This invention relates to a method for continous meldcasting of metallic strip, metallic matrix composites and laminated products by melding two thin liquidous/solidous layers together. If desired, reinforcing constituents, such as filaments or whiskers, may be introduced to the molten or solidifying metal. Alternatively solid or reticulated webs or strips of other materials may be introduced to form laminated products in continuous lengths. Additionally, the outer surfaces of the cast strip may be shaped into decorative or useful patterns. The invention also comprises a twodrum machine for producing the two thin liquidous/solidous layers and for melding them together with or without the reinforcing or laminating constituents.
To illustrate the advantages of the present invention aluminum and its alloys will be used as examples of the cast metal although this invention applies to other metals and alloys also.
Existing methods of producing aluminum or aluminum alloy strip start with a cast product from 0.25 of an inch up to 24 inches or more in thickness. If the cast product is over 0.375 of an inch thick, it must be hot rolled to a gauge which can be coiled, typically 0.09 to 0.375 of an inch thick. Such hot rolling equipment usually has a minimum capacity of 100,000 short tons per annum and likely costs more than $50 million. If the cast stock is less than 0.375 of an inch in thickness, it can be cold rolled directly to the required finished gauge. It is normally more economic to cast stock less than 0.375 of an inch in thickness (by continuous casting) and coil it. However, prior art continuous casting processes cannot efficiently cast thicknesses less than 0.25 of an inch. The continuous casting equipment to produce a typical gauge such as 0.30 inch thick stock will likely cost $5 million for a minimum capacity of 10,000 short tons per annum.
Whether starting with a thick ingot and hot rolling or with 0.3 inch thick continuous cast stock, it is necessary to use powerful high capacity cold rolling equipment for reducing the gauge to that required by most users (typically between 0.01 and 0.04 of an inch thick). Such cold rolling equipment typically has a capacity of 50,000 short tons per annum or more and typical facilities cost $50 million dollars. Thus, with existing processes it is impossible to produce the gauges of sheet required for most applications with an expenditure of less than about $55 million.
These existing metal casting processes also introduce many undesirable operating costs. In producing thick ingot much energy and cooling water are wasted during solidification, which energy is then replaced during ingot reheating to achieve the temperatures required for hot rolling. Energy is then additionally wasted when the hot rolled coil is allowed to cool for cold rolling. When continuous casting, energy and water are similarly wasted to the degree that solidification takes place at a gauge greater than that required by the application. For the hot and cold rolling reductions required by such processes it is necessary to use flood lubrication on the rolls. This flood lubrication contains oil and is a substantial environmental hazard and cost.
From the above review of existing casting methods, it is evident that for aluminum and many other metals a great deal of capital and fabricating costs could be saved by casting a strip of thickness closer to the final gauge required by the appIication.
Other economies occur if the production unit can be reduced in capacity and proportionately in capital cost, without major offsetting losses in production efficiency. Most of the margin between the cost of crude ingot and the delivered price of finished strip or sheet is represented by the sum of factory overheads plus administrative, selling, inventory, distribution and transportation costs. The actual cost of transformation from ingot to strip is typically less than 40% of the margin between ingot and the selling price of strip.
Thus, a low capacity, simple, cost effective, strip producing unit will reduce delivered costs of strip by permitting local production and in-house production of strip and sheet, which are now produced by shipping ingot from a smelter to a large rolling mill and then shipping the rolled product to the user. This principle that costs are dramatically reduced and usage is greatly expanded by local fabrication is illustrated by the history of the aluminum extrusion industry. In the United States, extrusions were produced after World War II by the three primary aluminum smelters plus two or three independent extruders. However, the selfcontained small extrusion press was developed, together with an improved alloy. A press could then be installed for less than $500,000. This feasibility of economical localized presses lead to a rapid expansion of the number of independent extruders so that by 1965 there were over 100. At the same time, there was a great expansion of aluminum extrusion usage which rose from being about 15% of sheet usage to being about 40% of sheet usage.