Laser applications to broaden.

Laser applications to broaden

Surface alloying, glazing, spot-welding, and chip-breaking are among new types of laser applications for metalworking expected to emerge during the upcoming decade. These and other applications are described in a recent, 280-pg report called Lasers in Materials Processing--A Summary and Forecast. The publisher is Tech Tran Consultants Inc, Lake Geneva, WI.

Surface alloying

"Laser surface alloying is related to laser cladding, but somewhat newer, and there is a major difference," says the report. "Rather than providing an overlay on top of a substrate, laser alloying seeks to directly modify the surface composition of the base metal. This is done by adding alloying agents to the base metal, which has been melted to a specific depth."

The resulting mixture produces a chemically altered layer at the surface of the metal, the report continues. This layer has desired characteristics such as increased hardness or corrosion resistance.

When this technique is used, a base metal can be selected because of cost or other considerations. Then the surface can be altered to produce the desired end-use capability.

"This technique is suitable when the way in which the surface of a part interfaces with its environment, on the one hand, is more important than the way the part as a whole interfaces."

The primary advantage of surface alloying generally, the report continues, is in material cost savings. The advantage of a laser is that a high surface quality can be achieved. Melting depth can be carefully controlled, and the resulting mixture is quite homogeneous. Further, there is little part distortion caused by concentated heat.

"The way in which alloying materials interact with the base material depends on characteristics of the laser beam used for treatment. The most important parameter involved is interaction time. If this is not sufficiently long, convection currents--driven by surface tension--are set up by temperature gradients on the surface of the melt pool. These currents control mixing within the molten zone, so final alloy composition and uniformity are dependent on them."

The relatively long interaction time (above 50 microsec) involved means convective alloying is associated with treatment by high-powered, continuous-wave [CO.sub.2] lasers, or sometimes by pulsed [CO.sub.2] lasers.

"Q-switched Nd:YAG lasers also have been investigated for surface alloying, as their ability to sustain relatively high average power has increased. When this type of laser is used, the interaction time is much shorter, on the order of 50 to 500 nanosec."

Laser alloying is potentially useful in treating compounds for enhanced corrosion and wear resistance, as on auto exhaust valves. Up to a third of the costs of high alloy steels is attributed to transition metal additions. With laser alloying, it could prove possible to obtain performance of a bulk alloy component, and greatly reduce consumption of alloying agents.

Laser alloying could be useful in preparing alloyed surfaces on steel components for steam power turbines, for instance. Another possible application is improvement of scuffing-wear performance of titanium. This would be accomplished by nitriding, carburizing, or a combination of the two.

Another possibility involves altering cutting tools, alloying only the surfaces to a high performance composition. To date, however, none of the applications mentioned has reached a production stage; all are still in development.

Glazing

Another process that uses lasers to melt metal is that of glazing, the report notes. Glazing is like surface melting, but has a different emphasis.

Melting seeks to refine surface grain structure mainly by suppressing nucleation of graphite granules by way of rapid solidification. Laser glazing carries this principle to an even greater extreme.

"The idea behind glazing is to totally suppress nucleation of all crystalling phases, so that an amorphous surface structure results. This requires extremely rapid cooling rates to bring the temperature down below the glass transition level, before onset of crystal nucleation."

To obtain increased cooling rates, the glaze layers are usually very thin, on the order of 0.1 mm or less. Both [CO.sub.2] and Q-switched Nd:YAG lasers have been investigated for glazing. [CO.sub.2] lasers tend to involve longer reaction times. This means that as subsequent, adjacent tracks are processed, heat input can sometimes initiate crystallization in the heat-affected zone of a previously heated track.

"Some alloys are susceptible to this behavior," says the report, "while others resist it. Also, beam traverse speed influences the results."

The Nd:YAG laser, operated in a Q-switched mode, has a much shorter interaction time. This makes recrystallization much less a problem. It also facilitates production of an entirely amorphous surface, essentially by forming a very large array of closely spaced spots.

Spot-welding

An approach to laser welding, devised as a substitute for resistance spot welding (RSW), has been developed by Avco-Lycoming and The Ohio State University. Called Laspot [R] , the prototype system was developed to closely resemble the physical setup and procedural sequence used in RSW.

"Pieces to be joined are inserted in a clamping system which applies enough pressure (around 100 lb) to ensure intimate contact between the parts," says the Tech Tran report. "This is similar to RSW, where electrodes clamp the pieces prior to welding, though clamping force is less in the Laspot [R] process."

A laser beam passes through the hollow upper "electrode" onto the work-piece surface. There the beam traces out a spiral or circular path. This has a diameter about equal to that of the indent diameter for an RSW. Beam operation is in the pulsed mode, because weld penetration and heat input can be readily controlled.

"Beam penetration can be visually verified by examining the underside of the weld. If the path of the laser beam is visible, then a full fusion weld exists.

"Samples of stainless steel and nickel alloys bonded by the Laspot [R] process showed consistent quality and repeatability. Weld shear strength was comparable to that for equivalent RSW welds, and exceeded applicable standards."

Chip-breaking

Another new, interesting application is use of a laser to improve machinability of difficult metals such as nickel-base superalloys, titanium alloys, and hardened steels. When traditional machining methods are used, the metal removal rate and productivity for these metals are limited. Factors causing limitation are high chip tempertures at high speeds, and high concentration of heat near the cutting tool.

"In cutting Inconel 718, for example, carbide tools are limited to speeds of less than 0.5 meters/sec. High-speed machining can help overcome this problem for some steels that lose strength at high temperatures. However, many metals retain their strength characteristics even at the high temperatures caused by high-speed machining. In these situations, tool life may be quite short."

The ideal machining situation would consist of the chip being removed becoming heated at the top, but remaining cool at the bottom where it comes in contact with the tool. The laser has been shown to be effective at doing this.

Either a CW or pulsed beam can be applied to the top of the chip, just ahead of the tool. The chip is heated only where the laser is applied, resulting in some softening of the metal.

The bottom of the chip remains cool, so there is less wear on the tool as it cuts. Using absorptive coatings sprayed on the part may increase absorption of heat from the laser, resulting in even more softening.

"A laser chip-breaking process has been developed in Japan," says the report. "It employs the beam from a high-power, Q-switched Nd:YAG laser to cut machining chips into short lengths as they are generated.

"Small chips are blown away from the workpiece by gas jets. The short chip lengths are expected to simplify chip management and reduce forces at the machining face."

Chipless machining

Two methods for laser-assisted chipless machining are being pursued at Hydronetics Inc. In the first, a computer is used to "slice" into thin layers of a 3-D shape. Under control of the computer, a laser cuts out the shapes of individual layers of thin metal sheets. The cut sheets are stacked, then laminated together to form the desired 3-D shape.

"In the other method," says the report, "powder metal is used instead of sheets. A thin layer of powder is laid down and compressed. Then a laser draws the image of the desired layer of the workpiece.

"Where the laser beam scans, it sinters the metal, forming a solid shape embedded in powder. Additional powder is deposited, compressed, and scanned to form the next layer. A fusing operation joins the new layer to the previous one."

This process is repeated until the final, desired shape has been completed. Powder metal that was merely compressed rather than fused by laser sintering is easily broken away, revealing the desired work-piece.

Other processes

In addition to the processes mentioned, other emerging processes covered are paint-stripping, ablation by vaporization, selective sintering of composites, film deposition, and modification of non-metals with melted metals.

Besides describing emerging processes, the report gives much useful, detailed information on basic laser technology, laser selection, and market trends. An appendix lists suppliers and trade organizations, and contains a glossary of laser terms.

PHOTO : Laser-assisted machining of hard-to-cut alloys is being explored at General Electric's

PHOTO : Research & Development Center, Schenectady, NY. In experimental cutting of titanium

PHOTO : alloys, GE researchers use a 400-W Nd:YAG laser that fires at rates to 300 pulses/sec.

PHOTO : Resulting temperatures on the metal surface go to 3300 C, causing the surface to soften

PHOTO : and partially vaporize. The treated alloy can then be removed at high speeds with

PHOTO : conventional cutting tools.