Death to False Metal: Why Aluminum isn’t for Everything

It’s happened again: someone has asked me to weld something out of aluminum that would probably be better made out of something else. Probably stainless.

Aluminum is a great material. It machines well. Its corrosion resistance, even in not-terrifically-corrosion-resistant alloys like 6061, is plenty good enough for many applications. Did I mention it machines well? It’s really a machinist’s dream: soft but not gummy, brittle enough to form chips nicely, and with tons of thermal conductivity to draw heat away from the tool.

The problems usually arise when someone switches from designing aluminum machined parts to designing aluminum welded parts, which is a different animal entirely.

1. The welding itself.

First off, aluminum welding is more challenging than welding many materials, including stainless and mild steel, for a few reasons:

  • You need bigger, more pricey welding equipment. Although its melting temperature (~1100° F) is actually lower than that of steel (~2600° F), aluminum’s thermal conductivity is roughly double that of mild steel, so you really have to pour heat into it to get it to melt, which means bigger, more complicated welders.
    • Aluminum TIG requires AC, not the DC used for almost all other TIG applications. All TIG machines have DC, but not all have AC. In addition, many AC-capable TIG machines have AC-specific adjustments: AC balance, AC frequency, and/or AC waveshape. This adds complexity.
    • Sometimes this equipment is so big that it needs expensive electrical installations. A Miller Syncrowave 250, for example, is a common TIG machine that works well for aluminum. But it requires 93 amps of 230V single-phase power in order to run at its rated output. Have you priced that kind of electrical work lately?
    • If you don’t have a welder with enough juice, you can pre-heat the aluminum, e.g. with an oxy-fuel torch, but this complicates things further, including cleaning (see below).
    • MIG is of course faster than TIG, but aluminum MIG is especially pricey, as aluminum has low stiffness and therefore poor feedability. A spoolgun is the cheapest way to get started (~$800 and up) but they’re finicky little things. A push-pull setup is far better, but the gun alone can easily cost $2,000. In contrast, steel wire will feed well in a sub-$200 MIG gun.
  • The welding technique is harder. Aside from some expert aluminum welders, most folks find aluminum pretty challenging, especially at first. I’ve always taught it after teaching mild steel and stainless.
    • With TIG, the filler metal addition rate is significantly higher than with steels, so filler metal must be fed more quickly. 6061 is prone to cracking unless the foot pedal is tapered off to minimize crater size. Aluminum TIG requires far more pedal manipulation than is required for other metals.
    • With MIG, there are a variety of possible issues, including burnbacks, birdnesting, and porosity (which in my experience is more common with aluminum MIG than with aluminum TIG, and is overall vastly more common with aluminum than with steel or stainless).
  • Cleanliness is everything. Like stainless, aluminum is highly reactive, so much so that what we think of as aluminum is really a block of aluminum with a layer of aluminum oxide covering it. The oxide layer is usually thin, but aluminum oxide is a vastly different material: it’s roughly 3x harder (Source) and its melting point is 3.5x higher (Source and source). Its removal is critical to success in aluminum welding. This is generally best done with a hand-held stainless steel scratch brush. There’s a bit of technique to the brushing, which is best learned in person from someone who knows how, and the brushes used ought to be dedicated to this specific purpose to prevent cross-contamination.
  • Aluminum is its own universe. Many of the work practices, techniques, and consumables that work well for other metals don’t work for aluminum.
    • Aluminum behaves differently than steels as it heats up. There’s no orange or red glow to indicate temperature.
    • It can be terribly difficult to assess the alloy content (and therefore properties) of an unknown aluminum. With steels, the main alloying element is carbon. Carbon content is a huge factor affecting weldability of steels, and conveniently, carbon content can be assessed easily, e.g. by spark test or hardenability test. No such simple tests exist for aluminum alloys, and yet aluminum alloys vary more widely than steels in both alloying contents and properties than steels do, including huge variations in weldability.
    • Selecting filler metals for aluminum can be more complex than with steels. There are dozens of aluminum filler metals available. The two most common ones (ER4043 and ER5356) behave quite differently, so a well-equipped shop needs both, but users must be careful not to mix them up, as they’re essentially identical.
    • In contrast, switching from, say, mild steel to stainless steel, requires only a new filler metal and some small changes in machine settings and/or  torch technique. Someone with 20 hours of mild steel TIG experience can pick up stainless TIG in only a couple hours, but this hasn’t been true of people making a switch to aluminum in my experience.

2. Strength-weight ratio

At this point, you may be thinking, “Sure, it’s a bit of trouble to weld, but aluminum has such a fantastic strength-weight ratio that it MUST be worth all this trouble!” And you’re right…except you’re also wrong.

Let’s consider 6061-T6, as it’s by far the most common flavor of aluminum. Typically 6061 is supplied in the -T6 condition, as 6061-T6. The “6061” part just describes the alloying contents: 97% Al, 1% Mg, 0.6% Si, 0.3% Cu, etc. But 6061 is a heat-treatable alloy, so its properties vary dramatically depending on the heat treatment it’s had. That little “-T6” is a temper designation; it tells us what heat treatment has been done. In the case of -T6, it means the 6061 aluminum has been through a series of heat treatment operations, including heating to ~980° F  for an hour, rapidly cooling in water, then heating again, for anywhere from one hour at 400° F to eight hours at 325° F.

Why does this matter? Because it means that even though 6061-T6 is super strong stuff, as soon as you weld it, all that fancy heat treatment goes out the window, along with almost half the material’s tensile strength:

  • Typical 6061-T6 ultimate tensile strength (UTS): 45 ksi
  • Typical 6061 ultimate tensile strength as-welded: 27 ksi
  • Minimum expected 6061 ultimate tensile strength as-welded: 24 ksi (Source)

Oops, you know that super strong 6061-T6 you just welded on? It just got 40-47% weaker. To be fair, this weakening happens on a gradient, with the weakest part being nearest the weld (see Figure 2 here for more). In fairness, if we weld together two sheets of 6061-T6, there’s typically at least some part of each sheet still at the pre-welded strength of 45 ksi. But weakness near welds is really unfortunate, because welds and stress concentrations tend to be co-located.

If the goal was to have a material that’s strong and light (i.e. high strength-weight ratio), then this post-weld weakening business is bad news, because your aluminum weldment hasn’t gotten lighter, but it has gotten weaker:

  • UTS/density, 6061-T6: 462
  • UTS/density, 6061-T6, typical as-welded: 277
  • UTS/density, 6061-T6, minimum expected as-welded: 246
  • UTS/density, A36 mild steel: 204
  • UTS/density, 304 stainless: 253
  • UTS/density, 4130 “CrMo” steel: 342

(These are unitless ratios; higher numbers indicate a material that’s stronger per unit of density. Note that neither A36, 304, nor 4130 lose significant strength when welded properly.)

The main takeways:

  • 4130 has a strength-weight ratio 23% higher than as-welded 6061-T6 typically does. The major downside of 4130 is corrosion resistance, and while paint is a great solution to that problem, it adds weight and doesn’t stick around forever.
  • Even 304 stainless (not known for its super strength) may have a slightly higher strength-weight ratio than as-welded 6061-T6. And 304 is both easier to weld and more corrosion-resistant than 6061-T6. Its main downside is its relatively poor machinability.

3. Added uncertainty

Worse yet, the strength loss in as-welded 6061-T6 is variable depending upon a host of factors. As this aluminum welding guide puts it:

The as-welded mechanical properties of the 6xxx base metals are very sensitive to welding variables such as heat input and joint design

I’ll go a little further. As-welded, weldments rendered in 6061-T6 can vary in strength (and therefore strength-weight ratio) by up to 88%, depending on factors including:

  • location relative to the weld (farther from the weld being stronger)
  • heat input
  • joint design
  • filler metal selection
  • welding process

Welding 6061-T6 introduces undesirable uncertainty about the strength of the base material, which is stacked on top of other uncertainties, e.g. about weld quality.

4. Some caveats

A couple notes for my more technical readers (all both of you).

It’s possible, though typically not at a prototype scale, to post-weld heat treat (PWHT) a 6061-T6 weldment and thereby restore it to roughly the -T6 condition, i.e. with roughly 45 ksi UTS. I doubt this is within the realm of possibility for my readers. It requires an oven that goes up to about 1000 F, can maintain a consistent temperature for hours, and is large enough to fit your weldment inside it. Also, you’ll need to be able to move the nearly-1000-degree weldment out of this oven to dunk it in water. Also, your weldment will distort as a result of all this heating and cooling.

There are aluminum alloys besides 6061, and some of them lose significantly less strength after welding. The 5000 series in particular is attractive for welding applications, for a variety of reasons cogently elucidated in the Maxal aluminum welding guide I’ve referenced repeatedly. Sourcing 5000 series alloys can be a little tricky, but they are generally better for welding applications than 6061. Many of the 5000s bend nicely, too, whereas 6061-T6 often just cracks when bent (try it, it’s fun). Be sure to choose the right filler metal; many 5000-series alloys are best welded with ER5000-series filler metals, as opposed to the ER4043 (and, recently, ER4943) often used when welding 6061.

Similarly, there are aluminum alloys for which the PWHT process is vastly simpler. Alloy 7005, for example, can be post-weld aged at temperatures as low as 200° F, restoring much of its pre-weld strength. But you probably won’t find some kicking around the shop–outside of industries like bicycle frames, 7005 is rare bird.

5. Aluminum isn’t all bad

I’m not saying you shouldn’t use the stuff. Just don’t assume it’s the perfect material for all applications because you’ve used it for machined parts. Know that when you switch from machining it to welding it, you’re adding some significant risk and complexity.

And material selection is always a matter of compromise. 304 stainless, for example, welds easily, looks cool, and has great corrosion resistance…but it’s a pain to machine, it’s a little heavy for its strength, and it’s expensive. 4130 would be considered downright magical were it not for a couple limitations: it needs paint in most applications, and it’s not nearly as machinable as 6061-T6.

Until a perfect material is developed, we’ll have to settle with an assortment of less-then-perfect options…including aluminum.

 

Training ≠ Manufacturing

2014-01-02-16-46-23
A factory in Faritabad, India, I visited in January 2014. This is not a training environment.

Recently I was at a customer site in the Midwest. These folks do…a lot of welding. They’re consistently one of the biggest 3 or 4 buyers of steel plate in the country. They buy so much plate that they don’t have to buy it in standard thicknesses like 1/2 inch or 3/8 inch; rather, they buy it in whatever size their engineers decide is best. One employee told me they can buy custom thicknesses like .433 inch cheaper than most companies can buy normal sizes like 1/2 inch. They buy a lot of plate, and then they do a lot of welding on it.

This site (one of dozens) is massive–easily the size of a couple of football fields. I was taken at one point to the “training area” at this plant. The training area felt like they had picked a random corner of the factory to stick it in.

It was loud. Not from the welding or the training that was happening, but from ambient noise.

It was poorly-lit. That is, there was more than enough light to see things happening around you, but nowhere near enough light for detail work, and welding training is most certainly detail work.

It was filthy. Not just in the sense that they needed to sweep up. More like there was just a lot of visual clutter around–old welding machines, spools of wire, work in progress, none of which were part of the welding training process.

Here’s the thing. Welding training is not welding. It’s not even manufacturing. It requires a different approach to do well.

The goal of a training center is to mold minds, not make widgets. The goal in a production environment is, primarily, efficiency; the main concerns are things like deposition rates and defect rates. The goal of a training center is to take untrained humans and turn them into (better-) trained humans. The goals are different, so the environments need to be different.

In a production environment, there shouldn’t be a lot going on besides production. As Peter Drucker put it, “A well-managed factory is boring. Nothing exciting happens in it because the crises have been anticipated and have been converted into routine.” The factory environment, then, doesn’t really need to foster communication beautifully. Communication isn’t the main priority; welding is. Yes, welders need to communicate, but not every second of the day, or even most seconds.

In a training environment, on the other hand, communication is everything. Training is communication. Communication moves the training process forward, and a lack of communication will hold it back.

So it’s imperative that a training area be quiet.

Similarly, it’s important for trainers and trainees to be able to see what they’re looking at. Details often matter in welding, so one might argue that good lighting is always necessary. But experienced welders can produce good work even under sub-optimal lighting conditions; this is in fact one way to define a welder as experienced. In my experience, inexperienced welders require good lighting to even do mediocre work.

There’s a school of thought that suggests that trainees should have to endure hardships identical to those they’ll experience on the job. I agree. In fact, the ideal training environment might expose trainees to hardships worse than those they’ll ultimately experience on the job. But, those hardships must be introduced gradually. Training is inherently stressful; adding hardships from the start only increases the dropout rate and makes the training take longer.

To sum up, let me contradict myself: in a sense, training actually is manufacturing, if we define “manufacturing” broadly. The goal is to produce something, even if that something is very different from a typical manufactured widget. As Peter Drucker said, we still need to anticipate crises and convert them to routine…it’s just that training involves a vastly different set of crises and routines. To be effective, training areas must reflect that.

Flipping Down Right: Using a Passive Welding Helmet

Part 1: Why use a passive helmet?
There are two types of helmets or hoods for arc welding: active and passive.
The active kind is auto-darkening; the lens starts out at a light shade (often shade #3), but when an arc is struck in front of it, the lens darkens (usually to a shade between #8 and #12). The darkening happens within a tiny fraction of a second–between 1/5,000 and 1/20,000 seconds.
Passive hoods are much simpler–the lens is always a dark shade like #10, and the user “flips” the hood down immediately prior to striking an arc. Looking through a passive hood, it generally looks completely opaque all the time, unless you’re looking at a bright light.
  • Passive hoods are cheaper.
    Fifty dollars will buy a passive hood that will likely last the rest of one’s life. I’m a big fan of the Jackson HSL-100. Even a high-end passive hood typically costs well under $100. They never need batteries. Fifty-dollar auto hoods are available, but they are far less durable than $50 passive hoods. Decent auto hoods start at around $80, and go up to many hundreds of dollars.
  • Passive hoods generally give a better view of the arc.
    I can weld just fine with my $200 Miller Performance auto helmet, but the view of the arc is a little clearer, and colors are rendered a little more realistically, with my $50 passive hood. More expensive auto hoods supposedly have better color rendition, but at a cost 5 or 10 times as much as a $50 passive hood. Further, a $50 passive hood has an industry-standard 4.5″ x 5.25″ viewing window–that’s 23.6 sq. inches, versus 9.2 sq. inches on Miller’s high-end auto-darkening Digital Elite hoods. Granted, a large viewing area is more of a necessity when using a passive hood than when using an auto-darkening, but it’s also just nice to be able to see a larger area.
  • Passive hoods are more reliable.
    There are a number of things that can go wrong with an auto hood–batteries wearing out and electronics being damaged by temperature or moisture are just the start. An auto hood has a limited number of sensors that “see” the arc and “tell” the lens to darken. It’s possible that one or more of these sensors can become blocked, such that the user can see the arc, but the sensor(s) cannot, causing an uncomfortable, distracting arc flash for the user. Pricier helmets have more sensors (typically up to four) to help mitigate this risk. Furthermore, these sensors must be sensitive enough that even low-amp welding will trip them, but not so sensitive that they trip due to exposure to lights indoors or sun reflections outdoors. This is a problem the helmet manufacturers have been working on for a while now, and again, pricier helmets have solutions like sensitivity adjustment dials and even modes that sense when an arc is struck by non-visual methods (e.g. Miller’s X-mode). Passive helmets work regardless of any blockages, and at any current level.

Despite all this, I own a Miller Performance auto-darkening helmet, which I use and recommend. Auto hoods are especially good for precise TIG work (where even the most careful flip-down might damage or move the work) and repetitive operations like tacking (flipping down 100 times an hour is time-consuming, tedious, and hard on the neck). They also come in handy when a passive hood simply won’t fit–I once had to weld on some I-beams in a house under construction; I was working so close to the ceiling that a passive hood would’ve made things even more difficult and cramped.

I also own a Jackson HSL-100 passive hood. I use it once in a while, partly to be sure I don’t forget how to use it. And I keep it around just in case my Performance fails, breaks, or runs out of batteries.

The two types of hoods are different tools for different applications. Ideally every welder would know how to use each. This article focuses on passive hoods, because nobody else seemed to be writing about them, and because auto hoods vary considerably more than passive hoods do.

Part 2: Setting up the passive hood

jackson headgear
This is the current (as of May 2013) headgear included with new Jackson HSL-100 passive welding helmets. I like this particular headgear–it’s simple, durable, and comfortable–but all headgear has roughly the same functions.
The big red knob that sits at the back of the user’s head and allows the user to fit the helmet to their head circumference or hat size.
  • The hat size adjustment is too loose when the hood falls off the wearer’s head during flip-down.
  • The hat size adjustment is too tight when it causes discomfort.
  • The user should have to loosen the hat size adjustment knob in order to remove the helmet.

The crown adjustment controls how high the headgear sits on the user’s head.

  • If the headgear sits too high on the user’s head, it can be impossible to keep the hood on the head during flip-down.
  • If the headgear sits too low, it can get in the way of safety glasses (which should always be worn under a welding helmet) or become uncomfortable when tightened.
  • Generally the front of the headgear should sit just above the eyebrows, where the skull narrows slightly.

The tension adjustment knobs control the speed of the flip-down.

  • They are both right-hand threaded (“righty tighty, lefty loosey”), and should always be adjusted in concert.
  • Should be set loose enough that the user can achieve a fast flip-down with minimal effort or movement. The helmet should come down quickly.
  • If the tension screws are set properly, the flip-down requires the use of only a few small muscles in the neck. The less the neck and head move during the flip-down, the less likely the hands are to move.
  • They are set too loose when the hood flips down on its own.
  • Even once adjusted perfectly, tension screws may need to be readjusted for a number of reasons, such as welding position–i.e. different tension may be required if welding in horizontal position (2G) versus in flat position (1G).
Part 3: Using the passive hood
  • The ideal flip-down is one in which the user’s hands remain exactly where they were prior to the flip-down.
  • The goal, therefore, is to keep the body motionless except for the neck.
  • The flip-down should not require much effort. Welding work is often hard work, and the flip-down shouldn’t add to user fatigue any more than necessary.
  • It’s a flip-down, not an up-then-down. Lifting the head up before nodding it down is usually a sign that the tension screws are too tight. An up-then-down motion can also take the user’s eyes off the work, which is sub-optimal.
  • Never use a hand to lower a passive hood. It’s a bad habit to get into. One can barely, sorta, kinda get away with flipping down by hand in some applications (chiefly GMAW), but not many. Learning to flip down well is part of learning to weld; starting with good habits shortens the process.
  • Good helmets (including the Jackson HSL-100) have a detent that locks the helmet in the flipped-up position. This feature should be used every time–that is, the helmet should be lifted all the way up so that it clicks and stays in place. The detent is immensely helpful in preventing the helmet from accidentally flipping down, especially when the user bends over to look down at a weldment on a table.
Part 4: Everything else
Your helmet is yours
If you’re serious about this welding thing, you need your own hood.
Don’t delay
Beginner welders often flip the hood down, then delay striking the arc for a few seconds. But with the hood down, the user can’t see anything–there is little to be gained when the user is “flying blind” like this. I tell beginners to think of the flip-down as occurring while they’re on the way to starting the arc; the finger should already be on the MIG gun trigger, or the foot on the TIG pedal. Prior to flipping down, everything should be ready for welding to begin, except that the hood is up instead of down. As soon as the hood comes flipping down, the arc should be struck. The only exception is if the welder detects that the flip-down was unsatisfactory, usually meaning that the hands moved (more on this below).
Process-specific tips
GMAW:
  • The tip of the electrode wire should generally be touching the work prior to and during flip-down.
  • Friction between the tip of the wire and the work can help in preventing movement of the torch during flip-down.
  • If the hands move slightly during flip-down, the user can sometimes use the tip of the wire to “feel” for the start location, e.g. the gap between two plates.

GTAW:

  • Unless scratch-start or lift-start is used, the electrode should never touch the work.
  • If in doubt, prior to depressing the pedal, the user can touch the electrode to the work (again, possibly “feeling” for the start location with the tip of the tungsten), then lift the electrode slightly before depressing the pedal to start the arc.
  • Advanced tip: Flashlight mode. When using a foot pedal for non-critical applications, the arc can be struck and kept at low amperage, then guided to the desired start location using the arc as a light source. Keeping the amps low (pedal barely depressed enough to keep the arc lit) helps prevent discontinuities like arc strikes, which are especially troublesome of crack-prone materials. This technique is undesirable for critical applications.

Get a sweatband
sweatband
This is a leather sweatband for welding helmets. It installs on the front of the headband in a couple seconds and is worth its weight in gold when doing any significant amount of welding work. Tillman makes this one, which is well worth the $2 asking price. Black Stallion/Revco also makes a cotton one. The foam typically sold with a helmet, even a nice one, is engineered for low cost, not comfort.

Seriously, get a $2 leather sweatband. Your helmet will feel more comfortable, espeically when it’s hot out.

Get a gold lens
For things like precision TIG, a gold lens is the gold standard of arc visibility. I like to say that looking at a TIG arc through a gold lens is like looking into the soul of the arc. Instead of the green or blue tint of a normal lens, colors look nearly true-to-life through a gold lens. But the difference between an auto-dark lens and a typical green passive lens is bigger, to me, than the difference between a green passive and a gold passive lens. It’s nice to have, but not a necessity for every welder.

Practice

Learning to flip-down a passive hood properly takes patience and practice. Even after years of welding, some flip-downs will go poorly; this is not a cause of great concern. It takes only a second to lift the hood up, evaluate what went wrong, and try again. Within a few minutes of learning how, almost anyone can set up and use a passive hood like a pro.