Metal Melting 101 - How To

Shop Talk

For better or worse thermally fused metal joints are an integral part of motorcycle construction, and despite some serious advances in liquid adhesive technology, are likely to remain so for the near future. That's not a bad thing; people have been welding metal together since roughly 3,000 BC so most of the procedures are fairly well established. In light of that, we thought it might be interesting to take a brief look at the various processes involved. The following won't make you a welder, but it should provide some insight into what it takes to keep all those loose pieces of steel, copper and aluminum that make up our motorcycle, flying in close formation.

Soldering is the joining of two metals, like or unlike by the use of a third metal that's entirely different from either of the two base metals. Which is a long winded way of saying that soldering fastens two metals together without melting them, by using a third molten metal to bind them together. Soldering in fact might best be described as "metal gluing."

Common solder is a mixture of tin and lead that melts at a relatively low temperature, by definition all soldering takes place at temperatures under 800 degrees F, and is normally sold in either a roll or less often these days by the stick or bar. Typically the solder will be described by a ratio like 60/40 or 50/50, the numbers referring to the amount of tin and lead respectively that make up that particular grade of solder.

Before a soldered joint can be made the joints have to be chemically cleaned with soldering flux to prevent dirt and oxidation from ruining the bond. Fluxes are available in liquid, powder, solid and paste form, or it may be embedded in the solder itself. Fluxes are further divided into two loose groups, acid, which is used for connections where corrosion won't present any particular problems, and rosin, which is a non-corrosive flux used for electrical joints.

Older motorcycles made extensive use of solder. The fuel and oil tank joints were soldered, solder was used to smooth out rough body work before the paint was applied, this is where the term lead sled comes from in hot rod circles, and fuel and oil lines had their ends soldered on. Additionally nearly all of the electrical connections were soldered in place as were the control cable nipples. Currently the use of solder is pretty much limited to securing crucial electrical connections, especially where a wire terminates at a circuit board or terminal and as a means of securing control cable nipples.

Like solder, brazing uses a third metal to create the joint without melting the base material, however the temperatures are higher, all brazing takes place above 800 degrees F, and the filler material, typically a brass alloy, stronger. An off-shoot of brazing is known as braze welding, but since the basic process is so similar we'll forgo any distinctions between the two processes. Brazed joints are stronger than soldered ones, and in some applications are just as sturdy as a weld, and may even be preferable. This is because the lower temperatures involved in brazing create little if any warping of the base metal and doesn't affect its heat treating. Brazing is also a handy way to join certain dissimilar metals, such as copper to steel or tool steel to low carbon steel.

When men were men and motorcycle frames were flimsy, quite a few manufacturers brazed their frames together, and given the technology of the day it worked rather well. Those days are long gone, but brazing is still a very handy, and low impact way to repair a lightly stressed part, I use it a lot to attach things like mounting bracket tabs when I don't want to get the base material too hot, or to occasionally repair small cracks in something like an exhaust pipe Y connector because the filler material is strong yet easy to blend in to the base metal with a minimum of grinding, after which it can be painted or plated.

At the OEM level you no longer see much brass used to assemble motorcycles, certainly no one is brazing their frame joints these days, although I wouldn't be too surprised if some of them were silver-brazing things like fuel line fittings together.

Welding might be defined as a process where two metals are joined together by heating them to the melting point, and then allowing them to flow together and fuse. Actually it's slightly more complicated than that, and generally involves the introduction of third metal to act as filler, but melting metal is essentially what welding is all about.

There are so many different types of welding that it'd take a book, to cover them all, and quite a few of them such as explosive welding, which uses high powered explosives to drive plates against each other fusing them together or forge welding, where two pieces of metal are brought to a white heat and then literally beat together, are a little impractical for our purposes so we'll just concentrate on the more common forms used in motorcycle construction and repair.

Oxygen/Acetylene Gas Welding
Torch welding is one of the oldest and most popular ways to join metal. When made by a skilled welder, a good gas weld is every bit as strong as anything made by a machine, and in most cases far more attractive, which is an important consideration when we're discussing motorcycles, with their many visible joints. The most common gas welding device is the Oxygen/acetylene torch. In this process oxygen and acetylene are stored separately in high pressure cylinders. Flexible hoses connect the tanks via pressure regulators to a torch equipped with adjustable flow control valves. The gas flows through and mixes in the torch, and is burned at the torch tip. The flame produced melts the base metal and the filler rod, which all combine to create the joint. Oxy-Acetylene torches are extremely versatile, they can be used to heat, cut, braze, solder and weld, and because of that they are found in every properly equipped repair shop in the world. Ironically because they are so common, and unfortunately so abused by guys that don't know how to properly use them, some armchair experts tend to look down on them, if they knew how to actually use them it'd be a different story.

The rigs do have some drawbacks. Gas welding is an art, and some people just can't get the hang of it. I've met more than one guy that could lay down a pretty credible weld using an ARC or MIG welder, yet couldn't get the hang of gas welding or brazing. This is one of the reasons why most welders of my generation were taught to use a torch before anything else. The feeling was that if you had a talent for the job, it'd show up in your torch work, if it didn't, well, you might make an OK production guy but you'd never be a real "welder." Secondly, it is a fairly slow process. That's rarely a problem if you're making a repair, or building a custom part, but it's a real issue if you're an OEM trying to churn out a couple of thousand frames a week. Lastly Oxy-Acetylene welding isn't a particularly easy or efficient way to weld aluminum, which is a drawback when it comes to some motorcycle work.

At the factory gas torches have long been replaced by other, more efficient welding methods but in the repair shop they're still one of the most popular ways to weld.

Gas Tungsten ARC Welding
When it comes to working on motorcycles Gas Tungsten ARC welding, more commonly known as TIG (tungsten inert gas) or Heliarc welding, is the uh, hot set up. TIG welders produce heat by drawing an electric arc between a tungsten electrode and the base metal. The heat creates a molten puddle into which a filler rod is fed as needed. A shielding gas, most often Argon or Helium, hence the term Heliarc, shields the welding puddle from oxidation. TIG welding is an excellent, in some cases the only, way to successfully weld materials like stainless steel, aluminum and titanium, which are all nice materials to use when you're building motorcycles, which is one reason why TIG welders see widespread use throughout the industry, especially in custom and race shops.

Perfected in the early 1940's, primarily to work on aircraft, early TIG machines were expensive and a little esoteric so only specialty welding shops and factories had them. But over the years they've become quite affordable and can now be found in every welding shop and in fact many dealerships where they're used to repair everything from broken cylinder fins to cracked frames.

TIG welding does present some problems, especially at the OEM level. For starters becoming a proficient TIG welder requires lot's of patience, a good deal of practice, and some natural talent, it is an art in every sense of the word so good TIG men are a little hard to find, and by the same token are generally paid a premium for their skills. It's also a fairly time consuming way to weld. If you're a small production run company, or only building a few hundred bikes at a time then having a few guys do nothing but stitch things together, makes perfect sense, and I think we'd all agree it also lends the finished product, be it a custom fuel tank, or a chassis a certain cachet. However if you're an OEM churning out widgets by the thousands then a robotic MIG welder that works morning, noon and night, as well as Sundays and holidays makes a lot more economic sense.

Gas Metal ARC Welding (GMAW)
What's that you've never heard of GMAW? I'm not surprised, the term's a little obscure, so we'll call it by its more popular acronym, MIG, (Metal Inert Gas).

In MIG welding an electric arc is created between the base metal and spool of wire that's fed through a gun like handle. An inert gas, usually Argon, Helium or Carbon Dioxide is used to shield the weld, the choice of the gas being dependent on the metal you're working with. Because the process is easy to learn and the spools can hold up to several thousand feet of wire the MIG process is very popular in production and repair shops, many motorcycle components including frames and fuel tanks are MIG welded either by robots or humans and the process can be used on steel and aluminum.

Early on MIG welders or "Squirt Guns" got kind of a bad rap. This was largely because the inexpensive bikes from Japan were stitched together with MIG welds and while the welds were structurally sound, they weren't very attractive. It wasn't unusual to see errant bits of wire left in the weld for example and the welds sometimes looked as if they'd been made by someone that in the middle of an on the job training program. Nevertheless, you rarely saw them break, at least not during normal use. Robotic MIG welders have largely replaced the human component and the process itself has been much improved over the years so these days it takes a trained eye to spot a MIG weld, and yeah, there are an awful lot of MIG welds on new motorcycles.

As an aside because a MIG machine is versatile, you can join everything from sheet metal, to steel plate with them, easy to use and relatively inexpensive to buy and run, they have become the most commonly used production welders in the world. Furthermore because inexpensive 110V machines are available, along with flux cored wire, which eliminates the need for a shield gas, many serious do-it-yourselfers have some sort of MIG welder in the garage.

ARC Welding
When many of us think of welding the DC ARC weld is what first comes to mind, usually along with a mental picture of manly men welding huge slabs of steel together to produce manly things like air craft carriers and bridges. ARC welders are intense, lots of heat is created and big welds can be made, but they can also be used to weld steel as thin as 1/16 of an inch thick, if you know what you're doing. If you don't you'll do more damage with an ARC or "stick" welder in five minutes than you can repair in a week.

ARC welders work by producing an electrical arc between the metal you're welding and the metal you're adding to the weld. A metal electrode is placed in an insulated holder that's connected to the positive terminal of a generator or rectifier capable of producing enough current to jump the gap between the electrode and the base metal. The base metal is connected to the ground side. When the electrode is struck against the base metal an arc is drawn which provides the heat needed to create a molten puddle. The electrode also melts and is used to fill the weld. Typically the electrode or rod is covered with a hard flux that gives the arc some stability and protects the finished weld. The slag from the flux must be manually cleaned from the weld after it cools off. ARC welds can be made using AC or DC current, and the polarity may be straight or reverse as the situation warrants.

ARC welding has a place, albeit a limited one in motorcycle construction and repair, however because the equipment is bulky, and the skills somewhat difficult to master it's of limited use and popularity. That being said it's my favorite form of welding and I've earned many a nice paycheck sticking everything from steel piers to dump truck bodies together with an old gas engine driven Lincoln ARC welder.

Down The Road
Whether or not future motorcycles will feature as many welded joints as current ones do is debatable. For sure gluing the things together would be cheaper, and yeah, there is some movement in that direction, in fact my 1994 DR 350 Suzuki has its aluminum swing arm stuck together with some sort of epoxy, and I understand some of the current aluminum sport bike frames are also constructed using glued joints.

However at least in the near term I think that welding will remain the preferred method of construction, it's proven technology, and it works extremely well which are reason enough at the OEM level. As for me, I've always enjoyed looking at the properly welded joints that keep my bikes together. Somehow a bike that was put together like some giant model car kit just wouldn't be the same.

Brazed joint
Gas welded joint
Soldered wires
Automated mig welder in action
MIG Welded frame
Motorcycle frame being TIG welded togther.