Weld tolerances and thickness

So, we’ve talked about gaps a few times, and if you’re not sick of hearing about them, thanks for your patience.  Yes, there are other weld tolerance concerns, so let’s talk about one of the other common tolerances that can wreak havoc on a qualified welding process.

Thickness.  No, I’m not referring to anyone’s skull, rather the tolerance of the cross section of the material at the point of welding.  Depending on the weld joint configuration (assume autogenous for today), there may be 2 or more tolerances to consider. 

  • The penetration tolerance – how deep the weld is and the +/- allowance. This will play a critical role in the strength, repeatability and achievability of the weld.
  • First component thickness tolerance – typically you will be welding two or more components together. With a full penetration weld, if your tolerance band here is wider than your weld process can readily accommodate, you can have over- or under-penetration with a whole host of problems – as always, a topic for a future post.
  • Second component thickness tolerance – if there is sufficient thickness tolerance on both first and second members, this can lead to significant mismatch between the components, again leading to a host of potential issues.
  • Additional components – weld backups, nearby features, etc. The tolerances of these need to be considered.  We spoke about compensating for weld shrinkage in an earlier blog post. If that isn’t taken into consideration, you risk all sorts of undesirable weld issues.

Now, if we’re looking at a partial penetration weld, we may not be as concerned about the overall thickness, but we will be much more interested in the mismatch between the components. 

Hopefully you’ve gleaned just a bit of useful information from this.  Please remember to consult with a welding expert every time you’re designing a component for welding.  It will reduce your stress in the design process and all subsequent operations.

Weld cracks: a lesson in the delicacies of metallurgy

One of the many interesting parts of this job is the detective work involved when a part isn’t welding properly.  Roughly six months ago I was tasked with my first major metallurgical engineering project, which showed that the metal in your parts may have some hidden surprises.

 

This particular issue involved a customer part that we had been laser welding mostly problem-free in sizable quantities for several years. Our production officer had set up the job in accordance with our process documentation, but observed a crack running from root to surface of the weld when he cut his first piece sample.

 

We performed a few additional cross sections and found that the penetration and hardness were well within the customer’s requirements. This led us down the troubleshooting path of singling out each variable on our end, one at a time. However, we found nothing on our end that influenced this type of weld cracking. Our process had not fluctuated, our tooling was in tip top shape, and our machine output was well documented with no fluctuation in power or beam quality.

 

Difficult weld combinations

This led me down the road of metallurgical investigation. These parts happen to be comprised of two dissimilar alloys – 440C stainless steel and tungsten. By all rights, these alloys really should not be weldable for a number of reasons, primarily because tungsten is incredibly brittle – it lacks the ability to flex and stretch during the heating and cooling phases of a weld. This, coupled with the extreme melting point differences of the two alloys, creates an incredibly difficult-to-weld combination.

 

Therefore, we had to examine what made the weld possible in the first place and, by eliminating variables,  figure out what had changed between the last several years of successful production and this particular order of parts. For that, we set off to gather as much information about the alloys as possible.

 

Piecing together a road map

We started with performing a coarse metallurgical analysis on the two components. Using our XRF gun (a portable, x-ray fluorescence device used to analyze the elemental composition of materials), we can see a breakdown of the heavy alloys in a customer’s part. The 440C showed all the elements we would expect to see in that alloy. The tungsten came up with large amounts of nickel and copper, with little else present. This correlated to the customer’s material certs, although it left us scratching our heads about how and why these elements would be alloyed with the tungsten. In comparing these alloying elements and the material certs, we were able to start piecing together a road map of what was going on.

 

The alloying elements had steadily decreased over the last several years according to the customer’s material certs. In addition, the yield strength gradually decreased while the tensile strength gradually increased, which effectively gives us a harder but more brittle alloy. We reached out to our partners at Mott Corporation to help us with a more in-depth look at what was happening to these parts.

 

SEM analysis

Mott brought us in to their metallurgy lab, which is equipped with an SEM (Scanning Electron Microscope capable of extreme magnification and elemental detection). The SEM analysis was able to show us the molecular structure of specific regions of the parent materials as well as the weld pool area. The images were stunning. The SEM effectively paints each alloying element in different colors so that we could see how the molecules were coalescing in our weld.

 

These photos show the laser melt weld pool on the left, a crack in the heat affected zone of the tungsten hardware, and the tungsten hardware at the right. The crack follows the intermetallic region (lower melting temperature phase containing W, Ni, and Cu) that encompasses the tungesten rich (high melting temperature phase).

 

We found that the tungsten was actually powder formed, with the nickel and copper being used as a sort of glue. In the region where the tungsten was not melted into the weld pool, the molecules appear as field stones with nickel and copper packed tightly between them. As we approached the weld region, the nickel and copper took on a liquid appearance like that of a river flowing around boulders of tungsten. We discovered that the nickel and copper were melting too soon, long before the tungsten did, which was allowing most of the “glue” to flow out of the powdered tungsten in the heat affected zone (HAZ) and into the weld pool. This left the tungsten nodules with too little glue to hold them together. Coupled with the increase in tensile strength, this gave us a metallurgical recipe for failure.

 

Utilizing this information, our customer was able to adjust the material requirements for their vendor and the product line was back up and running with the next shipment.

Soldering: not all flux is created equal

This image shows a part with corrosion (top) and without (bottom).

A recent issue with a soldering process for a customer has been the focus of our attention here at Joining Technologies for quite some time. Since it turned out to be a good learning experience, we thought it was worth sharing to explain issues that intermittently arise when soldering these types of assemblies.

The process I’m speaking about involves soldering a 26 gauge, bright soft steel wire to a flat solder terminal made of tinned copper.  Overall, the process is very sound with excellent bonding and visually acceptable solder joints.  However, we were finding corrosion and oxidation on the assemblies, which rendered some of the parts to be non-conforming.  Since the corrosion occurred infrequently, a deeper look at the process was required.  

We dissected every step of the process and eventually found that we were using a hydrochloric acid-based soldering flux.  This is something that never crossed our minds, but obviously sheds light into why we had the issues of corrosion and oxidation.  Even after following the manufacturer’s instructions for neutralizing the flux, the corrosion was still evident.

Upon calling tech support for the soldering flux manufacturer, we quickly found out that not all flux is created equal!  After explaining our application, a tech expert recommended a bromide-based flux as a replacement option, since we were dealing with stainless steel.  It’s also important to mention that certain soldering fluxes work better with soldering torches vs. soldering irons.  We use the soldering torch in our assembly process. 

Post-cleaning or rinsing is required for all types of flux.  Always follow the manufacturer’s rinsing instructions for the specific type of flux used, as the expert noted that flux is not supposed to be left on any material post-process.  The best time to rinse the materials of flux is immediately after soldering.  This is a very important part of the process that should not be overlooked.

The bottom line is, it’s important to know your material types as well as the difference between torch soldering and iron soldering!  Your choice of flux and a successful soldering process will depend on these factors. 

Questions about soldering or welding? Contact us at 860.653-0111

 

Joint design for welding: the pros and cons of groove joints

After you’ve selected the right material for your welding project, the next important consideration is joint design. We’ll be taking a look at the different types of joints and briefly discussing the pros and cons of each, especially in relation to laser and electron beam (EB) welding. Let’s start with one of the strongest joints, the groove joint, and what makes it so “groovy”:

First, a quick definition: Groove joints or square groove joints are a type of butt joint, with two flat pieces parallel to each other and butted together with a 100% weld joining the two pieces. Here’s an example:

Pros

  • High strength: Provides complete fusion, low stress, and 100% penetration.
  • Easy to machine
  • Good distortion control: Welds shrink evenly and are less likely to distort.
  • Easy to inspect

Cons

  • Geometry limited applications
  • Not suitable for applications with delicate items behind the weld, such as electronics
  • Not self-aligning – fixturing or a backer may be required
  • Sensitive to faying surface conditions

Additional considerations:

Fit up is important for groove joints, especially for laser and EB welding. Due to the energy density of these types of welding, the beam falls through large gaps.

Each type of joint has its advantages and disadvantages, but the biggest advantage of the butt joint and square groove joint is its strength. It can withstand stress better than any other type of weld joint. Pretty groovy, right?

Have questions about joint design for laser welding or EB welding? Ask one of our experts, or leave a comment in the space below.

Weld contamination part 2 – aluminum oxide

Aluminum oxide contamination is no big deal for that rusty yard art project, but can wreak havoc on critical applications like an axle shaft. (Spider by John Lucas, Jr.)

I’ve talked about weld contaminants in a previous post, but a recent conversation convinced me that it was a topic that warranted revisiting.

Aluminum oxide (AlO), in particular, is a nasty contaminant that is oft overlooked.

What exactly is AlO? Quite simply, it’s one of the most common abrasives used. It is present in most sandpaper, grinding disks, cutoff wheels, and even your favorite abrasive pads.

Why use AlO, and why is it so common? That, as well, is simple. It’s cheap, and it works. It’s a fantastic abrasive with low heat retention, making it ideally suited for all kinds of grinding processes.

Why is AlO a problem for welding? Ask a dozen welders, and you’ll get a dozen answers to this one.

Ultimately, the extent of the problem will be based on your part characteristics. If you’re welding up some rusty yard art with a stick welder, you’re probably not going to care that you just cut that shovel head and leaf spring with an AlO cutoff wheel. The residual contaminants from the parent material are going to contaminate the weld far more than the few embedded AlO particles.

Now, on the other hand, that alloy steel dragster axle shaft you’re about to have electron beam welded is going to care a whole lot about even the smallest defect. You might think that a couple of seemingly inconsequential grains of AlO abrasive couldn’t possible make the difference between a mid 5 second run and hitting the wall at breakneck speeds. That would be a dangerous assumption.

AlO melts at nearly 4,000 degrees (F), while steel melts at under 3,000 degrees (F). That means that those particles have the potential to sit buried in the resolidified weld as brittle little points of failure. No amount of post-weld heat treatment will reduce how brittle they are.

Now, let’s assume that we DO get the weld hot enough to melt the AlO particles. That should be OK, right? Not really. AlO is, as implied in the title, Aluminum and Oxygen, both of which will be liberated with sufficient heat. Neither of which you want in your steel.

As welders, we take extreme care to prevent the introduction of oxygen into our weldments. There are entire classes dedicated to this. For this blog, let’s just say oxygen is bad, mmm kay?

Aluminum is also something not desirable in steel. It forms brittle intermetallics that may or may not show up as weld defects. They WILL present in the form of unexpected strength and toughness characteristics.

One other way AlO can get you is by evaporating, but not dissolving into the parent material, causing porosity. Again, for your steel garden statue, no problem. For that axle, the results could be anywhere from embarrassing to straight-up dangerous.

What is the alternative? There are lots, but if abrasives are in your plan, consider Silicon Carbide as your final abrasive.

For more details, contact us at 860.653-0111 to reach out to an expert to go over some great options for your next artistic or high speed venture. You can also leave a question or comment in the space below.