One of the advantages that friction welding offers is the ability to weld dissimilar materials.
A friction welding machine. Inset (right) shows the intersection of two workpieces that are being rotated under pressure that will result in a weld.
For what Dan Adams, president and chief technology officer of Manufacturing Technology Inc. (mtiwelding.com) calls a largely overlooked and underutilized process, friction welding is making notable strides toward a more integrated and valued process for automotive and transportation manufacturing.
Notably, Lightweight Innovations for Tomorrow (LIFT; lift.technology), founded in 2014 by the University of Michigan, Ohio State University, and the Edison Welding Institute (EWI; ewi.org), will be installing a massive 75-ton linear friction welding machine MTI is building for delivery to LIFT’s Detroit-based demonstration facility in 2018.
Friction welding can be broadly categorized into three processes: rotary friction welding (spinning two tubes against each other at high speeds until they join), friction stir welding (where a spinning cone-shaped bit—a “pin tool”—stirs and melts the seam between two workpieces), and linear friction welding (where the machine tool rubs one workpiece against a stationary partner).
“The processes continue to improve, the challenge continues to be education and designing parts for the process,” Adams says. To that end, MTI partners with industry organizations such as EWI and universities including the University of Notre Dame to share ideas and expose more design, welding and manufacturing engineers to friction welding’s attractions.
A main appeal is the ability to efficiently join different materials, such as steel with low-carbon alloys or aluminum to copper. This can help attain a number of part-design goals, such as lightweighting or more efficiently using expensive work materials.
Another is weld consistency. MTI touts updating the controls on its friction welding equipment to more efficiently address “upset,” what it identifies as the amount of part shortening from the process.
Material differences, surface conditions, and interface squareness all can cause process variations for which the control can adjust energy and other variables. “Compared to traditional methods, MTI’s recently developed advanced controls for friction welding allow the welded components to achieve better length control and/or radial orientation than ever before,” says marketing and sales vice president Bob Besse. “This helps reduce the as-welded tolerances that may drive the requirement for additional manufacturing operations in the production line, ultimately reducing cost.”
Speaking of manufacturing lines, Besse also notes that advanced controls allow friction welding equipment to be automated and integrated into cells or production lines. Weld results can be consistently programmed for and variation in operator skills are taken out of the process, reducing process time.
More Powerful Pistons
MTI points to its collaboration with Federal Mogul (federalmogul.com) as an example of what can be done to advance automotive parts production. Seemingly continual legislative calls for reducing particle emissions and improving fuel economy were dictating better engine performance. Federal Mogul was looking to develop larger and more efficient piston designs for diesel engines in response.
What Federal Mogul calls its “Monosteel” piston has a strong box section gallery to withstand firing pressures, a large enclosed gallery to contain cooling oil, a full steel skirt and improved bearing surfaces to transmit the bearing loads. The galleries are hollow, allowing them to circulate oil through the piston during the operation. Results are decreased operating piston temperatures and significantly increased durability.
Dependent on the welds, that is. “We had an interesting challenge of developing the compromise between the inner and outer weld surfaces,” Adams says. “Normally, it’s just a factor of rotational speed, inertia energy and pressure to get the right temperature and the right bond parameters on a single surface. But now we have two surfaces—an inner and outer.”
The double-banded weld designs joining the skirt to the piston head allowed larger cooling galleries placed gallery much closer to the top of the piston. This enabled reducing the operating temperature at the top groove from approximately 280°C to 60°. Finite-element analysis used to inspect the friction welds basically tripled the Monosteel piston’s fatigue safety factor, from a previous 1.4 to greater than 5. Where peak cylinder pressure was advancing an average of 2.3 bar a year (starting at 110 bar for Federal Mogul aluminum pistons in the 1960s), the company predicts advancing at a 7.0 bar per year rate, hitting 250 bar in 2020 for the Monosteel pistons. It has friction welding machines in place in manufacturing lines in North America, Europe and Asia.
Skin in the Game
The linear friction welding machine MTI is building for the LIFT facility is large enough to join railcar walls to floors. “We have a lab machine on site in South Bend, but the LIFT machine will be able to do full-size demo components that you can test,” Adams describes.
Process development doesn’t end with delivering the machine, he adds. “We are available to consult on tooling, fixturing, engineering, and process development—much more of a partnership as opposed to a research and development exercise.” To that end, LIFT will be getting the machine, but MTI retains a percentage of the ownership so that it can demo parts and run prototypes for new and existing customers. “We wouldn’t have invested so much if we weren’t excited about it expanding the audience for friction welding,” Adams says. “It’s a game-changer for North America.”