Motion: Sensors help ensure stir-welding repeatability

The friction stir welding process was invented in 1992 at The Welding Institute (TWI) in Cambridge, U.K. The solid-state welding process, combines extruding and forging, and is not true welding. The method was not created specifically for aluminum but is definitely well-suited for it.

09/27/2007


The friction stir welding process was invented in 1992 at The Welding Institute (TWI)

In the process, a hydraulic motor is coupled to a spindle. The spindle is rotated, and at the end, a drill-bit-like pin with a shoulder attached is plunged into the joint line between two metal sheets. By applying load and rotational speed, the aluminum plasticizes, but never reaches its melting point, and is extruded around the pin. This allows the operator to stir molecules from one plate of aluminum into molecules of another plate of aluminum to create a very strong, solid-state welding joint that is durable as well as environmentally friendly—there is no generation of smoke or gases as there is with standard fusion welding.

When MTS Systems built its Istir Advanced Friction Stir Welding development system, it kept its sensor selection in the family by choosing MTS Sensors ’ Temposonics linear position sensors to ensure the precision and repeatability of its machines.

Istir PDS is the first fully instrumented friction stir welding system capable of performing load-controlled welds along three independent axes. It has reliably produced welds with double curvature, joined materials less than 1 mm and up to 30 mm thick, and joined together materials that vary in thickness.

In friction stir welding, the welder stays stationary and the material moves. The MTS Temposonics sensors are used for positioning axes X, Y, and Z as the aluminum is moved into position. There is an RH sensor (40 inches in stroke) embedded in an actuator for the X-axis that is used to position the head; An RP sensor with a “floating” magnet (80 in. per stroke) attached to the top of the Y-axis table to position the moving part fixture base; and two ball screws for the Z-axis with an RP sensor with floating magnet (24 in. per stroke) on each side to position the weld head vertically.

The friction stir welding process creates a much stronger bond than traditional fusion welding, which is why it is ideal for aerospace applications. It’s also the reason that the components that make up the finished machine must be highly accurate and reliable, as so many of the welds generated by these machines will be part of mission-critical applications.

MTS’ proprietary synchronization technology guarantees the most accurate dynamic position output while minimizing inherent system-related delays to produce smooth, precise velocity loop control./EN 61000-4-2/3/4/6, level 3/4 criterion A).  This is critical in this type of machine tool and robotic application where high power servodrives typically are used.

The R-Series SSI sensor provides the same serial encoder interface (Synchronous Serial Interface) widely used in the servocontrol industry for absolute position feedback. The SSI interface option was a key factor for MTS Systems in choosing the sensor, according to John Meyer, Program Manager, MTS Systems. “We had electronics and hardware already in place to work with the SSI interface, so finding the position sensor that could work with that protocol was key.”

The R-Series SSI output sensor includes options for synchronized 7,500 Hz update rates and position accuracy as low as +/-40 microns (+/-0.0016 in.) standard.

The R-Series model RH hydraulic or “rod”-style” products are designed specifically to be embedded into high pressure hydraulic actuators.

Edited by C.G. Masi , senior editor from information provided by Tempsonics





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