Wetting by alloys
Measurement of contact angle and surface tension of molten metals
In the manufacture and processing of metals, the surface of molten alloys comes into contact with other surfaces such as furnace walls, casting molds and soldered joints. Different grades of wetting are required depending on the application.
The surface tension and contact angle are characteristic of the wetting behavior of molten metals on the solid surface. We have developed accessories and measuring instruments for carrying out drop shape analysis at high temperatures up to 1750 °C for measuring these quantities. Our measuring equipment also provides options for carrying out measurements in vacuum or under the exclusion of oxygen. This enables actual process conditions to be simulated in order to optimize alloys and their wetting.
The lowest possible wetting of the furnace wall is required when recovering metals and alloys. Molten metal adhering to the wall is detrimental to heat transfer and reduces the yield. Measurement of the contact angle between molten metal and wall material provides information on the wetting behavior. If the wetting is too great, this can be counteracted by suitable control of the process in order to keep the furnace wall free.
The wetting of solder plays an important role in the joining of workpieces or when producing electronic assemblies. The solder must wet the solder point and remain on the printed circuit board, but must not run over a large area. The actual degree of wetting is indicated by the contact angle. Measuring this quantity can prevent expensive manufacturing errors and reduce scrap.
Casting molds must be perfectly wetted by the metal so that the mold cavity is exactly reproduced by the solidified melt. The contact angle between molten metal and mold therefore provides important information.
The very high surface tension of metals can cause the flow to stop during casting. As a result, the mold is not completely filled and the workpiece does not have the correct shape after solidification. As the surface tension and the wetting are highly dependent on temperature, the mold can be optimized by suitable control of the process. Automatic measurement of the temperature-dependent surface tension during a heating ramp with the Drop Shape Analyzer – DSAHT is helpful in this regard. A suitable process temperature can be determined or the composition of the alloy adjusted based on the measuring curve obtained.
Interatomic forces in the melt are responsible for the formation of surface tension. Measuring the surface tension therefore allows conclusions to be drawn concerning the bonding forces in alloys. As a metallurgical method, measurement of the surface tension helps to specifically affect the strength of alloys.