Come join us at AWS Welding Summit 2022 in The Woodlands, TX on August 24 – 26 8:00AM – 4:30 CT
Red-D-Arc’s Lori Kuiper, Product Manager of Orbital, Heating & Pipe, will be speaking on Pre & Post Heat Treatment and Emerging Trends in Welding. This will contain data that is valuable to your businesses future success. Afterwards, connect with Lori Kuiper and national account managers Chuck McCabe and Jonathan Forte.
Steel-Aluminum Welding Introduction (Applications)
Steel and aluminum (and its alloys) are among the most widely used structural materials in the world. Steel has been used by human beings for several millennia in construction, transportation, warfare and many other industries. Aluminum doesn’t have nearly the same tenure being only discovered by Hans Christian Ørsted in 1825 and the ore refining process being more complex than iron ore.
Ever since its discovery aluminum has gained extensive use due to its low weight compared to most metals. Due to the desire for more lightweight designs, aluminum is becoming increasingly employed. However, steel and other alloys have significantly higher strength and higher operating temperature than aluminum. The requirements for high strength from steel and lower weight in aluminum generate the need for steel-aluminum dissimilar joining.
Steel and aluminum are rarely, if ever, directly welded together via fusion welding like TIG and MIG welding due to the brittle intermetallic compounds that form in the fusion zone. The brittle intermetallic compounds may affect the ductility of the weld and high temperature properties of both metals. The specific intermetallic compounds formed largely depends on the specific steel and aluminum alloy being formed. Additionally, steel and aluminum have different coefficients of thermal expansion, so in elevated or cryogenic temperatures, the steel and aluminum will be subject to thermally-induced stresses since the metals will not expand at the same rate.
Bolting and Fastening
There are a few solutions if you find yourself in need of steel-aluminum welding. Starting with the simplest option, bolting and fastening is one way to join steel to aluminum while avoiding several of the drawbacks. As long as the steel-aluminum joint is primarily structural, bolting is one of the more cost-effective ways of joining. Bolting and fastening, however, is not recommended to for joints that must be seamless such as for a pipe joint because the joint is not airtight or watertight. Furthermore, bolting aluminum to steel requires electrical insulation in the presence of salt water or other conductors. A plastic washer will accomplish the electrical insulation goal. Keep in mind that using a plastic washer may limit your operating temperature.
Another well-known option is inserting an interlayer prior to welding. The purpose of the interlayer is to provide a “barrier” to direct interaction between steel and aluminum, preventing the formation of the intermetallic compounds. Additionally, an interlayer can help ease the thermal stresses induced by the difference in coefficient of thermal expansion (CTE) Thus, the interlayer must be thick enough to prevent interaction between iron and aluminum and, in the case of thermal and cryogenic, have a CTE between steel and aluminum.
Most interlayers for steel-aluminum joining are bimetallic transition inserts that can be purchased from many companies. Bimetallic inserts for steel-aluminum welding are typically aluminum in one section and steel in the other. When welding with bimetallic inserts, always consult the manufacturer documentation for proper utilization and details about the metallic insert composition, but most are compatible with MIG, TIG, and GMA welding.
Not that not all aluminum alloys and steels are the same or have the same weldability. For instance, some 7000 series of aluminum tend to be difficult to weld. Be mindful that commercially-provided bimetallic inserts are meant to fit specific structures and not necessarily suitable for more customized or irregular structures. Therefore, do proper research before purchasing a bimetallic insert.
Some academic articles report using a copper interlayer for joining steel to aluminum. Keep in mind that copper can form intermetallic phases with aluminum, some of which are beneficial to aluminum, and does not tend to do so with steel. Another interesting development was using a Al0.5FeCoCrNi interlayer to weld 6061-T6 aluminum to St-12 low carbon steel in an academic article published in the Intermetallics journal in 2020. Using a material other than marketed bimetallic inserts may have more flexibility in terms of accommodating customized or irregular structure and may come as a foil, welding wire, or powder.
The last common steel-aluminum joining remedy is to coat one or both sides of the joint with a different metal prior to welding. The most common one for steel-aluminum welding is hot dip aluminizing which coats the steel side of the joint with aluminum. When using this technique, only melt the aluminum and be sure that the arc does not touch the steel! Touching the steel with the arc will cause the aluminum coating and the steel to melt and react to form intermetallic phases. While the aluminum coating does adhere to the steel, it is not actually bonded and the strength of the joint is not as strong as steel-steel or aluminum-aluminum joints.
Other coatings can be used such as copper when welding aluminum to steel. For hot dip coating, it is generally a safer bet to coat the steel since aluminum would also melt in a pool of liquid copper. In general, dip coating is more flexible than using bimetallic inserts since dip coating can conform to most geometries.
Electroplating is a “cold” coating option that does not involve any molten metal and is not used as frequently for welding compared to the above options. Electroplating uses controlled electrolysis to transfer the desired metal coating from the anode (made of the same metal as the desired coating) to the cathode (the part being plated). Aluminum can be electroplated with copper, but steel cannot. If you choose aluminum as the piece to be electroplated, your joining options are typically limited to spot welding, high-precision laser welding, brazing, and emerging solid state welding processes.
Keep in mind that electroplating can be a rather slow process that can take several minutes and the surface area of the workpiece that can be plated depends on the size of the container holding the plating solution and the amount of current you can generate. When electroplating, having a current or voltage that is too high will cause the plating process to proceed out of control as seen by dendrites growing on the workpiece surface.
If the cathode and anode are not parallel to each other, then the electric field that governs the electroplating process will be uneven and so will the coating thickness. The plating may have poor adhesion of the process is not well controlled or the surface is not well cleaned. If the current or voltage is too low then the electroplating will either be slowed or not work at all. As such, electroplating is recommended only for relatively small, flat work pieces and not large structural pieces.
One other consideration to note is the geometric requirements of your welding operation. If welding something with a very specific geometry like a hollow part made by additive manufacturing, you may consider using a brazing over welding to avoid damaging the structure via torch brazing or induction heating. Keep in mind this may require a filler metal that is guaranteed to melt before the geometry-sensitive part will melt and is compatible with both materials.
When considering your steel-aluminum welding needs ask the following questions:
- What is the purpose of the steel-aluminum joint?
- What type of steel and aluminum are you welding?
- What are your load-bearing requirements?
- What are your thermal requirements?
- Are there any environmental factors to consider (i.e. thermal, corrosive, gas, etc.)?
Welding Automation: Arc Motion vs. Work Motion
The welding world consists of a broad spectrum of technologies suitable for a wide range of use cases, budgets, and levels of complexity. Between the two extremes of entirely manual and entirely automated processes is welding mechanization. Mechanization can be accomplished using devices that move the workpiece, move the arc, or both simultaneously. Regardless of the mechanical device used, the goals are the same: to make a welding operator’s work easier and more productive.
An Introduction to Welding Positions
To better understand the target applications of these devices, a quick refresher on welding positions can be helpful. Both the American Welding Society (AWS) and the International Standards Organization (ISO) use terms and designators to communicate the position of the axis of welding relative to the vertical and horizontal planes for both plate and pipe. The four fundamental welding positions for plate are:
- Flat (aka 1G/1F depending on if the weld joint is a Groove or Fillet weld, respectively)
- Horizontal (aka 2G/2F)
- Vertical (aka 3G/3F)
- Overhead (aka 4G/4F)
The four fundamental welding positions for pipe are:
- 1GR (rolled horizontally oriented pipe)
- 2G (vertically oriented pipe/horizontal welding axis)
- 5G (horizontally oriented pipe, vertical welding axis)
- 6G (~45° inclined pipe)
Welding in the flat and horizontal positions provides the greatest opportunity for the use of high-productivity welding parameters since the influence of gravity on the weld pool is not (or at least less) detrimental. Conversely, welding in the vertical and overhead positions often requires comparatively low amperages and wire feeds to fight gravitational influence, even if an all-position welding electrode or wire is used.
In-position welds are also favorable from an ergonomic perspective; out-of-position welding can be physically taxing, and the ability to mechanize an application provides an opportunity to locate the operator away from the welding arc in a more favorable location.
In a perfect world, every weld could be performed in the flat or horizontal welding positions and, in the case of a pipe, rotated during welding. “Work motion” devices allow the workpiece to be repositioned during or between welds and are also known as “positioners.”
- Weld rotators have a rotating platen that the workpiece can be mounted to using a chuck or fixturing. Often, the platen is designed to tilt approximately 90 degrees so that a workpiece oriented vertically is reoriented horizontally. These positioners are available in sizes ranging from benchtop to multi-ton heavy-duty units. Note that these positioners typically leave the workpiece cantilevered, which may constrain the maximum weight that can be placed on the unit before reaching rated capacity. These positioners are popular for applying flanges to process piping. In some applications, the welding cell can be designed to use a welding rotator to permit welding on one assembly while another is being welded.
Headstock & Tailstock
- Like weld rotators, headstock and tailstock positioners have one powered rotatable platen that allows the workpiece to be rotated about the horizontal axis. However, the workpiece bridges from the powered headstock to the idle tailstock so that the workpiece is supported and not cantilevered. These positioners are excellent for large, unwieldy weldments such as large boom assemblies or even entire railcars. With proper fixturing, the workpieces need not be cylindrical.
- Turning rolls also provide the rotational movement of a workpiece. Unlike weld rotators and headstock/tailstock positioners, the workpiece is not fixtured to turning rolls, it is simply cradled by them. These positioners are popular for performing circumferential welds on very large cylinders or pipes found in the oil, gas, and power generation industries
While work-motion devices such as welding rotators help make welding easier by placing the weld in the ideal position, arc-motion devices serve to make welding easier by mechanizing the actions of the welding operator. Some common arc-motion devices are:
- These devices consist of a vertical mast mounted to a horizontal boom. Typically, the vertical movement of the boom up and down the mast is powered using an electric motor. If the horizontal movement of the boom is powered as well, the device can be used to perform longitudinal welds. However, some manipulators do not have mechanized boom travel and exist primarily to hold the torch at a fixed point in space above a welding positioner. Small manipulators may be mounted to a base having castors or lift points that permit ease in moving the device to the workpiece.
- These devices are typically portable and consist of a track and carriage. The welding torch is mounted to the carriage, and the track is typically affixed to the workpiece using magnets, permitting both in- or out-of-position welding. Specially designed track torches can be affixed to pipes for circumferential welding.
- Seamers are specialized side-beam that has integrated workpiece clamps. The workpiece clamps are cantilevered over the equipment’s base, which allows loading and unloading from a single side. Seamers are especially popular for performing longitudinal welds on smaller-diameter tanks and pipes.
- Oscillators are an important accessory for many of the arc-motion devices described above. These devices can be affixed to a carriage or boom and permit the mounting of a welding torch. The oscillator is then able to finely manipulate the position of the torch—back and forth—similarly to how a welder would perform wide weave weld passes.
The adage “work smarter, not harder” can certainly be applied to the use of welding positioners and arc-motion devices. And because there is such a wide range of equipment types and capacities, finding a device that fits within your budget is not a complex task.
Furthermore, these devices can be rented, allowing you to invest in the best technology for the job when you need it. Sometimes, the best solution may be to integrate both a work-motion and arc-motion device into a single weld cell. Red-D-Arc Welder Rentals has automation specialists who are able to help you identify the best devices for your application.
Tools that Improve Pipe Welding Efficiency
Discovering new equipment is an excellent step in improving operational efficiency. However, the first step of any improvement is changing your thinking about the existing process.
What isn’t working about the “old way” of doing things? How does new equipment address those deficiencies? This article aims to share not just tools used to improve pipe welding efficiency but considerations to make about the welding process before researching equipment.