise particularly the copper based alloys which are used for making the tangle making the electrodes for the spot welding system can be of other copper cadmium based copper as silver copper chromium copper nickel copper beryllium copper cobalt copper zirconium and copper tungsten and the tungsten based alloys are also used for making the electrodes and these are mainly used where it is required to have very low via rate and the deformation tendency to the electrode is more so here if the deformation is to be controlled and wear is to be controlled then these technician base electrodes are are used but the cooling is the problem with these electrodes and these needs cooling through the using water circulation to maintain the temperature within the limits the common understand alloys which are used for making the spot welding electrodes are pure are made of pure tungsten or tango stencil over and tennyson copper alloys this is how cooling is done in jungle stone in the spot welding electrodes to maintain the temperature within the limits here temperature water is supplied from one side and it is taken out from another side this is the case of cooling for a spot welding purpose and if the electrodes for seam welding us to be cooled then these are used in form of rollers and the water is supplied from one side and it is taken out from the other side to maintain the temperature of the electrodes within the same limit electrode diameter affects the size of the Nugget which is produced and the Nugget size is determined by the thickness of the plates which are to be welded and if thicker plates are to be welded we will be going for the larger nugget size and to produce the last nugget size we have to supply large amount of heat and for that we need large electrode size so the selection of the electrode size to a great extent depends upon the nugget size which is to be produced which in turn depends on the material thickness which is to be welded theoretically negative to the diameter of the rivet which is to be used for joining the two sheets by the riveted joint to get the equivalent a strength so the diameter of the nugget and the diameter of the rivet it which is to be used for producing the joint by the riveted joined these two should be equal and and here you will see the nugget diameter which is normally used or which is normally produced for welding of a particular thickness of the plate and it is six times at T raised to the 1 by 2 or a square root of T here where T is the thickness of the plate and the electrode diameter can be the 5 into the T raised to the power 1 by 2 same for same the D also means that is the electrode diameter can also be obtained by using equation 2 point 5 plus twice of the T where T is the thickness of the seat which is to be welded in mm and the weldability of the of a given material by the spot welding to a great extent is a determined by the electrical resistivity thermal conductivity and the melting point these are the three main parameters that affect the weldability of the metal by spot welding and that is why the weldability of the metal by spot welding is calculated using equation which is based on the electrical resistivity our thermal conductivity K and melting point P for a given material if the electrical resistivity is high thermal and thermal conductivity and the melting point low then we will get the good thermal good weldability of the metal by the spot welding say here high electrical resistivity low K and low melting point results in high variability for metals like aluminium and copper which show high thermal conductivity and a low electrical resistivity are found difficult to well and that’s why the if the weldability is calculated then these materials so very low weld ability and the value ability can be calculated using equation like this weldability expressed in terms of percentage like is equal to the electrical resistivity in 200 divided by thermal conductivity of the material multiplied by the melting point where electrical resistivity are in micro ohms per centimeter relative thermal conductivity of the metal with respect to copper is a has been quantified and it has been tabulated in handbooks can be obtained and the melting point in degree centigrade the variability w calculated using every equation can be interpreted in terms of how easily we can weld a given material by the spot welding process if the W of material comes more than two it shows the excellent weld ability and if the W of material is in between point seven five and two then it is considered a good weld ability if the W value or comes less than 0.25 then it is considered of poor weld ability and for example the valid ability in terms of percentage for mild steel is found in range of or above the ten which indicates very good weld ability of the mild steel by the spot welding while that for aluminium lies in between one and two for the different alloys this w will different different aluminum alloys this will be in range of one to two means it is fairly good and for copper alloys further it is found less and if we see the applications of the spot welding it is widely used in electronics electrical aircraft at home well and hope Huma planchas in electric in Auto while industry particularly it is used for joining the body parts of the automobiles car body parts or the trucks and buses body parts are and joint and the tanks patrol tanks which are to be made are normally produced by the spot welding process the seam welding is the another important resistance welding process which is normally used to produce the leak-proof joint and in this process the overlapping seats are good between the two roller discs which acts as electrodes and the current is passed through these rollers and this current can be supplied continuously or in intermittent manner to form nugget means the the seats to be welded are passed through the rollers and the rollers will be routed continuously through a seat will be passed and these seats are to be joined and the when current passes through and the workpiece is to be joined heat is generated and nugget is formed at the at the interface of the work plates or the seats to be joined and the supply of the current can be continuous or intermittent when continuous current is supplied we get the overlapping words or nuggets otherwise we get the intermittent nugget so according accordingly as per the needs whether intermittent nuggets are to be formed or the overlapping nuggets are to be formed according to that the current is supplied in impulses or in newest manor schematic of the seam welding can be seen here these are the two rollers which acts as the electrode and one electrode is is affixed and another is a captive movable fixed means both rotate during the operation but one can be adjusted vertically to accommodate the place plates to be welded and during the route rotation the place will rotation of these rollers the plates will be passed through these rollers and these rotating rollers will also be acting as the electrode to supply the current to these plates which are to be very dead schematically here we can see from the side view if you see the section of this arrangement these are the this is the upper lower roller and this is a rower role lower roller and these are the two plates to be welded flow of current through these plates forms the nugget between them like this here we can see more clearly the this is the upper roller low row lower roller and these are the two plates which are to be welded and the seam is formed along this line here and if you see this diagram the rollers are rotated and the Nuggets are formed in series in overlapping manner this is the side view of the same diagram and it shows that the interface weld nugget is formed the dis overlapping of the Nuggets may vary from the 10 to 20 50 percent depending upon the purpose of the joint the Nuggets can overlap from 10 to 50 percent like for gas tight joints the overlapping can be 40 to 50 percent however for a water and other flip tight and liquid tight joints it may vary from 10 to 40 percent so when it is approaches around 50 percent means when overlapping approaches about 50% it is termed as continuous well and overlapping wells are used for air and water tightness and for gas tightness it can range from 40 to 50 percent here we can see that the Nuggets are intermittent and here the the Nuggets are overlapping and here the overlapping is more than 50 percent you can say that nugget is formed continuously these are the different shapes of the the rulers which are used in seam welding and these are the points which are to be considered the kind of pressure distribution required and the welding current which is to be used based on that we select a particular geometry of the roller to be used for application of the seam welding includes like making the patrol tanks for auto while seam welded tubes drums and other components for domestic applications and seam welding is relatively fast method for producing the high quality wells so here now we can see that the some of the limitations are also they are with these equipments like Cubans are costly and maintenance is also expensive and for these processes are mainly limited to the thin sheets where it exceeds cannot be processed by the resistance welding processes particularly seam welding and a spot welding so now here I can summarize this lecture here in this lecture we have seen the basic principle of the resistance welding processes and some of the technical aspects related to the spot welding and seam welding we have seen that these processes can be successfully used and are being successfully used in in production of in production of the components which are to be made by using thin sheets however these processes cannot be used for producing the joints of air it exceeds thank you for your attention

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