New developments in advanced welding

New developments in laser welding

6.1 Introduction

A laser is an outstanding invention of the twentieth century; a variety of lasers have been developed and applied in many industrial fields since Maiman announced laser oscillation in optically pumped ruby crystals in 1960.1 A focused laser beam is a heat source operated at extremely high power or energy density. It can heat, melt and evaporate any material and consequently produce a deep spot or bead weld at a high speed. It is expected that laser materials processing should play an important role in fundamental and advanced technology in the twenty-first century. Laser welding has received much attention as a promising joining technology because it encompasses high quality, precision, performance, and speed with good flexibility and low deformation or distortion. In addition it allows robotic linkages, reduced man-power, full automation and systematization.

High power lasers (listed in historical order) including CO2, lamp-pumped YAG, diode (LD), LD-pumped YAG, fiber and disk lasers have been developed as welding heat sources. Combined or hybrid heat sources using two or three lasers with the same or different wavelengths, or a laser and another heat source such as metal inert gas (MIG) and tungsten inert gas (TIG) have been employed to produce better weld beads at high speeds efficiently. Furthermore, a second harmonic Nd:YAG laser with a few milliseconds time period has recently been developed to melt a copper sheet easily (see Chapter 5.) There has been considerable research on laser welding in order to understand the phenomenon or to apply its technology in industry. Relevant titles include ‘Laser weldability and welding phenomena of materials such as high strength steels, Zn-coated steels, aluminum alloys, stainless steels, Ni-base superalloys and magnesium alloys’, ‘Spectroscopic analyses of laser-induced plasma/ plume, and elucidation of emission, absorption and scattering properties of plasma’ and ‘Development of monitoring and adaptive control systems for penetration and weld quality’.

In this chapter, the current state of laser sources for welding is described

in terms of their characteristics, merits, power, beam quality and general applications so that the developmental trend of laser apparatuses is understood better. Subsequently, interesting new results, novel interpretation of welding phenomena (including melt flows) and the formation and prevention of welding defects are briefly summarized; the discussion includes the laser welding of stainless steels, zinc-coated steels, aluminum alloys, magnesium alloys, dissimilar metals and plastics. New process developments in remote or scanner laser welding, in-process monitoring and adaptive control during laser welding as well as laser-arc hybrid welding are noted as possible future technologies. These new developments in laser welding are applied in several industries.

6.2 Strengths and limitations of current laser welding technologies

6.2.1 Characteristics, power levels and beam quality of typical lasers for welding

The characteristics, laser media, maximum/normal power levels and merits of typical lasers for welding are summarized in Table 6.1.2 Lasers have been developed to achieve higher power, higher beam quality and/or higher input - to-output efficiency. The CO2 laser can provide the highest output power with continuous emission (commercial units up to 45kW), while lamp - or LD-pumped YAG and fiber lasers can deliver 10kW class power. Fiber lasers can produce high power and will possibly replace high power CO2 lasers.

The beam quality is defined as M2 or BPP (beam parameter product in mm-mrad), as shown in Fig. 6.1.3 The beam waist of 0.2mm diameter is shown for various laser types with different BPP in Fig. 6.2.3 Fiber lasers are now believed to deliver the highest beam quality and the advantages of improved beam quality are summarized in Fig. 6.3.4 A higher power density can be obtained by a smaller spot size with the same optics, or the same power density can be achieved at lower laser power, leading to reduced cost, as shown in Fig. 6.3(a). The same spot size can be attained at a longer working distance (Fig. 6.3(b)) or with a slim optics of smaller diameter (Fig. 6.3(c)), leading to improved manipulation and enhanced processing operation capability.4 The correlation of beam quality to laser power for respective lasers is overlaid with the condition regimes for several material processing methods in Fig. 6.4.2-8 The beam quality of a laser worsens with an increase in power. Deep-penetration or high-speed welding can be generally performed with a high power laser of the 5 kW class, and it is understood that LD - pumped YAG, thin disk, CO2 and fiber lasers can provide high-quality beams. The quality of high power diode lasers is the worst, although their wall plug efficiency is the highest. The development of higher power CO2 or YAG lasers is at present fairly static and therefore intensive effort is focused on

Table 6.1 Characteristics, laser media, maximum/normal powers and merits of typical lasers for welding

CO2 laser (wavelength: 10.6 mm; far-infrared ray)

Laser media : CO2-N2-He mixed gas (gas)

Average power [CW] : 45 kW (maximum)

(Normal) 500 W - 10 kW Merit : Easier high power (efficiency: 10-20%)

Lamp-pumped YAG laser (wavelength: 1.06 mm; near-infrared ray)

Laser media : Nd3+: Y3Al5O12 garnet (solid)

Average power [CW] : 10 kW (cascade type max & fiber-coupling max)

(Normal) 50 W-4 kW (efficiency: 1-4%)

Merits : Fiber-delivery, and easier handling

Laser Diode (LD) (wavelength: 0.8-0.95 mm; near-infrared ray)

Laser media : InGaAsP, etc. (solid)

Average power [CW] : 10 kW (stack type max.), 5 kW (fiber-delivery

max.)

Merits : Compact, and high efficiency (20-50%)

LD-pumped solid-state laser (wavelength: about 1 mm; near-infrared ray)

Nd : Y3Al5O12 garnet (solid), etc.

13.5 kW (fiber-coupling max.)

6 kW (slab type max.)

Fiber-delivery, high brightness, and high efficiency (10-20%)

Disk laser (wavelength: 1.03 mm; near-infrared ray)

Yb : YAG or YVO4 (solid), etc.

6 kW (cascade type max.)

Fiber-delivery, high brightness, high efficiency (10-15%)

Fiber laser (wavelength: 1.07 mm; near-infrared ray)

Yb3+ : SiO2 (solid), etc.

20 kW (fiber-coupling max.)

Fiber-delivery, high brightness, high efficiency (10-25%)

the development of high-power diode, LD-pumped YAG or solid-state, disk and/or fiber lasers with higher beam quality.

The reflectance or reflectivity of near - or far-infrared lasers is high for most metals, but decreases with a decrease in their respective wavelength. Copper vapor lasers (l = 510nm) and the second harmonic Q-switched YAG lasers (l = 532 nm), which can melt and evaporate highly reflective metals such as copper, are used for metals drilling. Recently, the apparatus delivering the second harmonic YAG laser of 2W with 3 ms pulse width has been developed with the objectives of welding copper sheets and so on directly.9 Such direct welding of copper may well replace soldering. Typical lasers and their features are briefly described in the following paragraphs.

6.3 Effect of improved beam quality on focusing. (a) Smaller focus at constant aperture and focal length, (b) longer working distance at constant aperture and spot diameter, (c) smaller aperture ('slim optics') at constant focal diameter and working distance.

6.2.2 CO2 lasers

The highest average output power can be obtained with a CO2 laser. Lasers in the 2.5 to 7kW class are normally used in the automobile industry10-12 and 5 to 45kW class lasers are utilized in the steel and shipbuilding industries.13-15 Furthermore, remote or scanner welding technology is noted in some industrial fields, and high quality CO2 laser systems with output power up to 6kW levels are now used as heat sources for remote welding of car body components.16-18

Cross-sections of laser weld beads in Type 304 steel made at 10 to 40kW are shown in Fig. 6.5.19 Deeply penetrated welds of a keyhole type are produced. The penetration depth increases proportionally with an increase in the laser power. Porosity is almost always present in such deeply penetrated welds.19,20 In Ar or N2 shielding gas at high powers such as 5kW and more, Ar or N plasma which blocks laser energy reaching the plate is always or intermittently produced along the laser beam axis over the shot location by the coaxial gas flow torch or by the plasma control nozzle from an oblique angle, respectively.20 Examples of plasma formation affecting weld penetration during laser welding are schematically illustrated in Fig. 6.6. The tendency of gas plasma to form depends upon the laser power and the material.19-22 Therefore, in CO2 laser welding, He shielding or He-mixed gas should be generally used at more than 10 kW to avoid the strong interaction which takes place between a laser beam and Ar plasma in the case of Ar shielding

Porosity is generally less in full penetration welds than it is in partial ones.20 In partial penetration welding at low speeds, longer periods of interaction between the laser beam and the keyhole wall and the instability of a deep keyhole during welding are chiefly attributed to bubble generation, leading to porosity formation.20-22 The formation of bubbles and porosity can be

reduced by the selection of proper repetition and width of pulse modulation,20-

22 2122

22 as shown in Fig. 6.7.2122 In this case, the porosity is drastically reduced at

60 to 70% duty (6 to 7 ms pulse beam irradiation period), because of the suppression of bubble generation.

6.2.3 YAG lasers

A YAG laser can be oscillated in the mode of continuous wave (CW), normal or modulated pulsed wave (PW) or Q-switching. The PW or CW laser beam can be delivered through a GI or SI fiber. In the case of low power or low pulse energy, as indicated in Fig. 6.8,9 deeper penetration can be obtained by GI fiber than by SI fiber, because the former can provide higher power density under the focused conditions. At high laser powers, there is little difference in the penetration between GI and SI fibers, and SI fibers having higher damage thresholds are chiefly used at CW powers greater than 1 kW.

A normal pulsed YAG laser can be used in spot or seam micro-welding of small parts in the electrical and other industries,23 and moreover SI-fiber delivered laser apparatus in the 3 to 4.5kW class is widely used in the automobile industry.12 In deep-penetration, weld spots and beads produced with pulsed or CW lasers easily cause porosity and therefore, in the case of pulsed laser, saw-like or tailing pulse shape should be utilized to reduce porosity.24-26 Controlled saw-like pulse shapes can reduce porosity and underfilling in the spot weld by suppressing spattering and adjusting keyhole depth, as shown in Fig. 6.9.26 In the case of high CW powers, a system using two or three beams was developed for reduction of porosity.27,28

YAG lasers of 10 kW class can be obtained by one apparatus29 or fiber - coupled 3 sets of 3 or 4kW class lasers.30 Penetration depths are indicated as a function of welding speed; comparisons between CW and modulated PW are shown in Fig. 6.10.29 When the average laser output powers are equal, pulse modulation offers greater advantages in the production of deeply penetrated welds owing to its higher power density at lower welding speeds. On the other hand, deeper penetration of a weld bead at higher speeds, i. e. greater than 25mm/s, as shown in Fig. 6.10, can be achieved in CW mode.

6.10 Effect of CW and PW modulation on penetration depths as a function of welding speed.

6.2.4 Laser diodes (LD)

High power laser diodes (LD) or diode lasers, which can be used directly and/or in fiber-coupled mode, are commercially available in a maximum power output range up to 10 kW and 5kW, respectively.4,31-34 Diode lasers of low power levels are suitable for the welding of plastics. High power diode lasers of rectangular beam shape leading to moderate power density (which are generally called bad quality) are directly used to weld thin sheets of aluminum alloys or steels at high speeds. Furthermore, fiber-delivered diode lasers are employed for brazing of Zn-coated steels by using robots together, and some lasers can produce deeply penetrated keyhole-type weld beads in stainless steel at low speeds. It is generally accepted that diode lasers are suitable for the welding of plastics and thin metal sheets as well as the brazing, soldering, quenching, surface melting treatment and cladding of metals.

The development of bright high-power diode lasers is anticipated as compact, highly efficient heat sources for such processes as welding and brazing.4,34

6.2.5 LD-pumped solid-state lasers

Commercially available LD-pumped YAG lasers can deliver brighter and higher-quality beams at higher efficiencies than lamp-pumped YAG lasers. LD-pumped YAG lasers of 2.5 to 6kW power are used in the automobile industry.4 A 13.5kW laser system using three sets of 4.5 kW apparatus is in special use.

Rod-type LD-pumped Nd:YAG lasers with output powers of 8 and 10kW have been realized as a laboratory prototype in Germany and Japan and a slab-type LD-pumped Nd:YAG laser of 6kW power has been developed by PLM (Precision Laser Machining Consortium) in the USA.4,35-37 The welding results with the latter slab-type laser are compared with those with a lamp - pumped YAG laser in Fig. 6.11.37 Weld bead penetration can be extremely deep at low welding speeds. Bead welding was performed with a pulsed laser of focal length 350 mm and about 10 mm depth of focus at a high repetition rate (for example, 400 Hz) with a weaving process because of the narrow beam diameter; thereafter, cosmetic treatment might be required for underfilling due to the severe spattering caused by extremely high power density. Such a bright laser with high power density can be effective in the production of deep weld beads.

The above example with the slab-type bright laser may show exceptional penetration depths. In general, in welding with LD-pumped CW YAG lasers, the welding phenomena and imperfection formation tendencies are the same as those with lamp-pumped CW YAG lasers. The penetration depends mostly upon the power and its density.35

6.2.6 Disk lasers

Yb:YAG disk lasers are relatively new and expected to have higher power, higher efficiency and higher intensity (brightness).35 Disk laser systems of 1 and 4kW are commercially available and are delivered through 150 and 200 mm diameter fibers, respectively.6,7,35 The principle of the laser system are shown in Fig. 6.12. Figure 6.13 gives a comparison of weld penetration between 4kW rod and thin disk lasers.7 It is confirmed that deeper penetration is obtained with a disk laser of higher power density at higher speeds. Even a 1 kW class laser produces a deep keyhole-type weld bead with extremely

narrow width in stainless steel and aluminum alloy. It is expected that such a laser with superior beam quality will be utilized in place of high power CO2 or lamp-pumped YAG lasers. Moreover, a thin disk laser could be used as a heat source for remote/scanner welding with a robot.

6.2.7 Fiber lasers

Fiber lasers have good beam quality and are now recognized as being highly efficient, bright and high-power lasers. High power lasers for welding are being rapidly developed using pumping system and fiber coupling, as shown in Fig. 6.14.38 Deep weld beads can be produced with the fiber laser as well as with the LD-pumped rod YAG laser. The laser at 6.9 kW can provide a deeply penetrated weld at high speed.39 Moreover, it is possible to use fiber lasers as heat sources for remote or scanner welding in conjunction with robots, in place of high quality (slab) CO2 lasers.17 Fiber laser appliances of 10 kW or more are available and those of 100kW power levels are scheduled.