bcoppola

Preface: I'm not and never will be a framebuilder, nor do I know much about metalworking. Just Joe Average cyclist here, wandering into the shop...

I do know that aluminum can be and is often brazed for various applications. So I've wondered from time to time why welding seems to be the preferred method for aluminum frames vs. lugged and brazed?

Just curious.

Falanx

Aluminium, contrary to what you've been told is NEVER brazed. Brazing filler alloy melts at about 800 degrees, two hundred degrees above the melting point of aluminium alloys. By the time the filler had become mushy, the tubes would have become a puddle.

bcoppola

Aha, I think I got brazing confused with soldering...which renders my question moot!

I would not want to ride on a soldered frame, I suspect.

tomacropod

Lugged aluminium frames such as early models from Alan (Italy) were typically held together with epoxy (glue) much like lugged carbon frames. Some also had a threaded connection between tube and lug hence the term "glued and screwed" lugged alloy frame.

I'm not sure that there was much merit in this sort of design, largely because the tubing diameters were about the same as the steel tubing diameters of the day, making them incredibly flexy. I've never ridden one, that's just hearsay.

- Joel

Redman1959

I rode an Alan for a year while racing in college in 1980. I was pretty big even back then weighing about 180lbs. My coach helped me get a 531 Holdsworth pro since I flexed that frame so bad. There was a huge difference in the ride particularly on hills or sprinting.

I was coming back from a ride one afternoon and got stopped by a drwabridge. Someone offered to buy
the bike on the spot. When I told my coach he was angry that I hadn't sold it.

Redman

MichaelW

You can braze Al (to Al or other materials such as copper). It is not used in bike construction because lugless welding is quite good enough and a lot easier and cheaper and lighter. There are brazing materials for a wide variety of metals, you can even braze Ti if you feel the need.

Falanx

I'm afraid you can't. Silver solder is not brazing, soldering is not brazing, nickel-based filler rods are not brazing. Brazing requires brass based alloys.

Yes, you can braze titanium, because its melting point is above the melting point of your filler alloy, it alloys readily with copper and it dissolves its own oxide above 600 degrees. Aluminium's surface oxide film can only be removed with fluxes so strong that the will kill you if you inhale them, and brazing rod STILL melts above the tube's own point by at least 200 degrees. Please read my post.

Now, there are solders that will melt below the melting point of aluminium, but most of them wet aluminium very poorly, so even if you can dissolve the oxide, the bond will have diabolical shear strength and poor mechanical properties in general. Yes, they are marketed, and most of what you read in their marketing is garbage. The preponderance of the word brazing in their blurb is disheartening, because brazing is certainly what it isn't.

On the point of bonding, a number of epoxies are more than adequate in lugged construction, as some of the new Scotch ones, such as #490 show remarkable shear strength, and in a lugged construction, that's what you require. It was the method Raleigh used for assembling their MMC tubing on some M-Trax models.

Jonny B

Wow, I never knew brazing wasn't the same as soldering But brazing is a type of soldering right? As in, it uses the same (similar) theory and process.

NoReg

Well the actual definition I read was that brazing is basically soldering except the temp threshold is 850 degrees and above. Temperature is the defining difference, so is 849 soldering a whole ofther process than 851 degree brazing... In theory something different happens at the higher heat in that with steel the higher heat allows a greater degree of "penetration" into surface character of the material. And of course higher melting point braze is stronger than the much softer materials associated with lower temp processes.

"Silver solder" is a term comonly/correctly applied to hard silver brazing rod, so one has to be aware of that since things get misleading fast.

Here is some interesting stuff:

https://tinmantech.chainreactionweb.c...ds_and_flu.php

Falanx

Temperature cut-offs of ± 2 degrees are unheard of in any subset of materials engineering. It's called brazing because it uses brass. As I stated, they all fall under the heading of soldering. Industry definitions of this type are frequently picked by people who don't fully understand what they are talking about as they are chosen for marketing, not internation quality/science standard reasons.

I think a little clarification is in order here. I'm British and as such work only in degrees celcius or Kelvin. Farhenheit isn't any use to me, so the 850 degrees I talk about is 1560 degees farenheit. That definition of 850 F (455 C)is ridiculously low for a brazing definition, and almost any combination of light metal alloys can be made to melt at a temperature than low.

Silver solder is still a solder, though

You will note there is a usually a distinction made between copper brazing alloys and all other forms of commonly used jointing materials anywhere you purchase them from.

skeeter

to answer simply without all the *rigamaro* , yes it is.

silversmith

I love my old SR Litage. Its lugged & very light. Thanks to some inovative bracing on the lugs and the shape of the tubing its not flexy like a Vitus.

DannoXYZ 

I had a '80s Peugeot Comete which was a lugged alloy frame. It used neither brazing/soldering nor glue to hold the lugs to the tubing. Rather, the lugs were crimped and rolled on top of the tubing. Just slightly oversized tubing compared to steel frames of the day, so it was more flexible than steel, but still stiffer than a Vitus.

Serendipper 

My Miyata Alumnicross Cyclocross bike has a bonded lugged composite aluminum with steel chainstays and fork. It has no flex, and the whole bike weighs only around 19 pounds.

The top tube says 'Alumnitech'. I know Miyata left the US shortly after this bike was introduced, so I'm not sure if any other bikes were built as light and stiff. I have heard horror stories, none confirmed, about bonded frame failure. But I don't imagine my frame will start flexing anytime soon. I think the chainstays and fork in steel is the key, plus the Alumnitech bonding process, which appears to be superior to the vitus.

The lugs are especially beautiful, as I am a huge otaku of Japanese design. Very Wabi Sabi desu.