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WELDING PROCEDURE

Cold Starts

Because aluminum conducts heat so well, it will happily carry the weld heat off to distant parts of the panel, leaving the weld area wanting more. The first rule on aluminum is to weld hot and fast, so the cold start needs a trick.

Here's one: the run-on tab. Simply snip off a 2 x 4-in. piece of any odd bit of clean aluminum scrap that is at least as thick as what needs the weld, and lay it down at the starting position. Start the arc at the far end of the tab and run along it onto the seam or tear. (See photos, weld, No.5.) The hot start with both GMAW and GRAW reduces distortion and increases penetration. With GTA welding, too, a run-on tab enables the tungsten to preheat properly for a uniform bead. Preheating the tungsten on a separate block of aluminum, then moving quickly to the panel is another method.

Thin Sheet (less than .090" thick)

For nearly everyone except the rare virtuoso, MIG-welding thin aluminum is virtually impossible. However, if the backside of the panel is accessible but does not show, there is a trick- the back-up tab. By selecting another tab of clean aluminum scrap of appropriate thickness and size for the job at hand, you can effectively thicken or chill the panel to be welded.

This weld is made hot so that the penetration welds the back-up tab as well. (See photos, weld No.2.) The tab is then left in place after perhaps a bit of hammering to relive stress and level up the finish side. This permanent tab is especially helpful if fit-up is poor or a tear leaves a large irregular gap. However, the back-up tab need not be permanent.

If only some chilling (heat sink) is needed to prevent burn-through, a copper chill block is especially effective where a good heat sink is needed or as a precaution where burn-through is likely (See photos, welds No.1 and 3. Notice how well both the chill block and back-up tab restrain the penetration.)

Vertical Surface Welds

The rule is: On a vertical surface, weld up. This is because gas entrapment and loss of control of the molten weld pool do occur by welding down a vertical surface.

Is there really a secret method that absolutely positively prevents weld-cracking? Well, I've researched this issue from the top down and from the bottom up. I've spoken to highly qualified professionals and hideaway wizards on the auto, airplane and aerospace levels. This may come as a shock but, to date, there is no guaranteed method. However, you can choose to play it as you would a high-stakes, doubledown, one-card cut-and-draw on house odds in Nevada: analyze the situation very carefully, take all the pros and cons into account, and choose the best method for the job at hand. This will help you shave those odds and stack success on your side of the table. What follows is some useful information to help you make the decisions that will help AVOID THOSE CRACKS!

Crack Avoidance Guideline No.1: Choose the right filler metal for the job.

As metal heats to the melting point, it expands. As it cools, it shrinks back again. In fact, it shrinks back from the molten state even more than it expanded in the first place. Therefore, the net result of welding is shrinkage. This shrinkage causes panel distortion (warpage), oil cans, tight spots and loose spots, and is therefore an important reason to plan ahead.

Aluminum shrinks about twice as much as steel, so it has a great tendency to pull away from itself so much that it tries to crack. If the panel is loose or baggy to begin with, there is much less tendency toward cracking, because the panel can move to compensate. Conversely, if there is a lot of strain or rigidity in the part, it tends to crack more easily. Some aluminum alloys, usually those of high strength and hardness, are more prone to cracking, while those of a softer, lower yield strength are less prone to do so.

Filler metals as well are more or less crack-prone. Where there is a choice of filler, try a bit of each to test for this property. Generally, 4043 filler is less crack-prone because of its low solidus (the temperature at which it solidifies). However, 4043 is a brittle material and won't take much hammering. Filler alloy 5356 might prove more effective in some instances.

Sometimes, experience will prove better than the book, but it takes doing it-right or wrong-to get the experience.

The filler chart above is the most complete I've seen for autobody work. Don't be daunted by its complexity.

Crack Avoidance Guideline No.2: Keep the aluminum clean.

Right before welding, clean off the invisible oxide layer with a clean stainless toothbrush dedicated to that purpose. If oil, grease or questionable residue remains, solvent-wash the surface to be welded with non-chlorinated agents such as alcohol or lacquer thinner.

Filler Metal Alloy Selection Guide

NOTES:
(1) NR=Not Recommended
(2) The preferred filler alloy is shown first
(3) Filler alloy 4047 may he interchanged with 4043 filler alloy
(this guide is supplied by Paul Dickerson of the Aluminum Association, and has not been previously published.)

Crack Avoidance Guideline No.3: Stop-drill the ends of tears.

The simple practice of stop-drilling the ends of a tear with a 3/ 32-in. or I/8-in. hole can be very helpful in crack prevention.

Crack Avoidance Rule No.4: Clamp the joint carefully.

It may seem obvious to cinch down the panel with a few clamps and call it good, but WATCH OUT! Clamping requires a deft touch and both aluminum and cast iron suffer greatly from under-clamping or over-clamping. When cracks persist, look for clamps that are too tight. Tight clamps prevent shrinkage which, in turn, forces cracks to occur. If misalignment occurs, or if the damps are wobbly or loose, snug them up. The middle ground is simply to hold alignment while allowing the shrink age to pull the parts together!

Crack Avoidance Rule No.5: Tack-weld or skip-weld the joint first.

Tack-weld or skip-weld to align the gap, hold alignment and test for weld viability. Skip welds (longer than 1/2 in.) are better, in my opinion. The sequence of skips can make a difference in crack resistance in some cases. Think in these terms of balancing the stresses as you go. Many times, the first two tacks crack, only to have the third, fourth and sixth hold (persistence succeeds).

Crack Avoidance Rule No.6: End the weld properly.

The crater that is left at the end of a weld as the metal cools, contracts and shrinks in upon itself is called a crater crack. As a general rule, they are to be avoided. To avoid them, use the following:

MIG-GMAW:
(1) Use a run-off onto a tab at the end of the weld.
(2) Tail out- stop and accelerate backwards as you quit the weld.

TIG-GTAW
(1) Stop the travel and ease up on the current as you dab-in filler to bring up the center flush as the weld solidifies.
(2) Stop, restart and dab filler 1-3 times.
(3) Tail out- back up to the weld.

Crack Avoidance Guideline No.7 Pay careful attention to the weld bead geometry.

Convex beads are more crack-resistant than concave beads. A concave weld (ditch) displays insufficient filler and simply needs another pass to bring it up to at least slightly rounded (convex). (See page 90.)

Crack Avoidance Guideline No.8: Maintain a strong weld geometry.

Essentially, the geometry of a joint contributes greatly to its strength- whether it be for the finished strength of the part or, in this case, the strength of a joint's resistance to back-cracking. "T" welds, for instance, are susceptible to both weld undercuts and concave weld beads. Weld undercuts are an obvious source of weakness in any joint configuration and are caused by too much weld heat (amperage/current).

This concludes the eight-point checklist I have used to play the "beat the cracks" game when welding aluminum. As I mentioned in previous articles, I keep important notes like this near my tool box for handy reference.

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