Mike VanHouten sets up a workpiece for laser-beam welding

Accu-Mold used its laser welding unit to repair the chipped edge (above) of a lifter for a mold. Six layers of welds were applied using 0.0010 in. wire, then the lifter was polished and finished (below).

A 0.0010 in. wire is used to build up a worn edge and minimize post-weld machining.

Accu-Mold, Inc, Portage, Mich., gained ten percent more repair work and five percent more business in the mold making side of the firm since adding a laser-beam-welding machine.

These business boosts come from meeting customer needs faster and at a lower cost than ever dreamed. For example, a customer called on a recent Saturday with what he thought would be 48 hours of rework on a mold that he thought would cost $4,000 to $5,000. He brought the mold to Accu-Mold. A half an hour later he, left with his repaired mold. He paid $125.

Research pays dividends
In the past, Accu-Mold contracted with local welders for gas-tungsten-arc welding (GTAW) to repair molds, but as the firm's business grew these welders were not fast enough to keep up with the work.

"We needed to serve our customers quickly and accurately losing a mold on a production line costs our customers a lot of money per hour," Dave Martin, president of the 30-year-old company, says. The company's customers primarily are located in Michigan and Indiana, but it gets an occasional customer from as far away as Texas.

With local welders unable to keep up with demand, Martin said he and VanHouten started to research alternatives that had an acceptable risk. They considered GTAW precision equipment, then laser-beam welding, and found that laser beam welding was a better tool to use with complex molds that had finer details in their cavities.

They also considered such options as the capacity for larger workpieces and memory-tracking programs for repeat welds.

Martin and VanHouten also looked at the effect of laser welding on the hardness of the workpiece. A key question was: How much does welding change the Rockwell rating of a mold or die? A one to two point increase in Rockwell hardness can be allowed in a mold, but that increase could cause brittleness in a die.

Another study involved making prototypes of medical instruments on the laser-welding machine from 0.002-inch shim stock.

GTAW versus laser repairs
Plastic molds and dies require sharp, crisp edges to produce quality parts. Making repairs with an arc welding process, normally GTAW, results in a build up of weld metal that has to be machined to the specified dimensions.

The GTAW process can be applied in a microenvironment with constant-current power supplies and microprocessor controls. The heat input, in the 10,000 degrees F range, limits the applications because of the potential to cause damage to the mold's component, and the heat-affected zone may require heat-treating to return the metal to its pre-repair condition.

Access to the repair location often is smaller than the size of a dime and less than an inch deep, and that provides an access challenge to the GTAW welder. The welder must have an artist's touch to fit the torch and filler metal rod into the cavity at the correct place and at the correct angle, while still providing himself room to see the weld puddle. The welder's skill is vital to manipulate the arc gap, weld pool and filler rod.

The GTAW process on steel could run at 200 amps.

The same weld with a laser beam could run at 20 amps, placing less heat into the workpiece for a briefer period of time.

A laser-beam welding unit typically uses power in the 20-watt range, but is adjustable and has localized peak power for tool steel. It pulses between 5 milliseconds and 10 milliseconds with cooling at the same rate. This results in minimal softening of the mold, and reduces the heat-affected zone measures to a few tenths of a millimeter with minimal change in the structure, such as a coarse-grain zone or undercut.

Laser welding also allows the welder to select a filler metal and laser parameters that match the hardness of each mold or die. For example:

• A shorter laser pulse reduces the cooling time. This helps to increase the hardness of the mold and weld.
• Limiting the size of the melt pool and rapid solidification of the individual weld pulses minimizes the amount of base material drawn into the melt pool.
• The heat absorbed by the substrate is limited because the filler wire absorbs most of the laser beam, with a spot size between 0.0118 in. and 0.0236 in. (0.3 mm and 0.6 mm).
• And, oxidation of the substrate is limited because of the shielding gas usually argon or an argon-helium mix.

Martin and VanHouten decided to add laser-beam welding to the shop's repair capabilities, and selected a Trumpf PowerWeld laser-beam welding machine with a Koolant Kooler chiller. The unit they selected has a variable power source that ranges from 20 watts to 200 watts, to give them the wide selection they need. Because thermal conductivity and reflectivity, materials such as aluminum and copper require more energy than tool steels, and Martin and VanHouten wanted a versatile welding machine so they could handle applications for a range of material and part sizes.

The unit has a sit-down workstation, and can accommodate workpieces to 250 pounds. The operator views the part and the weld

location through a 16x binocular, and uses a joystick and footswitch to control the weld.

The workpiece rides on an air-cushioned plate for positioning, and has 10 in. x-axis, 10 in. y-axis, and 11.8 in. z-axis movement for the workpiece. The positioning plate slides out of the workstation to transfer workpieces.

The machine can store 24 parameter settings, and shielding gas flows automatically when welding begins.

Laser welding techniques
The Trumpf YAG laser eliminates weld sink, welds with fine details and provides precise amounts of steel addition for a near net shape. Most deep ribs (line of sight) can be welded without "hogging" out, and textured and diamond polished surfaces usually do not need color matching after welding or heat treatment.

"If I can reach a point with the beam I can weld inner contours and chamfers, deep cavities and accessible geometries," says VanHouten.

For example, if the specs call for an 0.080-in. build up on a worn edge, VanHouten said using GTAW welding he would lay down 0.0010 in. to 0.0020 in. more weld than specified to allow for machining. With laser welding he now can choose welding wires with diameters 0.024 in. to 0.0059 in. (0.60 mm to 0.15 mm) to create weld seams less than 0.039 in. (1 mm) in width with virtually no undercut.

Weld prep is similar to GTAW welding; VanHouten says he cleans the mold to remove plastic or metal chips and oils. He says he seldom has to grind a joint because he can laser weld on the existing surface.

"Fixturing is simple a rotating vice, a couple of clamps, and a piece of clay because of the adjustable work surface," VanHouten says, noting that the key to set up is ensuring that there is a level surface for the weld bead.

He positions the workpiece so he can see the weld bead through the microscope. He welds curves with short straight beads, the length of which depend on the circumference of the curve. On some jobs VanHouten manually turns a rotary vice as he welds. When the bead must follow a rise or indention in the workpiece, he lowers or raises the worktable.

"The key issue is to minimize pits and porosity as I lay down the filler rod," says VanHouten. He centers the laser beam on the wire so the greatest amount heat is at the center. He adjusts the power to minimize cold laps from poor fusion along the side of the wire, and to minimize the porosity and brittleness that can come from rapid solidification. He adjusts the laser-beam focus diameter to 1.5 times the rod size. Sometimes he sets the bead focus to cover the welded bead and smooth the edges. He typically sets the laser power at 120 watts, using 20 cfm of argon as the shielding gas.

VanHouten controls the welding speed with the joystick, and uses variable speed and pulse frequency to match the material weldability. Usually, weldability is a factor of reflection. The feed rate is derived from experience with the welding machine and metallurgy. The machine has the capability to store 24 weld parameters, and VanHouten uses that storage capacity for material and rod sizes, power duration and frequency.

VanHouten has a degree in welding technology and three years of on-thejob welding experience. He notes that training on the laser was a matter of learning the settings, and working with a microscope that reverses concave and convex images.

"GTAW welding is part skill, part art," VanHouten says, adding that laser welding is easier to learn than arc welding process.

The machine requires maintenance, and VanHouten keeps log for the cleaning of its optics and filters, and to note when the table is adjusted and lubricated. He spends nearly $400 on supplies and nearly $800 on maintenance items a year.

"We have limitations," VanHouten says. Workpiece weight is limited to 250 pounds and has to fit the table. The table size is 31.5 in. by 11.8 in. by 23.6 in. (800 mm by 300 mm by 600 mm).

Also, he said sometimes they are limited by the size of the repairs that must be made. "Some holes in the mold are just too small for the filler rod."

Martin said Accu-Mold uses its laser welding machine for 95 percent of its mold repair jobs, and that his company ships 90 percent of its mold repair jobs the day they are received, and all of them by the next day.

He said the laser-welding machine cost about $100,000. A machine that has the welding head moving costs about twice as much.

For more information, contact David Martin at Accu-Mold, Inc., 7622 S. Sprinkle Rd, Portage, Mich. Telephone number: 269-323-0388, or email: dave@accu-moldinc.com. For Trumpf Inc.: www.us.trumpf.com