Stirring up a better weld

Stirring up a better weld

ACCORDING TO UK-BASED TWI World Centre for Materials Joining Technology, a new friction-stir-welding (FSW) method could form better welds versus conventional friction-stirwelding techniques. Called Com-stir, the FSW technology applies both rotational and

A Com-stir weld in a lap joint measures 230% of the top-plate thickness. Conventional FSW would measure only 110%.

Com-stir, a new friction-stir-welding technique, applies orbital and rotary motion to a workpiece. The rotations can be opposing (as depicted in the illustration) or in the same direction.

Realmeca's RV 8SP is for high-speed machining of ultralight, complex parts. Photo courtesy of Realmeca.

The Skymill — basically an inverted milling machine — uses gravity to evacuate chips from the cutting zone. Photo courtesy of Sud Ouest Systðme.

Vartech researchers are developing a vapor-phase process to produce titaniumalloy powders at lower cost. Current activities include running tests on coating fibers and filaments and studying reactor thermodynamics, cost projections, and yield.

ACCORDING TO UK-BASED TWI World Centre for Materials Joining Technology, a new friction-stir-welding (FSW) method could form better welds versus conventional friction-stirwelding techniques. Called Com-stir, the FSW technology applies both rotational and orbital motion to the weld zone for high heating rates and good metal flow. TWI reports that the process forms wider welds than conventional techniques, particularly advantageous in lap and T-joint welding.

Com-stir improves weld quality over conventional FSW by making surface-oxide fragmentation more efficient and heat generation more uniform. In addition, the process applies lower torque to the weld zone, so operators need less workpiece fixturing. They may also find it easier to spot weld or weld dissimilar materials.

In conventional rotary FSW, the relative velocity of the tool increases from zero at the tip of the weld head to maximum velocity at the shoulder O.D. Comstir, on the other hand, modifies the velocity differential by selectively changing the relative rotational speed. This feature can be preselected or varied automatically by in-process control. Options range from an almost-complete orbital motion to a nearly complete rotary motion.

TWI reports that the next step for Com-stir will involve combinedmotiontechniques for machining. Such techniques could eliminate the need to use free-machining materials where materials are added to a metal to improve machinability — for instance, lead additions to steel and selenium to stainless steel. These additions often come at the expense of weldability, says TWI.

High-speed machining with a continental flair
MACHINE TOOL BUILDERS FROM around the world continue to advance high-speed machining (HSM) technology. French companies, for instance, are reporting that new work on machine structures, spindles, and toolholding are progressing quite well. The results of their labors will be on display at international tradeshows, including EMO-Milan 2003.

"With HSM, we must dare to work differently," advises FranÁois Lhuillier, technical director of Realmeca, based in Clermont en Argonne. "We must use a machine designed for HSM, give it a special spindle with a specific speed of rotation (35 to 45,000 rpm) and power (from 12 to 20 kW), equip the spindle with a special toolholder, carefully select the tools, and master CAD/CAM."

Realmeca has developed 5-axis machines for precise milling of ultralight, complex parts. The company makes the framework of its machines, such as its new RV 8SP high-precision VMC, out of composite materials that ensure structural rigidity, minimize thermal drift, and dampen vibration. It also uses digital-design techniques to improve dynamic control, which is often a weak point of HSM systems.

For the future, Realmeca says ultrahigh-speed systems will exploit the advantages of spindles with magnetic bearings. The company expects the next generation of HSM machines to boast 50,000-rpm/50-kW spindles and deliver feedrates as high as 164 ft/min and accelerations to 65 ft/sec2.

Several builders point to the vulnerability of the electric spindle as a weak point of HSM. "Controlling vibrations means increasing the rotation speed while preserving the spindle and improving surface finishing," says Andrè Greffioz, a specialist in structure calculations at Etude Logiciels Procèdès Spèciaux (ELPS) based in Figeac. Expert in behavior analysis of high-speed machine tools, the company has developed a range of software that optimizes operation by simulating vibration occurrences.

The company's new machine concept, called Skymill, is now being manufactured by Sud-Ouest Systðme, another Figeac-based company. Designed to machine structural parts, Skymill is basically an "inverted" 3-axis milling machine equipped with linear motors and operated by parallel kinematics. The spindle cuts from below an upper-mounted moving machine table.

CMW, a Vosges-based builder, also uses parallel kinematics in a concept it calls Hexapode. A lightweight head installs in place of a conventional milling head or a surfacing table on a traditional machine, and, according to CMW, delivers the speed, rigidity, and precision needed for HSM. Hexapode comes in three versions, depending on the rotation speed of the spindle.

In addition to other advancements, HSM systems will continue to employ linear motors in machine design. Capdenac-based Forest Linè, for instance, has abandoned ballscrew spindles in its Linèar Minumac line, opting instead for linear motors in all axes — including rotating ones. The company reports that these machines combine high speed and velocity (65 ft/sec2 acceleration). They also offer accurate trajectory for machining thin walls in aerospace parts.

"The direct-axis drive systems have many advantages," summarizes Yves Neboit Guilhot, sales engineer for Forest Linè. "No mechanical backlash, better rigidity, increased feed/speed, high acceleration, no wear and tear, low noise levels, and reduced maintenance."

For manufacturers needing HSM coupled with a large capacity, a new line of GX machine tools from Huron Technologie may fit the bill. The Illkirch-based company recently introduced the GX 45, which has 177 in. of capacity in one directional axis. In addition to this machine, Huron is currently selling the K2X, which serves as a transition to ultrahigh-speed machining (UHSM). The machine tool's spindle speed tops out at either 18,000 or 42,000 rpm, and its maximum acceleration is 20 ft/sec2.

More-affordable titanium?
ALTHOUGH TITANIUM IS MUCH stronger than aluminum, it is more costly — aircraft-quality titanium billets can sell at 10 the price of pure aluminum ingots. But a new production process could drive down the cost of titanium, according to Vartech Inc., Idaho Falls, Idaho. The company is building a new type of titanium reactor with funding from the Missile Defense Agency.

Titanium is expensive to produce because of its high reactivity with other elements such as oxygen and carbon, according to Dominic J. Varacalle Jr., president of Vartech. Currently, companies use lengthy steps to overcome the high level of reactivity. The conventional method, called the Kroll process, reacts liquid or gas titanium tetrachloride with magnesium metal (which by itself is expensive) to produce a crude, titanium-metal "sponge" mixed with magnesium chloride. Using a vacuum for electrorefining the titanium sponge removes oxygen, magnesium, and chlorine to purify the titanium metal for subsequent alloy production.

If a titanium alloy is intended for use in alloy powder — for application in aircraft or missile parts, for example — the desired titanium alloy must be melted and blasted with an inert gas such as helium or argon to atomize the material.

The Vartech reactor is reportedly simpler and cheaper to use than the Kroll process. It employs a vaporphase process to produce titaniumalloy powders directly from titaniumtetrachloride vapor. The technique could take place as a vacuum process, but Vartech operates its reactor under normal atmospheric conditions to reduce capital costs.

One of Vartech's key objectives is to make bulk titanium-alloy powders at costs of $3 to $5/lb. The company is currently searching for capital and partnerships, and it plans to eventually license out other titanium-production technology to companies such as large metal producers.

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