Lasers advance the precision welding art Essay

Lasers advance the precision welding art Welding with a laser has advanced the art of precisely joining andsealing a range of ferrous and nonferrous metals. When compared toconventional processes, laser welding enjoys many productive andquality-improving advantages. No welding rods, fluxes, or protectivematerials are needed, for example.

Dissimilar materials can be joinedor sealed as well. And warping, internal stresses, and cracks areminimized because the focused beam, with a spot size of 0.004 to 0.010dia, generates heat only in the weld area. Flexibility is another plus. The beam can be directed with mirrorsso it impinges on surfaces difficult to reach conventionally. Also, alaser can be made fully automatic by computerized numerical control (CNC).

Running in this mode, a laser welder greatly improves precision,repeatability, and productivity in processing complex shapes. Two operating modes The laser welding process can be performed two ways: continuouswave (CW), or deep penetration by pulsing the beam. In CW welding, which is mainly used to join sheets or plates, theheat source is focused at the workpiece surface. The operation resultsin fast welding speeds, but has the disadvantages of small weldbeadwidth and high heat input (and correspondingly large heat affectedzone). These drawbacks are overcome with deep penetration welding, whichis more widely used.

It’s accomplished by sinking a small keyholeinto the metal with the laser. Then, as the beam travels along the weldpath, the molten keyhole solidifies. Such welds result in deeppenetration with minimal heat input. Keyhole welding lends itself to processing difficult materials(such as aluminum) where high power densities are required to overcomethe metal’s reflectivity and thermal diffusivity. A technique usedto accelerate penetration, improve bead control and minimize theheat-affected zone is to enhance the beam’s pulse, i.e., anadditional high-voltage power supply is used to increase pulse power. Single, long pulses with enhanced edge spikes are ideal for weldingvarious metals.

The leading edge spike quickly melts the metal, whichis necessary to overcome its reflectivity. The remaining pulse energynow is absorbed readily by the molten puddle. One benefit of welding this way is that the time required to bringmetal to a molten stage is reduced considerably, and this lowers theheat input into the workpiece. The result is less distortion. Deeper, more discrete weld penetrations in metals are obtained bybeam pulsing. For example, a beam operating in the enhanced pulse mode(with a pulse length of 4 milliseconds and a repetition rate of 100pulses/sec) can produce a weld rate of 30 ipm. Figure 1 illustrates thedifference in performance between CW and enhanced pulse modes for a750-W CO2 laser welding AISI 304 stainless. With a standard gas mix of nitrogen, CO2, and helium flowingthrough the laser resonator, a CO2 laser designed for enhanced pulsingcan be operated up to 1000 pulses/sec.

Pulse rates higher than 2500pulses/sec are possible when a special, commercially available gasmixture is used. This mixture is needed to shorten the enhancedpulse’s decay time and eliminate pulse overlap. Cloud cover Wen a laser welds, a plume of metal vapor (plasma) forms above thespot at which the focused beam reacts with the work surface. It’spossible that this plasma can absorb most of the beam’s energy,allowing little or none of it to react with the workpiece.

To counter this, either argon or a mixture of helium and argon isused as a shield gas. These inert gases reduce the plasma halo andsurface oxidation, while aiding absorption of light energy at the joint. Argon is a suitable shielding gas for lasers operating in the CWmode; however, it ionizes too readily to be useful in beams of greaterpower density such as those produced by pulsing. Helium, or a mixtureof He and Ar, is used as a shielding gas for pulsed welding beams.

Helium has a sufficiently high ionization temperature to resist forminga large plasma cloud in the multikilowatt peak power levels produced byenhanced pulsing. A plasma formed by the beam of a pulsed laser is smaller than thatformed by a continuous wave with the same power input to the worksurface. This is because the plasma is allowed to dissipate (or relax)between pulses. On the edge The workpiece must be held correctly to maintain intimate contactwhen laser welding. Probability of a successful weld increases byknowing when to spend effort for edge preparation. Butt weld. The work material doesn’t require beveling at theedges.

Sheared edges are acceptable if they are square, straight, andsecurely clamped together. Lap weld. Square edges aren’t of great importance ifit’s either a full- or partial-penetration application, but airgaps between the workpieces will severely limit both penetration andwelding speeds. Flange joint.

It’s important that the workpieces havestraight, square edges. Also, it is necessary that they are securelyclamped together and be in precise transverse alignment with oneanother. Record is impressive Laser welding is a mature technology, and today’s industrialunits pack the kind of processing power needed to successfully handle avariety of metals and applications. Design advancements have enabledcost- and quality-conscious manufacturers to use industrial lasers onhigh-alloy metals.

Coherent General, with an Everlase Fast Axial(TM) CO2 laser, hassuccessfully welded aerospace material. Butt welding of 0.060-thickInconel was done using an inert shielding gas at rates up to 300 ipmwith good fit-up and alignment.

Inconel plate (0.125 thick) can bewelded at 40 ipm with full penetration. Similar results have been observed with titanium, with theexception that weld rates are somewhat slower with thinner materials.Titanium (0.

047 thick) was welded at 200 ipm, whereas 0.120-thick stockwas welded at 45 ipm, slightly faster than the Inconel. Skip welding of materials held in a T configuration also is beingdone. Here, the beam is directed at a 10- to 15-degree angle, withrespect to the cap of the T, and directly at the junction of the matingparts. Again, inert gas is used. Superior welds of 0.060-thick Inconelhave been done at 80 ipm. Further, lower power CO2 lasers have been applied to weld and seala variety of metals.

A lap weld joining a 416 stainless cap to a 310stainless body is shown in Figure 2. In another case, an electricallead-through was pulse welded in place without fracturing a ceramicinsulator that was close to the weld zone. A 575-W laser with a 2.5focal-length lens was rotated around each pin to produce a hermetic weldin 2 sec.

Penetration was almost 100 percent on this 0.025-thicktitanium workpiece. Processing was at 30 pulses/sec. In a finalexample, Figure 3 shows a 1.5-dia nickel-plated, cold-rolled steelbattery can with the top hermetically welded on a 0.

040 lip. There are many other success stories. This, however, doesn’tmean that lasers are a panacea. What it does mean is that they lendthemselves to applications previously considered impossible or verydifficult to weld conventionally. Photo: 1. Performance comparison of laser modes when welding AISI304 stainless. Photo: 2. This lap weld of a 416 stainless cap to 310 stainlessbody met the requirements for a cosmetic as well as a deep penetratingweld.

The joint is made with a 575-W laser using a 5 focal-length lensand a helium-gas shield. An enhanced pulse mode (100 pulses/sec) with 4milli-second pulses is used to achieve penetration of 0.065 at 30 ipm.The cross section shows no undercut at the edge of the weld bead, andbecause a shielding gas is used, there’s no oxidation. Photo: 3. Battery cans made of nickel-plated, cold-rolled steelare hermetically welded by lasers.

Weld time is 8 sec/can. A 375-Wlaser (with a 2.5 focal-length lens) in a pulsed mode (80 pulses/sec)was used to achieve 0.030 penetration.

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