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Ultrasonic cavitation as way to create impossible alloys?

I played around with ultrasonics now for a while and noticed that when it comes to certain things then logic seems no longer to apply.In the normal household you might find some ultrasonic cleaner and that's about it.A few people might have some distance measuring device or sensor array somewhere.As far as the normal human is concerned that is more than enough ;)Playing with certain metals like Bismuth or Gallium is not only but also a nice way to create nice alloys that you can play with even more.Take a portable and simple hydrogen supply as an example.Just make an alloy with lots of aluminium and a small amount of gallium.Cut it into strips, blocks or grind into a powder if you dare.Either way you just add water in a sealed container and get lots of pure hydrogen.The waste product is aluminium oxide, which has additional uses.The gallium itself is not affected by the reaction and can be reused many times.However, with some metals things are just different.As you might know it is hard to impossible to create certain alloys and other wouldn't make any sense.For example an alloy made from Calcium and iron...One of the big problems with alloys is that you need to have both metals in a molten form, then mix them properly and hope it turns out as planned.And well, if the metals in question just on't want to combine we cheat by using slats as a flux for example or by blowing hydrogen through the molten mix to act as a sacrificial binder until the metal cools down.Through ultrasonic cavitation we can not only clean surface, the same effect also destroys cells as the power from the implosion and the intense heat is more than what a cell can handle.There are even tests now to determine how safe and effective it would be to sterilise hospital equippment.A few seconds in an ultrasonic bath would safe the hours in the autoclave...On an industrial scale ultrasonic vibrations are used to weld plastic parts - like the head and tail lights on modern cars or just sealed plastic housings of any kind.With all this in mind my experiments with ultrasonic soldering made me wonder...Science papers state that that for example ceramics are not actually soldered.Appearently it is again hydrogen bonds provided by the ceramic or trapped air inside that provide the means to stick permantly.There is also an effect based on the implosion of the cavitation bubble.Here the solder literally is shot at well aboce ultrsonic speeds onto the surface of the ceramic.Together with the vacuum effect the solder is then pushed into the tiniest of cracks and cavities.Surface tension and other effects finally prevent the solder from just flowing off like it would do if we use just heat.What it means is that there is no real soldering at all happening.In reality it is like millions of big hydraulic presses would push the molten metal onto the surface.Going back to the fun of Gallium with Aluminium....Aluminium does not really go to well with steel.And gallium does not that good with steel either.Melting an Aluminium-gallium alloy is quite simple.With an excess of Gallium in the mix it should be possible to add fine steel powder (steel, not iron!).Of course it would neither mix well nor really melt at these low temperatures.With ultrasoic cavitation however we could force the stuff to not only mix but also create the same effect as used by ultrasonic soldering.The additional metals and minerals in a steel alloy should hopefully prevent any unwanted reactions in the final step...If the steel powder is ine enough then the assimilation of the steel into the aluminium-gallium mix would result in the breakdown of the steel.Once cooled and hard again the big question what would happen if we let water attack it?In theory all aluminium would react to form aluminium oxide and aluminium hydroxide.The gallium again would not be affected and as it is also bound to the steel should form a nice gallium-steel alloy.But what hapens to the voids where the aluminium was???The alloy would either be only affected on the surface or through cavitation and time all aluminium would be transformed.In the best scenario we would get a steel-gallium sponge where the voids are filled with alumium oxide.Forging such a mix could result in a ceramic steel..... !?? ;)Imagine a safe...There is always forceful ways to get in.Like drilling or using a big angle grinder.The pro might use a magnesium torch rod though....The common approach to improve penetration resistance is by filling a space between the outside and inside walls of a safe.Whatever you can imagine that is nightmare for your tools can be used, like thick glass plates, hardened steel bits, carbide studs, concrete with glass fibres....But even diamond tipped tools would already struggle if the steel itself would contain high amounts of a hard ceramic like aluminium oxide.The remaining gallium would also cause very high friction and through this heat - which these tools really can't stand unless you can provide water cooling as well.With the right balance of aluminium and gallium most of the original properties the steel had can be preserved.Just and idea though....

Topic by Downunder35m  


what is the relationship between cavitation of pumps and pressure of liquid and temperature of liquid?

I want to know the relation of cavitation and liquid pressure and temperature? 

Question by waila4    |  last reply


What would happen if you apply ultrasonic levitation to water ?

While tinkering with my latest creation, a 6L ultrasonic cleaner, I started to wonder...When using these actuators for cutting or welding purposes a horn is used to provide some amplification of the movement and to get the right shape for the tool.Oil seals, like used on drive shafts are no problem when attached to a node of the horn.And if you ever placed a small diameter horn into some water to test it you know the storm it creates.But what would happen if two horns are used on opposing sides of the container?At the correct distance the same static nodes like we see when levitating in air should appear.What would happen to the tiny cavition bubbles that shoot away from the horn?Am I correct to assume that, at least in theory, it should be possible to create a stable vacuum bubble between the horns?

Question by Downunder35m    |  last reply


homebrew ultrasonic cleaning solutions

I just picked up a branson 5210 2.5 gallon ultrasonic cleaner.i will be using it to clean aluminum motorcycle carbs that have been sitting and become encrudded.i have gotten a lot of suggestions as to what sort of home brew recipes work well, i was hoping people around here could tell me what they use or have heardi will be ordering some of the carb cleaning stuff from sharperteks website to compare it to different homebrew recipes. my goal is ease and lack of cost here. thankshttp://i9.ebayimg.com/01/i/001/43/54/91b8_35.JPG

Topic by 666mph    |  last reply


Universal ultrasonic driver circuit - help required

I would like to build a few, properly working, ultrasonic devices.For example an ultrasonic soldering iron and an ultrasonic soldering bath.But some small ultrasonic plastic welder or cutter is nice too :)If you ever had one of the above to play with you know why they are great to have.The development story so far:I managed to destroy several driver boards.The ones you find for cheap with 28 or40kHz transducers in your favourite online store.In the beginning I knew I will have a need to repair or replace these boards but no clue why.Take an ultrasonic cleaner and read the manual.There it is always pointed out that a low water level can destroy your toy.What does that exactly mean?The transducer needs to be kept in resonance, if the water level is too low or something havy sits right at the bottom of the tank the frequency drifts off too much.Very expensive untis can cope a bit better here, which gave me the idea for the universal driver.During my experiments with hoorns I noticed that it is very hard to get usable results without extensive computer simulations first.Just one mm too long or too short and literally nothing happens, go a bit further and a thin aluminium horn might start to crack under the stress.And in all these cases the driver overloads, in one cheap case to the point that the transducer fused together.Trying to examine these driver circuits while they operate turned out to be a total nightmare!Place the probe from the ocsilloscope literally anywhere and the thing goes out of tune already.By the way: Never coil up the wires going to your transducer.....Only way I found that somehow works is by adding a tiny transformer around the wire going to the transducer and to measure the voltage generated there.To make it short: Destructive testing provided the requirements a driver needs to match to keep the cost low.Reasons for the premature death of cheap driver boards:Almost all of these cheap drivers I could find generate the 28 or 40kHz signal from the mains voltage.Means it goes through a transformer to get the desired 50-80V and some witchcraft turns that into a more or less smooth DC voltage.This is then switched by some beefy transistors, mosfets or similar, depending on the circuit.The actual feedback happens with a tiny ring toroid, similar to what you use to drive a ZVS system.With this dirt simple design a fully tuned transducer - like when nothing is attached to it yet - would cause the driver to provide a voltage of about 6x of what the transducer is rated for.Thankfully in most cases the transducer survives this a couple of times while the transistors fry within about 3 seconds no matter how good the cooling.Slightly out of tune - like when mounted onto a cleaning tank - the resonant frequency is slightly off the tuned 28 or 40kHz.The driver compensates this through the tiny feedback transformer.But this only goes for a about 1-4kHz, drift away further and first the power drops, then the voltage spikes and it dies.The feedback is not able to shift the generated frequency enough as it is ultimately derived from the mains frequency of your grid.Reasons why a dedicated, low cost driver would open new possibilities:Imagine you need to make a horn or sonotrode for your transducer.Knowing that each half of it should be equal to a quarter wavelength of the operating frequency is nice and easy.But if you add something like a blade for cutting or you need some pressure for welding then calculated dimensions become useless.Programs to fully simulate complex sonotrode designs, especially if you need to add screws or blades are costly and out of reach for most of us.Even if you would have access you still need to know the material properties to know the speed of sound in the material and how much it can flex in various directions without being subjected to metal fatigue.For basically all hobby needs in terms of ultrasonic gadgets we are happy with a simple push pull motion.the same motion our transducer offers by default.And when it comes to attachments it turned out that quite stubby horns of light weight are a good compromise already.A 50-50 ratio of diameter and length works reasonably well in most cases.For example the standard 40kHz transducer of 45mm diameter is quite happy to work with a horn like this:45mm diameter on the thick end, 20mm diameter on the tin end.Thick part 40mm long, thin part 42mm long.The extra 2mm are for the manual tuning by filing or sanding it off until there is good cavitation happening when you put the end into water.This however is only good for simple testing purposes and some fun but as soon as you attach blades or a small pot with about 200grams of molten solder the tuning is way off and destroys the driver quickly.To be able to deal with different pressure levels on the working end or just a different mass that is attached the driver needs to "know" the new self resonant frequency.Basic idea for a dedicated driver:Please bare with me on this one as my developing days got severly neglected once I moved to the other side of the globe....Input should from a 12V power supply, preferably a PSU to keep costs and sourcing time low.The operating voltage for the transducer shall come from a simple switch mode supply.I was thinking of scrapping a PSU for the transformer and switching transistor.This however would provide about 120-160V on 240V mains with the transformer of a PSU.To match the required load changes it would be great to drive this first transformer by PWM means to regulate the output voltage with a potentiometer while keeping it steady within the set values.Basically like every cheap phone charger but with an output voltage that can be adjusted and kept regulated.The switching transistors for the transducer should be well over the required specs of an out of tune transducer.I guess capable of switching 600V should be sufficient.Main design change to the cheap driver boards would be the feedback.A hall effect sensor could provide the proportional voltage to the current going into the transducer.It would also provide the real operating frequency of the transducer for the feedback loop.The resulting real resonant frequency of the running system is then used to drive the switching transistors.As a result the transducer would always be driven at the exact right frequency no matter the load on the working end.These transducers still have a quite limited frequncy range due to the fixed counterweight on the back - it is optimised to be self resonant without the transducer being mounted.To explain this feature let me use a spring with a weight on it....You can move your hand up and down to make the weight swing up an down with the spring force.You can also push the weight to get the same effect.But if the weight would just expand and contract there would be no change in the spring force or position or the weight.Our transducer however is mounted to something and the weight on the back is heavier than what is on the front end of the transducer.As a result the weight is pushed back and forth and because all is fixed together this movement is transfered to for example your cleaning bowl.Without anything attached to the transducer it would literally start to rip itself apart until either the bolt or the ceramics fail.The feedback loop needs to prevent this by adjusting the switching voltage going to the transducer.Once too far out the system needs to shut off until it can reset.The frequency control is not that fragile.With the power controlled through the feedback even a wide drift in the operating frequency of about 5kHz would only reduce the effectiveness and amplitude of the moving horn/sonotrode.Sadly my skill set in circuits is not that good anymore to have the required parts in my head and to know how to combine them properly :(Why this concept is only really good for really basic applications:Professional solutions utlise often less than 20W of ultrasonic power for a soldering iron or scaler.For these devices the sonotrode/horn is spefically designed for the task at hand.Same goes for any possible attachments - without them these things don't do much at all.Finding these low power ceramic transducer rings for a good price is hard enough, making an amplifying horn even harder.But when using these quite big 50 or 100W transducers we find for cheap online we can compensate the lower amplitude with the added power of the transducer.Since we only need surface action but won't have to go through a few liters of liquid it might even be beneficial.Fun fact: A 40kHz transducer has the second harminc frequency at about 170kHz.Means we could design a driver for the second harmonic and enjoy total silence when working with it.Would also mean that the ultrasonic power would be much higher.Mass times acceleration and such things ;)If you want some ultrasonic cutter then you don't want to waste weeks and lots of money trying to come up with a working attachment to your transducer.Just keep it as short as possible and with about the same weight as the front part of the transducer.At least the driver desing would make it quite easy to design an amplifying horn by trail and error through reducing the lenght of the thin end until it really fits.Anyone with good circuit skills willing to volunteer? ;)

Topic by Downunder35m    |  last reply


Ultrasonic soldering bath

Making a working ultrasonic soldering iron is not as easy as I though it would be.Finding tanrsducer of suitable design and size is even harder.So I thought I start with something easier and share the thoughts here.If you need to solder impossible to solder things then quite often you could get away by wetting the entire area.For example the end of a wire or a lug where it won't matter that you can solder on the bottom as well as the top.Back in my days flux core solder was a rare and very expensive thing to find.So we had a little soldering pot and flux pot instead for working with lots of wires.Dip, dip, done....The pre-soldered wires where then easy to work with and the ramaining flux on then was enough.Doing this for metals like aluminium, stainless steel or even ceramics seems impossible at first sight.China offers cheap ultrasonic transducers including the required driver electronics for very littel money these days, despite the trade wars.The most obvious solution would then be to get a cheap and big enough soldering bath and to attach the transducer to it....Won't work though and if it does then not for long.Problem is firstly the heat transfered to the ceramic parts of the trandsucer and secondly the fact that most of these soldering baths use quite thick steel for the container.Add the that you deal with quite some grams of molten metal and you know where I am going.Building your own ultrasonic soldering bath to solder the impossible with ease!Project costs:40kHz transducer with driver board : about 50 bucks.Thin walled stainless steel bowl ( about 50 to 100ml but go bigger if you like) : about 2 bucks.Leftovers for an enclosure can be wood, plasic or your favourite 3D printer.Ultrasonic horn: About 500 bucks from your favourite engennering company or you need to make it yourself - I prefer the later.Main design considerations for the horn:We need something to keep the heat away from the transducer that also amplifies the power coming from it.That is why we can use a bowl or container that has a small bottom daimeter as the transducer if need be ;)There is a good reason a commercial horn costs a lot of money.They are preferably made from titanium and they need to perform as advertised right from the start.We substitude by using some aluminium round stock and a lathe.It is advisable to leave the transducer as it is!Do not take it apart to mount your horn directly onto the ceramics!Use a long enough set screw or include the required thread on your horn to mount it onto the transducer.If you prefer to use stainless steel doe to the lower heat conductivity then be my guest.The horn should have the same diameter as the mating part of the transducer for a quarter of the wavelength of the transducers frequency in the given material.Please look up how fast sound travels in your choosen material and calculate it properly.Having the lenght of the thick part right is quite cruicial.The thinner part that amplifies our movements should be about a quarter of the diameter of the transducer.For example: if the mating face of the tansducer is 40mm in diameter then the thin part of the horn should be 10mm.The length again is a quarter of the wavelength or the same as the thick part.Where thick meets thin please allow for a 3 to 5mm radius and make sure this area is nice and smothly finnished.Now, length is quite critical here....As we will mount our finnsihed actuator free hanging under the bath we need a feasable way to comapensate for our tolerances by creating our horn without a simulating software. I found that welding a short stub onto the container works best but with aluminum it is harder.I assume most will opt for welding a 6mm soft steel threaded rod onto the container.Either way the container surface must be kept flat for the mating surface of our actuator rod.So it is best to make the stud yourself or to use a suitable replacement - like using some flux and your stick welder for create a makeshift spot welder ;)If you decided on using steel for the horn then of course you can just mill a 10mm piece with a suitable thread and flat mating surface...What you want to end up with is a screw connection that has a flat mating surface and no empty spaces, fine thread prefered.Tuning the horn....The ensclosure is easy to make as a box, so the only thing to worry about is insulation but nothing to affect performance.So I just assume you have it all ready ;)With the horn at one quarter wavelength either end our thin end will be too long unless a short stud is used for a direct fit.So whatever you had to add for the part on your container or bowl need to be removed from he horns thin end.Try to keep the gad for the threaded part as small as possible as it affects the resonace.As things never turn out perfect the first try I prepare some thin steel washers - 100mm outer diameter in case you wonder and stick with the above example.I use a strong neodymium magnet and belt sander to create washers from very thin to slightly thinner ;)Taking off slightly more from the horns end will then allow toadd these washers if required - but please do a try as it is first when you think you got the measurements all right!For an aluminium horn you will of course use aluminium washers here.To do so fill the container with some water and place a sheet of thin alumiium foil on top of the water.Turn it on and within a few seconds you should see holes appearing in the fiol or even small fractures.If nothing but noise happens it is quite certain your rod will be a bit too long.Unscrew and take about one tenth of a mm off the thin end of the horns mating surface to shorten it.Try again with the foil and if no better remove some more material.Once you see some action try adding a layer of aluminium foil between the mating surfaces - screw it tight!The foil won't last long but if the action on the water is far better until it fails you know you took off too much.The washers come into place if the tuning won't work at all.Sometimes you can cut off a little bit again and again but the piece will remain too short ;)Especially if you have an aluminium horn and needed to use a steel screw on the bowl...So once the shortening of the horn fials you add a washer to get slightly above the original length and start replacing the differently thick washer until you find a sweet spot.The tricky part is over, now to solve the heating poblem...Using some glass seal as used on wood fire ovens not olnyl provides good insulation to our enclosure but also prevents the vibrations from spreading too far.As our hardware store won't just give use the little bit we need the rest can be used to insulate our container.Dending on the size and shape of your container I hope you decided to buy a container tha fits your heating element...I found that replacement coils for lab heaters work fine but some small fan heaters also use round heating elements instead if wire spirals.For a custom shape it is quite easy to use a coil of heatin wire rated for your mains voltage and a glass fibre sleeve for insulation.To keep it all in shape just wrap some steel wire over it - over the insulated coils of course.The temperature control can be as fancy as with a microcontroller or as simple as using a dimmer like I did.Most heating elements will go glowing red hot if the mains voltage is not reduced.It makes sense to limit the dimmer's movements accordingly by testing it.Just do it in the dark afeter exposing a small bit of the heating wire from the insulating sleeve.Once you see a faint glow coming dial it back a bit until you can see any glow - that should be the max setting.For a big bath or to save time you can of course crank it up to what the glass insulation can tolerate but be aware that solder can boil over!I do a temperature check either with a touch free IR thermometer of by checking how quickly some rosin boils off.If you need to dip bigger parts you need a higher temperature, so I think a digital or sensor temp control is not really required.Once you found a sweet spot to hold the solder temp long enough without getting too hot or cold just mark it for reference ;)Using the ultrasonic soldering bath correctly.Cavitation is what the work for us, so we only need to activate the ultrasonic part when we need it with a push button or food pedal switch.We do not use any flux or resin!That means if you used the bath for normal soldering and or resin then clean the remains off the surface first.A shiny and clean surface is best but the oxidisation will happen quickly so don't be too disappointed ;)Start by dipping in a clean copper wire.Some solder might stick but it won't look proper.Now dip it in again and while it is in push the button for about 3 seconds.Like magic, if tuned properly your wire is soldered and properly covered to where it was dripped in.Try the same with some slightly sanded or at least clean aluminium wire, but use the button right away for about 5 seconds.The wire should be coated with solder once more.You can try a glass rod or some stainless wire next but I guess the working principle is clear now ;)Not everything will bond with solder, especially not if it is not clean.A piece of glass with your fingerprint on it might just fail and some ceramics will only let the solder stick without actually bonding.You should always check the mechanical strength of your soldered connection before having to rely on it ;)And why would you need such a machine?Well, most people won't have any use for it.Those who do might not be able to afford a commercial model.And there is always those who just want it all...If you know why you need such a thing than you have an alternative now at a fraction of the cost.You only need a lathe or someone who can machine the horn for you.Another benefit is that for smaller containers it is possible to weld a small "bridge" over the top.Should be placed so the bottom is in the solder while top is above it.In many cases you will then be able to use this plate to heat up whatever you need to solder on.Like a glass plate where you would like to solder a wire to.Once up to temp turn the ultrasonic part on and use a normal soldering iron and flux flree solder.Works quite well for these small solar panel kits...Ok, and how far away is our cheap ultrasonic soldering iron?Not that far :)I already have a topic for this though....

Topic by Downunder35m    |  last reply


Soldering tips and tricks for complicated metals

Whether you are just a hobby builder or do your own electronics projects, you know how to solder...Then one day you find yourself in the position that your solder just does not want to stick...My first moment of total defeat happened when I was a teenager.Was building some simple motor with instructions from a book but substituted what I could...Ended up with some stainless steel contacts and being unable to solder my wires to them...If you ever had problems like this then read on ;)What are easy to solder metals?Basically everything that does not form an oxide layer on the surface and is able to bind with tin, lead or silver.Copper is one of the easiest metals to solder on but every plumber certainly knows how important a clean and corrosion free surface is.Any coating or alloy that prevents oxidisation or provides a harder surface usually means with normal, electornics solder we might be lost.Nickel for example can be a true pain and same for chrome.So lets start with the hard metals first.Steel, nickel, stainless...If the part size does not already mean trouble to get it hot enough, then we face the problem of how to "wet" it with our solder.Normal steel is usually fine if you give it a fine sanding right before the soldering, however getting the heat onto the part is crucial.Even something simple like a 5mm thick steel rod can be a pain with a normal soldering iron.I good way to cheat is to preheat the part or area with a blow torach on a soft flame - not a hot, blue flame.Try to do this away from the area you need to solder as the temperature difference usually causes some initial condensation on the surface.Most steels that play a vital role don't like to be overheated as it can affect the hardness an other things, so be careful here.Rosin core solder works fine on steel and it also indicates when the temperature gets too hot by boiling and smoking badly.If you still struggle to wet the surface try to scratch it with your solder - if it does not melt the surface is not hot enough.Nickel coatings are usually very thin and a slight sanding quickly reveals the layer underneath.If the metal used is not copper already then a copper layer will be electroplated on before the nickel coating.Either way the key is to get through the nickel without going through the copper, for example if steel contacts were used for durability reasons.After that soldering is as easy as directly onto copper.Steinless steel however can be a true pain, same by the way if you need to preserve the nickel coating as best as possible and can sand it off.Without using chemistry the only way I found is to use a stainless steel tip in the soldering iron.But as the preperation of one requires chemicals anyway we might start with them first.The passivating layer of layer or stainless steel can of course be pre-treated by sanding.Especially very shiny surface benefit from it.After this I prefer to wet the surface with Phosphoric Acid - you can find it in the harware store as "Rust remover".It is a food grade acid used in many of your favourite fizzy drinks, so skin contact is not a big deal - just wash it off.The phosphoric acid is not strong enough to break the oxide layer but it keeps air away.And once you start scratching the hot metal with your stainless steel soldering tip it will prevent a new oxide layer from forming.This method however requires a low temperature solder and quick work as the acid boils off quickly.In the plumbing section of your hardware store your find various fluxes for soldering.Look for something containing both Ammonium Chloride and Tink Chloride.Around here a common brand name is Bakers Fluid.Usually if it has a red danger label on it you will find the above ingredients on the lable somewhere.Be careful with it as it is very corrosive and harmful to your health!Good thing is that all remains can be washed off with just running water.What does it do though?Unlike the phosphoric acid, the chlrodies directly attack the metal.Especially once getting hot, so if in doubt wear proper protection as advised on the label!The oxide layer is not only being eaten away, there is also an ion exchange happening, so a product with more than 30% of zink chloride is prefered here.The zink binds with the stainless steel or nickel and provides an easier way to bond for the solder.Key is to work quickly and with precision!Flux paste is good for brazing but not so good for soldering.The flux liquid, unlike the paste will start to boil right when the metal get to soldering temperatures.That is if you use standard lead based solder, most lead free types should be ready a bit sooner.Start to scratch the metal with the solder and use a soft flame from the other side or close to the soldering area - do not apply the flame directly onto the flux covered area.Why? Well, the flux isolates the metal from the heat of the flame and it will boil off way before the metal gets hot enough ;)On smaller parts and when using the soldering iron create a small bubble of solder and keep scratching the surface while it heats up.In case the flux dries off apply a bit more before this happens!Once the solder starts to wet the metal a tiny bit it is usually very easy to spread it out to the desired size and shape.With the heat applied from the underside the solder will always flow to the area of most heat!Once done it is best to let the part cool down then to give it a good wash under running water to remove all remains of the flux.Failing to to do so will result in quick and ongoing corrosion, so do it properly...Aluminium, the bad metal...I encountered it first when I could not welding or brazing on a quite small part.Plus, of course, the problem of having to add a copper wire as well.Then again when I had to solder some aluminium wire.Acid won't work, chlorides only make it worse, so don't bother with either for aluminium.Standard rosin core solder also fails.But there is a suprisingly simple solution to the oxide problem on aluminium.Mechanical work...There are quite few videos out there showing how someone solders onto some aluminium foil.It is so simple because the foil is thin - use it to test your new skills.A thing though that is often done wrong is the surface preperation.It usually starts with a fine sanding - to remove the oxide layer.....The some oil is applied and soldering starts under the oil cover.And if pay attention then it is often a painful process of scratching with the soldering iron while trying to make the solder bubble wet the aluminium.That's why foil is so simple here....What happened in those videos?Quite simple: Aluminium oxidises right away while you sand it.Even if you are quick with the oil it already happened.So why not do the sanding after the oil was applied?A fibreglass pen or a stainless steel wire brush (usused on other things!) work quite well here.The oil prevents the air from attacking the aluminum.If in doubt use some clay and form a little dam around the soldering area to prevent the oil from running off.Petroleum jelly, vaseline and all other identical things work fine here same for clean engine oil.But you have to use rosin free solder, no flux core, just plain solder.If you don't have it simply melt some normal rosin core solder to a nice drop and clean the rosin off ;)Since there is no real oxide layer with this way of pre-treating the soldering and wetting happens right once the aluminium get hot enough to melt the solder.You might find it sticking nice right away but don't be fooled!You need to heat the aluminium until you actually see the solder forming a nice puddle.With careful sanding you create very clean boundaries.Other soldering tricks...Getting cholired based flux for a single job might be overkill.If you happen to have one of these tip cleaning stones for your soldering iron then you have what you need ;)Simply scrape some of it off and dissolve it is a tiny amount of water.Will only be ammonium chloride and requires more scratching on stainless steel but works...Preparing a stainless steel soldering tip sunds as easy as finding a suitable piece of wire and grindinga tip onto it.If you every changed the tip on a soldering iron them you know there is two types.The simple one for the cheaper irons uses a set screw or similar to hold the tip.The better ones are hold in place by a collar or other type of screw fitting.And well, those have a thicker part in their body.If you need to solder stainless steel more than once or twice it makes sense to buy a cheap but powerful soldering iron and to make sure it uses a straight piece of metal with no thicker parts to hold it in place.If you can't find some stainless steel wire or round bar of suitable thickness you can go slightly below or much thinner if you require a thin tip.Just make a copper or aluminium collar for the tip to hold it in place, like a sleeve to go around.Grind the tip to your desired shape before fitting it in....You won't need a mirror finnish and it can be helpful if the the surface is quite rough.After all, you want to scratch around on stainless steel with it and you can't harm it this way.To get a nice and clean cover of solder onto the tip you need the mentioned flux from above.Use a small cup and fill some of the flux in it so you can dip the tip of the soldering iron into it.If there is no temperature control start with a cold iron and the tip sanded off a last time right before dipping it into the flux.Use some clamps or whatever you feel like to help keeping the tip in place.If you get flux onto bits you don't want to cover with solder then wash off and try again.Turn the iron on observe the tip.As soon as you see tiny bubble forming take it out and quickly start rubbing your solder onto the tip.It helps to have a thick enough solder so you can apply some pressure here.And of course the solder should be nice and shiny and not covered by oxides...Special cases like titanium or othe metals that usually fail to bond with solder....Let's face it: whenever soldering is not feasable we are happy to revert back to crimping or screwing.Nothing wrong with it either and often the better option when it comes to being able to do a quick repair at a later stage.Most of thes special metals, including your favourite heating wire can still be solder using the right surface prep and flux but it really should be avoided if you can.And real bond like you get when soldering copper would only be on a surface level and mechanical strenght questionable.On a professional level ultrasonic soldering is used to make the impossible possible.The cavitation effect breaks through the surface oxides or passivating layers and the solder just wets the surface like it would be copper.On a hobby level things look different though.Unless you decide to build your own solar panels from scratch the investment into some low end ultrasonic soldering machine already set you back a few grand....There is a way to cheat on the cheap though if you are into experimenting and building things....More on that in my other topic about making an ultrasonic soldering tank. ;)

Topic by Downunder35m