Laser Welding of Jewellery

Laser machines for jewellery are compact, low-powered and safe, see Figure 1. They weld most alloys quickly, repeatably and precisely, but the efficiency of the laser process depends very much on the properties of the target material. The energy that is effectively used in the welding depends on the surface absorption of the target and is controlled by adjusting pulse intensity, duration and pulse frequency. The laser welding effectiveness depends on properties of absorption, reflection and any chemical reactions of the target material. Components to be joined, or upgraded (repaired) in the case of castings, are arranged under visual control or jigged, and exposed to one or more laser pulses. A stereomicroscope and cross hairs facilitate the positioning of the parts and help to target the exact position where the laser pulse will strike.

The laser welding machine is easier to use if the shape of the beam in the working zone is cylindri-cal; this is because the spot diameter does not change over a range of focus of several millimetres, see Figure 2. A typical laser pulse lasts from 1 to 20 millseconds and suitable adjustments can be made for various materials through trial and error, but certain heat flow data allow good predictions of suitable welding parameters. Material properties which need to be taken into account when welding jewellery alloys, including latent heats of melting and corresponding thermal conductivities, are available but these values are more accurately known at or near room temperature than around the melting point. Despite the high energy needed to melt platinum alloys, their relatively low thermal diffusivity allows the heat to be retained/concentrated at the target, so they can be efficiently welded. However, platinum jewellery alloys have casting temperatures around 2000⁰C and high Solidification rates, so the challenge is for compact laser machines to achieve this.

The very high intensity laser pulse generates a surface temperature well above the melting point of platinum over a very small diameter target spot. This allows controlled welding, under ideal conditions, as close as 0.2 mm from complicated and heat-sensitive component parts, such as hinges, catches, fasteners, settings, most precious stones, and even, with care, pearls and organic materials. Provided that the heat flow away from the target is limited, it is possible to retain heat treated or cold worked hardness in most jewellery alloys; this works particularly well with platinum jewellery alloys.

The settings given in Table II are based on a beam diameter of about 0.5 mm on the materials stated and may need adjustment for other compositions. The main control settings on the laser machine (power/energy, beam diameter, durations) have slightly different effects on any one material, as shown in Figure 3. Different materials can have very different values for thermal diffusivity, melting temperatures and latent heats of melting. The way these properties combine together has a marked effect on the energy intensity needed to produce an effective weld. Welding is achieved only when adequate heat is absorbed through the surface, not when the beam is reflected off the surface, so surface colour and reflectivity have to be taken into consideration. Where there is a combination of high reflectivity and high heat dispersion (for instance in silver and high carat golds), it is helpful to mark and darken the target spot or line with a dark blue or black felt tip pen or permanent marker. This effectively increases the absorption coefficient of the surface.

Why Platinum Responds well

The efficiency of a laser welding machine differs from alloy to alloy. While the same set of control parameters will result in the same power delivered in each welding pulse, the melting effect of each pulse depends on the proportion of the heat energy absorbed and then on the rate at which the heat is dissipated from the melting/welding zone. This is not simply a function of the thermal conductivity. Thermal conductivity is defined as the rate of heat transferred through a volume whose two extremes are at different but constant temperatures — a steady state. However, what matters more in dependable practical workshop technology, is how the heat is transferred from a hot-spot, such as where a welding torch touches a surface, through a mass whose temperature rises as a consequence, and thus where the ‘low’ temperature end is not at constant temperature. This property is best described by thermal diffusivity, still very dependent on conductivity but modified by the specific heat of the metal related to the volume:

The heat input parameters are:

Melting points for most platinum alloys are high, but thermal diffusivities are relatively low (Table III) so the laser is able to deliver enough energy to melt a very small focused spot at each pulse but with only a small heat affected zone. With the possible exception of Palladium, all the Platinum group jewellery alloys respond in the same way to identical settings of a laser machine. Slight differences in surface colour when melting in air (alloys containing copper and cobalt tend to be a little greyer) have little effect on the optimum settings. Gold and silver alloys have lower melting points but five to seven times higher capacity to transmit heat away from the target.

The units used in Table II are c.g.s. units. This is because they fit the scale of jewellery alloys better than SI units (which rates thermal diffusivity in J m² s^-1) and the interest here is in the order of the effect rather than calculating the actual heat flow. The data for the pure metals are known most accurately around room temperature (2, 3) and data for the alloys were calculated from the properties of the pure metals, based on alloy composition by weight. The fullest version of the data was used where available. For instance, data for Platinum, gold and silver have been more extensively studied than for, say, ruthenium. However, the data in Table III have been ratio-nallsed to two decimal places to give greater uniformity, while acknowledging that the accuracy at higher temperatures is questionable.

The laser beam does its most effective work at or near the melting point of the target metal so we should be more interested in data at and near melting point temperatures. Thermal conductivity increases with temperature: for Platinum from 0.171 at 300 K to 0.230 at 1800 K; and at the melting point it is around 0.24. Specific heat (more correctly, molar heat at constant pressure, C_p) within a single-phase region also increases with temperature according to a polynomial function:

For Platinum, a = 5.755, b = 0.00150.5, C. = –0.185 x 10^-6, so the specific heat increases from 0.0316 at 273 K to 0.0452 at 2046 K.

Density decreases by the cube of the coefficient of linear expansion with temperature, which for Platinum is 8.9 × 10^–6 This means that a cube of platinum, sides of 1 cm, at 273 K (21.40 g) would expand to a 1.016 cm-sided cube at 2046 K, or volume 1.049 cm³, which is almost 5% less dense — at 20.04. On these assumptions, the thermal diffusivity of Solid Platinum at the melting point is approximately 0.265 instead of 0.245, about an 8% increase. All the Platinum jewellery alloys have thermal diffusivities of the order of 0.23 to 0.27. A similar argument applied to silver, shows its thermal diffusivity decreases from 1.74 to 1.31 (near its melting point).

The heat energy contained within 1 cm³ cubes of liquid Platinum, gold and silver at 100 K above their respective melting points, are 1854, 956, and 878 cal, respectively. Although the heat contents of molten Platinum jewellery alloys are roughly double those of gold or silver, their thermal diffusivities are about one fifth of that for gold and one seventh of that of silver. So the rate of heat input to the target can still substantially exceed the rate of outward heat diffusion. In effect this means that the laser beam can be placed very close to delicate stones with Platinum, and that normally it is unnecessary to remove stones before making repairs. A skilled operator can weld the surface without scarring and most components can be polished to ‘near finish’ before welding. Alternatively, components can be ‘tack welded’, adjusted to the correct position and final welds made with laser settings that improve the cosmetic finish of the tack welds.

Another feature of the localised heating effect of the laser is that dissimilar alloys can be joined more readily than when using bulk melting. There are incompatible pairs of metals, but a laser welding machine can produce narrow weld zones where the change in colour or texture between the two components is sharper and better delineated than in alternate technologies. The most obvious common feature of a typical range of laser welded Platinum jewellery is that remarkably small sections, thin stampings and fine wire can be at least stitch welded with precision. More extensive welds and repairs of casting defects (see later section) can be made by a series of overlapping pulses. As the laser is limited to joints that can be hit by the direct beam, deep and undercut sites should be avoided.

Jewellery Design

Several components in the jewellery shown in Figure 4 have been elastically stressed to give springiness and rigidity. The tightly localised and limited heat diffusivity allows springy and hard components to be joined with little or no softening. This enables designs that make good use of lightweight springy sections or robust fasteners. The very limited heat affected zones also allow joining of more dissimilar alloys (assay rules per-rmtting) than would be possible with large scale melting. In good commercial practice all the components could have a high degree of finish prior to joining. Most of the high finish is preserved, and it is clearly easier before welding to clean up and polish separate components than finished pieces.

Upgrading and Repairs

There are probably as many laser welding machines used for upgrading castings as there are for making welded pieces. Some surface defects can be repaired at the fettling stage but small pinholes sometimes show up later during polishing. The expense to both the finisher and caster of returning such components for recasting is often avoidable by using the laser welder to upgrade the castings, particularly when casting and finishing operations are on the same site.

A small area of rough surface texture may be glossed by using a rapid repeat sequence of pulses with the laser beam set relatively wide and shallow. Small pinhole defects (around 0.25 mm) can be filled by similarly pulsing around the edge of the defect. Larger defects can be effectively filled with fine filler wire touched into the defect, cut to size with the laser beam striking the wire and then lev-elling the filler down to the original surface. The colour of the filler can usually be matched accurately to that of the casting. The principle of relatively low thermal diffusivities of Platinum jewellery alloys when used with a laser machine, whether for upgrading castings or for welding, are virtually identical.

Conclusion

Comparing jewellery alloys, the necessary heat inputs to melt Platinum alloys are high compared with gold and silver alloys, but thermal diffusivities are significantly lower. One effect is that heat is more localised around hot-spots than with gold and silver.

Most Platinum jewellery alloys show the relatively high stress necessary to exceed the elastic limit, followed by a high rate of plastic work hardening which also raises the ‘bend-back’ stress. Components may have useful strength and springiness in slender sections, and these extra properties acquired before laser welding can be retained after assembly.