Automotive sheetmetal and structural welding are vast topics, and it is not possible to give them anything approaching complete coverage here. This chapter concentrates on some of the fundamentals of welding sheetmetal sections, and on a few ploys that that may make this type of welding easier for you to perform, while producing superior results.
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It is important to note that welding thin metal sections is very different from welding bridge girders or thick plates. For one thing, welding thick metal pieces rarely involves having to worry about heat distortion and material warping. Thick materials resist distortion due to their bulk, and to their tendency to act as their own heat sinks. The main considerations in that kind of welding are penetration, bead deposit, bead shape, and strength. Concerns like welding through base materials and creating dropouts are remote. In non-structural panel welding, these issues become paramount, while strength is usually secondary.
You may have great skills for and success with stick welding thick sections, but these do not translate into gas or electric sheetmetal welding mastery. The skill set for this work is very different, and must be developed separately. If you are familiar with torch and/or electric welding, that may help you to learn sheetmetal welding. But aside from the fact that all of these forms of welding involve carrying a puddle of molten metal down a seam, and fusing it to the metal on either side of the seam, there is no automatic transfer of skills from heavy section welding to sheetmetal welding.
There is one commonality between stick or torch welding thick sections and welding sheetmetal. In both of them, a serviceable weld often, but not always, has a good-looking, even, penetrated, and uniform appearance. Unfortunately, most finished sheetmetal welds are unnoticed because, very quickly after their creations, they are almost invariably ground off and covered over with the likes of filler, primer, and paint. While the skill needed to perform good sheetmetal welds may equal, or surpass, the skill applied to visible welds, like those on motorcycle frames, you are far less likely to attain the fame, accolades, and down-right glory with a crowd of the adoring that the makers of those motorcycle welds receive. Still, you will see your raw sheetmetal welds, before they are ground and painted over, so you will have the opportunity to briefly appreciate your great work.
Types of Joints
Panel welding was discussed briefly in Chapter 3. Let’s now look at this topic in greater detail.
Welded joints for bodywork fall into three categories: butt, lap, and off-set lap. While it would sound democratic to say something like, “…each of these joints has its place in panel work, and each is a good approach,” that would not be accurate. Butt joints are the gold standard of welded panel joints. The other two types of jointure, particularly lap joints, are sometimes desirable. This usually is the case when they are used to duplicate factory lap joints. I suppose that it is true that butt joints are more difficult for novice welders to master, but once you learn how to weld them, they are not hard to achieve with good MIG or TIG welding equipment.
Butt joints are made with the edges of the sections butted end to end, against each other, with some amount of gap between them, to provide for expansion during welding. Lap joints are made by overlap-ping small amounts of metal, and welding the exposed edges of each section to the other, on one, or on both sides. This, of course, creates a double thickness of metal at the joint overlap, something that can be difficult to hide without using excessive amounts of filler.
Offset lap joints require the use of an offsetting tool to flange the edge of one of the sections to be joined. The other section edge is then slipped under the flanged area, and welded to it. This creates the appearance of continuous metal because the double thickness of the lap joint exists on only one side of the joined sections, and, naturally, it is that side that is chosen as the back side, and, therefore, hidden. Again, in offset lap joints, the weld is sometimes made on both edges of the lapped metal to seal the joint.
There are multiple problems with lap and offset lap joints. One is that you may have to weld the joints twice, if you want to seal them. Another approach is to weld the outside (or visible side) of a lap or offset lap joint, and then seal the inner side of the joint with seam sealer. Sealing lap joints is critical to preventing corrosion from forming in the laps, where capillary action invites moisture and electrolytes in for a corrosion bash.
If you double weld these joints, that is, weld them at both seams, you must apply more than twice as much heat to them as you would with a butt joint. In lap welding, one or both welds involve welding an edge to a flat, and this requires more heat than is used to make a butt weld, end-to-end. That extra heat is an invitation to local distortion and panel warping. Another problem with lap joints is that they may later show themselves through filler and paint, as a panel experiences vibration cycles. It takes many miles for this to occur, if it does occur, but it is a haunting possibility. Finally, there is no advantage to using lap and off-set lap joints, where situations do not mandate them, other than the misguided idea that they are easier to make than butt joints. The two exceptions are when you are duplicating a factory weld that was originally a lap joint, and when space and access considerations make butt welding undesirable, or impossible.
Butt joints usually return panels more closely to their original format than do the other two types of joints, and are simply the cleanest solution to the issues of laterally joining thin sections of sheetmetal. Certainly, when panel patching is the purpose of welding thin sections, butt joints are preferred.
Welding Smaller Pieces into Large Constructions
Sometimes large, complex constructions are welded up from smaller pieces. Many advanced practitioners of metal forming and fabricating tend to frown on this practice, preferring to make their fabrications from single pieces of stock metal. Still, it can be a useful approach, when limitations of your equipment and/or skills make single-piece fabrication impossible.
It is interesting to note that in the past, some OEM large panels were made from smaller pieces, welded together. While, for cost reasons, this practice is rare or extinct in modern volume produced light vehicles, it was common as recently as 15 years ago. Before that, the side framing panels of many cars were still welded up from as many as 20 separate pieces. That practice was replaced by stamping and roll forming techniques that made it possible to form these large, complex structures as single pieces.
Early fenders were often fabricated out of more than one stamping. For example, the drop skirt edges of very early automobile fenders were riveted to the bodies of those fenders. Later some large panels, like fenders and hoods, were gas welded, brazed, or electrically welded out of two or three smaller stamped pieces.
When you have to fabricate a panel or structure that is too large or complex, or both, for you to form it from a single piece, always remember that, as a last resort, you can form parts of it and then join them.
Of the things that I really hate in life, there are three that stand out: 1) the sound of a horse pulling its hoof out of deep mud, 2) the smell of the inside of a Russian horse doctor’s valise, and 3) trying to weld a moving target. It is that last one that I am usually able to avoid.
The way I avoid it is to properly fixture the pieces that are to be welded together. This means two things: providing and maintaining adequate fit-up gaps between them, and holding the pieces firmly in place for tack welding. Once you have done those things, you can tack weld attachments that maintain proper positioning and fit-up gaps for final welding.
There are many methods of fixturing pieces for welding. Which one you choose for your work depends partly on the situation and partly on your personal preference. For example, welding magnets might be adequate for holding a patch panel in place for tack welding, but are probably not the best approach to holding a whole rear-quarter section in position. Of the many fixturing methods, devices, and gadgets out there, the main ones are: locking pliers, welding magnets, edge clips, screw clamps, and various Cleco devices. Each of these represents a class of fixturing devices, and each comes in a great variety of styles, sizes, and configurations. Each also has application to most types of seams butt, lap, and offset lap.
Locking pliers are often referred to by the name given to this tool by the company that first manufactured it, Vise-Grips. Locking pliers are terrific for holding pieces in alignment while tack welds are made. They come in angled, long-reach, and pivoting-end designs, and represent the first line of holding parts in place. Specialized locking pliers devices are also available for many specific purposes.
The main limitation on the use of locking pliers to hold metal in position for welding is the reach of their arms. They may work well for you, when your positioning needs require holding at 12 inches, or even 16 inches, from the nearest accessible edge of what you are welding. But even at that reach, locking pliers tend to be pretty bulky and cumbersome. Beyond that reach, other methods of fixturing must be employed.
Edge clips are limited to holding the edges of panels and patches, but have the advantages of being quick to apply and to remove, and of offering very little obstruction to or interference with adjacent parts.
Wing nut clips can be positioned anywhere in a butt joint, regardless of their depth from its edges. They work well on straight-line joints, and inherently maintain a consistent fit-up gap. However, they do not work along curved joints because they hold edges unacceptably far apart in that application. Still, if you are butt welding along a straight line, these inexpensive clips work impressively well.
Many welding techniques apply to autobody work. Most of this welding is now electric, though gas welding is still sometimes used. MIG, TIG, and resistance (spot) welding are the main approaches covered here.
This method was once used to join autobody panel metal, both in production and for repair. This practice involved using small-diameter, coated welding rods that were specifically designed for sheetmetal work. Both AC and DC formats were employed. Stick welding sheetmetal required considerable skill, and yielded results that were often less than great. The main problem was that when stick welding was performed with the machines commonly associated with it, the process produced excessive heat for thin-section jointure, resulting in excessive drop-out and distortion. The time required to finish stick welds in sheetmetal was excessive, by today’s standards. This practice is obsolete.
Resistance or Spot Welding
This has been a mainstay of automotive construction since the 1930s. It uses no flux or filler, and is accomplished by applying a concentrated short circuit, and strong physical pressure, to a small spot on as many as three thicknesses of body metal to be joined. A combination of intense heat, created by maintaining a very-high-amperage short circuit at the point of the weld for a short interval, and considerable squeezing pressure on the two outer surfaces of the weld area by the welding electrodes, melts the spot into a fusion weld. Spot welds are quick and easy to make, and reasonably neat and strong. Modern light vehicles depend on as many as 3,000 or 4,000 spot welds to hold their structures together. In recent years, car manufacturers sometimes supplement spot welds with adhesives and anticorrosion treatments to add bonding strength, and to protect joints that are welded this way.
Some of the spot welders used in repair and restoration do not squeeze the weld area between two electrodes, but apply spot welding force and heat to one piece of metal that is grounded through the other piece to be joined to it. These devices are particularly handy where access to both sides of a spot weld area are difficult or impossible to achieve. There are also numerous attachments for MIG welders that mimic the strength, appearance, and configuration of resistance welds.
Another technique for duplicating the appearance and function of spot welds is button hole welding. This technique employs torch, MIG, or even TIG welding to join two sections together, one on top of the other, by welding through the top thickness of metal and into the surface of the metal under it. This must be accomplished without overheating the whole area, and ending up with an unsightly and embarrassing hole through both pieces. One trick to avoid this is to do a pseudo-button-hole weld by drilling a hole through the top section, and welding its rim to the metal under it.
Spot welds are often vulnerable to corrosion because the places where they are used are prone to attracting moisture between the joined pieces of metal through capillary action. This problem is made worse by the fact that spot welding tends to vaporize many of the steel treatments, like galvanizing, that are employed to protect sheetmetal. This problem is mitigated by using a weld-through primer between pieces that are spot welded. Such primers contain a very high percentage of zinc in their solids. This makes them conductive, and thus able to carry spot welding currents, and protects finished welds with ions from the zinc particles in the coating after a weld is made.
This has become the most common type of repair and custom auto-body welding. Its name derives from the term metal inert gas, which is really a misnomer. After all, so-called TIG welding (for tungsten inert gas) is also a metal-inert-gas welding process. Properly, what is called MIG should be called according to the American Welding Society (AWS) GMAW for gas metal arc welding. TIG is properly designated as GTAW, for gas tungsten arc welding. I’m glad to get all of that sorted out, thank you. Now, please forgive me, while I continue to use the vernacular terms, MIG and TIG, for no better reason than that everyone uses and understands them.
In the MIG process, a welding wire is continuously fed into the weld area the puddle as it is drawn along the weld seam. The wire carries current, and is surrounded at the weld, by an inert shielding gas that is fed there through the welding hose and gun, along with the wire. C-25 is the most common gas used for sheetmetal welding, in a 25-percent CO2 and 75-percent argon mixture. The gas acts like the heat-vaporized rod coating in stick welding. It shields the weld puddle and the cooling weld from most of the oxidation corrosion that would occur if the weld was made and cooled in a normal air environment.
The actual MIG process involves a cycle. As the mechanically fed wire contacts the puddle, it creates a direct short circuit with the grounded work piece. The heat generated by the short melts off the wire’s end, into the puddle, ending the short circuit. However, it is quickly reestablished, as more wire is fed into the puddle, creating the short arc cycle that is the basis of MIG welding. This all occurs at about 100 to 150 cycles/second, and produces the famously characteristic frying egg sound that is associated with MIG welding.
The most-often botched aspect of MIG welding is fit-up, the distance between the edges of the metal pieces that are being joined. In lap and offset lap joints, fit-up is not an issue because the joints are overlaps. But in butt welding, it is critical to leave adequate fit-up distance between butted edges. This space is consistently in the range between the thickness of a dime and a nickel. It may sound more difficult to carry a weld puddle down such a gap than down two more closely fitted edges. However, this is not the case.
What is beyond difficult, in fact all but impossible, is to get a good MIG butt weld when edges are fitted up too closely or in actual contact with each other. This is because the expansion from welding heat inevitably distorts too-closely-fitted metal edges so badly that it is very hard to weld them. It is also very difficult to straighten out panels welded that way.
There are several issues to master in order to do good MIG welding. You need to figure out ideal gas flow for your work. This is set with a regulator as the volume of shielding gas delivered. Try to use a two-stage regulator that employs a volume indicator in its second stage. About 10 to 20 CFM (at atmospheric pressure) is a good range in which to work when you are welding body metal with .023- to .025-inch wire. Practice and experimentation are your best guides in this matter.
Amperage settings are critical when MIG welding sheetmetal. Amperage is set by setting the wire feed speed. Different wires and different thicknesses of metal require different speeds. Manufacturers supply recommendations for this setting with their machines. It is, however, rough data, and you should plan to modify it for your own situations and according to your own experience.
Voltage settings are also critical. The best MIG machines have continuous voltage settings, while less-expensive ones rely on step settings. In either case, the voltage setting in MIG welding is somewhat analogous to how long an arc you hold in stick welding. It controls the format of the weld bead and thus its height, shape, and, to some degree, its penetration.
Somewhere between your general experience and getting frequent practice, you will get the hang of setting wire speed and voltage properly for MIG welding. Some machines set one, or even both, of these variables automatically. While many welds do not allow for practice, some do. If you can duplicate the conditions of a difficult weld with scrap metal that is similar to the material in the sections that you will be welding, and practice your difficult weld, it would be to your advantage to do so.
Some MIG welding machines have fine controls for things like stitch welding, that is, weld time on and time off, and burnback control, which is the time that the wire electrode remains energized after the gun trigger is released. These are handy features that can improve the convenience and quality of work, but they are not essential to doing good MIG welding.
One thing that is essential is to use good wire. MIG welding wire varies in quality. Some wires are junk, barely good enough to weld with, while others are a pleasure to use. Today, most of the major wire brands perform well. Still, different wires often have different features and advantages. For example, one wire may produce welds that are easier to grind, while another may lay down more uniform beads. Again, these are things with which you should experiment.
Other variables, like torch approach (forehand or backhand), distance from work, position, and angle, are best dealt with in manuals and other sources that are specifically devoted to MIG welding instruction.
This is probably the most skill-intensive type of welding, but presents an odd dichotomy. It can be understood in very complex and technical terms, but it can be performed beautifully with little knowledge of its technical aspects.
Here is an example: Back in the age when TIG welders did not have the modern sophistication of solid-state-generated square-wave forms and slope control, they relied on carbon bundle frequency generators for superimposed high-frequency currents to keep their arcs from stalling at the AC wave turnover point. I knew a man who operated one of those primitive TIG machines. He would not have understood much of the technical sentence above, but with the equipment of that period, he could deposit a uniform and well-penetrated weld bead on the business edge of a razor blade. It was no small feat. He understood little of TIG technology, but knew how to weld with it, instinctively and superbly.
The point is: You can interpret TIG welding in many ways and at many levels, comprehending and mastering as much or as little of the underlying technical issues as the spirit grabs you. Some people revel in the complexities of TIG waveforms and the possible adjustments to them. Others are greatly concerned with the shape of the TIG electrode and/or of its ceramic. Others almost intuitively know or remember how to make the best choices in these matters.
TIG is the most individualistic welding format that exists. Some operators prefer air-cooled torches; others prefer liquid-cooled units. The merits of each are often debated for jobs that involve the same materials and similar welds. Foot controls versus thumb wheels, and so forth, are hot topics. Also, there is specialization in TIG welding particular metals and alloys.
Unfortunately, is not possible to offer instruction in TIG welding in this book. However, if you are interested in doing ultimate-quality welding work on a variety of metals, from stainless and common steel to aluminum, TIG welding provides the best results. It takes some considerable commitments of money for equipment and of time to learn to use it, but you should at least consider making those commitments if ultimate welding quality is your objective. Don’t let the imposing technicalities of TIG welding scare you off if you are interested in this format. You do not have to understand it in technical detail to make great welds in a variety of situations. As with your personal computer, you can operate a TIG welder without a deep understanding of how it works. A good welding course at a vocational school will have you started down the path of mastering TIG techniques.
Oxy-Acetylene Gas Welding, Brazing and Braze Welding
These are older methods of joining thin metal sections that still have some application in today’s world of (mostly) electric welding. In most cases they will prove inferior, in one or more of several ways, to electric welding approaches. But there are times when you may find uses for torch welding and brazing techniques.
The oxy-acetylene flame, generated by mixing oxygen and acetylene gases to fuel a torch, is infernally hot about 6,300 degrees F at the cone tip of a neutral flame. A neutral flame has a perfect, combustible mixture of oxygen and acetylene for complete combustion of each gas, with no excess of either in the mix. This means that it is possible for that flame, in sufficient size, to melt the surface of steel, which begins to melt at around 2,700 degrees F.
In practice, oxy-acetylene welds are made by moving the flame, with its inner cone near the metal to be joined, angled at roughly 45 degrees to the surface, and oriented in a fore-handed direction (the direction that the weld is being made). The torch is moved along with a slightly oval or circular tip motion to make welds that have characteristics indicated by the approved ripple appearance in their weld beads. All of that takes some coordination and practice.
Actually, you can’t weld with a torch by melting and carrying a puddle down the seam. The two pieces can’t fill the seam without thinning them unacceptably. For that reason, a steel filler rod is applied to, and melted into, the puddle as needed to form a bead, as the puddle is carried down the seam by torch movement in that direction.
All of this is possible and has worked reasonably well for generations. On the other hand, MIG and TIG welding also have worked much better for fewer generations. The first disadvantage of torch welding thin sections is that it requires considerable skill, more than MIG welding, and about the same as TIG welding. Note that the considerations and manipulations of the TIG electric torch and filler rod are somewhat similar to those employed in gas welding.
Torch welding also imparts much more general heat to a weld area than either of the common electric welding formats. This means more distortion and more fun and games chasing collateral damage off the welding scene, after welding is completed. For those reasons, torch welding sheetmetal seams is rarely employed these days. Put simply, you can do better work with much less skill, knowledge, and effort with electric welding techniques.
Brazing and braze welding also have limited use in good autobody jointure practice. Like torch welding, these techniques are performed with an oxy-acetylene flame. In this case, a slightly carburizing flame is preferred. That is, a flame with a slight feather around its inner cone, caused by a richer-than-neutral amount of acetylene in the oxygen/acetylene mixture.
Brazing is somewhat like soldering with tin/lead-based and silver-based solders, but is done at higher temperatures (around 1,000 degrees F) and with filler rod that is a brass alloy or, less commonly, a bronze-based alloy. Note: Brass alloys are based on copper and zinc, while bronze alloys are based on copper and tin. Like soldering, brazing does not produce a fusion joint; that is, the molecules at the surfaces of the metals do not intermingle as they do in true fusion processes, like MIG, TIG, and torch welding. Instead, the brazing material is attracted by capillary action between the sections being brazed together. There is some surface mingling of braze and base metal molecules, but nothing like the alloying action that occurs in true fusion welding.
You might think that brazing is the low-temperature and low-distortion solution to the challenges of joining thin-metal materials. It might seem that all you have to do is bring joints up to a relatively low temperature, and flow brazing rod into the space between the pieces. And because brazing rod is very corrosion resistant, it should automatically seal areas like lap joints.
Alas, it isn’t that simple. Brazing materials typically don’t have the strength required for making sound butt welds, so their use is reserved for lap and offset lap joints. In fact, in years past, some automobile manufacturers used some brazed lap joints in their original constructions of cars and trucks. However, for most purposes, butt joints are more desirable. That eliminates brazing them.
Even with lap joints, the fit-up for brazing is critical to getting the right capillary flow of the brazing material into the joint. Without this factor, brazed joints tend to be too weak for automotive panel jointure. Maintaining proper fit-up gaps can be done in production, but usually is very difficult in repair and custom fabrication situations. Another problem with brazing is the flux that is used. It is most often borax based, and it can be persistently difficult to remove from finished joints. If some is left behind after cleaning, it does not take on primer and paint well. Finally, brazing fluxes have a tendency to cause hydrogen embrittlement in the metal adjacent to brazed joints, and this can cause cracking in that metal, as panels vibration cycle over miles and time.
Braze welding, unlike brazing, goes beyond capillary action and deposits a strengthening thickness (or bead) of brazing material in braze weld areas. This type of joint is stronger than simple brazing, but has all of brazing’s other drawbacks, mentioned above. There are some places where brazing and braze welding have application in sheetmetal work, particularly when they are used to repair or to replace factory joints that were originally brazed or braze welded. In the main, however, brazed and braze welded joints are perilously close to being substandard techniques for most panel jointure purposes today. While these techniques once may have seemed attractive, modern electric welding approaches have supplanted them.
Written by Matt Joseph and Posted with Permission of CarTechBooks