Laser welding with Nd:YAG lasers
Laser welding represents a new process being applied to an old industrial technique. It is a fusion welding process requiring no filler material, where parts are joined by melting the interface between them and allowing it to solidify.17 The process is not just an alternative to conventional welding processes, but rather it offers the engineers and designers greater flexibility in selecting components from materials, which are difficult or impossible to weld conventionally. Much literature now exists on laser welding and the reader may refer to Duley18 for the most up-to-date discussion and presentation on the theoretical and practical aspects of the process.
The main features of laser welding which make it an attractive alternative compared to conventional processes are:
• Precise narrow and deep welds can be produced with high metallurgical quality (weld bead less than 1mm and penetration up to 50 mm).19
• A small heat-affected zone reduces metallurgical damage and also allows welds to be made close to heat-sensitive components.
• The low heat input into the material obviates the need for complex jigging and allows distortion-free welding of thick to thin sections.
• High process speed-welding speeds in excess of 10 m/min can be achieved with materials of thickness about 1 mm.
• Flexibility allows one laser to be shared among a number of workstations.
• Post-weld treatment is not normally required.
• Welds can be performed in difficult geometries and dissimilar material thicknesses.
5.11 Principle of (a) conduction welding and (b) keyhole welding.
Laser welding is performed by one of two mechanisms illustrated in Fig. 5.11. In conduction welding, overlapping spots from a pulsed laser or from the beam of a continuous laser are absorbed by the surface of the material and the volume below the surface is heated by thermal conduction producing a semi-circular cross-section. This type of welding is usually confined to materials up to 2 mm thick. When the laser power exceeds 1 kW and power density exceeds 106 W/cm2 deep penetration welding is achieved. At this intensity level the rapid removal of metal by vaporisation from the surface leads to the formation of a small keyhole into the workpiece. The keyhole grows in depth because of increased coupling of radiation into the workpiece, through multiple reflections of the laser beam off the keyhole walls, and
5.12 Transverse sections of butt joint in stainless steel with a 3 kW Nd:YAG laser: (a) 5.7 mm thick @ 2.8m/min; (b) 8mm thick @ 0.12 m/min.
material vaporisation. The balance between the hydrostatic forces of the liquid metal surrounding it governs its existence and the pressure of vaporised and ionised material or plasma within it. A typical macrograph of a keyhole weld is shown in Fig. 5.12.
Sometimes plasma is ejected from the keyhole, forming a cloud above the workpiece. This plasma cloud can have a deleterious effect on the welding process because it can shield the workpiece from the laser beam leading to wider and shallower welds. To overcome the problem a shielding gas is normally employed both to suppress plasma formation and to protect the weld from oxidation. Gas flow rates are in the range 10 to 40 l/min depending on the laser power. Helium shielding gas is used when welding with high power CO2 lasers because of its high ionisation potential which inhibits plasma formation. Oxygen-free nitrogen is also an effective plasma suppressor but can cause embrittlement in some steels. Carbon dioxide gas can be used with pulsed lasers but is avoided with continuous lasers because it assists plasma formation. For the production of long welds in easily oxidised materials where an additional trailing gas cover is required to prevent oxidation, argon shielding gas can be used.
Laser welding, like other processes, requires the control of a number of operating parameters including power, mode, shielding gas and travel speed. There are many compilations of data that indicate the typical welding speeds
Weld penetration (mm)
5.13 Representative weld speeds as a function of penetration in mild steel for CO2 and Nd:YAG lasers.
and weld bead cross-sections, which may be expected.18 An example of welding speed as a function of penetration for a range of lasers and output powers is illustrated in Fig. 5.13. As a rule of thumb17 1kW of CO2 laser power at a welding speed of 1 m/min and focusing optic with an f number in the range 6 to 9 gives approximately 1.5 mm penetration in steel.
When considering laser welding the design of the joint and possibly the fabrication method of the whole product should be reassessed in order to gain the maximum advantage from the process. Laser welding requires high tolerances in gap control and joint positioning (Fig. 5.14). Using filler material can widen the tolerance field but, in practice, this is not very common because of the associated reduction in weld speed. In addition to the suitable preparation and alignment of the joint faces, they should be free from contaminants such as grease, paint, dirt and oxide scales. Residue from chemical degreasing and cleaning agents should be carefully removed since weld spatter and porosity can result if these substances are present on the workpiece.
In principle, a laser can also weld any material that can be joined by conventional processes. Illustrated in Table 5.1 is the weldability of metal pairs. In the welding of dissimilar metals, good solid solubility is essential for sound weld properties. This is achieved only with metals having compatible melting temperature ranges. If the melting temperature of one material is near the vaporisation temperature of the other, poor weldability is obtained and often involves the formation of brittle intermetallics.