New developments in advanced welding

Control of Nd:YAG laser welding

Both CO2 and Nd:YAG laser welding is carried out on those products where a high confidence level in the weld quality is necessary. Some of examples already mentioned are the welding of razors blades, heart pacemakers and automotive parts. The aspects that contribute to producing welds of respectable quality can be grouped under the following headings:

• materials

• joint design

• welding conditions

• in-process monitoring

• joint tracking

5.8.1 Materials

Variations in materials used can have a significant effect on weld quality. Some relevant variations are the following.

Surface quality

The laser welding process is mainly unaffected by variations in surface quality of the material unless the changes are sufficient to prevent the coupling of the laser beam. This commonly occurs with highly polished surfaces that increase the threshold power density for achieving a keyhole type process. Highly oxidised surfaces can produce porosity during welding.

Coating thickness

Variation in the coating thickness can alter the welding performance. The weld joints are characterised by blowholes or porosity.

Proximity of sealants

If sealants or adhesives are present on the joint line that is to be laser welded, disruption of the weld bead and excessive porosity will occur.

Pressing quality

The most common variation likely to be seen for the three-dimensional structures will be the quality of the pressed components. If the gaps between the joints to be welded are too big and cannot be compensated by the clamping operation, inconsistent weld quality will result.

5.8.2 Joint design

The effect of joint fit-up has already been explained. Fit-ups which leave gaps of > 10% sheet thickness will for butt, lap, hem or edge joints result in a weld undercut which will adversely affect weld properties and performance. Factors affecting joint configurations for laser welding are shown in Table 5.4.

5.8.3 Welding conditions

Focus position

Optimum focus position is dependent on weld joint geometry and weld strength requirements. The optimum focus position is typically that which yields the maximum weld penetration for the butt joint and weld width interface for the lap joint configurations. In general, the tolerance to focus position for laser welding of sheets is ± 0.5-2mm for Nd:YAG lasers, with the focus being on the top workpiece surface for the focal lengths 80-200 mm. When the focus positions outside these tolerances are used, the weld will show:

• Reduced penetration, if the focus position is above the workpiece surface;

• Greater tendency for the weld undercut, if the focus position is below the workpiece surface.

If the focus position is moved further into the workpiece, a loss of weld penetration will occur.

Welding speed

The welding speed is the parameter most often adjusted when defining optimum welding conditions. This takes into account such factors as laser power, laser mode, spot size and power density. Given that all the other parameters are constant, welding speed or weld penetration will increase with:

• increased average power (Fig. 5.23)

• improved beam quality

• small focused spot size (Fig. 5.24).

If the welding speed is too high, the weld is characterised by a loss of weld penetration and cracking, while with low welding speed the weld exhibits excessive drop trough, top bead undercut, a disrupted weld bead and there may be excessive porosity.

Shielding gas

The shielding gas fulfils two main roles, to provide protection against excessive

oxidation and to reduce plasma formation. The formation of plasma is more critical to welding when using the CO2 laser, as there is an interaction involving laser energy and the cloud of ionised gas above the weld, which reduces the penetration. Nd:YAG laser welding does not suffer plasma formation; however, when welding thick sections (>4mm) at slow welding speeds, there is a cloud of gas above the weld which can affect the quality of the weld.

The most frequently used cover gas is either helium or argon and typically

it is directed centrally at the laser/material interface; if there is an auxiliary tube design, it is directed towards the trailing weld (hot material). Helium is technically the most suitable shielding gas for CO2 laser welding due to its ability to suppress any plasma formation; in the case of Nd:YAG laser welding, helium gas can also be used for welding stainless steels, aerospace alloys and a range of aluminium alloys. However, due to its low mass, flow rates that provide effective protection from the atmosphere are high, especially for open, three-dimensional components. This factor, coupled with the high cost of helium, makes the use of other cheaper gases attractive.

For whichever type of shielding gas and delivery used, a too low gas flow is characterised by a heavily oxidised weld surface while too high a gas flow causes excessive weld undercut and disrupted weld bead. In most cases underbead shielding is not required for welding at speeds > 1 m/min. However, for stainless steels, nickel alloys, titanium alloys and aluminium alloys the use of underbead shielding is recommended in order to produce an acceptable underbead appearance.

Measurement of quality

With CO2 or Nd:YAG laser welding the relationship between process parameters and the weld quality is complex. In addition to problems caused by changes in the material composition and surface conditions, alignment errors can be significant, especially when welding large structures. Such errors can affect the weld quality and one approach to detect defect welds is to use in-process monitoring or seam tracking systems; this allows errors to be recognised as they occur.