diode lasers and fiber lasers have also been developed. Diode lasers of 2.5 kW power and 1 mm focus diameter have been demonstrated [19]. Conduction‐mode (surface melting) welding can produce full‐penetration welds with a depth–width ratio of 3 : 1 or better in 3‐mm‐thick sheets. Fiber lasers, on the other hand, can provide highly focused laser beams. The wavelength is 1.06 μm [20].
1.4.2.2 Reflectivity
The very high reflectivity of a laser beam by the metal surface is well‐known in LBW. As much as about 95% of the CO2 beam power can be reflected by a highly polished metal surface. Reflectivity is slightly lower with a YAG laser beam due to its lower wavelength. Surface modifications such as roughening, oxidizing, and coating can reduce reflectivity significantly [21]. Once keyholing is established, absorption is high because the beam is trapped inside the keyhole by internal reflection.
1.4.2.3 Shielding Gas
A plasma (an ionic gas) is produced during LBW, especially at high power levels, due to ionization of metal vapor by the laser beam. The plasma can absorb and scatter the laser beam and reduce the depth of penetration significantly. It is therefore necessary to remove or suppress the plasma [22]. The shielding gas for protecting the molten metal can be directed sideways to blow and deflect the plasma away from the beam path. Helium is often preferred to argon as the shielding gas for high‐power LBW because of greater penetration depth [23].
Since the ionization energy of helium (24.5 eV) is higher than that of argon (15.7 eV), helium is less likely to be ionized and become part of the plasma than argon. However, helium is lighter than air and is thus less effective in displacing air from the beam path. He‐10% Ar shielding has been found to improve penetration over pure He at high‐speed welding where a light shielding gas may be less able to displace air from the beam path [24].
1.4.2.4 Laser‐Assisted Arc Welding
As shown in Figure 1.32, laser‐assisted gas metal arc welding (LAGMAW) has demonstrated significantly greater penetration than conventional GMAW [25]. In addition to direct heating, the laser beam acts to focus the arc by heating its path through the arc. This increases ionization and hence the conductivity of the arc along the beam path and helps focus the arc energy along the path. It has been suggested that combining the arc power with a 5‐kW CO2 laser, LAGMAW has the potential to achieve weld penetration in mild steel equivalent to that of a laser 20–25 kW in power [25]. Albright et al. [26] have shown that a lower power CO (not CO2) laser of 7 W and 1 mm diameter can initiate, guide, and focus an Ar‐1% CO gas−tungsten arc.
Figure 1.32 Weld penetration in GMAW and laser‐assisted GMAW using CO2 laser at 5.7 kW.
Source: Hyatt et al. [25]. Welding Journal, July 2001, © American Welding Society.
1.4.2.5 Advantages and Disadvantages
Like EBW, LBW can produce deep and narrow welds at high welding speeds, with a narrow HAZ and little distortion of the workpiece. (Penetration depth can be up to about 20 mm, far less than in EBW.) It can be used for welding dissimilar metals or parts varying greatly in mass and size. Unlike EBW, however, vacuum and X‐ray shielding are not required in LBW. However, the reflectivity of a laser beam can be high. As with EBW, the equipment cost is very high, and precise joint fit‐up and alignment are required.
1.5 Resistance Spot Welding
The resistance spot welding (RSW) process is shown schematically in Figure 1.33. Two overlapping metal parts are squeezed tightly against each other by the tips of two opposing electrodes that are water‐cooled. The resistance to the electric current passing through the electrodes and parts can exist in the form of bulk resistance and contact resistance. The highest resistance usually is at the faying surfaces (represented by Point 4), thus causing local heating and melting at the faying surfaces to form a weld nugget after cooling.
Figure 1.33 Resistance spot welding: (a) overview; (b) resistance to electric current; (c) weld nugget.
The electrode clamping force need to be sufficient to keep the liquid in the nugget from being expelled, called expulsion or spitting. The lower density of liquid than solid causes volume expansion upon melting and promotes expulsion. Expulsion can leave insufficient liquid in the nugget to form a continuous solid weld, resulting in large voids and weakening the weld.
The key process variables include the welding current (usually several thousands to tens of thousands of amperes), welding time, clamping force, and electrode shape. The copper electrodes are often alloyed with refractory metals to improve erosion resistance. It is desirable to have a nugget with a diameter about 3.5–4 times the sheet thickness and a height about two‐thirds the combined thickness of the sheets [27]. To reduce erosion, some electrode tips are coated with special materials.
In welding dissimilar metals, e.g., Cu to steel as shown in Figure 1.34, heat balance may need to be considered. The much higher thermal conductivity of Cu than steel may prevents Cu from being melted sufficiently. So, Cu can be thicker and a smaller electrode tip area can be used to increase the current density. Alternatively, an electrode tip with high electrical resistance material such as tungsten or molybdenum can be used.
Figure 1.34 Resistance spot welding of dissimilar metals with highly different thermal conductivities.
1.6 Solid‐State Welding
1.6.1 Friction Stir Welding
FSW is a solid‐state joining process invented by The Welding Institute in UK in 1991 [28]. As shown in Figure 1.35, the tool typically has a shoulder (which can be flat or slightly concave) and a pin (also called a probe) at its lower end. For welding Al alloys, the tool is made of tool steel, e.g. H13 steel. For butt welding, the length of the pin is slightly less than the thickness of the workpiece. The pin is typically threaded to help push the surrounding material downward, while stirring it, to enhance the mixing action. An example of the tool is shown in Figure 1.35b. It is a H13 steel tool, with a shoulder of 15 mm diameter and a threaded pin of 6.0 mm diameter and 5.4 mm length beyond the shoulder. The shoulder is slightly concave.
