Welding Aluminum Problems

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Hi Kent -   I am building a prototype paintball gun from the ground up. I'm using 6061-T6, doing the machining myself. By necessity all the parts right now are "one of a kind", and so messing them up in the welding process can set me back some amount of time. (Think of this as a network of relatively small Al pipes I am welding together...)  

The internal pressures on most of the joints should never exceed 800 PSI; shearing strength is probably needed to be many times that but well within most normal joining that is rated for Al.  

I have a few questions:   I spent most of yesterday on a Square Wave Tig, just doing practice work on 6061 stock. I was not very successful; I could get a puddle going, but it had no penetration at all; the parts would simply fall away once I un-clamped them. Or they would mix, but then crack down the seam when they cooled. Or there would be two separate puddles, and they would refuse to mix. I now believe that this was because I wasn't nearly clean enough, but is there anything else I need to know here?  

Tig kinda makes me nervous, because screwing up means burning through a part, and thus going back to the mill and lathe to fab more parts. (However I have seen some joints on things like bicycle frames that makes me think that it could be really great...)   With the above in mind, will either soft-soldering or brazing do the trick? The brazing rod and flux I ordered from you seems to have a low enough melt point so that it's unlikely I'll burn right through my parts. Any advice here? General pointers? Thanks, Peter

The Tinman Respondeth:

Peter, For the aluminum puddling and but not flowing together: DIRT. Either a dirty ceramic cup, or tungsten, or filler, or material. Or the torch cap is loose or the "o" rings are bad, and it is leaking argon.

Now, for joining successfully without those nasty cracks, see:

Welding Aluminum Body Sheet

Crack Control (see CAG #2) 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.

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.

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 Guideline 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 Guideline 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 Guideline 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.

......................Once you have read this, try the methods. I cannot say brazing is the answer without analyzing the joints and testing. As for a "how to" on brazing and soldering, it is certainly worth a book-sized effort or a workshop. Briefly: fit, clean, and flux parts. Heat with soft flame until flux runs like water. Ease off on adding heat to keep heat constant or imperceptibly higher. Touch fluxed filler and heat together until filler runs.

BE CAREFUL not to overheat. Kent