Mold With Thermally Insulating, Protective Coating

The present invention relates to a mold, in particular a mold for casting aluminum and its alloys, the working surface of which features a thermally insulating, protective coating.

On casting metals in molds the melt is brought into direct contact with the mold in order to solidify the metal. For reasons of quality it is necessary to regulate very accurately the heat transfer during the time the melt first makes contact with the mold surface. If heat extraction is too strong, undesireable cold shuts are often observed in the cast product. Strong heat extraction through the mold at the start also produces considerable thermal stressing which can lead to thermal cracking of the mold work face.

A known method of regulating the heat transfer between melt and mold is to apply a thermally insulating, protective coating to the work face of the mold. Such coatings are made for example of ceramic materials which are applied to the mold face by high temperature spraying methods. Permanent ceramic coatings, however, yield only relatively short service lives and have high cost. Also known are thermally insulating coatings which are deposited on the mold face in the form of an aqueous suspension of fine, granular refractory material. In practice it has been found disadvantageous if the layer is not uniformly thick over the whole of the mold face as the rate of solidification at the start is also non-uniform, which can lead to flaws in the casting such as surface porosity and surface cracking. Also, commercially available coatings form on the work face of the mold a strongly adherent protective layer which has to be removed completely in a very labor intensive process before depositing a new layer.

In view of the above it is an object of the invention to develop a mold of the kind mentioned at the start bearing a protective coating which provides very good thermal insulation and which can be readily deposited uniformly over the work face of the mold and removed again from that face.

This object is achieved by way of the invention in that the protective coating comprises basically sub-micron metal oxide particles.

The use of sub-micron metal oxide particles to coat the work face of the mold makes it possible to build up thin layers with very low density and therefore low thermal conductivity. To achieve a specific thermal insulation, therefore, only a small quantity of metal oxide particles per unit work face area is necessary.

The mass of the protective layer of metal oxide particle material is preferably 0.002-2 mg/cm.sup.2 of mold facing, and the preferred particle size is 5-50 nm.

Particularly good results with respect to thermal insulation are obtained using a protective layer made up of sub-micron SiO.sub.2 particles. Other preferred metal oxides are Al.sub.2 O.sub.3, MgO, TiO.sub.2 and ZrO.sub.2. The oxides may be employed as single oxides or in mixture form.

The coating process can be carried out simply by wetting the work face of the mold with an aqueous sol containing a metal oxide, and subsequently evaporating off the water phase preferably by the application of heat.

In a particularly advantageous application of the process the work face of the mold is heated to a temperature of at least 60.degree. C. and then sprayed with or immersed in the aqueous sol, whereby these stages can be repreated several times. The density of the coating can be varied widely via the concentration of the aqueous sol, the spray time, and the number of immersion and drying cycles.

The protective layers deposited by this method on the work face of the mold have a density of around 0.2 g/cm.sup.3 which, for a mass of 0.002-2 mg/cm.sup.2 of work face, provides a layer which is 0.1-100 .mu.m thick.

The protective layer of sub-micron metal oxide particles exhibits adequate adhesion to the mold face throughout casting. Particles on the surface of the cast product or on the mold face can be readily removed after casting by means of compressed air or water jetting.

The coating of sub-micron metal oxide particles is suitable for all kinds of molds either smooth or roughened.

In the case of stationary molds such as in die casting molds and molds for casting pigs, after each cast the still hot work face of the mold, if desired after the removal of the worn layer, is usefully sprayed with the aqueous sol by jetting with compressed air or water.

The coating of the work face of continuous casting molds with continuously moving mold walls which have their work faces cooled by jetting directly with water can be carried out very simply by adding an aqueous sol of metal oxide to the cooling water.

Preferred, commercially obtainable silica sols which generally have a SiO.sub.2 content of around 10-30 wt.-% and if desired up to approximately 1.5 wt.% Al.sub.2 O.sub.3 can be diluted freely with water according to the thickness of coating wanted.

Further advantages, features and details of the invention are revealed in the following description of results from trials.

Spraying trials in which a 0.1% silica sol was sprayed onto a copper plate heated to about 100.degree. C. showed that a coating of 0.005 mg SiO.sub.2 /cm.sup.2 is obtained after spraying for only 3 seconds. To obtain a coating of 0.2 mg SiO.sub.2 /cm.sup.2 using a 1% silica sol, it was necessary to spray for 15 seconds.

After heating copper plates to about 100.degree. C., they were sprayed for different lengths of time with a 1% silica sol; this way it was possible to produce coatings of 0.002-2 mg SiO.sub.2 /cm.sup.2 on the copper plates.

Aluminum melts, at a temperature of 680.degree. C., were poured onto the coated copper plates. After the solidified metal had cooled, the dendrite arm spacing in the metal structure was measured. From this it was seen that already a coating of 0.002 mg SiO.sub.2 /cm.sup.2 of copper plate surface led to a considerable increase in the dendrite arm spacing compared with an uncoated plate which is to be attributed to the excellent thermal insulation provided by the protective layer of SiO.sub.2 particles.

After pouring aluminum repeatedly onto the coated surface, a gradual removal of the coating was observed due to SiO.sub.2 particles adhering to the solidified metal.