Within the next year-and-a-half, Dana Corporation's Spicer Driveshaft Division (Toledo, OH) should be producing a variety of bi-metallic driveshafts (and potentially other structures, like engine cradles and space frames) at high volumes using magnetic-pulse welding equipment. Until this point, successful magnetic-pulse welding has been limited to small tubes with diameters measuring in the fractions of an inch; Dana's new prototype machine can magnetically weld tubes the size of a typical driveshaft—several inches in diameter. While this news might not be that shocking, as magnetic-pulse theory has been around for decades and is used for a variety of automotive applications (such as forming oil filters and welding aluminum air conditioning tubing, for example), it actually represents a huge leap forward in the technology. The result: these driveshafts may soon be saving considerable weight on the gas-sipping SUVs of the future.
Magnetic-pulse welding isn't complicated. Take two cylindrical materials and fit them together with one material sleeved over the other. Surround this sleeve with an electrical current passing through an inductor coil, and the magnetic forces that are created will accelerate the outer material toward the inner, welding them together.
The process requires the outer material to be conductive, otherwise the magnetic force won't affect it. (This can be useful in that it is possible to weld through plastic or other non-conductive materials. For example, a metallic part can be welded to another part while still enclosed in a plastic housing without disturbing the housing.) The other necessity is that the current has to be delivered to the coil in a symmetrical fashion to create an even distribution of magnetic energy. If the magnetic force is not distributed symmetrically, unintended deformation occurs, usually jeopardizing the weld and ruining the part.
The stumbling block with magnetic-pulse welding, as far as Dana was concerned, is that it had never been optimized for larger diameter materials. The larger the material, the greater the amount of current necessary to create a magnetic force capable of accelerating the material fast enough to create a weld. Smaller diameter materials have relatively forgiving power requirements, i.e. lower amperage is much easier to control than higher amperage, specifically when it comes to switching the current. Instantaneously delivering the huge amount of current necessary to develop a symmetrical magnetic force capable of accelerating larger diameter materials was considered to be impractical, if not impossible. In the case of a four-inch diameter aluminum tube used in a driveshaft, for example, it is necessary to switch over one million amps in 100 microseconds to create the force to accelerate the tube at 900 miles per hour to produce a weld.
This is the prototype magnetic pulse welding machine, the brainchild of Dana's Boris Yablochnikov, Ph.D., lead scientist, Advanced Design. He has been working on this technology since the 1970s, previously in the former Soviet Union, and now in conjunction with Manufacturing Technology Inc. (South Bend, IN).
To transfer the current necessary to weld a large diameter material like a driveshaft, Yablochnikov developed an integrated switch/inductor coil. "What's different about this technology," says Jim Duggan, chief engineer, Advanced Design, "is that we're able to transfer energy at a very efficient level." This efficiency is Dana's secret to making magnetic-pulse welding work for large parts. It is worth noting that Dana claims a typical magnetic-pulse weld requires approximately 100 times less energy than an equivalent MIG weld.
Beyond the technological development, Manufacturing Technology is designing the production equipment, which uses precise tooling to hold the workpiece within the coil. This allows the straightness of a driveshaft to be directly associated with the tooling. (This is not possible with MIG welding because of the deformation of the workpiece caused by adding heat.) The result is that magnetic-pulse welding produces extremely straight and even welds, which is an advantage when the shaft is balanced. As Duggan explains: "If you can hold the tube in the machine straight, the part will be straight."
Of course, throughput of a production version of the magnetic-pulse welding machine has to be comparable to that of conventional MIG welding processes. Since the machine discharges its current to produce a weld in less than one second, its cycle time is much more dependent upon the fixturing of the workpiece and the recharge time than the actual welding time. The prototype machine can recharge its capacitors in roughly 10 seconds. Since this time can be used for fixturing, Duggan says that throughput should be at least as good as that of MIG welding.
This bi-metallic driveshaft has been constructed with a different configuration than a conventional one, made possible because of the magnetic-pulse welding process. In a conventional driveshaft, it is necessary to bolt a CV joint to a companion flange from the axle or transmission. This is done because it is impossible to MIG weld a heat-treated bearing element like a CV joint. "Since the CV joints are heat-treated, any thermal welding process would destroy the hardness and distort the joint," explains Duggan.
But since the magnetic-pulse process produces no heat, it allows the CV joint to be welded directly to the aluminum tube. This means that the flanges can be eliminated, which simplifies assembly. Duggan continues: "The elimination of the flanges could be possible by connecting directly to the male spline interfaces for a weight and part count savings."
This is just one example of the advantageous use of magnetic-pulse welding for joining aluminum and steel, thereby taking advantage of aluminum's lighter weight and steel's greater strength. "Marrying these materials allows us to design specific lightweight, compact components utilizing the benefits of the materials and the process," says Duggan. And you might call that a match made in Toledo.