Lec 15 – Laser beam welding

Lec 15 – Laser beam welding


hello i welcome you all in this presentation
on the laser welding related with the subject joining technologies for the metals you know
this is one of the radiation based processes another is the electron beam welding and both
these processes are known to offer the higher power density wherein right from the heating
to the melting and evaporation can be achieved by adjusting the power density suitably so in this presentation mainly i will be talking
about the laser welding laser welding is basically the abbreviation
of like light amplification by stimulated emission of radiations
so this process basically sources of energy using laser basically one of form of energy
is converted into the radiation energy or the electromagnetic radiations so energy may
be in form of electrical or thermal that is converted into the electromagnetic radiations
these radiations include all types of like ultraviolet radiations infra-red radiations
and also the visible light radiations so from these radiations basically the single
wave length radiations are obtained which are called monochromatic radiations which
are used for processing of the material so how it is done and how it is achieved that
is what we will try to see in this one basically the most common way
is to that the lamps mainly called flash lamps are used to excide the material mostly in
the form of ruby crystal and from this after the excitation the laser radiations are emitted
like this then they are reflected back using suitable reflectors here like this and then
they are brought to the parallel then through the lines it is focussed at the desired point
and this point becomes the workpiece so when they are targeted or applied or directed
over the surface of the workpiece so using flash lamps light energy is directed which
excites the material which may be in form of the gases or the ruby crystal gases like
helium or argon or carbon dioxide can be used these are the reflectors which will be reflecting
and focussing all these things towards the lenses and these lenses will finally be converging
it onto the surface of the workpiece and whenever it is directed over the surface
of the workpiece a kind of energy density or the power density that we get becomes extremely
high which may range from like say 10 to the power three to say 10 to the power eight power
mm square or more so in that case there is lot of flexibility to achieve a wide range
of the power density and this feature is exploited for the various purposes in processing of
the material which includes so by regulating the energy density we are
in position to use it for the heating purpose which is basically used for the like say laser
hardening material is heated rapidly at the surface and then by auto-quenching or by external
quenching it can be harden like say the surface is directed with the laser and here it gets
heated reaches to the rising temperature and then it is quenched off as laser moves away
from the directed surface and then this quenching results in the hardening
of the surface layer but for this it is necessary that material has a hardening characteristics
which is primarily say for medium or high carbon steels and cast iron so this method
is found to be suitable similarly this is also used for the melting purpose now melting
when the laser is used for the melting purpose we need much higher density as compared to
that for the heating and this can be used for the welding where
fusing of the fusion of the faying surface is achieved may be used for the brazing purpose
or may be used for the alloying of the surface layers like this and it is also used for the
controlled removal of the material which we can say that machining so this is what we normally call as a machining
and in this one basically the evaporation or ablation of the material over the directed
surface is achieved so that the controlled removal of the material takes place in order
to obtain the desired geometry and the sizes with surfaces of the material so the machining
welding or the melting and heating all can be achieved by adjusting the power density
of the laser being applied over the surface so there is one very general guideline kind
of the diagram which will help us to see the kind of approach which is used here in the
x axis we have the exposure times means the time for which the laser is applied so here
extremely a small say minus 10 ten 0 minus 10 ten to the power minus 4 and 10 to the
power zero and in the y axis we have power density or energy density which is in watt
per mm square so here say 10 10 to the power 4 10 to the
power 8 so if we see this is the band for the band which is used for the purpose of
the heating goes in like this and the band for the purpose of the metal so they are three
zones basically combination of the power density as well as x to the time can be used for the
different purposes this is the zone for the heating purpose means it is used for the heat
treatment purpose or softening of the material like bending so this location lies for the say bending
purpose where very short period the laser is directed so material gets soft and then
it is after softening bend by applying suitable pressure similarly these are the bends for
the heat treatment purposes as for as the power density and the exposure combination
is concerned and then we have the like say welding so welding lies somewhere here like
slightly more than 10 to the power minus 4 seconds and here 10 to the power 4 or more
power density and further higher power density and shorter
exposure periods are used for the purpose of alloying of the surface wherein the material
to be alloyed is led or over the surface and then laser beam is passed over the surface
so that the fusion control fusion of the base material and the material over led on the
surface fused together and results in the alloying of the surface and the brazing is used here for the lower
power density for the longer periods like say this is the kind of a zone for the brazing
purpose and then this is the area where so this band between the two corresponds to the
melting and then evaporation or ablation so here the lower power density for longer period
and higher power density for shorter period so more than 10 to the power 8 power density
is normally used for the evaporation and machining purposes so that somewhere here for the machining the
power density and exposure time combination for the machining purposes so basically if
higher is the power density we need the shorter exposure time for generating the heat depending
upon the purpose of the using laser we may use it for very short period like suggest
for softening purpose higher power for soft purpose for short period and somewhat the high power density for somewhat
longer period for the machining or the cutting purposes and for the brazing purpose lower
power density and the longer period similarly somewhat moderate power density and the longer
exposure period is used for the welding purpose so this is how this diagram gives the broad
idea about the kind of the exposure period and the power density is to be used for the
different purposes now how to get the suitable exposure time
for this purpose now we say this is the surface of the workpiece so now we can direct the
laser at particular point normally the using the cnc table over which workpiece is mounted
is moved with respect to the laser laser is now directed at one location and position
of the workpiece is adjusted using the control movement using the numerically control machine
tool so now the speed of the movement can be adjusted
suitably in order to have the required exposure time it may be very slow for longer exposure
period and it may be very fast for the shorter exposure period so basically the speed of
the movement means the relative speed of workpiece with respect to laser beam determines the
exposure period so normally what we do higher scanning speed laser scanning speed or the
workpiece is speed with respective laser is used for the short exposure periods so depending upon the combination of the power
density and the scanning speed suitable joints are made for example like say the laser we
may use of the co2 laser or nd yag laser this offers the short wavelength laser and this
is of the co2 laser is the high wavelength laser which is about 10 to the power 6 micrometer
and here it is 10 something micrometer so the short wavelength lasers they reflect less
as compared to the high wave length lasers so here if this is the surface of the workpiece
and we are directing the laser over the surface so the diameter over which laser can be focussed
is very small say in case of nd yag laser typically say 200 to 400 micrometer so the
beam is focussed over the surface it may be of a say it is common to have 1-2 kg watt
lasers while co2 laser can be as high as 25 kg watt capacity so what is of the laser metals allot in delivering
the power over the surface so higher is the power which is available with the laser it
can deliver lot of energy over the smaller area in short period so say for 1 kg watt laser or 500 watt laser
if it is focussed over the surface of the area over which it can be focussed say 200
micrometer for say typical value for the nd yag where here nd is neodymium yttrium garnet
aluminum garnet so these are the three abbreviation for the neodymium yttrium garnet aluminum
garnet laser this is one typical laser and in this case what we do laser is directed
over the very small so the melting takes place so you see melting will be taking place like
this so if it keeps on increasing the duration of this state the depth penetration will keep
on increasing up to certain limits so we have to identify that how to adjust the scanning
speed so that we get the suitable depth which will be sufficient for penetrating through
the thickness and once this depth is achieved we can start moving the laser as per the required
speed so the speed of the relative movement like
the relative movement between the laser and the workpiece is determined using suitable
so you can say the laser or workpiece speed whatever is used so it will help us in determining
how much energy is being delivered in unit length and then the power of the laser certainly
matters power of a laser so increase in power and decrease in speed will increase the power
being delivered or energy being delivered to the work piece and so it will increase the depth of the penetration
and the size of the weld which will be made now as i have said since the laser is directed
over a very small area and very lot of large amount of the energy is delivered in very
short period so amount of heat actually required for the
welding purpose or for fusion purpose is very small so here you see these are the two sheets
to be welded and the two sheets to be welded must be properly machined aligned this is
very important in case of laser accurately machined and aligned if any gap is left between
the two plates to be joined like see the gap is if say 300 micrometer between the plates
due to the machining accuracy or misalignment if the gap of 300 micrometer is left between
the two and we are using the laser of 200 micrometer than at that location you will
see that laser will just pass through the surface of the workpiece pass through the
gap between the plates to be welded without causing any heating so it is very important
that the edges of the plates must be properly machined they should be straight and aligned so that the gap between the plates to be joined
is very less or rather smaller than the diameter of the laser which is to be used so here if
the diameter is larger than the gap which is there than of course it will result in
the continuous melting of the faying surfaces and this melting will be so fast faster melting
using very less heat input so h-net is very small in case of the laser welding and if the h-net is very small means the fusion
is taking place like say this one if this is the side view here and the laser will be
generating very you can say not the volume of the weld metal produced is very small and
we can say aspect ratio is very high it means depth to the width ratio is very high and
the thickness for which the laser welding is normally used is 19 mm of the steel however using the high capacity lasers efforts
have been made to weld in single pass up to 32 mm also but it is not so common mainly
up to the 19 mm thickness plates have been welded in single pass so here the thing is
since it works very fast it melts very fast and using even very less heat input so the cooling rate experienced by the weld
metal is very high so high cooling rate results in certain favourable things and unfavourable
things also favourable things like high cooling rate results either very fine grain structure
which is good from the mechanical properties point of view sometimes even amorphous structures
also obtained but sometimes too high cooling rate can lead to the entrapment of the gases
in form of ferocity or if the material is hard enable then excessive hardening can cause
the cracking tendency in the hard enable steels so this is another problem related to the
high cooling rate fine it is good to have the fine grain structure for improve mechanical
properties sometimes it results in the amorphous structures also but since the cooling rate
is very high so there is a possibility that the gases will get interrupt or in case of
the hardening steel materials may harden too much so leading to the cracking tendency due
to the residual tensile stresses other effective features of the process are
like reduced heat input like one vector is this cooling rate another is since the weld
volume and haz both are less due to the reduced heat input so this is the so that related
expansion and contraction (()) (22:26) the volume of the metal which is heated during
the welding and then contacts during the cooling is the small so these in turn reduces the
residual stress volume so reduction in the residual stress decreases
the distortion tendency this is one thing since this entire volume is extremely heated
volume of the well metal which is produced and the heat effected zone which is produced
in case of laser welding due to the limited heat input for the fusion purpose is less
so it results in the reduced residual stresses as well as the distortion tendency further most of these joining in case of the
laser welding does not use the external filler just the fusion of the faying surfaces has
achieved so mostly it is autogenous weld in this case there is no use of no contamination
from outside in which form like no electrode no filler or no external gases however for
the good quality purposes we may use the shielding gases so shielding gases may help in improving
the quality of the weld joint by avoiding the interaction of the atmospheric gases with
the molten metal so these are some of the positives of the
laser welding apart from this we need to see the capability and the limitations of the
laser welding from the welding point of view so further no vacuum requirement in this process
and a very small tiny welds can be made this feature is extremely exploited in joining
of a very thin sheets for mass production purpose because the welding speed is high
thin seats in the controlled conditions can be welded effectively once the process parameters
so of course the power and speed these two need to be optimised properly so that we get
the required depth of the penetration and depth of the fusion however the depth of penetration and depth
of the fusion are significantly influenced by the process conditions and the molten being
welded therefore we need to ensure that the consistent penetration is consistently acute now some of the limitations are also they
are related with this process since the laser beam diameter is very small so it is important
that the workpieces or sheets or plates to be joined
are placed very accurately otherwise we will find that the beam has passed from somewhere
else so workpieces are placed very accurately to ensure that the beam passes through the
centre line of the welds and very limited penetration it is normally up to 19 mm only the true thickness
penetration weld can be made it is very common so it is like up to the 2 cm only it can be
welded and very good control over the process parameter is needed otherwise we may get inconsistent
penetration and another the issue related with these is that high reflectivity materials
like aluminium and copper these impose lot of difficulty related with the reflection because whatever beam is directed over the
surface it gets reflected and hardly say 5-10% of the energy is absorbed so the melting is
very difficult and high thermal conductivity further makes this melting of the copper and
aluminium difficult so that these materials require especial precaution in order to improve
the absorptivity and reduce the reflectivity it is common for this materials that efficiency
is very less like say 10% only so that is why some roughening of the surfaces
or application of the coatings like the black prints etc help in increasing the absorption
of the laser in case of the aluminium and the copper now we will see it from the penetration
or performance point of view we know that the laser is used in two ways
for the welding purpose one is the heat applied is transported through the conduction and
then melting takes place this is called conduction mode and another is key hold mode so what
is the difference in these and what happens when the laser is directed on the surface
of the work piece so for this purpose what we need to see is
one diagram here we have power density in terms of the watt per square inch in 10 to
the power 6 here like say two four six eight ten twelve like this and on the other hand
the penetration depth which is achieved in inches it is like 04 08 and like this 044
so when the power density is increased what we get like initially it keeps on increasing
linearly like this and then it jumps fast it increases rapidly with the increase in
the power density so this is the zone for the conduction mode and this is the zone for
the key hole mode so here the penetration the width is large and penetration is limited
width and penetration increases width is also high but thereafter will see the penetration
width does not increase much but the depth of penetration increases so this depth of penetration will keep on
increasing without increase in the width of the weld so this is what is obtained in key hole approach
but it imposes certain problems like high energy density results in the evaporation
of the metal and a metal vapours actually ionised an ionisation of the metal vapours
forms the plasma and this plasma absorbs the laser so whenever plasma is formed in this
gap the plasma absorbs the last and which in turn reduces the actual energy being delivered
at to the base metal and that in turn reduces the depth of penetration so depth of penetration is reduced due to
the plasma formation so this plasma formation must be taken care of or it must be suppressed
so either the energy density and the scanning speed parameters are controlled in such a
way that the plasma formation is avoided or if it is being formed in any case then the
flow of the inert gases during the welding is ensured in such a way that the plasma is
expelled from the whole which has been formed so a proper care is taken during this process
means directing the inert gases in such a way that the key hole is not disturbed otherwise
it will get collapsed it will adversely affect the weld joining process or the welding process
so this is what we have seen the depth of the penetration is affected by the energy
density and the formation of the plasma also affected so the difference metals of course
all of the different kind of the depth of the penetration so here now i will conclude this presentation
in this presentation i have talked about the basic principles how the laser beam can be
effectively used for the welding purpose what are the positives or the advantages of laser
welding and in what way it offers certain disadvantages for the laser welding and what
are the points must we kept in mind while using the laser welding for the consistent
and uniform penetration during the welding thank you for your attention

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