Laser Applications Newsletter
Laser Kinetics Inc.
Mtn. View, CA 94041
Issue 1, Vol. 2
LASER KINETICS IN 1998
It looks like a happy New Year from here, laser fans. We're looking at increasing volumes of business as the year opens. Our activities will remain in the area of laser material processing, and we are continuing with process development, system design, training and troubleshooting in this field. We are fortunate to be located in California's Santa Clara Valley, where talented engineers are available for work on specific projects. This lets us be very responsive to increased levels of business.
LASER PROCESSING TIPS
A very important use of lasers in production is the joining of thin metallic components with spot welds. Laser spot welds can be very small, fast and clean. Razor blades and hard drive suspensions are laser spot welded by the millions.
Conceptually, a spot weld is very simple: Put the parts to be welded in contact with each other and aim a focused laser beam at them until the material melts and fuses the parts together.
Problems, however, arise in each part of this simple description. One vital concern in spot welding is fixturing. Fixturing merely implements "put the parts to be welded in contact with each other". It can be extremely difficult to keep thin metal parts in contact since even a 50 micron gap can cause trouble in a lap joint where the material thickness is 100 microns. Our experience with fixturing is that it's hard, and that every new part has its own challenges. Formed parts have variations and are out of flat; thin metal wrinkles and lifts; clamp elements block the laser beam's access to the joint. Once your fixture does work, you may not be able to easily load and unload it. Eventually, after several trips to the toolroom, the fixture works. It pays to be relentless in making a good fixture, since poor fixturing causes more problems than anything else in laser welding.
Most laser spot welds are performed on lap joints. The fusion zone (the metal that melts) must go completely through one part and into the other to form a weld. This tends to limit spot welding to thin material, at least for the top element. Some spot welds are performed on butt joints, but these are almost always for staking (holding parts in position for subsequent manufacturing operations) rather than for structural purposes, since spot welds on butt joints are rather weak.
There are two broad categories of laser spot welds: conduction welds and keyhole welds.
A conduction weld is performed by heating the surface of the workpiece with a laser beam until it melts. After the surface melts, heat conduction into the material melts a zone beneath the laser spot. This zone is roughly hemispherical because heat loss is greater at the edge of the laser spot than at its center. Conduction welds, therefore, can't be very deep. Although it's possible to leave the laser on for a couple of seconds and make a large melt pool, the practical limit of penetration depth is around 1 mm.
Nd:YAG lasers are used for most conduction welds even though CO2 lasers are cheaper and safer. This is primarily because solid steel absorbs 1 micron YAG light better than it does 10 micron CO2 light. While reflectivity measurements are subject to great variation depending on surface finish and oxidation, steel reflects about 75% of YAG light and 95% of CO2 light until it melts. Then it reflects about 50% at either wavelength. For a YAG, then the absorbed power doubles when the surface melts. For a CO2, it increases by a factor of 10. This big jump means that once you melt metal with a CO2 laser, you will immediately vaporize it because so much more power gets transferred to the material. Vaporization drives the process from conduction welding to keyhole welding. Keyhole welds are deeper, rougher and more variable than conduction welds. The vaporized metal also generates more contamination than found in conduction welds, a serious problem for many assemblies. So, for smooth, repeatable conduction spot welds, a YAG laser is a much easier tool to use.
As noted above, once you start vaporizing metal you alter the welding process. The vapor generates a channel that penetrates the workpiece, creating fusion zones much deeper than they are wide. The vapor channel also ejects particulates, and the forces acting on the channel disturb the melt pool creating a rough surface. Either YAG or CO2 lasers will make keyhole spot welds as long as they are focused to produce an irradiance of 10 E6 watts/cm2 or better.
WELD ENERGY AND PULSE DURATION
For a conduction weld in stainless steel, it takes about 400 Joules to melt 1 cubic millimeter of metal (See LASER MATERIALS PROCESSING pg. 198 for more details). Since the weld shape is hemispherical, the energy is related to depth by the formula:
E=16 R **3
(for E in joules and R in mm)
Pulse durations usually range from 2 to 10 milliseconds, with the power varied to get the right energy. Long pulses have fewer metallurgical problems than short ones; many stainless steels tend to crack if welded with 2 millisecond pulses. Once you start to keyhole, as evidenced by UV light and acoustic emission from the weld, the efficiency rises dramatically and only about 50 J per cubic millimeter are needed. Since the weld is no longer hemispherical but becomes more cylindrical in shape, far greater penetration is achieved for any given energy.
Many Nd:YAG lasers allow their pulses to be shaped: The power can be varied during the pulse to optimize the process. In general, reflective materials such as copper are best welded with very high initial power followed by a lower welding power. Conduction welds in other metals do not benefit from an initial spike and, in fact, tend to have increased spatter if one is present.
Large conduction welds and most keyhole welds are improved by a "ramp-down" at the end of the pulse. Lowering the power gradually rather than shutting it off abruptly eliminates crater cracking.
LASER RELATED EVENTS
High-Power Lasers and Applications
January 24-30, 1998
San Jose, CA
The laser applications part of Photonics West, SPIE's big lasers and optics symposium. The program on solid state, gas and free-electron lasers is of interest to workers in laser applications who want to understand the latest developments on laser sources. More directly related to applications is the program on optical techniques in precision manufacturing, where we have a conference on laser applications in microelectronic and optoelectronic manufacturing. Important new developments in micromachining, laser ablation and electronic processing should be described here.
SPIE's phone no. is (360) 676-3290, fax is (360) 647-1445
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Call 650 575-4919 or e-mail us for more information.