• LACK OF JOINT PENETRATION

    A nonstandard term for INCOMPLETE JOINT PENETRATION. See Figure I-3. 

  • LACK OFFUSION

    A nonstandard term for INCOMPLETE FUSION

  • LAG

    A nonstandard term for DRAG, Thermal Cutting. 

  • LAMELLAR TEAR

    A subsurface terrace and step-like crack in the base metal with a basic orientation parallel to the wrought surface caused by tensile stresses in the through thickness direction of the base metal weakened by the presence of small dispersed, planar shaped, nonmetallic inclusions parallel to the metal surface.

    Lamellar tearing is a cracking phenomenon that occurs in welds joining rolled steel products. Lamellar cracks or tears occur most often during fabrication where weld shrinkage strains exceed the strength of the base metal in the through-thickness direction of the steel.

    All steels contain nonmetallic inclusions in varying amounts. Hot-rolled steels may contain other internal imperfections such as porosity, seams, or laminations. When the steel is rolled to the desired shape for fabrication, these inclusions and imperfections are elongated in the direction of rolling. Internal seams and tears may or may not be “healed” (welded together by the rolling action). These defects are likely to occur in thick sections where the mechanical deformation of the internal seams or tears may not be sufficiently worked to “heal” the defects. The specified strength of these steels is always measured in the direction of rolling. The strength of rolled steels in the through-thickness direction (perpendicular to the direction of rolling) is considerably less than the strength obtained in the direction of rolling.

    The contraction or shrinkage of deposited weld metal during cooling sets up localized strains in the base metal. These strains may exceed the strength of the base metal in the through-thickness direction, resulting in lamellar tearing. Welds that require the deposition of large amounts of filler metal tend to degrade the tensile properties of the base metal and contribute to lamellar tearing. Highly restrained joints are also susceptible to lamellar tearing and should be welded with caution.

    The design of welded joints must take into account the direction of rolling. Welding to members in the through-thickness direction must be avoided, if possible.

    Where this is unavoidable, the joint should be detailed to reduce the possibility of lamellar tearing resulting from welding. Figure L- 1 illustrates susceptible joint details.

    Recognizing lamellar tearing may be difficult since the tearing is internal, like underbead cracking. Corner joints and T-joints are the most susceptible to lamellar tearing, but joint details can be modified to minimize it. If there is any question that subsurface tearing exists, then nondestructive methods should be used to examine the base metal. 

  • LAMINATE

    A laminate is the composite metal product of two or more layers joined, usually by welding, to form a structural product. 

  • LAMINATION

    A type of discontinuity with separation or weakness generally aligned parallel to the worked surface of a metal.

    Metal defects with separation weaknesses are generally aligned parallel to the rolled direction of the fabricated section. These defects may result from elongated pipe, seams, or inclusions in the metal that are made directional during the mechanical working of the metal. 

  • LAND

    A nonstandard term for ROOT FACE. 

  • LAP

    A base metal surface defect (not caused by welding), appearing as a seam in the base metal, caused by folding over hot metal, fins, or sharp corners and then rolling or forging them into the surface. 

  • LAP JOINT

    A joint between two overlapping members in parallel planes. 

  • LASER

    A device that produces a concentrated coherent light beam by stimulated electronic or molecular transitions to lower energy levels. Laser is an acronym for light amplification by stimulated emission of radition.

    The laser beam is a focused, high-power, coherent, monochromatic light beam. The laser was independently invented in 1960 by two scientists, one at Bell Laboratories and the other at Hughes Aircraft. Most of the early application development was conducted by Bell Laboratories. The original laser device consisted of a ruby rod surrounded by a xenon flash lamp that excited the chromium atoms in the ruby to higher energy states. Simultaneously stimulated and returning to the ground state, the atoms emit an intense amplified light beam. See Figure L-2 for a schematic diagram of a ruby laser

    The rapid flashing of the xenon lamp produced a seemingly steady state of emitted light. Only a focused, monochromatic light beam was permitted to leave the device. Initial application was limited to the low power of the ruby laser.

    The three basic types of laser include solid state, gas discharge, and semi-conductor injection types. High power, pulsed outputs in the megawatt range are provided by solid state lasers. Gas discharge lasers use helium, neon, krypton, or xenon to provide low power output frequencies that are continuous. Semi-conductor injection lasers have limited power output, are dependent on liquid nitrogen operating temperatures, and do not need a flashlamp for exciting the atoms since they convert electricity directly into light.

    Early laser metal working applications were limited, but with the advent of higher-powered lasers, applications include welding, brazing, cutting, micro perforation, and metal removal. High-powered lasers can cut steel up to 25 mm (1 in.) thick. 

  • LASER BEAM AIR CUTTING (LBC-A)

    A laser beam cutting process variation that melts the workpiece and uses an air jet to remove molten and vaporized material. 

  • LASER BEAM BRAZE WELDING (LBBW)

    A braze welding process variation that uses a laser beam as the heat source.

  • LASER BEAM CUTTING (LBC)

    A thermal cutting process that severs metal by locally melting or vaporizing with the heat from a laser beam. The process is used with or without assist gas to aid the removal of molten and vaporized material. See LASER BEAM AIR CUTTING, LASER BEAM EVAPORATIVE CUTTING, LASER BEAM INERT GAS CUTTING, and LASER BEAM OXYGEN CUTTING.

    The source of heat for laser beam cutting is a concentrated coherent light beam that impinges on the workpiece to be cut. A combination of melting and evaporation provides the mechanism for removal of material from the kerf. High-power lasers have unique advantages for cutting applications, including capability to cut any metal and producing a narrow kerf and heat-affected zone. High cutting speeds are achieved, and the equipment is adaptable to computer control.

    A laser is a heat source with some unique characteristics. Relatively modest amounts of laser energy can be focused to very small spot sizes, resulting in high power densities. In cutting and drilling, these power densities are in the range of 104 to 106 W/mm2 (6.5 X l06 to 6.5 x 108 W/h2). Such high concentrations of energy cause melting and vaporization of the work piece material, and material removal is enhanced by a jet of gas. Depending on the material, a jet of reactive gas such as oxygen can be applied coaxially with the beam, improving process speed and cut edge quality.

    Among laser material processing applications, cutting is the most common process: its use has quickly grown worldwide. The first laser material processing application was drilling diamonds for wire drawing dies. Today, laser cutting and the related processes of drilling, trimming, and scribing account for more than 50% of the international industrial laser installations.

    A high-power C02 lasers in the range of 400 to 1500 W dominate the cutting area. Neodymium-doped, yttrium aluminum garnet (Nd:YAG) lasers are also used.

    Laser cutting has the advantages of high speeds, narrow kerf widths, high-quality edges, low-heat input, and minimal workpiece distortion. It is an easily automated process that can cut most materials. The cut geometry can be changed without the major rework required with mechanical tools: there is no tool wear involved, and finishing operations are not usually required. Within its thickness range, it is an alternative to punching or blanking, and to oxyfuel gas and plasma arc cutting. Laser cutting is especially advantageous for prototyping  studies and for short production runs. Compared to most conventional processes, noise, vibration, and fume levels involved in laser cutting are quite low.

    Metals which can be cut by the laser beam process include carbon steel, alloy steel, stainless steel, aluminum, copper and copper alloys, nickel base alloys, and titanium and its alloys. Nonmetals such as alumina and quartz can also be cut, along with organic materials, such as cloth and the spectrum of plastics. Some types of composite materials with organic matrices can be cut. Lasers have been successfully used to cut several types of metal-matrix composites.

    Laser Drilling- Hole diameters produced by laser beam drilling typically range from about 0.0025 to 1.5 mm (0.0001 to 0.060 in.). Depths achieved are usually less than 25 mm (1 in.) because of beam focusing limitations.

    The process produces clean holes with very small recast layers. When large holes are required, a trepanning technique is used where the beam cuts a circle with the required diameter.

    Drilling with a laser is a pulsed operation involving higher power densities and shorter dwell times than laser cutting. Holes are produced by single or multiple pulses. Laser drilling is a cost-effective alternative to mechanical drilling, electro-chemical machining, and electrical-discharge machining for making holes of relatively shallow depths.

    Laser drilling shares most of the advantages found in laser cutting. It is especially advantageous when the required bole diameters are less than 0.5 mm (0.020 in.) and when holes are to be made in areas inaccessible to conventional tools. Beam-entry angles can be very close to zero, a situation where mechanical tools are susceptible to breakage. The industrial laser drilling area is dominated by Nd:YAG lasers. 

  • LASER BEAM DIAMETER

    The diameter of a laser beam circular cross section at a specified location along the laser beam axis. 

  • LASER BEAM EVAPORATIVE CU'ITING (LBC-EV)

    A laser beam cutting process variation that vaporizes the workpiece, with or without an assist gas (typically inert gas), to aid the removal of vaporized material. 

  • LASER BEAM EXPANDER

    A combination of optical elements that will increase the diameter of a laser beam.

  • LASER BEAM INERT GAS CUTTING (LBC-IG)

    A laser beam cutting process variation that melts the workpiece and uses an inert assist gas to remove molten and vaporized material

  • LASER BEAM OXYGEN CUTTING (LBC-0)

    A laser beam cutting process variation that uses the heat from the chemical reaction between oxygen and the base metal at elevated temperatures. The necessary reaction temperature is maintained with a laser beam. 

  • LASER BEAM SPLITTER

    An optical device that uses controlled refection to produce two beams from a single incident beam. 

  • LASER BEAM WELDING (LBW)

    A welding process that produces coalescence with the heat from a laser beam impinging on the joint. The process is used without a shielding gas and without the application of pressure.

    The focused, high power coherent monochromatic light beam used in laser beam welding causes the metal at the point of focus to vaporize, producing a deep penetrating column of vapor extending into the base metal. Yttrium aluminum garnet (YAG) lasers are used for spot and seam welding of thin materials. For welding thicker materials, multi-kilowatt carbon dioxide gas laser systems are available. Such systems provide power densities of 10 kW/mm2 (6.5 MW/in.*).

    Continuous power provides a high power laser with deep penetration welding capability.

    Laser beam welding is a high-speed process ideally suited to automation, although it requires good joint fit-up. The high cost of equipment relegates applications to high-volume production or to critical weldments requiring unique characteristics. The equipment is very sophisticated but is designed for use by welding operators who may not be skilled manual welders.

    Process Advantages

    Major advantages of laser beam welding include the following:

    (1)Heat input is close to the minimum required to fuse the weld metal; thus, metallurgical effects in heat-affected zones are reduced, and heat-induced workpiece distortion is minimized.

    (2) Single pass laser welding procedures have been qualified in materials of up to 32 mm (1-1/4 in.) thick, thus allowing the time to weld thick sections to be reduced and the need for filler wire (and elaborate joint preparation) to be eliminated.

    (3) No electrodes are required; welding is performed with freedom from electrode contamination, indentation, or damage from high resistance welding currents. Because LBW is a non-contact process, distortion is minimized and tool wear is essentially eliminated.

    (4) Laser beams are readily focused, aligned, and directed by optical elements. Thus the laser can be located at a convenient distance from the workpiece, and redirected around tooling and obstacles in the workpiece. This permits welding in areas not easily accessible with other means of welding.

    (5)The workpiece can be located and hermetically welded in an enclosure that is evacuated or that contains a controlled atmosphere.

    (6) The laser beam can be focused on a small area, permitting the joining of smali, closely spaced components with tiny welds.

    (7) A wide variety of materials can be welded, including various combinations of different type materials.

    (8) The laser can be readily mechanized for automated, high-speed welding, including numerical and computer control.

    (9) Welds in thin material and on small diameter wires are less susceptible to burn-back than is the case with arc welding.

    (10) Laser welds are not influenced by the presence of magnetic fields, as are arc and electron beam welds; they also tend to follow the weld joint through to the root of the workpiece, even when the beam and joint are not perfectly aligned.

    (11)Metals with dissimilar physical properties, such as electrical resistance, can be welded.

    (12) No vacuum or X-ray shielding is required.

    (13) Aspect ratios (Le., depth-to-width ratios) on the order of 1O:l are attainable when the weld is made by forming a cavity in the metal, as in keyhole welding.

    (14) The beam can be transmitted to more than one work station, using beam switching optics, thus allowing beam time sharing.

    Process Limitations

    Laser beam welding has certain limitations when compared to other welding methods, among which are the following:

    (1) Joints must be accurately positioned laterally under the beam and at a controlled position with respect to the beam focal point.

    (2) When weld surfaces must be forced together mechanically, the clamping mechanisms must ensure

    (3) The maximum joint thickness that can be laser beam welded is somewhat limited. Thus weld penetrations much greater than 19 mm (0.75 in.) are not presently considered to be practical production LBW applications.

    (4) The high reflectivity and high thermal conductivity of some materials, such as aluminum and copper alloys, can affect their weldability with lasers.

    (5)When performing moderate-to-high power laser welding, an appropriate plasma control device must be employed to ensure that weld reproducibility is achieved.

    (6) Lasers tend to have a fairly low energy conversion efficiency, generally less than 10%.

    (7) As a consequence of the rapid solidification characteristic of LBW, some weld porosity and brittleness can be expected.

    Weld Processing Modes

    There are two distinctly different modes of energy transfer in laser welding which are commonly referred to as conduction mode welding and keyhole mode welding. It is the power density incident on the material surface, as well as the material properties, which ultimately determine which mode is present for a given weld.

    Conduction Mode Welding- In conduction mode welding, the laser beam does not produce sufficient vaporization pressure to displace the weld pool, form a cavity, and allow the beam to emerge directly at the root of the weld. Instead, the incident beam energy on

    the weld pool surface is transferred to the root of the weld solely by conductive and convective heat flow in the molten metal. For a given weld diameter, conduction limited welding has a maximum penetration value at which no further penetration can be obtained without creating a cavity. The maximum aspect ratio (pool depth divided by pool width) for conduction mode welding is between 0.5 and 1.0.

    Conduction mode welding can be obtained either with continuous wave lasers or with pulsed power lasers and with either low or high power. Selection of parameters and focusing optics that result in small vapor plumes and the absence of spatter are necessary to insure conduction mode welding.

    Keyhole Mode Welding. Keyhole mode welding occurs when the power density of the beam is about 106 W/ cm2 (6.45 X 106 W/in2) or greater. The material at the interaction point melts and vaporizes. The vapor recoil pressure, surface tension, and other phenomenon create a deep cavity. This cavity is a high-pressure region surrounded by walls of molten metal. As the workpiece moves relative to the beam, the cavity is sustained, and the molten metal flows from the front edge of the cavity around the sides of the cavity in a direction opposite to the travel direction, and solidifies at the trailing edge forming a narrow fusion zone or weld.

    Applications

    Laser beam welding is being used for an extensive variety of applications such as in the production of automotive transmissions and air conditioner clutch assemblies. In the latter application, laser welding permits the use of a design that could not otherwise be manufactured. The process is also being used in the production of relays and relay containers and for sealing electronic devices and heart pacemaker cases. Other applications include the continuous welding of aluminum tubing for thermal windows and for refrigerator doors.

    Successful laser welding applications include welding transmission components (such as synchro gears, drive gears and clutch housings) for the auto industry. These annular and circumferential-type rotary welds need from 3 to 6 kW of beam power, depending on the weld speed being employed, and require penetrations which typically do not exceed 3.2 mm (0.125 in.). Materials welded are either carbon or alloy steels. In some cases, such as the gear teeth, they have been selectively hardened before welding. There are many advantages to laser welding such assemblies. The low heat input provided by the laser does not affect the pre-hardened zones adjacent to the weld. Also, this low heat input produces a minimal amount of distortion so that precision stampings can often be welded to finished dimensions. Since the ease of automation and high weld-speed capability of the laser process makes it ideal for automotive-type production, a number of these systems have been installed in the automotive industry.

    Metals Welded

    Laser beam welding can be used for joining most metals to themselves as well as dissimilar metals that are metallurgically compatible. Low-carbon steels are readily weldable, but when the carbon content exceeds 0.25% martensitic transformation may cause brittle welds and cracking. Pulsed welding helps minimize the tendency for cracking. Fully killed or semi-killed steels are preferable, especially for structural applications, because welds in rimmed steel may have voids. Steels having high amounts of sulfur and phosphorus may be subject to hot cracking during welding. Also, porosity may OCCUI in free machining steels containing sulfur, selenium, cadmium, or lead.

    Most of the 300 series stainless steels, with the exception of free machining Types 303 and 303Se and stabilized Types 321 and 347, are readily weldable. Welds made in some of the 400 series stainless steels can be brittle and may require post weld annealing.

    Many heat resistant nickel and iron based alloys are being welded successfully with laser beams. Titanium alloys and other refractory alloys can be welded in this way, but an inert atmosphere is always required to prevent oxidation.

    Copper and brass are often welded to themselves and other materials with specialized joint designs used for conduction welding. Aluminum and its weldable alloys can be joined for partial penetration assembly welds and are commonly joined by pulsed conduction welds for hermetically sealed electronic packages. Joint designs must retain aluminum in tension.

    Refractory metals such as tungsten are often conduction welded in electronic assemblies, but require higher power than other materials. Nickel-plated Kovar is often used in sealing welds for electronic components, but special care is required to ensure that the plating does not contain phosphorous, which is usually found in the electroless nickel plating process commonly used for Kovar parts that are to be resistance welded.

    Dissimilar metal joints are commonly encountered in conduction welds where the twisting of conductors forms a mechanical support that minimizes bending of potentially brittle joints. Dissimilar metals having different physical properties (reflectivity, conductivity and melting points) are often joined in the welding of conductors. Special techniques such as adding extra turns of one material to the joint as opposed to the other may be required to balance the melting characteristics of the materials. Some of these concepts can also be applied to structural and assembly welds, but the possibilities are much more limited.

  • LASING GAS

    A gaseous lasing medium. 

  • LASING MEDIUM

    A material that emits coherent radiation by virtue of stimulated electronic or molecular transitions to lower energy. 

  • LAYER

    A stratum of weld metal consisting of one or more weldbeads. See Figure L-7.

  • LAYER LEVEL WOUND

    A nonstandard term for LEVEL WOUND. 

  • LAYER WOUND

    A nonstandard term for LEVEL WOUND.