Penetration-Mode HLAW. To fully utilize the benefits of anexpensive laser system, HLAW is conducted primarily in penetration mode. Thisis sometimes referred to as arc-assisted laser welding. In penetration-modeHLAW, the laser generates a keyhole in the metal. Both deep penetration andhigh processing speeds can be achieved with keyhole laser welding.
A keyhole isformed when a laser beam with sufficiently high-power density causes meltingand vaporization of the base metal. As the metal is vaporized, it rapidlyexpands and pushes away from the substrate. This expansion exerts a reactiveforce on the melted substrate called the evaporative recoil force. This recoilforce pushes the melted metal away to form a depression. The melted metal iscontinually pushed out until the depression has formed into a deep keyhole.The keyhole can be partially orfully through the thickness of the metal. In the steady-state condition afterthe keyhole is established, continual vaporization of the bottom and walls ofthe keyhole holds it open against the forces of surface tension and gravity.
The relationship of laser power density and travel speed dictates thepenetration and width of the keyhole for a given base metal. Power densities onthe order of 106 to 108 W/cm2 are typical for keyhole laser welding. 3. HYBRID LASER ARC WELDINGPARAMETERSKnowledge of each variable and theability to precisely control is necessary to consistently produce hybrid weldswith the desired quality. These variables can produce competing effects on theweld attributes, and balancing the performance of each variable is essential tosuccessful hybrid welding. Table 1 lists the general effects of each HLAW variableon hybrid weld attributes.
The effects listed are typical for welding butt jointsover 6mm (0.24in.) thick or for welding thinner sections at travel speeds above3 m/min (120 in.
/min).Hybrid laser arc welding isapplicable over a wide range of travel speeds. Generally, the determiningfactor for welding speed is the productivity requirement. As travel speedincreases, hybrid weld penetration will decrease.
To maintain the required weldpenetration at increasing travel speeds, more laser power and an increased rateof filler-metal deposition is required. If the existing laser equipment islimited in power, then a compromise must be made among travel speed, laserpower, and weld penetration. At travel speeds on the order of 4 m/min (160in./min) or greater, joint-filling capabilities from the GMAW system can belimited. Gas metal arc welding systems are inherently limited to a maximumcurrent output.
For a given electrode diameter, there is a maximum wire feedspeed at the maximum current rating of the GMAW power supply. This limitationcan lead to insufficient filler-metal addition at faster travel speeds. If therequired reinforcement or fillet size cannot be met for a given travel speeddue to the limitations of the GMAW power supply, the travel speed, GMAW source,wire diameter, joint design, or the number of passes must be re-evaluated.Additional GMAW torches with separate power supplies and wire feeders could beused to overcome deposition limitations.Process Orientation. The HLAW process can be oriented in two directions: arc leadingor laser leading. The GMAW process can be positioned behind or in front of thetraveling laser keyhole. If the GMAW process travels behind the laser beam, theHLAW process orientation is referred to as laser leading.
If the GMAW processtravels ahead of the laser, the HLAW process orientation is referred to as arcleading. Figure 1 illustrates the laser-leading and arc-leading processorientations.The main difference between the twoorientations is the angle of the GMAW torch with respect to the direction oftravel. Torch angle can have an effect on the deposited GMAW bead. In thelaser-leading HLAW configuration, the GMAW torch is traveling behind the laserbeam, positioned at a “push angle.” In the arc leading configuration, the torchis at a “drag angle,” traveling in front of the laser beam.
This difference intorch angle can produce different weld surface geometries. In the laser leadingorientation, the deposited weld bead is relatively wide and flat, with largeweld toe angles. With arc leading, the deposited weld bead is more narrow andconvex, with sharper weld toe angles. Torch angle can be adjusted for eachprocess orientation, but there is a limitation to how close the torch can bepositioned to the beam axis, due to the beam convergence angle and obstructionsfrom the laser-focusing optic assembly. Alternatively, the laser beam axis canbe tilted while the GMAW torch is positioned normal to the work.Another reported difference betweenthe two process orientations is in penetration.
If the laser beam is positionedin the arc depression of the GMAW process, the arc-leading configuration canprovide slightly more penetration for HLAW. However, there is conflicting datareporting that the laser-leading process provides deeper penetration. In eithercase, the reported gain in penetration is generally considered insignificantfor most manufacturing applications.