I have some small pieces of aluminum I wish to join, and have Al solder and flux. The flux has no directions but does say to apply to parts and do not overheat. Should the flux be wiped off before attempting to solder? When I try to leave the flux in place, it turns dark brown, smokes a lot, and seems to prevent the solder from adhering? Any useful suggestions?(Other than "try brazing" or "get a welder"...)
I'm not all that familiar with the term aluminum solder. I do have some of the low temp (750F?) aluminum rods that you occasioanlly see on late night infomercials and are sold at auto swap meets and flea markets. They work well on aluminum and pot metal and pretty much form a soldered joint.
I've fabricated some objects out of discarded aluminum sign stock with a combo of folded edges and seams filled with these low temp rods and ground/sanded smooth. The work has held up well in a vibration prone section of a motorcycle.
The rods I have don't need flux, but they do specify to not overheat. OA is too hot. Simple propane torch is recommended.
Try your solder without the flux and use low heat like a propane torch and see what happens. Make sure the joining surfaces are clean right before starting. Clean aluminum usually means degreased with acetone and wire brushed with a stainless brush never used for anything else.
It sounds as if you are heating the flux too much.
The process of soldering is essentially the same for all metals. The solder is a metal (usually an alloy) that has a lower melting point than the stuff to be joined. Basically, you just heat the work (the bits you want to join) until its hot enough to melt the solder which turns to liquid and wets the work. When you remove the heat, the solder freezes and the joint is made. The important part is that the liquid solder _wets_ the work. Just like water won't wet a greasy or dirty surface, the solder won't wet a dirty surface, that's why you must ensure that both the joint and the solder are clean. That's the theory bit.
In practise its a bit more complicated, 'cos what happens is that the heat causes both the work and the solder to combine with the oxygen in the air to create oxides. The oxides act as a barrier, rather like dirt, that prevents the solder from wetting the joint. The trick is to stop the oxygen from getting to the joint. That is the job of the flux. As you heat the joint the flux flows over it to shield it from the oxygen. When the joint reaches the proper temperature the solder melts and the joint is made.
Some metals, aluminium and stainless steel for example, oxidise very rapidly (in seconds) at normal room temperature, so flux for these metals has the extra job of getting rid of existing oxide as well as preventing new stuff from forming. This is the reason that these metals are, traditionally, difficult to solder.
The final problem is that no flux is perfect. If you heat them too much they lose their properties and the solder refuses to stick. If you heat the solder and flux directly you will almost certainly destroy the flux. The secret is to clean the joint and flux it, then heat the _joint_ until its hot enough to melt the solder. If you just heat the solder it will melt and turn into a ball, the tendency is then to apply more heat which tends to burn the flux. If the _joint_ is hot enough to melt the solder it will flow into the joint all by itself.
In summary then. Use the correct flux for the solder and metal you are using. Clean the joint properly, apply the flux and heat the joint until the solder flows. A good trick is to place a little bit of solder on the joint before applying the heat, then, when you heat the joint enough to melt the solder you can apply more solder if required.
Thanks Gary, Here's the real life problem... After cleaning the Al with a clean wire brush, I apply the flux. I heat the joint and LONG before the joint has reached the melting temp. of the solid solder, the flux begins to smoke and turn black forming a nice hard barrier between the workpiece and the applied solder. As things currently stand, it seems that I can't help but scorch the flux, to my chagrin. When all is said and done, I'm left with 2 separate pieces of nicely scorched Al. BTW, the solder and flux were purchased as a kit.
It also helps to use a stainless steel wire brush and scrub the aluminum surface you are attempting to wet THROUGH a puddle of molten solder. The solder shields the freshly brushed surfaces from air and prevents new oxides from forming.
Years ago I used to astound doubting Thomases by getting regular 60-40 electrical solder to wet and bond to aluminum using that trick.
I was tought by a very smart guy who occassionally shares his wisdom on this newsgroup that the same trick helps to get "maximum bonding" of epoxy to metal. Smear the epoxy on the degreased metal and then wire brush right through it.
That sounds like the organic flux used with Harris Alsolder500 and Allstate Strongset 509. The flux must be present when soldering for the solder to wet the aluminum These materials work well if you can avoid burning the flux, but that's not easy to do. The trick is to keep flame away from the flux. If the flame ever touches the flux, yer screwed. Either heat by conduction from an unfluxed area, or use a hot air gun.
Some easier-to-use aluminum solders are: aerosolder , see
use #585 flux
The fluxes used with these materials work OK with a torch. I like the aerosolder best. That flux turns water clear at about the right heat for soldering, which is about 780F. It's a lot easier to use than the organic stuff. There is no sodium flare with these fluxes, so you don't need special goggles as in aluminum brazing.
I don't like the "miracle rod" or "rub on rod" , as Aladdin #31 "three-in-one" and others for aluminum, though they do work well on whitemetal. They basically are whitemetal! I think the other materials mentioned here make much better and stronger joints.
As others have mentioned, buy a stainless "toothbrush" from a welding store for cleaning the metal. They're under 2 bux. Anchor is one good brand. The Chinese ones are so soft they're about useless.
You don't say what solder, flux, or type of heat you are using. Some aluminium solder I've used requires no flux at all. Here in UK its called Technoweld, but its not really welding. In USA its probable got another name. The technique, as others have mentioned, is to first 'tin' the joint by first scrubbing it with a stainless toothbrush and then scratching it with a stainless toothpick while applying heat until the solder melts. It works by mechanically removing the oxide from the material while it is shielded by the solder. When both parts of the joint have been tinned, you hold them together and apply heat to melt the solder to make the joint. You can add more solder as required - once the joint is tinned it won't oxidise. It works very well, but is no good at all for 'capillary' soldering where the solder runs into small fluxed spaces by capillary attraction. I suspect this where the name Technoweld comes from 'cos welding is similar in this respect.
Which is less than what a cup of coffee costs these days at most "sit down" restaurants here in Red Sox country these days.
I finally admitted to myself that I was getting old when I realized that just the sales tax on a cup of coffee here in Taxachusetts now costs about twice as much as a cup of coffee cost me when I first began drinking it. (5¢)
BTW, a couple of ounces of acetone still costs far less that $2 and will clean uncured epoxy off a small wire brush well enough for reuse...
Paint tin full of cellulose thinners, (or any tin with a solvent tight lid). Immerse all epoxy contaminated tools, brushes, what have you after wiping off as much as you can with paper towel. Wait till the epoxy on the job is well set, remove tool, scrub off soft cheeze consistancy epoxy traces from tools. Rinse off with a little thinners, put the rinseings into the can and seal it till next time. When there is too much cheezy epoxy buildup in the bottom scrape out and dispose of safely.
Yes I am a cheap b&$$?&£d too . . . :-) but acetone is too expensive here in small quantities. Oddly enough its only a couple of euros a litre in france :-(
Agreed, but watch out for tools with plastic handles, some of them don't like those kind of solvents.
Don't call yourself cheap Ian, do as I do and just tell folks you are "value oriented".
I was born a couple of years after "The Great Depression" ended here in the USA, to parents who had just suffered through the worst of it.
I guess their lessons weren't lost on me, as I often find myself telling my critics (Including my somewhat younger bride.) that I hate wasting stuff. That's usually when I encounter something like never touched perishable foods rotting at the back of our overstuffed kitchen refrigerator.
They say you can't take it with you, but as yet I've never met someone who's come back from there to tell me if that's true or not.
Just yesterday SWMBO and I had occasion to swap cars for part of the day and she cell phoned me while driving mine to say, "This car is a POS, when are you going to get a new one?" Said vehicle is a 1990 Caddy with only 110K miles on it which starts and runs fine. So what if I can see the floor pan shining up through the hole in the carpet where my right heel sits, the loose baffle inside the muffler rattles enough to turn heads when idling at a stop, and there's a wide band of duct tape covering the torn leather where my backside goes? Everything else about the car, including the AC, still works just fine for me.
Perhaps I should loosen up and heed the sign on the office wall of the Cadillac dealer in the next town which reads, "If you don't drive a new Cadillac, your heirs will."