Tooling Corner : Laser deposition technology: One option for mold repair

Editor?s note: Laser-deposition welding is an important alternative to more conventional mold-repair techniques. It is beginning to find acceptance in U.S. mold shops, having previously gained a foothold in Europe. Richard Hendel, product manager for Rofin-Sinar, describes what laser-deposition welding is and how it works.

Laser deposition welding technology is beginning to find its place in modification and repair of molds. A typical application would be the repair of an injection mold constructed of cold work steel, which is subject to heavy wear on the edges caused by processing of glass-fiber-reinforced material. The chipped or rounded edge areas can be laser-deposit welded to fill cracks, using a wire diameter of .4 mm. After repair, the insert and the mold have a service life at least equal to that of the original component. The welded deposit has a height of about .5 mm and a hardness of 52 Rockwell C. Hardnesses of more than 60 Rockwell C are possible.

One successful user, for example, is The Tech Group, Grand Rapids, MI, which is currently using laser-deposition welding for mold repair. The Grand Rapids plant, which has 58 employees and uses 30 injection machines ranging from 28 to 250 tons, is devoted exclusively to medical parts. These parts, which include components for open-heart surgery, minimally invasive surgery, and pediatric surgery, are generally small, tight-tolerance, short-run products. Quality is critical. In fact, the entire shop operates under Class 100,000 cleanroom conditions.

The business includes a tool shop with four full-time moldmakers. Since it molds medical parts, the tool steel alloys for its molds must be entirely beryllium-free. A benefit of using laser-deposition welding is that the process can handle beryllium-free alloys, as well as stainless steel, standard tool steels, and aluminum.

According to David Guth-rie, a moldmaker at Tech Group, 85 to 90 percent of repairs are for small components, such as shutoffs on slides, and core pins. Formerly the company subcontracted laser welding, but to save time, effort, and money, the company has now brought the process in-house. With the technology available in-house, laser engineering changes and repairs to molds can now be made in hours rather than days, the company reports. After doing a welded repair, a mold part goes back to the tool shop for final machining, and then back into the mold. Frequently, only the damaged part of a mold needs to be removed for repair, allowing the mold to remain in the machine.

Laser Beams and Wires

The StarWeld tool from Rofin-Sinar that The Tech Group is using is a laser deposit welder for use with wire-type filler material. It is a pulsed Nd:YAG welding laser in a Class I laser housing with a motorized x-y-z table and stereo microscope. The defect in a tool is filled using a laser beam and a wire-shaped filling material. The wire is guided so that the laser beam causes the wire and the surface of the workpiece to melt simultaneously. The total height to be deposited is achieved through the use of multiple layers. After welding, the mold component is machined to final dimensions.

The advantages of this technique vs. arc welding or microplasma deposit welding are in the higher heating and cooling speeds and the lower melting volumes. The thermal effects on the base material are less extreme, and the risk of distortion, weakening, and crack formation is considerably reduced. In addition, better layer characteristics can be achieved through fine-grained structures or by using filling materials with different alloy characteristics. Considerably finer structures can be achieved with laser-assisted techniques, since filler wires can have a diameter of as little as 100 µm.

This work has typically been carried out with TIG or microplasma welding. Also, resistance bonding with powders is suitable for mold repair when low cohesion powers are required.

Relatively new to the market is direct metal deposition (DMD) with high-power (2 to 4 kW) CO2 and YAG lasers. (See Table 1, p. 24, for a comparison of tool repair methods.)

Deposition welding with a laser has some advantages over the other three methods. The laser beam can be well focused, heating the workpiece and filler metal (welding wire) in a very precise area. Heating of the workpiece can be minimized, resulting in limited distortion. Refinishing work is reduced to a minimum. In addition, hardnesses up to 60 Rockwell C can be achieved, depending on the base material.

Laser Hardware

The StarWeld laser welding machine has two large sliding doors that provide an opening of 800 mm. Workpieces weighing up to 350 kg with a volume of 125 liters can be inserted from the front with a forklift truck or from above with a crane. The sliding doors and side covers can be removed to accommodate even larger workpieces.

The operator can feed in the welding wire manually and operate the joystick through ergonomically positioned armholes. Workpiece guidance takes place on an x-y table. The workpiece is positioned accurately with the assistance of a stereo microscope and a joystick. The joystick is controlled proportionally (the degree by which it is moved determines the speed of the axes.) When the joystick is turned, the z-axis is traversed. The correct position of the welding area is reached with maximum image sharpness in the microscope. The set laser parameters are fed into the stereo microscope. Up to 50 preprogrammed parameter records can be called up using the joystick.

The operator can check and alter the parameters at any time, without looking away from the microscope or removing hands from the workpiece enclosure. All functions of the laser device, such as the laser pulse, shutter control, power unit control, temperature control, focusing, safety circuit, storage of data records, and diagnosis and test routines, are carried out by a high-performance microprocessor. A membrane keyboard is fitted for entering all the different kinds of data. The values for the capacitor bank voltage, pulse frequency, pulse width, pulse energy, pulse energy per cm (surface energy relevant for welding), pulse shape, text messages, and other data can be shown on a display.

The laser beam source is a Nd:YAG laser with an average power of 100W. The pulse times required for deposit welding are between 2 and 10 msec, depending on the material. With pulse powers of up to 50J, maximum pulse densities of several tens of mW/cm can be achieved on the surface of the component. This is more than sufficient for fast work, even with thicker welding wires. The machine comes with an integral water/air cooling system and an air filter for welding fumes.

A patented optical system that can be swiveled on both axes allows work to be carried out on vertical walls and undercuts. An optimized and error-tolerant welding quality results from a special laser resonator. A high depth of focus allows for a large tolerance in the vertical fluctuations in the surface of the component in relation to the focal spot. A constant application volume of filler wire is provided by a specific software mode. During manual processing of the workpiece, this mode allows the overlap relationship of the welding pulses to be kept constant by coupling the pulse frequency to the feedrate.

Laser-assisted deposition welding with wire-type filler material is a relatively new technology. It offers advantages over conventional methods with regard to processing quality and precision. Because it can be implemented in the form of a manual machining system that is simple and safe to operate, its use is becoming more widespread, particularly in small and medium-sized businesses.

Rofin-Sinar Inc., Plymouth, MI

Richard Hendel

(734) 455-5400; www.rofin.com