Weld shrinkage is a common challenge for both experienced and amateur welders in the metal fabrication industry. Applying heat to any metal will cause it to expand and contract, resulting in warping and distortion. This inherent characteristic can be a major problem when a completed fabrication needs to be perfectly flat and achieve extremely tight tolerances.
Despite efforts by some engineers to develop a model to determine the amount of weld shrinkage in a given application, there is no formula. That’s because no two welders will perform exactly the same; the slightest difference in technique will result in variances. Plus, there are other factors at play, and something as seemingly small as warmer or colder ambient temperatures can make metal materials act differently.
It leaves many wondering how to prevent weld shrinkage and how to ensure their metal fabrication company of choice can deliver a project to specification. Consider these tips and ask the right questions.
Weld Mapping
Weld mapping is a critical part of ensuring each item is manufactured the same way each time so you get the same end result. A weld map is exactly that: it maps out every step in the process, including which size fillet weld to use (when joining two perpendicular pieces) or where to start and what weld sequence to follow.
Consistency matters; you don’t want one item going through production one way and then the next one coming down the line differently. In cases like this, distortion is pretty much inevitable and you’ll be fighting to keep things straight. If you need to fabricate multiple identical items and you don’t map out your steps, you’ll have a process that can’t be repeated the next time.
Clamping and Fixtures
One of the most common methods for maintaining flatness and dimensions is to use clamps, braces, and fixtures. Strongbacks are also used to maintain proper alignment and hold a weldment in position. Once completed, the strongbacks and bracing are removed. Metal components might also be temporarily tack welded to a welding table so they don’t move, or parts might be tabbed into a large opening or cavity to help dissipate heat to minimize warpage.
The design of the weld fixtures is a skillset all its own. Engineering teams spend a lot of time planning and designing fixtures that are strong and able to maintain tolerances. Part of that design process is ensuring that welders have enough space to maneuver between the fixtures with their weld guns.
Machining
A customer might request prefabricated structural materials as part of a project, such as bars, angles, or channels. Right out of the box, however, these components rarely meet the tight tolerances outlined in quality data packages. Half-inch metal plate, for example, will not be precisely a half-inch thick across the entire surface.
Prefabricated parts have a wide range of variances and often require machining a thicker or larger part down to the required tolerance. Even with the methodical process of mockups, fixtures, clamps, and other methods, a part might need to be machined down to the proper dimension once completed. Likewise, holes, slots, and other access points should only be machined once everything is assembled to ensure proper placement.
Create a Mockup
Creating a simulation can provide insights into how materials might shrink. For example, welding a small mockup of a joint using the same thicknesses as the final design can help a welder determine the average shrinkage and translate those findings to the larger piece. While this method doesn’t guarantee the exact tolerance, it can get very close. This method is just another piece of the puzzle that’s used in combination with other methods and machining to achieve precise tolerances.
Sub Weldments
Engineering drawings for a complex electrical enclosure typically show an entire fabrication in its completed form. However, a project needs to be broken down into bite-sized pieces, or sub weldments, which are then machined and assembled to the acceptable tolerance. The process of putting it all together can involve a lot of welding, straightening, and sometimes machining, followed by more of the same.
There is an entire team on the shop floor that is responsible for straightening and ensuring that weldments are perfect. Sub weldments help fabricators ensure that each smaller piece meets the exact specification prior to moving on to the next step and placing it into the final cabinet.
Material Selection
Knowing how the material you’re using will perform, such as aluminum vs. steel, is a critical aspect of controlling weld shrinkage. If you end up with cracks in your weld, you’ll need to gouge it out and redo the joint, adding more heat and more distortion.
In general, steel is easier to manipulate into position and you can use heat to aid in the straightening process. In the defense industry, aluminum is more commonly used due to its lightweight characteristics, however. Steel might be used for large watertight doors that can be 15 feet or more, which might shrink an inch or so over that span, meaning it will need to be oversized and then trimmed to size.
Types of Welds
Knowing how the various types of welds and specified filler materials react in a given application helps to inform how much distortion might occur. Is it a full penetration weld that requires beveling or a fillet weld? If it is a fillet, what size is appropriate? A ⅛-inch, ¼-inch, or ¾-inch fillet? Answers to these types of questions will inform how to proceed. Larger fillets typically require more than a single pass to remain secure, meaning more heat and more distortion. There’s also the danger of overwelding, especially among inexperienced welders who haven’t perfected their technique.
Manual Straightening
Manual manipulation is another method for maintaining tolerances. Press brakes, porta power units, and good old-fashioned hammers and mallets are just a few of the tools used to accomplish this. Aluminum weldments that need to be straightened requires you to move the material nearly twice as far compared to steel. This is due to the amount of spring back you get.
Experienced Fabricators
Customers need to feel confident that their metal fabricator has the expertise to deliver repeatable results and that items will always be as square and true as they need them to be. It’s not always that easy. In fact, we had one customer for a defense project send their engineering staff and inspectors to our facility to find out our processes and techniques for keeping a large, complex electrical enclosure as flat and straight as we did. Turns out, we were the third vendor they tried, and they appreciated the results compared to other operations. When they received the first enclosure, it was a perfect fit and went together like the piece of a puzzle they expected it to be.
As you can see, there’s no substitute for experienced welders, but it goes beyond that. It takes an entire team including engineers, material handlers, machinists, assemblers, straighteners, and more to fabricate complex projects to extremely tight tolerances. If you’ve been challenged with finding a precision metal fabrication company for complex defense or commercial applications, contact the team at Fox Valley Metal-Tech to talk through solutions.
